Fuels and Lubricants Handbook

Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing George E. Totten, Editor Section Editors Steven R. Westbrook Rajesh J. Shah

ASTM Manual Series: MNL37WCD

ASTM International 100 Barr Harbor Drive PO Box C700 # iMTBttmrmtM. West Conshohocken, PA 19428-2959 Printed in the U. S. A.

Library of Congress Cataloging-in-Publication Data

Fuels and lubricants handbook: technology, properties, performance, and testing/George E. Totten, editor; section editors, Steven R. Westbrook, Rajesh J. Shah, p. cm.—(ASTM manual series; MNL 37) Includes bibliographical references and index. ISBN 0-8031-2096-6 1. Fuel—Testing—Methodology. 2. Fuel—Analysis. 3. Lubrication and lubricants—Analysis. I. Totten, George E. II. Westbrook, Steven R., 1956-III. Shah, Rajesh J., 1969-IV. Series. TP321.F84 2003 662',6—dc21

2003049604

Copyright © 2003 ASTM International, West Conshohocken, PA. All rights reserved. This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher.

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Printed in Glen Bumie, MD June 2003

Foreword This publication, Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing, was sponsored by ASTM Committee D02 on Petroleum Fuels and Lubricants and edited by George E. Totten, G. E. Totten & Associates, LLC, Seattle, Washington. The section editors were Steven R. Westbrook, Southwest Research Institute, San Antonio, Texas; and Rajesh J. Shah, Koehler Instrument Company, Bohemia, New York. This publication is Manual 37 of ASTM's manual series.

Contents Preface—George E. Totten, Steven R. Westbropk, and Rajesh J. Shah

ix

I. PETROLEUM REFINING PROCESSES FOR FUELS AND LUBRICANT BASESTOCKS—^o/esfe / . Shah, Section Editor Chapter 1—Petroleum Oil Refining Marvin S. Rakow

3

II. FUELS: PROPERTIES AND PERFORMANCE— Steven R. Westbrook, Section Editor Chapter 2—Liquefied Petroleiun Gas Roberts. Falkiner

31

Chapter 3—Motor Gasoline B. Hamilton and Robert J. Falkiner

61

Chapter 4—Aviation Fuels Kurt H. Strauss

89

Chapter 5—^Automotive Diesel and Non-Aviation Gas Turbine Fuels Steven R. Westbrook and Richard he Cren

115

Chapter 6—Introduction to Marine Petroleum Fuels Matthew F. Winkler

145

III. HYDROCARBONS AND SYNTHETIC LUBRICANTS: PROPERTIES AND PERFORMANCE—/?a;es/i/. Shah, Section Editor Chapter 7—Hydrocarbon Base Oil Chemistry Arthur J. Stipanovic

169

Chapter 8—Hydrocarbons for Chemical and Specialty Uses Dennis W. Brunett, George E. Totten, and Paul M. Matlock

185

Chapter 9—Additives and Additive Chemistry Syed Q. A. Rizvi

199

Chapter 10—Synthetic Lubricants: Nonaqueous Thomas F. Buenemann, Steve Boyde, Steve Randies, and Ian Thompson

249

Chapter 11—Environmentally Friendly Oils Hubertus Murrenhoff and Andreas

267 Remmelmann

Chapter 12—Turbine Lubricating Oils and Hydraulic Fluids W. David Phillips

297

vi

CONTENTS Chapter 13—Hydraulic Fliiids Willie A. Givens and Paul W. Michael

353

Chapter 14—Compressor Lubricants Desh Garg, George E. Totten, and Glenn M. Webster

383

Chapter 15—Refrigeration Lubricants—Properties and Applications H. Harvey Michels and Tobias H. Sienel

413

Chapter 16—Gear Lubricants Vasudevan Bala

431

Chapter 17—Automotive Lubricants Shirley E. Schwartz, Simon C. Tung, and Michael L. McMillan

465

Chapter 18—Metalworking and Machining Fluids Syed Q. A. Rizvi

497

Chapter 19—Petroleum Waxes G. Ali Mansoori, H. Lindsey Barnes, and Glenn M. Webster

525

Chapter 20—Lubricating Greases Thomas M. Verdura, Glen Brunette, and Rajesh Shah

557

Chapter 21—Mineral Oil Heat Transfer Fluids John Fuhr, Jim Oetinger, George E. Totten, and Glenn M. Webster

573

Chapter 22—Non-Lubricating Process Fluids: Steel Quenching Technology Bozidar Liscic, Hans M. Tensi, George E. Totten, and Glenn M. Webster

587

rv. PERFORMANCE/PROPERTY TESTING PROCEDURES— Steven R. Westbrook and Rajesh J. Shah, Section Editors Chapter 23—Static Petroleum Measurement Lee Oppenheim

635

Chapter 24—Hydrocarbon Analysis James C. Fitch and Mark Barnes

649

Chapter 25—Volatility Rey G. Montemayor

675

Chapter 26—Elemental Analysis R. Kishore Nadkami

707

Chapter 27—Diesel Fuel Combustion Characteristics Thomas W. Ryan HI

717

Chapter 28—Engineering Sciences of Aerospace Fuels Eric M. Goodger

729

Chapter 29—Properties of Fuels, Petroleum Pitch, Petroleum Coke, and Carbon Materials Semih Eser and John M. Andresen

757

CONTENTS Chapter 30—Oxidation of Lubricants and Fuels Gerald J. Cochrac and Syed Q. A. Rizvi

787

Chapter 31—Corrosion Maureen E. Hunter and Robert F. Baker

825

Chapter 32—Flow Properties and Shear StabiUty Robert E. Manning and M. Richard Hoover

833

Chapter 33—Cold Flow Properties Robert E. Manning and M. Richard

879 Hoover

Chapter 34—Environmental Characteristics of Fuels and Lubricants Mark L. Hinman

885

Chapter 35—Lubrication and Tribology Fundamentals Hong Liang, George E. Totten, and Glenn M. Webster

909

Chapter 36—Bench Test Modeling Lavem D. Wedeven

963

Chapter 37—Lubricant Friction and Wear Testing Michael Anderson and Frederick E. Schmidt

1017

Chapter 38—Statistical Quality Assurance of Measurement Processes for Petroleiun and Petroleum Products Alex T. C. Lau

1043

Index

1061

vii

Preface There are many books on various aspects of fuels and lubricant chemistry, applications, and testing. However, few focus on testing and none provide extensive, in-depth coverage on fluid properties and testing methodologies together. And while there are numerous national and international standards that deal with specific testing procedures appropriate for fuels and lubricants, it is generally beyond the scope of these procedures to provide an extensive discussion of the principles behind the tests and their relationship to the properties themselves. Therefore, there is a strong need to address these informational shortcomings in the Fuels and Lubricants industry, which is one of the most significant tasks undertaken in this work. The ASTM Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing is an extensive, in-depth, well-referenced handbook that provides a detailed overview of various testing methodologies and also provides a thorough overview of the applications-related properties being tested. Since this manual provides an overview of all of the ASTM and important non-ASTM test procedures relating to the application areas addressed, it is an excellent companion text to the Annual Book of ASTM Book of Standards, or it is an invaluable reference manual on its own. The organization of the ASTM Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing is based approximately on the committee structure of the ASTM D.02 Petroleum Fuels and Lubricants Committee and the standards for which each committee is responsible. The information in this text is subdivided into four sections: I-Petroleum Refining Processes for Fuels and Lubricant Basestocks; II-Fuels; Ill-Hydrocarbons and Synthetic Lubricants; and IV-Performance/Property Testing Procedures. This manual contains thirty-eight chapters covering the following topics: • An overview of petroleum oil refining processes • Liquified petroleum gas (LPG) • Aviation, automotive diesel, non-aviation gas turbine, and marine fuels • Gasoline and oxygenated fuel blends • Petroleum hydrocarbon base oil chemistry • Synthetic hydrocarbons • Environmentally friendly fluids including those formulated from vegetable oil and synthetic ester basestocks • Lubricating oils for turbines, compressors (industrial and refrigeration), gears, and automotive applications • Metalworking fluids • Petroleum waxes • Lubricating greases • Oils used in non-lubricating applications: heat transfer fluids and metal quenchants • Detailed discussion on: static petroleum measurement, volatility, elemental analysis, fuel combustion characteristics, oxidation, corrosion and viscosity • Properties of coke, petroleum pitch, and carbon materials • Hydrocarbon structural analysis procedures • An extensive discussion of lubrication and wear • Environmental and toxicity testing • Statistical quality assurance testing procedures Essentially, all of the numerous important applications and test methods involved in the Fuels and Lubricants industry are discussed and referenced here. We strongly believe that this book will be an invaluable resource for anyone working in this industry, especially since this information is not available in any other single source. ix

X

PREFACE The preparation of a text of this scope was an enormous task involving many people. The editors are deeply indebted to the authors for their h a r d work and incredible patience. Specicd thanks go to Monica Siperko and Kathy Demoga at ASTM for their continued support and encouragement throughout the development of this text. The editors are especially indebted to their families for many evenings and weekends lost to this project. We Eilso acknowledge the vital support of Southwest Research Institute and the Koehler Instrument Corporation. George E. Totten General Editor G.E. Totten & Associates, LLC. Seattle, WA, USA Steven R. Westbrook Section Editor-Fuels Southwest Research Institute San Antonio, TX, USA Rajesh J. Shah Section Editor-Lubricants Koehler Instrument Company Bohemia, NY, USA

Section I: Petroleum Refining Processes for Fuels and Lubricant Basestocks Rajesh J. Shah, Section Editor

MNL37-EB/Jun. 2003

Petroleum Oil Refining Marvin S. Rakow^

THIS CHAPTER PRESENTS AN OVERVIEW of t h e m o d e m , integrated

oil refinery, a n d how it separates a n d processes crude oil and other hydrocarbon feedstocks into the required array of liquid fuels a n d other products. A description of an overall, simplified refinery flow plan serves to introduce the subject and terminology unique to the industry. I m p o r t a n t crude oil properties a n d test methods, along with a perspective on past, current, a n d likely future fuel product quality and demand, is presented. Each important refinery process is described in sufficient detail to appreciate its purpose, operating characteristics, yield a n d quality parameters, a n d current and future utilization in light of anticipated fuels product trends.

GENERIC REFINERY FLOW PLAN Figure 1 is a simplified block flow diagram of a fully integrated refinery utilizing the major process options in genereJ use. Each actual plant will incorporate those options that meet crude throughput and quality, as well as product dem a n d and quality, while striving for m i n i m u m capital and operating cost. Since most refineries reach their current configuration by periodic revamp and expansion, it is both likely and economically feasible for two refineries with similar feed and product slates to have a different mix of processes to satisfy all technical a n d economic parameters. Crude oil first undergoes physical separation by distillation (ASTM Test Method for Distillation of Petroleum Products, D 86-99a) to yield various boiling range streams as dictated by the chemical processing to follow (Table 1). The atmospheric distillation unit usually separates about one-half of the crude oil into the indicated cuts ranging from the low boiling gases through a gas oil. The final boiling point of the atmospheric gas oil can range from as low as 340°C t o as high as 410°C. Vacuum distillation of the atmospheric distillation unit bottoms (ASTM Test Method for Distillation of Petroleum Products at Reduced Pressure, D 1160-95) usually separates another 30% of the crude oil as vacuum gas oils having a fined boiling point ranging from 500-575°C. Selection of downstream processing units is greatly dictated by demand and specifications of the different products, and, particularly, increasing sulfur removal requirements. One of the key considerations is gasoline demand. For exam-

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pie, in the United States, one hundred volumes of crude oil are converted into about fifty volumes of gasoline (the ratio is about 42% o n a weight basis). Yet, crude oil typically has only 15 to 20% of its hydrocarbons in the gasoline boiling ramge. The fluid cataljrtic cracking unit (FCCU) is the main process that provides the additional "gasoline" and is thus a key process unit in the U.S. refinery flowsheet. The gasoline to crude oil ratio is substantially lower in all other parts of the world; thus FCCU, while in place worldwide, plays a significantly lesser role in non-U.S. refineries. Similar comparisons are summarized for other refinery processes in Table 2. In order of increasing boiling range, description and utilization of the distillation fractions are as follows: • CI to C4 gases—The crude oil gases, along with gases created by the downstream process chemistry, cu^e separated by carbon n u m b e r and hydrocarbon type in one o r more light ends plants (also called vapor recovery units, o r "sats" and "unsats" gas plants). Impurities, the main one being H2S, a r e removed, with final separation providing t h e product disposition shown in Table 3. • LSR—The light (straight run) gasolines, in most refineries, are blended directly t o t h e gasoline pool. If a refiner is squeezed for octane number, o r if LSR octane quality diminishes d u e t o refiner response to changing gasoline specifications, the octane n u m b e r of this stream cem be increased by the isomerization process. • HSR—The heavy (straight r u n ) naphtha, whether from crude oil or other process units, is too low in octane number to be economically blended in gasoline. Thus, all HSR is processed through the catalytic reforming unit (CRU) wherein the hydrocarbon composition is changed or "reformed" to a higher octane n u m b e r stream. • Kerosine—This fraction, after appropriate cleanup (treating), is directly sold to the kerosine and jet fuel markets and is also used as a blending component for the lighter fuel oils and diesel fuels. Since the jet fuel market now exceeds the kerosine market in parts of the world, many refiners now label this stream the "jet" cut. • Atmospheric Gas Oil (AGO)—The gas oils are generally those distillable streams heavier than kerosine. The name was derived in the early industry days when these oils were thermcJly cracked to produce olefin-containing "illuminating" gases for street lighting, for example. Depending on the final boiling point of the AGO a n d the downstream process options, t h e atmospheric distillation unit m a y jdeld one AGO c u t o r two. The lighter portion m a y b e hydrotreated to produce low sulfur diesel, while the heavier cut, hytrotreated or not, is typically processed in the FCCU.

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4

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

C, + C, GASES

REF. FUEL

PROPANE

•LPG •ALKYLATE -MTBE •GASOLINE

•LPG

BUTANES

CRUDE

LSR

A T M D I

S T

ATM V

ISOMERIZATION

-GASOLINE

NAPH

HYDROTREATING

KERO

HYDROTREATING

GAS OIL

•

CAT REFORMING

•JET FUEL

HYDROTREATING

GASES

RESID

— No. 2 FO/DIESEL

Q

MTBE ALKYLATION

L V A C D I

s

•GASOLINE

GASOLINE

GASOLINE FLUID

LVGO

LCO

No. 2 FO

CATALYTIC HCO CRACKING

HVGO

DO

T

No. 6 FO •CARBON BLACK H, PLANT

HYDROCRACKING

— No. 6 FO — ASPHALT VAC RESID

•REFORMER FEED •JET FUEL

•VISBREAKING •DEASPHALTING •COKING •HYDROCRACKING

FIG. 1—Refinery block flow diagram.

Vacuum Gas Oils (LVGO and HVGO)—i:\iese cuts are usually processed in the FCCU. They can be processed in a hydrocracking unit (HCU) wherein they can be converted into catalytic reformer feed, jet and diesel fuel, and for production of lubricating oil base stocks. Vacuum Resid—The residuum portion of crude oil is the non-distillable cut, boiling above, say, 575°C. It cannot be distilled without causing substantial thermEd destruction of its hydrocarbons. This cut is sold to the asphalt and heavy fuel oil markets, or processed by the options shown in Fig. 1.

The need for sulfur removal is on an ever-increasing path as average crude oil sulfur content continues to increase and product sulfur is forced to lower levels by environmental legislation. Sulfur removal, today, is accomplished in "hydroprocessing" units. Hydrotreating, wherein the goal is to remove sulfur (and other heteroatoms) with minimum conversion of the hydrocarbon feed stream to lower boiling molecules, is the most widely utilized. Alternatively, hydrocracking is utilized to remove heteroatoms and concurrently convert varying amounts of "high boilers" to lower boiling streams.

CHAPTER 1: PETROLEUM TABLE 1—Crude section cuts. Carbon Number Name/Other Names Gases Light straight run gasohne light n a p h t h a LSR gas condensate Heavy straight run naphtha Heavy naphtha Naphtha Reformer feed Kerosine or kerosine jet Light atmospheric gas oil furnace oil diesel Heavy atmospheric gas oil AGO gas oil Light and heavy vacuum gas oils LVGO and HVGO Vacuum resid" vacuum bottoms short resid vacuum reduced crude asphalt

C1-C4 C5-C6

Approximate Boiling Range, "C -160-0 25-90

C6-C10

85-190

C9-C15

160-275

C13-C18

250-340

C16-C25

315^10

C22-C45

370-575

C40+

500+-565 +

"The corresponding bottoms from the Atmospheric Crude Tower is called: Atmospheric Resid Reduced Crude Atmospheric Bottoms Topped Crude Long Resid

OIL REFINING

5

the maximum possible hydrogen. To compUcate matters, in crude oil some carbon atoms are also bonded to sulfur, nitrogen, oxygen and metals, as well as trace amounts of other elements. The four hydrocarbon types, based on the above characteristics, are: Name Paraffin Olefin Naphthene Aromatic

Chain/Ring

Saturated/Unsaturated

Example

Chain Chain Ring Ring

Saturated Unsaturated Saturated Unsaturated

Hexane Hexene Cyclohexane Benzene

The four types are listed in the order shown after chemical analysis, and are usually referred to by the term PONA analysis (ASTM Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption, D 1319-98). In chain hydrocarbons, all carbon atoms can be linked in a straight row; these are the "normal" versions, such as the example of a six-carbon hydrocarbon, normal hexane (n-hexane). "Isomers" of n-hexane exist as branched-chain versions; as a group, these are labeled isoparaffins, for example i-hexanes. They are commonly connotated as nC6 and iC6s. When a hydrocarbon type analysis is made that includes a normal/isoparaffin split, the normals keep the letter P, the isos take the letter I, and the olefin/aromatic order is reversed, resulting in the acronym PIANO. This same split is not attempted for the normal and branched-chain olefins because of the greater number of potential olefin structures and the lesser impact on refinery process options.

HYDROCARBON TERMINOLOGY There cire four basic tjrpes of hydrocarbon chemical structures. Their differences relate to the bonding between carbon atoms and whether the carbon bonds create a chain- or ringshaped molecule. The number of electrons in the outer shell of a carbon atom requires each atom to have four bonds to be stable. This four-bond requirement can be peirtially met by a carbon atom forming single or multiple bonds with adjacent carbon atoms. Nearly all hydrocarbon molecules encountered in refining chemistry consist of all singly bonded carbons, or contain pairs of doubly bonded adjacent carbons; the remaining bonds cu-e satisfied with hydrogen. When eill carbon-carbon bonds in a molecule are single, it is called saturated, because it contains the maximum possible amount of bonded hydrogen. When there cire double bonds present, the hydrocarbon is labeled unsaturated, since it holds less than

Region Asia/Pacific Western Europe Eastern Europe/FSU Middle East Africa North America So. America/Caribbean

TABLE 3—Disposition of refinery C1-C4 gases. Carbon Number

Chemical Name

CI C2

Methane Ethane Ethylene Propane Propylene

C3 C4

End Uses

n-Butane i-Butane Butylenes

Refinery Fuel Gas (RFG) RFG RFG, PetrochemicEils Liquefied Petroleum Gas (LPG) Petrochemicals, Alkylation Unit, Polygas or Dimerization Unit LPG, Isomerization Unit, Gasoline Blending Alkylation Unit Alkylation Unit, MTBE, Polygas or Dimerization Unit, Petrochemicals, Isooctane Unit

TABLE 2—^Worldwide refining capacity as percent of crude. Weight Percent on Crude Capacity Crude Oil Distillation, Vacuum Hyd retreating Million mt/year Distillation FCCU CRU + Hydrocracking

1020 740 540 300 160 1010 340

23 36 35 34 15 46 43

4110 Source: Oil & Gas Journal Worldwide Refining Report for January, 2001.

13 15 8 5 6 33 19

9 13 12 9 11 19 5

41 58 38 37 25 68 29

Coke

0.4 0.5 0.8 0.3 0.2 4.5 1.8

6

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

By chemical naming convention, all saturated hydrocarbons, the paraffins and naphthenes, end in the letters "ane," while all unsaturated hydrocEirbons, the olefins and aromatics, end in the letters "ene." Each of the four species has different properties and reactivity, and this plays a major role in refinery unit selection and operation. The amount of the various heteroatoms and the m a n n e r in which they are bound to carbon also is critical to refinery operation. These factors will be apparent throughout the remainder of this chapter as well as in other parts of this manual.

C R U D E OIL

almost always less dense than water, which has a specific gravity of 1.0. Worldwide, average API Gravity is about 31 (sp. gr. = 0.87) and ranges from a high of about 45 for "light" crudes to a low of 8-15 for the "heavy" crudes, tar sand oils, and the like. Aromatics are the densest of the hydrocarbon types and are therefore lowest in API, while peiraffins Eire generally highest in API gravity. As boiling point of the crude oil increases, a r o m a t i c s content increases. Thus, high API crudes likely have lower concentrations of high boilers and most likely less aromatics, while low API crudes are generally higher in aromatics and in high boilers. Sulfur

Hydrocarbon Composition Crude oil is composed of aromatic, paraffin and naphthene hydrocarbons. Minor amounts of olefins may be present. Average aromatics content is about 50%; however, it can range from as low as 2 5 % in light paraffinic crudes to as m u c h as 75% in heavy oils, such as those recovered from Canadian tar sands. The paraffin/naphthene ratio is widely ranging and is qualitatively identified via the labeling of many crudes as "paraffinic" or "naphthenic." Hydrocarbon composition affects many considerations in selecting processing options, or meeting product specifications, as shown by the following examples:

Worldwide, weight average crude oil sulfur content is about 1.25% with commercial production ranging from 0.1-6%. Crudes low in sulfur are labeled "sweet" while those higher in sulfur £ure designated "sour." The terms derive mainly from the foul, or sour, odor of one of the sulfur species, the mercaptans, as well as the corrosiveness of hydrogen sulfide and the mercaptans. The split between sweet and sour was at 0.5% when average sulfur was m u c h lower than today; now, the crossover is more likely in the 0.5-1.5% range and is usually designated by each refinery depending on its history and capability to process higher sulfur crudes. Barring the discovery of major low sulfur crude oil pools, it is expected that average sulfur will continue to modestly increase.

Aromatics Because of their inherent molecular stability, they are the bane of refining. Aromatics are the most difficult compounds to thermally crack, hydrogenate, and desulfurize. They are increasingly "unwanted" species in fuel products because they are the hardest to completely combust, thus contributing more to pollution and equipment maintenance. As product aromatics content is regulated to lower levels, refinery process severity and costs will increase. Aromatics do have value in the refinery flow plan as a high-octane gasoline blending component from the catalytic reforming unit and in the recovery of chemical grade aromatic feedstocks for the petrochemical industry. Paraffins

and

Naphthenes

These constituents are more readily processed in the refinery. Each offers "positive" and "negative" features, but the effects of these species are not as severe as for the aromatics. As an example, paraffins crack cleanly and thus yield m i n i m u m coke, but, in fuel blends, they raise pour point and are more difficult to octane-upgrade in the CRU. Density The density of chemiceJ compounds is expressed as specific gravity (sp. gr.). In the petroleum industry, this weight/volu m e relation is expressed worldwide as "API (American Petroleum Institute), or commonly as "API Gravity", per the formula "API = (141.5/sp.gr.) - 131.5 (ASTM Test Method for API Gravity of Crude Petroleum and Petroleum Products, D287-92). The formula was adapted from the Baume density equation used for sulfuric acid. Its value is the creation of a whole n u m b e r scale to more readily discern one crude oil from another, rather than using decimals, since crude oils are

Nitrogen Generally, nitrogen content is about 5-20% of sulfur content in most crudes. This is a helpful circumstance because nitrogen removal requires greater process severity than sulfur removal since nitrogen is mostly bound in ring form while a greater percent of sulfur is bound in chain form. Heteroatom removal from rings is more difficult than from chains. Many California crude oils are high in nitrogen content, often equaling sulfur content, and thus require greater process severity than other crudes of equal sulfur content. Metals Organometallic compounds are predomineintly found in the vacuum resid. Important metals are nickel, vanadium, copper and iron, with nickel (Ni) and vanadium (V) usually in highest concentration. Ni + V levels in resid range from a few p p m to as m u c h as 1000 ppm. The metals, particularly Ni, have an adverse effect on catalyst performance and have specified maximum limits in products such as heavy fuel oil and petroleum coke. This can affect crude selection options for many refinery process configurations and product slates, as well as in process selection for future refinery revamp/expansion projects. Oxygen Acids, particularly naphthenic acids, and phenols constitute the main organic oxygen compounds in crude oil. A substantial portion are in the naphtha, kerosine, and gas oil boiling ranges. These compounds are readily eliminated via hydrogenation, and will pose less concern as more hydrotreating is

CHAPTER added to the flow plan to achieve greater sulfur removal. If these streams are not hydrotreated, the acids are usucJly removed by many variations of caustic treating.

PRODUCTS The main refinery products, except for the gas streams shown in Table 3, are automotive gasoline, kerosine/jet fuel. No. 2 and No. 6 fuel oils, and No. 2 diesel fuel. Changes to the refinery flow plan have mainly been driven by changes in crude oil quality, and in specifications and demand for these products. It should be noted that all industry fuels and lubricants are presented in this manual including a complete presentation and discussion of test methodology and properties. Gasoline The first meaningful specifications evolved in the 1920s. From this beginning until 1990, the two key control properties have been "octane number" and "volatility." Octcine number, the measure of the engine to resist the rapid "pinging" noise, called "knock," due to improper combustion, steadily increased as engines became larger and more powerful, peaking in the eeirly 1970s. The refinery met the required octane appetite of the car population by instcJling processes that produce high octane streams, such as the FCCU, CRU, and alkylation and isomerization. In addition, organolead compounds, mainly tetraethyllead (TEL), were used to add about eight octane numbers to the gasoline pool. Two laboratory octane n u m b e r test methods are used to measure and control this gasoline quality parameter, the Research Octane N u m b e r (RON) and Motor Octane N u m b e r (MON), per ASTM Test Methods for Research Octane Numb e r a n d Motor Octane N u m b e r of Spark-Ignition Engine Fuel, D 2699-97a and 2700-97, respectively. Their different operating conditions, detailed in this manual, act to delineate and bracket the potential for knock in the automobile population. In most of the world, RON is the value the consumer sees at the "pump," while in the United States and Canada, the average of both the RON and MON is the guiding specification. The first significant response to pollution from vehicular emissions came with the passage of the U.S. Clean Air Act in 1973. This legislation stipulated that the m a i n pollutants emitted from the automobile tailpipe; namely, u n b u m e d hydrocarbons, carbon monoxide and nitrogen oxides, had to be reduced by 90%. The automotive industry chose to use a catalyst to comply with this regulation. This "catalytic converter" requires platinum, or other noble metal such as palladium, to effect the desired emissions reduction chemistry. Platinum's activity, however, is adversely affected by lead. As a result the oil industry had to remove lead from gasoline and would have lost the eight octane numbers from the addition of TEL. The Act also required an increase in fuel economy from 12.5 mpg (5.35km/l) to 27.5 mpg (11.77km/l). This led to a reduction in vehicle weight and power, which in turn reduced octane appetite by about four numbers. Thus, the net effect on the U.S. oil industry from 1974 through the mideighties was to add about four octane numbers to the base gasoline pool. This was achieved mainly by the addition of

1: PETROLEUM

OIL REFINING

7

process technology such as catalytic reforming and alkylation. However, the industry also introduced organic oxygenates to gasoline blends, initially alcohols and then ethers, since such compounds have high octane quality (Table 4). Of the three utilized alcohols, the only surviving member is ethanol, which is used in the U.S. through the help of government price subsidy. Concern for water solubility, corrosion, and emissions problems from the alcohols, verified or not, drove the U.S. refining industry to the ethers. The first, a n d still dominant, ether was methyl tertiarybutyl ether (MTBE), which was initially provided by ARCO Chemical converting tertiarybutyl alcohol, a by-product from their propylene oxide process, to the isobutylene feed needed to make the ether. Volatility requirements can be described as the control of the boiling range of the gasoline to assure that the automobile operates properly during startup, warm-up, and hot operation. Gasoline specifications assure that there is a proper balance among the C5 through C l l hydrocarbons that constitute the gasoline boiling reuige, and that the final boiling point is limited. Volatility control also includes the need to add liquefied n-butane to provide sufficient vapor pressure (ASTM Test Method for Vapor Pressure of Petroleum Products (Reid Method), D 323-99a) to ensure cold engine startup. This results in "seasonal" volatility grades wherein butane content and boiling range Eire varied to match expected ambient temperature. As industry used the oxygenates to meet octane number requirements, data showed oxygenates can reduce pollutants such as carbon monoxide (CO) and nitrogen oxides (NO^). And, as analysis of the effect of various gasoline constituents on tailpipe emissions became better defined, it became apparent that other compositional changes would be beneficial. This was addressed by passage of the U.S. Clean Air Act Amendments in 1990. This legislation, and subsequent regulations such as those enacted in Ccdifomia, has substantively changed the entire composition of gasoline (Table 5), per ASTM Specification for Automotive Spark-Ignition Engine Fuel, D 4814-99. As a result, the refinery has undergone continuous change to meet the cleaner fuel product requirements. These changes, initiated in the U.S., are being adopted throughout most of the world; in some instances, legislation in other locales is resulting in even more severe specifications (Table 6).

TABLE 4—The oxygenates. Blending Actual RVP, kPa RON MON

Source

Name

Alcohols Methanol Natural Gas Ethanol Crops t-butyl alcohol Propylene Oxide By-Product Alcohol Ethers MTBE TAME ETBE TAEE DIPE

133 130 109

99 96 93

54 27 17 6

118 111 118

100 98 102

Olefin

Methanol Isobutylene Methanol Isoamylene Ethanol Isobutylene Ethanol Isoamylene (Propylene + Water)

"Average of RON and MON.

32 16 12

104"

8 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Jet Fuel

TABLE 5—History of U.S. gasoline specifications. Pre-1974 Regular Grade RON MON Pb, g/1 Premium Grade RON MON PB, g/1 RVP, kPa Oxygen, %w Benzene, %w, max 90% Dist., °C, max Aromatics, %w Olefins, %w Sulfur, p p m w

94 86 0.8 100 90 0.9 62-103 0

1992-1994

1974-1991 Remove Pb, Increase Fuel Economy

91 83 0.1

"

98 88 0.1

1995+ 91 83 0 98 88 0 50-90 2.0-2.7* 0.8-1.0 165-180 20-35 4-10 30-150

2.7*

0.3-1.0"

185-190 30-45 8-15 300-500

'^Optional use for octane number improvement. 'Mandatory in locales requiring "oxy fuel" or reformulated gasoline.

TABLE 6—Gasoline specifications in U.S. and Europe. California

United States 1998-2000 2004

Locale Year Oxygen, %w MTBE, %w Benzene, %w Sulfur, p p m w Aromatics, %v Olefins, %v

2.0-2.7 11.0-14.8 0.95-1.0 150-350 25-32 6-12

0.0-2.7 Unknown 0.8-1.0 30-150 20-30 4-9

Europe

1996

2000

2005

2.0 11.0" 0.8-1.0 30 25 4-6

2.7 14.8 1.0 150 35 14

2.7 14.8 1.0 10-30 30-35 12-14

''MTBE banned by 2003, but enforcement is currently on hold.

Today's military and commercial aircraft use jet fuels that are produced almost exclusively from the kerosine fraction of crude oil or similar boiling range cuts from certain refineiy processes (Table 7). Key requirements focus on sulfur and aromatics contents, clean burning characteristics, and storage stability (ASTM Specification for Aviation Turbine Fuels, D 1655-99). Increasingly, refineries use hydrotreating technology, as well as crude slate selection, to meet specifications and aircraft power demand. Legislative reduction in sulfur and aromatics is not currently anticipated. F u e l Oil The various fuel oil grades are shown in Table 8 (ASTM Specification for Fuel Oils, D 396-98). The major products are No. 2 and No. 6 fuel oils. No. 2 fuel oil, commonly called furnace oil or home heating oil, is the fuel used for residential and small commercial oil heating systems. No. 6 fuel oil is commonly called industrial or heavy industrial fuel oil when used by utilities for electricity and steam generation. It is called Bunker, or Bunker C, fuel when used by the marine industry for those ships powered by "furnaces." Nos. 1, 4, and 5 are specialty products. For example, No. 1, also known as stove oil, is a kerosine-type blend for small stovelike non-atomizing b u r n e r s a n d some farm equipment, while Nos. 4 and 5 are lower viscosity industrial fuel oils for smaller commercial furnaces such as those used by schools and hospitcds. Sulfur content is an important quality parameter. While most No. 2 fuel oil is specified at 0.5%, maximum, (ASTM Test Methods for Sulfur in Petroleum Products, D 129-95, D 1266-98, and D 2622-98, for example) m u c h product has considerably lower sulfur because of the n e w low sulfur diesel requirement. Many refiners have found that the added cost to desulfurize No. 2 fuel oil to match diesel specifications may be offset by reducing the n u m b e r of separate products, providing increased product blending options and reducing storage and distribution costs. No. 6 fuel oil sulfur ranges from 0.25-3.5% depending on end use and locale. It is anticipated that average "six-oil" sulfur content will decrease over time.

TABLE 7—Jet fuel specifications. 775-840 Density, kg/m^ 204 10% Dist, °C, max Final Boiling Point, °C, max 288-300 Flash Point, °C, min 38-60 Aromatics, %v, max 20-25 Sulfur, %w, max 0.3-0.4 MercaptEin Sulfur, %w, max 0.002 Freeze Point °C, max - 4 0 to -47

TABLE 8—Fuel oil specifications. Grade Uses Density, kg/m^, max 90% Dist, °C, max Flash Point, °C, min Pour Point, °C, max Sulfur, %w, max Viscosity mm^/s @ 40°C min max mm^/s @ 100°C min max

No. 1 Stove and Farm

No. 2 Home Heating

850 288 38 -18 0.3-0.5

876 338 38-55 -6 0.3-0.5

1.3 2.1

1.9 3.4

No. 4

No. 5

No. 6

Light Industrial

Medium Industrial

Heavy Ind'l & Marine

55 -6

55

60 0.25-3.5

5.5 24.0 5-9 9-15

15 50

CHAPTER TABLE 9—Diesel fuel specifications. No. 1

No. 2

No. 4

Uses

Farm, City Bus

Automobile Truck, Railroad

Railroad, Marine, Stationary Engine

90% Dist, °C, max Sulfur, ppmw, max Cetane No., m i n Flash Point, °C, min Aromatics, %w, max

288 10-5000 40-55 38-55 10-35

338 10-5000 40-55 52-55 10-35

20 000 30 55

Grades

Diesel Fuel Key diesel fuel specifications and major uses eire shown in Table 9 (ASTM Specification for Diesel Fuel Oils, D975-98b). The U.S. Clean Air Act Amendments also required the reduction of sulfur to 0.05% (500 ppmw), maximum, for over-theroad usage; namely, buses, trucks, and automobiles. The legislation did not require lower sulfur for other use such as the railroad and marine industry. Again, as with the fuel oils, m u c h No. 1 and 2 diesel is made "low sulfur" for all markets to minimize product distribution costs. Driven initially by European and California legislation, diesel fuel sulfur is being further reduced toward 10-50 ppmw, requiring more severe hydrotreating. While the U.S. law did not substantially constrain aromatics content, California law did, requiring reduction of aromatics to as low as 10%. Worldwide adoption of this level of dearomatization will be difficult to achieve near-term, but a trend toward lower aromatic content diesel is occurring. This is being affected in part by requiring higher Cetane N u m b e r diesel fuel (ASTM Test Method for Cetzuie Number of Diesel Fuel Oils, D613-95). Aromatics decrease "cetane rating." P o u r point (ASTM Test Method for Pour Point of Petroleum Products, D97-96a) and cloud point (ASTM Test Method for Cloud Point of Petroleum Products, D5771-95) are importctnt parameters to assure proper flow of fuel oils and diesel fuels under all anticipated ambient temperatures. Pour point must be low enough to prevent solidification of the entire fuel while cloud point must be low enough to assure that normal paraffins will not solidify and block fuel flow in piping or through nozzles.

CRUDE OIL PREPARATION AND SEPARATION A refinery may process just one crude oil or u p to as many as a few dozen, depending on factors such as captive crude supply, access to ocean and river delivery, and pipeline supply logistics. At a refinery handling multiple crudes, individucil crude oils are usually fed sequentially over variable lengths of time. Some of the crude oils may first be blended in intermediate crude storage tanks to reduce the effects of substcintial changes in crude oil quality on the process units. In almost all refineries, the crude oil slate is processed as in Fig. 2. Crude oil is first "descJted" to reduce entrained and dissolved salts, mainly sodium, m a g n e s i u m and calcium chlorides, to a n acceptable level a n d to remove "debris" such as mineral matter resulting from transportation. The crude oil is then physiccJly separated by distillation in atmospheric

1: PETROLEUM

OIL REFINING

9

a n d v a c u u m towers/columns/crude u n i t s to provide the streams in Table 1. Many refineries incorporate a preflash column before the atmospheric tower to remove a portion of the lower boiling components. This improves subsequent cut point control and flexibility in the atmospheric tower, and can increase m a i n c o l u m n capacity. A complex heat exchange network is used to warm the crude to desalter temperature and then to furnace inlet temperature. Desalting is accomplished by mixing 100 parts of crude oil with about 3-10 parts of water at about 120-140°C. Emulsifier or demulsifier additives may be used to aid either the mixing of crude and water or separation of the two after mixing. Electrically charged plates may also be used to coalesce water to aid in separation. The desalted water is then sent to wastewater treatment and, if necessary, for stripping of undesired contaminants such as benzene. After further heat exchange, usually to 210-240°C, the crude is sent through multi-pass furnaces to attain atmospheric column inlet temperature in the range of 340-410°C. Then, by control of top column reflux a m o u n t and temperature, reboil, and p u m p a r o u n d / p u m p d o w n parameters, the desired cuts are obtained. Atmospheric column bottoms are usually sent to the vacu u m tower to recover the vacuum gas oils to a final boiling point (FBP) as high as 575°C, although 540-565°C may be more common. The ability to reach desired FBP depends on m a x i m u m furnace outlet temperature a n d its concomitant coke laydown on the furnace coils vs. economic run length, and attainable vacuum. Vacuums in the range of 7-25mm Hg absolute are usually achieved using steam ejector systems or vacuum p u m p s .

PROCESSES FOR GASOLINE AND DISTILLATE YIELD A N D QUALITY Fluid Catalytic Cracking This is the predominant process for converting high boiling gas oils a n d resid into lower boiling streams, mostly gases, a gasoline blending component and a light gas oil for No. 2 fuel oil blending, fts place in the refinery is mainly driven by the yields and compositions of the product components, the ability of the process to vary product yield/composition, a n d lower capital investment cind operating costs compared to alternative process options. Figure 3 depicts the process operating principles. Many equipment configurations and catalysts are employed, but the essence of the process is reactor and regenerator "vessels," vapor phase fluidization of the catalyst in each vessel, flow of catalyst between the two vessels, thermal cracking of the feed in the reactor and removal of coke from the catalyst in the regenerator. Much of the design variations cire related to engineering improvements and the increased processing of poorer quality feed, which in t u r n has been enabled by greatly increased activity of today's catalysts. Fresh feed and, if desired, heavy product recycle, are introduced into the reactor. The feed vaporizes and thermally cracks in about 1-3 s at about 525-550°C, to yield a potential variety of product s t r e a m s exemplified in Table 10. Coke forms in the reactor section due to rapid polymerization of olefins a n d aromatics as cracking chemistry occurs. This

10 MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK Condenser

—•0—iS^S—I I

Atmospheric Distillation

*•'Wet Gas •*LPG

Reflux

• • Light "B-' Water r'stabifeation'l Straight Run -*• and. ^ L.SpJtttinjtJ_*

xr

~ ) P Stripper

Naphth: Kerosene

Crude

t

Stripper Steam

-Water

* Diesel

Preheat Desaiteri "Gas Oil

Steam

Brim Preheat L f i j J Preflash Drum

Vacuum Distillation

Furnace^»i=J -••Topped Crude

Steam

Non-condensat)les and steam

PreCondenser High Pressure Steam NonC.W. condensables O

P

-

T

QHot Wbll Accumulator [) ^ Heavy Gas Oil

» Naphtha

Sour Water

Topped Crude Q Ejectors

Coil Steam

Stripping > Vacuum Resid Steam FIG. 2—Atmospheric and vacuum distillation units.

coke "lays down on" the fluidized catcJyst particles, diminishing their activity. The "coked-up" catalyst is continuously removed from the reactor section and transported with air to the regenerator wherein controlled combustion converts the coke into a "flue gas" consisting mainly of carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and sulfur and nitrogen oxides (SOx and NOx) resulting from the presence of sulfur and nitrogen in the feed. This combustion raises the catalyst temperature to 680-760°C which, in turn, supplies most or all of the heat to raise fresh feed to its reactor temperature as the "hot" regenerated catalyst is continuously removed from the regenerator and re-mixed with fresh feed. In fact, many FCC units operate in heat balance, meaning that all heat to raise the feed to reactor temperature is supplied by the regenerated catalyst.

CO, a pollutant, must be removed from the regenerator flue gas. Even if CO emissions were not an issue, economics of maximizing heat recovery would dictate oxidation of CO to CO2. This is accomplished by the use of combustion additives in the regenerator, improved regenerator design and operation, and external secondary combustion in a CO or waste heat boiler. SOx and NOx pollutants face refinery emission limits. Acceptable levels in the flue gas can be achieved with regenerator additives, various flue gas scrubbing processes, or by installing "cat feed hydrotreaters" to reduce the sulfur and nitrogen content in the FCCU feed. Run length between scheduled turnarounds for inspection and maintenance is 3-5 years. Longer runs have resulted from ongoing improvements to the overall design that include multistage regeneration, improved cyclones and feed

CHAPTER nozzles, and taking advantage of favorable FCCU process economics. Factors favoring economics include the ability to produce high yields of high-value gas and gasoline from low value feed, substantial yield flexibility resulting from catalyst selection and variable operating parameters, operation at essentially atmospheric pressure, and no addition of hydrogen as is required in the hydr©cracking process. The cyclone systems in both the reactor and regenerator are mechanical devices to collect the catalyst carried out of the dense phase fluidized beds by the respective cracked product and flue gas vapors. Although thousands of tons of catalyst circulate each day, cyclone efficiency is so close to 100% that losses are kept in the 1-10 ton-per-day range. The losses from the reactor are returned via the downstream product recovery train. Losses from the regenerator are collected and then disposed, thus creating the need for daily catalyst replacement. Early catalysts were silica-alumina "clays" incapable of processing resid and limited in activity. Today's catalysts contain various highly active zeolites, combined with appropriate silica-alumina bases, which permit the processing of heavier, dirtier feed, as well as provide increased yields of

Reactor Vapor

Reactor

Reactor Riso'

Regenerator /

/

Spent-Catalyst Stripper

Combustor Riser Catalyst Cooler (Future) Combustor

Spent-Catalyst Standpipe

RecirculatedCatalyst Standpipe

RegeneratedCatalyst Standpipe

CooledCataiyst Standpipe (Future)

Chargestock

t Combustion Air UitGas FIG. 3—Fluid catalytic cracking unit.

\

1: PETROLEUM

OIL REFINING

11

TABLE 10—FCCU yields. Operating Mode

Gasoline

Gas

Distillate

Reactor Products, %w C1-C2 C3s C4s Gasoline Light Cycle Oil Heavy Cycle Oil Coke on Catalyst

3 6 11-12 46-52 15-19 7-8 6

6 22 20 27 12 5 8

2 4 8 38 35 9 4

light products without the overproduction of coke. Catalyst formulations can be chosen to maximize gasoline yield and octane number, gas yield and olefin content; tolerate resid metals; and resist higher regenerator temperature resulting from additional coke p r o d u c t i o n from the higher carbon residue level in the resid portion of the feed. Reactor effluent is taken to the "cat main fractionator" wherein the main separation occurs to yield the olefin containing gases, gasoline blending streams, a light gas (cycle) oil, and a heavy gas (cycle) oil. If the heavy gas oil has the right properties, it, or a heavier portion of it, may be sold as feed to produce carbon black for the manufacture of tires and printing inks. If so, it will be labeled carbon black oil, decant oil, slurry oil, or clarified oil. It is the heavy gas oil that is used as the "washing" stream at the b o t t o m of the fractionator to remove the catalyst to the settler and then back to the reactor. The gases undergo further absorption and distillation to achieve the desired separation for various end uses. Some refiners select feed, catalyst and operating conditions to maximize olefin gas yield at the expense of gasoline and/or light cycle oil yield. This has been done to meet growing demand for propylene as a petrochemical feedstock and to provide butylenes for MTBE and alkylation feed. The "cat" gasoline, a major gasoline blending component used throughout the world, is treated for mercaptan sulfer removal prior to product blending for odor and corrosion control. Removal of other sulfer compounds, or just the sulfur atoms themselves, will be required to meet future demand for low sulfer gasoline. While desulfurization via hydrotreating will r e m a i n a d o m i n a n t technology for some time, other sulfer removal technologies, for example utilizing extraction or oxidation, are in various stages of commercicdization. The light cycle oil is mainly blended to No. 2 fuel oil. As feeds to the "cat" unit get dirtier, and as demand for cleaner fuels grows, the refiner will be faced with an increasing need to reduce the sulfur, nitrogen, and aromatics content of this stream, if for n o other reason than to provide blending flexibility into the diesel fuel markets. Heavy cycle oil yield is very low in today's operation; most refiners blend this cut to No. 6 fuel oil, while some recycle it to extinction in the reactor. It was this recycle of the FCCU gas oil in the early days of this process that gave rise to labeling them light and heavy cycle oil in addition to light and heavy gas oil. Key fluid catalytic cracking process variables include: • Catalyst circulation rate, commonly called the "cat-to-oil ratio" • Reactor temperature • Catalyst activity • Feed residence time

12

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

The cat to oil ratio, in the weight range of 4 to 15, can be varied within the Umits of maintaining the necessary pressure balances and vessel vapor velocities so that catalyst flow and bed fluidization are properly maintained. Reactor temperature will vary with cat-to-oil ratio, regenerator temperature and catcdyst cooling, and from feed preheat for those units having feed furnaces. Process severity is mainly defined in terms of conversion of the heavy feed into gas and gasoline, and is expressed as 100 minus the product streams boiling above gasoline. Conversion typically ranges from 50-85%. Lower conversion is desired to meiximize light cycle oil yield for blending No. 2 fuel oil during the oil heating season, while higher conversion is set to meet gasoline a n d olefin gas deniEmd. Catalytic R e f o r m i n g This process is an essential part of every refinery that desires to use the heavy straight run naphtha fraction from crude, or similar boiling range material from a n o t h e r process unit such as the hydrocracker, as a gasoline component. The feed, usually a C6-C10 cut, is too low in octane n u m b e r to be economiccdly blended to the gasoline pool. The "cat reformer" converts low octane n u m b e r peiraffins and naphthenes in the feed into "high octane" aromatics in the product. Typical feed unleaded RON is 40-65 while product is 94-103. There Eire three main reactor configurations, two of which are shown in Fig. 4; all accomplish essentially the same chemistry. • Semiregenerative—Fixed bed reactors, all operating in series. • Cyclic—Fixed bed reactors in series; one out of service on rotating basis for catalyst regeneration. • Continuous—Moving bed reactors in series with "continuous" catalyst regeneration. The evolution from the original semiregenerative reformer to today's continuous unit was driven by "competing chemistry" within the process and by the need for higher octane n u m b e r product to help replace the lost octanes from lead. Hydrogen must be added with the naphtha feed to minimize coke formation and laydown on the catcdyst and achieve economic run length, since catalyst performance is readily diminished by coke. Higher operating pressure aids in reducing coke formation chemistry. However, the goal is to maximize the conversion of naphthenes to aromatics since higher eiromatics content translates into higher octane number reformate. Lower operating pressure increases aromatics yield since it makes it easier for hydrogen to "escape" from the naphthene. The continuous process is thus able to operate at lower pressure because the resulting higher coke make can be removed "continuously." In actuality, a catalyst particle tcikes about five days to migrate from the top of the first reactor to the bottom of the last; such r u n length in the fixed bed "semi-regen" reformer would be unacceptable. The original catalyst that enabled this chemistry consisted of an alumina base promoted with 0.25-0.75% platinum. Today's catalysts may contain just the platinum promoter; may be bi- or tri-metallic using additional metals such as rhenium and tin; may have the alumina base converted to the more acidic chloride form by treatment with chlorine or chloroform, for example; or may utilize appropriate zeolites. In any

case, the sensitivity of the catalysts to coke, and to deactivation from heteroatoms such as sulfur and nitrogen, requires that the naphthas first be hydrotreated. Fresh hydrotreated naphtha feed plus recycle hydrogen is heated to reactor temperature of about 500-530°C in a furnace and fed to the first reactor. The over-riding chemistry is endothermic dehydrogenation of naphthenes to aromatics. Thus, as the feed progresses down through the catalyst, as m u c h as 60°C cooling occurs from top to bottom in the first reactor, reducing activity and hence aromatics yield. The net effect is that octane n u m b e r gcdn in one reactor is not enough to make the process attractive. Therefore, the reactor effluent is reheated in a second furnace, or in a second coil within the same furnace, back to 500-530°C and fed to a second reactor, and so on, until desired octane product is attained. Cat reformers usually consist of three reactors although as many as five are utilized. Typical yields at two product RON levels are shown in Table 11. The net hydrogen produced from aromatics formation is the m a i n source of hydrogen in the refinery for its desulfurization and related needs. Refineries are in "hydrogen balance" if all such hydrogen can be provided by the reformers. If not, refiners purchase hydrogen or build their own hydrogen plants. Figure 5 depicts the main process parameters of catalytic reforming; namely, the "yield-octane" relationship. Higher octane n u m b e r d e m a n d requires higher reactor temperature and/or longer residence time. Both will cause more of the naphtha feed to crack into gas and thus reduce gasoline 3rield. "Reformer severity" is uniquely defined as the unleaded RON of the C5 + liquid product, the second major gasoline blending component. In fact, the gasoline streams from the fluid cat cracker emd cat reformer make u p about two-thirds of the world's gasoline. Significant changes in feed composition to the reformer a n d in downstream utilization of the reformate are occurring, related to the move toward cleEmer fuels. The major source of benzene in gasoline is catal3^ic reformate. The trend is to reduce benzene to a m a x i m u m of 1%, with the potential for even lower limits being legislated in parts of the world. When the p u s h for lower benzene content began, worldwide benzene levels ranged from as low as 1.6% in the U.S. to as high as 5% in pEirts of Europe and Asia. Refiners can reduce benzene concentration as follows: 1. Remove benzene precursors from the feed. This is accomplished by increasing the initial boiling point of the heavy naphtha feed, or, in essence, raising the cut point between light and heavy straight run naphtha. This results in less feed available to the reformer and lower octane quality in the resultant higher boiling range light straight r u n stream. 2. Leave the benzene precursors in the feed. In this case, refiners can then distill the C6 cut from the reformate. Benzene can be extracted from this cut for chemical sale, or the benzene can be reacted with olefins to produce alkylbenzenes. The C6 cut can also be hydrotreated to convert benzene to cyclohexEine with a resultant loss in octane number. Clcciner gasoline also mcindates lower final boiling point and lower total aromatics. This requires the refiner to eliminate a substantial portion of high boiling fraction from the naphtha feed and, in some cases, to r u n the reformer at lower

i

CHAPTER 1: PETROLEUM OIL REFINING Semiregenerative Catalytic Reforming Unit Net hydrogen Compressor

Recycle hydrogen

Off Gas

^^-^c^O

Heavy Naphtha

C3s&C4s

Separator

Reformate

Continuous Catalytic Reforming Unit

Spent Catalysi Product 'Separatiofi Naphtha & Recycle Gas

FIG. A—Semiregenerative and continuous catalytic reforming units.

13

14 MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

severity to yield less total aromatics. Beside the loss of octane number and yield, these changes likely yield less by-product hydrogen, a costly ingredient needed for desulfurization chemistry. Catalytic reformate also plays an important role in the petrochemical industry by being a major source of benzene, para-xylene, ortho-xylene, and toluene primary feedstocks. The recovery of these aromatics is illustrated in Fig. 6. Total reformate, or a narrower C6-C8 cut, is fed to a liquid-liquid extraction column where it is contacted countercurrently with a solvent with known selective solubility for aromatics. The main solvents are sulfolane or di- to tetra-ethylene glycols. The aromatics, called the extract, are then distilled from the reusable solvent. The rejected paraffins and naphthenes, called raffinate, must find a new home which can include the solvent/spirits market or as feed to ethylene plants running on naphtha; recycling the raffinate to a high severity catalytic reformer is usually not economically attractive. The "pure" aromatics in the extract are then separated in a series of distillation columns, except for meta- and para-xylene, which have virtually identical boiling points. Their separation is accomplished either by crystallization, which takes advantage TABLE 11—Catalytic reforming unit yields. Reformer Severity

102

94

Yield, %w Hydrogen CI + C2 C3 iC4 nC4 C5+ Reformate

3.0 3.7 3.5 1.8 2.5 85.5

1.7 1.0 1.8 1.4 2.1 92.0

of their widely different freeze points, or by adsorption with molecular sieves, which takes advantage of their different molecular diameters. Isomerization This process mainly converts normal paraffins into isoparaffins. The technology is employed to increase the octane quality of light straight run gasoline/naphtha by converting npentane and n-hexane into mixed C5-C6 isoparaffins. The process is also used to convert n-butane into isobutane to either provide feed to the alkylation process or as a step in the process to make MTBE when isobutylene is not available. A simplified process flow diagram is shown in Fig. 7. Fresh feed, along with recycle hydrogen, is heated to reactor temperature of 180-260°C by heat exchange or a furnace. The feed then flows down through a fixed bed of catalyst that is similar to reforming catalyst. Operation may be oncethrough, or with peirtial or total recycle of unconverted nparaffin. The recycle can be recovered using distillation to separate lower boiling isoparaffin product from the normals, or adsorption with molecular sieves to separate smaller diameter n-paraffins from isoparaffins. Recycle yields a greater octane number increase at the expense of higher capital investment and operating cost. Octane improvement generally ranges from 5-25 numbers depending on feed composition and process configuration. Because of the low reactor temperature, thermal cracking of the feed is very low, in the order of 1-2% compared to 12-20% in the CRU. However, the sensitivity of the catalyst to coke laydown still requires the presence of hydrogen to minimize coke formation and hence provide satisfactory activity and run length.

96

High Aromatics Feed

2 g>

Low Aromatics Feed

+

O

76 94

96

98

100

RON without Pb FIG. 5—CRU yield/octane relationship.

102

104

CHAPTER

1: PETROLEUM

OIL REFINING

15

'^Raffinue

Bxtfictio ^Tnadngind Fnctiamtioii Feed

LSGBflD E«EXTRACTOR S'STRlPfCR RC - RECOVERY COLUMN SR - SOLVB^TREOENERATOR FIG. 6—Solvent extraction unit for btx recovery.

Alkylation The reaction of an olefin with an aromatic or isoparaffin is labeled alkylation. Benzene is alkylated with ethylene to eventually produce styrene, and with propylene to eventually produce phenol. These "petrochemical" technologies are found in some refineries. But the "alky" process that is part of the typical refinery flow plan (Fig. 1) is the reaction of C3-C5 olefins with the paraffin isobutane. The process requires that the paraffin be branched to provide a reactive tertiary carbon atom, bonded to only one hydrogen. Normal paraffins, having only primary or secondary carbon atoms, are not sufficiently reactive. Isopentane could be used, but it is already a clean, high-octane liquid contained in the C5 part of the gasoline pool. Thus, the gas isobutane becomes the obvious choice since it is reactive, not part of the gasoline pool, and reasonably available in the refinery as a crude oil component, as well as in the C4 product from processes s u c h as the FCCU, CRU, a n d hydroprocessing units. The chemistry can be accomplished with any olefin. Ethylene is not used because it is expensive to recover, operating pressure would have to be considerable since the process is carried out in the liquid phase, and the resulting alkylate octane quality is poor compared to processing higher molecular weight olefins. Propylene is a n "alky" feed in many units, but its use competes against its value as a petrochemical feed for polypropylene, propylene oxide, and linear alpha olefins. The butylenes are the predominant alky feed because of their

availability, cost t o separate into chemical grade components, and high octane quality of the alkylate. However, as the light olefin content of gasoline diminishes due to evolving legislation, refiners may also find it economically attractive to use amylenes as alky feed, instead of converting them to lower octane pentanes in a hydrotreater. In many refineries, there is insufficient isobutane to match the availability of olefins from the FCCU and cokers. The missing iC4 can be produced by isomerizing available nC4, by purchase from the natural gas condensate pool, or from nearby refineries lacking an alky plant. The catalysts that are commercially employed are 98-99% sulfuric acid (H2SO4), or concentrated hydrofluoric acid (HF) because the chemistry requires a very strong acid. Commercialization of solid-type catalyst is in progress. The process in concept is simple: pressurize the system to liquefy the hydrocarbon feeds, mix them with the strong liquid acid, and separate the alkylate from the catalyst. The process in reality is complex because of acid carryover into the alkylate; formation of acid-hydrocarbon molecules; significant exothermic heat of reaction; the need for substantial excess isobutane to suppress olefin polymerization; high catalyst consumption and utility cost in the case of H2SO4; and safety considerations, especially with hydrofluoric acid. Sulfuric acid alkylation m u s t r u n at low temperature, namely, about 5-10°C, in order to minimize olefin polymerization, acid consumption, and by-product yield. This requires refrigeration of the catalyst and feed, and substantial

16 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

£

4—Mafenp Hydiogra

O

• • [ ^

ri c:^

X .J^

iall:^^

OM

a

?

sr

LEGEMD D-DRYER R-REACrOR ST • STABILIZER

C5-C

booMnli FIG. 7—Isomerization unit.

mixing energy because of the acid's high viscosity. Most alky units used sulfuric acid from the commercialization of the process in the 1940s until the 1970s, the period when energy cost was low. In addition, H2SO4 consumption is substantial at about 0.1 kg per kg alkylate. Since the early 1970s, most new units have been H F alky plants to counteract higher energy cost. H F alkylation operates at about 25-50°C negating refrigeration, acid viscosity is so much closer to that of the hydrocarbons that designs can accomplish the chemistry in piping or in simpler mixing vessels, and acid consumption is only about 0.001 kg per kg alkylate. However, the use of H F poses a more serious safety concern than H2SO4 in the event of a n acid release. A sulfuric acid spill will cause soil and g r o u n d w a t e r contamination; however, hydrofluoric acid, which boils at 20°C, will release a vapor cloud of H F as it escapes emd depressurizes. Releases of H F at a n u m b e r of U.S. refineries in the 1980s led to significant re-engineering to assure process safety and "save" the process from legislative extinction. A simplified flow diagram is shown in Fig. 8 for an H F alkylation unit. Key highlights include separation of the acid from

alkylate, cooling of recycle acid, recovery and recycle of isobutane, cleanup of alkylate, and separation of products. Typical product yields, quality, and disposition are shown in Table 12. MTBE Oxygenates were introduced into the gasoline pool in the 1970s to help replace the lost octanes from lead additives. Their use has now been mandated in many parts of the world based on data showing their ability to reduce harmful vehicle emissions. The majority of worldwide use of MTBE is by commodity chemical p u r c h a s e . However, a substantial a m o u n t is made within the refinery gate, in particular in the United States. Typically, the MTBE unit is installed in front of a n alkylation unit. The C4 cut from the FCCU is first fed to the MTBE unit where the isobutylene is reacted with purchased methanol. The effluent C4 paraffin and olefin stream, minus isobutylene, is then sent to the alky plant. MTBE has helped the refiner meet gasoline d e m a n d and octane, and is being adapted for blending worldwide. In fact, during the 1980s and much of the 1990s, it was the fastest

CHAPTER 1: PETROLEUM OIL REFINING FEED PRElKEATMEm-

REACTION SECTION

BOSIRIFFER

DEFKOPANIZER

17

HFSnUFPER

Mixed Batane*

OUfin Feed "j

»

DiyingAnd Diokfin Saianlioa

FIG. 8—HP alkylation unit. TABLE 12—HF alkylation unit yield and quality. Olefin Feed

C3 + C4

C4

Yields, %w 4-10 Propane n-Butane 2-5 3-6 80-93 Alkylate 76-90 Alky Bottoms" 3-8 3-8 1 Alky Tar* 1 Alkylate Dist, °C Initial BP 40 40 75 10% 70 100 30% 90 105 100 50% 110 70% 105 125 90% 120 195 Final BP 190 96 RON, without Lead 93 MON, without Lead 92 94 ^Too high boiling for gasoline. Usually blended to diesel and fuel oil. ' A "waste" stream of high molecular weight. May be processed in cracking units, burned as fuel, or disposed.

growing "chemical," with usage now surpassing 20 million mt per year. However, the use of this ether and perhaps other ethers could be reduced or banned because of concern for contamination of drinking water from gasoline storage tank leakage, as detected in parts of the U.S. Should this happen, refiners will again have to find substitute streams to replace lost octane and "barrels." This issue could become economically critical to the refiner and consumer as the industry simultaneously reduces gasoline and diesel fuel sulfur, as well as gasoline aromatics and olefins. TAME, the ether produced by reacting isoamylene with methanol, has entered the refinery picture because it can provide an opportunity to reduce the C5 olefin content of gasoline. A number of MTBE plants have been revamped to handle the higher boiling olefins. However, if MTBE use is

banned, it is likely that TAME will suffer the same fate. Ethanol continues to be used, particularly in the United States and Canada, and some refiners are substituting ethanol for the ethers. Other technologies utilizing isobutylene, beside alkylation, are being added to the refinery flowsheet. One example is the dimerization of isobutylene to isooctene, followed by its conversion to isooctane via hydrogenation.

PROCESSES FOR HETEROATOM REMOVAL AND AROMATIC/OLEFIN SATURATION The main technology for removing unwanted heteroatoms such as sulfur and nitrogen, and for reducing aromatics euid olefin concentration in refinery liquid streams, is by contacting the stream with hydrogen in the presence of a catalyst, and at appropriate temperature and pressure. The vocabuIciry to identify the numerous applications is extensive and not universally consistent. The most common generic name is hydroprocessing. This has been further divided into three categories; hydrotreating, hydrorefining, and hydrocracking, to delineate process severity for desired product cleanliness, as well as feed and product boiling range. Currently, hydrotreating and hydrocracking are the two most widely used names for this technology, with the trend to place hydrorefining within the hydrotreating category. Process severity, with respect to the above vocabulary, is defined by percent conversion (Table 13), or how much of the feed is thermcdly cracked into lower molecular weight, lower boiling product. Conversion is usually defined as 100 minus the amount of product in the same boiling range as the feed. The vocabulary also expands to include the word desulfurization, since sulfur reduction has been and remains the key need for hydroprocessing, in addition to denitrogenation and dearomatization. Some of the myriad names and abbrevia-

18

MANUAL

37: FUELS

AND LUBRICANTS

HANDBOOK

TABLE 13—Hydroprocessing terminology. Conversion, %w Hydrotreating (HT) Hydrorefining (HR)

"0" As little as possible

Hydrocracking (HC) Additional Terminology HDS HDT HDN HDA

25 +

NHT DDS MHC ARDS VRDS

Objectives Cleanup without conversion Cleanup of higher boiling streams with minimum conversion Cleanup + Conversion

Hydrodesulfurization Hydrotreating Hydrodenitrogenation Hydrodearomatization

(HDA is also used for Hydrodealkylation of Toluene to Benzene)

Naphtha Hydrotreater Distillate Desulfurizer Mild Hydrocracking Atmospheric Resid Desulfurization Vacuum Resid Desulfurization

tions for this technology are also listed in Table 13. The refinery streams that can/must undergo hydroprocessing, and the reasons for the process selection are summarized in Table 14. An example of a simplified process flow diagram is shown in Fig. 9. Fresh feed, with hydrogen recycle and make-up hydrogen for the a m o u n t c o n s u m e d by the hydrogenation chemistry, are heated to reaction temperature in a furnace and then fed downflow through one or more fixed catalyst bed reactors in series. After the desired reactions have occurred, the remainder of the process involves the recovery of a high purity hydrogen recycle stream, separation of any cracked hydrocarbon gases from the desulfurized liquid product, as well as distillation of the liquid into appropriate cuts. Start-of-run r e a c t o r t e m p e r a t u r e ranges from 2 8 5 400°C a n d pressure from 900-20 000 kPa; generally, the higher the boiling range axid "dirtiness" of the feed and the greater the desired cracking severity and dearomatization, the greater is the required temperature, pressure and n u m b e r of reactors, as well as chemical hydrogen consumption. Excess hydrogen via the recycle is needed to minimize coke formation and laydown on the catalyst; this recycle is about 3-5 times the expected hydrogen consumption.

cess of hydrogen to minimize coke production, this runaway can increase reactor temperature by u p to 60°C per minute, creating an unsafe operating condition. Refineries therefore usually cap furnace outlet temperature at about 435-440°C. Thus, processing heavier, dirtier feed at higher severity, which requires higher start-of-run temperature, results in shorter r u n length and/or multiple reactors. The metal promoters on the alumina base end up in their oxide form from the drying of the finished catalyst. After being in the reactor for a short time the metal oxides will convert to sulfides from the hydrogen reducing atmosphere, combined with the production of hydrogen sulfide. The oxide form is overactive and can cause temperature excursion, increased coking, and partial catalyst deactivation from changes in the alumina and metal promoter structure. Thus, cobalt-moly and nickel-moly catalysts must be "presulfided" or "presulfurized" prior to the start-of-run. This can be done in-situ by adding compounds such as dimethyldisulfide to a feed to condition the catalyst, for about one to two days, or it can be done ex-situ wherein the catalyst is pretreated by the manufacturer, and then shipped and loaded to assure that the metals remain in the sulfide form.

The catalysts comprise the widest selection among all the refinery processes because this technology is used to "clean" all refinery streams from light straight r u n gasoline through vacu u m resid. The two most common formulations consist of combinations of cobalt and molybdenum or nickel and molybd e n u m promoters on an alumina base. The "cobalt-moly" catalyst, as it is typically called, is preferred for sulfur removal while the "nickel-moly" combination is active for nitrogen removal and some aromatics saturation. Some catalysts also utilize nickel-tungsten. Noble metal catalysts such as platinum as well as zeolites are now employed, in particular for deep deciromatization. Cobalt-moly and nickel-moly catalysts contain in the range of 3-25% promoter metals depending on feed properties and planned hydroprocessing severity. Hydroprocessing is exothermic and increases with hydrogenation severity. Reactor outlet temperature in a fixed bed reactor can be as m u c h as 35°C greater than inlet temperature when processing vacuum gas oils and resid. Further, as the run proceeds, slow b u t inevitable loss of catalyst activity due to coke and metals laydown occurs and is compensated by raising furnace outlet temperature. Hydrogenation reaction rate increases with temperature and at about 470°C, can result in a "runaway" reaction. Since there is always an ex-

At the end of a run, burning the coke off can regenerate the "coked up" catalyst. This also is accomplished in-situ or exsitu. Ex-situ removal more efficiently removes the coke, but at increased cost. If performed in-situ, the catalyst will usually remain in the reactor for 2-4 runs, after which refiners will d u m p the spent catalyst and replace it with fresh material. Spent catalyst is usually then taken to hazardous waste disposal. If the catalyst has been used to process resid, it may be sent for nickel and vanadium recovery or to a process that can remove enough of the feed metals to reactivate the catalyst and allow its reuse. Hydrogen consumption and throughput rate, often called "space velocity," are two of the key process and economic variables. As the boiling range of the feed, and thus its heteroatom and aromatics content, increases, it can be expected that hydrogen consumption will increase a n d throughput will need to decrease to increase residence time. A similar response occurs for a given feed as conversion increases. Examples of yield, hydrogen consumption and residence time for different feeds are shown in Table 15. Hydroprocessing will continue to play a n ever-increasing role in worldwide refining. The factors driving this conclusion are:

CHAPTER 1: PETROLEUM OIL REFINING

19

TABLE 14—Hydroprocessing options. Stream Lt Str Run Gasoline Hvy Str Run Naphtha Coker Naphtha

B.P.°

Sulfur

For the Reduction of Nitrog. Arom.

Diolef.

Acids X X

FCCU GasoHne Catalytic Reformate Kerosine/Jet Lt Attn Gas Oil Atm Gas Oil Coker Gas Oil

X

Vac Gas Oil Vac Gas Oil

Resid

End Use Isomerization Unit Feed Cat Reformer Feed Gasoline Blending, Cat Reformer Feed Low Sulfur Gasoline Low Benzene Gasoline Jet Fuel, Low Sulfur Diesel Low Sulfur Diesel Gasoline Blending, Cat Reformer Feed, Jet Fuel, Diesel Fuel FCCU Feed Gasoline Blending, Cat Reformer Feed, Jet Fuel, Diesel Fuel, Lube Oil Base Stocks Cat Reformer Feed, Jet Fuel, Diesel Fuel, FCCU Feed, Coker Feed, No. 6 Fuel Oil

"Boiling Point Range HEATER IffTSTQ. REACTOR

lEAlER 9 0 STTO. REACTOR

STABIUZER HlACnONATOR LightEmli

FIG. 9—Fixed bed hydroprocessing unit. The sulfur content of crude oil has been increasing over the years and is expected to continue that trend, barring major discoveries of low sulfur crudes. The sulfur content of fuel products, as we know them today, has been decreasing and this trend is expected to accelerate as the world seeks cleaner fuels.

3. The aromatics content of fuel products has been decreasing and this trend should continue and expand worldwide. The technology and catalysts to achieve these results are available; the only missing ingredient is money. If gasoline, jet fuel, and diesel fuel continue as the major sources of transportation fuels, then the industry will be forced to uti-

20 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 15—Hydroprocessing unit yields. Feed Density, kg/m^ Sulfur, p p m w Nitrogen, p p m w Initial Boiling Ft., °C 50% Distilled Final Boiling Ft. Products Naphtha Yield, %w Density Sulfur Nitrogen Jet/Diesel Yield Density Sulfur Nitrogen Atm Gas Oil Yield Density Sulfur Nitrogen Hydrogen Consumption, %w of Feed

FCCU LCO + Coker Gas Oil

FCCU LCO

Crude VGO

940 22 400 940 220 275 375

935 28 000 1 000 365 470 575

980 8600

5

24 730 «. * ^^

'N:

>

(B

s o

^

^% !>* X

c o

Hi

*

f

"1

\

GAS

Q. V

oc

0)

c o a

V

\ k

CD U O

TJ

O)

'

£ (0 (0 (0

-i -» a- 0 _

o

"« 1

Q. (S

>

r 8

o

a> a

E (U

§§l

8

I

8S S

o •• "O Olid 'aimtajil iodoA

2oS 3

o ot

o

a.

33

10000

100

300

200

400

500

600

n-Butane Rsfsrenes Stales:

^

^

v

T / G

H(sat liqO'100 Btu/lb # T(trp) SAp

MEAW-OUTT FNCUMATIC PWTOM S c h e m a t i c D i a g r a m of t h e Scuffing L o a d Ball-on-Cylinder Lubricity Evaiuator (not including Instrumentation)

MANDREL ASSEMBLY •

SCREWS W REQ'a BUTTON HEAD ••8-32

DRIVESHAFT ASSEMBLY

Ring and Mandrel Assembly (Cylinder) Parameter

Test Conditions Value

Fluid volume 50 ± 1.0 mL Fluid temperature 25 ± 1°C Conditioned air^ 50 ± 1 % relative tiumidity at 25 ± 1°C Fluid pretreatment: 0.50 U min air flowing through and 3.3 L/min air flowing over the fluid for 15 min Fluid test conditions: 3.8 L/min air flowing over the fluid Cylinder rotational speed 525 ± 1 rpm Applied Load Break-in period 500 g Incremental-load test 500 to 5 000 g Single-load test user defined' Test Duration Break-in period 30 s Wear tests 60s *Fifty percent humidity should be achieved using equal volumes of dry and saturated air. The SLBOCLE has a water column through which air passes and it is assumed to be saturated when it exits this column. "The applied load for the single test Is set at the pass/fail requirement for the fuel being evaluated. FIG. 1 0 — T e s t e q u i p m e n t a n d t e s t c o n d i t i o n s f o r s c u f f i n g l o a d b a l l - o n cylinder lubricity e v a i u a t o r ( D 6 0 7 8 ) .

CHAPTER 5: GAS TURBINE FUELS

131

Lubricity Task Force is also working on possible revisions to the standard HFRR test to make it more sensitive to low levels of lubricity additive.

Test Plata Loading

Aromatics

Schematic Diagram of HFRR (not including instrumentation) Test Conditions Fluid volume Stroke length Frequency Fluid temperature

Relative humidity Applied load Test duration Bath surface area

2 ± 0.20 mL 1 ± 0.02 mm 50 ± 1 Hz 25 ± 2°C or 60 ± 2°C >30% 200 ± 1 g 75 ± 0.1 min 6 ± 1 cm^

FIG. 11—Test equipment and test conditions for high frequency reciprocating rig (D 6079).

The inclusion of a single fuel specification in the main table of specification D 975 for Grade No. 2 requires further research because: 1. the correlation of the data among the two test methods and the fuel injection equipment needs further clarification, 2. both methods in their current form do not apply to all fueladditive combinations, 3. the reproducibility values for both test methods are large. In the meantime, the following information may be of use and serve as a general guideline to fuel suppliers and users. Westbrook and coworkers recommended that users monitor their fuel injection pumps for possible trends of abnormal wear rates if the fuel has a scuffing load value between 2000 and 2800 g in Test Method D 6078 [44]. According to this paper, fuels with values below 2000 g will, in all probability, cause accelerated wear in fuel lubricated rotary-t5Tpe fuel injection pumps. It should be noted that a properly-additized fuel might provide protection for fuel-wetted components and yet not produce significant D 6078 test results as compared to the non-additized fuel. Work at ISO indicates that a fuel with a 450-micron wear scar diameter or lower value at 60°C in Test Method D 6079 (380 micron at 25°C) should protect all fuel injection equipment [45]. Other SAE publications present data to show that some fuels and fuel/additive combinations can have values above this level and still provide sufficient lubricity protection to the equipment. Pump stand testing of fuels, although more expensive and time consuming, is a more accurate means of evaluating the lubricity of diesel fuel. At the time of this writing, the ASTM Diesel Fuel Lubricity Task Force is working on the development and standardization of a pump stand test method. The

Diesel fuel contains many types or classes of compounds including paraffins, naphthenes, olefins, and Eiromatics. Compounds that contain heteroatoms such as sulfur, nitrogen, and oxygen are also present. Aromatics warrant discussion because 1) they have an effect on the combustion quality of the fuel, 2) typically, they are the only hydrocarbon type listed in diesel fuel specifications (including D 975), and 3) increased amounts of aromatics can have a negative impact on vehicle emissions. It is well known that an increase in the total aromatics content of a diesel fuel can (and usually does) have an adverse effect on the ignition quality, i.e., cetane number of the fuel. Several methods are available for the measurement of aromatic content. They eire described below: • ASTM D1319, Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption—Approximately 0.75 mL of sample is introduced into a special glass adsorption column packed with activated silica gel. A small layer of the silica gel contains a mixture of fluorescent dyes. When the entire sample has been adsorbed on the gel, alcohol is added to desorb the sample down the column. The hydrocarbons are separated in accordance with their adsorption affinities into aromatics, olefins, and saturates. The fluorescent dyes are also separated selectively, with the hydrocarbon types, and make the boundaries of the aromatic, olefin, and saturate zones visible under ultraviolet light. The volume percentage of each hydrocarbon type is calculated from the length of each zone in the column. This test method was originally developed for the analysis of gasoline (spark ignition engine fuel). It is for determining hydrocarbon types over the concentration ranges from 5-99 volume % aromatics, 0.3-55 volume % olefins, and 1-95 volume % saturates in petroleum fractions that distill below 315°C. The test method may apply to concentrations outside these ranges, but the precision has not been determined. Samples containing dark-colored components that interfere in reading the chromatographic bands cannot be analyzed. D 1319 was applied to and specified for diesel fuel usually because no other suitable method was available. As suitable methods became standardized they grew in use but have not replaced D 1319 in D 975. This is because the requirement for aromaticity currently included in D 975 comes from the requirement in 40 CFR Fart 80. Since federal law requires D 1319, it is the method listed in D 975. Method D 5186, described below, is more appropriate for diesel fuel and is often used in place of D 1319. However, in case of dispute, D 1319, by virtue of it's status as the legislated method, is considered the referee method. • ASTM D 5186, Standard Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography—A small aliquot of the fuel sample is injected onto a packed silica adsorption column and

132

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

eluted using supercritical carbon dioxide mobile phase. Mono-aromatics and polynucleEir aromatics in the sample are separated from non-aromatics and detected using a flame ionization detector. The detector response to hydrocarbons is recorded t h r o u g h o u t the analysis time. The chromatographic areas corresponding to the mono-aromatic, polynuclear aromatic, and non-aromatic components are determined and the mass % content of each of these groups in the fuel is calculated by area normalization. This test method covers the determination of the total amounts of mono-ciromatic and polynuclear aromatic hydrocarbon compounds in motor diesel fuels, aviation turbine fuels, and blend stocks by supercritical fluid chromatography (SFC). The range of aromatics concentration to which this test method is applicable is from 1-75 mass %. The range of polynuclear aromatic hydrocarbon concentrations to which this test method is applicable is from 0.5-50 mass %. • ASTM D 6591, Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates, High Performance Liquid Chromatography Method with Refractive Index Detection—A known mass of sample is diluted in the mobile phase, a n d a fixed volume of this solution is injected into a high performance liquid chromatograph, fitted with a polar column. This column has little affinity for the non-aromatic hydrocarbons while exhibiting a pronounced selectivity for aromatic hydroccirbons. As a result of this selectivity, the aromatic hydrocarbons are separated from the non-aromatic hydrocarbons into distinct bands in accordcince with their ring structure. At a predetermined time, after the elution of the di-aromatic hydrocarbons, the column is back flushed to elute the polycyclic aromatic hydroccirbons as a single sharp band. Method D 2425 offers a more detailed analysis but requires considerable investment in i n s t r u m e n t a t i o n a n d sample preparation time. For these reasons, it is not typically used for routine ctnalysis of diesel fuel. • ASTM D 2425, Standard Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry—This test method covers an analytical scheme using the mass spectrometer to determine the hydrocarbon types present in virgin middle distillates 204-343°C (400-650°F) boiling range, 5-95 volume % as determined by Method D86. Samples with average carbon n u m b e r value of paraffins between C 12 and C 16 and containing petraffins from C I O and C 18 can be analyzed. Eleven hydrocarbon types are determined. These include: paraffins, non-condensed cycloparaffins, condensed di-cycloparaffins, condensed tricycloparaffins, alkylbenzenes, indans or tetraJins, or both, C„H n.io (indenes, etc.), naphthalenes, CnH n.14 (acenaphthenes, etc.), CnH „.i6 (acenaphthylenes, etc.), and tri-cyclic aromatics. Method D 5292 also offers more information than D 1319 or D 5186. However, the results are reported in mole% rather than mass or volume percent, which are normally required in specifications. • ASTM D 5292, Standard Test Method for Aromatic Carbon Contents of Hydrocarbon Oik by High Resolution Nuclear Magnetic Resonance Spectroscopy—This test method cov-

ers the determination of the aromatic hydrogen content (Procedures A and B) 8ind aromatic carbon content (Procedure C) of hydrocarbon oils using high-resolution nuclear magnetic r e s o n a n c e (NMR) spectrometers. Applicable samples include kerosines, gas oils, mineral oils, lubricating oils, coal liquids, and other distillates that are completely soluble in chloroform and Ccirbon tetrachloride at ambient temperature. For pulse Fourier-transform (FT) spectrometers, the detection limit is t5^ically 0.1 mol % aromatic hydrogen atoms a n d 0.5 mol % aromatic carbon atoms. For continuous wave (CW) spectrometers, which are suitable for measuring aromatic hydrogen contents only, the detection limit is considerably higher and typically 0.5 m o l % aromatic-hydrogen atoms. The reported units are mole percent, aromatic- hydrogen atoms and mole% aromatic-Ccirbon atoms. This test method is not applicable to samples containing more than 1 mass % olefinic or phenolic compounds. This test method does not cover the determination of the percentage mass of aromatic compounds in oils since NMR signals from both saturated hydrocarbons a n d aliphatic substituents o n aromatic ring compounds appear in the same chemical shift region. For the determination of mass or volume% aromatics in hydrocarbon oils, chromatographic or m a s s spectrometry methods can be used. It should be noted that there are several standard methods for the analysis of aromatics. Each method yields a slightly different result and each is considered appropriate in different situations. One reason for this apparent inconsistency is that since a single molecule can contain several chemical moieties, it is possible to include it in several hydrocarbon classes. For example, a molecule could contain an aromatic ring, a pciraffinic side chain, and a naphthenic ring. How should this molecule be classified? A hierarchy was established to address this situation. Under this hierarchy, aromatics are on top, then olefins, followed by naphthenes, and finally paraffin. Using this hierarchy, the example compound would be considered an aromatic compound. The level of aromatics in the fuel is also important as it relates to the potential for elastomer and seed swell problems. This is especially true for older vehicles/fuel systems. Depending on the type of elastomer, prolonged exposure to relatively high levels of aromatics, followed by a sudden decrease in the amount of aromatics, can cause elastomeric seals to shrink and thus leak. If the elastomers are too old and have taken a set, they Cctn also crack or break. This phenomenon was widely seen in late 1993 and early 1994 when mcindated reductions in fuel sulfur and aromatics content went into effect. In most instances, the problems were solved by installing new seals made of less sensitive elastomer. Heat Content The heat content or heat of combustion of a fuel is the amount of heat produced when the fuel is burned completely. Gross and net heats of combustion are the two values measured for the heat of combustion. The gross heat of combustion is the quantity of energy released when a unit mass of fuel is burned in a constJint volu m e enclosure, with the products being gaseous, other than

CHAPTER 5: GAS TURBINE FUELS 133 water that is condensed to the hquid state. The fuel can be either hquid or soUd, and contain only the elements carbon, hydrogen, nitrogen, and sulfur. The products of combustion, in oxygen, are gaseous carbon dioxide, nitrogen oxides, sulfur dioxide, and liquid water. • The net heat of combustion is the quantity of energy released when a unit mass of fuel is burned at constant pressure, with all of the products, including water, being gaseous. The fuel can be either liquid or solid, and contain only the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. The products of combustion, in oxygen, are carbon dioxide, nitrogen oxides, sulfur dioxide, and water, all in the gaseous state. For c o m p o u n d s with the same n u m b e r of c a r b o n atoms, heat content increases as you go from aromatics to naphthenes to paraffins, on a weight basis. The reverse order is correct if you measure on a volume basis. The same is true for fuels. Denser fuels, such as diesel, have higher heat content on a volume basis. Less dense fuels, such as gasoline, have higher heat content on a weight basis. Chevron has reported typical heat content values that demonstrate this relationship (see Table 5). Heat of combustion is usually reported in units of megajoules per kilogram (MJ/kg). Conversion factors to other units are given in Table 6. Heat of combustion can be estimated by calculation from selected properties or measured using b o m b calorimetry. The m e t h o d s typically used for diesel fuel are discussed below. • ASTM D 4868, Standard Test Method for Estimation of Net and Gross Heat of Combustion of Burner and Diesel Fuels— This test method covers the estimation of the gross and net heat of combustion of petroleum fuel. The calculations use the fuel density, sulfur, water, and ash content. The equations for estimating net a n d gross heat of combustion used in this method were originally published in the National Institute of Standards and Technology (NIST) Publication No. 97. The equations are: Calculate the gross heat of combustion of the fuel corrected for the sulfur, water, and ash content in accordance" with the following equation: Q^ (gross) = (51.916

8.792^2 X 10- ') [1 (x+y

+s)] + 9.420s

where: Q

d X y 5

= gross heat of combustion at constant volume, MJ/kg, = density at 15°C, kg/m^, = mass fraction of water (% divided by 100), = mass fraction of ash (% divided by 100), eind = mass fraction of sulfur (% divided by 100).

TABLE 5—Typical density and heat content value of different fuels. Fuel

Density, g/cm^

Regular gasoline Premium gasoline Jet fuel Diesel fuel

0.735 0.755 0.795 0.850

Net Heat of Combustion, Btu/lb

18 18 18 18

630 440 420 330

Net Heat of Combustion, Btu/gal

114 116 122 130

200 200 200 000

TABLE 6—Conversion factors for heat of combustion values. 1 cal (International Table calorie) = 4.1868 J 1 Btu (British thermal unit) = 1055.06 J 1 cal (I.T.)/g = 0.0041868 MJ/kg 1 Btu/lb = 0.002326 MJ/kg

Calculate the net heat of combustion of the fuel corrected for the sulfur, water and ash content in accordance with the following equation: Qp (net) = (46.423 - 8.792^^ x 10"* + 3.70d X 10"^) X[l

- (x+y

+ s)] + 9.4205 + 2.449;c

where: Qp = net heat of combustion at constant pressure, MJ/kg, d = density at 15°C, kg/m^, X = mass fraction of water, y = mass fraction of ash, and s = mass fraction of sulfur. This test method is useful for estimating, using a minimum n u m b e r of tests, the heat of combustion of burner and diesel fuels for which it is not usually critical to obtain very precise heat determinations. This test method is purely empirical. It is applicable only to liquid hydrocarbon fuels derived by normal refining processes from conventional crude oil. This test method is valid for those fuels in the density range from 750 to 1000 kg/m^ and those that do not contain an unusually high aromatic content. High aromatic content fuels will not normally meet fuel specification criteria for this method. This test method is not applicable to pure hydrocarbons. It is not intended as a substitute for experimental measurements of heat of combustion. According to the m e t h o d the estimation of the heat of combustion of a hydrocarbon fuel from its density, sulfur, water, and ash content is justifiable only when the fuel belongs to well-defined classes for which a relationship between these quantities have been derived from accurate experimental measurements on representative samples of these classes. Even in these classes, the possibility that the estimate may be in error by large amounts for individual fuels should be recognized. This test method has been tested for a limited number of fuels from oil sand bitumen and shale oil origin and has been found to be valid. The classes of fuels used to establish the correlation presented in this test method are represented by the following specifications: 1. D 396 Fuel Oils Grades 1, 2, 4 (light), 4, 5 (hght), 5 (heavy), and 6 2. D 975 Diesel Grades 1-D, 2-D, and 4-D 3. D 1655 Aviation Turbine Jet A, Jet A-1, and Jet B 4. D 2880 Gas Turbine Grades 0-GT, 1-GT, 2-GT, 3-GT and 4GT 5. D 3699 Kerosine Grades 1-K and 2-K • ASTM D 4809, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method)—This test method covers the determination of the heat of combustion of hydroccirbon fuels. It was

134 MANUAL 3 7: FUELS AND LUBRICANTS

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designed specifically for use with aviation turbine fuels when the permissible difference between duplicate determinations is of the order of 0.2%. It can be used for a wide range of volatile and nonvolatile materials where slightly greater differences in precision can be tolerated. Under normal conditions, the method is directly applicable to such fuels as gasoline, kerosine, Nos. 1 and 2 fuel oil, Nos. 1-D and 2-D diesel fuel and Nos. 0-GT, 1-GT, and 2-GT gas turbine fuels. The increased precision is obtained through the improvement of the CcJorimeter controls and temperature measurements. • ASTM D 240, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter—This test method covers the determination of the heat of combustion of liquid hydrocarbon fuels ranging in volatility from that of light distillates to that of residual fuels. Under normal conditions, this test method is directly applicable to such fuels as gasoline, kerosine, Nos. 1 and 2 fuel oil, Nos. 1-D and 2-D diesel fuel, and Nos. 0-GT, 1-GT, and 2GT gas turbine fuels. This test method is not as repeatable and not as reproducible as Test Method D 4809. In this method the net heat of combustion is represented by the symbol Q n and is related to the gross heat of combustion by the following equation: Q„ {net, 25°C) = Qg {gross, 25°C) - 0.2122 X H where: Q„ (net, 25°C) = net heat of combustion at constant pressure, MJ/kg Q„ (gross, 25°C) = gross heat of combustion at constant volume, MJ/kg H = mass % hydrogen in the sample

Total Sulfur The test methods for measuring total sulfur in diesel fuel, as prescribed in D 975 are: • ASTM D 2622, Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry—D 2622 is prescribed for the measurement of total sulfur in Grades Low Sulfur No. 1-D and No. 2-D. This test method covers the determination of total sulfur in liquid petroleum products and in solid petroleum products that can be liquefied with moderate heating or dissolved in a suitable organic solvent. The applicable concentration range will vary to some extent with the instrumentation used and the nature of the sample. Optimum conditions will allow the direct determination of sulfur in essentially paraffinic samples at concentrations exceeding 0.0010 mass%. • ASTM D 129, Standard Test Method for Sulfur in Petroleum Products (General Bomb Method)—The sample is oxidized by combustion in a bomb containing oxygen under pressure. The sulfur, as sulfate in the bomb washings, is determined gravimetrically as barium sulfate. D 129 is the prescribed method for the determination of total sulfur in Grades No. 1-D, No. 2-D, and No. 4-D. This test

method covers the determination of sulfur in petroleum products, including lubricating oils containing additives, additive concentrates, and lubricating greases that cannot be burned completely in a wick lamp. The test method is applicable to any petroleum product sufficiently low in volatility that it can be weighed accurately in an open sample boat and containing at least 0.1% sulfur. • ASTM D 4294, Standard Test Method for Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy—This test method covers the measurement of sulfur in hydroccirbons such as naphthas, distillates, fuel oils, residues, lubricating base oils, and nonleaded gasoline. The concentration range is from 0.05-5mass%. • ASTM D 5453, Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Motor Fuels and Oils by Ultraviolet Fluorescence—This test method covers the determination of total sulfur in liquid hydrocarbons, boiling in the range from approximately 25—400°C, with viscosities between approximately 0.2 and 10 cSt (mm/s^) at room temperature. This test method is applicable to naphthas, distillates, motor fuels and oils containing 1.0 to 8000 mg/kg total sulfur. • ASTM D 1266, Standard Test Method for Sulfur in Petroleum Products (Lamp Method)—This test method covers the determination of total sulfur in liquid petroleum products in concentrations from 0.01-0.4 mass %. A special sulfate analysis procedure is described in the method that permits the determination of sulfur in concentrations as low as 5 mg/kg. • ASTM D1552, Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method)—This test method covers three procedures for the determination of total sulfur in petroleum products including lubricating oils containing additives, and in additive concentrates. This test method is applicable to samples boiling above 177°C (350°F) and containing not less than 0.06 mass% sulfur. Two of the three procedures use iodate detection. One employs an induction furnace for pyrolysis, the other a resistance furnace. The third procedure uses IR detection following p5Tolysis in a resistance furnace. The sulfur content of diesel fuel is known to affect particulate matter (PM) exhaust emissions because some of the sulfur is converted to sulfate particles in the exhaust. The amount that is converted varies by engine; but reducing total sulfur produces a linear decrease in PM in nearly all engines. Fuel sulfur can also adversely affect cylinder wear (through the formation of acids) and deposit formation (many sulfur compounds are known deposit precursors). Copper Strip Corrosion The test method for copper strip corrosion is D 130. • ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test—^A polished copper strip is immersed in a given quantity of sample and heated at a temperature and for a time characteristic of the material being tested. At the end of this period the copper strip is removed, washed, and compared with the ASTM Copper Strip Corrosion Standards (this is an adjunct available from ASTM Headquarters).

CHAPTER 5: GAS TURBINE FUELS 135 The copper strip corrosion test covers the detection of the corrosiveness to copper of aviation gasohne, aviation turbine fuel, automotive gasohne, natural gasoline, or other hydrocarbons having a Reid vapor pressure no greater than 124 kPa(18psi). Crude petroleum contains sulfur compounds, most of which are removed during refining. However, of the sulfur compounds remaining in the petroleum product, some can have a corroding action on various metals and this corrosivity is not necessarily related directly to the total sulfur content. The effect can vary according to the chemical types of sulfur compounds present. The copper strip corrosion test is designed to assess the relative degree of corrosivity of a petroleum product. It is very rare to find a commercially available diesel fuel that fails the D 130 test.

such as amyl nitrate, hexyl nitrate, or octyl nitrate, causes a higher carbon residue value than observed in untreated fuel, which can lead to erroneous conclusions as to the coke-forming propensity of the fuel. Test Method D 4046 can detect the presence of alkyl nitrate in the fuel. The carbon residue value of burner fuel serves as a rough approximation of the tendency of the fuel to form deposits in vaporizing pot-type and sleeve-type burners. Similarly, provided alkyl nitrates are absent (or if present, provided the test is performed on the base fuel without additive) the carbon residue of diesel fuel correlates approximately with combustion chamber deposits. The carbon residue value of gas oil is useful as a guide in the manufacture of gas from gas oil. In a gas turbine it can be an indication of the tendency to form carbon deposits in the combustor.

Carbon Residue

Ash

Carbon residue is the residue formed by evaporation and thermal degradation of a carbon containing material. The residue is not composed entirely of carbon but is a coke that can be further changed by carbon pyrolysis. The term carbon residue is retained in deference to its wide common usage.The test method for carbon residue, as listed in the diesel fuel specification is D 524. • D 524, Standard Test Method for Ramsbottom Carbon Residue of Petroleum Products—The sample, after being weighed into a special glass bulb having a capillary opening, is placed in a metal furnace maintained at approximately 550°C. The sample is thus quickly heated to the point at which all volatile matter is evaporated out of the bulb with or without decomposition while the heavier residue remaining in the bulb undergoes cracking and coking reactions. In the latter portion of the heating period, the coke or carbon residue is subject to further slow decomposition or slight oxidation due to the possibility of breathing air into the bulb. After a specified heating period, the bulb is removed from the bath, cooled in a desiccator, and again weighed. The residue remaining is calculated as a percentage of the original sample, and reported as Ramsbottom carbon residue. Provision is made for determining the proper operating characteristics of the furnace with a control bulb containing a thermocouple, which must give a specified time-temperature relationship. This test method covers the determination of the amount of carbon residue left after evaporation and pyrolysis of an oil, and is intended to provide some indication of relative coke-forming propensity. This test method is generally applicable to relatively nonvolatile petroleum products that partially decompose on distillation at atmospheric pressure. Petroleum products containing ash-forming constituents as determined by Test Method D 482 will have an erroneously high carbon residue, depending upon the amount of ash formed. Values obtained by this test method are not numerically the same as those obtained by Test Method D 189, or Test Method D 4530. Approximate correlations have been derived (Fig. 12) but need not apply to all materials that can be tested because the carbon residue test is applicable to a wide variety of petroleum products. The Ramsbottom Carbon Residue test method is limited to those samples that are mobile below 90°C. In diesel fuel, the presence of alkyl nitrates

Ash is the non-combustible material in a fuel oil. It can be present as either solid material or oil or water-soluble metallic compounds. These solid particles are the same as those often designated as sediments. The concern for fuel systems is that these solid particles can result in wear and erosion ultimately resulting in substandard or failing performance. The test method for ash is D 482. • ASTM D 482, Standard Test Method for Ash from Petroleum Products—The sample, contained in a suitable vessel, is ignited and allowed to bum until only ash and carbon remain. The carbonaceous residue is reduced to an ash by heating in a muffle furnace at 775°C, cooled and weighed. This test method covers the determination of ash in the range 0.001-0.180 mass %, from distillate and residual fuels, gas turbine fuels, crude oils, lubricating oils, waxes, and other petroleum products, in which any ash-forming materials present eire normally considered to be undesirable impurities or contaminants. The test method is limited to petroleum products that are free from added ash-forming additives. Knowledge of the amount of ash-forming material present in a product can provide information as to whether or not the product is suitable for use in a given application. Ash can result from oil or water-soluble metallic compounds or from extraneous solids such as dirt and rust. Low-Sulfur Diesel Fuel and Dyed Diesel Fuel The Clean Air Act Amendments of 1990 established standards for highway diesel fuel. The standards, in part, made it illegal as of October 1, 1993, to manufacture, sell, supply, or offer for sale diesel fuel for highway use that has a sulfur content greater than 0.05% by weight (this amount is also commonly expressed as 500 ppm). Similarly, it is illegal for any person to use fuel that has sulfur content greater than 0.05% by weight in any highway vehicle. EPA also requires diesel fuel not intended for use in highway vehicles be dyed in order to segregate it from highway fuel. Internal Revenue Service (IRS) regulations require that tax-exempt diesel fuel be dyed regardless of the sulfur level of the fuel. The original EPA regulation mandated the addition of a blue dye to fuel with greater than 500-ppm total sulfur. However, the Federal Aviation Administration soon ex-

136

MANUAL

37: FUELS AND LUBRICANTS

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100 80 60 40 30 20

6

i s s t—

CJ

£Q_ i.r

^—^. *^

4 3 2

• « •

is ^5

l±i *o

eo Q=

1.0 0.8 0.6

2 §

0.4 0.3

^ S

0.2

g

;

1

u

0.02 0.03

0.06

I

0.10 0.08 0.06 0.04 0.03 0.02

0.01 0.01

0.10

0.2 0.3 0.4 0.60.81.0

2

3 4

6 8 10

20

30 40 60 80 100

CONRAOSON CARBON RESIDUE, PER CENT BY WEIGHT (ASTM D 189)

NOTE 1—^All dimensions are in millimetres. FIG. 12—Correlation of conradson carbon residue (D 189) with ramsbottom carbon residue (D 524).

pressed their concern that blue-dyed fuel might be confused with the most c o m m o n grade of aviation gasoline, which is also dyed blue. Based on this, the EPA changed the requirements to the use of red dye. The EPA now requires "visible evidence of the presence of red dye" to identify high sulfur fuels intended for off-road use. This typically requires that oil companies add a level of red dye equivalent to 0.75 pounds per 1000 barrels of a solid Solvent Red 26 dye standcird. Solvent Red 26 was selected as the dye standard because it is a unique chemical and available in pure form. Diesel fuels are actually dyed with liquid concentrates of Solvent Red 164 because this dye is more fuel soluble and less costly than the standard. Solvent Red 164 is a mixture of isomers that are very similar to Solvent Red 26, except the former incorporates alkyl chains to increase the solubility in petroleum [46]. Under the EPA regulations, any red dye seen in the fuel of a vehicle operating on-road triggers a n analysis of the fuel's total sulfur content. Penalties are assessed based on the measured sulfur content of the fuel, rather than the mere presence of red dye. The IRS tcikes a slightly different path with its regulations. They require that tax-exempt diesel fuels, both low sulfur and

high sulfur, have a m i n i m u m level of Solvent Red 164 that is "spectrally equivalent to 3.9 pounds per 1000 barrels" of Solvent Red 26. This is over five times the a m o u n t required under the EPA regulations. The IRS holds that the excessive dye amount is required to allow detection of attempted tax evasion even after a five-fold dilution of the dyed fuel with undyed fuel. In practice, diesel fuel is taxed as soon as it leaves a terminal unless it has been dyed. The change, in 1993, to low sulfur diesel fuel for on-road use brought numerous problems. Some of these include: • A marked increase in the number of fuel lubricity related failures of fuel-wetted engine components. This is primarily attributed to the fact that the hydrotreating required to remove the sulfur also removes naturally occurring fuel components that would have improved the lubricity of the fuel. • The requirements to dye the fuel at the early stages of the distribution process m e a n that dyed fuel is often transported through pipelines. While not a c o m m o n occurrence, red dyed diesel fuel has been known to contaminate other fuels in the system. This occurs either through actual mixing of the diesel with the other product or contamination of the other product with red dye residue on the wzJls of the

CHAPTER pipeline following a shipnient of dyed fuel. In the cases where the contaminated fuel has been aviation fuel, the result is usually the requirement to dispose of the contaminated fuel since most jet engines are not certified to operate on fuel with red dye. • Under the EPA regulations, kerosine used for home heating and other off-road applications must contain the red dye. Since this fuel is not tcixed, the IRS does not require the presence of the dye. Unfortunately, the evidence regarding the possible health effects of using red-dyed fuel in un-vented kerosine heaters is minimal. Therefore, m a n y users of these heaters are reluctcuit to use red-dyed kerosine. However, if the user wants un-dyed kerosine, they must pay the tax so that it will not have to be dyed u n d e r the IRS regulations. Understandably, m a n y kerosine heater users are upset about having to pay the tcix. The laws do allow a refund of the tax, however, and many users (and in some cases, suppliers) are taking the necessary steps to reclaim those monies. There is also work underway to develop information on the potential health effects of burning the dyed fuel in un-vented heaters. Users are encouraged to check with their fuel suppliers for additional information. • Seal swell and elastomer compatibility problems brought about by the reductions in ciromatic content in low sulfur fuels. This is discussed in the section on aromatics. • It should be noted that the reductions in allowable sulfur have also had some positive effects. Aside from the obvious improvements in engine emissions, the hydrotreating required to remove the sulfur often means that the fuel has significantly better stability characteristics (through the removal of precursors). The concomitant removal of aromatics can also bring some improvement to the ignition quality of the fuel. The requirement to reduce sulfur levels in diesel fuel is now a "fact-of-life" throughout the world. At the time of this writing, the EPA is proposing legislation to reduce the maximum allowable sulfur level to I S p p m b y J u n e 1, 2006 [47]. The primary impetus for this continued reduction in sulfur is the need to protect exhaust-treatment devices installed on diesel engines, many of which are poisoned by sulfur. World Wide Fuel Charter The World-Wide Fuel Charter was jointly developed by the E u r o p e a n Automobile Manufacturers Association (ACEA), the Allieuice of Automobile Manufacturers, the Engine Mcinufacturers Association (EMA), the Japan Automobile Manufacturers Association (JAMA), and numerous associate mem-

5: GAS TURBINE

FUELS

137

bers and supporting organizations [48]. In a letter dated April 2000, the members stated that the "Charter was first established in 1998 to promote greater understanding of the fuel quality needs of motor vehicle technologies and to harmonize fuel quality world-wide in accordance with vehicle needs." The Charter contains four categories of gasoline and diesel fuel as follows (see Table 7 for a listing of selected diesel fuel properties): • Category 1: Markets with n o or minimal requirements for emission control, based primarily on fundamental vehicle/engine performance concerns. • Category 2: Markets with stringent requirements for emission control or other market demands. For example, meirkets requiring US Tier 0 or Tier 1, EURO 1 and 2, or equivalent emission standards. • Category 3: Markets with advanced requirements for emission control or other market demands. For example, markets requiring US California LEV, ULEV, and EURO 3 and 4, or equivalent emission standards. • Category 4: Markets with further advanced requirements for emission control, to enable sophisticated NOx and particulate matter after-treatment technologies. For example, markets requiring US California LEV-II, US EPA Tier 2, and EURO 4. Premium Diesel Fuel As discussed above, environmental regulations promulgated under the Clean Air Act Amendments have resulted in significant changes to the automotive diesel fuel manufactured and sold in the United States. These changes, coupled with rapidly changing engine technology, created the need to address several fuel properties to ensure proper performance, while cJso minimizing engine maintenance problems. There is also a segment of the automotive diesel fuel market that believes that they can benefit from a fuel supply with properties different from or in addition to, the m i n i m u m ASTM D 975 specifications. Many fuel suppliers sell such fuels at a higher price. As a marketing tool, this fuel is often called "premium diesel fuel." Other terms or descriptions have also been used. At the time of this writing, two major groups have proposed definitions for premium diesel. Those two groups are the Nationeil Conference on Weights and Measures (NCWM) and the Engine Manufacturers Association (EMA). In both cases, the proposed premium diesel is based on varying one or more fuel properties. To ensure that the fuel consumer gets a "premium" product for the higher price, the National Conference on Weights and Measures (NCWM) took steps to develop a standardized

TABLE 7—Selected property specifications from world-wide fuel charter. Property

Category 1

Category 2

Category 3

Category 4

Cetane Number, min Cetane Index, min Sulfur, max, mass % Lubricity, HFRR scar dia @60°C, (xm Particulates, mg/L Total Aromatics, mass %

48 45 0.50 400 No Requirement No Requirement

53 50 0.030 400 24 25

55 52 0.003 400 24 15

55 52 Sulfur-free 400 24 15

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TABLE 8—Diesel fuel properties referenced in NCWM definition of premium diesel fuel. Heating Value, Gross, Btu/gallon, min

D240

Cetane number, min Low temperature operability

D613 D 2500 or D4539

Thermal stability, 180 min. 150°C, reflectance, min Fuel injector cleanliness Flow loss, % max CRC rating, % max

D6468

138 700

47.0 2°C maximum above the D 975 tenth percentile minimum ambient air temperature 80%

L-10 Injector 6.0 10.0

definition of premium diesel. An NCWM task force composed of representatives from the oil industry, additive manufacturers, independent labs, and government agencies, with the assistance of ASTM, prepared a set of requirements to define premium diesel. In late 1997, the NCWM task force recommended that a fuel must meet any two of the five criteria listed in Table 8 before it can be labeled "premium diesel." The NCWM adopted the plan at its 84th conference in 1999 [49]. The definition became a model law and was automatically adopted by some states; elsewhere, it will only become effective if a state specifically adopts it. The EMA issued a Recommended Guideline (FQP-IA) for a premium diesel fuel. The proposed values are listed in Table 9. From the EMA consensus position: This diesel fuel is considered to be "premium" insofar as it may assist in improving the performance and durability of engines currently in use and those to be produced prior to 2004. It is not intended to enable diesel engines to meet any emissions standard or, in general, to improve engine exhaust emissions... It is intended as a "living document" in that, as other needs or test procedures are identified, the recommendation will be upgraded [50]. The most significant aspects of this Consensus Position are its requirements for a minimum fuel lubricity, increased cetane number, improved cold weather performance, detergency, thermal stability, minimum energy content, and specifications regarding overall fuel "cleanliness." Alternative Fuels As in the early 1980s, research with alternative fuels is again on the increase. Whereas the earlier work centered on the suitability of alternative fuels to power diesel engines; most of the current work is evaluating the potential these fuels offer to reduce engine emissions [51]. Fischer-Tropsch Liquids and Biodiesel are the fuels that seem to show up the most in the current literature and reports. Fischer-Tropsch Synthesis is the process whereby natural gas or coal is converted into hydrocarbons. The product hydrocarbons are usually upgraded to middle distillate products such as kerosine and diesel fuel. Typically Fischer-Tropsch diesel fuels have high cetane numbers, often greater than 70 cetane, no aromatic compounds, no sulfur, and a density of around 0.78 kg/L [52]. The Fischer-Tropsch liquids have

been evaluated as a diesel fuel and as a blend component with conventional petroleum diesel fuel. Schaberg, et al. [53] tested two variations of the Sasol distillate fuels, a 2-D diesel fuel, a CARB (California Air Resources Board) diesel, and three blends of the Sasol fuel with the 2-D fuel. The Sasol fuels produced significantly lower engine emissions than the 2-D and CARB fuels. The fuel blends reduced emissions in proportion to the amount of the Sasol fuel in the blend. Other resesirchers have shown similar improvements in regulated emissions, with the use of FischerTropsch fuels, as well [54-56]. The most significant potential problem associated with the use of these fuels is lubricity. Fischer-Tropsch fuels have very poor lubricity properties. There may also be some elastomer/seal swell problems, especially in older fuel systems, since these fuels have no aromatic compounds. Biodiesel is also a potential alternative to conventional, petroleum-derived diesel. Biodiesel is a renewable source of energy. In the United States, Biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, designated BIOO. Biodiesel is registered with the U.S. EPA as a fuel and a fuel additive under Section 211(b) of the Clean Air Act. There is, however, other usage of the term biodiesel in the market place. Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerin. The finished biodiesel derives approximately 10% of its mass from the reacted alcohol. The alcohol used in the reaction may or may not come from renewable resources. Biodiesel blend is a mixture of biodiesel fuel with petroleum-based diesel fuel designated BXX, where XX is the volume % of biodiesel.

TABLE 9—EMA recommended guideline (FQP-IA). Property

Flash point, °C, min Water Eind sediment, % vol, meix Water, ppm max Sediment, ppm max Distillation, °C, % vol recovery 90% max 95% max Viscosity, 40°C, cSt Ash, % mass, max Sulfur, % mass, max Copper corrosion, max Cetane number, min Cetane index, min Ramsbottom carbon on 10% residue, % mass, max API gravity, max Lubricity, g, min Accelerated stability, mg/L, max Detergency CRC rating, max Depositing test, % flow loss, max Low temperature flow, °C

Test Method

D93 D2709 D 1744 D 2276 or D5452 D86 D445 D482 D2622 D 130 D613 D4737 D524 D287 D 6078(1) D2274 L-10 Injector D 2500 or D4539

Requirements

52 0.05 200 10 332 355 1.9^.1 0.01 0.05 3b 50 45 0.15 39 3100 15 slO.O s6.0 (2)

NOTE: Alternatively, lubricity can be measured by D 6079 with a maximum wear scar diameter of 0.45 Jim at 60°C. Diesel fuels must pass the Cloud Point (D 2500) or Low Temperature Flow Test (D 4539) at the use temperature.

CHAPTER 5: GAS TURBINE FUELS 139

OH O

JL \ ^ ^

J. +

"^^lalyst

HO

3(GH30H)

^

3(RCOOCH3) + OH

O.^ ^ O

T

Triglyceride

+

3 iVIetlianol

"^^*^'^'

3 methyi-estiiers +

Glycerol

R is usually 1 6 - 1 8 carbons with 1 - 3 0 = 0 bonds.

FIG. 13—Reactions of vegetable oil to form methyl-esthers.

TABLE 10—Detailed requirements for biodiesel (BlOO)." Property

Flash point (closed cup) Water and sediment Kinematic viscosity, 40°C Sulfated ash Sulfur^ Copper strip corrosion Cetane number Cloud point Carbon residue" Acid number Free glycerin'^ Total glycerin^

Test Method''

Limits

Units

D93

100.0

min°C

D2709

0.050

max % volume

D445

1.9-6.0"

mm 2/s

D874 D2622 D 130

0.020 0.05 No. 3

max % mass max % mass max

D613 D2500

40 Report to customer 0.050 0.80 0.020« 0.240«

min °C

D4530 D664

max % mass max mg KOH/g % mass %mass

"To meet special operating conditions, modifications of individual limiting requirements may be agreed upon between purchaser, seller, and manufacturer. ''The test methods indicated are the approved referee methods. Other acceptable methods are indicated in 5.3. "SeeXl.3.1. '^Other sulfur limits can apply in selected areas in the United States and in other countries. "Carbon residue shall be r u n on the 100% sample (see 5.2.10). 'See Annex 1 for test method. A gas chromatographic technique is being converted to a standard test method. *The test m e t h o d is u n d e r ASTM consideration by Subcommittee D02.04.OL.

Soybean oil is the largest source of biodiesel in the United States, however, oil from other plants is sometimes used. Biodiesel is a mixture of fatty acid methyl esters. The oils are combined with methanol in a process known as transesterification (Fig. 13). The resulting mixture of fatty acid methyl esters has chemical and physiccJ properties similar to those of conventional diesel fuel. Provisional Specification 121 is the ASTM specification for Biodiesel Fuel (BlOO) Blend Stock for Distillate Fuels. (At this writing, ASTM is working to approve PS 121 as a standard specification.) Table 10 contains the detailed re-

quirements for BlOO as found in PS 121. Diesel engines can run on B1 GO; however, most of the testing in this country has been done on blends of biodiesel a n d low sulfur diesel. A blend of 20% biodiesel with 80% low sulfur diesel (B20) has been tested in numerous applications across the country. The limited testing thus far completed has shown that this fuel produces lower emissions of particulate matter, hydrocarbons, a n d carbon monoxide t h a n conventional diesel fuel. NOx emissions can be slightly higher thcin with conventional diesel, unless the fuel system injection timing is optimized for B20. BlOO has good lubricity properties and contains essentially no sulfur or aromatics. However, it has a relatively high pour point, which could limit its use in cold weather. Biodiesel is biodegradable, but this property may lead to increased biological growth during storage. Biodiesel is also more susceptible to oxidative degradation than petroleum diesel. Other eJtemative fuels that have been investigated for use in diesel engines include ethers, alcohols, naphtha, and various gaseous fuels. Each of these has some advantage (such as reduced engine emissions) associated with its use. However, much work remains to be done with these fuels, including building a distribution infrastructure, before they will be widely used in diesel engines.

GAS TURBINE FUELS This section discusses the fuels used in non-aviation (industrial) gas turbine applications. The specification for industrial gas turbine fuels is D 2880, Standard Specification for Gas Turbine Fuel Oils. Table 11 contains the specific requirements for the fuels covered by D 2880. Comparison of the specifications for diesel fuels and gas turbine fuels shows that gas turbine fuels actually have fewer requirements. Many of the individual property requirements of both specifications are equivalent for corresponding grades. This is demonstrated in Tablel2. The main differences are due to operational difference of diesel versus gas turbine. As an example, diesel fuel has a cetane number requirement whereas

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TABLE 11—Detailed requirements for gas turbine fuel oils at time and place of custody transfer to user.' ASTM Test Method"

Property

Graded'' No. 0-GT f 0.05

No. l-GT'^

No. 2-Gr

No. 3-GT

No. 4-GT

38 (100) 38 (100) 55(130) Flash point °C (°F) min D93 66 (150) 0.05 0.05 Water and sediment D2709 1.0 % vol max D1796 1.0 Distillation Temperature °C (°F) D86 90 % volume recovered 282 min max 288 338 Kinematic viscosity 2 mm/s^ D445 At40°C(104°F) f 1.3 1.9 5.5 min 5.5 max 2.4 4.1 50.0 At 100°C (212°F) max 50.0 Ramsbottom 0.15 0.35 Carbon residue on 10% distillation D524 0.15 Residue % mass, max 0.01 0.01 D482 0.01 0.03 Ash % mass, max Density at 15°C k g W D 1298 max 850 876 -18 D97 -6 Pour point«°C (°F) max "To meet special operating conditions, modifications of individual limiting requirements may be agreed upon between purchaser, seller, and manufacturer. ''Gas turbines with waste heat recovery equipment may require fuel sulfur limits to prevent cold end corrosion. Environmental limits may also apply to fuel sulfur in selected areas in the United States and in other countries. "The test methods indicated are the approved referee methods. Other acceptable methods are indicated in 6.1. ''No. 0-GT includes naphtha, Jet B fuel and other volatile hydrocarbon liquids. No. 1-GT corresponds in general to specification D 396 Grade No. 1 fuel and D 975 Grade 1-D diesel fuel in physical properties. No. 2-GT corresponds in general to Specification D 396 No. 2 fuel and D 975 Grade 2-D diesel fuel in physical properties. No. 3-GT and No. 4-GT viscosity range brackets specification D 396 Grades No. 4, No. 5 (light). No. 5 (heavy), and No. 6, and D 975 Grade No. 4-D diesel fuel in physical properties. "Under United States regulations. Grades No. 1-GT and No. 2-GT are required by 40 CFR Part 80 to contain a sufficient amount of dye Solvent Red 164 so its presence is visually apparent. At or beyond terminal storage taniks, they are required by 26 CFR Part 48 to contain the dye Solvent Red 164 at a concentration spectrally equivalent to 3.9 lbs per thousand barrels of the solid dye standard Solvent Red 26. 'when the flash point is below 38°C (100°F) or when kinematic viscosity is below 1.3 mm^/s at 40''C (104°F) or when both conditions exist, the turbine manufacturer should be consulted with respect to safe handling and fuel system design. *For cold weather operation, the pour point should be specified 6°C below the ambient temperature at which the turbine is to be operated except where fuel heating facilities are provided. When a pour point less than — 18°C is specified for Grade No. 2-GT, the minimum viscosity shall be 1.7 mm^/s and the minimum 90% recovered temperature shall be waived.

TABLE 12—Comparison of specification requirements for selected distillate fuels. Parameter

D396

D975

D 2069

D 2880

D 3699

Flash point Water & sediment Distillation Viscosity Carbon residue Ash Copper strip corrosion Density Pour point Sulfur Cetane # Cloud pt. Freezing point Burning quality Saybolt color NOTE: An asterisk indicates the property is included in the specification.

gas turbine fuel does not. Because the specifications are so similar, most fuels sold under one specification would also meet the requirements of the other. For this same reason, the properties and test methods for diesel fuel, as discussed above, are equally applicable to gas turbine fuels. Gas Turbine Fuel Requirements Industrial gas turbines are basically the same as aviation gas turbines in operation. A simple gas turbine has three major

components: compressor, combustor, and turbine. The purpose of the compressor is to raise the pressure of the operating fluid usually a ratio of 10 to 20 to 1. It is desirable to accomplish this pressure increase as efficiently as possible to maximize the available thrust or horsepower, because the efficiency determines how much horsepower is required for the compression. The purpose of the combustor is to raise the temperature of the operating fluid. The combustor inlet temperature depends on the pressure ratio and efficiency of the compressor. In more complicated cycles some of the heat in the exhaust is recovered and used to increase the combustor inlet temperature, which reduces the required temperature rise across the combustor and thus the amount of fuel required. Current combustor outlet temperatures are in the 1093-1482°C (2000-2700°F) ranges. The combustor must accomplish this temperature rise efficiently. Current combustion efficiencies are in the 99%-f range at most operating conditions. Additionally the outlet temperature profiles have specific requirements. A gas turbine combustor operates at fuel-air ratios less than stoichiometric and below the lower flammability level. Figure 14 is a simplified diagram of how the air is introduced in a conventional combustor i.e., not a low emission combustor. The figure shows the distribution of air for a combustor with an overall air to fuel ratio of 70 to 1. About four parts of air per part of fuel are introduced in the swirler to help stabilize the flame zone. Then, in the primary zone, 12 parts of air per part of fuel are introduced to provide enough

CHAPTER 5: GAS TURBINE FUELS

141

COMBUSTOR AIR DISTRIBUTION FULL LOAD

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air to provide approximately the stoichiometric amount of air required, making a total of 16 parts of air per part of fuel. In the secondary zone an additional 6 parts of air per of part of fuel are introduced making a total of 22 parts of air per part of fuel, which completes the reaction. The rest of the air (48 parts of air per part of fuel) is used to cool the combustor walls and to dilute the air-stream temperature down to the design turbine inlet temperature. The major difference between an aviation gas turbine and an industrial gas turbine is that the aviation generates thrust to propel the airplane by exhausting hot gases at high temperatures and velocities. The aviation turbine section only has to generate enough horsepower to drive the compressor. In an industrial gas turbine the gases that would be exhausted in an aviation gas turbine are expanded across additional turbine stages to generate shaft horsepower that can be used to drive generators, pumps, gas compressor, etc. Many industrial gas turbines use gaseous fuels but others use a variety of liquid fuels ranging from naphtha to residual oils. Aviation gas turbine fuel requirements are quite narrow because of the varying operating conditions (altitude, temperature, etc.), which impose limitations on volatility, viscosity, distillation range, etc. Industrial gas turbines are usually stationary. This means atmospheric conditions do not change as drastically as with aviation gas turbines. The operating conditions for the combustors for the two applications vary. Many industrial gas turbines operate at or necir design point for extended periods where an aviation gas turbine operates at take-off (full power) for a short time eind then the power level is reduced to cruise for the duration of the flight. In the following paragraphs, selected requirements specific to gas turbine operation will be discussed. The first consideration is light-off (initiating the combustion process and accelerating the engine to idle). Factors such as the tjrpe of fuel injection system, fuel viscosity, and fuel volatility are important. In earlier gas turbines, the fuel injectors or nozzles were of the pressure atomizing type simileir

to those used in some heaters. Lower flow nozzles of this type are particularly sensitive to viscosity, which is why many engine company specifications have a meiximum viscosity limit regardless of temperature. More modern fuel injection systems, which utilize air to assist the atomization of the fuel, are less sensitive to viscosity but some limit is still required. Another factor to be considered for light-off is the volatility of the fuel. The initial boiling point of the fuel must be considered because, even if the fuel is well atomized, if it is too heavy, light-off might not consistently occur. Smaller gas turbines ( x: g

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FUEL STORAGE SYSTEMS Steam, diesel, and gas turbine marine fuel storage, settling, and transfer systems are very similar. Figure 10 illustrates a steam propulsion plant with a 600 cSt residual fuel and diesel oil fill, transfer, and storage system. A diesel engine propulsion plant with its fill, storage, transfer, and centrifuged purification system is illustrated in Fig. 11. The marine fuel transfer system provides the following functions: • To transfer marine fuel from any storage tank to any other tank, such as to the settling tank. • To strip solids and water from the bottom of the settling tank(s) through the stripping and drain connections. The marine residual fuel and marine diesel fuels are bunkered through deck fill connections that should have sampling connections installed to allow continuous, representative fuel samples to be taken as the fuel is taken aboard. Heated tanks are used for storing marine residual fuels, and unheated tanks for marine diesel fuels and gas oils. From the residual fuel storage tanks, the transfer pumps forward the fuel to the settling tank(s). The residual fuel settling tanks begin the fuel treatment process by settling gross water and solids to the bottom. As soon as the settling tank is filled, it is heated to 80°C, but not higher than 10°C below the fuel's flash point. By heating the fuel, the viscosity is reduced, the gravity settling process is enhanced, and the fuel deaerates as well. Once settled, the fuel is forwarded to the boiler fire front, after additional heating, as illustrated in Fig. 12. The settling tank bottom drain connections permit the removal of settled water, sludge, and solids to the sludge tank. In the fuel transfer system, all piping should be traced, heated, and insulated to prevent the marine residual fuel from solidifying in the piping. The residual fuel storage tanks all have heating coils to control tank temperature and to maintain the fuel at 10°C above the pour point until it is transferred to a settling tank. From the safety standpoint, experience has proven that explosive atmosphere can collect in the tank headspace even though the fuel oil temperature is well below the flash point. This condition exists when cracked marine fuels contact hot heating coils that are significantly hotter than the bulk fuel temperature. Further, if shore side petroleum waste products have been mixed with the marine fuel, the lower flash point of the waste products could greatly lower the flash point of the bulk marine fuel. Even worse, this lower flash point may not be reported if the shore side sample was taken prior to the introduction of the waste. Therefore, the crew must fre-

TO MARINE PETROLEUM FUELS

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quently inspect and maintain the condition of the flame arrester screens on the fuel tank vent lines, and careful and safe use of the ullage equipment is imperative. In a steam propulsion plant system, a duplex heater set heats the fuel to the atomizing viscosity prior to the fuel reaching the boiler fire front. In a diesel engine propulsion system, the settling tank delivers heated residual fuel to the purifier heater set prior to the centrifugal separator(s). It is important that the residual fuel temperature to the separator stably remain at 98-99°C to maintain high separator efficiency and to prevent boiling of the water (in the fuel) within the separator. Service tanks or day tanks provide an additional opportunity to further settle water and solids from the heated residual fuel. The service tank can be filled by the centrifugal separators and provides additional time for deaeration.

FUEL TRANSFER SYSTEMS The fuel transfer pump(s) is provided to move fuel from storage to settling tank(s). The positive displacement transfer pump(s) is protected by coarse suction strainers, pressure relief valves, and pump bypass lines. The flow rate of the transfer pump is established by the engine's fuel consumption rate and the capacity of the settling tank. The operational flexibility of the transfer system is provided by the arrangement of the valves in the system. This valving can permit the fuel to be pumped from any storage tank to any settling tank or to other storage tanks.

STEAM PLANT FUEL SERVICE SYSTEM The simplest of the service systems is that of a steam plant. It consists of storage, settling, and service tanks; transfer and service pumps; and heater sets (see Fig. 10). The treatment system consists of a heated settling tank that allows solids and gross water to fall to the bottom of the tank [4]. The service system is also very simple, consisting of a pump, a heater, and a pressure (or flow) regulating controller. Most modern systems incorporate a quick-closing fuel valve to shut down fuel flow if there is a flame out, loss of combustion air, or loss of fuel pressure. Two fuel service pumps, with one in standby, are provided and each is capable of supplying the total fuel flow plus an additional msirgin, with the excess flow diverted back to the settling tank. Service pumps are typically of the positive displacement type that are fitted with a pressure relief bypass, remote shut downs, and isolation valves (for servicing). The pressurized fuel flows to the service heater sets where the temperature is increased to provide for the proper fuel viscosity for atomization. The two heater sets (one in standby) are steam heated shell and tube or plate type heat exchangers, each with the capability of increasing the fuel to 145°C. It is important to use properly sized heat exchangers and lowpressure steam supply to prevent overheating the fuel as it passes through the heat exchanger. After the heater sets, the fuel passes through duplex strainers, a viscometer, and a flow meter and then to the burner management system at the boiler Are front. All residual fuel piping must be trace heated (usually by steam) and insulated.

158 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

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CHAPTER 6: INTRODUCTION The marine diesel fuel storage (see Fig. 11) provides a separate fuel supply for cold boiler start up and for the emergency diesel generator. A small diesel fuel service tank is used to provide clean fuel for emergency operation when neither steam nor electricity is available. This diesel service tank is located high in the machinery space so that gravity will supply enough fuel pressure to start the boilers and diesel generators in an emergency.

DIESEL PLANT FUEL SYSTEM Diesel engine power plants require more intensive and complex fuel systems than those of steam plants. The contaminants, water, solids, and debris that may have been allowable in a steam plant must be removed prior to diesel engine operation. When burning residual fuels in diesel engines, the basic principles of settling, pumping, heating, and treating are common to slow speed and medium speed diesels alike. A typical residual fuel and diesel fill, transfer, storage, and treatment system for diesel engine power plants can be seen in Fig. 11. All residual fuel piping should be insulated and, if the viscosity exceeds 380 cSt at 50°C, trace heating should be included under the insulation. Whenever emergency shutdowns occur to a diesel engine operating on residual fuel, the viscous fuel will remain in the piping and heaters, and also in the injection pumps and high pressure piping. If all piping and components have trace heating and insulation, it is not a problem since the residual fuel can be reheated and its temperature controlled by recirculation prior to restart. If the emergency will keep the engine shut down for a lengthy time, the residual fuel should be purged from the piping with marine diesel fuel and this will eJlow the trace heating system and the recirculating pump to be secured. Additionally, if Einy maintenance is planned for the fuel injection system, the work will be faster and easier with marine diesel fuel in the piping system. When bunkering residual fuel and marine diesel fuel, both enter through sepjirate deck connections. Both deck connections must incorporate sampling equipment to permit continuous, representative sampling during the entire lifting. This is considered the point of custody transfer and these samples will be key should a quality dispute arise. As was the procedure in steam plants, marine residual fuels for diesel plants are transferred to the settling tank(s) by a transfer pump through a coarse strainer. The best practice is to duplex transfer pumps to prevent pumping problems. If a demulsifier chemical is to be used to aid in water or particulate removal, the demulsifier should be metered into the suction strainer ahead of the transfer pump. Demulsifier chemicals work by facilitating the separation of water and solids from the residual fuel, and this process begins in the settling tank(s). After the settling tank(s), the centrifugal purifiers and clarifiers, from two to four centrifuges, are typically installed to treat the residual fuel. These units include supply pumps, heaters, and automated controllers. Centrifugal separators, set up as purifiers and clarifiers, are widely used. They are considered a reliable and efficient method for treating and cleaning marine diesel fuel and marine residual fuels if properly maintained and adjusted. Centrifugal purifiers have the clear advantage of being capable of removing large quantities of water and particulates. The centrifuge ser-

TO MARINE PETROLEUM FUELS

161

vice pumps, fuel heaters, sludge tank, and interconnecting piping must be designed to match and support the needs of the centrifuge. The piping system must be configured to allow centrifuge operation in parallel or in series as either purifier/purifier, clarifier/clarifier, or purifier/clarifier. The centrifugal processed residual fuel then passes through very fine duplex filtration units that remove abrasive catalytic particles (cat fines) that pass through the centrifuges. After these fine filters, the residual fuel enters the service/day tank. From the service tank, the residual fuel is forwarded to the diesel engine through the residual fuel service system as seen in Fig. 13. The service system raises the residual fuel temperature up to the fuel injection temperature as controlled by a viscometer. An advanced service system will include a homogenizer to treat the residual fuel just prior to injection into the diesel engine. The service system flows two or three times the maximum fuel consumption of the diesel engine. Older fuel service systems are atmospheric pressure mixing tanks, while modem practice is to pressurize the entire system, remove the mixing tank, and add a mechanical deaerator to eliminate gases and water vapor from the injection pump discharge flow. The marine diesel fuel system moves fuel from storage to the service tank by a transfer pump or by a centrifugal purifier. The fuel can be transferred from the service tank to the emergency diesel generator service tank by either transfer pump or centrifugal purifier.

DIESEL FUEL SYSTEM COMPONENTS Centrifuges The recommended centrifuge flow capacity is the quantity that can be treated at the highest separating efficiency. This flow capacity is based upon the d5Tiamic viscosity of the residual fuel at the temperature of separation. The maximum separation temperature has an upper limit of 98°C. There is a risk of the loss of the water seal in the centrifuge due to the formation of steam bubbles. A thorough review of each centrifuge manual will determine recommended maximum flow rates for any given residual fuel. The rule is to reduce the fuel flow through the centrifuge to slighfly above the fuel consumption of the diesel engine. Avoid the temptation to rush the fuel through the centrifuge and into the service tank so it overflows back to the settling tank and is recentrifuged. Peak centrifuge operation efficiency occurs when the flow is reduced to increase the residence time of the fuel within the centrifuge while maintaining a stable fuel temperature of 98°C. Two properly sized, correctly adjusted and operated, selfcleaning centrifuges are considered absolutely necessary to provide a reliable diesel fuel treatment system. Most diesel engine warranties become invalid if adequate centrifuges are not used effectively. The following fundamental principles are necessary to establish and maintain effective centrifugal separator procedures for marine residual fuels. The centrifugal separators are the foundation of the diesel fuel treatment system. • To treat contaminated marine fuels, supplementary systems are required in addition to the centrifuges. These supplementary systems consist of fine filtration, demulsifier

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thenic molecules are insoluble in the extraction solvent while multi-ring aromatics (polynuclear aromatics), sulfur and nitrogen compounds, olefins, and other undesirable species are dissolved in the polar solvent phase. After phase separation, the non-polar phase containing the base oil is stripped of residual polsir solvent and is usually hydrogen treated as discussed below. Furfural, phenol, and N-methyl-2-pyrolidone (NMP) are commonly used as extraction solvents [10,11].

Propane Deasphalting

Hydrogen Refining/Hydrocracking

For certain heavy distillates and vacuum residua, base oils of very high viscosity (Bright Stocks) can be refined through propane deasphalting [10]. In this process, liquefied propane under relatively high pressure is used to precipitate insoluble, high molecular weight aromatic hydrocarbons such as asphaltenes and to extract other compounds (typically termed "resins"), which could be deleterious to the performance of the base oil being refined. Sulfur and nitrogen compounds as well as certain metals can also be removed by propane deasphalting.

Although solvent refining is effective in improving the quality of distillates in the manufacture of base oils, a significant yield loss is normally associated with the extraction process. This loss can be economically unacceptable for distillates derived from relatively poor crude oils that contain high concentrations of heavy aromatic compounds. Fortunately, it is also possible to improve the quality of a raffinate through various catalytic processes in which high-pressure hydrogen (1500 - 4000-1- psi) is employed to saturate aromatic molecules while cracking other large molecules to smaller compounds of molecular weight appropriate for base oils [10]. Olefins as well as sulfur and nitrogen containing compounds are also reacted and removed by hydrogen processing. Because aromatics are converted to saturated compounds, hydrogen processing normally increases VI.

Solvent Refining For basestocks of low to moderately high viscosity, solvent refining has been employed extensively and on a worldwide basis; this refining technology is probably the most popular method. Solvent refining removes undesirable polar and highly condensed aromatic molecules from the distillate and, in doing so, significantly increases VI. In this process, distillate is mixed with an insoluble polar solvent that creates a two-phase system. Ideally, desirable paraffinic and naphTABLE 5—Distillation cuts of crude oil [14, 15]. Fraction

Boiling Range (°C)

Ethane, butane, propane gases Light naphtha Heavy n a p h t h a Gasoline Kerosine Stove oil Lights gas oil Heavy gas oil Lubricating oils Vacuum gas oil Resid

400 425-600 >600

Kinematic Viscosity at 100°C (cSt) 3 4 6 8 10 12 14 16 18

SUS Viscosity at 100°F 70 100 200 310 430 560 710 870 1000

Dewaxing Essentially, all crude oil and base oil distillates contain a small fraction of linear paraffins (wax) [14] that can crystallize at low temperatures. For engine oils, transmission fluids, and other lubricants that must remain liquid and pumpable to temperatures approaching -40°C, it is necessary to remove as much wax as possible from the basestock. To accomplish this task, at least two techniques are commonly used to remove wax: solvent dewaxing (SDW) and catalytic dewaxing (CDW). For SDW, the base oil is first diluted in a solvent (toluene, for example) and then the mixture is added to a wax non-solvent (typically ketones such as methyl ethyl ketone-MEK). The mixture is then chilled to low temperatures to precipitate wax as a solid and is then collected by filtration. SDW is a batch process. In the CDW procedure, a shape selective catalyst is exploited to "crack" the paraffins into smaller, volatile segments that can be fractionated by distillation. CDW is a continuous process and, as a result, en-

172

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

joys m a n y advantages over SDW. However, since CDW is more efficient in removing wzix and other branched pEiraffins than is SDW, the resulting base oil VI is usually reduced since paraffins contribute to high VI. More recently, several processes have been developed that isomerize wax to brsmched isomers that do not crystallize as readily as linear peiraffins. As a result, VI is preserved by the hydroisomerization process cind very high VI oils (>120 VI) can be made directly from wax as a feedstock. Several API Group III base oils, to be defined below, are synthesized from a pEiraffin hydroisomerization procedure. Hyfinishing (Hydrofinishing) At the conclusion of the refining steps detailed above, base oils still may contain small quantities of sulfur, nitrogen, organic acids, and partially hydrogenated aromatics that can cause the base oil to exhibit poor color stability and an increased tendency to form sludge or other insolubles [10]. Historically, base oils were "finished" using either a sulfuric acid procedure or clay contacting. Unfortunately, both of these procedures jield potentially hazardous byproducts that require careful disposal to avoid environmental contamination. As a result, most m o d e m refineries use a catal3^ic hydrogenation process called hyfinishing to remove trace levels of impurities from base oil. Hydrogen pressures are typically 500-1000 psi, which are below the levels required for hydrocracking. Hyfinishing decreases the aromatic content of a base oil slightly while significantly lowering sulfur and nitrogen levels. Byproducts of hyfinishing include H2S, NH3, and CO2, which result from the cracking and hydrogenation of undesirable organic molecules containing S, N, or O heteroatoms. B a s e Oil R e - r e f l n i n g The reclamation of base oil from used engine oils and industrial lubricants has become a n important process from both an environmental protection and economic perspective. In the U.S., a Presidential Executive Order spurred the development of re-processing technology in October 1993, requiring that all federal agencies establish procedures to purchase lubricants containing at least 2 5 % re-refined base oil. Many municipalities have adopted this requirement for contract purchases of lubricants as well. Although a n u m b e r of processing m e t h o d s can be used to recycle spent lubricants, most commercial re-refiners utilize at least three stages: heating to remove water and residual low molecular weight hydrocarbons (fuels), distillation to separate hydrocarbons from polar oxidation products (sludge, varnish, other insolubles), residual fuel components, organometallic additives, etc., followed by hydrotreating to reduce sulfur, nitrogen, and polynuclear (multi-ring) aromatic levels. In at least one commercial re-refining process, a p r o p a n e deasphalting stage is employed to remove undesirable by-products etnd additives prior to re-distillation. N a p h t h e n i c vs. Paraffinic B a s e Oils The widely used term "naphthenic" base oils represents those stocks produced from naphthenic crude as defined by the U.S. Bureau of Mines Classification of Crude Oils [16]. In this

system, crude fractions that boil in the range typical of base oils (250-275°C and 275-300°C; at 1 atm) are naphthenic if they have a n API gravities of < 3 3 a n d < 2 0 , respectively. Paraffinic oils for the same boiling ranges provide API gravities of > 4 0 and > 3 0 . API gravity (ASTM D 287) is sensitive to the relative ratio of paraffins, cycloparaffins, and aromatics in a crude or base oil. Higher paraffinic cheiracter increases API gravity. A significant feature that distinguishes most naphthenic crudes is a very low natural level of linear paraffins or wax, which enables the production of base oils with very low pour points and excellent low temperature fluidity, essential in some applications such as refrigeration oils. Naphthenic base oils also have a relatively high aromatic content, which has limited their use in other applications where higher toxicity is a concern. Due to improvements in dewaxing methodology and concern over the potenticd adverse health effects of naphthenic base oils, the market share of these stocks has steadily decreased over the past decade (see Ref 11 and earlier issues of the annual NPRA survey).

B A S E OIL CHARACTERIZATION Hydrocarbon Type Analysis The chemiCcd composition of lubricant base oil is strongly dependent o n a n u m b e r of factors. These include crude oil source, molecular weight range (generally higher molecular weight crudes are richer in multi-ring aromatics), the refining process (solvent vs. hydroprocessing), the degree of refining, and the effectiveness of the finishing process. Determining the chemical compositional profile of base oil is usually initiated with a chromatographic separation procedure to isolate the aromatic and saturate fractions of the sample. ASTM method D 2549 involves an open colu m n procedure using a bauxite column packing with polar and non-polar organic solvents to elute the aromatic and saturate fractions, respectively. In this procedure a sample of base oil is applied to the top of the column, which is then flushed sequentially with solvents of increasing polarity. Saturated molecules, including both paraffins and naphthenic molecules, bind to the column less strongly than ciromatic compounds and, as a result, they elute from the column with n-pentane (non-polar) while the aromatics require "stronger" polar solvents (chloroform/ethyl alcohol) to be eluted. Typical aromatic and saturate levels determined by D2549 for a series of base oils, refined by different methods, are given in Table 7. It may generally be concluded that base oils contain a preponderance of saturate compounds and that the level of aromatics for a solvent refined base oil increases with viscosity grade largely because heavier distillates contain higher

TABLE 7—Composition of base oils. Refining Process - SUS Viscosity Grade

% Saturates

% Aromatics

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70-90 65-75 60-70 50-60 90-100 95-100 60-70

10-30 25-35 30-40 40-50 0-10 0-5 30^0

CHAPTER 7: HYDROCARBON levels of aromatics. Further, hydroprocessing techniques, specifically hydrocracking and hydroisomerization, drastically reduce aromatic level. In addition, base oils processed from naphthenic crudes typically exhibit a high aromatic content and a very low level of paraffins (90

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of secondary or tertiary hydrogen atoms. The rate of free radical propagation is Eilso very dependent on ring geometry and adjacent function groups as shown in Table 16 for phenyl radicals [33]. As oxidation proceeds, propagation reactions involving peroxy radical addition to other hydrocarbon molecules typically result in the formation of polar compounds and higher molecular weight species, which may ultimately increase the viscosity of the oil. The combination of oligomeric and polymeric radicals during the termination phase of oxidation can also cause a molecular weight increase and viscosity enhancement. Studies on the autoxidation of n- hexadecane (linear CI8 paraffin) at temperatures of 160-180°C have illustrated that Ccirboxylic acids, hydroperoxides, ketones, alcohols, and esters Eire all formed over a period of hours. It is interesting to note, however, that this "model system" exhibited a significant increase in viscosity while only a "modest" increase in molecular weight was observed [31]. A useful oxidation model for thin hydrocarbon films on metal surfaces has been developed by Professor E.E. Klaus a n d his co-workers at the Pennsylvania State University [34-36]. They have shown that if Molecule A is oxidized it forms Molecule B that initially has the same molecular weight as A. At this stage, both A and B can volatilize to A'

and B', respectively, which can Hmit further reactivity by evaporation from the hquid phase system. As oxidation continues, B undergoes condensation-type polymerization reactions to yield molecule P that has a high molecular weight but is still soluble in the hydrocarbon liquid phase. As a result, P is capable of significantly increasing the viscosity of the lubricant being oxidized. As a polymer, P is not volatile and remains in bulk solution and in contact with the metal surface. Through further oxidation and thermal decomposition, P becomes an insoluble deposit of very high molecular weight on metal surfaces. This model has been effective in predicting deposits in both gasoline and diesel engine tests as well as high temperature oxidative oil thickening in many applications. Although hydrocarbon oxidation can have a profound effect on the properties of a lubricant, this process can, fortunately, be controlled by the use of antioxidant additives. These molecules will be discussed more extensively in another chapter of this manual. Very generally, antioxidants function either by scavenging radical species or by decomposing peroxides to unreactive products. Radical scavengers include hindered phenols such as t-butylphenol, quinines, and certain amines. Zinc dialkyldithiophosphate, a common antiwear compound, also serves in a peroxide decomposing capacity as well.

I m p a c t o f S u l f u r C o m p o u n d s o n B a s e Oil Chemistry Throughout the evolution of base oil refining and processing technology, sulfur content has been employed as an indicator of product quality and a predictor of lubricant performance. In this context, the t e r m "sulfur" relates to soluble, organosulfur compounds that occur naturally in crude oil in contrast to elemental, yellow sulfur. Current domestic base oil sulfur concentrations typically range from 2000 Desulfurized Base Oil E +1.4% cetyl phenyl sulfide 190 Desulfurized Base Oil E + 1% bis(phenylethyl) sulfide 50 0.53% Organosulfur 170 Base Oil A (8 VI, 0.53%S) Desulfurized Base Oil A None >5000 Desulfurized Base Oil A 1% dicetyl sulfide 90 Desulfurized Base Oil A 2% dicetyl sulfide 5000

compositional variables constant. They observed that the high temperature oxidation rate of the sulfur-free oil was m u c h higher than the original sulfur-containing base fluid. In these experiments oxygen uptake by base oil was monitored over time (hours) at 171 °C under atmospheric pressure. Typical data are provided in Table 17. Table 17 illustrates that desulfurized base oils exhibit very poor oxidation inhibition compared to the original base oil and that certain sulfur compounds, specifically organosulfides, can significantly reduce the rate of oxidation at high temperature. Sulfur-containing aromatic molecules, however, such as dibenzothiophene, did not appear to act as antioxidants and may actually increase the rate of oxidation slightly (data not shown). Further work reviewed by Harpp et. al. [32] revealed that sulfides in themselves are not antioxidants but rather they become active when oxidized to — SO2, — SO3 and related organo-sulfonic acids. These authors also report that thiophenes (or thioaromatics) have "no stabilizing effect on hydrocarbon oil oxidation" and they were not able to identify a molecular mechanism through which antioxidation could occur. More recently, using both statistical and neured network modeling techniques, Stipanovic, Smith, and co-workers [3,4] have shown that base oil thioaromatic content can be directly correlated to an increase in oxidation and deposit formation level observed for crankcase engine oils in the ASTM Sequence HIE and VE gasoline engine tests. These observations were attributed to an increase in the rate of free radical propagation in the engine oil at high temperature based on data reported by Russel [33]. As shown in Table 18, the free radical hydrogen a t o m abstraction rate is m u c h higher for thiophenes than other hydrocarbons for at least two types of radicals that could occur in hydrocarbon systems. As a result, it is reasonable to conclude that propagation reactions can be accelerated by the presence of thioaromatic molecules, such as benzothiophene, although data specific to alkylperoxy radicals, of significance in hydrocarbon oxidation, are not readily available. The role of sulfur compounds as natureJly occurring base oil stabilizers has some very interesting implications. It is well known that untreated, sulfur-free synthetic base oils, such a polyalphaolefins, oxidize more readily in tests such as the Rotary Bomb Oxidation Test (RBOT;ASTM D 2272) than conventional mineral oils that contain sulfur compounds. However, if appropriate antioxidants are added to both t5^es of base fluids, the PAOs typically respond with better longterm oxidation stability. Severely hydrocracked, low sulfur Group II and III base oils exhibit similar behavior.

TABLE 18—Relative reactivity of hydrocarbon radicals. Substrate Species: Radical

CH3 - X X =

Phenyl* Phenyl* Phenyl* Phenyl*

Alkyl Phenyl Benzothiophene 2-Thiophene

RCH2* RCH2*

Phenyl 2-Thiophene

Relative Reaction Rate per Hydrogen Atom 1 9 14 15 65 210

TABLE 19—Impact of basic nitrogen compounds on the oxidation of hexadecane (Cu catalyst, 0.6% butylated hydroxy toluene (BHT) added as an antioxidant). Compound Hexadecane (HD) HD + 3-n-butylpyridine HD -1- 2,4-dimethylquinoline HD + 2-methylindole

HD + Carbazole HD + Phenanthridine

Effective Nitrogen Concentration (ppm)

RBOT Lifetime (min.)

10 5 2 9 4.5 2 11 6 0.2 10 10

596 216 248 296 306 324 405 501 516 578 576 440

Nitrogen C o m p o u n d Reactivity Nitrogen containing molecules found in base oils can also accelerate oxidation and deteriorate the useful lifetime of lubricants. More specifically, "basic nitrogen" compounds (socalled proton acceptors) such as various pyridine derivatives, can act at very low concentrations (below 10 p p m based on N) in deteriorating oxidative stability [6,38,39]. Oxidation lifetime for the straight chain paraffin hexadecane, a base oil "model compound," in the presence of small quantities of "basic nitrogen" are shown Table 19. The data clearly illustrate that such species can promote oxidation rate in the ASTM D 2272 RBOT procedure. These results illustrate that the molecular structure of the "basic nitrogen" compound influences the oxidation reaction. More specifically, Yoshida et. al [38] have found that reducing the pKb of the nitrogen group enhances the overall rate of oxidation. In addition to these model compound studies, statistical and neural network modeling methods have demonstrated that RBOT lifetimes for prototype industrial oils (hydraulic

178

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fluids and turbine oils) also decrease with increasing base oil basic nitrogen content. Further, thermal sludge formation and oxidation onset determined by a high pressure differential scanning calorimetry technique indicated a deterioration with higher levels of basic nitrogen [6]. Olefins Although crude oils Eire generally relatively low in olefin content [10], base oil processing techniques can introduce olefins, especially at high temperatures, due to "cracking" reactions. In the presence of heat or UV light, olefins can polymerize to form higher molecular weight products that can color the base oil or actually cause sediment. In general, olefins can be removed during the process of hydrofinishing or by clay treatment discussed above [10].

CHEMICAL COMPOSITION/LUBRICANT PERFORMANCE CORRELATIONS As previously discussed, base oils contain a broad spectrum of paraffinic, cycloparaffinic, and aromatic molecules, the distribution of which varies with crude source and refinery processing. More importantly to the lubricant chemist and engineer, each of these hydrocarbon types can exert a different effect on the ultimate physical and chemical properties of the base oil and, ultimately, the lubricant from which it is formulated. Table 20 illustrates the relationship between hydrocarbon type and performance for several important properties. B a s e Oil R h e o l o g y a n d C o m p o s i t i o n The flow properties, or rheology, of a lubriccint strongly reflect the composition of the base oil used in its formulation. Comparing hydrocarbons of similar molecular weight, paraffins (especially linear paraffins) provide the highest positive contribution to base oil VI while aromatics and naphthenes, particulcirly multi-ring structures, strongly decrease VI. Base oils of high VI Eire generally preferred for lubricants because they provide higher viscosity at high temperatures and lower viscosities at low temperatures, provided they are properly treated to inhibit wax crystallization. In the absence of effective pour point depression, higher VI oils containing elevated levels of wax display poor low temperature fluidity as wax crystals form network structures that Eire resistant to flow. In most cases, polymeric pour point depressant additives eliminate wax crystallization and, u n d e r these circumstances, base oils rich in paraffins can exhibit excellent rheological properties at low temperatures.

In engine oils, poor cold temperature fluidity can reduce pumpability to the extent that oil stcirvation causes catastrophic engine failure. To protect against this occurrence, the SAE J300 Engine Oil Viscosity Classification specifications include two ASTM tests, D 3829 and D 5293, which measure both the high shear viscosity and pumpability, respectively, of engine oils to assure that a motor will start and have sufficient oil fluidity to assure good lubrication. A low temperature viscosity limit is also included in the specifications for many geeir oils and transmission fluids. This viscosity is determined using a very low shecir Brookfield Viscometer operating at —5 to —40°C, depending o n viscosity grade. The specific test method is described in ASTM D 2983. Composition/Performance Correlations for Engine Oils Gasoline cind heavy duty diesel engine oils probably represent the most sophisticated lubricant formulations in terms of physical a n d chemical requirements and, subsequently, their additive packages are very complex. Engine oils must provide a fluid lubricating film for sliding metal-to-metal interfaces at high t e m p e r a t u r e s while neutralizing acidic combustion gases; minimizing oxidation and corrosion; suspending insoluble combustion a n d lubricant oxidation byproducts; reducing wccir and the tendency to foam; etc. Since each of these characteristics can be influenced by base oil composition, considerable attention has been focused on relating specific molecular components to engine performance. Murtay and co-workers [1,2] were among the first to use base oil compositional data derived from mass spectrometry to develop statistical correlations between oxidation in the ASTM Sequence IIIC and HID oxidation engine tests and base oil hydrocarbon type distribution. These authors observed that a regression function including saturate content plus total sulfur concentration predicted viscosity increase very well for a series of engine oils formulated to be API SE quality. In this function, higher levels of saturates and sulfur enhance performance. Viscosity Index edone was not found to be a good predictor of performance for oils originating from different crude, sources, but it did correlate reasonably well for samples derived from a single crude. Murray also demonstrated that during the course of oxidation reactions, the level of both saturates and aromatics decreased while the concentration of polcir compounds increased significantly. For example, in the ASTM Sequence IIIC engine test, a SAE lOW-30 oil that was originally 74.1% saturates, 13.6 % aromatics, and 4.9% polars exhibited an end-of-test composition of 46.1% saturates, 7.9% aromatics, cind 33% polars.

TABLE 20—Performance characteristics of base oil components. Base Oil Proprety

ParaiBns

Naphthenes

Aromatics

Viscosity Index (VI) Low Temperature Fluidity" Low Temperature Fluidity* Pour Point Oxidation/Thermal Stability Solvent for Additives

Excellent Poor Excellent Poor Excellent Poor

Poor-Good Good Good Good Poor-Good Good

Poor Good Good Excellent Poor Excellent

"Unadditized. ''Treated with a pour point depressant.

CHAPTER Roby and co-workers were also successful in developing statistical correlations between base oil composition and performance in the ASTM Sequence HID and VD gasoline engine tests [40]. For high temperature oxidation in the HID procedure, it was learned that no single base oil parameter provided a good correlation to viscosity increase but a regression equation including nitrogen content (ASTM D 4629), olefin n u m b e r (ASTM D 460), sulfur content (ASTM D 1552) and saturates (column gradient elution chromatography) provided an excellent fit (R-squared = 0.97) to the observed data. For oxidation control, high levels of saturates and sulfur compounds were beneficial while elevated olefin and nitrogen levels contributed to poor performance. In the Sequence VD gasoline engine test, which eveJuates deposit formation at relatively low levels of oil oxidation, the individual variables discussed above all showed good correlations (R-squared values > 0.79) to varnish formation with higher nitrogen, olefin, and sulfur content being detrimental. Average engine varnish ratings in the Sequence VD improved with increasing base oil saturates content, however. More recently, base oil effects in the Sequence HIE and VE engine tests have been studied [3,4] using Partial Least Squares (PLS) and neural network modeling methods. These authors evaluated the engine test performance of a group of different base oils (approx. 12 oils) formulated into engine oils using similar additive technology. For the Sequence HIE oxidation test, viscosity increase and piston skirt varnish ratings generally improved with paraffin content while reacting negatively to high levels of multi-ring naphthenes, multi-ring aromatics, and thioaromatics. Total sulfur content was found to reduce viscosity increase and piston deposits in the Sequence HIE, consistent with its antioxidant effect discussed earlier. For the Sequence VE test, total sulfur, thioaromatics, and multi-ring aromatics all enhanced the formation of varnish deposits. Using the PLS modeling protocol described in Ref 3, it is possible to predict the engine test performance of lubricants, assuming a c o m m o n passenger car engine oil additive technology, using base oil compositional features as input. In this fashion, the potential Sequence VE varnish ratings for engine oils formulated from a population of base oils representing typical U.S. production were predicted. Ratings are based on a visual examination of a n u m b e r of engine pairts where 10.0 corresponds to a totally clean part, 5.0 is the m i n i m u m "passing" rating, and 60%), while Group I and II products provided intermediate performance because they contained less paraffins and higher levels of naphthenic ring compounds (>40%). Since the cycloparaffin ring structure is sterically very bulky, it is especially sensitive to applied forces and contributes significantly to increasing the viscosity-pressure coefficient for base oils. Planar aromatic rings appear to pack with less difficulty. Under pressure, paraffins are easily compressed and can actually be induced to crystallize if pressures are sufficiently high. In the design of CVT fluids, molecules are synthesized to optimize their steric bulkiness [46]. A number of other procedures are also available to calculate the viscosity-pressure coefficient for base oils from com-

Group I -*- Group 11 -A- Group III -¥r Group IV —- Naphthene

100

200

300

400

500

600

Pressure (MPa) FIG. 2—Viscosity - pressure relationships for base oils at 100°C (nominaiiy 4 est at 100°C, 1 atmosphere pressure).

CHAPTER positional data and/or other bulk fluid properties [44,49]. Roelands [44] has shown that the viscosity-pressure relationship for base oil can be predicted solely from atmospheric viscosity (TJO) a n d a knowledge of the percentage of carbon atoms in a aromatic ring structure (CA) and the percentage of naphthenic (cycloparaffinc) carbons (Cn). Johnston determined that the pressure-viscosity coefficient could be calculated from ambient pressure fluid density and the viscosity/temperature relationship (specifically viscosity at two temperatures is needed) [49]. More recently, Spikes and co-workers have shown that the thickness of a lubricant film under conditions of elastohydrodynamic lubrication (EHD) can be related to a by the following expression obtained from a high-speed ultrathin film interferometry technique [45]:

7: HYDROCARBON

BASE

OIL CHEMISTRY

181

and solubilize this plasticizer is an important consideration. In other cases, certain base oil molecular fractions can actually dissolve into the rubber matrix causing it to swell. Although most lubricant base oils are relatively inert in their ability to deleteriously interact with a variety of elastomeric materials commonly in use, high aniline point products, such as Group IV polyalphaolefin (PAO), can cause elastomer cracking after long periods of exposure at high temperatures due to a loss in plasticity. In many cases, a so-called seal swell agent can be successfully added to PAO to maintain good seal characteristics. At the other extreme, low aniline point naphthenic base stocks can cause seals to swell excessively also creating operational problems. As a result, for any base oil system, rubber compatibility should be evaluated carefully in formulating a lubricant product.

h oc U 0-*^ 7, 0*" a 0 "

where: h = film thickness measured by interferometry, U is the mean entrainment speed, and TJ is the low pressure dynamic viscosity. B a s e Oil S o l v e n c y E f f e c t s In any lubricajit formulation, base oils are required to dissolve polar additive compounds and to ultimately disperse polar oxidation products that are formed during use. For this reason, the so-called aniline point of a base oil can be a critical parameter in defining its compatibility with additives and the byproducts of use. Aniline point is determined by ASTM D 611 a n d represents the t e m p e r a t u r e at which aniline, (C6H5MNH2), a polar aromatic compound, becomes miscible with a hydrocarbon base oil. At low temperatures, base oil and aniline are not miscible but as temperature is raised they become a single phase at the aniline point, commonly expressed in °F. In general, base oils of low aromatic/high saturates content have high aniline points (>230°F, 110°C), conventional solvent refined base oils have m o d e r a t e aniline points (200-215°F, 93-102°C) and naphthenic base oils have a very low aniline point (^ >^ '>^ ^i ^^°"

HHHHHH|!,,!,HHHHHH

;o

Oleic Acid FIG. 1—A general representation of a typical additive molecule.

tering the strength of the polar functionahty or by changing the size of the hydrocarbon chain. Changing the strength of the polar functionality alone is difficult and has its limitations. Changing the size of the hydrocarbon group, on the other hand, is much easier. In practice, both strategies are used. Whether an additive performs its function on the surface or in the bulk lubricant depends on its polar to non-polar ratio. With the strength of the polar moiety constant, additives with small hydrocarbon groups have higher polar to non-polar ratio than those with large hydrocarbon groups. As a consequence, EP agents and rust inhibitors, which require more surface activity, have small hydrocarbon groups; and dispersants and detergents, which require a higher solubility in oil, contain large hydrocarbon groups. Except in very few cases, a connecting group or a link is necessary to connect ("tie") the two functionalities together. The importance of such a group is described in detail in the dispersants section. StabilizersADeposit Control Agents This class of additives controls deposit formation. These additives do so by inhibiting oxidative breakdown of the lubricant to deposit precursors and by suspending those already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism and dispersants and detergents do the suspending part. Oxidation Inhibitors All lubricants, by virtue of being hydrocarbon based, are susceptible to oxidation [7,8]. Many lubricants contain three major components: base oil, additives, and viscosity modifier, all of which have the susceptibility to oxidize. Each tj^e of basestock, mineral, vegetable, or synthetic, has a stable threshold beyond which stabilizers or oxidation inhibitors are needed to retard oxidation. In terms of oxidative stability, synthetic oils eire the most stable and vegetable oils are the least stable. The oxidative stability of mineral oils is intermediate between the two. Most lubrication applications expose lubricants to oxygen in some manner. Oxygen reacts with hydrocarbon molecules

201

that make up the lubricant. The reaction sites, in order of decreasing ease of attack, are benzylic, allylic, tertiary alkyl, secondary alkyl, cind primary alkyl hydrogens. The result is the formation of hydroperoxides and peroxy or other radicals. The rate of oxidation of hydrocarbons in addition depends upon the amount of oxygen, presence or absence of nitrogen oxides (NOx, a general term used for NO and NO2), ambient temperature, and the presence or absence of metal ions. Hydrocarbon oxidation is a three-step process, and comprises initiation, propagation, emd termination (Fig. 2). Initiation involves the attack of atmospheric oxygen or of nitrogen oxides (NOx) on hydrocarbon molecules. The result is the formation of hydroperoxides (ROOH) and alkyl (R*) and peroxy (ROO') radicals. During the propagation stage, hydroperoxides decompose either on their own or in the presence of metal ions to alkoxy (RO') and peroxy radicals. These react with the lubricant hydrocarbons to form a variety of additional radicals and oxygen containing compounds, such as alcohols, aldehydes, ketones, and carboxylic acids [see the chapter on Oxidation]. Aldehydes and ketones are highly reactive and can form polymers in the presence of acids, such as nitric and sulfuric acids. These acids result from the interaction of nitrogen oxides and sulfur oxides, products of combustion, with water. Carboxylic acids attack iron, copper, and lead, present in the mechanical equipment, to form metal carboxylates that further increase the rate of oxidation. During the termination stage, the radicals either self-terminate or terminate by reacting with oxidation inhibitors [8]. Oxidation inhibitors (InH) circumvent the radical chain mechanism by promoting deRH

+

O2

ROOH

RH

+

O2

ROO-+H

RH

+-NO,

'

Initiation

R- + HNOx

ROOH

>v

RO- + OH-

ROOH + Fe+2

-*-

RO- •»• OH' + Fe+3

ROOH + Fe*'

-*•

ROO- + Fe*^ + H*

ROO - ••• RH

-*•

ROOH ••• R-

ROO-

-*-

R- ••• O2

RO - •!• RH

-*•

R O H -I- R-

OH- + RH

•*-

H2O

ROO- + ROO-

•*-

RR + 2O2

ROO- + InH

•*-

ROOH + In-

RO- + InH

-^

ROH + In-

R- + InH

-*•

R H ••• I n -

+

> Propagation

R-

y

>-

Termination

RH = Hydrocarbon

ROOH = Hydroperoxide

RO- = Alkoxy Radical

R - = Alkyl Radical

InH = Inhibitor

In- = Inhibitor Radical FIG. 2—Oxidation mechanism.

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HANDBOOK

composition of hydroperoxides and taking reactive radicals out of the oxidation process by reacting with them. Oxidation inhibitors can be classified as hydroperoxide decomposers, radical scavengers, or metal deactivators, depending upon the mode of their controlling action. Sulfur containing compounds, such as sulfides a n d dithiocarbamates, and phosphorus containing compounds, such as phosphites and dithiophosphates, act as hydroperoxide decomposers. Nitrogen a n d oxygen containing c o m p o u n d s , such as arylamines and hindered phenols, act as radical scavengers [8,9]. These chemicals convert chain-propagating hydroperoxides and radicals to innocuous products. Some inhibitors, such as dithiophosphoric acid derivatives and dithiocarbamates are extremely potent oxidation control agents. This is because they act both as hydroperoxide decomposers and as radical scavengers. They in addition possess antiwear properties. Transition metals can act as both oxidation initiators (promoters) and oxidation inhibitors, depending upon their oxidation state [8]. They act as promoters if they facilitate the formation of free radicals, and they act as inhibitors if they remove free radicals from the oxidation process [10]. For example, heavy metals, such as iron and lead, and their salts are well known as oxidation promoters [11,12]. Metal deactivators, another class of oxidation inhibitors, are used to control oxidation under such circumstances. These inhibitors, primarily used in fuels, form complexes with metal ions via chelation, thus taking t h e m out of the chain reaction. Ethylenediaminetetraacetic acid derivatives and N,N-disalicylidene-l,2-propanediamine represent the m o s t p o p u l a r members of this class. The structures of common inhibitors

are provided in Fig. 3. Synthetic methods for polysulfides, phosphites, and dithiophosphoric acid and dithicarbamic acid derivatives are described in the antiwear and extreme pressure agents section. Those for synthesizing alkylphenols are provided in the detergents section. Hindered phenols, such as 2,6-di-?-butyl-4-methylphenol (BHT), a n d diaryla m i n e are p r e p a r e d by reacting phenol, alkylphenol, or diphenylamine with an olefin in the presence of a Lewis acid catalyst. The salicylidene meted deactivator is a product of phenol, formaldehyde, and ethylenediamine. Some oxidation inhibitor combinations are synergistic. That is, they reflect an effect greater than the additive effect of two or more inhibitors. Such combinations usually, but not always, consist of compounds that intercept oxidation by two different mechanisms. An example is the combination of a sulfur compound with an arylamine or a hindered phenol [13]. Oxidation inhibitors cire used in almost all lubricants, with gasoline and diesel engine oils and automatic transmission fluids accounting for ~ 60% of the total use. High temperature and high air exposure applications require a higher level of oxidation protection. Zinc diaJkyl dithiophosphates are the primary inhibitor tj^pe, followed by eiromatic amines, sulfurized olefins, and phenols. A n u m b e r of tests are used to assess a lubricant's oxidation stability under accelerated oxidation conditions. ASTM Sequence IIIE/IIIF (viscosity increase), ASTM Sequence VE/VG (sludge and varnish formation), and CRC L-38 (bearing corrosion) tests are used for engine oils. The CRC L-60-1 test is used for gear oils a n d the ASTM D 943 and ASTM D 2272 tests are commonly used for turbine oils.

1. Hydroperoxide Decomposers R-S-(S)rS-R

X = 0 or 1; R = alkyl or functionalized alkyl

DialkyI polysulfide RQ.,,S (R0)2R, H

:>-c'

Zn

RO'%

CHo

R DialkyI hydrogen phosphite

Alkylphenol

Zinc dialkyi dithlophosphate

2. Radical Scavengers

Methylene coupled dlthlocarbamate

3. lUletal Deactivators

2,6-DI-f-butyl-4-methylphenol

Alkylated diphenylamine

[BHT (Butylated hydroxytoiuene)] N, N-Disalicylldene-1,2-propanediamine FIG. 3—Commonly used oxidation inhibitors.

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY

203

Hydroperoxide

RCH2CH(CH3)2 + O2 CH3 _,OH R

8 o

—-

» Fluid-film Lubrication

.ZN. P (A) RELATIONSHIP OF VISCOSITY (2), SPEED (AO, AND LOAD (P) TO FRICTION AND FILM THICKNESS

FIG. 22—Types of lubrication.

BOUNDARY LUBRICATION Performance essentially depends upon the quality of the boundary film

(B) GRAPHIC REPRESENTATION OF

LUBRICANT FILM THICKNESS IN DIFFERENT LUBRICATION REGIMES

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY

4R0H

+ P2S5

- •

2

.^' (R0)2P,

\

215

+ H2S

SH DialkyI Dtttiiophosphoric Acid

2 (RO)2P.

+

.^'

(R0)2P\ .

ZnO

SH DiaikyI Dithjophosphoric Acid Activated Olefin

Zn

+

H2O

Zinc DialkyI Dithioptiosphate

2 CH2=CH—< OR

i^°l<

2 (ROhP,

Zn J2

5 Cn2 CH2

Zinc Diaryl Oithiophosptiate

C.

OR DialkyI Ditliiophosphate Ester THERMAL STABILITY:

Aryl > Primary Alkyl > Secondary Alkyl ANTIWEAR ACTION: Secondary Alkyl > Primary Alkyl > Aryl FIG. 23—Syntiiesis of dialkyi dithiophosphoric acid derivatives.

The film-formation by these additives is a two-step process. The adsorption of the chemical onto the metal surface occurs first. It is followed by the formation of chemically reactive species due to thermal decomposition or hydrolysis. Antiwear Agents—Zinc salts of dithiophosphoric acids are the most widely used antiwear agents. These salts, in addition to providing antiwear protection, act as oxidation and corrosion inhibitors. They find major use in gasoline and diesel engine oils and industrial lubricants. Zinc dialkyi dithiophosphates or zinc diaryl dithiophosphates cire S3aithesized by reacting the respective dithiophosphoric acids with zinc oxide. The dithiophosphoric acid derivatives that do not produce ash on combustion (ashless) can be prepared by reacting the dithiophosphoric acids with alkylene oxides, such as ethylene oxide or propylene oxide, or with materials that contain activated double bonds, such as cilkyl acrylates and methacrylates. The synthetic scheme to prepare these materials is shown in Fig. 23. Dithiophosphoric acids are products of reaction of an alcohol or a phenol with phosphorus pentasulfide. Thermal and hydrolytic stability of these products depends upon the nature of the organic group. Dialkyi dithiophosphates derived from primary alcohols are more thermally stable than those derived from secondary alcohols, and are used extensively in formulating gasoline and automotive diesel engine oils. Diaryl dithiophosphates, although thermally the most stable in this family, are hydrolytically the least stable and, with some exceptions, are not very effective antiwear agents. Therefore, they do not get much use.

Dithiophosphoric acid derivatives decompose, generally below 200°C, to form thiols, olefins, polymeric alkyl thiophosphates, and hydrogen sulfide [28,29]. The antiwear performance of these derivatives depends upon their thermal stability, which in turn depends upon whether the alkyl groups are primary or secondary. Primary dialkyi dithiophosphates decompose via an alkyl transfer mechanism to form zinc monoalkyl dithiophosphate and an alkyl thiophosphate ester. Through a series of steps, these materials are converted into zinc phosphate and trialkyl tetrathiophosphate, along with a variety of other products [28]. Trialkyl tetrathiophosphate appears to be the major thermal decomposition product, as shown by ^'P nuclear magnetic resonance (NMR) spectroscopy. Secondary alkyl zinc dithiophosphates lose an olefin via ;8elimination to form a product with free dithiophosphoric acid functionality. This product can further decompose by the loss of hydrogen sulfide or another olefin to form a thioanhydride and a variety of other products. Trialkyl tetrathiophosphate is again the major product. The aromatic zinc dithiophosphates are believed to decompose by a free radical mechanism to phenol and a number of phosphorus and sulfur-containing products. Besides the thermal mechanism described above, these additives can also decompose oxidatively to form products that are potent oxidation inhibitors. The details of their oxidation-inhibiting properties were discussed in the oxidation inhibitors section and the oxidation chapter of this manusil. It is important to note that the oxidation inhibiting action of these additives is indepen-

216

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

dent of the nature of the alkyl group, but their antiwear action is not. Aliphatic zinc dialkyl dithiophosphates have better antiwear performance t h a n a r o m a t i c derivatives. And among ahphatics, the secondary alcohol derived are better than those that are primary alcohol derived. Extreme Pressure Agents—^Alkyl and aryl disulfides and polysulfides, dithiocarbamates, chlorinated hydrocarbons, dialkyl hydrogen phosphites, a n d salts of alkyl p h o s p h o r i c acids are the c o m m o n extreme pressure (EP) agents. Polysulfides are synthesized from olefins either by reacting with sulfur or sulfur halides, followed by dehydrohalogenation. Sulfurization of olefins with elemental sulfur, or sulfur and hydrogen sulfide, yields organic sulfides and polysulfides [28,30]. Dialkyldithiocarbamates are prepared either by neutralizing dithiocarbamic acid (resulting from the low-temperature reaction of a dialkylamine and carbon disulfide) with bases, such as zinc oxide or antimony oxide, or by its addition to activated olefins, such as alkyl acrylates [31]. The synthesis of these materials is described in Fig. 24 and Fig. 25, respectively. Alkyl and aryl phosphites are obtained by reacting an alcohol or a phenol with phosphorus trichloride or by a transesterification reaction [32]. Alcohols a n d phenols react with phosphorus pentoxide to yield a mixture of an alkyl (aryl) phosphoric acid and a dialkyl (diaryl) phosphoric acid [33]. These acids, when treated with bases, form salts. Alkyl phosphates can also be prepared by the oxidation of phosphites. The preparation of eJkyl phosphites is outlined in Fig. 26 a n d of alkyl phosphates is outlined in Fig. 27. The extent of EP protection in equipment depends upon the conjunction temperature of the two metal surfaces in contact [34]. Figure 28 shows a direct correlation between the conjunction temperature and the degree of EP protection needed. The equipment that operates at low speeds and high loads generally requires more EP protection than equipment that operates at high speeds and low loads. This is because

the former generates higher temperatures as a consequence of the increased friction. Disulfides and polysulfides decompose o n metal surfaces at t e m p e r a t u r e s above 200°C to form a protective sulfide layer. The thickness of this layer depends on the quantity and the lability of sulfur in the additive. Sulfurized fatty oils and sulfurized olefins are the most commonly used products in this class. Chlorine-containing compounds provide protection under boundary lubrication conditions, via the formation of a metal chloride film. A detrimental aspect of chlorine-based E P agents is the formation of hydrogen chloride in the presence of moisture, which can cause severe corrosion problems. Chlorinated paraffins with 40-70% chlorine by weight were once popular. However, environmental concerns about the negative effects of chlorine are limiting the use of these additives. Phosphorus c o m p o u n d s react with the metal surface to make a metal phosphite or a metal phosphate protective film. Such a film forms at a m u c h higher temperature than that formed by sulfur EP agents. Tricresyl phosphate is the bestknown phosphorus EP agent. Dialkyl hydrogen phosphites [35] and phosphonic and phosphoric acid salts are other examples of such EP agents. As mentioned earlier, the EP mechanism can be considered a two-step process. The first step involves adsorption of the EP agent onto the metal surface and the second step involves its chemical reaction with metal to form the EP film. After being adsorbed on the surface, these materials thermally decompose to reactive m e r c a p t a n s or p h o s p h o r u s compounds that form the EP film. Probable mechanisms by which zinc dialkyl dithiophosphates form EP films is depicted in Fig. 29. Organic halides, often not used in m o d e m formulations because of the environmental concerns, form an iron halide protective film, by reacting with the metal surface via a similar mechanism [36]. Organic polysulfides are

R—CHa—CH=CH2 Olefin

+

Sg

I Heat R—CH=CH—CHa

R—CH=CH—CHz

RCHz

Sx

+

I +

Sa

I

I R-CH2-CH-CH3

R—CH2—CH—CH2

f

RCH Organic Poiysutfides + R

Dithiolethione FIG. 24—Olefin sulfurization.

R—CH2—CH—CH3

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY

RjNH

+

RzN —ct

CSj

Oialkylamine

217

SH

Carbon Disulfide

Ditliiocarbamic Acid

lUletai Oxide or IHydroxidey

CH2=CH—COOR

Activated Olefin

/•/

M = Zn or Sb X = 2 or 3

R2N — C '

R2N—C

M

SCH2—CH5-COOR Metal Dithiocarbamate

Dithiocarbamate Ester

FIG. 25—Synthesis of dithiocarbamic acid derivatives.

P

PCI,

+

3 HCI

Triaryl Phosphite

.^ (R0)2P.

PCI3

3 ROH Alcohol

{PhO)3P

+

2 H C I + RCI

DialkyI Hydrogen Phosphite

+

2 ROH

+

H2O

•

^ (R0)2P.

+

DialkyI Hydrogen Phosphite

Triphenyl Phosphite

(CH30)2P^

+

2 ROH

H Dimethyl Hydrogen Phosphite

(RO)2p^

+

3PhOH Phenol

2CH3OH

Diallcyl Hydrogen Phosphite

Methanol

FIG. 26—Synthesis of alkyl and aryl phosphites.

converted into dicJkyl disulfide, which reacts with the metal to form the metcJ sulfide EP film [37,38]. These inorganic films, which are only a few molecules thick, have low shear strength and are removed during the movement of the surfaces in contact. This situation is represented in Fig. 30. Removal of the EP film can expose fresh metal, and the film-forming process is repeated. Each time the film is removed, the metal is removed with it. One way of looking at the process of EP protection is the controlled wear of rough surfaces, as shown in Fig. 31. In general, formulators use different tjrpes of EP agents in combination because of the possible synergism [36,39]. The synergism between sulfur and chlorine-containing EP agents is shown in Fig. 32, where average scar diameter is plotted as

a function of the applied load [36]. When only disulfide is used, weld occurs at a load of 250 kg, and the scar diameter is about 2.15 mm (Graph A). When a similar level of alkyl chloride is used, the weld load stays the same but the scar diameter improves to 1.74 mm (Graph B). Combining the two types of EP agents to deliver an amount equal to that in the previous cases increases the weld load to 350 kg and decreases the scar diameter to 1.6 mm (Graph C), thereby indicating a synergism between the two chemistries. A further increase in the amount of disulfide and chloride shows weld resistance beyond the load of 500 kg (Graph D). Similar sjoiergism exists between phosphorus and sulfur chemistries [39]. The formation of active new compounds may be responsible for the synergism.

218

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

o

3R0H Alcohol

+

P2O5

—

II RO—P—OH +

Phosphorus Pentoxide

OR

o

II RO—P—OH I OH

DialkyI and MonoalkyI Phosphoric Acids

R'NHa

Metai Base

• / Metal Salts

Amine Salts

P=0 Triaryl Phosphite

Triaryi Phosphate

FIG. 27—Synthesis of alkyl and aryl phosphates. 40C-

Many effective extreme pressure and antiwear additives are corrosive to metals. Therefore, lubricants using them are typically formulated to optimize a balance between corrosivity and extreme-pressure and antiwear protection.

O S^35fl|

52 3 (B a> 30C Q.

E

ffi I-

I 25q

lilHPI^a

u c 5*200

o u

[ZESSIS 15a-

FIG. 28—Extreme pressure (EP) protection requirements vs conjunction temperature.

Some additives in a formulation can diminish the effectiveness of EP/AW agents. These include surface-active additives, such as certain friction modifiers, oxidation inhibitors, rust inhibitors, metal deactivators, detergents, and dispersants. These components either irreversibly adsorb on the surface and interfere with the EP mechanism, or they form complexes with EP agents, thereby rendering them inactive [40,41]. The same is true of some highly polar basestocks. This type of antagonism is quite common for some lubricants, such as gear oils, where EP agents form the core of the formulation. Antiseize additives are a separate class of antiwear additives, which perform independently of temperature. They improve boundary lubrication by forming the protective film through deposition. Molybdenum disulfide and graphite, common additives of this type, are generally used in greases, some industrial oils, and various break-in lubricants.

Rust and Corrosion Inhibitors Corrosion is a general term used to describe the destructive alteration of metal by chemical or electrochemical action of its environment. It primarily involves a heterogeneous reaction, which causes a metal to change from its nascent form (metallic state) to an oxidized form (ionic state). All metals except noble metals are thermodynamically unstable under atmospheric conditions and get converted into their oxidized form. On the other hand, noble metals, such as gold, platinum, iridium, and palladium, are resistant to attack by the environment and are therefore found in nature in the free form. There are many types of corrosion, but a lubricant supplier is primarily concerned with corrosion in the presence of electrolytes (electrochemical corrosion) and in the absence of electrolytes (chemical corrosion). Common electrolytes that lead to electrochemical corrosion include water, acids, alkalis, and salts. Chemical substances that cause chemical corrosion include acids, alkalis, and sulfur. In alloys, corrosion can be selective or nonselective. It is selective if a particular metal is corroded in preference to others. It is nonselective if all metals in the alloy are corroded at the same rate. Electrochemical corrosion involves the reaction of metals in the presence of electrically conducting solutions, or electrolytes, and occurs in two stages: the anodic process and the cathodic process. In the anodic process, metal goes into solution as ions with extra electrons left over. The process can be considered an oxidation. The cathodic process involves the reaction of thus generated electrons with water and oxygen to form the hydroxide ions. The process can be considered a reduction. In solution, metal ions combine with hydroxide ions to form metal hydroxides, or hydrated oxides.

V

RO.

RO

K RO.

FeO Fe OFe Fe FeO Fe OFe Fe fFeO Fe sii„„ OFe Fe FeO Fe OFe Fe FeO Fe \^''''' OFe Fe FeO Fe

S>>ii

RS

-Olefin

J

OR I S=P—S—Zrw I O

EP Film containing Sulfur and Phosphorus Compounds

^

K s=p—s—ziw-X Fe

Fe or Fe203

RO

POLYMER

Adsorption

Fe

or Fe203

Reaction

FIG. 29—The mechanism of boundary film formation by zinc dialkyi dithiophosphates.

FIG. 30—Protective boundary film vs shear.

Original Profile

Worn Profile FIG. 31—Controlled wear of asperities to produce submicron debris.

200

300

Applied Load (l- Lubricant

Part A

mmxm

'^^•^^^^'^^^^^^^^^'^i^^'^^^^^^'^^^i^ Copper Oxide

SOLUTION

s s s s SSSSS

Corrosive Materials

•^^ s ^SSs

Parte Deactivator Film

s '^ss -Lubricant

"SSR Cu O Cu O Cu O Cu O Cu O Cu OCu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O Cu O

Oxide l^yer

(a)

vProtective '^Film Nc. JJ-R

KyN-fl

N>yN-R

KyN-R

1 < ~Cu 0"Cu 0"Cu b'Cu 3.0% oil and are translucent. Their appearance is a function of the particle size. The performance specifications of metalworking fluids are established by the OEMs and end-users. Test methods to evaluate performance of these fluids are not well standardized. Tests that are presently used or can be used to judge the suitability of metalworking fluids include: • Corrosion Teste—Copper Strip (ASTM D 130), Turbine Oil Rust (ASTM D 665), Aqueous Cutting Fluid (IP125), Filter Paper Chip Breakpoint (IP287), Aluminum Cup Stain, Humidity Cabinet Rust (ASTM D 1748), Salt Spray (MIL-B117-64), Cleveland Condensing Humidity Cabinet (ASTM D 2247) •Extreme Pressure—Four-Ball Wear (ASTM D 4172), Timken (ASTM D 2782), Four-Ball EP (ASTM D 2783), Falex EP (ASTM D 3233) • Stability~¥oam Tendency/Stability (ASTM D 892, IP312), Panel Coker, Demulsibility (ASTM D 1401), Emulsion Stability (IP263), Aquarium Biostability • Miscellaneous—Color (ASTM D 1500), GM Quenchometer (ASTM D 3520), Thread Tapping (Lubrizol test). Pipe Threading (Lubrizol test). Stick-slip (Cincinnati Milacron test), Bijur Filtration, Falex #8, SLT (Draw Bead Simulator), Reichert Test, and Tapping Torque Test (ASTM D 5619)

• Wear and Extreme-pressure Tests—Denison T-50 Vane pump and P-46 Piston Pump; Vickers 35VQ-25 Vane and Vickers V104C Vane Pump; FZG EP/antiwear [DIN 51354 (Part 2)]; Four-Ball EP (ASTM D 2783); and Four-Ball Wear (ASTM D 4172) • Oxidation Jesfs—Turbine Oil (ASTM D 943); Sludge (ASTM D 4310); and Rotary Bomb (ASTM D 2272) • Corrosion Tesfs—Turbine Oil Rust (ASTM D 665) and Copper Strip (ASTM D 130) • Miscellaneous Tests—Turbine Oil Demulsibility (ASTM D 1401); Hydrolytic Stability (ASTM D 2619); Cincinnati Milacron Thermal Stabihty; Denison TP 02100 Filterabihty; Foam ASTM D 892; Air Separafion (DIN 51381); and Seal Compatibility [DIN 53538 (Part 1)]

Miscellaneous Industrial Oils Compressor oils, refrigeration oils, turbine oils, circulating oils, slideway lubricants, and rock drill lubricants make up this group. Compressor and refrigeration oils are used in compressors to reduce friction and act as a seal separating low and high-pressure areas. Turbine and circulating system oils are used to lubricate steam and gas turbines for marine and stationary applications. Circulating oils are used in systems where large quantities of heat must be removed and where heavy contamination of oil occurs. As a result, these oils must possess excellent air and water separation properties and good oxidative stability over the duration of their use. Slideway and drill lubricants are used to lubricate guiding surfaces on the bed of a machine along which a table or a carriage moves and to lubricate pneumatic equipment. These oils must perform under extreme temperatures, high loads, moisture, and poor ambient air quality. They therefore possess both the EP activity and the rust and corrosion-inhibiting properties. ISO viscosity grades and U.S. Military and OEM performance requirements usually specify these lubricants. Turbine oils are classified as R & O oils, non-EP oils, and EP oils. R & O oils are formulated to provide rust and oxidation protection and EP oils are formulated to provide EP protection, depending upon the intended end use. EP oils contain R & O packages enhanced with antiwear additives. R & O oil performance specifications are established by OEMs, such as U.S. Steel, Cincinnati Milacron, Denison, General Electric, and organizations, such as U.S. Military, AFNOR (Association Francais Petroles de Normalisation) and DIN (Deutsche Industrie Norm). Specifications for non-EP oils are established by General Electric, British Government, and DIN and for EP oils the OEM Brown Boveri plays an active role [74]. Common tests for these oils are as follows.

Metalworking

Fluids

Metalworking fluids are used to convert metal into a component or a piece. These fluids are of four basic t3^es, straight oils, soluble oils, semisynthetic fluids, and synthetic fluids. Soluble oils, semisynthetic fluids, and synthetic fluids are waterbased and differ from one another, mainly in their oil content and emulsion type [76]. Straight oils are devoid of water. They are mineral oil-based (primarily hydrotreated naphthenic basestocks) and usually contain sulfur, chlorine, and phosphorus-derived additives. Soluble oils are water emulsions of mineral and/or fatty oils. An emulsifying agent or surfactant is used to form these emulsions. Synthetic fluids are oil-free and are simply solutions of additives in water. Since most organic materials are hard to dissolve in water, high polarity of the additives is necessary for solubility. Soaps or other surfactants are sometimes added to help in this regard. Semisynthetic fluids are in between soluble oils and synthetic fluids as far as their oil content is concerned. In

242

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

R & O Oils • Wear Tests—Vane P u m p (ASTM D 2882), Denison P-46 Piston Pump, Four-Ball Wear (ASTM D 4172) • Oxidation Tests—Rotary Bomb (ASTM D 2272), Turbine Oil (ASTM D 943), 1000-hour Sludge (ASTM D 4310), Cincinnati Milacron Heat, FTMS 5308.6 • Corrosion Tests—Rust (ASTM D 665), Copper Strip (ASTM D 130) • Miscellaneous Tests—Turbine Oil Demulsibility (ASTM D 1401), Neutralization N u m b e r (ASTM D 974 and D 664), Foam (tendency/stability) (ASTM D 892), and Air Release (DIN 51381, ASTM D 3427) Turbine Oils • Oxidation Tesfs—Turbine Oil (ASTM D 943), 1000-hour Sludge (ASTM D 4310), IP280 TOP, IP280 Sludge, Rotary Bomb (ASTM D 2272), FTMS 5308.6, and Universal Oxidation Test (ASTM D 5846) • Corrosion Tests—Rust (ASTM D 665), Copper Strip (ASTM D130) • Wear Test—FZG (A/8.3/90), a n d Four-Ball EP (ASTM D 2783), Falex (ASTM D 2670), Ryder gear tests to fulfil U.S. Military requirements. • Miscellaneous Tests—Viscosity Index (ASTM D 2270), Flash Point (ASTM D 93), Pour Point (ASTM D 97), Neutralization N u m b e r (ASTM D 974 and D 664), Air Release (DIN 51381, ASTM D 3427), F o a m (tendency/stability) (ASTM D 892), Demulsibility (ASTM D 2711), and Turbine Oil Demulsibility (ASTM D 1401) Greases The use of this lubricant goes back to ancient times [62]. Lubricating grease is defined as a "solid-to-semifluid products of dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may be added (ASTM D 288)." Such ingredients include additives that impart other desirable properties, such as EP, water resistance, etc. The lubrication function is carried out by the small amount of oil that is released during equipment operation. Because of their semisolid nature, greases are used when fluid lubricants are inefficient, the need for lubrication is infrequent, and/or the lubricant is required to maintain its original position in a mechanism. Greases are formulated from both synthetic and mineral oil basestocks by using a thickening agent and selected additive packages. The thickener, usually a metal soap (a carboxylic acid salt) and sometimes a gelled basic sulfonate,

serves to immobilize the lubricant until service application causes it to be released. The lubricant contains additives which reduce friction and prevent wear. Greases perform the same basic functions as their fluid counterparts but, in view of their high viscosity, they do not perform cooling and cleaning functions efficiently. Based on thickener, greases can be classified as simple-soap, complex-soap, and nonsoap. Simple-soap greases contain lithium (Li), sodium (Na), calcium (Ca), barium (Ba), or aluminum (Al) fatty acid carboxylates. Complex-soap greases contain metal salts of fatty and nonfatty acid mixtures. Nonsoap greases may contain either inorganic compounds or orgeinic compounds as thickeners. Greases are described by the National Lubricating Grease Institute (NLGI) consistency grades and NLGI Service Classification System for automotive use, first implemented in 1991 [77], Consistency grades are 000, 00, 0, and 1-6 and are based on degree of hardness and ASTM Worked penetration rcinge @ 25°C. NLGI service classifications are LA and LB for chassis use and GA, GB, and GC for use in wheel bearings. Prior to this classification, the SAE recommended practice, published in the SAE information report J310, was used for this purpose. The report, first introduced in 1951, had several revisions, the most recent of which occurred in 1993 [78]. Tests associated with NLGI Service Classes, provided in the ASTM 4950 Automotive Grease Specification, are listed below. • Shear Stability—Mukistroke Penetration (ASTM D 217), Roll Stability (ASTM D 1831), Wheel Bearing Leakage (ASTM D 4290 and D 1263) • Oxidation Resistance—Bomb Oxidation (ASTM D 942), High Temperature Life (ASTM D 3527), High-Temperature Performance (ASTM D 3336) • Water Resistance—Water Washout (ASTM D 1264), Water Spray-off (ASTM D 4049) • Bleed Resistance—Oil Separation, static (FTM 321.3), Pressure Oil Separation (ASTM D 1742) • Extreme Pressure/ Antiwear—Four-ball EP (ASTM D 2596), Timken Method (ASTM D 2509), Four-ball Wear (ASTM D 2266), Fretting Protection (ASTM D 4170), SRV Test (ASTM D 5706 and D 5707) • Corrosion—Rust Test (ASTM D 1743), E m c o r (IP 220), Copper Corrosion (ASTM D 4048) • Pumpability—Low-temperatu re Torque (ASTM D 4693) Mobility (US Steel LT37) • Msce/ZaneoMS—Elastomer Compatibility (ASTM D 4289) Dropping Point (ASTM D 566 or D 2265)

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY

243

ASTM AND OTHER STANDARDS ISO No. 2592:1973 2719:1988 6293:1983 3016:1994

IP No. 36/84 (89) 34/88 136/89 15/95

2160:1985

154/95

D 217-97 D 445-01

2137:1985 3104:1994

50/88 71/97

D 482-00 D 566-97 D 664-95 (2001)

2176:1995 6619:1988

132/96 177/96

D 665-99

7120:1987

135/93

D D D D

808-00 874-00 892-01 942-90 (1995)

3987:1994 6247:1998

163/96 146/82 (88) 142/85 (92)

D D D D

943-99 972-97 974-01 1078-95

4263:1986

280/96

6618:1997

139/93 195/90

3839:1978

130/92

D 1298-99

3675:1993

160/96

D D D D

6614:1994 2049-1996

412/96 196/97

D D D D D D

ASTM No. 92-01 93-00 94-00 97-96a 129-00 130-94(2000)

D 1091-00 D 1092-99 D 1159-01 D 1218-99 D 1263-94 (1999) D 1264-00

1401-98 1500-98 1742-94 (2000) 1743-01

D 1744 366/84

D 1748-00 D 1831-00 D 2070-91 (2001) D 2161-93 (1999) D 2265-00

239/97

D 2266-91 (1996) D 2270-93 (1998) D 2272-98

2909:1981

226/91 (95)

D 2500-99 D 2509-93 (1998)

3015:1992

219/94 326/83 (88)

D 2596-97 D 2602-86 D 2603-01

Test Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Flash Point by Pensky-Martens Closed Cup Tester Test Method for Saponification N u m b e r of Petroleum Products Test Method for Pour Point of Petroleum Products Test Method for Sulfur in Petroleum Products (General B o m b Method) Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Cone Penetration of Lubricating Grease Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Test Method for Ash from Petroleum Products Test Method for Dropping Point of Lubricating Grease Test Method for Acid Number of Petroleum Products by Potentiometric Titration Method Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Chlorine in New and Used Petroleum Products (Bomb Method) Test Method for Sulfated Ash from Lubricating Oils and Additives Test Method for Foaming Characteristics of Lubricating Oils Test Method for Oxidation Stability of Lubricating Greases by the Oxygen B o m b Method Test Method for Oxidation Characteristics of Inhibited Mineral Oils Test Method for Evaporation Loss of Lubricating Greases and Oils Test Method for Acid and Base Number by Color-Indicator Titration Test Method for t h e Determination of Distillation Characteristics of Volatile Organic Liquids (ASTM Procedure Now Obsolete) Test Method for Phosphorus in Lubricating Oils and Additives Test Method for Measuring Apparent Viscosity of Lubricating Greases Test Method for B r o m i n e N u m b e r s of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration Test Method for Refractive Index a n d Refractive Dispersion of Hydrocarbon Liquids Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases Test Method for Determining the Water Washout Characteristics of Lubricating Greases Practice for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products Test Method for Water Separability of Petroleum Oils and Synthetic Fluids Test Method for ASTM Color of Petroleum Products (ASTM Color Scale) Test Method for Oil Separation from Lubricating Grease During Storage Test Method for Determining Corrosion Preventive Properties of Lubricating Greases Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent (Discontinued 2000) Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet Test Method for Roll Stability of Lubricating Grease Test Method for Thermal Stability of Hydraulic Oils Practice for Conversion of Kinematic Viscosity to Saybolt Universal Viscosity or to Saybolt Universal Viscosity Test Method for Dropping Point of Lubricating Grease Over Wide Temperature Range Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel Test Method for Cloud Point of Petroleum Products Test Method for Measurement of Load-Carrying Capacity of Lubricating Grease (Timken Method) Test Method for Measurement of Extreme-Pressure Properties of Lubricating Grease (Four-Ball Method) Test Method for Apparent Viscosity of Engine Oils at Low Temperature Using Cold-cranking Simulator (Replaced in 1993 with D 5293) Test Method for Sonic Shear Stability of Polymer-Containing Oils

(Continues)

244 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ASTM No.

ISO No.

IP No.

D 2619-95 D 2622-98 D 2670-95 (1999) D 2710-99 D2711-01a D 2782-01 D 2783-88 (1998)

293/97

D 2882-00 D 2893-99 D 2896-01

3771:1994

D 2983-01

276/95 267/84

D 3228-96 D 3233-93 (1998) D 3336-97 D 3339-95 (2000)

7537:1989

BS7393

3679:1983 3680:1983

303/83 (88)

4265:1986

149/93

D 3427-99 D 3520-88 (1998) D 3527-95 D 3825-90 (2000) D 3828-98 D 3829-93 (1998) D 4047-00 D D D D

4048-97 4049-99 4170-97 4172-94 (1999)

293/97

D 4289-97 D 4290-94 (1999) D 4310-98 D 4377-00 D 4485-01 D 4624-93 (1998)

356/93

D 4627-92

287/94 125/82

D 4628-97 D 4682-87 (1996) D 4683-96 D 4684-99 D 4693-97 D 4739-96 D 4741-00 D 4857-Ola D 4858-00

276/95 417/96

Test

Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-Ray Fluorescence Spectrometiy Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method) Test Method for Bromine Index of Petroleum Hydrocarbons by Electrometric Titration Test Method for Demusibility Characteristics of Lubricating Oils Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Timken Method) Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method) Test Method for Indicating Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in Constant Volume Vane Pump Test Method for Oxidation Characteristics of Extreme-Pressure Lubricating Oils Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method Test Method for Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex Pin and Vee Block Methods) Test Method for Life of Lubricating Greases in Ball Bearings at Elevated Temperatures Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration Test Method for Air Release Properties of Petroleum Oils Test Method for Quenching Time of Heat-Treating Fluids (Magnetic Quenchometer Method) Test Method for Life Perforraance of Automotive Wheel Bearing Grease Test Method for Dynamic Surface Tension by the Fast Bubble Technique Test Method for Flash Point by Small Scale Closed Tester Test Method for Predicting the Borderline Pumping Temperature of Engine Oil Test Method for Phosphorus in Lubricating Oils and Additives by Quinoline Phosphomolybdate Method Test Method for Detection of Copper Corrosion from Lubricating Grease Test Method for Determining the Resistance of Lubricating Grease to Water Spray Test Method for Fretting Wear Protection by Lubricating Greases Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method) Test Method for Elastomer Compatibility of Lubricating Greases and Fluids Test Method for Determining the Leaking Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions Test Method for Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration Specification for Performance of Engine Oils Test Method for Measuring Apparent Viscosity by Capillary Viscometer at HighTemperature and High-Shear Rates Test Method for Iron Chip Corrosion for Water-Dilutable Metalworking Fluids Test Method for Analysis of Barium, Cadmium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption Spectrometry Specification for Miscibility with Gasoline and Fluidity of Two-Stroke Cycle Gasoline Engine Lubricants Test Method for Measuring Viscosity at High Shear Rate and High Temperature by Tapered Bearing Simulator Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature Test Method for Low-Temperature Torque of Grease-Lubricated Wheel Bearings Standard Test Method for Base Number Determination by Potentiometric Titration Test Method for Measuring Viscosity at High Temperature and High Shear Rate by Tapered-Plug Viscometer Test Method for Determination of the Ability of Lubricants to Minimize Ring Sticking and Piston Deposits in Two-Stroke-Cycle Gasoline Engines Other Than Outboards Test Method for Determination of the Tendency of Lubricants to Promote Preignition in Two-Stroke-Cycle Gasoline Engines

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY ASTM No. D 4859-97

ISO No.

IP No.

D 4863-00 D 4927-96 D 4928-00 D 4950-95 (2000) D 4951-00 D 4998-95 D 5133-99 D 5182-97 D 5185-97 D 5291-96 D 5293-99a D 5302-01

D 5480-•95 (1999) D5481- 96 D 5533-•98 D 5579- 01 D 5 6 1 9 00 D 5662 D 5706- 97 D 5707. D 5760- 95 D 5763- 95 D 5800 00a D 5844. 98 D 5846- 99 D 5862- 99a D 5949- 96 D 5950- 96 D 5966- 99 D 5967- 99a D 5968 00a D 5985- 96 D 6082- 00 D6121- 01 D 6202 -01 D 6335 D 6375 99a D 6417 99

334/93

245

Test Specification for Lubricants for Two-Stroke-Cycle Spark-Ignition Gasoline Engines-TC Test Method for Determination of Lubricity of Two-Stroke-Cycle Gasoline Engine Lubricants Test Methods for Elemental Analysis of Lubricant and Additive Components— B a r i u m , Calcium, P h o s p h o r u s , Sulfur, and Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy Test Methods for Water in Crude Oils by Coulometric Karl Fischer Titration Classification and Specification for Automotive Service Greases Test Method for Determination of Additive Elements in Lubricating Oils by Inductively Coupled Plasma Atomic Emission Spectroscopy Test Method for Evaluating Wear Characteristics of Tractor Hydraulic Fluids Test Method for Low T e m p e r a t u r e , Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) Test Method for Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by Inductively Coupled Plasma Emission Spectroscopy (ICP-AES) Test Methods for Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Test Method for Apparent Viscosity of Engine Oils Between - 5 and — 35°C Using the Cold Cranking Simulator Test Method for Eveduation of Automotive Engine Oils for Inhibition of Deposit Formation and Wear in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light-Duty Conditions (Sequence VE) Test Method for Engine Oil Volatility by Gas Chromatography Test Method for Measuring Apparent Viscosity at High-Temperature and HighShear Rate by Multicell Capillary Viscometer Test Method for Evaluation of Automotive Engine Oils in Sequence IIIE, SparkIgnition Engine Test Method for Evaluating the Thermal Stability of Manual Transmission Lubricants in a Cyclic Durability Test Test Method for Comparing Metal Removal Fluids Using the Tapping Torque Test Machine Test Method for Determining Automotive Gear Oil Compatibility with Typical Oil Seal Elastomers Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, Linear-Oscillation (SRV) Test Machine Test Method for Measuring Friction and Wear Properties of Lubricating Greases Using a High-Frequency, Linear-Oscillation (SRV) Test Machine Specification for Performance of Manual Transmission Gear Lubricants Test Method for Oxidation and Thermal Stability Characteristics of Gear Oils Using Universal Glassware Test Method for Evaporation Loss of Lubricating Oils by the Noack Method Test Method for Evaluation of Automotive Engine Oils for Inhibition of Rusting (Sequence IID) Test Method for Universal Oxidation Test for Hydraulic and Turbine Oils Using the Universal Oxidation Test Apparatus Test Method for Evaluation of Engine Oils in Two-Stroke Cycle Turbo-SuperCharged 6V92TA Diesel Engine Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) Test Method for Pour Point of Petroleum Products (Automatic Tilt Method) Test Method for Evaluation of Engine Oils for Roller Follower Wear in Light-Duty Diesel Engine The Method of Evaluation of Diesel Engine Oils in T-8 Engine Test Method for the Corrosiveness of Diesel Engine Oil Test Method for Pour Point of Petroleum Products (Rotational Method) Test Method for High Temperature Foaming Characteristics of Lubricating Oils Test Method for Evaluation of the Load Carrying Capacity of Lubricants Under Conditions of Low Speed and High Torque Used for Final Hypoid Drive Axles Test Method for Automotive Engine Oils on the Fuel Economy of Passenger Cars and Light Duty Trucks in t h e Sequence VIA Spark Ignition Engine Test Method for Determination of High T e m p e r a t u r e Deposits by ThermoOxidation Engine Oil Simulation Test Test Method for Evaporation Loss of Lubricating Oils by Thermogravimetric Analyzer (TGA) Noack Method Test Method for Estimation of Engine Oil Volatility by Capillary Gas Chromatography (Continues)

246 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ASTM No.

D 6443-99 D 6481-99 D 6483-99 D 6557-00 D 6593-00

D 6616-01 D 6618-00 D 6681-01 D 6709-01

ISO No.

IP No.

Test

Test Method for Determination of Calcium, Chlorine, Copper, Magnesium, Phosphorus, Sulfur, and Zinc in Unused Oils and Additives by Wavelength Dispersive X-ray Fluorescence Spectrometry (Mathematical Correction Method) Test Method for Determination of Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray Fluorescence Spectrometry Test Method for Evaluation of Diesel Engine Oils in T-9 Engine Test Method for Evaluation of Rust Preventive Characteristics of Automotive Engine Oils (Ball Rust Test) Test Method for Evaluation of Automotive Engine Oils for Inhibition of Deposit Formation in a Spark-Ignition Internal Combustion Engine Fueled with Gasoline and Operated Under Low-Temperature, Light Duty Conditions (Sequence VG) Test Method for Measuring Viscosity at High Shear Rate by Tapered Bearing Simulator Viscometer at 100°C Test Method for Evaluation of Engine Oils in Diesel Four-Stroke-Cycle SuperCharged IM-PC Single Cylinder Oil Test Engine Test Method for Evaluation of Engine Oils in a High Speed, Single-Cylinder Diesel Engine—Caterpillar IP Test Procedure Test Method for Evaluation of Automotive Engine Oils in the Sequence VIII SparkIgnition Engine (CLR Oil Test Engine)

REFERENCES [1] Obert, E. F., "Lubrication," Ch. 16, Internal Combustion Engines and Air Pollution, Intext Educational Publishing, NY, 1968, pp. 633-677. [2] Modler, R., Anderson E., and Yoshida Y., "Lubricant Oil Additives," Specialty Chemicals, Strategies for Success, Vol. 9, SRI International, December 1996. [3] Rizvi, S. Q. A., "Additives: Chemistry and Testing," Tribology Data Handbook—An Excellent Friction, Lubrication, and Wear Resource, CRC Press, Boca Raton, FL, 1997, pp. 117-137. [4] Klamann, D., Lubricants and Related Products—Synthesis, Properties, Applications, International Standards, Verlag Chemie, Hamburg, 1984. (a) "Analysis and Testing," Ch. 10, pp. 218-247. (b) "Additives," Ch. 9, pp. 177-217. (c) Appendix A, pp. 437-442. [5] Gergel, W. C, "Lubricant Additive Chemistry," Presented at the International Symposium on Technical Organic Additives and Environment, Interlaken, Switzerland, 24-25 May 1984. [6] Ford, J. F., "Lubricating Oil Additives—^A Chemist's Eye View," Journal of the Institute of Petroleum, Vol. 54, July 1968, pp. 188-210. [7] Schilling, A., Motor Oils and Engine Lubrication, Scientific Publications, Great Britain, 1968. [8] Ingold, K. U., "Inhibition of Autoxidation of Organic Substances in Liquid Phase," Chemical Reviews, Vol. 61, 1961, pp. 563-589. [9] Johnson, M. D., Korcek, S., and Zinbo, M., "Inhibition of Oxidation by ZDTP and Ashless Antioxidants in the Presence of Hydroperoxides at 160°C," Lubricant and Additive Effects on Engine Wear, SP-558, Fuels and Lubricants Meeting, San Francisco, CA, 31 Oct.-3 Nov., 1983, pp. 71-81. [10] Al-Malaika, S., Marogi, A., and Scott, G., Journal ofApplied Polymer Science, Vol. 33, 1987, pp. 1455-71. [11] Abou El Naga, H. H. and Salem, A. E. M., "Effect of Worn Metals on the Oxidation of Lubricating Oils," Wear, Vol. 96, 1984, pp.267-283. [12] Vijh, A. K., "Electrochemical Mechanisms of the Dissolution of Metals Eind the Contaminants Oxidation of Lubricating Oils Under High-temperature Friction Conditions," Wear, Vol. 104, 1985,pp.l51-158. [13] HambUn, P. C, Kristen U., and Chasan D., "A Review: Ashless Antioxidants, Copper Deactivators, and Corrosion Inhibitors, Their Use in Lubricating Oils," Lubrication Science, Vol. 2, 1990, pp. 287-318.

[14] Kreuz, K. L., "Gasoline Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 55, 1969, pp. 53-64. [15] Lachowicz, D. R. and Kreuz, K. L., "Peroxynitrates. The Unstable Products of Olefin Nitration with Dinitrogen Tetroxide in the Presence of Oxygen. A New Route to a-Nitroketones," Journal of Organic Chemistry, Vol. 32, 1967, pp. 3885-3888. [16] Kreuz, K. L., "Diesel Engine Chemistry as Applied to Lubricant Problems," Lubrication, Vol. 56, 1970, pp. 77-88. [17] Covitch, M. J., Graf, R. T., and Gundic, D. T., "Microstructure of Carbonaceous Diesel Engine Piston Deposits," Lubricant Engineering, Vol. 44, 1988, p. 128. (b) Covitch, M. J., Richardson, J. P., and Graf, R. T., "Structural Aspects of European and American Diesel Engine Piston Deposits," Lubrication Science, Vol. 2, 1990, pp. 231-251. [18] Kombrekke, R. E., Personal Communication, Research and Development, The Lubrizol Corporation, Wickliffe, OH. [19] Boner, C. J., "Theory of Action and Performance," Ch. 8, Gear and Transmission Lubricants, Reinhold Publishing Company, NY, 1964. [20] Bhushan, B. and Gupta, B. K., "Physics of Tribological Materials," Ch. 3, Handbook of Tribology; Materials, Coatings, and Surface Treatments, McGraw-Hill, Inc., NY, 1991. (c) Buckley, D. H., "Properties of Surfaces," CRC Handbook of Lubrication, (Theory and Practice of Tribology), Vol. II, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 17-30. [21] Lansdown, A. R., "Extreme Pressure and Anti-wear Additives," Ch. 12, Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 269-281. [22] O'Brien, J. A., "Lubricant Additives," CRC Handbook of Lubrication, (Theory and Practice of Tribology), Vol. II, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 301-315. [23] "Engine Service Classification System and Guide to Crankcase Oil Selection," API Publication 1509, American Petroleum Institute, Washington, D.C., 1996. [24] Oliver, C. R., Renter, R. M., and Sendra, J. C, "Fuel Efficient Gasoline-Engine Oils," Lubrication, Vol. 67, 1981, pp. 1-12. [25] Jayne, G. J., Matthews, B. M., and Thomas, A. S., "Hypoid Gear Oils for the 1980s," Ch. 19, Performance and Testing of Gear Oils and Transmission Fluids, R. Tourret and E. P. Wright, Eds., Heyden and Son, 1981, pp. 307-319.

CHAPTER 9: ADDITIVES AND ADDITIVE CHEMISTRY [26] Feng, I. M., Perilstein, W. L., and Adams, M. R., "Solid Film Deposition and Non-Sacrificial B o u n d a r y Lubrication," ASLE Transactions, Vol. 6, 1963, pp. 60-66. [27] Schiemann, L. F. and Schwind, J. J., "Fundamentals of Automotive Gear Lubrication," SAE Paper 841213, Fuels and Lubricants Technology: An Overview, SP603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 107-115. [28] Jones, R. B. and Coy, R. C , "The Thermal Degradation and EP Performance of Zinc Dialkyl Dithiophosphate Additives in White Oil," ASLE Transactions, Vol. 24, 1981, pp. 77-90. (b) Jones, R. B. and Coy, R. C , "The Chemistry of Thermal Degradation of Zinc Dialkyl Dithiophosphate Additives," ASLE Transactions, Vol. 24, 1981, pp. 91-97. [29] Brazier, A. D. and EUiot, J. S., "The Thermal Stability of Zinc Dithiophosphates," Journal of the Institute of Petroleum, Vol. 53, 1967, pp. 63-76. [30] Bateman L. and Moore, C. G., "Reaction of Sulfur with Olefins," Ch. 20, Organic Sulfur Compounds, Vol. 1, N. Kharasch, Ed., Pergamon Press, NY, 1961, pp. 210-228. [31] T h o m , G. D. and Ludwig, R. A., The Dithiocarbamates and Related Compounds, Elsevier Publishing Company, NY, 1962. [32] Gerrard, W. and Hudson, H. R., "Organic Derivatives of Phosphorous and Thiophosphorous Acids," Ch. 13, Organic Phosphorus Compounds, Vol. 5, G. M. Kosolapoff and L. Maier, Eds., Wiley Interscienee, NY, 1973, p. 21. [33] Cherbuliez, E., "Organic Derivatives of Phosphoric Acid," Organic Phosphorus Compounds, Vol. 6, G. M. Kosolapoff and L. Maier, Eds., Wiley Interscienee, NY, 1973, p. 211. [34] Fein, R. S., "Boundary Lubrication," CRC Handbook of Lubrication, Theory and Practice in Trihology, Vol. II, Theory and Design, Richard E. Booser, Ed., CRC Press, Boca Raton, FL, 1983, pp. 49-67. [35] Forbes, E. S. and Battersby, J., "The Effect of Chemical Structure on the Load-carrying and Adsorption Properties of Dialkyl Phosphites," ASLS Transactions, Vol. 17, No. 4, 1974, pp. 263-270. [36] Dorinson, A., "The Additive Action of Some Organic Chlorides and Sulfides in the Four-Ball Lubricant Test," ASLE Transactions, Vol. 16, No. 1, 1973, pp. 22-31. [37] Forbes, E. S. and Reid, A. J. D., "Liquid Phase Adsorption/Reaction Studies of Organo-sulfur Compounds and Their Load-Carrying Mechanism," ASLE Transactions, 1973, Vol. 16, No. 1, pp. 50-60. [38] Plaza, S., "Some Chemical Reactions of Orgsinic Disulfides in B o u n d a r y Lubrication," ASLE Transactions, Vol. 30, No. 4, 1987, pp. 493-500. [39] Kawamura, M., Moritani, H., Esaki, Y., and Fujita, K., "The Mechanism of Synergism Between Sulfur- and Phosphorus-type EP Additives," ASLE Transactions, Vol. 29, No. 4, 1986, p p . 451^56. [40] Rounds F. G., "Additive Interactions and Their Effect on the Performance of a Zinc Dialkyl Dithiophosphate," ASLE Transactions, Vol. 2 1 , No. 2, 1978, pp. 91-101. [41] Rounds, F. G., "Some Effects of Amines on Zinc Dialkyl Dithiop h o s p h a t e Antiwear Performance as Measured in Four-Ball Wear Tests," ASLE Transactions, Vol. 24, No. 4, 1981, p p . 431-440. [42] Speller, F. N., Corrosion—Causes and Prevention, McGraw-Hill Pubhshing, Columbus, OH, 1935. [43] Cooper, A. R., "Molecular Weight Determination," Concise Encyclopedia of Polymer Science and Engineering, J. I. Kroschwitz, Ed., Wiley Interscienee, NY, 1990, pp. 638-639. [44] R a w e , A., "Molecular Weights of Polymers," Organic Chemistry of Macromolecules, Marcel Dekker, NY, 1967, pp. 39-54. (b) Deanin, R. D., Polymer Structure, Properties, and Applications, Cahner Books, NY, 1972, p. 53. [45] Baczek, S. K. and Chamberhn, W. B., "Petroleum Additives,"

247

Encyclopedia of Polymer Science and Engineering, Second Edition, John Wiley and Sons, NY, 1988, Vol. 11, p. 22. [46] MuUer, H. G., "Mechanism of Action of Viscosity Index Improvers," Tribology International, June 1978, pp. 189-192. [47] Watson, R. W. and McDonnell, T. F., Jr., "Additives—The Right Stuff for Automotive Engine Oils," Fuels and Lubricants Technology: An Overview, SP. 603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 17-28. [48] Stambaugh, R. L„ "Viscosity Index Improvers and Thickeners," Chemistry and Technology of Lubricants, R. M. Mortier and S. T. Orszulik, Eds., VCH Publishers, Inc., NY, 1992, pp. 124-159. [49] Becher, P., Emulsions: Theory and Practice, American Chemical Society Monograph Series, Ch. 6, Reinhold Publishing Corporation, NY, 1957, pp. 209-231. [50] Karsa, D. R., "Industrial Applications of Surfactants," Industrial Applications of Surfactants—An Overview, D. R. Karsa, Ed., Published by Royal Society of Chemistry, Cambridge, England, 1987. [51] Hancock, R. I., "Macromolecular Surfactants," Surfactants, T. F. Tadros, Ed., Academic Press, San Diego, CA, 1984, pp. 287- 321. [52] Lubricant Additives and the Environment, CEFIC, Brussels, Belgium, 1993, an ATC (Technical Committee of Petroleum Additive Manufacturers) Technical Publication. [53] Fuel Additives and the Environment, CEFIC, an ATC (Technical Committee of Petroleum Additive Manufacturers) Technical Publication, Brussels, Belgium, 1994, [54] De Hoffmann, E., Charette, J., and Stroobant, V., Mass Spectrometry: Principles and Applications, John Wiley & Sons, NY, December 1996. (b) Colthup, N. B., Daly, L. H., and. Wiberley, 5. E., Introduction to Infrared and Raman Spectroscopy, 3rd edition. Academic Press, San Diego, CA, September 1990. (c) Macomber, R. S., A Complete Introduction to Modem NMR Spectroscopy, John Wiley & Sons, NY, December 1997. (d) Derome, A. E., "Modem NMR Techniques for Chemistry Research," Vol. 6, Tetrahedron Organic Chemistry Series, J. E. Baldwin and P. D. Magnus, Eds., Pergamon Press, Oxford, 1993. [55] Hsu, S. M. and Cummings, A. L., "Interactions of Additives and Lubricating Basestocks," Lubricant and Additive Effects on Engine Wear, SP - 558, Fuels and Lubricants Meeting, San Francisco, CA, 31 Oct.-3 Nov. 1983, pp. 61-70. [56] Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1998, and the later revisions. [57] Rein, S. W., "Viscosity-I," Lubrication, Vol. 64, No. 1, 1978, pp. 1-12. (b) Rein, S. W., "Viscosity-II," Lubrication, Vol. 64, No. 1, p p . 13-32, 1978. (c) "Viscosity," Lubrication, Vol. 52, No. 3, 1966, pp. 2 1 ^ 8 . [58] SAE J300: "Engine Oil Viscosity Classification," Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. [59] SAE J1536: "Two-stroke Cycle Engine Oil Miscibility/Fluidity Classification," Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. [60] SAE J306: "Axle and Manual Transmission Lubricant Viscosity Classification," Society of Automotive Engineers, Warrendale, PA, 1985. (b) "Revision to SAE J306 Approved," Lubrizol NewsLine, Vol. 16, No. 3, June 1998. [61] SAE J2227: "International Tests and Specifications for Automotive Oils," Surface Vehicles Information Report, Society of Automotive Engineers, Warrendale, PA, July 1998. (b) "ACEA Issues New Engine Oil Specification," Lubrizol NewsLine, Vol. 16, No. 3, June 1998. [62] Rizvi, S. Q. A., "History of Automotive Lubrication," SAE Technical Paper 961949, Presented at Fuels and Lubricant Meeting, San Antonio, TX, 14-17 Oct. 1996, Society of Automotive Engineers, Warrendale, PA. [63] SAE J183: "Engine Oil Performance and Engine Service Classification (Other Than "Energy Conserving")," SAE 1995 Hand-

248 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

[64] [65] [66] [67]

[68]

[69]

[70]

book, Society of Automotive Engineers, Warrendale, PA, 1995, and the later revisions. Sullivan, T., "API Snuffs out GF-2," Lube Report—Industry News from Lubes-n-Greases, Vol. 2, No. 14, 2002. McFall, D., "GF-4 Oil Due in One Year," Lube Report—Industry News from Lubes-n-Greases, Vol. 2, No. 14, 2002. McFall, D., "CI-4 Diesel Oil: On Time, On Target," Lubes-nGreases, Feb. 2002, p p . 6-12. SAE Standard J2116: "Two-stroke-Cycle Gasoline Engine Lubricants: Performance and Service Classification," Approved July 1993, Society of Automotive Engineers, Warrendale, PA, 1994. Deen, H. E. and Ryer, J., "Automatic Transmission Fluids— Properties and Performance," Fuels and Lubricants Technology: An Overview, SP. 603, Society of Automotive Engineers, Warrendale, PA, October 1984, pp. 117-127. Artman, D. M. and Copes, R. G., "ATF From Performance Challenges to Market Opportunities," Presented at the 1994 NPRA National Fuels and Lubricants Meeting, 3-4 Nov. 1994. Graham, R. and Oviatt, W. R., "Automatic Transmission Fluids—Developments Toward Rationalization," Presented at CEC 1985 International Symposium, Wolfsburg, Germany, 7 J u n e 1985.

[71] "Lubricant Service Designations for Automotive Manual Transmissions a n d Axles," API Publication No. 1560, American Petroleum Institute, Washington DC, 1981. [72] Sutherland, J. M., "Proposed Automotive Gear Lubricant Categories: Their Impact on the Industry," Presented at NPRA National Fuels and Lubricants Meeting, Houston, TX, 2-3 Nov. 1989. [73] "Progress is Slow on PG-1 and PG-2," Lubrizol NewsLine, Vol. 11, No. 1, January 1993. [74] (a) "Ready Reference for Lubricant and Fuel Performance," Publication 1288 240-94R1, The Lubrizol Corporation, Wickliffe, OH, 1999/2002. (b) Reference Library, Lubrizol web site, www.lubrizol.com. [75] "Tractor Wet Brake and Wet Clutch Friction Properties," W. K. S. Cleveland, NLGI Sopkesman, July 1987, p p . 135-138. [76] Laemmle, J. T., "Metalworking Lubricants," American Society of Metals Handbook, Friction, Lubrication, and Wear Technology, S. D. Henry, Ed., ASM International 1992, Vol. 18, pp. 139-149. [77] "NLGI Lubricating Grease Guide," National Grease Institute, Kansas City, MO, 1987. [78] SAE J310; "Automotive Lubrication Greases," SAE 1995 Handbook, Society of Automotive Engineers, Warrendale, PA, 1995.

MNL37-EB/Jun. 2003

Synthetic Lubricants— Non Aqueous Thomas F. Buenemann, and Ian Thompson^

^ Steve Boyde, ^ Steve Randies, ^

POLYOL AND DIESTERS

Chemistry and Manufacturing General Features

ALTHOUGH THE DEVELOPMENT OF SYNTHETIC ESTER LUBRICANTS is

In addition to their good properties at extreme temperatures, synthetic esters have other desirable characteristics including good lubricity, high viscosity index, low volatility, and compatibility with s t a n d a r d lubricant additives a n d basefluids. The fundamental chemistry is flexible and a wide range of raw materials is available, which means that ester base fluids can be designed having a wide raxige of viscosities. Other key properties such as biodegradability can also be controlled by molecular design [3]. Consequently, synthetic esters have found use in many applications outside aviation. Examples include automotive crankcase a n d gear oils, 2stroke lubricants, industrial gear oils, hydraulic fluids, textile yarn lubricants, metal cutting and rolling fluids, air compressor lubricants, and refrigeration compressor lubricants [4].

Raw

' Senior Application Manager and ^ Technology Manager, Uniqema Lubricants, P.O. Box 2, 2800 AA Gouda, Buurtje 1, Gouda, BE 2802, Netherlands.

Materials

The raw materials used in the manufacture of synthetic esters for lubricant applications are derived from a variety of sources, both natural and synthetic. Aromatic acids or anhydrides are manufactured by the oxidation of the corresponding hydrocarbons [6,7]. Alpha, omega diacids are also generally produced by oxidation. The diacids used most widely to synthesize ester lubricant fluids are adipic, azelaic, sebacic, and dodecanedioc acids, as well as C36 dimer acid. Adipic and dodecanedioc acids are derived from petrochemical feedstocks.

249 2003 by A S I M International

Groups

Esters are defined as the class of chemical compounds containing the ester functional group. They are normally manufactured by reaction of a carboxylic acid with an alcohol (Fig. 1), optionally in the presence of an esterification catalyst, and with elimination of water [5]. The properties of the product esters can be controlled by appropriate selection of the raw materials used. The final product properties are mainly dependent on the molecular weight, the n u m b e r of ester groups per molecule, and the degree of branching in alkyl substituent groups. A mixture of raw materials may be used to deliver the precise combination of properties required. There are three main classes of synthetic esters currently in use as lubricant base fluids: aromatic esters, aliphatic diesters, and polyol esters. Aromatic esters, shown in Fig. 2, are manufactured by the reaction of an aromatic di or poly acid or anhydride, such as phthalic anhydride, tiimellitic anhydride, or pyromeUitic anhydride, with a monoalcohol or mixture of monoalcohols. Diesters, shown in Fig. 3, are manufactured by the reaction of an aliphatic alpha, omega diacid with a monoalcohol or mixture of monoalcohols. Polyol esters, shown in Fig. 4, are manufactured by reaction of a diol or polyol having the neopentyl structure, such as neopentyl glycol, trimethylol propane or pentaerythritol, with a monoacid or mixture of monoacids. Oligomeric esters, generally known as complex esters, can be manufactured by reaction of a diol or polyol with a di or polyacid/anhydride, with a monoacid or monoalcohol to act as capping reagent. This allows preparation of materials having higher average molecular weights and consequently higher viscosities, than can be achieved with simple diesters or polyol esters. However, complex esters also have a distribution of molecular weights, which means that their volatility characteristics are not as good as simple esters of the same number average molecular weight.

relatively recent, the use of esters as lubricating fluids is as old as h u m a n technology. Before suitable mineral oils became widely available as a b3^roduct of the petroleum based fuels industry, lubricants were based on natural fats or oils, which are either triesters of glycerine with natural fatty acids, e.g., tallow and olive oil, or long chain monoesters of fatty acids with fatty alcohols, e.g., sperm whale oil. The use of synthetic esters as high performance lubricating fluids was originally driven by the development of the gas turbine or jet engine in aviation. Aviation turbines have a higher operating temperature than the piston engines that preceded them, and jet aircraft are capable of operation at m u c h higher altitudes, where the ambient temperature is very low. Consequently, lubricants for aviation turbines are required to have both very good high temperature stability and good low temperature flow properties. It was found that mineral oils and synthetic hydrocarbons did not deliver the required combination of properties, and diesters were adopted as the lubricant base fluid of choice for early aviation turbines [1]. As gas turbine technology developed, the operating temperatures increased further, and diesters have been largely substituted in aviation applications by polyol esters, which have even better thermal stability [2]. Despite intensive research into alternative chemistries, polyol esters remain the base fluid of choice for aviation turbine lubricants.

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whereas azelaic, sebacic, and C36 dimer acids are derived from natural fatty acids [8]. The monoalcohols used in diesters are mainly manufactured by carbonylation of olefin feedstocks. These alcohols are also used in the manufacture of plasticizers and ethoxylate surfactants. They are generally commercially available as a mixture of isomers and/or carbon chain numbers. Exceptions include 2-ethylhexanol, a derivative of butanol, which is a single isomer, and the linear C-even alcohols (eg Cg, Cio alcohol), which are manufactured by chemical reduction of naturally occurring fatty acids [9], The mono acids (fatty acids) used in polyol esters may be derived either from petrochemical or natural sources. Naturally occurring fatty acids are almost always linear and have an even number of carbon atoms. Lower carbon numbers

Polyol esters

Q

R'"

R"-

I

CK

O ^

R = C4-C17linearor branched aikyi groups

^R

R" PE; Pentaerythritol; R' = R" = R'" = CH20COR TMP; Trimethylol propane; R"» R" = CH20C0R, R" = Et NPG; Neopentyl glycol; R' = CH20C0R, R" = R" = Me FIG. 4—Examples of polyolesters.

Acid

^

£st(^r:'

OH Fsterififaqtion ^ ^ ' t ^ R' n

nWater

FIG. 1—Chemistry of esteriflcatlon.

Aromatic esters o

0

I

I ^ I

II Phthalate O

0

Trimellitate

II °

R = C8 - CI8 linear or branched alkyl groups FIG. 2—Examples of aromatic esters.

Diesters

R'

>^' % C — ( ^ " 2 > n - \ . /

0 ~ R '

n = 4 - adipates n = 7 - azelates n - 8 - sebacates n = 10 - dodecanedioates R' - C8 - C13 linear or branched alkyl groups FIG. 3—Examples of diesters.

(Cs—Cift) are normally fully saturated, while higher carbon numbers (Cig—C22) have olefinic unsaturation. Petrochemical fatty acids may be either linear or branched. Linear petrochemical acids are manufactured by the oxidative carbonylation of linear olefins, which are themselves derived from ethylene and are therefore C-even (an even number of C-atoms/molecules). Carbonylation adds a single carbon atom, so linear petrochemical acids are normally C-odd (an odd number of C-atoms/molecules) [8]. The neopentyl polyols used in polyol esters are manufactured by reaction of an aldehyde with formaldehyde, followed by chemical reduction [10]. As noted below, the neopentyl structure confers superior thermal and oxidative stability as compared to other polyols, such as butane diol or glycerol, and neopentyl polyol derived materials are preferred in lubricant applications for this reason. The only significant exception to this rule is for glycerol esters, which are also widely used in lubricating applications, though only those where thermal and oxidative stability is not a key performance requirement. Some of these glycerol esters are simply purified vegetable oils, which are not normally classified as synthetic esters. Others, known as mid-chain triglycerides (MCTs), are produced by chemical reaction of purified short chain natural fatty acids with glycerol. The oxidative stability of MCTs is inferior to that of neopentyl polyol esters, but they are used in applications where very high biodegradability is required, or where products are required to be manufactured only from renewable raw materials. Manufacturing Technology Esters are manufactured by reacting the desired acid and ailcohol with an esterification catalyst, if required, and reacting at an elevated temperature [11]. The esterification reaction is reversible, and consequently it is necessary to drive the equilibrium over to the desired product by removal of water. The reaction temperature and pressure are therefore selected so that water can be removed by distillation as it is formed. High temperatures also increase the rate of reaction, but very high temperatures may lead to undesirable side reactions or discoloration. Esterification reactions are, therefore, normally conducted at a temperature in the range 200-250°C. For diesters and aromatic esters, the reaction is generally conducted in the presence of an excess of the monoalcohol. ASTM D 974-97 Standard Test Method for Acid and Base

CHAPTER 10: SYNTHETIC Number by Colorimetric Titration (DIN 51 559 Part 1) is used to measure the acid value, and when the acid value of the reaction mixture has reached the target value, implying that all of the acid groups have reacted, the excess monoalcohol is removed by distillation. For polyol esters, the reaction is usually carried out in the presence of an excess of the monoacid. In this case, the progress of the reaction is controlled by monitoring the hydroxy] value according to ASTM E 326 Standard Test Method for Hydroxyl Groups by Phthalate Esterification. When the target value is achieved, the excess monoacid is removed by distillation. Esterification catalysts are normally used to accelerate the rate of the esterification reaction. Conventionally used homogeneous catalysts include complexes of transition metals, especially titanium and tin, and strong acids, e.g., toluenesulfonic acid. Heterogeneous catalysts such as acid ion exchange resins may also be used [11]. The fact that acids catalyze the esterification process can be exploited in the manufacture of polyol esters, where the mono acid reagent, present in excess, may itself serve as the catalyst. The crude ester produced by chemical reaction and distillation of excess raw materials normally requires further processing to render it suitable for use as a lubricant base fluid. It is particularly important to remove all traces of the esterification catalyst, since the esterification catalyst is also capable of acting as a hydrolysis catalyst, which would have a detrimental effect on the hydrolytic stability of the product. Where titanium catalysts are used it is normal to wash the crude ester with water, which hydrolyzes the catalyst to form insoluble titanium oxide, which can be removed by subsequent filtration. Further treatments may be applied to reduce acid value; reduce water content, which is determined by ASTM E 1064, Standard Test Method for Water in Organic Liquids by Coulometric Karl-Fischer Titration (DIN 51 777 Part 1); or to remove color, which is measured by ASTM D 1209-97, Standard Test Method for color of clear liquids (Platinum-Cobalt scale) formed during reaction. Finally, the product is filtered to remove particulate impurities. Physical Properties Viscosity Probably the most important characteristic of a lubricant base fluid is the viscosity measured by ASTM D 445-97, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (DIN 51 550). The ester group does not significantly increase viscosity as compared to a hydrocarbon chain. Consequently, esters typically have viscosities similar to those of hydrocarbons of comparable molecular weight and degree of branching [12]. The viscosity of ester fluids can be controlled over a wide range by appropriate choice of molecular structure, and current commercially available materials cover the range of viscosities at 40°C from 5 cSt for simple diesters up to >1000 cSt for complex esters [13,14]. As for all fluids, the temperature dependence of viscosity (Viscosity Index, VI) is calculated by ASTM D 2270-93, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C (DIN/ISO 2909) which is a function of the extent and type of branching in the molecule. VI can be controlled by molecular design as for viscosity [12],

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Low Temperature Characteristics The low temperature properties of esters, as for all lubricating fluids, depend on the VI, and on the tendency to form waxy solids [15], As noted above, the VI can be controlled by appropriate molecular design. Linear substituents are desirable to give a high VI and therefore control the increase in viscosity with increasing temperature. Wax formation tendency is dependent on the presence of saturated linear hydrocarbon chains above a critical chain length. Linear saturated hydrocarbon substituents containing approximately eight or more carbon atoms will generally lead to tendency to wax formation in the temperature region of interest, i.e., — 50°C to 0°C. Ester base fluids intended for low temperature applications generally have a balance of linear and branched substituents to achieve both performance criteria which is measured by ASTM D 97-96a, Standard Test Method for Pour Point of Petroleum Products (DIN 51 597). Volatility The ester group carries a permanent electric dipole. This means that an intermolecular force due to dipolar interaction exists between the ester molecules, in addition to the normal molecular van der Waals forces which are present between hydrocarbon groups. Consequently, intermolecular forces are stronger in ester fluids, and esters have significantly lower vapour pressure, and therefore higher flash points than hydrocarbons of similar molecular weight and viscosity [4] determined by ASTM D 92-97, Standard Test Method for Fire and Flash Point by Cleveland Open Cup (DIN 51 376). Chemical Characteristics Thermal Stability Sjmthetic esters all show very good thermal stability as compared to mineral oil, but diesters are significantly less stable than polyol esters. This is because diesters can decompose via a /3-elimination reaction, which forms an acid and an olefin. This pathway is not possible for polyol esters due to the neopentyl structure, which has no hydrogen atoms, attached to the /3 carbon of the alcohol residue [4]. Hydrolytic Stability As noted above, the esterification reaction is reversible, and esters can, in principle, react with water to regenerate the acid and alcohol raw materials. This reaction is known as hydrolysis. The rate of hydrolysis under normal service or storage conditions is very low, and hydrol3?tic stability is rarely an issue in practice. However, it can be a cause of concern in some applications as the acids produced by hydrolysis could potentially act as corrosive agents. Test methods have been developed to characterize the rate of hydrolysis, i.e., ASTM D 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids. In this test, 75 g of fluid and 25 g of water eire sealed in a beverage bottle with a copper strip present for 48 h at 93°C (200°F). At the end of the test, the oil and water layers are separated and insolubles are weighted. Viscosity and acid numbers are also determined. However, these test methods must be used with caution as the rate of hydrolysis depends on factors such as the structure and purity of the ester, reaction conditions, and the nature of additives present. An understanding of these rela-

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tionships can be used to select appropriate ester lubricant formulations for a given application [16]. Biodegradability Esters generally exhibit higher aerobic biodegradability than corresponding hydrocarbons, because microorganisms produce lipase enzymes which catalyze ester hydrolysis. This converts the ester into alcohol and acid, which have higher water solubility and can be further degraded by enzyme catalyzed oxidation reactions. The degree of initial biodegradability of ester fluids is highly correlated with the hydrolytic stability. Ultimate biodegradability is also dependent on the linearity of the hydrocarbon subsituents. Esters derived from reactants containing fully linear alkyl substituents (and in particular those containing olefinic unsaturation) such as polyol esters derived from natural fatty acids, e.g., trimethylolpropane oleate (TMPO), generally biodegrade particularly rapidly. In contrast, a r o m a t i c esters, particularly those containing highly branched alkyl groups, biodegrade more slowly. However, even some branched aromatic esters can meet the criteria for ready biodegradabilitiy according to OECD 301B [17]. OECD 301B is currently the most widely used test method for ready biodegradability of waterinsoluble organic substances [18]. However, another closely related test m e t h o d is ASTM D 5864-95, S t a n d a r d Test Method for Biodegradability of Organic Substances, which is less stringent in that the latter allows for preacclimation of the microbial inoculum, and therefore represents a less severe test regime.

Application and Performance Characteristics Although slightly more polar, most esters are fully miscible with mineral oils or synthetic hydrocarbons. Due to their higher polarity, they are generally better solvents for polar materials, such as many standard lubricant additives used in mineral oil based formulations. Consequently, esters are frequently used as a component of mixed base fluids containing polyalphaoleflns, which are poor solvents for polar additives. However, the relatively higher solvency of esters may be a disadvantage in formulations containing polar additives, such as antiwear and EP agents which are required to adsorb on the metal surface. The additive treat rate in an ester-based fluid may need to be higher than in a mineral oil formulation in order to ensure an effective surface concentration of the additive [19]. The miscibility of esters with more polar materials also allows their use as mixtures with other polar synthetic basefluids, e.g., poly alkylene glycols, which are not generally miscible with mineral oil. The inherent oxidative stability of esters is similar to that of synthetic hydrocarbons. They therefore require the use of antioxidant additives to limit the rate of oxidation in service. However, esters generally show very good response to standard antioxidants and can easily be formulated to give good oxidative stability [4]. Four stroke cycle crankcase formulations require good thermal and oxidative stability, low volatility, and good low temperature pumpability. These requirements can be met by the use of mixed poly (alphaolefin)/ester base fluids. Diesters

and polyol esters are typically used. The more polar ester serves to enhance solubility of less polar additives in PAO, and confer seal swelling properties, as well as superior thermal stability. PhthaJate esters have been used where a low cost seal swellant is required. Diesters based on C36 dimer acid and polyol esters are frequently used in two stroke cycle formulations to provide low smoke properties and biodegradability for spilled or uncombusted oil, particularly in marine applications. Hydraulic fluids are formulated using synthetic polyol esters or diesters where a combination of good oxidative stability and biodegradability is required. Organic esters also have better flame retardancy than mineral oils, although not as good as phosphate esters. As noted above, polyol esters remain the lubricant base fluid of choice for aviation turbine lubricants, even in military applications. Polyol ester- derived aviation turbine lubricants are generally formulated containing aminic antioxidants and ashless phosphate ester antiwear agents. There is continuing interest in increasing the upper temperature limit for aviation turbine lubricants, but the thrust of current research has moved away from investigation of alternative base fluids towards optimization of ester chemistry. The relatively recent development of hydrofluorocarbon (HFC) compatible refrigeration compressor lubricants has led to increased demand for polyol esters [20]. Concern over the environmental impact of the ozone depleting chlorofluorocarbons (CFCs) previously used as refrigerant fluids has led to international legislation prohibiting their use. This has resulted in a change to alternative refrigerants including the zero ozone depletion potential HFCs. The HFCs are not miscible with the hydrocarbon lubricants that were generally used in CFC systems. Lubricant immiscibility was found to cause poor oil return leading to lubricant starvation in the compressor and fouling of low temperature heat exchange surfaces. Consequently, it has been necessary to develop new HFC-miscible synthetic lubricants. Polyol esters have been generally adopted as the preferred basefluid for most HFC compatible refrigeration applications.

POLYALKYLENE GLYCOLS Polyalkylene glycol (PAG) is a generic name used to describe a family of products formed from the polymerization of one or more alkylene oxides. Such products are also known as polyethers, polyoxyalkylene glycols, polyalkylene glycol ethers, polyglycols, and PAGs [21,22]. PAGs are an extremely versatile family of products, which can exhibit a wide range of physical and chemical properties. They are excellent lubricants in their own right [22-25], which makes them the fluid of choice for a large n u m b e r of engineering and lubricant applications. The history of PAGs is a long one. The earliest reported polymerization of ethylene oxide dates back to 1863 [26], with the first commercial products (polyethylene glycols) available in 1939 [27]. The range of viscosities now covered by PAGs is vast, making these products suitable as lubricants in their own right, but also as thickeners and lubricity improvers in water based systems.

CHAPTER 10: SYNTHETIC LUBRICANTS~NON AQUEOUS 253

4

4.5

5

log (molecular weight)

FIG. 5—^Typical molecular weight distribution for a polyalkylene glycol. Chemistry and Manufacturing PAGs are linear or branched chain polymers, which contain ether linkages in their main polymer structure. They are produced by the polymerization of one or more alkylene oxides, such as ethylene oxide (EO, C2H4O) cind propylene oxide (PO, CsHgO), though butylene oxide (BO, C4H10O) and higher oxides can also be used. They can be considered to consist of three parts: the initiator, the alkylene oxide or polyether section, and the terminal or end group as shown below. initiator

polyether section (CH2-CH(R)-0)„

end group -

R2

(R is H or alkyl; R' and R^ are H, alkyl, acyl, etc; X is O, N, S, etc; n is an integer 1, 2, 3, etc.) The three distinct parts of the polymer are variable, and it is therefore possible to produce an infinite n u m b e r of different products, each with its own unique properties. The skill in making PAGs is to select the appropriate combination of initiator, polyether section and terminal group to give the desired properties. Since the PAG is in fact a "statistical polymer," the product will contain a series of polymers w i t h a distribution of polyether chain lengths, centered around "n," and distribution of molecular weights, as shown in Fig. 5. The molecular weight distribution is measured by the modified ASTM D 5296-97, Standcird Test Method Gel Permeation Chromatography (GPC). As "n" and the length of the polymer increases, so does the viscosity, thus giving access to a wide range of viscosity. The R group is H for EO, CH3 for PO, etc. The initiator must be an "active hydrogen" containing compound R ' X — H ; thus X must be an atom of the type O, N, S, etc. Alcohols are the most extensively used initiators, and include monohydric alcohols (i.e., containing 1 OH group), such as butanol, or polyols (i.e., containing two or more OH groups), such as ethylene glycol, glycerol—water can be considered as a dihydric alcohol. Increasing the functionality of the initiator is particularly useful when trying to build high molecular weight, as there are more reactive OH ends for addition, and thus the reaction rate is increased. The higher OH concentration also increases the polarity of the polymer, which is useful for particular applications; the OH value is typically measured using ASTM E326, Standard Test Method for Hydroxyl Groups by Phthalate Esterification. Lower polarity can be achieved by the use of long chain aliphatic or aromatic alcohols; this is an option for achieving mineral oil miscibility of PAGs [28].

The polyether section may contain one or more alkylene oxides. Single oxide derived PAGs ("homopolymers") are widely used and relatively simple to manufacture. When two or more alkylene oxides are used—"copolymers"—these will be incorporated as blocks ("block copolymers," e.g., EO—EO—PO—PO—EO—EO—EO, by sequential reaction of the oxide), or r a n d o m l y ("random copolymers," e.g., PO—EO—EO—PO—EO—EO—PO—EO, by the reaction of a mixture of oxides). These different polyether configurations lead to somewhat different properties. For example, blocks copolymers tend to have worse low temperature properties than r a n d o m copolymers. The principle benefits from including EO are increased water solubility, reduced solubility with non-polar species (e.g., hydrocarbons), and increased stability. The principle benefits from including PO are improved low temperature performance, reduced polarity, and thus increased solubility of lubricant additives a n d nonpolar species. In most PAGs, the end group is a hydrogen atom, and this is adequate for most applications. Reacting this active hydrogen further (endcapping), by etherification or esterification, can yield beneficial effects. Typical end groups of this form include aliphatic and aromatics (by etherification) and alipathic ester groups [29]. Nuclear magnetic resonance (NMR) spectroscopy is a key analytical technique for the chemical structural analysis of PAGs, including the determination of EO/PO ratio, initiator and end group analysis, polymerization sequencing (random/block), and molecular weight [30,31]. Synthesis Most PAGs are manufactured via a three step process involving: catalyzation of the initiator; reaction with the alkylene oxide(s) ('alkoxylation'); and post-treatment to remove the catalyst or adjust the p H (see Fig. 6), the reaction of a n alcohol (ROH) with EO. The initiator is typically reacted with a catalytic amount of a base, such as potassium hydroxide, to produce the alcoholate, RO — K + . The water produced may either be left in at this stage, or may be removed under vacuum. The oxide addition Catalyzation: R-O-H + KOH-

- * R-O-K+ + H2O

Reaction with oxide:

R-OK* + CH2-CH2"

•R-0-CH2-CH,-0-K+

AK R-O-CH2 - CH2 - O- K+ + n-1 (CHj - CH2 ) Post-treatment: R-(0-CH2 - CHj )„ - O - K+ + 'H+'

R-(0-CH2-CH2)„-0-H FIG. 6—Reaction scheme for PAG production.

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reaction is normally carried out in a stainless steel reactor at elevated temperatures (tjrpically 80-150°C) and at high pressures (up to 10 bar). The alcoholate is a strong nucleophile, and this approaches the carbon adjacent to the ring oxygen of the alkylene oxide. An O—C bond is formed, and the epoxide ring is opened; the ring oxygen then acts as the nucleophile for the next alkylene oxide addition. As the oxide is consumed in forming the polymer, more oxide is fed into the reactor, generally at a rate to maintain reactor pressure. A key side reaction that occurs during propoxylation is the formation of unsaturation by the isomerization of the PO unit. This reaction limits the viscosity attainable with pure propoxylates. In terms of reactivity, EO has a higher reaction rate than PO and higher oxides, and reactivity is enhanced by the use of more basic catalysts. Depending on the requirements of the final application, when the poljTiierization reaction is complete, the final product may be subjected to a catalyst removal step. This is achieved by demineralization, using a magnesium silicate, and neutralization with an acid, both of which are followed by filtration. Both techniques work by replacing the metal ion (in this example K"*") with a hydrogen ion (H"*"). Sometimes the catalyst is left in the product, and the product is treated with an acid to adjust its acid value, which is measured by ASTM D 974, Standard Test Method for Acid and Base Number by Colorimetric Titration (DIN 51 559 Part 1); here, no filtration is then used. Typical quality control measurements, carried out during production of PAGs, include determination of viscosity by ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (DIN 51 550), density by ASTM D 70, Test Method for Density of Semi-Fluid Bituminous Materials (Pycnometer method), flash point by ASTM D 97, Standard Test Method for Fire and Flash Point by Cleveland Open Cup (DIN 51 376), pour point by ASTM D 92, Standard Test Method for Pour Point of Petroleum Products (DIN 51 597), OH value by ASTM E326, Standard Test Method for Hydroxyl Groups by Phthalate Esterification, and water content by ASTM E 1064, Standard Test Method for Water in Organic Liquids by Coulometric Karl-Fischer Titration (DIN 51 777, Part 1).

PAGs show a broad viscosity range, from a few mm^/s at 40°C up to a few hundred thousand, as a result of the build in molecular weight from a few hundred to several tens of thousands. Over the wide range of PAG viscosity (for a consistent structural t3^e), the trends in the other physical properties tend to vary in a fairly linear fashion. Their key properties are water solubility, and excellent viscosity index (VT) determined by ASTM D 2270, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C (DIN/ISO 2909) together with very good pour point and flash point. The main route to adjusting the physical properties is by adjusting the initiator and the EO/PO ratio, as shown in Table 1. For example, by changing the initiator from a diol, to a mono hydric alcohol (compare products a and b in Table 1), there is an increase in viscosity index. Likewise, increasing the EO/PO ratio also results in an increase in VI. This is important in applications relying on little change in viscosity over a range of temperatures (e.g., worm gear lubrication). Excellent pour and flash points are maintained across a wide viscosity range, but are again influenced by EO/PO ratio. Increasing the molecular weight leads to increased VI and flash point, but also increased pour point. PAGs may be either water soluble or water insoluble depending on their structure. Increasing the EO content increases the water solubility. Linked with water solubility is the so called "cloud point," (or inverse solubility temperature), which is measured by ASTM D 2024, Standard Test Method for Cloud Point of Petroleum Products—PAGs show inverse solubility in water. Below the cloud point, normal solubility is observed, but at the cloud point, the PAGs come out of solution and eventually separate. The solubility and the cloud point increase as the EO content increases, and decrease with increasing molecular weight. PAGs tend to be soluble in relatively polar materials and their excellent solubility in the ozone benign hydrofluorocarbons, such as Rl 34a (1,1,1,2-tetrafluoroethane), has considerably aided the transition from the chlorofluorocarbons in automotive air conditioning systems [32]. Chemical Characteristics

Physical Properties The physical properties of a selection of commercially available PAGs are shown in Table 1, to illustrate the range of physical properties.

Compared to mineral oils, PAGs have good thermal stability, but poor oxidative stability as shown by thermal gravimetry (23,25,28,30). However, one of the key characteristics of PAGs is that when they decompose, the final decomposition

TABLE 1—Typical physical properties of PAGs (with ASTM methods).

VI

Mol Weight

Density (g/cm^) @20°C

Cloud Point CC)

Flash Point Open Cup ASTM D 92 (°C)

D2270 103 204 184 242 157 225 287 408 489

350 1900 2000 2600 1200 1650 4500 12500 25000

D70 0.9573 0.9940 1.0035 1.0031 1.0951 1.0564 1.0574 1.0908 1.0905

D2024 insol insol insol insol >100 59 53 81 76

D92-97 80 225 230 232 254 230 230 240 240

Viscosity (mm?/s)

Functionality of Initiator

EO/PO Ratio

40°C

100°C

ASTM Method Mono Mono (a) Di(b) Di Tri (c) Mono (d) Mono Di Tri

0:1 0:1 0:1 0:1 3:1 1:1 1:1 3:1 3:1

D445 11 126 142 387 127 132 1050 19500 45000

D445 3 22.5 22.2 65 18 25 180 2400 6500

NOTE: the molecular weight is calculated from Mol Weight = functionality of initiator * 56100/OH value.

Point Pour Point CO D97 -53 -36 -36 -23 -28 -42 -28 4 7

CHAPTER 10: SYNTHETIC products are all volatile, with the result that there is no carbonaceous solid or liquid residue, which can be proven by ASTM D 189-97, Standard Test Method for Conradson Carbon Residue of Petroleum Products. This is a major advantage in many functional fluid applications such as high temperature chain oils and compressor fluids. In the absence of air, PAGs are stable up to around 250°C. At this point polymer chain scission occurs, which releases free radical ends, which further decompose by depolymerization to produce volatile components (eddehydes, ketones, alcohols, alkenes, alkanes, CO2) and lower molecular weight polymers [30]. In the presence of air, PAGs are stable up to around 180°C. Decomposition is initiated by an oxygen molecule attaching itself randomly along the polymer chain. A radical is then formed, which rearranges, causing the polymer chain to break and lose its end group. This again leads to a reduction in molecular weight and production of similar volatile components to the thermal breakdown. By adjusting the chemical composition and by the use of antioxidants, the stability of the PAG can be improved significantly. Increasing the EO content (compare the flash points of product "c" and "d" in Table 1) and increasing the molecular weight of the polymer tend to increase its stability. Aminic and phenolic antioxidants, especially in combination, provide significant increase in stability to PAGs [33]. PAGs have good hydrolytic stability, which is measured by ASTM D 2619-88, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids, since they do not contain hydrolytically labile chemical groups. When released into the environment, PAGs tend to slowly biodegrade, helped by their affinity for water (and thus ability to disperse in the environment) and their ability to oxidize and fragment into smaller and more biodegradable structures [22]. Rapid biodegradability of PAGs, as measured by such tests as the OECD 301 and 302, is not inherent across the range, but, is achievable by careful structural modification, especially towards the lower viscosity end of the range [34]. In general, biodegradability is favored by linear components (i.e., EO rather than PO), and by reduction in molecular weight. In terms of general toxicity, PAGs are generally classified as low hazard, and some have approvals as indirect food contact lubricants [35]. Due to their highly polar nature, which gives them a very strong affinity for metal surfaces, PAGs have excellent inherent lubrication properties [23]. The lubricating film formed between moving metal parts remains intact even during very difficult operating conditions, such as high temperatures and loads. However, their polar nature does require that care must be taken in the choice of compatible materials, particularly paints and some elastomers [25], which is determined by ASTM D 471-98el, Standard Test Method for Rubber Property-Effect of Liquids. Formulations using standard additive chemistry can also be hampered by solubility problems and potential reduction in activity—the PAG may compete for the metal surface with the additive. Increasing the molecular weight and the PO content does improve this situation by reducing the polarity. Their hygroscopicity requires care to be teiken regarding ferrous metal corrosion, which is measured by ASTM D 665-99, Standard Test Method for Wear Preven-

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255

tive Characteristics of Inhibited Mineral Oil in the Presence of Water. Anti-corrosion agents are available. Application and Performance Characteristics Due to the flexibility in their chemical structure and properties, and their inherent lubricity, PAGs are used in a wide range of application areas, as illustrated by Fig. 7. Very high viscosity PAGs are used extensively in HFC (Hydraulic Fluid C type) water based fire resistant hydraulic fluids [36,37] where they perform the function of thickening the water, suppressing its pour point, and providing lubricity determined by ASTM D 2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and NonPetroleum Hydraulic Fluids in Constant Volume Vane Pumps. The water provides the fire protection [38]. The key parameter is fire resistance and the performance standards are developing apace [39,40]. A similar lubricity improving function is delivered by PAGs in water based metal working fluids, which additionally use the cloud point phenomenon of the PAG to boost lubricity: as lubricity of the fluid begins to fail, metal-metal contact causes the temperature to increase, which heats the solution above the cloud point of the PAG, which is determined by ASTM 2024-65, Standard Test Method for Cloud Point of Nonionic Surfactants. This temperature rise results in the PAG coming out of solution and being released into the contact to provide enhanced lubricity. The low coefficient of friction and excellent viscosity/temperature or VI characteristics of the PAG lend it to the lubrication of the gecirs and bearings, particularly to heavily loaded worm gears used within the plastics, rubber and paper industries [41-43]. Tests for gear oil performance include the FZG Test [DIN 51 354] and ASTM D 2782-94, Standard Test Method for Measurement of Extreme-Pressure, better known as the Timken Test. The efficiency of the worm gear is related to the friction between the wohn drive and the gear wheel. Contact between these involves a high level of sliding resulting in increases in operating temperatures—ideal operating conditions for the PAG. Combining this with their high level of tolerance for water ingress delivers excellent performance. The compression of process gases presents a unique technical challenge [44]. Due to the intimate contact between lubricant and gas, there can be dissolution of the gas into the lubricant, resulting in a reduction in viscosity of the lubricant, and as a worst case, washing of the lubricant from the metal surfaces requiring lubrication. Owing to the polar nature of PAGs, they perform extremely well in the lubrication of compressors for low polarity process gases (e.g., methane, ethylene, nitrogen). Since there is very little dissolution of the gas into the lubricant, the viscosity and lubricity is maintained [44]. PAGs are also suitable for use as air compressor lubricants, especially when combined with esters [46,47]. Since any build up of sludges or deposits in the compressor can lead to ignition of the lubricant (for the proof of low carbon residues ASTM D 189-97 Standard Test Method for Conradson Carbon Residue of Petroleum Products can be used), the clean decomposition characteristics of the PAG (ASTM D 189) make this a very attractive alternative to mineral oil based air compressor lubricants.

256 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

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CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 257 POLY (ALPHA-OLEFINS)

/-I

Polyalphaolefiin fluids, or PAOs, are synthetic, saturated hydrocarbons that are manufactured by a two-step process from hnear alpha-olefins, that are themselves manufactured form ethylene. These synthetic hydrocarbons are generically described by their viscosity at 100°C. The most common commercially used PAOs are 2, 4, 6, 8, and 10 cSt and the higher viscosity grades 40 and 100 cSt. PAOs have excellent physical properties when compared with conventional mineral oils, a wide operating temperature range, including high flash and firepoints, high viscosity indices and low volatilities. When compared with certain natural and synthetic esters, PAO fluids have excellent thermal, oxidative, and hydrolytic stability. Since the mid 1980s PAOs have quickly gained market share in the synthetic lubricant base fluid market, particularly in Europe [48]. The major application area was and continues to be the automotive sector for crankcase oils answering the quest for tighter specifications for lower oil volatility. Today, automotive crankcase lubrication is still the main application area for PAOs globally. Other automotive application sectors are now two-stroke cycle engine oils, automatic transmission oils, gear oils (multigrade), and greases. Industrial applications include: hydraulic oils, compressor oils, heat transfer fluids, and food grade oils and greases. The milestones of the technical history are marked by three patents, one by Brennan [49] at Mobil Oil in 1968 and two by Shubkin [50,51] of Ethyl Corporation in 1973. Brennan first described a process for oligomerization of alpha-olefins using a BF3—ROH catalyst system [49] whereby the combination of the selected process conditions and the catalyst system yielded a product consisting of a mixture of oligomers with a high concentration of trimers. Shubkin showed that instead of R—OH as a co-catalyst, H2O [50] or alcohols and carboxylic acids [51] could be also used in combination with BF3 to produce oligomers of uniform quality. The U.S. Army and U.S. Navy took an early interest in this new synthetic base fluid. In July 1970 the MIL-H-83282 specification for fire resistant hydraulic fluids based on PAOs was established in cooperation with the industry [52]. Chemistry and Meinufacturing As the name implies, Polyalphaolefines are synthesized up from alphaolefines. The starting material of the chemical

CH2 = CH2

Catalyst >•

BF3

Unsaturated

R-CH=CH2

>•

ROH

Unsaturated Oligomers + H2

aigomeric Mixture

Dimer Trimer Tetramer Pentamer Higher

NIorPd

DIstilatlon

Oligomers

DImer Trimer Tetramer Pentamer Higher

Saturated Paraffinic Hydrocarbons

Viscosity Grades

FIG. 8—Chemistry of PAO manufacture [55].

Qimsr

FIG. 9—Typical PAO components [55].

synthesis is ethylene as shown in Fig. 8. The reaction products of the first step are linear unsaturated alphaolefins (LAO). These LAOs are used for the manufacture of a variety of chemicals, mainly as detergents. For the synthetic lubricant base fluids (PAOs 2-10 cSt), mainly 1-Decene is oligomerized with the catalyst BF3 and a protic co-catalyst such as water or an alcohol. For the higher viscosity PAO 40 and PAO 100 Ziegler-Natta (Aluminium chloride based), catalysts are used. The unsaturated oligomer mixture is hydrogenated using either Nickel or Palladium catalysts. Finally, a distillation step is applied to remove unreacted monomers and to separate the various product grades. Sometimes the distillation step is carried out prior to hydrogenation [54]. The tjrpical molecular structures of a dimer, trimer and a tetramer of 1-decene are shown in Fig. 9. Every oligomer is branched and is present as a number of different isomers. Though 1 -decene is the pre-dominantly used alphaolefine for PAO manufacturing, shorter or longer chain length olefines may also be used. In particular, 1-dodecene has recently gained importance for the synthesis of PAO 5 and PAO 9 [57]. Tailor made products can also be obtained by changing reaction vsiriables such as temperature, time, catalyst concentration, co-catalyst t3^e, and concentration and distillation conditions [57]. Physical Properties The physical properties of the common commercially available PAOs are shown in Table 2. The given properties are typical values and do not represent values for PAOs from a particular supplier. The low viscosity PAOs 2-10 have excellent low temperature properties which make them very suitable for applications in cold climate. The average viscosity index (VI) calculated by ASTM D 2270, Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and lOOX (DIN/ISO 2909) for PAOs is 135. This has the advantage that the viscosity changes much less with increasing temperature compared to a product with low VI. For PAO 2 no VI is given because VI is undefined for fluids having a kinematic viscosity ofless than 2.0 cSt at 100°C. The advantage of a high VI is that addition of viscosity index improvers is not required for the formulation of lubricants for many applications. Also, the addition of pour point depressants is often not necessary as the pour points for PAOs are very low. In Table 2 some other physical properties are given which are important to the lubricant formulator. The flash point is important for safety reasons and the given flash points in Table 2 are at least as high

258

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

TABLE 2—Properties of polyalphaolefines. Parameter

Test Method"

PA0 2

PA0 4

PA0 6

PAO 10

PAO 40

PAO 100

KV @ 100 °C cSt KV @ 40 °C cSt KV @ - 4 0 °C cSt Viscosity Index Pour Point, °C Flash Point, °C Noack, % Loss

ASTM D445 ASTM D445 ASTM D445 ASTM D2270 ASTM D79 ASTM D92 DIN 51581

1.80 5.5 310

3.90 16.8 2540 122 -69 215 12.0

5.90 31.0 7800 137 -63 225 6.7

9.60 45.8 19000 134 -54 264 2.0

40.0 395

100 1250

150 -34 280 0.8

170 -20 290 0.6

-63 >155 99

"ASTM D 6375 and D 5800 are alternate procedures.

TABLE 3—The oxidative stability of motor oil qualities studied using High Pressure Differential Scanning Calorimetry (PDSC). Additive-free Base Oils

Mineral oil based lube Synthetic lubricants Polyalphaolefin Polyolester Diester 1 Blends of synthetic lubricants Polyalphaolefin/polyolester 80/20 blend Polyalphaolefin/diester 80/20 blend Base oils with additive Mineral oil based lube Mineral oil/polyalphaolefin based lube 55% mineral oil/22% PAO Mineral oil based diesel engine lube Polyalphaolefin/esteroil 80/20 blend

T (onset) [°C]

187 187 210 198 196 196 254 260 262 274

as those of mineral oil of the same viscosity. Volatility is measured by the standard NOACK volatility test at 250°C for 1 h. Except for PAO 2, all other PAOs have very low volatility, which makes them very suitable for high temperature applications and engine oils to reduce the need for "topping-up." Low volatility is also an important property for a fluid to retain the original viscosity during its working life. The highly viscous PAO 40 and PAO 100 hsted in Table 2 are similar to low viscosity PAOs and have good viscometric properties and allow operation over a broad temperature range. New PAOs with m e d i u m viscosity grades 5, 7, and 9 and very high viscosity grades, u p to 3000 cSt, and have recently been developed as custom-synthesized products [56]. Chemical Properties and Performance Characteristics Oxidation stability is one of the most important properties of automotive lubricants, which are mainly responsible for the oil renewal time. Essiger [57] investigated the oxidation stability of different motor oil qualities with High Pressure Differential Scanning Calorimetry (PDSC) as shown in Table 3. PAO without anti-oxidants is as stable as mineral base oil, blends with synthetic ester, a n d shows superior oxidation stability. In the presence of anti-oxidants, such blends were significantly more stable than mineral oil based engine lubes. The use of PAO is also c o m m o n today in high performance greases. The lifetime of such greases is said to be three to five fold c o m p a r e d to mineral oil based products. Wunsch recommends operating temperatures of u p to 150-160°C for Lithium-12-hydroxystearate greases based on PAO [57].

Low toxicity in general, and biodegradability of the low viscous PAOs according to the CEC L-33-A-93 test, are other important benefits making these products versatile for incidental food contact and environmentally acceptable lubricants

OTHER SYNTHETIC BASE STOCKS Although mineral oils and the synthetic base fluids described in the sections above, together with solid lubricants, satisfy the performance requirements of the majority of lubricating applications, there r e m a i n m a n y applications where a different combination of properties would be desirable. A wide variety of other chemical classes have been proposed as candidate synthetic base fluids to cover the real or perceived shortcomings of more established fluids. Many of these developments have been targeted towards high performance aerospace applications, e.g., military aircraft and satellites, where satisfactory performance under extreme conditions of t e m p e r a t u r e , high vaccuum, a n d high radiation m a y be required, and unit cost is less significant [3]. Most of the proposed chemistries, e.g., silicate esters, silahydrocarbons, titanate esters and phosphazines, appear not to have found any real application. However, others, in particular silicones, perfluoroalkyl ethers, polyphenyl ethers and cycloaliphatic hydrocarbons have found significant niche applications and are discussed briefly in this section.

Silicones Silicones are polymers containing the siloxane (Si—O—Si) backbone structure with pendant alkyl side chains, normally methyl groups. The chemistry is well established and silicone fluids have been commercially available since the 1940s [59,60]. They are manufactured by hydrolysis of dialkyldichlorosilane themselves p r e p a r e d by reaction of methyl chloride (or other alkyl chlorides) with silicon metal. Trialkyl monochlorosilanes are introduced into the reaction in controlled stoichiometry to act as end capping reagents and control the molecular weight. Silicone fluids can be prepared with viscosities ranging from < 1 to > 500 000 cSt. They are characterized by low pour points, low surface tension, high compressibility and little change in viscosity with temperature. The standard viscosity index calculation is not appropriate for silicones and other materials with such low temperature coefficients of viscosity. Methyl groups m a y be substituted by other functional groups to modify the inherent properties of the basefluid.

CHAPTER 10: SYNTHETIC These substituents include phenyl groups (for improved oxidative stability) and trifluoropropyl or tetrachlorophenyl groups (for lubricity) Silicones are highly fire resistant and have very low volatility, good thermal stability, and very good chemical resistance. This makes them well suited to some highly demanding functional fluid applications, e.g., heat transfer oils and transformer dielectric fluids. However, the load bearing characteristics of poly (dimethyl siloxane) Eire very poor and metal contacts lubricated by silicones tend to seize under boundary lubrication conditions [59]. For this reason, simple silicones are rarely used as base fluids, except in some speciality greases where they serve as stable carriers for additives and solid lubricants. Such greases are used in aviation and automotive industries to lubricate linkages, bearings and bushings, and instrument components [3]. Silicones are also used in textile applications as fibre, thread and yam lubricants, and modified silicones are used as textile treatments for improved resistance to staining. Traditional lubrication applications of silicones are further limited by the fact that they are not generally miscible with mineral oils, or other synthetic base fluids. This does, however, make them suitable for use as both profoaming and antifoaming additives in more conventional base fluids. Use of silicone fluids or additives in some manufacturing environments where goods are painted or coated is discouraged because minute traces of silicones can interfere with the spreading and adhesion of paints. Silicone fluids are essentially non biodegradable and although of low toxicity, they are likely to be persistent in the environment. Consequently, the environmental impact of dispersive applications is a cause of some concern. Perfluoroalkyl Ethers (PFAEs) PFAEs have structures basically similar to those of PAGs, but with all the hydrogen atoms replaced by fluorine. They were originally developed in the 1960s for aerospace applications and have subsequently found some use in other applications, particularly where resistance to oxidation or other chemically aggressive environments is required. Four different classes of PFAEs have been commercialized, for which the abbreviations D, K, Y, and Z have become generally accepted. These differ somewhat in their chemical structure, and consequently show slight differences in performance, but all show broadly the same characteristics as compared to other classes of base fluids. PFAE-K and PFAE-Y have quite similar structures, consisting of a —(CFa—CF(CF3)—O—) repeat unit, although with different manufacturing routes, which lead to some differences in properties. PFAE-K is prepared by anionic polymerization of hexafluoropropylene oxide, whereas PFAE-Y is prepared by polymerization of hexafluoropropene in the presence of oxygen. PFAE-Z contains a —(CF2—CF2)—O—) repeat unit and is prepared similarly to PFAE-Y, using tetrafluoroethene, rather than hexafluoropropene. PFPE - D contains a —(CF2—CF2—CF2—O—) repeat unit, and is manufactured by ring-opening polymerization of tetrafluorooxetane, followed by exhaustive fluorination to convert the remaining C—H bonds to C—F bonds.

LUBRICANTS—NON

AQUEOUS

259

All types are manufactured as poly disperse polymers, which are then fractionated to give the molecular weight and viscosity ranges of interest. PFAEs are available with viscosities ranging from about 5 to about 500 cSt 40°C. Viscosity indices vary according to the chemical type, with the K and Y types having lower VT than the D and Z types. Pour points are generally low. High temperature volatilities depend on the molecular weight spread of the particular grade, but are generally low [60]. The main chemical characteristic of PFAEs is outstanding resistance to oxidation by air. They are also resistant to chemical attack by corrosive chemicals including strong acids and alkalis and oxidants such as fluorine and hydrogen peroxide. As oxidative degradation is one of the most frequent limiting factors for lubricant life, this means that PFAEs can have very long lifetimes in service, and are well suited for sealed for life applications. Like silicones, PFAEs have a tendency to exhibit seizure under boundary conditions, and, also like silicones, they are not miscible with standard lubricant additives, so the problem cannot be alleviated by formulation with conventional antiwear agents [3]. Thermal stability of PFAEs under ideal conditions in contact with glass is very good, but the benefit is often not realized in real life applications as thermal decomposition is catalyzed at lower temperatures by metcil fluorides, particularly those of aluminium and ferrous metals. Metal fluorides are apparently produced by reaction of PFAEs with metal oxides under conditions of tribological contact. The same effect can lead to oxidative corrosion of some metals by PFAEs under air. Both thermal decomposition and oxidative corrosion are inhibited in the absence of oxygen [59]. The very high cost of PFAEs has restricted their use to high value applications where their properties are essential or where greatly extended oil life can justify their use on economic grounds. The initial applications were in spacecraft, where the very high chemical resistance of PFAEs was required. The largest current application is in specialty vacuum pump oils for use in contact with reactive chemicals in electronics manufacture. PFAEs are cJso used in formulation of greases for some sealed for life bearings.

Polyphenyl Ethers Polypheny] ethers (PPEs) consist of benzene rings joined by ether links, with the ether links in bridging monomer units being arranged in meta geometry. They were developed during the 1950s for high thermal, oxidative, and radiation stability [61]. PPEs are manufactured by reaction of phenols and halides. The simplest member of the family is diphenyl oxide; longer chain analogues are available up to six benzene rings and are coded according to the number of benzene rings and ether groups they contain; thus 5P4E contains five benzene rings and four ether linkages. PPEs have very high pour points and most are not liquids at room temperature. For example, the only example which is currently commercially available, 5P4E, crystallizes at 43°C in the pure state, and therefore must be mixed with other fluids to extend the liquid range and inhibit crystallization, or used only in applications where it will re-

260

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

main at elevated temperature [62]. In addition, PPEs have very low viscosity indices, so it is necessary to accept high viscosity at lower temperatures in order to have adequate viscosity at the elevated temperatures of operation. [3] PPEs are also characterized by high surface tension (ca. 50 dyn/cm, as compared to 30 dyn/cm for typical mineral oil). This means that they do not wet metal surfaces, or migrate from the point of application. Thermal stability of PPEs is good, but oxidative stability is not outstanding. PPEs have been used as aviation gas turbine lubricants for supersonic military aircraft [61]. The main current application for PPEs is as ultra-high vacuum diffusion pump fluids, where lubricity is not important and the good high temperature characteristics can be exploited. They are also used in formulation of radiation-resistant greases. Alkylated Cyclopentanes Multiply-alkylated cyclopentanes (MACs) were introduced as candidate lubricating fluids during the 1980s. MACs are manufactured by reaction of cyclopentadiene, in the form of dicyclopentadiene, with an alcohol, followed by catalytic reduction of alkylated cyclopentadiene initial product [63,64]. A wide range of MACs have been reported, although apparently only one member of the class, tris(octyl dodecyl) cyclopentane, has been commercialized. Viscometric properties are typical for hydrocarbons and similar to those of PAOs. The main distinguishing characteristic of MACs is that they are essentially monodisperse and their volatility is therefore substantially lower than mineral oil or PAO of similar viscosity. Viscosity and viscosity index can be adjusted by appropriate selection of the chain length and degree of branching of the alcohol raw material, and by the degree of substitution obtained. MACs are miscible with mineral oil and with standard lubricant additives. Tris(octyl dodecyl) cyclopentane has a particularly low pour point and low volatility, and is used in spacecraft applications [63]. Cyclohexane Derivatives The cyclohexane derivatives are a class of fluids which are targeted, not at high temperature applications, but rather at automotive transmission applications requiring high traction. A range of such fluids were commercialized during the 1970s, although only one such material is currently available [65], namely 2,4-dicyclohexyl-2-methylpentane, which is manufactured by catalytic hydrogenation of the linear dimer of alpha methyl styrene [63]. In most respects, the cyclohexane derivatives behave similarly to other synthetic hydrocarbons of similar molecular weight, but have the distinctive characteristic that they exhibit high traction coefficients under the very high pressures experienced in an elastohydrodynamic (EHD) contact [63]. It is believed that the rigid regular structure of the cyclohexyl component leads to a very high volume change on melting, and consequently a very strong pressure dependence of the temperature of solidification. Thus, under extreme pressure, these fluids transiently solidify in the contact, forming a solid pad in the contact, which readily transmits lateral force and resists formation of shear slip planes.

SUJVIMARY Synthetic lubricants possess superior performance capabilities compared to mineral oils. There are three main reasons why synthetic lubricants are selected in preference to mineral oils: • improved stability, usually oxidative, • improved lubricity, usually at very high or very low temperatures and • reduced environmental impact, usually high biodegradability or reduced toxicity. Oxidative

Stability

The superior oxidative and thermal stability of synthetics over mineral oil is historically the main reason why they are used. It is very difficult to give precise figures for the exact temperature at which decomposition for a synthetic lubricant occurs as this is affected by several variables, the main ones being: • exposure time • tj^e and amount of additives present (antioxidants, metal passivators/chelators etc.) • availability of oxygen (air) • ability of the lubricant to remove heat • which metals the lubricant is in contact with (catalysis and potential deposit formation at metal surfaces) and • the presence of system contaminants (e.g., acidic components) There are several different ways of measuring the oxidative stability of a lubricant: • Decomposition temperature as measured by a change in: viscosity, acid value, hydroxyl value (ASTM D 943, Turbine Oil Stability Test), amount of oxygen consumed (ASTM D 2272, Rotary Bomb Oxidation Test), heat flow change (Differential Scanning Calorimetery), etc., under specified conditions • Temperature and nature of deposit formed on decomposition (e.g.. Panel Coker FTM3462 or Wolf strip tests UK 359, ASTM D 189, Conradson Carbon Residue, DIN 51 551, Pneurop DIN 51352-2) • Volatility of a lubricant at set temperature and times (e.g., ASTM D 5800, Noack test, Thermogravimetric Aneilysis) The life of oil at a particular temperature depends on the amount and type of degradation, which is acceptable, which in turn depends on how much performance can be allowed to deteriorate. Any chart comparing the relative stability of synthetic lubricants will therefore be quite arbitrary and highly dependent on the specific oil, test conditions, and application [65]. For example, oxidation tests on pure basestocks can show that esters have an oxidative stability similar or slightly worse than that of mineral oil. The reason for this is that mineral oil contains impurities that can act as anti-oxidants. Esters tend to only show their remarkably better stability when compared to mineral oil, provided the oils are formulated with antioxidants. Figure 10 gives an indication of oil life versus temperature for a range of formulated synthetic lubricants. Useful lifetimes are based on filed experience from a range of applications and should only be used as a crude approximation of actual service life in a specific application. As the previous dis-

CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 261 cussion has shown, oil hfe will change dependent on the exact temperature regime. Table 4 gives a useful qualitative overview of the volatility a n d deposit forming tendencies of various oils. Depositforming tendencies of synthetics can be highly dependent on several p a r a m e t e r s not necessarily connected with chemistry. For example, lubricants with a high polarity can help solubilize and disperse decomposition products leading to lower deposits. Polymers should have a narrow dispersion of molecular weight to avoid the lower molecular weight component volatilizing. Polymers such as Polyalkylene Glycols (PAGs) and Poly Iso Butylene (PIBs) tend to be very stable u p to a certain t e m p e r a t u r e a n d then rapidly degrade. The volatility and deposit forming tendencies can be highly dependent on the presence of metals. Metals act as a catalyst a n d therefore aid decomposition for specific chemistries.

Their volatility is therefore highly dependent on the test temperature. The types of additives used also play an important role. Therefore, the table should only be used as a rough guideline. The generation of heat from friction causes the temperature of the oil film to increase. This higher temperature reduces the viscosity of the oil. As the oil's ability to remove heat is increased, this may lead t o lower operating temperatures. A lower temperature will reduce the decrease in viscosity of the lubricant and also reduce the oxidative degradation of the lubricant, potentially increasing the life of other components in the system. The heat transfer of various lubricants can be compared by using a simplified version of the Sieder and Tate equation given below [69]. This equation is applicable to areas of turbulent flow. The equation can be further simplified for areas of laminar flow. LrO.bl pO.8^0.33

ha snnHEtic HnmocAfaoNS Where: h = K = p = X= Cp =

SUGON^

too

aoo

300

TEMPERA-niRE ' C FIG. 10—A comparison of oil lifes for a range of synthetic lubricants [66].

TABLE 4—An overview of the volatility and depo forming tendencies of a range of synthetic lubricants evaluated using a variety of tests. Lubricant Mineral oil WON PA0 6 Alkyl benzene 150 Esters PAGs PIBs Silicones Fluorocarbons

Volatility at 250 "C Poor Good Fair Very good Poor Very poor Excellent Good to excellent

Deposit Formation at 250°C Poor Fair Fair Excellent Very good Good Very good Very good

heat transfer coefficient of lubricant components thermal conductivity density viscosity specific heat capacity at constant pressure

Lubricants with good heat-transfer characteristics generally have high specific heat capacity, high thermal conductivity, high density, and a low working viscosity. T5rpical data for several lubricant classes are given in Table 5. PAGs and polyol esters, due to their polar nature and their superior lubricity, should be able to lubricate the system at lower bulk viscosities than their mineral oil equivalents, improving heat transfer still further [69]. Lubrication As with oxidation, the lubricity of a synthetic lubricant is highly dependent on the operating regime. Most wear tends to occur during; start-up, slow-down to stop, overheating, or overloading. Polar lubricants such as esters have greater affinity for metal surfaces than mineral oil and are less likely to drain to the sump [69]. Such lubricants are therefore more likely to maintain a lubricant film on start-up. Low temperature viscosity is also an important technical criterion. Cold starts, for instance, is the prime cause of engine wear and an effective lubricant film can be only be maintained by immediately effective lubrication circulation. Lubricants that have poor low temperature flow properties can take a significant time to reach the parts that require lubrication. For automotive applications, this can be evaluated using the cold crank

TABLE 5-

Lubricant Type

Specific Heat Capacity at100°C Calg-' °C^' (ASTM E 1269)

Thermal Conductivity at 100°C m W m " ' °C"'

Density at 100°C gcni~^ (ASTM D 70)

Viscosity at 100°C in cPs (ASTM D 445)

Lube Heat Transfer Coeft.

Mineral Oil PAO 6 [67] Polyol ester [68] PAG

0.52 0.55 0.55 0.46

127 144 150 185

0.82 0.77 0.93 1.02

6.49 4.54 5.58 7.75

7.33 9.14 9.91 9.21

262 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK simulator test (ASTM D 2602). An example of the benefits synthetics can bring is given in Table 6. The viscosity of a lubricant has a marked effect on wear (viscosity being related to film thickness). Viscosities of lubricating oils are often quoted at 40°C (ISO grade) or 100°C (ASTM D 445). In reality the viscosity u n d e r operating conditions is the controlling factor. Provided the stability of the lubricant is sufficient, the ability of the lubricant to resist viscosity dilution is dependent o n the Viscosity Index (ASTM D 2270) and the ability of the lubricant to remove heat. The viscosity index provides a measure of the rate of reduction of viscosity with temperature. The viscosity pressure coefficient measures the rate of viscosity increase with increased load. The ability of the lubricant to resist overloading is therefore highly dependent on the viscosity pressure coeffecient of the lubricant. Table 7 gives an overview of the viscosity indices and viscosity pressure coefficients for a range of synthetic lubriccints. In certain compressor applications, a proportion of the gas u n d e r compression can become dissolved in the lubricant (e.g., refrigerants, hydrocarbons), thereby reducing the lubricant viscosity below the recommended viscosity required for lubrication. This problem can be solved to a certain degree by using high viscosity grade lubricants (up to ISO 680 mineral oil). However, at low temperatures these lubricants are difficult to p u m p . In addition, gas streams may wash the lubricant off the cylinder walls, resulting in wear. PAGs and PIBs have been used in hydrocarbon applications because of their ability to resist this dilution. Where good solubility of the lubricant is required, e.g., refrigeration applications, the excellent lubricity of PAGs and polyol esters has been used to compensate for the reduction in viscosity. Table 8 gives Ein TABLE 6—Review of cold crank simulator viscosities for fluids with a viscosity of 4cSt fluids at 100°C.

Lubricant

Viscosity at 100°C icSt) (ASTM 445-97 DIN 51550)

Cold Crank Simulator Viscosity at -25°C (MPa.s) (ASTM D 2602) DIN 51377)

Mineral Oil SN 100 PA0 4 Polyol Ester

3.8 3.9 4.5

1300 500 550

TABLE 7—^Viscosity indices and viscosity pressure coefficients for a range of synthetic lubricants. Viscosity

Type of Lubricant

Index (ASTM 2270)

Naphthenic Mineral oils Parafflnic Mineral oils PAO DI & Tri esters Polyol esters PAGS Alkyl benzenes

0-80 80-120 120-150 50-150 50-170 150-280 RMM = 1000 Hydrogenated polybutenes listed under 178.3740

10: SYNTHETIC

LUBRICANTS—NON

AQUEOUS

263

TABLE 11—Relative cost of synthetic lubricants versus mineral oil. Lubricant Mineral oil PAOs Esters PAGs PIB Alkyl benzene Silicones Polyphenylethers Fluorocarbons

Relative Cost to Mineral Oil 1 2-5 2-10 2-6 2-4

2-3 10-50 50-250 75-300

tions and transformations. Synthetics will therefore always cost more than mineral oil. Their relative prices as opposed to mineral oil are given in Table 11. Although more expensive than mineral oil, synthetics can lead to a major reduction in system production and running costs and thereby quickly repay their initial costs. For example, synthetics can lead to reduced lubricant consumption, lower maintenance and less plant downtime, reduced disposal costs, and longer equipment lifetimes. The growth of mineral oil has been stagnating over the last few years. The higher performance, reduced environmental impact, and the increasing potential to reduce costs using synthetics has allowed them to show strong year on year growth in many sectors. In short, the commercial importance of synthetic lubricants is set to increase significantly in the future.

ASTM STANDARDS No. D70 D86 D91 D92 D93 D94 D97 D 130 D 189 D445 D471 D524 D664 D665 D873 D892 D 892 (Option A) D943 D972 D974 D D D D

1209 1218 1298 1401

Subject Density Distillation Precipitation n u m b e r (Sludge Formation) Flash Point - Cleveland Open Cup Flash Point - Pensky-Martins Saponification Number Pour Point Copper Strip Corrosion Carbon Residue - Conradson Viscosity - Kinematic Elastomer Seal Compatability Carbon Residue - Ramsbottom Acid Number, Potentiometric Rust Prevention Potential Residue Foaming Characteristics (Sequence IHI) Foaming Characteristics (Sequence IIII) (Option A) Turbine Oil Stability Test (TOST) Volatility - Evaporation loss Acid/ Base N u m b e r by Color Indicator Titration Color - APHA Refractive Index Density Emulsion Characteristics

264 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK D 1500 D 1744 D 1748 D2070 D2270 D2272 D2500 D2602 D2619 D6375 D2670 D2711 D2780 D2782 D2783 D2983 D3233 D3427 D3827 D4052 D4172 D4739 D4742 D5001 D5133 D5185 D5191 D5481 D5800 D6079 D6082 D 6082 (Option A) E326 E 1064 E 1269

Color - ASTM Water Content by Karl Fisher Rust Protection Thermal Stability Viscosity Index Rotating B o m b Oxidation Test Cloud Point Cold Cranking Simulator Hydrolytic Stability (93°C) Volatility - NOACK at 250°C / TGA method Falex PinA^ee Demulsibility Characteristics Gas Solubility Timken OK Load EP, 4-Ball E P Viscosity -Brookfield Falex Pin/Vee Air release Gas Solubility Specific Gravity Four ball wear Base N u m b e r Thin Film Oxygen Uptake Test Ball on Cylinder Lubricity Evaluation Brookfield - Scanning Elemental Analysis Vapour Pressure, Reid Viscosity - High T e m p e r a t u r e High Shear by capillary Volatihty - NOACK at 250°C / TGA method High Frequency Reciprocating Rig Foam, High Temperature Foam, High Temperature (Option A) TMC Certified Phthalate Esterification Coulometric Karl-Fischer Titration Specific Heat Capacity

OTHER STANDARDS No. D I N 51 5 5 0 DIN/ISO 2909 D I N 51 5 9 7 D I N 51 3 7 6 D I N 51 5 5 9 P a r t 1 DIN 51 777 Part 1

Subject K i n e m a t i c Viscosity K i n e m a t i c Viscosity Index P o u r P o i n t of P e t r o l e u m P r o d u c t s Cleveland Open C u p Calorimetric Titration Coulometric Karl-Fischer Titration

REFERENCES [1] Barnes, R. S. a n d F a i n m a n , M. Z., Lubrication Engineering, 1957, p. 454. [2] Smith, T. G., "Neopentyl Polyol Esters," Synthetic Lubricants, R. C. Gunderson and A. W. Hart, Eds., Reinhold, NY, 1962, p. 388. [3] Randies, S. J., "Esters," Synthetic Lubricants and High Performance Functional Fluids, R. L. Rudnick and R. L. Shubkin, Eds., Marcel Dekker Inc., NY, 1999, p. 63.

[4] Randies, S. J., "Refrigeration Lubes," Synthetic Lubricants and High Performance Functional Fluids, R. L. Rudnick and R, L. Shubkin, Eds., Marcel Dekker, NY, 1999, p . 563. [5] Randies, S. J., "Esters," Synthetic Lubricants and High Performance Functional Fluids, R. L. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, NY, 1999, p . 63. [6] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 18, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p. 991. [7] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 18, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p . 991. [8] Johnson, R. W., "Dibasic Fatty Acids," Fatty Acids in Industry, R. W. Johnson and E. Fritz, Eds., Marcel Dekker, NY, 1982, p. 327. [9] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, 4th Edition, John Wiley & Sons, Inc., NY, 1991, p . 893. [10] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1, 4th Edition, John Wiley, NY, 1991, p . 913 [11] Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 9, 4th Edition, John Wiley, NY, 1991, p . 755. [12] Briant, J., Denis, J., a n d Pare, G., Rheological Properties of Lubricants, Editions Tecnip, Paris, 1989. [13] Emkarate Esters for Synthetic Lubricants, Product Brochure, ICI Performance Chemicals, Middlesbrough, UK, 1997. [14] Lubricant Esters, UnichemaBV, Gouda, Netherlands, 1997. [15] Boyde, S., Journal of Synthetic Lubrication, Vol. 18, 2001, p . 99. [16] Boyde, S., Journal of Synthetic Lubrication, Vol. 16, 1999, p . 297. [17] Scholz, N., Diefenbach, R., Rademacher, 1., and Linnemann, D., "Biodegradation of DEHP DBF DINP," Bulletin of Environmental Contamination and Toxicolology, Vol. 58, 1997, p. 527. [18] OECD Guideline for t h e Testing of Chemicals, Ready Biodegradability, OECD 301, Organisation for Economoic Cooperation a n d Development (OECD), Paris, adopted 17 July 1992. [19] Boyde, S., Randies, S. J., and Gibb, P., "The Effect of Molecular Structure on Boundary and Mixed Lubrication by Synthetic Fluids—an Overview," Lubrication at the Frontier, D. Dowson, et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 799. [20] Randies, S. J., "Refrigeration Lubes," Synthetic Lubricants and High Peformance Functional Fluids, L. R. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 563. [21] Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 6, John Wiley & Sons, Inc., NY, 1991, pp. 225-269. [22] Matlock, P. L. a n d Clinton, N. A., in 'PAGs' in Synthetic Lubricants and High Performance Functional Fluids, R. L. Shubkin, Ed., Marcel Dekker, Inc., NY, 1999, pp. 101-124. [23] Klamann, D., Lubricants and Related Products, Verlag Chemie, Weinheim, Germany, 1984. [24] Emkarox Physical Properties, ICI Corp., London, 1997. [25] Polyalkylene Glycols, Properties and Applications, ICI Corp., London. [26] Wurtz, A., Annales de Chimie et Physique, Vol. 69, 1863, p. 330. [27] McClelland, C. P. and Bateman, R. L., Chemical Engineering News, Vol. 23, No. 3, 1945, p . 247. [28] Thompson, R. I. G., Eastwood, J., and Stroud, P. M., "The Development of High Performance Carrier Fluids for Detergent Fuel Additive Packages," Proceedings of the 11th International Colloquium, Technische Akademie, Esslingen, Germany, 1998, p. 2485. [29] Briant, J., Denis, J., and Pare, G., Rheological Properties of Lubricants, Editions Technip, Paris, 1989, pp. 155-163. [30] Yang, L., Heatiey, F., Blease, T., and Thompson, R. I. G., "A Study of the Mechanism of the Oxidative Thermal Degradation of Poly(ethylene oxide) a n d Poly(propylene oxide) Using ' H and '^C-NMR," European Polymer Journal, Vol. 32, No. 5, 1996, pp. 535-547. [31] Headey, F., Luo, Y.-Z., Ding, J.-F., Mobbs, R. H., and Booth, C , Macromolecules, Vol. 21, 1988, p . 2713.

CHAPTER 10: SYNTHETIC LUBRICANTS—NON AQUEOUS 265 [32] Sanvordenker, K. S., "Materials Compatibility of R134a in Refrigerant Systems," presented at The American Society of Heating, Refrigeration and Air-Conditioning Engineers, Winter Meeting, Jan. 1989. [33] Hamblin, P., "Oxidative Stabilisation of Synthetic Fluids and Vegetable Oils," Journal of Synthetic Lubrication, Vol. 16, No. 2, 1999, pp. 157-181. [34] Moxey, J. R., "Process for the Preparation of Polyoxylalkylene Block Copolymers," European Patent 0 570 121B1, Europena Patent Office, Munich, Germany, 10 Feb. 1999. [35] Chapter I, Food and Drug Administration, Department of Health and H u m a n Services (Continued), Part 178—Indirect Food Additives: Adjuvants, Production Aids, and Santizers— S u b p a r t D—Certain Adjuvants a n d P r o d u c t i o n Aids—Sec. 178.3570, Lubricants with Incidental Food Contact, Food and Drug Administration, U.S. Government Printing Office via GPO Access, Pittsburgh, PA, April 2002. [36] ISO 6743: Industrial Oil Class L Classification Part 4: Family H (Hydraulic Systems), International Organization for Standardization, Geneva. [37] Hodges, P., Hydraulic Fluids, John Wiley & Sons, Inc., NY, 1996. [38] Doc. No. 4746/10/91 EN, Requirements and Tests AppUcable to Fire-Resistant Hydraulic Fluids used for Power Transmission and Control, 7th ed., Comite Europeen des Transmissions Oleohydrauliques et Pneumatiques, Luxembourg, 1994. [39] Bock, W., "Moderne Schwerentflammbare u n d Umweltschonende Hydraulikflussigkeiten in Industrie und Bergbau," Tribologie und Schmierungstechnik, Vol. 46, 1999, pp. 22-28. [40] Test Standard for Specification Test Standard for Flammability of Industrial Fluids—Class Number 6930, Factory Mutual Research Corporation, Johnston, RI, 2000. [41] Kussi, S., "Eigenschaften von Basisflusigkeiten fur Synthetische Schmierstoffe," Tribologie und Schmierungstechnik, Vol. 33, 1986, pp. 33-39. [42] MoUer, U. J., "Grenzen and Moglichkeiten fur Syntheseole und Konventionelle Oele," Erdoel und Kohle, Vol. 23, 1970, p p . 667-673. [43] Moller, U. J. a n d Boor, U., Lubricants in Operation, Verlag, Duesseldorf, 1986. [44] Lilje, K. C , Short, G. D., and Miller, J. W., "Compressors and Pumps," Synthetic Lubricants and High Performance Functional Fluis, R. L. Shubkin, Ed., Marcel Dekker, Inc., NY, 1999, pp. 539-562. [45] Short, G. D., "Development of Synthetic Lubricants for Extended Life Rotary-Screw Compressors," Lubrication Engineering, Vol. 40, 1984, pp. 463-470. [46] Carswell, R. and McGraw, P. W., "Rotary Screw Compressor Lubricants," U.S. Patent 4,302,343, granted to the Dow Chemical Company, Midland, MI, 1981. [47] Ward, E. L., McGraw, P. W., and Appleman, T. J., "Lubricants for Reciprocating Air Compressors," U.S. Patent 4,751,012, granted to the Dow Chemical Company, Midland, MI, 1988. [48] Benda, R., BuUen, J., and Plomer, A., "Base Fluids for High-Performance Lubricants," Journal of Synthetic Lubricants, Vol. 13, No. 1, 1997, p. 41. [49] Brennan, J. A., "Polymerisation of Olefins with BF3" U.S. Patent 3,382,291, to Mobil Oil, Washington DC, 1968. [50] Shubkin, R. L., "Process for Producing a Ce-Cie N o r m a l ALPA-Olefin Oligomer Having a Pour Point Below About 50°F," U.S. Patent 3,763,244, to Ethyl Corp., Washingotn DC, 1973. [51] Shubkin, R. L., "Synthetic Lubricants by Oligomerisation and Hydrogenation," U.S. Patent 3,780,128 to Ethyl Corp., Washington DC, 1973. [52] Szydywar, J., "Synthetische Kohlenwasserstoffe, Speziell Polyalphaolefine," Schmiertechnik + Tribologie, Vol. 28, No. 4, 1981, p. 124.

[53] Plomer, A., Senior Scientist, Personal communication, BP Amoco Chemicals, Feluy, Belgium, October 1999. [54] Benda, R., Market Development Manager, Personal communication, BP Amoco Chemicals, Feluy, Belgium, October 1999. [55] Rudnick, L. R. and Shubkin R. L., "Lubrication at the Frontier," D. Dowson et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 10. [56] "PAO a n d Synthetic Esters," Product Literature, Mobil Chemicals, Chemical Products, Edison, NJ, 1998; "Polyalphaolefins," Product Literature, BP Amoco Chemicals, Chicago, IL, 1987. [57] Wunsch, F., "Synthetische Schmierstoffe - Heute und Morgen," Proceedings of the 9th International Colloquium, Technische Akademie Esslingen, 1996, p. 2271. [58] Randies, S. J., Lubrication Engineering, Vol. 22, 1957, p. 82. [59] Quinn, C , Traver, F., and Murthy K., "Silicones," Lubrication at the Frontier, D. Dowson et al., Eds., Proceedings of the 25th Leeds-Lyon Symposium on Tribology, 1998, Elsevier, NY, 1999, p. 267. [60] Bell, G. A., Howell, J., and DelPasco, T. W., "Perfluoroalkyl Polyethers," Synthetic Lubricants and High Performance Functional Fluis, R. L. Rudnick a n d R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 215. [61] Joaquim, M., "Polyphenyl Ether Lubricants," Synthetic Lubricants and High Performance Functional Fluis, R. L. Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 239. [62] "OS 124, Polyphenyl Ether Fluid by Santovac," Commecial Literature, Santovac Fluids, St Charles, MO, 1999. [63] Casserly, E. W. and Venier, C. G., "Cycloaliphatics," Synthetic Lubricants and High Performance Functional Fluis, R. L, Rudnick and R. L. Shubkin, Eds., Marcel Dekker, Inc., NY, 1999, p. 325. [64] Venier, C. G. and Casserly, E. W., Lubrication Engineering, 1991, Vol. 47, p. 586. [65] "Santotrac Traction Lubricants," Commercial Literature, Findett Corporation, St Charles, MO, 1999. [66] Landsdown, A. R., High Temperature Lubrication, Mechanical Engineering Publications Ltd., London, 1988. [67] M. J. Neale, Ed., Tribology Handbook, Butterworth, London, 1973. [68] Shubkin, R. L., "Polyalphaolephins," Presented at the Advanced Synthetic Lubricants Education Course, STLE National Meeting, Calgary, Alberta, Canada, May 1993. [69] "Midel 7131," Transformer Fluid Brochure, M & I Materials, 1993. [70] Randies, S. J. and Whittaker, A. J., "Compressor Fluids—^Value Creation Using Synthetics," International Conference on Compressors and Their Systems, Paper C542/029/99, ImechE Conference Transactions, 1999, p . 127. [71] Aderin, M., Spikes, H. A., and Caporiccio, C , "The Elastrohydrodynamic Properties of S o m e Advanced Non-Hydrocarbon Based Lubricants," Lubrication Engineering, Vol. 48. No. 8, August 1992, pp. 633-638. [72] CRC Handbook of Lubrication, Vol. 2, CRC Press, Boca Raton, FL, 1988, p. 235. [73] Guangteng, G. and Spikes, H. A., "Boundary Film Formation by Lubricant Base Fluids," Paper 95-NP-7D-3, Presented at The 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [74] Chang, H. S., Spikes H. A., and Bunemann, T. F., "The Shear Stress Properties of Ester Lubricants in Elastrohydrodynamic Contacts," Journal of Synthetic Lubricants, Vol. 8, No. 3, 1991, p. 258. [75] Gunsel, S., Lockwood, F. E., and Westmorland, T., "Engine Oil Oxidation—Correlation of ASTM III-D and III-E Sequence Engine Tests to Bench Tests" Presented at the SAR International Fuels and Lubricants Meeting and Exposition, Baltimore, MD, 1989, SAE Paper No. 892164, Society of Automotive Engineers, Warrendale, PA.

266 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [76] Gunsel, S. and Pozebanchuk, M., "Elastrohydrodynamic Lubrication with Polyester Lubricants and HFC Refrigerants," The Air Conditoinaing and Reirigeration Technology Institute, Project No. 670-54400, Report No. DOE/CE/23810-102, April 1999. [77] Smeeth, M. and Spikes, H. A., "The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastrodynamic Lubrication?," Paper 95-NP-7D-2, Presented at the 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [78] Summers-Smith, D., An Introduction to Tribology in Industry, The Machining Publishing Co., Brighton, England. [79] CONCAWE (Conservation of Clean Air and Water Europe), "The Collection, Disposal and Regeneration of Waste Oil and Related Materials," Report 85/33, The Hague, 1985. [80] Betton, C. I., "Lubricants and Their Environmental Impact," Ch. 13, Chemistry and Technology of Lubricants, 2"^ Edition, R. M.

Mortier and S. T. Orszulik, Eds., Blackie Academic and Professional, London, 1997. [81] Smeeth, M. and Spikes, H. A., "The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastrodynamic Lubrication?," Paper 95-NP-7D-2, Presented at the 50th Annual Meeting, STLE, Chicago, IL, 14-19 May 1995. [82] Summers-Smith, D., An Introduction to Tribology in Industry, The Machining Publishing Co. Ltd., Brighton, UK, 1969. [83] CONCAWE (Conservation of Clean Air and Water Europe), The Collection, Disposal and Regeneration of Waste Oil and Related Materials, Report 85/33, The Hague, 1985. [84] Betton, C. I., "Lubricants and Their Environmental Impact," Chemistry and Technology of Lubricants, 2nd Edition, R. M. Mortier and S. T. Orszulik, Eds., Blackie Academic and Professional, London, 1997.

MNL37-EB/Jun. 2003

Environmentally Friendly Oils Hubertus Murrenhoff^ and Andreas Remmelmann^

A Cs

d FM

g P Q T V V

P DIN EP HEES HEPG HETG HFA HFC NZ ppm TMP

VI WGK C CI CO2 H H2O N O VCI

Units ,2 m area J/(kgK) heat capacity mm diameter N force gravity m^/s bar (lO^N/m^) pressure flow m^/s °C temperature m^ capacity Ns/m^ dynamic viscosity mm'^/s kinematic viscosity kg/m^ density Deutsches Institut fiir Normung Extreme Pressure Hydraulic-Environmental group ES (synthetic ester) Hydraulic-Environmental group PG (polyglycol) Hydraulic-Environmental group TG (tri-glycerid) Hydraulic-Fire-resistant group A (water-based) Hydraulic-Fire-Resistant group C (water glycol) Neutralization N u m b e r Parts-Per-Million tri-methylol-propan Viscosity-Index Water Hazard Class Carbon Chlorine Carbon Dioxide Hydrogen Water Nitrogen Oxygen Verband Chemischer Ingenieure

Pressure Media In a variety of technical systems, hydraulic drives are the preferred alternative because of their versatility and efficiency. To perform the drive functions over the life of the system, the hydraulic fluid must be considered as a system component during the design stage. Due to a societal increase in ' Executive Director, Institute of Fluid Power Transmission and Control, IFAS, University of Teciinology Aachen, Steinbachstr. 53, D52074 Aachen, Germany. ^ Product Engineering Hydraulics, John Deere Works Mannheim, Windeckstr. 90, 68163 Mannheim, Germany.

environmental consciousness in the late 1980s, regulatory agencies began d e m a n d i n g that these fluids be rapidly biodegradable and nontoxic. This excluded mineral oil from consideration. Currently vegetable oils, some synthetic esters, and polyglycols are being used for these applications [1-10]. Since the properties of those base fluids differ substantially from those of conventional mineral oils, extensive testing is required. To qualify the fluids for the intended applications, special laboratory tests are required to ensure that they will withstand the pressures and t e m p e r a t u r e s encountered. Many of the chemical and physical test procedures were developed for mineral oils and aren't applicable for these new types of hydraulic fluids. Parallel to the development of base fluids, new additives are being developed. This is necessary for two reasons: 1. Existing additives used so far in mineral oil based pressure media lead to a deterioration of the performance properties in rapidly biodegradable oils. 2. Existing additives contain toxic substances leading to a significant deterioration of the biodegradability of the fluid [11,12-14]. Functions and Requirements In a hydraulic system, the pressure media has many functions. Fig. 1. Transmitting the hydraulic power is the main function. For this purpose, the pressure media connects the generator (pump) and the motor (cylinder) via the enclosed volume and carries the pressure that is determined by the load [15]. This main function of a pressure media can be accomplished in principle with any fluid; but additional requirements result in a substantial limitation of the fluids to be used. To ensure a long lifetime of the hydraulic components, wear protection is of central importance. Moreover, the fluid must protect the component surfaces against corrosion and it must be compatible with the elastomers used as sealing material. Based on the above-mentioned tasks, a variety of demands are m a d e on hydraulic fluids, see Fig. 2. To transmit hydraulic power and guarantee high load stiffness, the fluid must exhibit a high bulk modulus. This is another prerequisite for optimal control in open and closed loop hydraulic systems [14]. The task of protecting hydraulic components against wear and corrosion creates very high demands on the fluid [33]. It must exhibit a high increase of viscosity vs. pressure, thus producing a self-enhancing effect in the tribological contact

267 Copyright'

2003 by AS'I M International

www.astm.org

268

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

FIG. 1—Functions of hydraulic fluids.

FIG. 2—Demands on hydraulic fluids.

for high loads [16]. In addition, the fluid must provide good lubricating film wetting between surfaces moving relative to each other that reduce wear and stick slip. Materials used in hydraulic components such as bronze or other alloys are susceptible to corrosion. Consequently, the fluid should contain only a m i n i m u m a m o u n t of free acid, if any. The by-products created by the aging process must be neutralized or adsorbed by additives contained in the fluid to eliminate any acid build-up. In addition, the fluid properties must vary only slightly over extended time [31]. Elastomers and other non-metal components may also be attacked by these by-products. Increased elastomer swelling may result in complete decomposition and sealing failure. Mechanical and volumetric losses in hydraulic units are responsible for heat generation. This heat is partially released to the environment by convection. The pressure media itself, however, accounts for the major part of the heat loss. To limit the temperature increase of the fluid, it must possess high heat capacity and thermal conductivity. These demands are directly related to the use of the pressure media in a hydraulic system. The development of fluids is significantly influenced by external requirements. For ex-

ample, fluids used in underground mining or in steel mills must be fire resistant to reduce the dcuiger of fire. An additional requirement is environmental compatibility, which has led to the development of another pressure media group [19,22,35,38]. Types of Pressure Media Different types of hydraulic fluids are shown in Fig. 3. Each one is developed for a specific application. The most comm o n type in use today is based on mineral oil. Fluids are blended with additives to inhibit aging, weeir and to reduce friction and corrosion. For special application requirements, additives with detergent and dispersent characteristics are used to suspend solid particles as well as water, in some cases u p to 5% water. By utilizing special refining methods, it is possible to improve the viscosity t e m p e r a t u r e performance of the base stock. Group II API Base Oils can be blended to produce good tribological characteristics and long-term stability. State of the art refining technology ensures constant production of prime quality base stocks. The technology to develop additives is readily available.

CHAPTER 11: ENVIRONMENTALLY

FRIENDLY OILS

269

FIG. 3—Different types of hydraulic fluids [24,25].

TABLE 1—Groups of biologically fast degradable pressure media. Fluid

Base Fluid

Saturation

Origin

HETG HEES

Native ester Synthetic ester

Unsaturated Unsaturated Saturated

HEPG

Poly-glycol

Native materials Native materials Chemical industry Chemical industry

New groups of fluids have been developed to provide good fire resistance, rapid biodegradability, and low toxicity. Mineral oils are classified based on their performance, which is different from readily biodegradable pressure media, which are classified according to base fluid composition. There are three groups of biodegradable fluids: natural esters (type HETG), synthetic esters (type HEES), and poly-glycols (type HEPG) as listed in Table 1. HETG base stocks are derived from vegetable oils such as rapeseed and sunflower. These base fluids have a limited temperature range. New additives have been developed to meet the expected application temperature ranges. Their poor thermal stability is due to the amount of unsaturated carbon containing acids found in these natural esters. HEES base stocks can be produced from various materials including natural esters. The use of natural esters usually leads to unsaturated synthetic esters. Performance of the fluids is superior to HETG fluids due to a more uniform molecular structure and the use of different alcohols. Using completely saturated esters, the resulting fluids exhibit very stable aging characteristics. Additives used speciflcally for this type of fluid have also been developed. Based on the many choices of acids and alcohols available

for the production of synthetic esters, a wide variety of technical performance properties is possible. Early synthetic esters possessed a chemical structure similar to rapeseed oil. HEPG fluids is the third fluid type and is the only type that is water-soluble. This can be an advantage for the biological degradation in water. On the other hand, there is the danger of fluid-contaminated water penetrating more deeply into the soil layers, thus reaching ground water. For that reason, in some countries, polyglycols are not considered environmentally friendly fluids.

CHEMICAL B A S E S O F N A T I V E A N D SYNTHETIC ESTERS Functional Groups and Elementary Compound Pressure media based on native and synthetic esters consist of carbon-hydrogen bonds as do mineral oils. However, the structure differs significantly from that of mineral oil, which explains the different properties and performance characteristics exhibited by these fluids. Examples of the different functional groups can be seen in Fig. 4. The characteristic group for an alcohol is the hydroxy group (OH-group). In a case where the OH-group of aliphatic alcohols is connected to a carbon-atom, the alcohol is described as primary alcohol. In a case where the OH-group is connected to a carbon-atom in the center of the molecule, it is considered secondary alcohol. Tertiary alcohols are those with an OH-group connected at a branch site in the molecule. The hydroxy group of alcohols is responsible for their higher boiling points compared to those of the comparable

270

MANUAL

37: FUELS AND LUBRICANTS hydroxyl-group

carbonyl-group

HANDBOOK

alkyl-group

Mechanism of Esterification

H

0

—c—

C—H

OH

1

H carboxyl-group

aldehyde O

O

C—H

C—OH

ketone 0

II

carboxylic acid

R1-C—R2

alcohol

ester

O

O

R 2 - C — OH

R^-OH

R^-O—C — R2

FIG. 4—Important functional groups of organic chemistry.

R^ \

-.1

O—H

+

\

R' O—H

In Fig. 8, the general equation of a reaction for the production of esters is shown. This chemical reaction produces ester and water. Water must be removed during reaction to achieve a complete conversion of the alcohol reaction with the acid to produce a n ester (by shifting the equilibrium to the right). The equilibrium constant (K) for the ester producing reaction is [30]:

\

-.1 R' +\ * O—H - - 0 — H

-

FIG. 5—Hydrogen bond of alcohols, [22-30].

alkanes, because of the shared hydrogen bond of the hydroxyl groups. A strong polarity of the OH-connection is based on the electro-negativity of oxygen. The result is a positive shifting of the hydrogen a t o m so that the hydrogen bonding connection shown in Fig. 5 becomes feasible. The possibility for hydrogen bonding is the reason for the still unlimited miscibility of simple alcohols with water. However, with increasing size of the non-polar organic alcohol residue, this characteristic decreases. The abbreviation R used in Fig. 5 represents a n alkylgroup. This is a n acylic saturated carbon-hydrogen compound, called an aJkane, from which a hydrogen molecule is split off. According to their bonding capability, alcohols are considered to be monovalent, bivalent, or trivalent. To produce a synthetic ester hydraulic fluid with good performance properties, carboxylic acids are used. The carboxy g r o u p COOH is the functional g r o u p of carbon acids. Its nomenclature is based on the combination of the carbonyl and hydroxy group. Carboxylic acids with long-chain Rgroups attached are called fatty acids. The capability of carbon acids to hydrogen bond as with alcohols, is based on their chemical structure. This is the reason for their relatively high boiling point, which is comparable to alcohols due to its dimeric structure resulting from the hydrogen bonds shown in Fig. 6. Small carbon acids like the corresponding small alcohols are still soluble in water. However, the solubility diminishes with increasing molecular size. When carboxylic acids contain only single bonds, they are considered saturated. Unsaturated acids contain at least one double bond. The n u m b e r of double bonds influences chemical properties. Furthermore, carboxylic acids are distinguished by different isomers resulting in different chemical and physical properties (Fig. 7).

[ester] • [water] [acid] • [alcohol]

K

Where: "[ ]" indicates the concentration of the reactants (acid and alcohol) or products (ester and water) in mole/liter. It is desirable to maximize the concentration of ester and to minimize the concentration of acid and alcohol. One way to do this is to remove water during the reaction. Thus, to maintain equilibrium, more water must be produced, which decreases the concentration of acid and alcohol resulting in increased water production. Similarly, increasing the alcohol concentration will decrease the acid and increase the desired ester content. The rate of reaction is increased by the increasing temperature or by the addition of an acid catalyst such as H2SO4 (typically 5-10% based on the weight of the acid carboxylic). Other, even stronger, acids may also be used. The rate of the esterification reaction increases with increasing catalyst concentration The objective of ester production is to quantitatively complete transition of acid into ester. This means that the concentration of ester should be as high as possible. The removal of water from the reaction mixture may be facilitated by the addition of a solvent such as toluene. In this case, water is removed from the reaction mixture by azeotropic distillation. H —O

R ' - Cy .

+ O— H

,0—H—O C — R^

•^

R^ - C

O

C—R^ O—H—O

FIG. 6—Hydrogen bond of carbon acids.

cis-configuration

trans-configuration

H

H

H

I

I

I

C=C

H FIG. 7—Isomers of carboxylic acids, cis and trans structure.

carboxylic acid

aicohd

ester

water H

O R^-C — OH

R} - O H

^

R' - O — C — R 2

FIG. 8—Chemical reaction of ester production.

I

O—H

CHAPTER 11: ENVIRONMENTALLY The mechanism of the acid catalyzed production of ester from carboxyhc acids and primary or secondary alcohols is based on the addition of a proton to the oxygen of the carboxyl group. Producing a mesomer-stabilized cation and the Catom of the carboxyl group becomes positive. By adding the nucleophilic oxygen of the alcohol to this carbon atom, the ester is finally produced by separation of water and protons; see Figs. 9a and 9b. Tertiary zJcohols are not esterified in this way because the hydroxyl group will undergo acid catalyzed elimination producing undesirable alkenes, as shown in Fig. 10.

FRIENDLY OILS

271

1. step: alcohol protonation H R3C-OH

+

R3C-O

H

2. step: carbenium formation H R3C-0

- >

R3C

+

H2O

H 3. step: electrophilic carboxyl group attack 1. step: carbonyl oxygen protonation

o 1 II

O

OH'

^

R^ - C

+

^ H

1 II —*•

R^ - C — O H

I

~*'

R^ - C — O H

4. step:proton migration 0^—CR,

OH*

OH

R^ - G

+

1

R 2 --OH

R^- - c — 0

~*

H

/

R^ - C — O *

- C = 0 H " '

O—CR3

O—CR3

R^-C=O

+

H

R^1 - CI — O — R 2

Alcohol Bonds

L V 4. step: water split off OH R1 ^ - CI — O — R^

Ri

FIG. 10—Ester synthesis with tertiary-alcohols.

OH

H

O—CR3

-

5. step: proton removal

R^-C=OH'

3. step: proton shifting OH

R1^ - CII— O H

+

FIG. 9a—Ester synthesis with primary and secondary alcohols under acid conditions [22-30].

O—R"" R,1' - C

+

H.O

II . OH 5. step: proton split off 0 —R-" 1

R^ - C

//

\ OH

Rfi^

I OH

2. step: alcohol addition

R' - C

"^

R^ - C

OH

I

o'^CRj

+

H

O—R"

FIG. 9b—Ester synthesis with primary and secondary alcohols under acid conditions.

By variation of the acid and ester reagents, a vast array of possible ester products and a wide variety of chemical properties is achievable. These varieties, however, are limited by the demand for a good biological degradability as well as by other technical requirements with regard to viscosity, viscosity temperature dependence, and its stability under hydraulic load. The different alcohols depicted in Fig. 11 are used for the production of hydraulic fluids. Trimethylolpropane is a colorless, crystalline substance easily soluble in water. This is the stacking material used for polyols by reaction with ethylene oxide and propylene oxide, which are other reacted to form polyurethanes. By esterification with carboxylic acids, ester lubricants with a high viscosity index (VI) are produced. Glycerin is also a trifunctional alcohol and is very soluble in water. This is the main reason for its wide use in drugs and cosmetics. When esterified with carboxylic acids, ester-based lubricants with a high VI are produced as well. These esters can also be produced from natural materials such as rapeseed oil through a pressing process.

272

MANUAL 37: FUELS AND LUBRICANTS

trimethylolpropane HH

H

I

I

I

I

ditives. Mono and glycerin esters, however, exhibit limited chemical stability at high temperatures and low temperature performance. High temperature stability and low temperature performance properties can easily be achieved with di-carboxylic acid and polyol ester. Carbon-6 to carbon-12 di-carboxylic acid esters are increasingly important as engine and compressor lubricants. Polyol-esters are used in huge quantities as turbine oils for aviation applications where they are exposed to extreme temperatures. Good oxidative stability is

glycerin OH

OH

I

I

C— H

H—C — H H

I

H-C—C H

HANDBOOK

-C — O H

-OH

i

H

H H—C—H

H—C—H

I

I

OH

OH

trimethylolethane

pentaerythritol OH

OH

I

vinegar acid

I

H— C— H

stearic acid

H—C — H

O H

H

H

I

I

I

-C — OH

-c-

I

I

H

H

OH-C-

-OH

I

OH neopentylglycol

I

-C

I C

C—H

I

H

"*^17'^35

oleic acid 1x unsaturated cis-configuration O

H

H

OH-C—C^H,-^H

I

-c-

H

H—C—H

I

II

I

H

OH

H

O

H—0—C—C—H

I

"T"

-C—H

H

H H — C—H

"•^8^17

linoleic acid 2x unsaturated cis-configuation O

H

H

H

OH-C—C,H;

OH

For esterification of those alcohols to lubricants, carboxylic acids of vegetable and animal origin are used. Examples of the different acids used in hydraulic fluids are illustrated in Fig. 12. Ester basestocks, which are suitable as lubricants or pressure media typically possess a carbon number of about 18-30. These may be derived from natural sources. Achievable viscosities are within the range of about 22-100 mm'^/s. For this application, esters based on fatty acids are considered. They can be divided into five ester groups: mono, glycerin, dicarboxylic, polyol, and complex, as shown in Fig. 13. Mono-esters are prepared from linear monocarboxylic acids, which are reacted with branched or linear alcohols. The main applications of monoesters are in metaJ working such as for lubricants for cold and hot rolling. Of special importance within the group of mono-esters are the unsaturated methyl esters from which extreme pressure (EP) additives may be produced by reaction with sulfur or sulfur-containing functionality. Glycerin esters of rapeseed oils, particularly when the rapeseed oil has a high iodine number (high unsaturation), are used to synthesize such ad-

'^5^n

H

FIG. 11—Survey of alcohols for lubricants.

Carboxylic Acids

H

-C—C^C-

I

Pentaerythritol is a tetrafunctional alcohol with five carbon atoms. The four primary hydroxyl groups allow a relatively easy esterification with different acids. These esters are very stable against biological degradation in an aqueous solution.

H

FIG. 12—Survey of acids for lubricants.

dicarboxylic acid ester

mono aster

O

0

0

II

II-

II

R1-0—C—R2

R1-0—C—R2-C—0—R3

polyol ester (TMP-ester) O

glycerin ester 0

II

II

CHj-O—C—R1

CHj-O—C—R1

C,H,-C•*

Q[_|__Q

Q

H—C

-R1

CI H j - O — C — R 1

CJ H j - O — C — R 1 complex ester 0

0 II II CH^-O—0—R1

C^riy - 0 - - C — R 2 0

C,H,-( ; — C H j - o — c — -R1

C,Hg<

II

-CH,- 0 — C — R 2

I

CH,-0—C—R1 - C — O — C H ,

II

O

FIG. 13—Different ester types for lubricants [30].

CHAPTER 11: ENVIRONMENTALLY FRIENDLY OILS 273 achieved because of the unsubstituted beta-carbon atom and the primary hydroxyl groups.

CHEMICAL PROPERTIES OF NATURAL AND SYNTHETIC ESTERS Properties of esters are determined by the alcohol and acids from which they are derived. Various polyol-esters have been prepared by using alcohols for the production of lubricants similar to the triglycerides of rapeseed oil. Viscosity and Viscosity Properties Viscosity properties are important when esters are used as lubricants. The acid being esterified does not only determine the viscosity of an ester molecule. The viscosity of esters products increases with increasing molecular weight of the alcohols or their number of hydroxyl groups. This is due to the increasing structural possibilities for carboxylic acid components (Fig. 14). Pour points for these esters exhibit the same dependency as observed with viscosity. Lowering the pour point requires a short-chain branching of the alcohol with tertiary carbon or hydrogen-atoms. On the other hand, those molecular structures lead to a decreased oxidative stability of the alcohol. Therefore, neo-pentyl-polyols are especially advantageous for the production of lubricants, since they contain exclusively primary hydroxyl groups and they are branched. Acids used for the production of lubricants may also be the same as those of natural oils and fats with 16-18 carbon atoms, which produce the required classes of hydraulic fluid viscosity ISO VG 22 to 68. To obtain high oxidation stability, unsaturated acids are not desirable, since they represent a preferred point of attack for reaction with oxygen. However, double bonds also create a distortion of the ester molecule. This distortion positively influences the cold flow properties of the ester. The ester produced from acid with a monofunctional alcohol and acids containing double bonds will still flow at a temperature of 5°C, whereas completely saturated ester become solid at a temperature of 75°C (Fig. 15).

The position of the double bond within the fatty acid has no significant influence on the pour point. However, slight differences can be observed depending on the degree of distortion imparted by the double bond of the molecule. Depending on the position of the double bond, the distance between molecules increases, resulting in slightly different pour points (Fig. 16). Besides the number of double bonds, their steroconfiguration has a decisive influence on the viscosity properties. Ester with cis- double bonds will flow at low temperatures, whereas fluids with trans- double bonds exhibit pour point at comparable temperatures. Branching exerts similar influence as double bonds. With increasing branching at a constant carbon number, the cold flow behavior of the ester increases and viscosity decreases. In contrast, an increased chain length with the same structure results in an increase of viscosity. Reduction of the pour point is also obtainable by using esters with acid mixtures. These mixtures do not crystallize as readily thus leading to a decreased pour point. With the more complex esters there is no correlation between viscosity and carbon number, since the viscosity increase is compensated by branching, due to increasing carbon number.

g 60"o Q. 3 O

ziU

-40 -

6:0

8:0

10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 fatty acid [C-number: double bond]

FIG. 15—Pour-point of different esters with the same alcohol [20].

45 40

"

'*

^

—

^>

iC

FIG. 34—Temperature influence on oxidation stability.

test parameter

150

temperature:

• S1

-.125 E

110

>!ioo « 75

64

o

1 50 •a "S o 25

84

77

.a

i

150 iC

atmosphere:

RH

6,25 bar oxygen

46 limit:

24

®

1,75 bar pressure drop

catalyst:

Cu

Fe

283

Cr CrN Ti TiN materials and coatings

without

FIG. 35—Catalyst influence on oxidation stability.

^S variable

284

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

FIG. 36—Oil aging test rig [8].

test parameter: temperature: variable

I

pressure: S y\ variable volume flow:

® 300 bar 300 bar 150 bar 300 bar 300 bar 150 bar 90°C 60°C eO-C 90°C 60°C 60°C

GS1

GS1

GS1

S1

SI

20 l/min

water content: ^ < 0,03 % 4 HaO test duration:

S1

1000 hrs

FIG. 37—Change of TAN and viscosity after aging test runs.

The influence of high temperature and pressure indicates a high specific load on the fluid. Comparing the aging influence of pressure and temperature shows that the temperature exhibits the greatest influence on the aging. The influence of pressure is evidently less, at least as determined from tests performed at reservoir temperatures of 90°C and 60°C. Decreasing the reservoir temperature by 30°C reduces the aging process four-fold in the fluid as indicated by viscosity change. On the other hand, a 50% reduction of pressure yields only a very small reduction of the aging processes and rates. Absolute variation of fluid properties during tests are greater than conventional mineral oil based fluids.

Test runs on the aging test bench confirmed the very good oxidation stability of the more advanced fluids formulations (Fig. 38). Fluid S3, in particular, exhibits an aging stability comparable to mineral oils. The excellent stability is indicated by the constant viscosity throughout testing time. A comparison of results of the formulated fluids with their base fluids shows that the additives used and the base fluid correlate well with each other. Additives increase the aging resistance of the base fluid significantly. Table 3 illustrates the effect of additional fluid characteristics. Variations of these characteristics are only of minor importance with respect to the aging performance. Therefore, a graphical presentation is not shown.

CHAPTER

Aging

Rapidly biodegradable pressure media are primarily used in mobile hydraulic equipment, since they are often used in ecologically sensitive environments. These machines are often exposed to atmospheric influences so that there is always the danger of water ingression into the hydraulic system. Due to their chemical structure, synthetic esters tend to be hydrolytically unstable in the presence of water (see the Hydrolysis section above). Thus, evaluation of hydrolytic stability is of central importance for the assessment of the usability of rapidly biodegradable pressure media.

28

1

test parameter:

1

temperature: 90 °C

• viscx3sity

24

EBAN •5 X

IV.

g20

CO

•§12 >,

1^-

^°g.

1 8

I

o

o_

m

FRIENDLY

u

o

O 00_

1

lOCM

1^

o o"

GS1 GS2 GS3 S1 S2 S3 fluid generations

S4

pressure: 300 bar

0)

volume flow:

(0

®

o> c

V

I 4

z <

20 l/min

water content: ^ <

0,03 %

W

H,0

test duration:

HLP

1000 hrs

FIG. 38—AN and viscosity changes of different fluid generations after aging test runs.

TABLE 3—Important fluid properties. Property

Density Viscosity @40X Viscosity @ 100°C Viscosity index Pour-point Flash-point FZG-test Vickers pump test AN

Corrosion Steel

Alternative Test Methods'" ASTM^ ISC^

Unit

HEES 46

HLP 46

DIN Standard"

kg/m3 nim2/s

ca. 920

ca. 880 46

51 757 51562T1

D 1298 D445

3104

7,1

51562T1

D445

3104

100

ISO 2909

D 2270

2909

-27 220 12 60 bar, separate hydraulic and lubrication systems are used. Where the design involves separate systems, conventional turbine oils of ISO viscosity grades (VG) 32-68 are used for bearing lubrication and hydraulic oils of ISO VGs 32 and 46 for the hydraulic system-except for low temperature environments where oils of ISO VGs 10 and 22 may be used in the gate hydraulic control. (ISO Standard 3448 or ASTM D 2422, Standard Classification of Industrial Fluid Lubricants by Viscosity System, classify industrial lubricants by viscosity grade. The grade number corresponds to the mid-point of a viscosity range extending to ±10 % of the mid-point value and is measured in centistokes at 40°C.) Bulk oil temperatures are in the range of 40-55°C for the lubrication system. Under these conditions the stress levels in the system are low and the oil normally lasts the life of the turbine.

^ Private communication with H. Moeller, Elsam, Nordjyllandsvaerket, Denmark.

CHAPTER

12: TURBINE

LUBRICATING

AC main AC auxiliary DC emergency iube oil lube oil pump lube oil pump pump

OILS AND HYDRAULIC

FLUIDS

299

Oil purifier

FIG. 1—Lubricating oil system for a single-shaft combined-cycle turbo generator. Removal of excess water by centrifuge takes place every six months. Wind Turbines Wind turbines are a relatively recent development in which the rotation of a propeller is coupled to a generator either directly or via a gearbox. The power output currently varies according to location. Typically, land turbines have an output of 300-1000 kW while offshore turbines are larger with a capacity of u p to 2-3 MW [15]. Units of 5 MW output are currently under development [16]. Two basic types of propeller design are in use: those with a horizontal axis (otherwise k n o w n as p i t c h turbines) a n d those with a vertical axis (stall turbines). The former type uses one (or even two) gearbox(es) [17] and a generator. The gearbox is oil lubricated while the roller bearings of the generator are grease lubricated. A separate hydraulic system containing a conventional ISO VG 46 hydraulic oil (40-60 L capacity with a pressure of about 60 bar) for altering the pitch of the propeller may also be included. Some designs have demonstrated it is possible to avoid the use of a gearbox, and with these the propeller directly drives a multi-pole generator. The early gearboxes on machines of < 5 00 kW had a capacity of about 125 L and relied on splash lubrication with a n ISO VG 220 gear oil at temperatures of 80-90°C. For turbines with an output of >500 kW, forced lubrication systems are used a n d system capacities for the larger machines have since risen to about 200 L.

Problems initially arose with the use of conventional turbine oils in t e r m s of reduced oil life, deposit formation, micro-pitting and bearing failure and led to the use of polyalphaolefin (PAO)-based oils owing to their better high temperature stability. In spite of this the average life of the gearbox oil in the small units was still only about 1-2 ycctrs. The oil life has increased since temperatures were reduced to

llling using loling pump

C( Dllng water

-M-

Emptying>

illing using ^ s n ianual pump

FIG. 3—Typical steam turbine control fluid system. valves has caused many turbine fires [18,19]. As a result the industry, as long ago as the mid-1950s, introduced the concept of a separate hydraulic system containing a fire-resistant fluid [20]. Although there are alternative ways of reducing the fire hazard, e.g., by providing pipe-in pipe systems, fire-resistant hydraulic fluids are now widely used in electro-hydraulic governor control and emergency stop-valve systems. In order to reduce the volume of fluid used and the capital cost of a separate system, the pressures in these circuits are normally m u c h higher than when oil is used. Currently 160 bar is the m a x i m u m found, but an increase to 200 b a r is being considered. A typiccJ hydraulic control circuit diagram is shown in Fig. 3. Fire-resistant hydraulic fluids based on triaryl phosphates are now used in over 1000 large steam turbines worldwide. More recently the same fluids have been developed for use as a combined hydraulic fluid and main bearing lubricant for both small and large steam turbines u p to 1000 MW and the operating experience obtained since the early 1980s has shown them to be highly successful in this application [21]. Although most industrial gas turbines use mineral oil or synthetic hydrocarbons as the hydraulic fluid and lubricant in a single system, there are applications where fire-resistant hydraulic fluids are used in a separate system (in mediumlarge sized units) or as the operating media for both hydraulic and lubrication systems. As an example of the latter, their use in gas pipeline turbo-compressors has provided in-

creased safety and reduced downtime since 1958 and they are now widely used in this application in North America [22]. In the 1970s they were introduced for use in the control and lubrication systems of 70 MW sets for power generation [23]. A t5rpical lubrication system for an industrial gas turbine is shown diagrammatically in Fig. 4. Aero-derivative gas turbines were introduced during the Second World Weir and, while mineral oils were initially used for lubrication, it was soon realized that both the low temperature properties and the high temperature stability of the oils then available were inadequate for the more powerful engines that were being developed. Synthetic ester fluids were therefore introduced a n d have remained the most widely used type of fluid in this equipment for both aviation and industrial applications where they function as a combined hydraulic fluid and lubricant for the turbine [24]. Mineral oils are still used in some of the older or smaller aero-engines for military aviation, and in industrial applications where the thermal/oxidative stresses are lower. Where aero-derivative units are used for mechanical drive applications, the hot exhaust gases drive a power turbine that is attached to a compressor or a gearbox/generator, etc. In this application, a synthetic ester product lubricates the engine, while the power turbine and the "driven" equipment are normally lubricated with mineral oil in a separate system. When used for pumping natural gas, the power turbine cind compressor Eire often lubricated with a fire-resistant fluid.

302

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

IEGBO):

A A A • H

^

Q

^

Mr

mm MSuwIlF DfMn

->

Q )f>RESSUnE 'TRANSMITTEfl

TgwlflCSTiKfOMAt

FIG. 4—Typical lubrication system for an Industrial gas turbine. Reproduced with permisson of Solar Turbines, Inc., San Diego, CA.

There has also been some limited use of fire-resistant fluids in the hydraulic systems of aero-derivative gas turbines, but as yet there is no significant experience with these fluids as lubricants for this equipment. The severity of lubricating oil service (and hence operating life) varies considerably and depends on a number of factors including: • System design • The duty cycle, for example continuous or intermittent operation • Oil stability • The quality of system maintenance • The quality of oil or fluid maintenance • Top-up rates.

The system design determines the degree of thermal and oxidative stress to which the fluid or oil is subjected. In most turbines the thermal loading on the lubricant Eirises through heat conducted along the rotor to the bearings (shaft temperatures in large steam turbines can reach 320°C at the bearings), as a result of the heat generated through frictional and viscous losses in the bearings, and during compression in the pumps. Additionally, in gas turbines, Icirge quantities of sealing air at temperatures of 200-350°C [26] are drawn into the bearing and form an aerated mixture with the lubricating oil as it drains back to the tank—an ideal environment for promoting oxidation and foaming. The inlet temperatures of industrial gas turbines have risen over the years from around 700°C in

CHAPTER 12: TURBINE LUBRICATING 1950 to 1500°C in 1998 [27,28], as shown in Fig. 5 [28], while compressed air temperatures have also increased with the trend toward higher compression ratios. Together this would suggest a significant increase in the heat being dissipated via the engine structure. Certainly gas turbine bearings are now operating at high stress levels. For example, temperatures of up to about 115°C are found for plain bearings and up to about 300°C for roller beeirings. The various sources of heat and its removal for a t5rpical gas turbine bearing are shown in Fig. 6. Unfortunately, although the heat input at the bearings has steadily increased, the volume of fluid available for heat removal has not. If anything, oil volumes have been reduced as a consequence of reducing the size and therefore the cost of the system. This has been achieved by increasing circulation rates (with an adverse effect on aeration) and, where possible, by more effective cooling. In most cases however, the result has been an increase in oil return line temperatures, in the case of recent estimates for steam turbines, by 10-15°C [29]. As oxidation rates approximately double with every ten degrees rise in temperature, it is hardly surprising that tur-

1940

1950

1960

OILS AND HYDRAULIC FLUIDS

303

bine oil lives, particularly for gas turbines, are now giving cause for concern. Since the thermal/oxidative stress on the oil or fluid is a factor of temperature, time and the extent of air contact, the faster the oil temperature (and the air content) can be reduced, the lower the amount of resulting fluid degradation. A cooler in the return line is therefore preferred. The presence of air depends to a large extent on tank design (including the location of return lines, baffles, and sieves to remove entrained air), but it also depends on fluid circulation rates, the level of fluid/oil in the tank, etc., Guidance on the basic system design parameters of turbine lubricating oil systems (including the use of suitable materials of construction; design features of the reservoir and pump train; the appropriate use and location of coolers, valves and filters, etc.) is available for steam and industrial gas turbines when using conventional turbine lubricating oil. This appears in such standards as ASTM D 4241, Standard Practice for the Design of Gas Turbine Generator Lubricating Oil Systems, ASTM D 4248, Standard Practice for Design of Steam Turbine Generator Oil Systems, and the American Petroleum Institute (API) Standard

1970 Year

1980

1990

2000

FIG. 5—Development of gas turbine capacity, inlet/outlet temperature and efficiency. Reproduced by permission of Verelnigung der Grosskraftwerk Betreiber, Essen, Germany.

304

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

614, Lubrication, Shail-Sealing, and Control-Oil Systems for Special-Purpose Applications, b u t is t h e n sometimes ignored. In fact, tank residence times have decreased in recent years in the pursuit of lower costs, making it more difficult for the air to be released and with an adverse effect on oxidative stability [29]. The duty cycle is important where the turbine experiences frequent stops and starts or where the unit is used at maxim u m power for only a short period, as in aviation applications. If aircraft gas turbine oils were continually subjected to the same stress as found on take-off, the oil life would be substantially reduced. By contrast, the long periods of continuous operation in base-load thermal or nuclear power generation are less demanding. The stability of the oil or fluid will obviously play an important peirt in determining its operating life and may help identify the most appropriate type of product for the application. Depending on the quality of fluid maintenance (which is discussed in more detail later) and also system maintenance, for example minimizing air and water leeiks into the oil, the life of the lubricant can be extended considerably. The importance of reguleir, planned, maintenance cannot be overemphasized in the pursuit of trouble-free operation. Lastly, top-up rates will determine the rate at which new fluids, and therefore additives, are replenished. Normally, top-up rates are fairly lo\? for steam turbines (about 3-10% per year), but much higher veJues have been reported in the past (up to 27%) and the beneficial effects of such high topu p rates in extending oil life have been investigated [30]. In

industrieJ gas turbines with some mineral oil types, top-up rates can rise to as high as 3 3 % per year while aero-engines in aviation consume on average about 0.25 L per hour or a complete replacement charge in about 24-130 h of engine operation. In industrial operation the c o n s u m p t i o n may be lower due to the fitting of more efficient oil demisters. In the past, an oil change normally took place when equipment builders' recommendations on used fluid performance were exceeded. These would normally include limits on acidity, water, viscosity increase, and dirt levels. When these values were reached or exceeded, the oil or fluid was considered too degraded or contaminated for continued use. Today, filtration techniques are available which can readily reduce the levels of some contaminants, e.g., water and particulates. This improvement, together with the possibility of monitoring the depletion of additives and then re-inhibiting when necessary, makes it possible to extend life substantially in some applications. The volume of oil used in t u r b i n e lubrication systems varies considerably. While a figure of 270 L per MW used to be quoted as typical, steam turbine lubrication systems today contain in the region of 1 0 0 ^ 0 0 L per Megawatt for turbines of 500°C), mineral hydraulic oil has been almost completely

replaced by fire-resistant fluids based on triaryl phosphates in order to avoid fires arising from oil escaping at high pressure and coming into contact with steam pipes. The following is a brief summary of the basestock types and additives currently in use and their advantages/disadvantages. A more detailed account of the different oil types used in hydraulic fluids and their properties, which is also largely applicable to turbine oils, is available from other references [33]. BASESTOCKS Hydrocarbon Oils Petroleum oils are complex mixtures of many different chemicals and their relative amounts vary considerably from one crude source to another. The components do, however, fall into a limited number of categories, principally straight- and branched-chain saturated hydrocarbons (also known as paraffins), cyclic saturated hydrocarbons (also known as cycloparaffins or naphthenes), and unsaturated cyclic hydrocarbons (otherwise known as aromatics), examples of which are shown in Fig. 7. Additionally, small amounts of impurities, consisting mainly of cyclic derivatives of nitrogen, sulfur and oxygen, and polar materials such as naphthenic acids, may be present. Fig. 8 shows typical examples of such heterocyclic compounds. Each hydrocarbon component influences the properties of base oil to an extent dependent on its concentration (Table 2 [34]). The above impurities are present in solvent-refined oil in small quantities, typically 0.1-0.5 %, but occasionally in much larger amounts, and they can also impact the performance of the fluid particularly in terms of stability and lubrication. Much lower amounts of impurities, if any, are present in the hydro treated oils. The need to quantify the different components in lubricating oils has resulted in the development of several analj^tical test procedures. Currently these include ASTM D 2425, Standard Test Method for Hydrocarbon T5rpes in Middle Distillates by Mass Spectrometry; ASTM D 3238, Standard Test Method for Calculation of Carbon Distribution and Structural Group Analysis of Petroleum Oils by the n-d-M Method; and ASTM D 5443, Standard Test Method for Paraffin, Naphthene, and Aromatic Hydrocarbon Type Analysis in Petroleum Distillates Through 200°C by Multi-Dimensional Gas Chromatography. While the D 3238 method is easiest to apply, being based on refractive index, density and molecular weight, it is more restrictive in its application with limits on the total ring content and the ratio of aromatics to naph-

306

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

Type

n-paraffin

Viscosity index

Structure

\

/

\

/

^

/

^

high

^Ho

high

CH, CH,

iso-paraffin

CH, CHo

CH,

CH

naphthene

moderate

aromatic

low

FIG. 7—Typical hydrocarbon structures.

^ - - ^

Dibenzothiophene

^"^^

I

(High T e m p e r a t u r e Method); ASTM D 2622, Standard Test Method for Sulfur in Petroleum Products by X-Ray Spectrometry; or ASTM D 4927, Standard Test Method for Elemental Analysis of Lubricant and Additive Components— Barium, Calcium, Phosphorus, Sulfur a n d Zinc by Wavelength-Dispersive X-Ray Fluorescence Spectroscopy. In order to provide general guidance on the identification and selection of lubricants, ASTM has issued D 6074, Standard Guide for Characterizing Hydrocarbon Lubricant Base Oils. This document recommends methods for analyzing the composition of base oils, describes their important chemical properties, and discusses the toxicological requirements, including regulations covering the presence of undesirable components such as polynuclear aromatics.

H

Conventional Solvent-Refined Types Pyrrole

1,7- phenanthrone

FIG. 8—Impurities typically present in solvent refined mineral oils. thenics. The levels of impurities such as nitrogen and sulfur are not available from these tests and this information has to obtained from other procedures, e.g., ASTM D 3228, Standard Test Method for Total Nitrogen in Lubricating Oils a n d Fuel Oils by Modified Kjeldahl Method and ASTM D 1552, S t a n d a r d Test Method for Sulfur in Petroleum Products

In order to produce oils that meet industry requirements it is necessary to remove or substantially reduce the level of those components that adversely affect performance, and this has been achieved by various refining techniques. As indicated above, the oil produced by the solvent refining of a crude paraffinic basestock (Fig. 9 [34]) is still in major use for both steam and industrial gas turbine applications. In this basestock, about 45-60% of hydrocarbons are in the form of saturated straight- or branched-chain paraffins and monocycloparaffins, but there is still a significant a m o u n t of

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 307 TABLE 2—Effect of composition on base stock properties. Chemical Component n-Paraffin Iso-Paraffin Naphthene Aromatic Polar compounds

Viscosity Index Very High High Low Low Low

Pour Point (High/Low) High Low Low Low Low

Response to Antioxidants Good Good Good Some poor Poor

Oxidative Stability Good Good Average Average/poor S is antioxidant; N and O are pro-oxidants

Volatility (High = Poor, Low = Good) Good Good/average Average Poor Poor

ReprintedfromHandbook for Hydraulic Fluid Technology, 2000, p. 716, courtesy of Marcel Dekker Inc., NY.

Crude oil

Dewaxing

Solvent extraction

Atmospheric/vacuum distillation

Conventional base oil

Gas oil

i

T

Wax

Extract

FIG. 9—Solvent refining process. Reproduced with permission of Petro-Canada Lubricants, Mississauga, Canada. TABLE 3—Chemical composition of lubricant base oils. Oil Reference

A

B

C

D

E

F

G

API Category

I

II

II

II

III

III

Description

I Solvent Refined

Solvent Refined

Hydro-Cracked

Hydro-Cracked

Hydro-Cracked

Dewaxing

Solvent

Solvent

Solvent

Solvent

Iso-

Solvent

Iso-

Mass Spec. Analysis Paraffins, n- & isoMonocycloparaffins Polycycloparaffins Aromatics Thiophenes Paraffins -1Monocycloparaffins

25.7 20.8 27.9 24.9 0.7 46.5

29 25 31.7 14.2 0.1 54

23.7 30.8 39.1 6.4 0.0 54.5

21.6 32.8 37.6 8 0.0 54.4

30.2 30.5 35.3 4 0.0 60.7

32.6 34.2 32.9 0.6

76.1 14.7 9.2

66.7

90.8

Severely Hydro-Cracked

Severely Hydro-Cracked

Reprinted with permission of Petro-Canada, Mississauga, Canada unsaturated ring structures (Table 3 [35]). The refining process removes wax (mainly high molecular weight paraffinic compounds), most of the aromatic hydrocarbons, as well as some of the polar compounds containing oxygen and nitrogen, products that would otherwise have significantly reduced the stability of the oil. However, small amounts of sulfur-containing c o m p o u n d s , e.g., thiophenes, r e m a i n and these can be beneficial in terms of increasing the stability of the base. These basestocks are classified as Group 1 products by the API Classification of Base Oils according to their viscosity index, sulfur content and the content of saturated hydrocarbons (Table 4 [36]).

Hydrocracked/Hydrotreated Basestocks In view of the demand from industry for oils with better oxidation stability, as operating conditions become more severe, attention has turned to processes that can remove yet more

TABLE 4—American Petroleum Institute classification of base-stocks. Base Stock Sulphur, Saturates, Viscosity Group (wt%) (wt.%) Index Group I >0.03 and/or Ultra pure base oils Crude oil

Atmospheric/vacuum distillation

HTU = hydrotreatment unit

"Wax

HTU2

Hydro Isomerization

FIG. 10—Hydrotreatment plus hydroisomerizatlon process. Reprinted with permission of Petro-Canada Lubricants, Misslssauga, Canada.

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS

309

TABLE 5—A comparison of some physical properties of 6.0 cSt PAOs with solvent-refined and hydrocracked oils of similar viscosity. Parameter

Test Method

API Group Viscosity, cSt at 100°C at 40°C at - 4 0 ° C Viscosity Index Pour Point (°C) Flash Point (°C) Evaporation Loss (NOACK), %

ASTM D445

ASTM D 97 ASTM D 92 DIN 51581

PAO IV 5.98 30.9 7830 143 -64 235 6.1

160 Hydro-Treated

240 Neutral

200 Solvent Neutral

II

I

I

5.77 33.1 SoUd 116 -15 220 16.6

6.98 47.4 SoUd 103 -12 235 10.3

6.31 40.8 Solid 102 -6 212 18.8

Very High Viscosity Index Oils III III 5.14 24.1 Solid 149 -15 230 8.8

5.9 NA 127 -12 225 6

NOTE—Reprintedfrom:Synthetic Lubricants and High Performance Functional Fluids, 1992, p. 13, courtesy of Marcel Dekker Inc., NY. Viscosity at 40°C and 100°C) and flash points (by ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup); very good low temperature viscosities (ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids) and p o u r points (ASTM D 97, Standard Test Method for Pour Points of Petroleum Products), and a wide operating temperature range [42]. A comparison of the physical properties of PAOs with other hydrocarbon products of the same viscosity is given in Table 5 [43]. In view of the lack of aromaticity, additive solubility can be a problem. PAOs also have limited dispersency and do not penetrate rubber seals to cause swelling (as assessed by FEDSTD-791, methods 3604 and 3633, or by ISO 6072). As aresult, it is necessary to blend PAOs with small a m o u n t s of a sealswelling agent (usually a carboxylate ester) to avoid leakage. PAO-based gas turbine oils have been available for many years [39] where the higher prices could be justified in terms of smaller volume and better stability. They have not, as yet, m a d e a n y significant penetration of the steam turbine oil market [41].

Synthetic Ester Fluids Apart from the very early days of operation, carboxylate or synthetic esters (possibly in c o m b i n a t i o n with polyglycolether thickeners) have been the only products used for the lubrication of aero-derivative gas turbines in aviation applications. This market, like the others, is fragmented into different applications, this time in terms of the lubricant requirements of the turbopropeller, turbofan (civil) or turbojet (principally military) engines (Table 6) and into the different technical requirements of civil and military aircraft operation. The turbopropeller engine, for example, employs a reduction gear to accommodate the propeller. This necessitates the use of a higher viscosity fluid to provide a thicker lubricating oil film with the ability to reduce wear at heavy loads. At the other end of the spectrum, the military requirement to start an engine quickly at low temperatures requires the use of "lower" viscosity oils for turbojet engines. As a result, specifications for these oils currently cover four viscosity levels: 3, 4, 5, and 7.5 cSt at 100°C. Diesters, the lowest viscosity oils used in this application, were introduced after the World War II, initially for military applications. They were also used for the early commercial turbo-jets and in blends with polyglycolethers, for turbopropeller aircraft [24,26]. Diesters are produced by the reaction of an alcohol and an acid [44], viz

TABLE 6—^Aviation lubricant types currently commercially available. Engine Type Turbo-propeller Turbofan Turbojet

Engine Lubricant Composition Diester -I- Thickener 'High' Viscosity Polyol Ester 'Low-Medium' Viscosity Polyol Ester

TABLE 7—A comparison of minersil oil and carboxylateester properties. Mineral

Mineral Lubricant

Oil

Ester

Oil

Kinematic viscosity (cSt)at210°F 2.5 3.3 -40°F 3000 1700 -65°F 25 000 11000 Viscosity Index 75 154 Pour Point ("F) 350 m i n for ISO 32 or 46 grade products (ASTM D 4304). Higher values, e.g., > 4 5 0 min are required by some turbine builders. 2. ASTM D 943, Standard Test for Oxidation Characteristics of Inhibited Mineral Oils, also known as the TOST (turbine oil stability) test, is the second principal test for steam turbine oils. This involves heating the fluid together with water at 95°C in the presence of iron and copper catalysts, while air is passed through. At regular intervals the acid n u m b e r in the fluid is measured and the test terminated when the level reaches 2.0 mg KOH/g. In contrast to the RBOT procedure, and depending on the base stock and activity of the stabilizers, this test can take several thousand hours to complete. Minimum requirements for ISO 32/46 viscosity grades are normally 2000-2500 h. Fig. 16 shows

316 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

FIG. 14—A view of the ASTM D 2272 pressure vessel, recorder, and sample container with catalyst.

FIG. 15—Typical chart recording pressure changes in the RBOT test.

CHAPTER 12: TURBINE LUBRICATING the arrangement of fluids and metal specimens in the oxidation cell used for this test. As the length of the D 943 method can make product development with this procedure a very time-consuming process, some specifications (e.g., MIL-L-17331) have reduced the duration to 1000 h but added limits on metal content and sludge. To meet the requirements for a procedure in which sludge was determined, ASTM introduced D 4310, Standard Test Method for Determination of the Sludging Tendencies of Inhibited Mineral Oils. This procedure utilizes the apparatus of the D 943 method but after 1000 h the test is concluded and the fluid filtered through a 5-fim filter in order to determine the weight of sludge produced. Optionally, the change .'S'^ia k^

OILS AND HYDRAULIC FLUIDS

317

in weight of the catalyst coil may be measured and, of course, the increase in acidity may be determined. Another goal of test method development in this Eirea has been to find a method that could be used for both steam and gas turbine oils. Although the FED-STD-791 oxidation test methods are quite widely used (see below), they are regarded primarily as tests for aviation oils in view of the types of metal catalysts used and the high temperatures normally specified. When these conditions are used with mineral oils, the results are regarded as "somewhat inconsistent" [54]. For these reasons, it was eventually decided to specify a dry test procedure, which involved passing air through the oil or fluid in the presence of copper and iron catalysts, but at the lower temperature of 135°C. This formed the basis for ASTM D 5846, Universal Oxidation Test for Hydraulic and Turbine Oils. In this case the acidity and sludge are monitored and the test is terminated when the acid number reaches 0.5 mg KOH/g or the level of sludge (determined by rating the deposits left on a filter paper after placing a drop of used fluid at its center), becomes unacceptable. Since its introduction, the test has been of particular interest for gas turbine oil development in view of the availability of more stable basestocks. The stability testing of gas turbine oils, particularly aeroengine oils, has traditionally been at much higher temperatures than for steam turbine oils in view of their more severe operating environment. Most of the tests used in this area have been variations on the FED-STD-791, Method 5308, Corrosiveness and Oxidation Stability of Light Oils, which, in its original version, involved oxidizing the fluid in the presence of six different metal specimens for 164 h at 120°C. A modified version of this test still features in U.S. Navy specification MIL-PRF-23699F in which test conditions of three days at 175°C, 204°C, or 218°C are required. Although initially used in the U.S. Air Force specification MIL-L-7808 it was later replaced by FED-STD-791 method 5307, which has a test duration of four days and possible temperatures of 248-680°F (120-360°C) in the presence of seven metal specimens. The test (slightly modified) also features in the latest USAF specification, MIL-L-27502, for a high temperature engine oil in which conditions of two days at 220°C and 232°C are specified. Unlike the 5308 test in which the fluid and catalyst coupons are evaluated only at the end of the test, in the 5307 procedure, fluid samples are taken during the test and monitored for viscosity and acidity increase. ASTM D 4636, Standard Test Method for Corrosiveness and Oxidation Stability of Hydraulic Oils, Aircraft Turbine Engine Lubricants, and Other Highly Refined Oils, was later issued as a combined 5307/5308 method offering three alternative procedures and the ASTM method is now also specified as an alternative to the 5307 procedure in MIL-PRF-7808L.

FIG. 16—Metal catalyst, fluid, and water arrangement in the ASTM D 943 oxidation test.

The metal specimens used in the oxidation tests are normally (but not cJways) in electrical contact as this is how they are found in the system and corrosion is frequently accelerated by bringing together metals of different electrical potential. The arrangement of the metal specimens in the FEDSTD-791, method 5308 and Alternative Procedure 2 in ASTM D 4636 is shown in Fig. 17. Mineral gas turbine oils are also tested by a modification of the 5308 procedure. General Electric, for example, specifies this method for its high temperature gas turbine oil specifi-

318

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK bine oils meeting ISO VGs 32 and 46. To increase the test severity the duration was extended from 3-9 days for most oils. This resulted in the mineral oils starting to degrade significantly.

Rust and Corrosion Inhibitors

FIG. 17—Metal catalyst arrangement used in the FEDSTD-791 method 5308 oxidation test.

cations, GEK 32568 and GEK 101941, which call for a temperature of 175°C while reducing the duration to three days. Because of the importance of deposit formation in aircraft lubricants, the accurate measurement of the amount of dirt or sludge produced during the high temperature oxidation tests is as important a feature of these tests as the generation of acidity and increase in viscosity. Oxidation test conditions for phosphate esters tend to fall between those of mineral oil axid synthetic esters though, because of their tendency to hydrolyze, the tests specified are dry tests. Typical test conditions are 164 h at 120°C in the presence of copper and iron catalysts (ISO 15595) or three days at 175°C (FED-STD-791, m e t h o d 5308 modified) as found in b o t h ASTM D 4293, S t a n d a r d Specification for Phosphate Ester Based Fluids for Turbine Lubrication and the General Electric specification GEK 28136 for phosphate ester fire-resistant gas turbine lubricants. Table 13 [27] compares the oxidation stability under stemdard and more severe FED-STD-791 test conditions of uninhibited butylatedphenyl phosphate esters and commercially available mineral gas tur-

Rust is a continuing problem in turbines when carbon steel is present because water contamination of steam turbine oils and fluids is very difficult to avoid. Acid is also invariably present in degraded oils and this can attack certain metals. Although the engine in aero-gas turbines is usually constructed of stainless steel or exotic alloys, carbon steel can still be present in the auxiliary equipment and the oil tank. In order to avoid rusting and corrosion, certain chemicals have been found to protect the metal surfaces. For steel, such inhibitors are usually highly polar materials, e.g., organic acids, esters or amides (Fig. 18), which form an adsorbed film on the surface of the metal that physically hinders the transfer of water to the metal surface. This necessitates careful formulation to avoid interaction with other surface-active materials, such as antiwear additives, and to minimize the impact on foaming/air release properties. In view of the fact that many turbines operate in saline environments, rust protection is frequently required against salt water. This is a more severe requirement than distilled water protection. Rust inhibitors may also be used in polyol and phosphate esters. However, the polarity and film-forming tendency of the latter when uninhibited already offer some protection against rusting, which is enhanced by acidic degradation products. If the phosphate is used in conjunction with adsorbent media for controlling the level of acidic degradation products, careful selection of the inhibitor is required as the adsorbent solids can remove both acidic and basic additives. The most widely used procedure for evaluating rust inhibitors is ASTM D 665, Standard Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. This involves stirring together 300 mL of the fluid and 30 mL of either distilled or synthetic seawater for 24 h at 60°C. A cylindrical test specimen of carbon steel is immersed in the mixture and, at the conclusion of the test, is assessed for the amount of rust produced. Ratings vary from a "pass" where no rust is found, through light rust ( 5 % covered). If rusting is going to occur it is thought that it will appear within the first 4 h of the test and proposals are currently being considered by ASTM to reduce the test duration to this period. In addition to steel, there are other metals that can be attacked by degraded oils and by some additives, such as sulfur-containing extreme-pressure additives. Of these, copper is by far the most important, not only because of its common use in system construction but also because it may catalyze the breakdown of oils and fluids when present as soluble salts at concentrations of < 4 0 p p m [52]. The mechanism normally involves acid attack on the metal with the formation of metal salts, which then dissolve in the oil. However, copper is also susceptible to attack from sulfur either present in the oil or, more commonly, released from extreme-pressure additives as they degrade. This can result in the formation of sulfidecontaining deposits. Fortunately certain types of chemicals

CHAPTER 12: TURBINE LUBRICATING

OILS AND HYDRAULIC FLUIDS

319

TABLE 13--Comparison of the oxidation stability of mineral gas turbine oil with butylated phenyl phosphates under extended FED-STD-791 test conditions. Test Duration (days)

Viscosity Increase

ISO VG 32 phosphate (TBPP)

3 6 9

ISO VG 46 phosphate (TBPP)

3 6 9

Gas turbine oil A (ISO VG 32)

3 6 9

3 4.4 3.8 6 6.4 7.3 3.6 4.2 25

Gas turbine oil B (ISO VG 46)

3 6 9 3 6 9

Fluid

Gas turbine oil C (ISO VG 46) Gas turbine oil D (ISO VG 46) Turbine industry limits ASTM D 4293 limits

3 6 3 3

(%)

18.3 24.3 49.5 23.4 28.6 80.6 13.4 21.2 - 5 to +15 - 5 to +20

Acidity Increase (mg KOH/g) 0.11 0.37 0.85 0.27 0.33 1 0.44 0.8 5.3 2.2 3.1 5 3 4.5 7 2 5.1 2.5 max 3.0 max

C12H22—CH — C O O H

Metal Weight Change (mg/cm^) Fe

Cu

Cd/Fe

Al

Mg

-0.11 0.02 -0.01 -0.01 0.11 -0.04 0.0 0.03 0.14 0.03 0.21 0.56 0.02 0.04 0.18 0.06 0.24

-0.17 0.04 0.1

-0.03 0.02 0.03 -0.01 0.17 0.18

-0.07 0.0 0.0 -0.11 0.08 -0.11 0.0 0.0 0.09

-0.03 0.02 -0.02 -0.02 0.13 0.07

where R, is

0.03 0.29 -0.02 0.02 0.04 0.01 0.08 0.13 0.04 -0.03 -0.1 -0.47 0.02 -0.13

0.05 -0.05 -11.7

0.03 0.03 0.05

0.01 0.03 0.06 0.06 0.07 0.33 0.02 0.01 0.04

-0.04 0.02

0.0 0.52

0.01 0.35

-0.04 0.02 -0.15 -0.1 0.07 0.96

0.05 0.02 0.32

+-CH2CH2O-J-H

I CH2—COOR1

C12H22—CH — C O O H

where R2 is a polyamine residue

CH2—CONHR2 FIG. 18—Typical structures for turbine oils rust inhibitors.

can protect the metal from either type of attack by the formation of a protective layer of up to 5000A thick [55]. These products, known as metal passivators are, for turbine oils and fluids, chiefly of the triazole family (Fig. 19), and the effect of their activity is to substantially extend the life of fluids in copper-catalyzed oxidation tests. Their effectiveness is readily demonstrated in oxidation tests such as ASTM D 2272 and ASTM D 943. Apart from their performance in these and other metal-catalyzed oxidation tests, the corrosive tendencies of oils towards copper is most frequently assessed by ASTM D 130, Standard Method of Test for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test. In this test, a copper strip is normally immersed in the oil for 3 h at lOO'C, but more severe conditions are also used depending on the application. Following the immersion period the strip is solvent cleaned cind visually rated for corrosion by comparing the surface color or tarnish with the ASTM Copper Strip Corrosion Standards, which represent increasing levels of tarnish axid corrosion. Normally turbine oil specifications call for a maximum rating of lb, which is slight tarnish.

Unfortunately, triazole derivatives titrate as acid and therefore contribute slightly to the acidity of the fresh fluid, a factor that has to be considered when meeting specification limits on fresh fluid. Antiwear and Extreme-Pressure Additives Antiwear additives are compounds added to an oil to reduce the wear occurring between the surfaces in sliding contact. They are normally effective at light to medium loads, for example in pump operation. These additives, of which neutral triaryl phosphates are perhaps the most well known examples, initially form strongly adsorbed layers on the surface. As the temperature increases due to the relative movement of the surfaces, a chemical reaction takes place with the surface. The mechanism early suggested involved the formation of a lower melting eutectic of iron and iron phosphide, which flowed into the gaps between the asperities and therefore helping to provide a greater surface area to cany the load. This idea was later rejected in favour of a metal phosphate layer that assisted lubrication [56]. The latest work,

320 MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

methylene-bis-benzotriazole

tolutriazole

N N

I CH2NR2

N-alkylated benzotriazole

N-alkylated 1,2,4, triazole

FIG. 19—Structures of some metal passivators used in turbine oils and fluids. however, suggests the formation of a self-regenerating polyphosphate layer possibly in the presence of amorphous carbon [57]. As the temperature increases still further a point is reached where the phosphate film breaks down and cannot prevent metcil to metal contact. In order to sustain even greater loads, for example to provide adequate gear lubrication, it is necessary to generate higher melting films, which are normally achieved by adding extreme pressure (EP) additives, particularly sulfur in the form of a sulphur carrier, e.g., ZDTP, sulfurized fats or olefins. Combinations of phosphorus and sulfur are also widely used and cover a wider temperature range than each element individually, providing both antiwear properties and EP performance over a wide range of applied loads. Figure 20 shows the structures of some typical antiwear and extreme pressure additives. Most specifications for hydrocarbon turbine oils do not require antiwear performance. There are also normally no load-carrying requirements for steam turbine oils except for extreme pressure steam turbine oils, e.g., in marine applications, but they do exist for gas turbine oils, and where the oil is used for the lubrication of both a steam and gas turbine as in some combined cycle applications. These requirements are not particularly onerous, typically lying between FZG failure load stage values of 5-8 while EP turbine oils require a failure load stage of 12 + . Micro-pitting resistance may aJso be required for this more severe application and is also ceirried out on the FZG gear rig using the Forschungsvereinigung Antriebstechnik (FVA) method 54/I-IV. The other application to require gear testing is aero-engine

lubrication. These oils have traditionally required performance on the Ryder Gear Tester (ASTM 1947—now discontinued—or FED-STD-791, method 6508). Increasingly, because of better precision, the test devised by the German Forschungsinstitut fiir Zahnrad und Getriebe (FZG) is being used and this method has been standardized by ASTM as D 5182, Standard Test Method for Evaluating the Scuffing Load Capacity of Oils. Both the FED-STD 791 method and the FZG procedure involve "a recirculating power loop principle, also known as a four square configuration, to provide a fixed torque (load) to a pair of precision test gears." A schematic is shown in Fig. 21. The drive gearbox and the test gearbox are connected through two torsional shafts. Shaft 1 contains a load coupling used to apply the torque through the use of known weights hung on the loading arm. The procedure involves operating the machine at a constant speed (1450 rpm) and oil temperature (90°C) for a fixed period at successively increasing loads until the failure criteria are reached. This is "when the summed total width of scuffing or scoring damage from all 16 teeth is estimated to equal or exceed one gecir tooth width." The FVA micro-pitting test referred to above is in two parts. First, a load stage test is carried out in which the test oil is run at each load stage between 5 and 10 for a period of 16 h. This is followed by endurance test involving 80 h at Load Stage 8 followed by 5 periods of 80 h at Load Stage 10. An assessment is made of the amount of pitting or profile deviation on each tooth at the end of each test period. For aviation applications, the Ryder Gecir Test is still used with the MIL-PRF-23699F specification in accordance with

CHAPTER 12: TURBINE LUBRICATING FED-STD-791 method 6508, in which the performance of the test oil is evaluated in comparison with reference oils giving known failure loads. In this case the load-carrying ability is defined as the "gear tooth load at which the average percent of tooth area scuffed is 22.5 %." The variability of the results on this equipment requires eight determinations of the reference oil and six determinations of the test oil. The performance is judged acceptable if the average of the six results is not less than 102% of the reference oil. In the MIL-PRF7808L standard, however, it is now possible also to use the ASTM D 5182 method where a minimum load stage failure of 5 is required. Only an average of two tests is required on this equipment.

OILS AND HYDRAULIC FLUIDS

Antifoams The inhibition of foaming is essential to ensure the correct lubrication of pumps and bearings and, in hydraulic systems, the transmission of power. Foam can also result in a rapid loss in oxidation stability of the oil or fluid and, under the worst conditions, can cause fluid loss from the tank. It is therefore important that air is lost quickly from the fluid and this can be assisted by the addition of very small quantities (usually a few parts per million) of specific chemicals known as antifoams that reduce the surface tension on the bubble envelope [58]. The most widely used of these are the silicone fluids. Chemically, these are polydimethylsiloxanes, which

where R is typically CjjCj orC4

triaryl phosphate

dialkyl phosphite where R is C4-C8

triphenylphosphorothionate

RO^

OH.HNR'2

321

R'zNH.HO'

OH.HNR'2

amine phosphates FIG. 20—Structures of typical antlwear/extreme pressure additives used in turbine oils and fluids.

322

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

are available in a range of viscosities, b u t the most active products are those that are not soluble but dispersible in the oil or fluid. Soluble siloxanes do not have the same activity at the air/fluid interface. Other problems associated with these additives include the difficulty in obtaining a homogenous dispersion—the polymers tend to accumulate at the surface and then deposit on the walls of the tank so that over time the efficiency of the additive is lost. At "high" concentrations they can also have an adverse effect on air release properties and, if the turbine oil escapes into the turbine, there have been reports of the build u p of silicon on the turbine blades. As a result, the trend in mineral turbine oils is away from the use of silicones and towards other products such as polyglycolethers and polyvinylethers, but larger quantities of these are normally needed. The laboratory test procedure (ASTM D 892, Standard Test Method for the Foaming Characteristics of Lubricating Oils, or the ISO equivalent method, 6247) involves passing air at a fixed rate through either a spherical crystalline alumina diffuser or a cylindriccd sintered steel diffuser immersed in the oil or fluid contained in a measuring cylinder (Fig. 22). These

1 != pinion 2 = gear wheel 3 = drive gears 4 = load clutch

diffusers produce bubbles of a known and consistent size. After passing air through the fluid for 5 min at 24°C (TS^F), the volume of foam (tendency) is noted immediately and then also after 10 min (stability). The test is repeated at 93.5°C (200°F) and then again at 24°C to check if the foaming behavior has changed (possibly due to loss of cintifoam) as a result of the exposure to high temperature. The results are quoted in terms of the tendency/stability values obtained at each of the three temperatures. Values are normally much lower at the higher temperature and hence limits under this condition are more severe. Because of the importance of foaming and its related topic, air release, it is further discussed in the section on Performance Requirements, Their Significance and Evaluation. The performance of the oil is usually retained as long as the additives remain in solution a n d are not depleted owing to removal from the fluid as volatiles, by partitioning into any free water, by precipitation, or by natural usage. If the additive is removed by any of these means, the rate of degradation, for example, can suddenly accelerate. If the oil monitoring is not adequate it can result in the oil exceeding its operating limits

5 = locking pin 6 = lever arm iNWn weights 7 = torque measuring clutch 8 = temperature sensor

Schematic section of FZG Test Rig torque measuring clutch

shaft 2

test gears

drive motor

drive gears load clutch shaft 1 FIG. 21—Diagram of the FZG Test Rig.

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 323 suggested a quantity of about 360000 tons of turbine oils, excluding aviation eind marine gas turbine requirements, were sold worldwide. Obviously, with a global market of this size there are a large n u m b e r of suppliers including both multinational companies and smaller, national oil companies. One of the "problems" associated with the supply of mineral turbine oil is that the are produced in different parts of the world from different crude sources. Although attempts have been made by the oil industry to minimize these differences by introducing "formula numbers" or product identification numbers that identify the product by certain physical characteristics, e.g., viscosity, flash point and pour point, etc., it is still possible for the base material to vary in its composition. As a result there may be variations in the performance, particularly stability, of finished products sold under the same trade name. This has caused some turbine builders to abandon formal approval lists in order to avoid liability claims and to rely on the local supplier to convince the end user that a product is available that meets the specification requirements.

FIG. 22—The development of foam in the ASTM D 892 test procedure. unnoticed. It is n o w widely accepted that with m o d e m turbine oils, condition monitoring is important; this aspect will be examined in more detail later.

THE CLASSIFICATION OF TURBINE LUBRICANTS AND HYDRAULIC FLUIDS In order to clarify the different applications for turbine lubricants and hydraulic fluids a n d hence ensure that the correct type of oil or fluid is used, ISO Standard 6743-5, Lubricants, Industrial Oils And Related Products (Class L)— Classification—Family T (Turbines), has been issued. This classifies the type of turbine, e.g., steam or gas, and then subdivides the application according to whether the requirement is for normal service or for a special application as, for example, in high temperature service or high load carrying ability. A further subdivision is according to viscosify. Table 14 shows the current classification and typical applications. At present the classification does not extend to lubricants for water, wind, or aviation gas turbines although the standard is currently under revision.

TURBINE OIL AND FLUID STANDARDS The turbine oil market is one of the largest segments of the industrial oil market. An estimate of consumption in 1998^ ^ Private communication, D. J. Whitby, Pathmaster Marketing Ltd., Woking Surrey, UK.

In order to ensure that the quality of the turbine oil supplied meets certain minimum standards, and to form the basis for purchasing agreements, performance specifications have been issued by international and national standards organizations as well as by the turbine builders and some end users. ASTM, for example, publishes D 4304, Specification for Mineral Lubricating Oil Used in Steam and Gas Turbines. As a result, the industry is now one of the most heavily specified. The major specifications currently used by industry are listed in Tables 15 and 16 for the main turbine applications. While past practice was for separate specifications on hydrocarbonbased steam and gas turbine oils, the current trend is to issue a combined document as, for example, in D 4304. At present there are n o separate specifications for water turbine oils, but requirements are sometimes included in standards for steam and gas turbine lubricants, e.g., ISO 8068, Petroleum Products and Lubricants-Petroleum Lubricating Oils for Turbines (categories ISO-L-TSA and ISO-L-TGA)-Specifications. Although there has recently been significant consolidation within the power generation industry, some of the specifications listed are still published under the n a m e of the previous builder. Some simplification of the list is therefore to be expected. When seeking approval, the oil or fluid supplier would first approach the turbine builder or end user with a request for their product to be examined against the specification. Tests would be carried out by the specifier (in-house or at an independent laboratory) to ensure that the product was technically acceptable. If successful, approval for field trials or commercial use would be given. In the former case it might be necessary to obtain several years of operating experience before full approval was given. Where no formal approval process existed, the supplier would have to convince the user that the product met the requirements, normally by providing the results of independent laboratory tests. Although most specifications are primarily concerned with the quality of new fluid as delivered, some manufacturers also specify the quality of any flushing fluid to be used and cilso the limits on fluid performance in use. The latter aspect will be referted to in more detail in the discussion on monitoring used fluid quality.

324 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 14—Classification of lubricants for turbines (ISO Standard 6743-5). Application Steam, direct coupled or geared to the load

Gas, direct coupled or geared to the load

Control system

Specific Application

Composition and Properties

Symbol ISO-L-

Typlcal Applications

Normal service

Highly refined petroleum oil with rust protection and oxidation stability

TSA

Special properties

Synthetic fluids with n o specific fire-resistant properties

TSC

Fire-resistant

Phosphate ester lubricant

TSD

High load-carrying ability

Highly refined petroleum oil with rust protection, oxidation stability, and enhanced load-carrying abilityoxidation stability, and enhanced load-carrying ability

TSE

Power generation and industrial drives and their associated control systems. Marine drives where improved load-carrying ability is not required for the gearing Power generation and industrial drives and their control systems where special properties of the fluid are advantageous, for example oxidation stability, low temperature properties. Power generation and industrial drives and their associated control systems with need for fire- resistance Power generation and industrial drives; marine geared drives and their associated control systems where the gearing requires improved load-canying ability.

Normal service

Highly refined petroleum oil with rust protection and oxidation stability

TGA

Higher temperature service

Highly refined petroleum oil with rust protection and improved oxidation stability

TGB

Special properties

Synthetic fluids with no specific fire-resistance properties"'*

TGC

Fire-resistant

Phosphate ester lubricant

TGD

High load-carrying ability

Highly refined petroleum oil with rust protection, oxidation stability and enhanced load-carrying ability

TGE

Fire-resistant

Phosphate ester control fluid

TCD

Aircraft^ Hydraulic"

Power generation and industrial drives and their associated control systems. Marine drives where improved load-carrying ability is not required for the gearing Power generation and industrial drives and their associated control systems where high temperature resistance is required due to hot spot temperatures Power generation and industrial drives and their control systems where special properties are advantageous, for example oxidation stability, low temperature properties. Power generation and industrial drives and their associated control systems with need for fire-resistance Power generation and industrial drives; marine geared drives and their associated control systems where the gearing requires improved load-carrying ability Steam, gas, hydraulic turbine control mechanisms where the fluid supply is separate from the lubricant and fire-resistance is needed

TA TH

"These products may not be compatible with petroleum-based products. 'This category Includes synthetic hydrocarbons as well as other chemical types. "Classifications for these categories have not been established. Reproduced with permission of the International Organization for Standardization, Geneva, Switzerland.

Specifications typically include r e q u i r e m e n t s o n b o t h the c h e m i c a l a n d p h y s i c a l c h a r a c t e r i s t i c s of t h e l u b r i c a t i n g oil o r h y d r a u l i c fluid. T h e y a l s o i n c l u d e ( b u t o f t e n fail t o differentiate b e t w e e n ) the so-called "type" tests t h a t identify a general level of p e r f o r m a n c e a n d r o u t i n e t e s t s u s e d for r e g u l a r q u a l ity c o n t r o l . T h e f o r m e r r e q u i r e m e n t c o u l d i n c l u d e l o n g t e r m

o x i d a t i o n t e s t s (for e x a m p l e t h e A S T M D 9 4 3 o x i d a t i o n t e s t referred to above) o r large-scale fire-resistance tests t h a t are clearly impossible and/or u n n e c e s s a r y to evaluate on a b a t c h p r o d u c t i o n b a s i s . A n e x c e p t i o n t o t h i s s i t u a t i o n is for m i l i t a r y a v i a t i o n s p e c i f i c a t i o n s , e.g., M I L - P R F - 2 3 6 9 9 F w h e r e t h e Ryd e r G e a r T e s t is p e r f o r m e d o n t h e first b a t c h of e a c h c o n t r a c t

CHAPTER 12: TURBINE LUBRICATING awarded and the bearing test performed on the first three full-scale production batches of any newly qualified oil. The co-existence of both international and national standards together with those of the individual turbine builders and end users is, perhaps, surprising. Although the publication of a European (EN) Standard requires the withdrawal of any competing national standcird, this is not the case for ISO (International Organization for Standardization) standards.

OILS AND HYDRAULIC FLUIDS

325

ISO/EN/national standards are also minimum requirements and, in some cases, may not be severe enough to meet the manufacturer's or user's requirements. In such cases the latter's specifications may still be used. National and international standards also require considerable time for revision, a process that may be inadequate for manufacturers seeking to make rapid changes in technical requirements to respond quickly to the concerns of industry.

TABLE 15—International and national specifications for turbine oils and hydraulic fluids. International or National Specifications International Standards Organisation International Electrotechnical Commission European Standard Canada China France Germany Japan India Russia UK USA

Steam Turbines

Industrial Gas Turbines

ISO 8068

Fire-Resistant Hydraulic Fluid

Aero-Derivative Gas

ISO 8068

CD 10050 lEC 61221 EN61221

3-GP-357Mb DL-571

DL/T 571-95 AIR 3514A AIR 3517

DIN 51515 nS-K-2213 IS 1012 Tp-22CTU 38.101821 Tp-22BTU 38.401-58 BS489 ASTM D 4304 ASTM D 4293

In preparation JISK-2213 IS 1012

DIN 51518 COST 12245-66 COST 13076-86

ASTM D 4304 ASTM D 4293

TABLE 16—Turbine builders and utility specifications for lubricating oils and hydraulic fluids. Turbine Builder/Utility Alstom Power (France/UK) Alstom Power Mannheim (Germany) Alstom Power (Switzerland) Alstom Power (Sweden)

Alstom Energy Lincoln, (UK) Alstom Power (Germany) Ansaldo BHEL EDF Fuji Electric General ElectricMedium & Large Steam Turbines General Electric-Industrial Steam Turbines Hitachi Kawasaki Mitsubishi Heavy Industries

Steam Turbine Oil Specification

Industrial Gas Turbine Oil Specification

Aviation Gas Turbine Oil Specification

NBAP50001A

SBVPRlOOl C

DIN 51515

HTGD 690149 VOOOIK

H G T D 9 0 117V001Q 812101 812102 812106 812107

HTGD 690 149 VOOOIK 81 23 00

812101 812102 812106 812107 65/0027

QM44-101/B IS 1012 HN 20-S-30 JISK-2213 GEK 46506D

QM44-100/B 602W917 ST 22007 HN20-S-41 Siemens TLV 9013.04 GEK 32568C GEK 101941 GEK-28136A*

D50TF1-S4

Solar Toshiba Siemens Westinghouse

GEK 46357E

165A974CE JISK-2213 GEK 4506D JISK2213 769 45192 JISK2213

GEK 32568 (mod) GEK 101941 (mod) H T G D 9 0 117(ABB) JISK2213

GEK 46357E

STM-1840

National Power (UK) Pratt and Whitney Siemens

Fire-resistant Hydraulic Fluid Specifications

Procurement Specification 207001, Part 9 521CTypell TLV9013/04 TLV 9016 03/02 (hydraulic oil) JISK-2213 55125Z3

TLV 9013/04

TLV:9012 01/05

ES 9-224 GEK 32568C 55125Z3

GEK 46357E 53740AL

326

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TABLE 17—Military specifications for turbine oils and hydraulic fluids. Country

Steam Turbine Oil

Belgium

BN-PO-175A

Canada France

3GP-357-Mb STM 7220B

Italy UK

MM-O-2001 Def Stan. 91-25/3

USA

MIL-L-17331H

Industrial Gas Turbine Oil

Aviation Gas Turbine Oil

BA-PO-lOeA" BA-PO-115A° BA-PO-103B'' BA-PO-109C AIR3514/A AIR3515/A-B'' AIR3516/A" Def. Stan 91-93 Def.Stan 91-94 Def.Stan 91-97/1" Def.Stan 91-98/1 Def.Stan 91-99/1" Def.Stan 91-100 Def.Stan 91-101 MIL-L-aOSlC MIL-PRF-7808L MIL-PRF-23699F

' Denotes a mineral oil-based product. The remainders are synthetic-based lubricants.

Table 17 lists some of the current military specifications for turbine oils. Where no standard is listed for a specific country it is probable that one of the standards developed by other countries (particularly the USA and the UK) is adopted for use. As can be seen, there are no military standards available for industrial gas turbines or fire-resistant turbine hydraulic fluids. The reason is that few industrial gas turbines are used in this application because of limitations on size and weight. There are military applications for fire-resistant fluids but these are not in turbines. One other complication in the chart is that there are several specifications available for aviation turbine lubricants. These are divided between mineral oil (extreme pressure and non-extreme pressure grades) and synthetic-based products. The latter are further subdivided according to the different fluid viscosities.

PERFORMANCE REQUIREMENTS, THEIR SIGNIFICANCE AND EVALUATION As might be expected in an application that may be using the latest technological advances, the technical requirements can be extensive and severe. This is also a result of providing a resource on which industry and the consumer totcdly depend and on which the safety of large numbers of the traveling public rely. Table 18 illustrates the technical requirements for: 1. A steam or industrial gas turbine oil without additional load carrying capacity (T5^e 1 oils listed in ASTM D 4304, Standard Specification for Mineral Lubricating Oil Used in Steam or Gas Turbines). 2. An aero-derivative gas t u r b i n e lubricant (MIL-PERF23699F, Lubricating Oil, Aircraft Turbine Engine Oil, Synthetic Base, NATO Code N u m b e r 0-156, standard grade). 3. A fire-resistant turbine control fluid (lEC 61221, Petroleum Products and Lubricants-Triaryl Phosphate Ester Turbine Control Fluids (Category ISO-L-TCD)—Specifications.

In each case the data in the table is given on one viscosity grade only as all these specifications contain multiple requirements. These would include, for example, a range of different viscosities; applications with additional load-carrying requirements; or, in the case of the aviation lubricant, grades with corrosion inhibiting or thermal stability requirements. For information on the grades not mentioned, reference should be made to the appropriate specification. As will be seen, there is commonality in some physiccd and chemical tests but considerable differences exist as far as "performance" or "type" tests (for example, stability a n d load-carrying requirements) are concerned. The following comments explain the background to the incorporation of the different performance requirements into the specifications and outline the procedures available for their determination. Table 19 lists the common ASTM, Federal Standard Method, or ISO standard tests used in turbine oil specifications; however, not all standards for the same property are identical. Viscosity The viscosity of a liquid can be regarded as its resistance to flow, but is more precisely a measure of the interned friction as the molecules move relative to one another. The property is important as it enables us to rank oils and fluids in terms of their relative "thickness" and to define the appropriate "level of thickness" necessary to ensure the presence of an adequate lubricating film in the p u m p , bearings, or gears throughout the operating temperature range of the equipment, including low temperature start-up capability. It was noted earlier, for example, that military requirements necessitate the use of a low viscosity lubricant for engine start-up in very cold climates. At the other extreme, when using mineral oils in turbines operating in high temperature environments, it may be necessary to use higher viscosity products to achieve adequate lubrication. However, if a lubricant has too high a viscosity, cavitation will occur while too low a viscosity can result in p u m p slippage and internal leakage (Fig. 23

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 327 [59]). Also related to viscosity are volatility and, to a limited extent, flash point properties. The most c o m m o n procedure for measuring kinematic viscosity (ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids) involves measuring the time for a fixed volume of liquid to flow under gravity through a calibrated glass capillary viscometer. Although in the past, 100°F and 210°F were standard tempera-

tures for measuring viscosity, the most widely-used temperatures are now 40°C and 100°C, and these are cJso the basis for the determination of viscosity index and viscosity classifications (see below). The former temperatures, however, are still frequently quoted and care is needed to avoid confusing the different data. It is possible to measure viscosity over a wide temperature retnge using this method and Fig. 24 illustrates the viscosity/temperature relationships for the major fluids

TABLE 18—A comparison of the technical requirements for turbine oils and fire-resistant hydraulic fluids. Test Method Property

Viscosity Grade Appearance (20°C) Kinematic Viscosity (cSt) at 40X

Steam/Industrial Gas Turbine Oil

Aero-Derivative Gas Turbine Oil

Fire-Resistant Hydraulic Fluid

ASTM

32 Clear and bright

22

46

D2422

3448

D445

3104

28.8-35.2

41.4-50.6

-6 180

>23 4.9-5.4 13000 6 -54 246

D97 D92

3016

Report

1.0

100°C

-40X Vise. Change after 72 h at -40°C (%) Pour point (°C) Flash Point (X) Density at 15°C Total Acid No (mg KOH/g)

max mEix max min max max

-18 1.2 0.1

FED-STD-791 etc.

ISO etc.

3675 6619

D974 SAE ARP 5088

Foaming Tendency/Stability Sequence 1 Sequence 11 Sequence 111 Air Release at 50°C Chlorine Content (mg/kg) Water Content (%) Volume Resistivity at 20°C (Mfim) 60247 Emulsion Characteristics at 54°C min. to 3 ml emulsion Evaporation Loss (%) Rust Preventing Properties —Procedure A Rubber Compatibility (% swell) Corrosiveness to Copper Rating Fluid Compatibility-sediment (mg/L) Trace Metal Content (ppm) Shear Stability-Viscosity Loss at 100°F (%) Bearing Deposition-demerit rating Thermal Stability & Corrosivity at 274°C -Viscosity Change (%) -Acid Number Change (mg KOH/g) -Metal Weight Change (mg/cm^) Oxidation Stability h to acid number 2.0 Oxidation Stability-min. to 175kPa drop Oxidation Stability Viscosity change (%) Total Acid Number Change Sludge Content (mg/100 mL) Wt. change (mg/cm^)-Fe Wt. change (mg/cm^)-Cu Wt. change (mg/cm^)-Ag Wt. change (mg/cm^)-Mg Wt. change (mg/cm^)-Ti Wt. change (mg/cm^)-Al Oxidation Stability Acid number increase (mg KOH/g) Wt. loss-Fe (mg) Wt. loss-Cu (mg)

D892 max max max m£ix max max mm

6247

5

150/0 25/0 150/0 6 50 0.1 40

D3427

9120 15597 760 lEC

30

15

D 1401

6614

50/0

25/0 25/0 25/0

10

D972 D665

25-May

D6546 D 130

Pass

3604/3433

6072

3403 Atomic Emission Spectr.

max max

20 Various (1-11)

max

4

max

80

max max max min

5.0 6.0 4.0 2000

D943

min

350

D2272

D2603 3410 3411

5308 max max max max max max max max max max max max

- 5 to + 25 3.0 25/50 ±0.2 ±0.4 ±0.2 ±0.2 ±0.2 ±0.2 15595 1.5 1.0 2.0

328

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

TABLE 19—A summary of the principal test methods for evaluating turbine oil properties. Property

ASTM Method

ISO Procedure

Color Kinematic Viscosity Low Temperature Viscosity and Viscosity Stability Viscosity Index Viscosity Classification Density-hydrometer method Density-digital density meter Acid Number-Colorimetric Potentiometric

D 1500 D445 D2532

3104

D2270 D2422 D1298 D4052 D974 D664

2909 3448 3675 12185 6618 6619

Water Content

D 1744

Pour Point Evaporation Loss Flash/Fire Points-Open Cup Autoignition Temperature Spray Ignition

D97 D972 D92 D2155/E659

760 12937 3016

Hot Manifold Ignition Wick Ignition Foaming Air Release Demulsification Shear Stability-Oxidation Stability

D5306 D892 D3427 D 1401 D2711 D943 D2272 D5846

2592 3988 15029 20823 14935 6247 9120 6614

Others (FED-STD-791 Unless Specified) 102 305 307 9111

5105 5106 SAE ARP 5088 3253 201 351 1103 1152 Factory Mutual Std. 6930 6053 3213

5307/5308 Hydrol5^ic Stability s h e a r Stability-diesel injector Shear Stability-sonic oscillator Shear Stability-tapered bearing Thermal Stability Particle Contamination-optical Particle Contamination-microscopic Particle Contamination-gravimetric Sizing particles Trace Sediment Copper Corrosion Rust Preventing Characteristics Corrosion prevention Seal Compatibility Fluid Compatibility Electrical Resistance 4-Ball Wear Test Vane P u m p Test Gear Test-Ryder Gear Test-FZG Bearing Test Deposition Test

D2619

15595 15596 CETOPRP 112H

D2603/D5621 D2070 F661 F313 D4898 D2273 D 130 D 665/D 3603 D 1748

CEC L-45-A-99 2508/3411 4402 4407/8 4405 4406 2160 4404 6072

D 1169 D4172 D2882 D 1947 D5182

used in turbine fluids and lubricants. Very low temperature viscosities (which are required for aviation gas turbine lubricants in view of their potential operation in very cold environments) are measured by a slightly different procedure. This involves not only a determination of viscosity at - 4 0 or -65°F but also a cold "soak" to check whether the fluid changes viscosity on prolonged exposure to low temperatures (ASTM D 2532, Standard Test Method for Viscosity and Viscosity Change After Standing at Low Temperatures of Aircraft Turbine Lubricants). This test is in addition to a requirement for stability at -18°C for six weeks without crystallization, separation, or gelling.

3009

3004/3010 5325 4011 3604/3432 3403 lEC 60247 CETOP RP 67H

20763 14635

6508 DIN 51354 3410 5003

Other ASTM standards that involve the measurement of viscosity include the determination of viscosity index (ASTM D 2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C). Viscosity Index is a way of representing the change in viscosity of an oil or fluid with temperature relative to two reference mineral oils arbitrarily assigned viscosity indices of 0 and 100. The lower the change of viscosity, the higher the Viscosity Index. Additionally, the classification scheme ASTM D 2422, Standard Classification of Industrial Fluid Lubricants by Viscosity System, (equiveJent to ISO 3448) enables fluids and lubricants to be classified according to their viscosity at 40°C (in centistokes)

CHAPTER

12: TURBINE

in a series of eighteen grades. These range iirom 2-1500 cSt and the width of each band is ± 1 0 % of the mid-point. This scheme is widely used to identify the level of viscosity required for different applications. The ASTM D 4304 turbine oil standard, for example, has limits on fluids u p to an ISO 150 viscosity grade while at least one major steam turbine builder specifies an ISO VG 100 hydraulic oil. However, the common viscosity grades for both hydraulic cind lubricating oils in industrial steam and gas turbines Eire 32 and 46 while products with ISO viscosity grades 10 and 22 are found in aero-engine oil applications. Pour Point/Low Temperature Storage Stability In addition to avoiding thickening at low temperatures, either in use or in storage, it is also important to ensure that the fluid in service does not solidify or precipitate. Such behavior could lead to pumping problems or operational failure. Conventional solvent-refined mineral oils tend to precipitate wax at low temperatures; the hydrocracked products m u c h less so and not at all with PAOs. In comparison, the synthetic esters may, if wet, become turbid at low temperatures and also more viscous on storage. Phosphates normally solidify without precipitation b u t may also become turbid in the presence of water. Additive incompatibility at low temperatures can also adversely influence the low temperature behavior of the oil or fluid and it is mainly for this reason that aviation gas turbine lubricants are tested for extended storage stability. The qualification sample for approval against MIL-PRF-23699F, for instance, is stored at temperatures ranging from - 4 0 ° C to +60°C for a period of three yeeirs. If at any time during this period the fluid fails to meet any of the technical criteria, the approved is withdrawn. The normal procedure for determining p o u r point (ASTM D 97, Standard Test Method for Pour Point of Petroleum

(mm2/sec) 10 000 5000 Cavitation

m

8

2000 4Jpp6^viscosity limit 1000 500 Slow respoRse-

X3

(0 • >

,g 'Jam

ro E

(D C

7)

o

100 50 20 10 5

Low efficieaiey=

3 S 3 5' CD 3

Q. CD O.

i

3 CO (D

Reduced volumetRelfticiency -4i^3lv3Q0S

1 FIG. 23—Significance of viscosity for tlie operation of a typical hydrostatic system. Reproduced from "Hydraulic Fluids," with kind permission of Butterworth-Helnemann, Oxford, UK.

LUBRICATING

OILS AND HYDRAULIC

FLUIDS

329

Products) involves cooling a fixed volume in a cylindrical container with a thermometer inserted just below the surface. As the t e m p e r a t u r e is lowered, the flow behavior is noted by holding the container horizontal for 5 s and watching for movement in the meniscus. The pour point is that temperature w h e n the fluid ceases to flow under these conditions and to which three degrees centigrade has been added. Typical limits for Type 1 turbine oils (of all viscosities) given in ASTM D 4304 are - 6 ° C maximum. Acid N u m b e r The acidity present in the new oil or fluid arises from two sources: 1. Residual acidity from manufacture (in the case of the ester-based products), 2. From the presence of additives—usually acidic rust inhibitors and/or metal passivators, but also some antioxidants. A high level of acid in a n unused, inhibited, fluid is therefore not necessarily indicative of a poorly processed product or one that has aged significantly on storage. The latest version of MIL-PRF-23699, for example, has a limit of 1.0 mg KOH/g. On the other haind the presence of a high level of acidity cem be disadvantageous to surface active properties, for example foaming (ASTM D 892) and air release (ASTM D 3427). It may cause corrosion (ASTM D 665) and promote oxidation (ASTM D 2272). The level of acid in new, uninhibited, phosphate esters is tighfly controlled because this may contain strong acid, with a p H of < 4 , which can catalyze the hydrolytic degradation process. A low initicd acidity in esterbased products therefore ensures good storage stability, a satisfactory condition on filling into the system and a longer operating life. As will be shown later, the acidity level is a n important indicator of fluid "health." Monitoring this parameter is therefore essential to ensure the trouble-free operation of the system. Two techniques are used for the measurement of acidity or acid n u m b e r of turbine oils and fluids. Both involve neutralizing the acid by a base (potassium hydroxide) of known strength in a measured quantity of fluid. One procedure uses indicators that change color when the sample is neutralized (ASTM D 974, Standard Test Method for Acid and Base Number by Color Indicator Titration). The other method measures the acid n u m b e r by plotting the changes in the potential of a glass electrode as the alkali is added. The amount required to produce an inflexion point on the graph is taken as the neutralization point. Where inflexion points are not readily defined, a predetermined end point, for example a p H of 11, is frequently used (ASTM D 664, Standard Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration). While b o t h procedures give similar results on fresh fluids, their accuracy, peirticularly reproducibility, is acceptable rather t h a n good. For example, in the colorimetric technique where the initial acidity lies below 0.1 m g KOH/g, the reproducibility is 0.04 m g KOH/g. More difficult still is the accurate determination of used fluid acidity where the fluid color has darkened or when the fluid has been dyed. This makes indicator end points more difficult to detect and precision may suffer with increasing acidity. The trend is therefore towards the use of the potentiometric method for

330 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 5.0

(0

o .52 "c

o

E

(U

c

200

0

10

20

40

55

65

75

85

degrees farenheit FIG. 24—Viscosity/temperature data for different turbine oil and fluid.

100

CHAPTER 12: TURBINE LUBRICATING fluids in service. For aviation lubricants a potentiometric method has been developed by the Society of Automotive Engineers (SAE-ARP 5088) which is now included in MIL-PRF23699F. In the past, determination of acidity was traditionally carried at the close of an oxidation test. However, because the major criterion for failure in the D 943 method was the time taken to reach a certain level of acidity, it was necessary to follow the development of this parameter throughout the test. This required regular sampling of up to 20 g of fluid per determination. For long-term tests this could significantly reduce the fluid volume in the test and would therefore disturb the relationship between catalyst surface area and the fluid and also between water and the fluid, making it difficult to compare test results. To reduce the effect of sampling on the oxidation test conditions, methods of measuring acidity on much smaller samples were developed. Currently ASTM D 3339, Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Indicator Titration, and D 5770, Standctrd Test Method for Semi-quantitative Micro Determination of Acid Number of Lubricating Oils During Oxidation Testing, are used. The former method uses up to 5 g of fluid while the latter uses only "drops" of fluid. The D 5770 procedure is regarded as less precise than D 974 or D 664 and is not recommended for monitoring oils in service. Its principal use is therefore likely to be product development and the method advises that "each laboratory shall develop its own criteria for determining when to switch from this method to a more precise test method for acid number." Figure 25 shows typical automatic titration equipment used for determining total acid number on a series of samples. Specifications vary considerably in their requirements

OILS AND HYDRAULIC FLUIDS

331

on initial acidity. ASTM D 4304 has no limits but values are to be reported. Some turbine manufacturers also have no requirement while others call for a limit of 0.2 mg KOH/g maximum for non-geared units and 0.3 mg KOH/g for geeired turbines. Higher limits are found for the ester-based aviation lubricants. MIL-PRF-7808L, for example, specifies 0.3 or 0.5 mg KOH/g max. while at the other extreme, the specification limit on fire-resistant control fluids is typically 0.1 mg KOH/g maximum. Water Content Water is a problem contaminant for all tjrpes of fluid. It can be present in either dissolved or dispersed form and, in the case of major contamination, may form a completely separate layer. When the main reservoir contains mineral oil, free water normally falls to the bottom of the tank owing to the different density and requires removal by means of the vaJve on the tank base or via a sediment drain to the tank side. If the fluid has a density greater than that of water, as in the case of phosphate ester, the free water layer will be on top of the fluid in the tank and should be skimmed or siphoned off. Other methods used for free water removal include centrifugal separation and water absorbing filters. To remove dissolved water, vacuum dehydration is recommended. The latter technique may take the form of an extractor on the tank encouraging the flow of a stream of dry air across the surface of the oil or fluid. Alternatively, a vacuum dryer may be used on a by-pass to the main oil tank in which a thin film of the oil or fluid is exposed to a counter-current of dry air. This technique reduces the water level more rapidly. Although water contamination is more likely on steam turbines, it can

FIG. 25—Equipment for the measurement of acid number by automatic titration.

332

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37: FUELS AND LUBRICANTS

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also occur on gas turbines as these are occasionally washed to remove deposits forming on blades, etc. The solubility of water in turbine oils is very low—about 30-100 mg/kg at ambient temperatures, and excess water in new mineral oils can usually be detected visually as it causes turbidity. With esters, the solubility is m u c h higher, in the range of 2000-2500 mg/kg at ambient temperatures although levels in service are normally in the region of 500-1500 mg/kg. Small amounts of water can normally be tolerated, and are common in used steam turbine oils. While dissolved water can cause a reduction in viscosity, hydrolysis of additives (or even of the ester fluids themselves) and hence a loss of stability, etc., it is dispersed or free water that is the usual cause of emulsification, rusting, and possible extraction of additives. With ester fluids, hydrolysis is the normal form of degradation and the acid formed can promote further degradation. Monitoring the water content of oils or fluids is therefore a n essential part of any fluid maintenance program. The procedure used for detecting water has to be capable of accurately measuring very small amounts and the technique used is to titrate a measured a m o u n t of fluid or oil with standard Karl Fischer reagent to an electrometric end point. The most c o m m o n method is ASTM D 1744, Standard Test Method for the Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent. Precision has been established for levels between 50 and 1000 mg/kg, but the method is frequently used for detecting m u c h higher values, peirticularly on used ester fluids. Limits are not frequently specified for new mineral turbine oils in view of the low solubility of w a t e r a n d the reliance on visual appearance (free w a t e r causes turbidity). Occasionally limits of 0.01-0.02% w/w (100-200 mg/kg) are quoted while, for fire-resistant control fluids and aviation esters, the values are significantly greater (0.1 % w/w or 1000 mg/kg), reflecting the higher solubility and the hygroscopic (water absorbing) nature of these fluids. Density Measurement of density or specific gravity is one way of identifying the use of the correct fluid and ensuring that the correct volume of fluid is purchased if the product is supplied by weight. It is also a n important factor for designers in determining the p o w e r required for p u m p i n g the oil or fluid around the system and in arranging for removal of free water. Phosphate esters, for example, have densities about 30% greater than mineral oils and require more power for circulation. Also, ciny free water in the tank will accumulate on top of the phosphate. This fluid t3rpe will hold more dirt in suspension than other fluids, which necessitates better filtration initiedly, but offers the possibility of a cleaner system in operation. Density or relative density (specific gravity) values for both transparent and opaque liquids Ccin be obtained either by the use of a hydrometer (ASTM D 1298, Standard Practice for Density, Relative Density (Specific Gravity) or API Gravity of Crude Petroleum and Petroleum Products by Hydrometer Method, or the ISO 3675 equivalent), or by a digital densitometer (ASTM D 5002, Standard Test Method for Density and Relative Density of Crude Oils by Digital Analyzer, for which the ISO equivalent method is 12985). Values are normally measured at 15 or 20°C, but can be calculated at other tem-

peratures from knowledge of the values at these or two other temperatures. API Gravity is a special function of relative density, and is obtained by hydrometer measurement carried out at 60°F (15.6°C). This test is not normally applied to non-hydrocarbon oils and gives values that are numerically significantly higher than the other procedures mentioned. Density limits featuring in specifications depend on the type of fluid being used. Mineral oils, for example, are typically quoted at 0.9 kg/1 maximum at 15°C while the API gravity for an ISO 32 gas turbine oil is 29-33.5. Triaryl phosphates have a substantially higher density and limits on this fluid would typically be 1.17 kg/1 maximum for an ISO VG 46 product. F o a m i n g a n d Air R e l e a s e Air is a "contaminant" of every turbine oil and fluid. Whether or not it causes problems it depends largely on whether it is soluble in the oil, or present as dispersed bubbles. System design, circulation rates, fluid cleanliness, etc., are also factors to be considered. Dissolved air is not normally a concern but if it comes out of solution when pressure is reduced locally it may cause p u m p cavitation. Dispersed air tends to be a problem if it is not readily released from the bulk of the fluid in the tank and compression of the bubbles by the p u m p does not cause their dissolution. Under such conditions dispersed air can cause a loss of control due to a change in the compressibility of the fluid, result in increased oxidation, and may have an adverse effect on lubrication. Foam is the accumulation of air bubbles surrounded by a thin film of oil and occurs at the surface of the liquid. Air release measurements, by contrast, are made in the bulk of the liquid where a relatively thick film of oil separates the air bubbles. There are a n u m b e r of factors relating to fluid properties, system design and use that can influence foam formation. These include tank design and circulation rates [60], fluid viscosity, surface tension, vapor pressure, contamination, bubble size [61], air leaks on suction lines, or simply too low an oil level in the tank. Some foam is inevitable as air is released from the oil or fluid when the pressure returns to ambient. In most systems the presence of a small a m o u n t of foam can be tolerated without any significant adverse effect on fluid/system performance. In extreme cases, excess foam may reduce lubrication performance, induce bearing vibration, cause fluid oxidation or loss of fluid, and create m u c h inconvenience to maintenance staff. In hydraulic systems operating at high pressures, the presence of dispersed air will depend on the system pressure, the size of the air bubbles, fluid temperature, and the compression time in the p u m p . Where the bubble is small, the compression time relatively slow, Eind the pressure Eind temperat u r e high, there is only a slight chance of bubbles being circulated that could affect the compressibility of the fluid euid hence the response time of the system. Conversely, if the bubbles do not readily dissolve under pressure, not only is there a risk of a loss of control but there may also be a n associated lubrication failure, fluid oxidation, and also the possibility of "dieseling." This is a phenomenon where compression/ignition takes place inside the bubble with resulting fluid degradation [62]. In turbine lubrication systems, pressures are normally too low for this phenomenon to occur but

CHAPTER 12: TURBINE LUBRICATING it is occasionally found in high pressure hydrauHc systems resuUing in a rapid darkening of the fluid, an increase in acidity, and the development of a "humt" odor. In addition to the D 892 foam test method discussed above, military aviation lubricant specifications also call for the use of a dynamic foam test, FED-STD 791, method 3214 in which the oil is heated and circulated around a loop consisting of a pump, oil heater, air injection orifice, foam test cell, and associated instrumentation. As the oil is circulated at a temperature of 80°C or 110°C, the sample is aerated for a period of 30 min at a fixed rate with measurements being taken of the foam, etc., every five minutes. At the end of the test, the foam level, oil pressure and oil volume are noted together with the time taken for the first patch of clear surface to appear or the foam remaining after 5 min. The test is then repeated with a different air-flow rate or at a higher temperature. The equipment is able to operate at low pressures thereby simulating the effect of high altitude flight. Typical foam limits for mineral turbine oils vary significantly and can also depend on whether the application is in a steam or gas turbine. Limits in the latter application are usually stricter in view of the high aeration of the gas turbine oil as it leaves the bearings. For both steam and gas turbines.

OILS AND HYDRAULIC FLUIDS

therefore, foaming tendency/stability values of 50/0 mL. for all conditions (as defined in the ASTM D 892 test described above) are required in ASTM D 4304 while, for steam turbines, the builder's requirements can be as high as 450/10, 50/10, and 450/10 mL for the three test sequences. In gas turbines, some equipment memufacturer's limits are 10/0, 20/0, and 10/0 mL. For fire-resistant control fluids, limits are also usually fairly severe at 50/0 ml. (all sequences) ranging up to 150/0, 50/0, and 150/0 mL. For aviation gas turbine oils, limits are also severe with the MIL-PERF-23699 requiring 25/0 mL metximum at all temperatures. Unfortunately, the laboratory test cannot simulate the conditions that are found in the turbine; such is the influence of system design. As a result, the test data should be treated as indicative of a trend in fluid behavior and any significant change, particularly the generation of stable foam, be initially confirmed by investigating the behavior in the tank itself. Air release properties (ASTM D 3427, Standard Test Method for the Air Release Properties of Petroleum Oils, or ISO 9120) are similarly measured by saturating the fluid (normally at 50°C, but other temperatures are also possible) with air bubbles and then measuring the time it takes for the fluid to return to an air content of 0.2%. Figure 26 shows the

c (D

Group 1 mineral oil

Groups mineral oil

333

ISOVG32 polyol ester

ISOVG46 PAO

ISOVG46 phosphate ester

FIG. 26—The effect of temperature on air release values for different turbine oils and fluids.

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air release properties over a range of operating temperatures for various turbine oils and fluids. Typical specification requirements for mineral oils, e.g., as required by ASTM D 4304, vary from 5 min (ISO VG 32), through 7 min (ISO VG 46) to 17 min (ISO VG 150) for nonEP oils and up to 25 minutes for products with improved load-carrying capacity. As would be expected, air release values increase with viscosity and this is reflected in these limits. Turbine manufacturers tend to have more severe requirements with 3-5 min m a x i m u m frequently specified for both steam and gas turbines. The air release properties of phosphate esters vary significantly with chemical type. The "natural" phosphates such as trixylyl phosphates tend to have very low air release values of about 1 min while the synthetic types (ISO VG 46 grade) are in the region of 5-8 min, hence specification limits for fire-resistant control fluids are frequently of the latter order. It is probable that m a n y fluid-related problems could be avoided if there was a greater awareness of the impact of system design on air entrainment.

Chlorine Content Limits on chlorine content are required with phosphates in order to avoid servo-valve erosion [63]. In fact it is the chloride ion (CI ) level that is critical and this can arise either as a trace contaminant from the manufacturing process (from the use of phosphorus oxychloride) or by air-bom contamination. Another source is the incorrect use of chlorinated solvents for cleaning system components. Although the chlorine in solvents is not initially present in ionic form it is thought that the solvents break down thermally in the system to release chloride ion. While limits are placed on new fluid levels (normally 50 ppm), the content of chlorine in fresh fluid is usually significantly lower (typically 20-30 p p m ) . In spite of the importance of the level of chloride ion in the fluid, it is rarely specified for new fluid and the limits that have appeared in the past are at much lower levels, e.g., 0.5-2 ppm. The accurate measurement of very low levels of chlorine is not easy and the standard b o m b calorimeter tests are not sufficiently accurate, as the combustion of phosphates by macro techniques is usually incomplete. ASTM D 808, Standard Test Method for Chlorine in New and Used Petroleum Products (Bomb Method), a typical bomb calorimeter process, for example, is only valid for chlorine levels of 0.1-50%. Instead, microcoulometry (e.g., IP proposed method AK99), a combustion process which uses very small amounts of sample is preferred, or possibly X-ray fluorescence (ISO 15597). The basis of all the calorimeter methods is the titration with silver nitrate of the chloride ion produced by combustion. Due to the purity of phosphates in commercial production this parameter is rarely a problem today and, as will be shown later, is controlled in use by special filtration techniques.

Fire-resistance Tests These properties are the most important requirements for the phosphate esters and for the small quantities of polyol esters that are supplied as hydraulic fluids into this application. The origins of fires in turbines are divided almost equally between the lubricant and hydraulic fluid [64] and are normally a re-

sult of contact with a hot surface. An obvious example of this would be a hot steam pipe, where temperatures are now reaching 610°C. However, there are other potential sources of ignition such as bearing housings and gas turbine exhausts that may be hot enough to ignite oil vapor, if not the liquid itself [65]. Unfortunately, there is no single test that can satisfactorily predict fire behavior and several tests are necessary to obtain a more complete assessment of performance. Much depends on the form in which the fuel (in this case the oil or fluid) is present. For example, it is easier to ignite the fluid in the vapor state as the access to oxygen is greater (combustion is an oxidation reaction with the release of heat in the form of a flame). The evaluation of fire resistance by tests that simulate the hazard is therefore of crucial importance. While many tests have been devised to assess this property, three types are widely used: 1) Spray ignition behavior, 2) Hot surface ignition, and 3) The flammability behavior when the fluid is absorbed onto a substrate [66]. Simple flash and fire points (ASTM D 92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup) are used to identify very flammable (and volatile) materials but, for products of low volatility such as are used as fire-resistant fluids, flash and fire points do not necessarily relate to the performance of these fluids in other, more hazard-related, tests. A high fire point, for example, does not automatically mean good performance in spray or hot surface ignition tests (see Table 8 and [66]). Today, flash and fire point tests are used more for quality control purposes rather than for measuring fire resistance [66]. As indicated above, in turbines, the main hazard is a hot surface. Where the surface is lagged, the risk increases in view of exposure of thin films of oil or liquid to oxygen in the lagging. The wick test (below) does not properly simulate this condition as the oil film in the lagging (unless saturation is reached) is thinner and the access to oxygen is greater. However there is currently no standard method that accurately reproduces this condition. The test methods that are most widely used to assess fire resistance include the following: Spray Ignition

Tests

These are of two types: 1) Those that attempt to ignite the fluid by spraying at elevated t e m p e r a t u r e and pressure through a n open flame, noting whether ignition occurred and, if so, whether burning continued once it had moved away from the ignition source (ISO 15029, Parts 1 and 3, Petroleum and Related Products—Determination of Spray Ignition Characteristics of Fire-resistant Fluids). 2) Measuring the heat emitted by the burning fluid (Factory Mutual Test Standard 6930 for the Flammability of Industrial Fluids and ISO 15029-2). It might seem strange to assess the fire resistance of so-caJled fire-resistant fluids by deliberately igniting them, but "fire-resistant" in this context does not mean non-flammable. Most, if not cdl, fire-resistant fluids will combust if contacted by a high-energy source for a sufficient period of time. Past spray tests tended to be pass/fail methods with poor precision but the latest methods, such as ISO 15029-2 and the new Factory Mutual procedure, are able to rank fluid behavior with acceptable precision and generally in the same or-

CHAPTER

12: TURBINE

LUBRICATING

OILS AND HYDRAULIC

FLUIDS

335

exhaust channel anemometer exhaust gas thermocouple (Tp/Tex)

nozzle

\_

ambient air thermocouple

atomising air hydraulic fluid

FIG. 27—Schematic representation of the test chamber used for the ISO 15029-2 Spray Ignition Test. Reprinted with permission of the Health and Safety Laboratory, Sheffield, UK. der. Figures 27 and 28 are diagrammatic representations of the equipment used to carry out the above tests. Hot Surface Tests Although the latest spray ignition tests can indicate the relative fire resistance of fluids, the results tell us nothing about the temperature at which the fluid would ignite on contact with a hot surface. It is therefore important to obtain some idea of this aspect from appropriate procedures. Currently only one test is specified for turbine fluids, the determination of autoignition or s p o n t a n e o u s ignition t e m p e r a t u r e , by ASTM D 286 or D 2155 (both now obsolete) or by E 659, Standard Test Method for Autoignition Temperature of Liquid Chemicals. In each of these procedures a small, measured a m o u n t of the test fluid is inserted into a heated glass container (conical or round-bottomed flask). The temperature at which the fluid ignites with the production of a visible flame was the result recorded in the two earlier tests while the most recent variant also observes the production of "cool flames," which occur at lower temperatures than conventional hot flames. The recent procedures have resulted from attempts to obtain a more even temperature distribution throughout the container, while replacing the original molten lead bath, which was a health hazard. Major differences are found between the results of the three tests with the trend towards lower values with the later methods [66]. The reason for the differences in results is mainly due to the increased size of the container and therefore to the increased oxygen concentration present [67]. Unfortunately, the reduction in autoignition temperature values with recent methods has occasionally led the user to believe that an inferior product was being supplied and as a result data is still quoted on the earlier methods, sometimes together with the figures from the latest test. A further hot surface ignition method (The Hot Manifold Test), based on FED-STD-791 Method 6053, is currently being developed by ISO as Standard 20823 and may be preferred to autoignition temperature in the future as the conditions more obviously simulate a n industrial hazard. Wick Tests The flammability behavior of the fluid when it is absorbed into a substrate (or wick) can be assessed by ASTM D 5306,

S t a n d a r d Test Method for the Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids, which, as the title suggests, measures rate of flame travel along the wick. Alternatively, ISO 14935 records the time it takes for the burning fluid to self-extinguish after removal of the ignition source. I n the case of the ISO standard, the ignition source is an oxy-propane flame while in the ASTM method, matches are used to ignite the wick. Such a test might simulate the ignition of the fluid when soaked into cloth waste. In both tests, the unused fluid is usually evaluated whereas in reality the fluid may have been in contact with the absorbent over a period of time allowing volatile components, such as water, to escape. This could significantly alter the behavior of the fluid. Obviously where such tests are required it is expected that the fluid would meet the criteria for a "pass" irrespective of its condition. Figures 29 and 30 show the equipment required for the two tests. Water Separability/Demulsibility Tests As noted earlier, water is a common contaminant of steam turbine oils. It may arise as a result of the penetration of seals by steam, from cooler leaks, condensation in the tank or from steam valves dripping onto hydraulic actuators, etc. Although gas turbines are normally drier due to the absence of steam and the higher operating temperature, they also use waterfilled coolers and may be periodically washed with water to reduce the build-up of deposits within the turbine. The oil tanks for some gas turbines are also located out-of doors and may not be protected from the elements. Consequently, undesirable levels of water in turbine oils and hydraulic fluids can occasionally build u p . Free water is found more frequently with mineral oils because of its m u c h lower solubility in this medium. Water can affect turbine oil in m a n y ways: it promotes rusting and subsequent wear, causes a reduction in the oil stability—^possibly by partition of the additives into the water—and, in the event of the formation of an emulsion, has a n adverse effect on lubrication performance and may cause filter blockage [68]. The effects of water can be enhanced by the presence of other contaminants such as oxidation products, dirt, rust and other additive-treated mineral oils [69]. With both phosphate and polyol esters, the effect of water is

336

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fire products collector

1.25 m

nozzle

®

®

^ y - / ^

flexible ^/connectors

T

1.4 m

propane burner floor

MdNn '

sample pressure vessel

propane

load cell

/7777Z FIG. 28—Factory Mutual Research spray ignition test equipment. Reprinted with permission of Factory IVIutual Research, Norwood, MA.

to cause hydrolysis and acidity generation with a potential adverse effect on stability, rusting and corrosion performance, and surface active properties like emulsification. A high water content in the long chain polyol ester hydraulic fluid has also been found to promote bacterial growth which, in severe cases, has resulted in fluid gelatification [70]. Water must therefore be removed from the fluid or oil as quickly as possible. In steam turbine systems free water can be removed using a centrifuge and by filters coated with water-adsorbing polymers. Dissolved water is removed using either chemiccd adsorbents, for example a molecular sieve, or

by applying a vacuum to the fluid. Physical methods for drying used oil and fluid are practiced widely but currently the use of chemical adsorbents is only found with fire-resistant fluids. Although the behavior of the fluid in use is more critical due to the likely presence of contaminants and fluid degradation products, new fluid is tested to ensure that emulsification does not initially occur as a result of the use of surfaceactive products such as corrosion inhibitors. The method most widely used to assess this aspect of performance is ASTM D 1401, Standard Test Method for Water Separability of Petroleum Oils and Synthetic Fluids, (ISO

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRA ULIC FLUIDS 337

ceramic fibre cord

chart recorder 50 g weight

1

50 g weight

FIG. 29—Diagram of apparatus for determining linear flame propagation.

reservoir

wick

stop holes for adjustment < •

TF5^ burner position for setting flame height r

|l |l |l burner may be raised and lowered in clamp

stop bar from propane regulating valve FIG. 30—Equipment used for the ISO 14395 wicl( flame persistence test.

338 MANUAL 37: FUELS AND LUBRICANTS

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6614) where the fluid at 40°C is stirred with an equal volume of water and the time taken for separation of the resulting emulsion (if formed), is noted. Typical limits for separation to a residue of 3 mL emulsion are 30 min maximum (mineral oils) and 15 min maximum for phosphate esters. Stability Characteristics Oxidative Stability All fluids are assessed for their oxidative stability as their service lives depend to a large extent on this property, and the effects of fluid degradation in terms of acidity generation, viscosity increase, and the production of deposits can have an important impact on the system performance. The principal methods used for evaluating oxidative stability were indicated earlier in the discussion on the testing of antioxidants. These methods are additionally used to assess the effects of other additives, e.g., rust inhibitors on oxidation stability and assessing the performance of complete formulations. In addition to the Universal Oxidation Test designed to overcome the long test duration associated with the ASTM D 943 test, other approaches have been or are being evaluated. These include a dry version of the TOST and, for research and development, quality control and residual life assessments, high pressure differential scanning calorimetry (ASTM D 6186, Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scan-

50

ning Calorimetry (PDSC), [71]) and voltammetric techniques [72,73]. The PDSC method involves heating a known (small) quantity of the oil or fluid under pressure in oxygen at a fixed temperature until an exotherm or release of heat occurs. The time to the onset of the exotherm is reported as the induction time at the specified temperature. Although it is an extremely rapid test, "no correlation has yet been established with service performance." The Remaining Useful Life of a Lubricant Evaduation Technique (RULLET) has recently been developed to assist the maintenance of military aircraft engine oils. In order to determine the remaining useful life, it is first necessary to thermally and oxidatively stress a fresh oil in the laboratory at 150-175°C and monitor the chemges in acidity and viscosity as the test proceeds. When the changes in these values start to deviate from a steady increase, this point is taken as the end of the useful life in service and intermediate data on the oil are used to predict the percentage remaining life in service (Fig. 31 [73]). This method has good repeatability and apparently correlates well with RBOT and PDSC tests. Cyclic voltammetric techniques can be used to quantify the remaining antioxidant concentration in used oil or fluid [73]. This technique involves the extraction of the antioxidants by a solvent containing a dissolved electrolj^te. The solvent/electrolyte system will vary depend on the type of fluid and antioxidant being analyzed. Using the solvent/electrolyte, a voltage is applied across an electrode system consisting of a

Hours of stressing time 100 150 I l__

200

250 1-22

21

--20

--19 (^ o 18 b To

178 ' '

in

--16

-Lis 200

150

100

IHours of remaining useful life FIG. 31—Percent remaining useful life (RUL), viscosity (40°C) and total acid number against h of stressing time and remaining useful life at 175°C for a typical aircraft engine oil. Reproduced with permission of STLE, Park Ridge, IL.

CHAPTER 12: TURBINE LUBRICATING "glassy" carbon electrode, a platinum reference electrode, and a platinum auxiliary electrode. The voltage of the auxiliary platinum electrode is scanned from 0 to 1.5 V at 0.2 V/sec. Data is produced by measuring the current through the cell as a function of the applied potential and the different antioxidants produce a typical "signature" from which the composition can be deduced. Comparison of the current produced for each antioxidant with reference data on solutions containing known amounts of the stabilizer will reveal the level of the antioxidant in the used oil. It was found possible to correlate the remaining oil life with the residual antioxidant concentration, and both techniques can be of benefit in applying predictive maintenance. The importance of avoiding deposits in aviation gas turbine lubricants is such that an additional full-scale bearing test that simulates engine behavior is specified for these oils. Details of the test used to assess this aspect of performance are given in FED-STD-791 method 3410 and involve lubricating a heated 100 mm diameter roller bearing with the test fluid at an elevated temperature (between 300-400°F or 149_204°C) for 100 h. At the end of the test the bearing is assessed for deposits against a "demerit rating scale" while the weight of filter deposits and changes in oil viscosity and acidity are also measured. Thermal Stability Tests for this parameter fall into two categories: 1. Those carried out in the absence of air, e.g., FED-STD-791 method 3411 (despite the fact that it is almost impossible to find a condition where the oil or fluid is heated in the complete absence of air). 2. Those where oxygen is present and may cause further degradation over and above the physical and chemical changes resulting from the thermal degradation of the oil. An example of this type of test is ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils. The Federal standard method is currently only used in aviation turbine oil specifications, e.g., MIL-PRF-23699F, probably for historical reasons and to provide additional support for the oxidation stabihty data in applications where the temperatures are very high. This procedure involves sealing a portion of the fluid together with a steel specimen in a glass ampoule under vacuum, heating for 96 h at 274''C and then determining the changes in viscosity, acidity, and the weight of the metal catalyst. Limits of 5% on viscosity change and 6.0 mg KOH/g on acidity increase are called for in the above specification. The ASTM method involves heating the fluid (200 ml) in an open beaker together with iron and copper catalyst rods for 168 h at 135°C. At the completion of the test the color of the rods, which is the principal evaluation criterion, is assessed and the amount of sludge produced, determined. This procedure is used primarily to evaluate the stability of mineral hydraulic oils and is strongly influenced by the presence of additives. However, the standard notes, "No correlation of the test to field service has been made." By contrast the FEDSTD-791 method could be used to discriminate between ester types that display different levels of stability depending on their chemical structure. Diesters, for example, are able to find a decomposition path at lower temperatures due to their ability to more readily form a cyclic intermediate [74].

OILS AND HYDRAULIC FLUIDS

339

Hydrolytic Stability This property assesses the fluid stability in the presence of moisture. Normally the effect of water is to generate the acid and alcohol (or phenol) from which the ester was originally formed. All esters are sensitive in varying degrees to water and, since hydrolysis is the normal mode of breakdown, it is an important factor in determining the Hfe of both phosphates and the polyol ester fluids used in hydraulic and aero-engine lubricant applications. In steam turbine systems particularly, water contamination of the hydraulic fluid and lubricant is often found and requires regular monitoring. As noted earlier (Table 9) the structure of phosphates affects their hydrolytic stability and may influence the selection of product for use in steam (wet) or gas (dry) turbine applications. The test procedure most commonly used for assessing hydrolytic stability is ASTMD 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method). In this test the fluid and water are mixed together in a rotating sealed container for 48 h at 93°C in the presence of a solid copper catalyst, followed by measurement of the acidity level in both the fluid and the water layers and the metal weight change. Procedures are available for fluids that are more or less dense than water and Fig. 32 shows the bottle with oil, water, and catalyst coupon.

n li

FIG. 32—The ASTM D 2619 beverage bottle container with oil sample (upper layer), water, and copper catalyst.

340

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For steam turbine control fluids, ISO 15596 is used. It is a static procedure that is uncatalyzed and the acidity increase of both fluid and water layers is measured. In this test, a limit on total acidity increase of 0.5 mg KOH/g is specified. Shear Stability Shear stability is an additional requirement that is part of some specifications for non-hydrocarbon oils. This is the resistance of the fluid to shear stress as the fluid is forced through a neirrow aperture and which can result in the breakdown of long-chain molecules into smaller units. It is of particular importance for hydraulic applications in view of the high pressures and very fine tolerances involved in pumps and valves, particularly when the fluid contains a high-molecularweight polymer, as is the case with polyol ester fire-resistant hydraulic fluids [66]. However, no specification requirements yet exist for the shear stability of these fluids in steam or industrial gas turbine applications. Limits do exist, however, for some aero-gas turbine oils, e.g., in the MIL-PRF-23699F specification. This requirement is thought to date back to the submission of thickened diester lubricants against early versions of the specifications. The instability of these fluids resulted in the addition of a limit on this parameter. Turbine oils (of all t5rpes) are themselves quite shear stable. Specifications for fire-resistant fluids based on phosphate esters have, for many years, stipulated that polymeric thickeners are not to be used as additives. This was due to their removal by the conditioning media (usually fullers earth) with the resulting blocking of filter cartridges and a marked reduction in fluid viscosity. Shear stability has traditionally been measured by two techniques viz resistance to high-frequency sound and circulation under pressure through an orifice. Sonic Shear The technique specified in MIL-PRF-23699F requires exposure under prescribed conditions to vibrations from a sonic oscillator followed by a measurement of the change in viscosity (ASTM D 2603, Standard Test Method for Sonic Shear Stability of Polymer-Containing Oils or ASTM D 5621, Standard Test Method for the Sonic Shear Stability of Hydraulic Fluid). The scope of the early version of this method, which was last reviewed in 1998, indicated that while useful for quality control purposes, it did not show good correlation with the service behavior of polymer-containing oils and may rate different types of thickener in a different order to the diesel injector test. The latest method however, states, "Evidence has been presented that a good correlation exists between the shear degradation that results from sonic oscillation and that obtained in a vane pump test procedure." The principle of the test is first to degrade a reference fluid with a sonic probe at 0°C using sufficient power for 12.5 min to produce a viscosity decrease at 40°C of about 15%. Then, using the same power setting that gave the above level of viscosity change, to irradiate the test fluid at 0°C for 40 min followed by a determination of viscosity. The initial and irradiated viscosities at 40°C are reported. Resistance to Shear in a Diesel-Injector Nozzle The second procedure involves measuring the viscosity loss at 100°C after the fluid has been circulated through a diesel

injector nozzle set at a predetermined lifting pressure, for example 17.5 MPa (175 bar) for 30 cycles, (ASTM D 3945, Standard Test Method for Shear Stability of Polymer-Containing Oils Using a Diesel Injector Nozzle). More recently, ASTM D 5275, Standard Test Method for Fuel Injector Shear Stability for Polymer-Containing Fluids has been published. The latter method was originally Procedure B of the D 3945 method but was separated after tests showed that the two procedures in the earlier method often gave different results. Although the principle of the two methods is the same, the equipment differs and the latest test uses conditions of 20.7 MPa (207 bcir) and 20 cycles. In both cases, reference fluids are required to calibrate the equipment prior to evaluating the test samples. A concern over the lack of field correlation with the results from the diesel injector rig has recently led to the proposal in Europe to use a tapered roller bearing as the means for shearing the fluid. This is Co-ordinating European Council (CEC) method L-45-A-99, "Viscosity Shear Stability of Transmission Lubricants" that promises improved precision and better correlation with field data. Rusting and Corrosion Behavior The requirement for an oil or fluid to prevent rusting (of ferrous metals) and corrosion (of non-ferrous metals) is an existing part of nearly all hydrocarbon turbine oil specifications. While most rust tests are carried out at typical bulk fluid temperatures of 35-60°C, high temperature corrosion performance requires evaluation either by oxidation/corrosion tests or by specific high temperature tests in non-oxidizing conditions such as those specified by the aviation turbine builders. Some tests, e.g., gas turbine "hot end corrosion" cannot be satisfactorily simulated in the laboratory and require evaluation in an engine test. The most widely-used test for assessing rusting characteristics was reported earlier as ASTM D 665, Standard Test Method for Rust Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. In spite of the title, the method is also used for non-hydrocarbon oils and there is provision for the testing of products such as phosphate esters, which are heavier than water. Although it may be difficult to distinguish between the ratings for "moderate" and "heavy" rusting in this test, this is largely academic, as specifications normally call for a "Pass," that is, the absence of rust. One of the limitations of the D 665 method is that it evaluates rusting on vertical surfaces only. In reality, horizontal surfaces that can retain droplets of water are more prone to rusting. A method has therefore been devised "to indicate the ability of oils to prevent rusting and corrosion of all ferrous surfaces in steam turbines." This is ASTM D 3603, Standard Test Method for Rust-Preventing Characteristics of Steam Turbine Oil in the Presence of Water (Horizontal Disk Method). The test involves immersing a horizontal steel disk and a vertical steel cylinder in a stirred mixture comprising 275 mL of the oil and 25 mL of distilled water for 6 h at 60°C. At the end of this period the specimens are washed and inspected for rust. The test is carried out in duplicate and no rust must be seen in either of the two tests for the oil to be considered a pass. The above procedures only consider rusting in fluid contact with the metal surface whereas vapor-phase rusting is

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also possible, for example above the liquid layer in the tank. This condition is simulated in ASTM D 5534, Standard Test Method for Vapor-Phase Rust-Preventing Characteristics of Hydraulic Fluids. With the exception of cin additional steel specimen, which is located in the vapor space above the liquid, the apparatus and test conditions are identical to those found in ASTM D 3603. The method is divided into two parts, the first relating to fluids where water is the continuous phase and the second to fluids where oil or other water-free fluids (except phosphates) are in the continuous phase. In the latter case the water necessary to cause corrosion is contained in a bcciker located below the specimen for assessing vapor-phase corrosion. Results are reported as a pass or fail. The introduction of a corrosion-inhibited grade of fluid in the MIL-PRF-23699 specification has also resulted in the incorporation of a humidity cabinet test (ASTM D 1748, Standard Test Method for Rust Protection by Metal Preservatives in the Humidity Cabinet.) This procedure involves immersing steel plates in the fluid, allowing them to drain and then exposing t h e m to water vapor in the cabinet. The time tciken for the production of rust is monitored. In use, metals of different t3rpes may be in contact with one another and the potential difference of the metal couple in these conditions can influence gcJvanic corrosion, particularly with water-based fluids. An attempt to simulate this condition for non-aqueous fluids is found in the CETOP Method RP 48H, which is also being published as ISO 4404 Part 2. In this test, selected metal pairs in electrical contact, after cleaning and pre-weighing, are half immersed in the fluid for 28 days at 35°C in a covered beaker. At the end of the test period the weight change is measured and examination made of the specimens for signs of corrosion, including the surface above the liquid layer, which will indicate whether vapor-phase corrosion has tciken place. Hydraulic or lubricating oil systems containing phosphate esters can be made from either mild or stainless steel and, while rusting can occur with these fluids, it is not a common occurrence and is normally limited to the area above the liquid layer in the tank where condensation occurs. If the tcink interior is coated with phosphate, this forms a protective layer and reduces the possibility of rusting. In cases where the mild steel surface does not come into contact with the fluid, condensation may be prevented by maintaining w a r m fluid in the tank and/or passing dry air over the surface of the liquid in the reservoir. As a result it is now standard practice to leave the interior surfaces of hydraulic systems unpainted when using phosphate esters. In the past, epoxy coatings were recommended, but unless the coating can be cured in position, the long-term resistance of this type of paint cannot be guaranteed. However, due to the reduced risk of rusting with these fluids in turbine applications, the frequent use of stainless steel systems and the possible removal of inhibitors by the adsorbent solids used in fluid conditioning to maintain low levels of acidity, there is normally no rusting requirement in the specification for this type of fluid. With regard to non-ferrous metals, the corrosion of copper/brass as is found, for example, in turbine oil coolers, can be a problem for all oil types, as the dissolution of this metal will catalyze oil oxidation. The potential for oils to cause corrosion of copper and its alloys and the use of ASTM D 130 for assessing this condition was mentioned above. While metal

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passivators can minimize this problem, use of stainless steel or titanium coolers—or even dry cooling—is now possible. Lead, silver, a n d magnesium, which are also prone to acidic corrosion, may need special protection when used. These metals have been used in aero-engine systems as lightweight casings for components (magnesium) and in solder (lead a n d silver). To prevent magnesium corrosion the surface is painted and propyl gallate has been used as an inhibitor for lead. A test for lead corrosion still features in MILPRF-7808L. This is FED-STD 791, method 5321, which involves immersing lead and copper specimens in the fluid for 1 h at 325°F (162°C) during which time air is passed through. At the conclusion of the test the weight change of the lead panel is measured. The same specification also requires a corrosion test to be carried out on silver and bronze using FED-STD-791, method 5305. This is a high temperature test carried out at 450°F (232°C) in which strips of each metal cire immersed in the oil for 50 h followed by an assessment of the weight change. In systems using phosphate esters, a l u m i n i u m surfaces need to be hard anodized to prevent attack by acidic degradation products. Lubrication The lubrication performance of a n oil or fluid can be regarded as its ability to reduce weeir on metal surfaces sliding relative to one another under load. It is a combination of the performance of the fluid base stock and the additive package. All the finished fluids discussed so far are acknowledged to have good lubrication performance and for most steam and gas turbine applications there are no additional lubrication performEince requirements. Occasionally there is a call for a 4-ball wear test—ASTM D 4172, Standard Test Method for Wear Preventative Characteristics of Lubricating Fluid. This involves measuring the wear produced on three ball bearings held stationary in a cup containing the test fluid. A fourth ball is rotated in contact with the stationeiry bccirings while a load (normally 40 kg) is applied for a period of 1 h (see Fig. 33). For oils needing extreme pressure performance, a gear-test failure load may be specified. As indicated above, this is now carried out according to ASTM D 5182, S t a n d a r d Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG VisucJ method), or in aviation oils by the Ryder Gear Test.

Cleanliness The importance of clean oil, to ensure that small orifices in valves remain clear, that bcciring surfaces cire undamaged, and that p u m p s and motors r u n smoothly, is now well accepted. Dirt can EJSO play a part in catalyzing oil and fluid degradation by stabilizing foam and increasing oxidation. It may also promote emulsification in the presence of water and increase the conductivity of the fluid. Limits on cleanliness have therefore become steadily tighter over the years. At the same time the variety Eind efficiency of filters has improved [53]. Today, turbine lubrication systems use 6 fim (j8 = 200) filters while hydraulic systems generally have finer filtration, typically 3 jam (jS = 200) filters to remove particles as small as 3.0/im "absolute" in view of the use of fine toler-

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Top ball turned by drive motor

3 lower balls clamped

Thermocouple

A/V\AAAA^

Heater

FIG. 33—Schematic of the four-ball wear test apparatus.

ance valves, such as servo-valves, where clearances can be of the order of 3-5 microns. Several methods are available for the determination of particulate levels. These fall into the categories of automatic counting by light interruption techniques, manual counting of particles deposited on a filter paper, and gravimetric procedures. Light interruption methods are probably the most widely used of the above and involve the passing of a fixed volume of the fluid across a light beam. When a particle interrupts the beam a record is made and the size is registered depending on the extent to which the beam has been blocked. The particle counters will automatically allocate the reading to one of several pre-programmed size ranges depending on the range required by the specification, for example, to a range of, >5)U,m, 5-15)am, 15-25jam, 25-50Atm, 50-100/j.m, etc. While it is a relatively quick way of counting particles, it is not the most precise. It presupposes that the sample is homogenous, i.e., the particles are evenly dispersed and that they are regularly shaped—assumptions that are entirely theoretical. Light interruption techniques can also count products dispersed in the fluid, e.g., antifoams. The ASTM method using this technique is F 661, Standard Practice for Particle Count and Size Distribution Measurement in Batch Samples Using an Optical Particle Counter, while the equivalent ISO standard is 4402. The membrane filtration methods are more accurate. In the manual counting method, e.g., ASTM F 312, Standard Test Method for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters, a known volume of fluid is filtered through a membrane filter, the surface of which is divided into squares of known and equal size. The number of particles in a representative selection (at least 10) of these squares is then counted manually and sized and scaled-up to produce an approximate total number on the filter pad. (ISO 4407/4408 are similar manual counting methods.) This procedure is very time-consuming and is also

based on the assumption that the deposit is distributed evenly over the filter. Potentially the most accurate procedure, and sometimes used as a referee method in cases of dispute, is the gravimetric method in which a known volume of fluid (usually 100 mL) is passed through a filter disk with a pore diameter of 0.45^lm or 0.80/Am and the weight of the deposit accurately determined (ASTM D 4898, Standard Test Method for Insoluble Contamination of Hydraulic Fluid by Gravimetric Analysis). ISO 4405 is based on the same technique. Again the method does not indicate the size of the particles, a factor that can be of use in determining the possibility of damage to equipment and may also suggest the source of the contamination. For example, the presence of sub-micron sized particles can be indicative of "dieseling" in the hydraulic system. The ability of the light interruption technique to size particles in bands or ranges allows the development of a classification system based on the maximum number of particles in each band. Original attempts to rank the contamination levels of hydraulic fluids were published as ASTM, SAE, and NAS standards. However the distributions used were largely geometric progressions that were unrelated to the actual pattern of particle distribution found in hydraulic fluids [76]. As an alternative, ISO subsequently published a standard, 4406, Hydraulic Fluid Power-Fluids-Method For Coding Level of Contamination by Solid Particles, the latest version of which (1999) classifies particles into ranges of >4 ^m, >6 ^m, and > 14 ^tm, respectively and, depending on the numbers of particles in each range, allocates a "scale number." This makes possible a numerical description for the particulate levels in both new and used fluids. For example the scale number 15 corresponds to 160-320 particles/mL and scale number 12 to between 20 and 40 particles/mL. Most turbine oil and fluid specifications still have to be updated and are using the previous version of the ISO classification based on the number of particles >5 /i,m and >15 yam in size. They typically call for scale numbers of 17/14 or 18/15

CHAPTER 12: TURBINE LUBRICATING for turbine lubricants and 15/12 for the control fluids. However, such is the pace of change that the SAE 749D classification procedure, which was disowned by SAE in 1971, is still occasionally specified. In order to obtain meaningful data from any of these tests it is essential that the correct sampling procedure, in terms of where and how to take a sample from the system or container, be used. ISO 4021, for example, indicates how samples should be taken from the lines of an operating system. ISO 3170 details suitable sampling procedures from containers while ISO 3722 details how containers should be cleaned before use, a point that is frequently overlooked and can invalidate particle count data. It is also important to ensure that the sample bottle is compatible with the fluid being sampled. Trace Metals The introduction of trace metal analysis into turbine oil specifications is now found quite widely, but for different reasons. In mineral turbine oils the trend to ashless products, and particularly away from the use of zinc dialkyldithiophosphate as an antioxidant/extreme pressure additive, has led to the introduction of limits on zinc content. Current specification limits can be very low (about 2 ppm meix.) or somewhat higher levels (100 ppm max.). In aviation gas turbine lubricants, limits of 2-10 ppm, depending on the metal concerned, have long been in place. This is to minimize contamination, to avoid a possible catal5^ic effect on oxidative breakdown, and because certain organo-metallic additives, used in the past as extreme pressure additives, precipitated on storage. Today, the data also functions as baseline or reference information for the spectrographic oil analysis programs (SOAP), which are used to monitor the condition of used military aviation gas-turbine lubricants. With phosphate esters, the final processing step in manufacture may involve contact with alkaline media to lower the acid number and the resulting presence of soluble metal salts can promote foaming and an increase in air release properties [77]. In use, phosphate esters are normally treated by an adsorbent medium to remove acidic degradation products. However, the most widely used adsorbents, fullers earth and activated alumina, contain compounds that react chemically with the acids to form soluble metal salts [78]. These salts can adversely the surface-active properties of the fluid and therefore need occasional monitoring. One method used for measuring low levels of metal contaminants in hydrocarbon oils is ASTM D 4951, Standard Test Method for Determination of Additive Elements in Lubricating Oils by Inductively-Coupled Plasma Atomic Emission Spectrometry. With phosphate esters, this method can give incorrect results on certain metals (e.g., sodium) and for this medium at least, atomic absorption spectrometry (AAS) is preferred. The ASTM procedure for AAS (D 4628) is currently targeted towards the analysis of barium, calcium, magnesium, and zinc but can easily be adapted to detect other metals. Volume Resistivity One requirement that is specific to phosphate ester control fluids is the measurement of volume resistivity. This param-

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eter, which is the reciprocal of conductivity, has been found to relate to the tendency of the fluid to produce servo-valve erosion [63]. This is an electrochemical process caused by the development of a "streaming current" close to the valve surface arising from fluid flow across the valve. As well as being influenced by the chemical structure of the fluid, it also depends on the presence of contaminants such as water, acid, chloride ions, dirt, and metal soaps. Even polar additives can reduce the resistivity. The problem initially arose when polychlorinated biphenyls were used as fire-resistant fluids in turbine control systems and investigators linked the phenomenon with chlorine content and low resistivity. A minimum limit on resistivity of 5 or 10 by 10' ohm.cm. (50 or 100 Mi2m) was therefore introduced and is now part of all fire-resistant control fluid specifications. This property is measured by ASTM D 1169, Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids. Internationally, the method specified is lEC 60247, Measurement of Relative Permittivity, Dielectric Dissipation Factor, and d.c. Resistivity of Insulating Liquids. Both procedures involve measuring the resistance between the terminals of a test cell when a specified voltage is applied. Accurate measurement of the test temperature is important as resistivity is very sensitive to this property. While the lEC method precisely specifies the voltage gradient, the ASTM procedure allows considerable latitude in this key aspect. In order to improve the correlation between the methods, the voltage gradient along with other test variables such as electrode gap and test temperature should be the same. In view of the effect of small amounts of impurities on the results, it is important that no additional contamination of the sample takes place. Consequently, the sampling of the fluid and the cleanliness of the sample container are very important when evaluating this parameter. Figure 34 shows a general view of the test equipment used for determining volume resistivity, while Fig. 35 is a diagram of a typical test cell. Compatibility with System Materials In selecting system components for use with both mineral and synthetic turbine oils, it is important to ensure that the fluid is compatible with the constructional materials, that is contact with the fluid should not result in significant changes to the physical or chemical properties of the material. Small changes are to be expected in some materials and, indeed, may be beneficial. For example, the controlled swelling of a seal will help it fill the cavity and maintain its performance at high pressures. The main concerns in this area are the behavior of seals, paints, and gasket materials in the presence of the operating fluid. Inadequate compatibility with a seal, for example, may cause the swelling or softening of the rubber due to penetration by the oil or fluid, with subsequent fluid leakage. The use of unsuitable paints could cause softening, flaking and subsequent filter blockage, perhaps leading to pumping problems. Occasionally there are incompatibility problems with metals. Phosphate esters do not "wet" the surface of aluminium and therefore this metal is unsuitable as a bearing material for these fluids. Hydrocarbons are generally less searching than synthetic fluids in this area although the PAOs are problematic owing

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to their lack of seal swelling and necessitate the use of a cocomponent (usually a carboxylate ester) as a seal-swelling agent. The high operating t e m p e r a t u r e s experienced in a n increasing n u m b e r of applications, for example aviation gas turbine lubricants, place severe stress on conventional elastomers. Consequently, the use of fluorocarbon (and, to lesser extent, perfluoroelastomer) seals is now widespread. Phosphates eire also very selective with regard to seals and paints, but fortunately a suitable range of materials is now available. Examples of suitable elastomers for turbine oils and fluids are given in Table 20. A general procedure for testing for the compatibility of seals in non-aerospace applications is detailed in ISO 6072, Hydraulic Fluid Power-Compatibility between fluids cind standard elastomeric materials, while the aero-gas turbine oil specifications require performance against FED-STD-791 methods 3604 and 3433 or procedures listed by the equipment manufacturers. An ASTM standard entitled "Standard Test Methods and Suggested Limits for Determining the Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications" is currently in development. Table 21 illustrates the conditions recommended for testing different elastomers in the ISO 6072 standtird. The list indicates the principal types of elastomers used with the different fluids, but is not exhaustive. All the procedures involve suspending a rubber specimen of known volume in the oil or fluid u n d e r fixed conditions of temperature and test duration. This is followed at the end of the test by a second measurement of the volume to determine the percentage swell that has occurred. Additional measurements may be made of the chemges in elongation at break and

tensile strength. Depending on the rubber type and application, the test temperature may vary significantly. Fluorocarbon and perfluorocarbon rubbers are, for example, evaluated at higher temperatures than nitrile and neoprene. However, tests at high temperatures, e.g., 150°C as currently appear in the latest draft ISO 6072 standard, may not be representative of practice since, at this temperature, the basestocks and additive packages may not be stable and could give erroneous results. Until recently, all methods specified relatively short periods of immersion, usually u p to one week. It is now accepted that this period may be too short for the degree of swell to have stabilized, and in the latest version of ISO 6072, tests of 1000 h in length are now proposed in addition to the shorterterm exposure. No s t a n d a r d m e t h o d s in turbine oil specifications are known for evaluating paint compatibility and this aspect is generally left to the paint manufacturers w h o u n d e r t a k e long-term tests. The aspects of compatibility discussed above relate only to the fluid/constructional material interface. There is EJSO the aspect of the compatibility of different oils and fluids with each other. Should fluids from different manufacturers be mixed? This aspect is viewed differently by different industries. On the one h a n d it is a condition of militciry aviation oil specifications that all approved products should be compatible with one another. If this were not the case it could impair the combat readiness of aircraft. In the industrial power generation market some equipment builders specify that a new fluid must be compatible with the residue of the previous charge without any adverse effect on the new oil. However,

FIG. 34—A general view of the test equipment for measuring d.c. resistivity.

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345

090 1

1

1

1 = inner elecrode 2 = outer etecrode 3 = guard ring 4 = inner elecrode 5 = silica washer 6 = silica washer 7 = minimum level of liquid

Volume of liquid is about 45 cm^ All surfaces in contact with liquid have a mirror finish Dimensions in millimetres FIG. 35—Example of a three-terminal resistivity cell. Reprinted with permission of the International Electrotechnical Commission, Geneva, Switzerland.

nothing is indicated regarding topping-up an existing fluid with a product from a different supplier, probably because such an occurrence is unusual. Obviously, mixing fluid tjrpes that are not completely miscible should be avoided. Some concern has also been expressed regarding the mixing of products that have the same base material but different additive packages. This is because of possible interaction of the different additives resulting in a reduction in performance. Very often, the attitude of the turbine builder is not to allow the use of a second oil (for top-up) during the warranty period of the turbine. Afterwards this would only be accepted if

the builder had investigated the compatibility of the two fluids cmd/or the user had accepted the risk associated with mixing. In some cases oils can be mixed without problems but laboratory tests are advisable to check the effects of physical and chemical (in)compatibility, e.g., the impact on stability and the surface active properties (e.g., foaming and air release). The test used by the aviation turbine oil industry, FED-STD-791 method 3403, Compatibility of Turbine Lubricating Oils, involves checking the miscibility with reference ester-based lubricants. Mixtures of the test oil and the reference lubricants at three concentrations are heated in an oven

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for 1 week at ZZl^F (105°C) followed by a determination of the sediment produced. Volatility In military aviation applications where a low viscosity oil is used in a high temperature environment, for example the 3 cSt oils specified in MIL-PRF-7808L, it is necessary to ensure that the oil is not too volatile. This is determined in ASTM D 972, Standard Test Method for Evaporation Loss of Lubricating Greases and Oils. This procedure involves passing air at 400°F (205°C) over the surface of a weighed quantity of oil heated to the same temperature and held in a test cell of specified dimensions. After 6'/2 h the weight loss of the sample is determined. In order to ensure that the equipment is providing the appropriate level of severity, a test is carried out with a reference liquid and, if necessary, a correction factor applied. The test limit is 10% maximum. Assuming that the new fluid meets all the requirements laid down in the specifications it may still be necessary to arrange trials in the turbine builder's equipment. These can require several years of satisfactory operation and be very costly to arrange and carry out. TABLE 20—Elastomer compatibilities of different turbine oils and fluids. Phosphate Mineral Oil Seal/Hose Material and PAOs Ester Neoprene Nitrile Chlor-sulphonated polyethylene Epichlorhydrin Styrene-butadiene Ethylene-propylene Butyl Fluorocarbon Perfluoroether PTFE

Polyol Ester

Yes Yes Yes

No No Yes''

No Yes" Yes

Yes No No No Yes Yes Yes

No No Yes" Yes Yes* Yes Yes

No No No No Yes Yes Yes

"Variations in the composition of these elastomers in particular can cause differences in fluid compatibility and the seal manufacturers should be consulted before use. ^Fluorocarbon rubbers are compatible with aryl phosphates, but not with alkyl phosphates.

THE IMPORTANCE OF SYSTEM CLEANLINESS One emphasis in this chapter has been on ensuring the cleanliness of the oil or fluid. However, there is no advantage in having a clean fluid only to fill it into a dirty system. The performance advantages of fresh fluid would soon disappear in these circumstances. There are two situations where attention to the system cleanliness is most important. The first is during commissioning of the new equipment or after major overhauls when the system is flushed. This procedure will remove dirt, preservative oils and greases, cleaning solvents, and other residual matter from cleaning and assembly operations, e.g., welding slag and cleaning rags. Effective flushing may involve removing or blanking-off system components and separately cleaning them while dividing the pipework into sections and circulating a flushing fluid through each section, preferably under turbulent flow conditions to remove as much debris as possible. Some builders remove rust by "pickling" the pipework by circulating a dilute solution of acid followed by a water wash and then dry with a current of warm air. In the past, chlorinated solvents have been used to remove oil films on components during assembly and residual product in the hydraulic system has subsequently caused servovalve erosion [63]. As a result, use of this tjrpe of solvent is now banned and has been replaced by hydrocarbons or, in some cases, by water-based products. Preservative oils containing inhibitors that prevent rust during the transportation and storage of the equipment can cause foaming of the operating charge if they are not adequately removed. The second important use of a flushing charge is when replacing badly degraded fluid, which, in addition to containing acid, may have resulted in deposit formation. In this case it is important to remove all but traces of the previous fluid and any associated deposits by a thorough flush and, if necessary on examination, a manual cleaning of components including the tarrk interior. More detailed guidance on the cleaning and flushing of lubrication systems is given in ASTM D 6439, Standard Guide for the Cleaning, Flushing and Purification of Steam, Gas

TABLE 21—Conditions recommended for testing the compatibility of elastomers with different turbine fluids. Suitable Temperature ISO Duration of Test Elastomer °C± 1 Test" h ± 2 Fluid Classification HH, HL HM, HR HV

NBRl, 2 HNBRl FKM2

100 130 150

Aryl phosphate esters

HFDR

FKM2 EPDMl

Polyol esters

HFDU

Mineral oils

Synthetic hydrocarbon

HEPR

168

1000

150 130

168

1000

NBRl, 2 HNBRl FKM2

100

168

1000

HNBRl FKM2

130 150

168

1000

"The test duration of 1000 h is recommended for evaluation of elastomer compatibility with highly critical fluids. NBR = nitrile (acrylonitrile butadiene) HNBR = hydrogenated nitrile EPDM = ethylene propylene FKM = fluorocarbon Reprinted courtesy of the International Organization for Standardization, Geneva, Switzerland.

CHAPTER 12: TURBINE LUBRICATING and Hydroelectric Turbine Lubrication Systems and, for hydraulic systems, in ASTM D 4174, Standard Practice for the Cleaning, Flushing and Purification of Petroleum Fluid Hydraulic Systems. Both documents relate specifically to hydrocarbon oils and discuss the types of contamination found in practice, how they can be controlled, guidance on suitable flushing procedures, oil sampling techniques, and condition monitoring. For non-hydrocarbon fluids, information is to be obtained from the fluid manufacturers or other appropriate standards, e.g., lEC 60978, Maintenance and Use Guide for Triaryl Phosphate Ester Turbine Control Fluids, which provides similar information except for recommendations on flushing. The fluid used for flushing is often the operating charge, but this is expensive if a second charge of fluid is used for normal operation. If only small amounts of the flush fluid are left in the system after flushing and draining, the use of a chemically similar material, usually the base stock for the operating charge may be considered. However, if significant quantities of the flush fluid remain in the system when the operating charge is filled-in, the use of a cheaper fluid could be counter-productive. The use of flush fluids that are a different chemical type to the operating charge is to be avoided in view of concerns with regard to their compatibility with system components and the fact that it is very difficult to remove residues from the system. Depending on the level of contamination acquired during the flush, it may be possible to re-use the flushing fluid several times.

TURBINE OIL AND FLUID MAINTENANCE EPRI Report CS-4555 [79] comments that "failures of steam turbine bearings and rotors cost the utility industry an estimated $150 million/year" and "one third of these failures involve contaminated lubricants or malfunctioning lubricant system supply." It is unfortunately the case that many of the operating problems found with turbine oils and hydraulic fluids are due to poor maintenance, occasionally compounded by inadequate system design. As operating conditions become more severe, the importance of regular maintenance (both system and fluid) increases. This comes at a time when utilities are achieving cost reductions by shedding jobs and trying to increase equipment availability. As a consequence, there is a distinct possibility that non-essential maintenance will suffer and the improvements in oil performance available as a result of recent technical developments will not be fully utilized. An essential part of all planned maintenance is the ability to carry out regular fluid monitoring. This involves taking representative samples from the system under strictly defined conditions as indicated, for example, in the above ASTM and ISO Standards and then evaluating critical parameters to determine the current condition of the material. Implicit in such a scheme is the availability of limits on these parameters and recommendations on appropriate action should they be reached. Also important is the frequency of sampling. This will vary depending on the particular parameter involved, the stress on the fluid in use, whether

OILS AND HYDRAULIC FLUIDS

347

or not the system is being commissioned, and if operational problems are being experienced. Using the data produced to establish trends in performance can assist in identifying the source of problems and when action may be needed [80]. Unlike the extensive range of specifications available on fresh oil or fluid, the situation with regard to the used oil is less complicated. Many, but not all, of the turbine builders incorporate used oil requirements into their new oil specifications. A few have separate standards and some of the oil or fluid suppliers offer guidance in the absence of information from the turbine manufacturers. Where no builder recommendations exist, guidance is also available from national/international standards. For mineral turbine oils there are ASTM D 4378, Standard Practice for In-service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines; the lEC Standard 60962, Maintenance and Use Guide for Petroleum Lubricating Oils for Steam Turbines; and the very detailed EPRI report referred to earlier [79]. For fire-resistant hydraulic fluids, lEC 60978 and ISO 7745, Hydraulic Fluid Power-Fire-resistant Fluids-Guidelines for Use, are available, while for aero-engine oils there are recommended limits on acidity and viscosity increase from the turbine builders, but otherwise no published requirements. The rapid rate of top-up of oil in this application—even in industrial applications—means it is unlikely that the physical changes in fluid quality would normally reach values of concern. The above industry guides are comprehensive documents which typically examine the reasons for fluid degradation and its impact on fluid performance, identify fluid sampling techniques, give appropriate tests for monitoring both new and used oil behavior, as well as a proposed schedule for their use. Table 22, which is taken from ASTM D 4378, identifies the turbine properties of gas and steam turbine oils that are monitored in service, their warning limits and appropriate action to be taken in the event that these limits are exceeded. Quite a wide range of tests can be used to monitor fluid degradation, etc. These can be divided into primary tests that check for specific degradants or contaminants and secondary tests, the results of which are influenced by the degradant or contaminant. An example would be the measurement of water, which can be determined directly and in esters, its presence may be implied from a reduction in viscosity or a rapid increase in acid number. Acidity is probably the most important property monitored on a regular basis, but is the parameter most frequently disputed. As indicated earlier, of the two techniques used for determining the acid number of both new and aged fluid or lubricant, only the potentiometric method is really suitable for aged fluid [81]. This is because the fluid darkens on ageing and it becomes more difficult to estimate the endpoint in the colorimetric method. Some fire-resistant fluids are also dyed to ensure that they are not confused with mineral oils and for such fluids this technique is also unsuitable, particularly when new. Other important properties that are routinely measured for all Quids include viscosity, water content, and particulate levels. In addition, rapid changes in color and appearance can be indicative of developing problems. A measure of the residual life or stability of the product may also be advanta-

348 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

Test

TABLE 22—Interpretation of test data and recommended action for mineral turbine oils. Steam (S) Oil Life Warning Limit or Gas (G) (Running Hours) Interpretation

Action Steps

s

up to 20 000 h up to 3000 h

This represents above normal deterioration Possible causes are: a) system very severe b) antioxidant depleted c) wrong oil used d) oil contaminated

Investigate cause. Increase frequency of testing—compare with RBOT data. Consult with oil supplier for possible reinhibition.

0.3-0.4 mg KOH/g

S, G

at any time during life of oil charge

Oil at or approaching end of service life, c) or d) above may apply

Look for signs of increased sediment on filters and centrifuge. Check RBOT. If less than 2 5 % of original, review status with oil supplier and consider oil change. Increase test frequency.

RBOT

less than hcJf value on original oil

S

up to 20 000 h

Above normal degradation

Investigate cause. Increase frequency of testing.

RBOT

less than half value on original oil

G

up to 3000 h

Above normal degradation

Investigate cause. Increase frequency of testing.

RBOT

less than 2 5 % of original oil

S, G

at any time

Together with high acid no. indicates oil at or approaching of service life.

Resample and retest. If same, consider oil change.

Water content

exceeds 0 . 1 %

at any time

Oil contaminated. Potential water leak.

Investigate and remedy cause. Clean system by suitable method.'' If still unsatisfactory, consider oil change or consult oil supplier.

Cleanliness

exceeds accepted limits"

at any time

Source of particulates may be: a) make-up oil; b) dust or ash entering the system; or c) wear condition in system.

Locate and eliminate source of particulates. Clean system oil by filtration or centrifuge or both.

Rust test D 665, procedure A*

light fail

up to 20 000 h

a) the system is wet or dirty or both b) the system is not maintained properly (e.g., water drainage neglected, centrifuge not operating.)

Investigate cause and make necessary maintenance changes. Check for rust. Consult oil supplier regarding reinhibition if test result unchanged.

Rust test D 665, procedure A*

light fail

S, G

after 20 000 h

Normal additive depletion in wet system. Maintenance

Consult oil supplier regarding reinhibition.

Appearance

hazy

S, G

at any time solids or both

Oil contains water, and remedy. Filter.

Investigate cause or centrifuge oil or both.

Color

unusual and rapid darkening

S, G

at any time

This is indicative of: a) contamination b) excessive degradation.

Determine cause and rectify.

Viscosity

5% from original oil viscosity

S, G

at any time

a) Oil is contaminated, b) oil is severely degraded, or c) higher or lower viscosity oil added.

Determine cause. If viscosity is low, determine flash point. Change oil if necessaiy.

Flash point

drop 30°F or more compared to new oil

S, G

at any time

Probably contamination.

Determine cause. Check other quality parameters. Consider oil change.

Foam test D 892

exceeds following limits tendency: 450 stability: 10

S, G

at any time

Possibly contamination or antifoam depletion. In new turbines residual rust preventatives absorbed by oil may cause problems

Rectify cause. Check with oil supplier regarding reinhibition. NOTE: plant problems often mechanical in origin.

Acid no. Increase over new oil

0.1-0.2 m g K O H / g

Acid no. Increase over new oil

Sequence 1

G

S, G

"Definition of suitable cleanliness levels depends on turbine and user requirements. ^Satisfactory for land turbines. '^Appropriate methods may include centrifuging, coalescence, or vacuum dehydration.

CHAPTER

12: TURBINE

geous. This can be obtained either by determining the RBOT value, an indication from analytical methods of the level of antioxidant concentration, or by the use of voltammetric techniques. For phosphate esters, volume resistivity may additionally need to be monitored in order to avoid the possibility of servo-valve erosion. One basic difference between the use of fire-resistant fluids/lubricants ajid hydrocarbon oils and synthetic esters is that, in order to remove the acid that is normally formed as a result of fluid hydrolysis or oxidation, the fire-resistant fluids are purified or reconditioned in situ. This involves passing the fluid on a by-pass loop from the m a i n reservoir through an adsorbent solid that removes acid and chloride from the fluid and also acts as a fine particle filter. For many years fullers earth has been used for this purpose but has often resulted in the formation of soluble calcium and magnesium soaps, which have h a d an adverse effect on fluid foaming a n d air release properties a n d w h i c h eventually precipitate in the form of gels in filters and in parts of the system where the fluid is cooled [80]. In an attempt to avoid this problem, other treatments have been used, notably activated alumina and blends of purified activated eJumina with alumino-silicates. The latter is a definite improvement but can still eventually result in the dissolution of sodium and aluminium in the fluid unless the acidity is kept very low [80]. More recently, the use of ion-exchange media h a s shown great promise a n d now enables the life of the fluid to be extended almost indefinitely [78] with a very positive effect on fluid maintenance costs and almost eliminating the need for fluid disposal.

FUTURE TRENDS Many of the developments discussed in the course of this chapter will continue to influence the development and availability of turbine lubricating oils. Some of t h e m were the subject of presentations at an ASTM Symposium entitled "Turbine Lubrication in the 21st Century." The papers presented are now published and serve as a useful reference to the subject [82]. Where possible, the turbine builders will continue in their sccirch for higher operating efficiencies that will result in even higher operating temperatures and greater thermal and oxidative stress on the lubricant. This, in turn, will accelerate the move from solvent refined oils towards severely hydrocracked oils and synthetic hydrocarbons. It may also accelerate a move toward fire-resistant lubricants particularly as the dcingerous "cocktail" of higher temperatures and reduced staffing could result in an increase in the frequency of oil fires. The d e m a n d for greater e q u i p m e n t availability and reduced outages in order to improve the financial returns to the IPPs will also result in a demand for longer oil/fluid lives. The increase in operational severity will place even greater emphasis on stability, but the advent of in-service monitoring of stabilizer levels, re-inhibition of the existing charge, the possibility of in situ reclamation by better filtration, and removal of developed acidity by by-pass treatment will extend oil life still further. As a result of the anticipated longer oil life there will be increased competition for the initial fill. This will substantially

LUBRICATING

OILS AND HYDRAULIC

FLUIDS

349

reduce the operating margins for new business cind encourage the suppliers to provide "cradle-to-the-grave" service agreements whereby they maintain the fluid throughout its life, supply top-up material as needed, and eventually dispose of the degraded product. The development of in situ condition-monitoring techniques will assist station staff in determining the level and timing of necessary fluid maintenance. The trend to environmentally friendly fluids, where required, may also develop—^particularly for water turbine applications, where some experience has been obtained with biodegradable fluids [83]. One recent development not previously mentioned that may have a future impact on the lubrication of gas turbines has been the successful evaluation in a static engine test of a vapor-phase lubricant based on a tertiarybutylphenyl phosphate [84,85]. Very small amounts of the lubricant are vaporized and react with the metal surface of a ceramic bearing to form a polymeric film that can sustain a load. The immediate interest appears to be for missiles and other unmanned aero-space vehicles because of the reduced weight penedty with the smaller volume of liquid required, b u t wider use in the longer term may be possible. Acknowledgments The author gratefully acknowledges the kind assistance (and patience) of R. Coombes and G. N. Kay of Alstom Power, M. Dennis of Rolls Royce, D. Irvine of Petro-Canada Lubricants, G. Jones of G.E. Energy, E. Letterman of General Electric Power Systems, H. Moeller of Elsam, M. Morris, Consultant, Dr. T. Okada of ExxonMobil, Dr. L. Quick of Siemens A. G., J. Pankowiecki of Siemens Westinghouse a n d B. Rayner, Consultant. Lastly, thanks are cJso due to Alan Holt of Great Lakes Chemical Corp. for the photographs and to Alan Watson for editorial assistance.

REFERENCES [1] Smith, D. J., "Private Ownership of Electric Power is More Efficient and Reliable than Public-Owned Plants," Power Engineering International, March/April 1995, pp. 25-28. [2] Kurtz, D., "Great Expectations," Power Engineering International, May 1999, pp. 33-37. [3] Lane, J., "IPPs Open Up New Markets," Power Engineering International, April 1998, pp. 27-30. [4] Council Directive 89/392/EEC, Official Journal, L183 of 29th June 1989, Office for Official Publications of the European Community, Luxembourg. [5] Dodman, K., "Efficiency Will be the Greatest Issue," International Power Generation, September 1997, pp. 51-53. [6] Collins, S., "Gas Turbine Power Plants," Power, June 1994, pp. 17-31. [7] Paterson, A. N., Simonin, G., and Neft, J. G., "Turbine Blading Materials Boost," International Power Generation, July 1996, pp. 65-67. [8] Curtis, T., "GE 2500+ Power Turbines," Turbomachinery International, May/June 1995, pp. 42-44. [9] Anon., "Micros-a New Gas Turbine Market," Turbomachinery International Handbook, 1997, pp. lli-lS. [10] Ashmore, C, "Power, Light and Cheap Heat," International Power Generation, September 1997, pp. 54-57. [11] Makansi, J., "Combined-Cycle Powerplants," Power, June 1990, pp. 91-126.

350 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [12] Smith, D. J., "Combined-Cycle Gas Turbines: The Technology of Choice for New Power Plants," Power Engineering International, May/June 1995, pp. 21-26. [13] S w a n e k a m p , R., "Gas-Turbine/Combined Cycle Power Systems," Power, June 1995, pp. 15-26. [14] Swanekamp, R., "Single-Shaft Combined Cycle Packs Power in at Low Cost," Power, January 1996, pp. 24-28. [15] Anon., "Two Prototypes for Large-Scale Wind Turbines," International Power Generation, May 1995, pp. 37-38. [16] Anon., "Towards the 5 MW Turbine," Supplement to Modem Power Systems, October 2000, p. 36. [17] Lakkenborg, J. "Understanding the Mechanisms," International Power Generation, November 1996, p. 45. [18] Hall, D. T., "Turbine Generator Fire Protection Overview," presented sX American Power Conference, Chicago, IL, April 1986. [19] "Evaluation of Fire-Retardant Fluids for Turbine Bearing Lubricants," Electric Power Research Institute Report NP-6542, Palo Alto, CA, 1989, pp. 2-1 to 2-7. [20] Schober, J., "Fire-Resistant Hydraulic Fluids," The Brown Boveri Review, Vol. 53, No. 1/2, 1966, pp. 142-147. [21] Schenck, K., Hoxtermann, E., and Hartwig, J., "Operation of Turbines with Fire resistant Fluids, Including the Lubricating System," VGB Kraftwerkstechnik, Vol. 77, No. 6, 1977, pp. 412416. [22] Dufresne, P. D., "14 Million Hours of Operational Experience with Phosphate Ester Fluid in a Gas Turbine Main Bearing App.lication," Proceedings of the 11th International Tribology Colloquium, Esslingen, Germany, January 1998, pp. 1447-1452. [23] Smith, A. N., "Fire Resistant Lubricants," General Electric Publication GTU-5 78, 1978. [24] Knipple, R. and Thich, J., "The History of Aviation Turbine Lubricants," SAE Paper 810851, Society of Automotive Engineers, Warrendale, PA, 1981. [25] Anon., "Turbine Lubricants for Steam, Gas, Wind and Water Turbines," Industrial Lubrication and Tribology, September/October 1994, pp. 8-15. [26] Byford, D. C. sind Edgington, P. G., "The Development of Special Oils for the Power Plants of Supersonic Transports," Proceedings of the Eighth World Petroleum Congress, 1971, pp. 101-110. [27] Phillips, W. D., "Triaryl Phosphates-The Next Generation of Lubricants for Steam and Gas Turbines," ASME Paper, 94JPGC-PWR-64, American Society of Mechanical Engineers, NY, 1994. [28] Hoxtermann, E. and Richter, P., "Failures of and Damage to Gas Turbine Components," VGB PowerTech, Vol. 80, No. 10, 2000, pp. 51-54. [29] Kondo, H., " Recent Trends in Turbine Oils," Japanese Journal of Tribology, Vol. 35, No. 9, 1990, pp. 969-979. [30] DenHerder, M. J., "Control of Turbine Oil Degradation During Use," Lubrication Engineering, Vol. 37, No. 2, 1981, pp. 67-71. [31] Moeller, H., "Lubricants for Gas Turbines," Proceedings of the 11th International Tribology Colloquium, Technische Akademie Esslingen, Germany, January 1998, pp. 379-382. [32] Ashman, L. A., Vetrone, J., Curts, L., and Johnston, A., "Advantages of Turbine Fluids Blended with Hydro-treated Base Oils: Exceptional Oxidative Resistance, Filterability, Air and Water Separation," presentation at the 54"" Annual Meeting of the Society of Tribologists and Lubrication Engineers (STLE), Las Vegas, May, 1999. [33] McHugh, P., Stofey, W. D., and Totten, G. E., "Mineral Oil Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 1999, pp. 711-794. [34] Hoo, G. H. and Lewis, E., "Base Oil Effects on Additives Used to Formulate Lubricants," Adv. Prod. Appl. Lube., Proceedings of the International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 326-33. [35] Henderson, H. E., "Base Oils for Engines and Drivetrains of the

[36]

[37]

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[41]

[42]

[43]

[44]

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[47]

[48] [49]

[50]

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[56]

Future," Supplement to Proceedings of the 12''' International Tribology Colloquium, Esslingen, Germany, Jan 2000, pp. 41-53. "Engine Oil Licensing and Certification System," American Petroleum Institute Publication 1509, 14* ed.. Appendix E, Section E.1.3, Washington, D.C., Dec 1996. Ashman, T., "Advantages of Turbine Oils Blended with Hydrotreated Base Oils," Presented at the 54"' Annual STLE Meeting, Las Vegas, May 1999. Galiano-Roth, A. S. and Page, N. M., "Effect of Hydroprocessing o n Lubricant Base Stock Composition a n d P r o d u c t Performance," Lubrication Engineering, Vol. 50, No. 8, pp. 659-664. Deckman, D. E., Lohuis, J. R., and Murphy, W. R., "HDP for Industrial Lubricants," Hart's Lubricants World, September 1997, pp. 20-26. Zielinski, J., "Supersyn Polyalphaolefins-a New Generation of Synthetic Fluids," Presented at the 54"' Annual STLE Meeting, Las Vegas, May, 1999. Benda, R., BuUen, J, V., and Plomer, A., "Polyalphaolefins-Base Fluids for High Performance Lubricants," Journal of Synthetic Lubrication, Vol. 13, No. 1, 1996, pp. 41-57. Fefer, M., Henderson, H. E., Legzdins, A., and Ruo, T., "Oxidation of Polyalphaolefins and Severely Hydro-Cracked : Kinetics of Peroxide Formation-Part III," presented at the 54''' Annual STLE Meeting, Las Vegas, May, 1999. Rudnick, L. R. and Shubkin, R. L., "Poly(a-olefins)," Synthetic Lubricants and High Performance Functional Fluids, L. R. Rudnick and R. L. Shubkin, Eds., Marcel Dekker Inc., NY, 1999. Randies, S. J., "Esters," Synthetic Lubricants and High-Performance Functional Fluids, L. R. Rudnick and R. L. Shubkin, Msircel Dekker, NY, 1999, pp. 63-101. Fowler, B. T., "Diesters as High Temperature Lubricants," Wear, Vol. 15, 1970, pp. 97-104. "Aviation Gas Turbine Lubricants-Military and Civil Aspects," SAE Special Publication 633, Proceedings of the Aerospace Technology Conference, California, October 1985, Society of Automotive Engineers, Warrendale, PA. Markson, A. J., "Future Gas Turbine Lubrication-A Formulators View," Proceedings of the 10th International Tribology Colloquium, Esslingen, Germany, January 1996. Rayner, B. R., Private presentation to Rolls-Royce pic, Derby, UK, 17 May 2000. Phillips, W. D., "A Comparison of Fire-resistant Hydraulic Fluids for Hazardous Industrial Environments -Part 1," Journal of Synthetic Lubrication, Vol. 14, No. 3, 1998, p p . 211-235. Phillips, W. D., "Phosphate Esters," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 1999, pp. 1025-1093. Marino, M. P. and Placek, D. J., "Phosphate Esters," CRC Handbook of Lubrication and Tribology, Vol III, Monitoring, Materials, Synthetic Lubricants and Applications, E. R. Booser, Ed., CRC Press Inc., Boca Raton, FL, 1994, pp. 269-285. Rasberger, M., "Oxidative Degradation and Stabilisation of Mineral Oil Based Lubricants," Chemistry and Technology of Lubricants, R. M. Mortier and S. T.Orszulik, Eds., Kluwer Academic Publishers, Dordrecht, The Netherlands, 1997, pp. 98-142. Graham, J. and Leonhardt, H., "Development of Lubricating Oils for Combined Cycle Applications," Proceedings, 11''' International Tribological Colloquium, Technische Akademie Esslingen, Germany, January 1998, p p . 1487-1500. "Lubricating Oil Recommendations for Gas Turbines with Bearing Ambients Above 500°F (260 °C)," General Electric Specification GEK 32568C, July 1993. Chadwick, D. and Hashemi, T., "Adsorbed Corrosion Inhibitors Studied by Electron Spectroscopy; Benzotriazole on Copper and Copper Alloys," Corrosion Science, Vol. 18, 1978, p p . 39-51. Godfrey, D., "The Lubrication Mechanism of Tricresyl Phosphate on Steel," ASLE Preprint 64 LC-1, Park Ridge, IL, 1964.

CHAPTER 12: TURBINE LUBRICATING OILS AND HYDRAULIC FLUIDS 351 [57] Forster, N. H., "High Temperature Lubrication of Rolling Contacts with Lubricants Delivered from the Vapor Phase and as Oil-Mists," WL-TR-97-2003, 1997. [58] Tourret, R. and White, N., "Aeration and Foaming in Lubricating Oil Systems," Aircraft Engineering, May 1952, pp. 122-130, 137. [59] Hodges, P. K. B., Hydraulic Fluids, Arnold, Great Britain, 1996, p. 46. [60] Staniewski, J. W. G., "The Influence of Mechanical Design of Electro Hydraulic Steam Turbine Control Systems on Fire-Resistant Fluid Condition," Lubrication Engineering, Vol. 52, No. 3, 1996, pp. 255-258. [61] Backe, W., and Lipphardt, P., "Influence of Dispersed Air on the Pressure Medium," Proceedings of the Institute of Mechanical Engineers, paper 091116, London, 1976, pp. 77-84. [62] Hatton, D. R., " Some Practical Aspects of Turbine Lubrication," Canadian Lubrication Journal, Shell Canada Products Co., Vol. 4, No. 1, 1984, pp. 3-8. [63] Phillips, W. D., "The Electrochemical Erosion of Servo Valves by Phosphate Ester Fire-Resistant Hydraulic Fluids," Lubrication Engineering, Vol. 44, No. 9, 1988, pp. 758-767. [64] Kaspar, K., "Einsatz von synthetischen schwerbrennbaren Hydraulikflilssigkeiten im Schmier- u n d Steuerkreislauf von Dampfturbosatzen," Der Maschinenschaden, Vol. 50, No. 3, p p . 87-92, 1977. [65] Schenck, K,, Hoxtermann, E., and Hartwig, J., "Operation of Turbines with Fire-Resistant Fluids, Including the Lubricating System," VGB Kraftwerkstechnik, Vol. 77, No. 6, 1997, pp. 412-416. [66] Phillips, W. D., "Fire Resistance Tests For Fluids and LubricantsTheir Limitations and Misapplication," Fire Resistance of Industrial Fluids, ASTM STP 1284, G. E. Totten, and J. Reichel, Eds., ASTM International, West Conshohocken, PA, 1996, pp. 78-101. [67] Howells, P., "Measurement of Autoignition Temperature," BP Technical Report 31 737/M, British Petroleum Company, Sunbury-on-Thames, UK, 31 January, 1976 [68] Reichardt, H. U., Fischer, R., a n d Schiilert, G., "Wasser im Turbinenol-Einfluss, Eigenschaften und Bestimmungsmoglichkeiten," Schmierungstechnik, Vol. 18, No. 11, 1987, pp. 335338. [69] Li, T-D. and Mansfield, J. M., "Effect of Contamination on the Water Separability of Steam Turbine Oils," Lubrication Engineering, Vol. 51, No. 1, 1995, pp. 81-85. [70] Rockwell, J., "The Slime Intermission," Herald Tribune, 27 May 1992. [71] Bowman, W. F. and Stachowiak, G. W., "New Criteria to Assess the Remaining Useful Life of Industrial Turbine Oils," STLE Pre-print 96-NP-4F-1, STLE Annual Meeting, Cincinnati, 1996. [72] Kaufmann, R. E., "Rapid, Portable Voltammetric Techniques for Performing Antioxidant, Total Acid Number (TAN) and Total Base N u m b e r (TBN) Measurements," Lubrication Engineering, January 1998, pp. 39-46.

[73] Kauffman, R. E., "Remaining Useful Life Measurements of Diesel Engine Oils, Automotive Engine Oils, Hydraulic Fluids, and Greases Using Cyclic Voltammetric Methods," Lubrication Engineering, Vol. 51, No. 3, 1995, pp. 223-229. [74] Critchley, S. W. and Miles, P., "Synthetic Lubricants-Selection of Ester Types for Different Temperature Environments," Proceedings of the Industrial Lubrication Symposium, London, March 1965. [75] Anon, "Oil Filters and C o n t a m i n a n t Control," Industrial Lubrication and Tribology, November/December 1993, p p . 14-19. [76] SAE J1165: "Reporting Cleanliness Levels of Hydraulic Fluids," SAE Recommended Practice, Society of Automotive Engineers, Wareendale, PA, July 1979. [77] Staniewski, J. W. G., "Operating Experience with Fire-Resistant Phosphate Esters in Steam Turbine Electro-Hydraulic Control Systems," presented at the EPRI/NMAC Lubrication Workshop, Cleveland, OH, June 1994. [78] Phillips, W. D. and Sutton, D. I., "Improved Maintenance and Life Extension of Phosphate Esters using Ion Exchange Treatment," Proceedings of the 10th International Tribology Colloquium, Technische Akademie, Esslingen, Germany, January 1996, pp. 405-432. [79] "Guidelines for Maintaining Steam Turbine Lubrication Systems," Electric Power Research Institute Report No. CS-4555, Palo Alto, CA, 1986. [80] Brown, K. J. and Staniewski, J. W. G., "Condition Monitoring and Maintenance of Steam Turbine-Generator Fire Resistant Triaryl Phosphate Control Fluids," STLE Special Publication SP27, Proceedings of the 1989 Condition Monitoring and Preventative Maintenance Conference, May 1989, pp. 91-96. [81] Christopher, S. a n d Marson, A. J., "Development of a Test Method for the Determination of the Total Acidity in Polyol Ester and Diester Gas Turbine Lubricants by Automatic Potentiometric Titration," Proceedings of the 11th International Tribology Colloquium, Technical Akademie, Esslingen, Germany, 1998, pp. 121-127. [82] Turbine Lubrication in the 21" Century, ASTM STP 1407, W. R. Herguth and T. M Warne, Eds., ASTM International, 2001. [83] Boehringer, R. H. and Ness, F., "Lubrication of Hydroelectric Turbine Thrust Bearings with a Diester-Based Synthetic Lubricant," Journal of Synthetic Lubrication, Vol. 6, No. 4, 1989, pp. 311-323. [84] Rao, A. M. N., "High Temperature Vapour Phase Lubrication," Proceedings of the 11th International Tribology Colloquium, Technische Akademie, Esslingen, Germany, January 1998. [85] Van Treuren, K. W., Barlow, D. N., Heiser, W. H., Wagner, M. J., and Forster, N. H., "Investigation of Vapor-Phase Lubrication in a Gas Turbine Engine," ASME Transactions, Journal of Engineering for Gas Turbines and Power, Vol. 120, April 1998, pp. 257-262.

MNL37-EB/Jun. 2003

Hydraulic Fluids W. A. Givens^ and Paul W. Michael^

T H E PRIMARY PURPOSE OF A HYDRAULIC FLUID is to

Where: K = Bulk modulus Vo = Original volume AP = Pressure change Ay = Change in volume

transfer

power. The concept of fluid power is based on a principle articulated by Blaise Pascal, which is usually given as follows: "Pressure applied to an enclosed fluid is transmitted undiminished to every portion of that fluid and the walls of the containing vessel" [1]. Within the context of fluid power, pressure is related to the force acting on a confined fluid as illustrated in Fig. 1 [2]. This principle has given rise to mode m hydraulics, which entails highly engineered systems for efficiently controlling fluid flow to transfer energy and accomplish work. The heart of any hydraulic system is the pump, which pulls in fluid from a reservoir by creating a vacuum at its inlet and then forces the fluid through its outlet, usually against pressure created by flow controllers and/or actuators downstream of the p u m p . Pumps, actuators, and other system components have surfaces that move relative to each other, often at high speeds, pressures, and temperatures. These components require cooling and lubrication for efficient performance and durability. Consequently, hydraulic fluids not only must transmit power, they serve critical functions as lubricant and heat transfer medium.

Heat Transfer Heat is generated as a by-product of normal operation of a hydraulic circuit. Friction between the moving parts of a p u m p or hydraulic motor, as well as friction between the fluid and surfaces of valves, pipes, and other circuit devices generates heat. In addition, heat is generated in a hydraulic system as a result of the dissipation of the potential energy of pressurized fluid [8]. As a hydraulic fluid is circulated through a system, heat is transferred from high temperature areas to coolers, reservoirs, and other regions of the circuit where it is dissipated. As can be seen in Table 1, typical specific heat and thermal conductivity values for hydraulic oils are a fraction of that of water [4]. These factors are an important consideration in sizing hydraulic system coolers because the inherent cooling efficiency of petroleum based hydraulic fluid is less than that of water. ASTM D 2717, Test Method for Thermal Conductivity of Liquids and ASTM D 2766, Test Method for Specific Heat of Liquids and Solids are used to determine these properties of fluids.

P o w e r Transfer To transfer power efficiently, a hydraulic fluid must exhibit minimal compressibility. Low compressibility allows all of the pressure applied to the fluid to be available for direct and effective transmission to system components such as motors, cylinders, or other actuators. The compressibility of a fluid is generally discussed in terms of its "bulk modulus," which describes the change in fluid volume as a result of applied pressure [3]. The bulk modulus of a fluid, which is the reciproccd of compressibility, is described by Eq 1. There are a n u m b e r of m e t h o d s available for estimating the isothermal secant bulk modulus of a fluid based upon its viscosity and density characteristics [4,5]. As depicted in Fig. 2, the bulk modulus for oil also varies with temperature [6]. For petroleum oils, compressibility is often assumed to be 0.5% for each 1000 psi pressure increase u p to 4000 psi [7]. Bulk modulus {K) = -Vo

(\PI\V)

Lubrication The durability of hydraulic equipment depends to a large extent upon the lubricating properties of the fluid. As a lubricant, the key function of the hydraulic fluid is to reduce friction between contact surfaces. A reduction in friction lowers contact t e m p e r a t u r e s a n d wear. This is accomplished through a combination of hydrodjoiamic and boundary lubrication mechanisms. The hydrodynamic lubricating properties of a fluid are governed by its physical properties while boundciry lubrication is a function of fluid chemistry. A discussion of hydraulic fluid wear testing is presented in the Wear Protection section of this chapter.

(1)

TRENDS

* Exxon Mobil Research & Engineering, Paulsboro Technical Center, 600 Billingsport Rd., Paulsboro, NJ 08066. ^ Benz Oil, 2724 West Hampton Avenue, Milwaukee, WI 53209.

A brief outline of major trends in the motion control industry, particularly with respect to hydraulic equipment design and fluid requirements, is presented as a backdrop for the discussion of hydraulic fluids test methods. As motion con-

353 Copyright'

2003 by A S I M International

www.astm.org

354 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

Force

Area

F = force in pounds p = pressure in pounds / sq. incli (psi) A = sq. in.

FIG. 1—Relationship of force, pressure, and area in fluid power. Any one of the parameters equals the other two in the relationship depicted by the triangle. TABLE 1—Thermal conductivity and specific heat values for oil and water.

40

^

30

M Q.

Oil Water

z CO

Thermal Conductivity Btu/h/ft^/F/Ft @ 212°F

Thermal Conductivity W/m-K @373K

Specific Heat BTU/lb°F @68°F

Specific Heat J/kg • K @293K

0.08 0.39

0.14 0.67

0.47 1.0

1966 4184

20

I 10 3

m 100

200

300

FIG. 2- -Effect of temperature on the bulk modulus of petroleum fluid.

trol technology advances, there is a trend towards higher performance and efficiency. For hydraulic equipment, this translates into a concentration of horsepower in smaller components. There are a n u m b e r of reasons for such a trend. Equipment manufacturers are looking for ways to minimize raw material usage and cost. Users of the equipment demand smaller systems for better space utilization in industrial environments cind compact multifunctional capabilities in mobile equipment. These advancements in mechanical design along with e n c r o a c h m e n t of environmental, health, and safety regulations fuel the following trends: • Hydraulic equipment builders will continue to push comp o n e n t manufacturers to design parts to a c c o m m o d a t e high pressures a n d t e m p e r a t u r e s . F o r example, hoses, valves, and other fittings will continue to evolve in terms of materials used as well as actual functional design.

Smaller c o m p o n e n t s will m e a n smaller p u m p displacements [cubic inches or cc per p u m p revolution]. To maintain flow rates at present or higher levels, p u m p speeds will be increased [cubic inches/minute = displacement X speed (rpm)]. Smaller reservoir sizes will mean shorter fluid residence times and will therefore dictate use of hydraulic fluids with improved air release characteristics. Smaller dimensional clearances will be required. These smaller clearances will dictate more stringent fluid cleanliness requirements to prevent abrasive wear from particulate c o n t a m i n a n t s a n d failure of servo or proportional valves. Fluid cleanliness will increasingly be emphasized as an effective way of increasing equipment durability and controlling warranty costs. As a result, users will move to finer filtration and specify pre-filtered hydraulic fluids [9]. Consequently, the filterability of the hydraulic fluid will continue to grow in significance. (Filterability is described in section 4.6.) Quieter hydraulic systems will be required in order to meet workplace noise restrictions and compete with electric motors. Reduction of noise levels in hydraulic equipment has been attained by the insulation that absorbs the noise. This insulation results in higher system temperatures, as heat is not as readily dissipated. Components and actuators, such as cylinders, will be designed with tighter seals to increase efficiency and reduce

CHAPTER

13: HYDRAULIC

FLUIDS

355

leakage. The effects of this trend include increased stress on seal materials and cylinder chatter resulting from reduced lubrication between seal and cylinder wall. In addition, certain applications will require fill-for-life systems that translate into lower maintenance and disposal costs. Consequently, fluids will remain in a system for longer periods, since meike-up fluid is not required. • A growing awareness of the environmental impact of chemicals will lead to further restrictions on performance additives eind base stocks used in lubricants. As a result, lubricant p r o d u c e r s are required to address such issues through alternative (usually more costly) chemistry and the development of environmentally friendly (nontoxic/biodegradable) lubricants. • The hydraulic fluid industry has evolved from the use of plain water in hydraulic systems to the use of advanced fluid technologies that continue to evolve as performance requirements become more stringent and equipment designs become more sophisticated [10]. Due to environmental health and safety issues, hydraulic systems are once again being designed to employ pure water as hydraulic fluid [11].

Solvent refining yields base oils that fall into Group I while hydroisomerization and deep hydrogenation processes yield low sulfur, high paraffin content Group II a n d Group III base stocks. Because of their lower aromatic and sulfur content, hydraulic fluids formulated from Group II and Group III base stocks typically have superior oxidation stability. However, more highly refined stocks tend to be less effective at dissolving additives. Not only is additive solubility a concern, additive chemistries and their functional mechanisms may be b o t h synergistic a n d antagonistic. Thus, additive chemistry must be ceirefully balanced to achieve optimum performance. In the following section, test methods for evaluating key fluid properties such as oxidation stability, wear prevention, and corrosion inhibition are discussed. These methods have been developed to measure characteristics of hydraulic fluids that are thought to correlate to performance in "real-life" applications as well as gage additive response for the fluid formulator. In order to provide a link between fluid tests and additive chemistry, a description of the generally accepted functional mechanisms of additives is also included.

PETROLEUM BASE STOCKS

FLUID CHARACTERISTICS AND PERFORMANCE

Most hydraulic fluids consist of a base fluid and additives that are designed to i m p a r t chemical characteristics and functionality to the finished product. Operating conditions and equipment builder specifications generally dictate the type of fluid that is needed and thus, the kind of base stocks and additives employed. In petroleum based hydraulic fluids the typical concentration of additives is less than 3.0% by weight. Paraffinic oils are the primary base stock utilized in hydraulic fluids but other materials, from polyglycols to vegetable oil, serve as the basis for formulating hydraulic fluids. From a historical standpoint, solvent reflned paraffinic oils have been the most widely used base stock for hydraulic applications. In recent years alternative refining processes such as catalytic isomerization and deep hydrogenation have been developed to yield higher purity base oils that are better suited to withstand severe operating conditions [12]. These base stocks are categorized by the American Petroleum Institute (API) according to their composition and viscosity index [13]. Groups I through III consist of crude derived base oils while Group IV is reserved for synthetic polyalphaolefins. Low viscosity index naphthenic oils and other base stocks that do not meet Group I through IV criteria are classified as Group V. The API Base Oil classification is described in Table 2. TABLE 2—API base oil classifications. Category Group I Group II Group III Group rV Group V

Composition < 9 0 % Saturates or > 1 0 % aromatics £ 9 0 % Saturates or < 1 0 % aromatics > 9 0 % Saturates or < 1 0 % aromatics All polyalphaolefins (PAO) All others not included in Groups 1,11, m or IV

Suli^ir >0.03%

Viscosity Index 80-120

10,000

CHAPTER 13: HYDRAULIC FLUIDS 357 Temperature Light, catalyst

_

Initiation

RH

Propagation

R • + O2

-•

ROO*

Peroxy radical

ROO• + RH

-•

ROOH + R*

Hydroperoxide

ROOH

-*•

RO • + • OH

Alkoxy radical

RO» + RH

->

ROH + R •

Alcohol

• OH + RH

->

H2O + R •

Water

Branching

Termination

Alkyl radical

R • + ROO •

Alcohols

RO • + ROO •

Aldehydes

ROO • + ROO •

Ketones

RO • + R •

Acids

R« + R«

Longer chain hydrocarbons

FIG. 4—Reaction scheme for liquid hydrocarbon oxidation.

Hydraulic Oil RO

S

S

RO

S—Zn—S

OR

OR

Base Oil (Paraffinic) and Additives

Machines and Outside Environment

T Reaction withi

Thermal P^°''^^ Deterioration Degradation witii Water ZnSq RO

Decomposition Oxidation Reaction with l\^etai Ions

Polyphosphates 0

0

OR

0- -Zn—0

OR

\ ^ RO

Oxidation Products and Metal Soaps

Wear Particles, Dust, Rust, Water andOtliers

T

Sludge FIG. 5—Mechanism of sludge formation by zinc dialkyldithiophosphate.

358 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK TABLE 4—Cincinnati machine thermal stabihty test performance requirements. Property

Condition of steel rod Visual Deposits Corrosion Condition of copper rod Visual Corrosion Condition of fluid Viscosity Sludge Total acid number

Requirement

No discoloration 3.5 mg maximum 1.0 mg maximum 5 rating maximum 10.0 mg maximum 5% change maximum 25 mg/100 mL max ±50 % maximum

species react with oxygen and non-oxidized oil to form additional free radicals, which propagate the oxidation process. This generally accepted mechanism is described as free radical chain reaction and is illustrated by the steps shown in Fig. 4. Antioxidants interrupt this chain reaction and thus, reduce the rate of oxidation and the resulting viscosity increase and acid and deposit formation. There are two general mechanisms by which these additives inhibit oxidation. The antioxidants are therefore categorized as primary or secondary, depending u p o n the m e c h a n i s m of oxidation inhibition. Primary antioxidants, commonly referred to as "free radical scavengers," react with the peroxy radicals and hydroperoxides to form inactive compounds (Fig. 6) [21]. Examples of primary antioxidants include hindered phenols and aromatic amines. Secondary antioxidants, commonly referred to as "peroxide decomposers," react with hydroperoxides or peroxy radicals to form less reactive compounds. Examples of secondary antioxidants include sulfur a n d / o r p h o s p h o r u s c o m p o u n d s a n d metal dithiophosphates (Fig. 7). Antioxidants genereilly function in the bulk lubricant and are consumed as they do their job [22].

Detergents

IDispersants

Detergents and dispersants are used to delay formation and subsequent deposit of insoluble oil degradation species. The terms detergent and dispersant are often used interchangeably, but are generally differentiated by their composition and primary functionality. Detergents are metallo-organic compounds that neutralize acidic deposit precursors, while dispersants are predominantly organic chemicals that keep insoluble materials dispersed a n d suspended in the lubricant. The t e r m "ashless" dispersants, m e a n i n g non-metallic, is used to further differentiate dispersants from detergents. Some detergents have the ability to disperse and suspend insolubles, while some dispersants are capable of neutralizing precursors of deposits. Typical lubricant detergents include barium, calcium, and magnesium phenates, phosphates, salicylates a n d sulfonates. Ashless dispersants are typically alkyphenol-based or alkyl succinimides.

(R0)3P?-0—O3 (R0)3P + R'OOH H (R0)3P=0 + HOR' FIG. 7—Secondary antioxidants such as the phosphite compound depicted above inhibit oxidation by decomposing hydroperoxides. This prevents the oxidation process from progressing beyond the branching stage In the reaction mechanism.

o» + R00» ROO^^R FIG. 6—Reaction scheme for primary antioxidants. Primary or freeradical trapping antioxidants work by donating a hydrogen radical H* to the peroxy radical formed during mineral oil oxidation. Due to steric hindrance, the antioxidant radical does not attack mineral oil molecules, i.e., R-H bonds. Consequently, the radical chain is terminated.

CHAPTER 13: HYDRAULIC FLUIDS Wear Protection Reduction of friction and prevention of wear is the fundamental purpose of a lubricant. Lubricants reduce friction in machine components by producing a physical or chemical barrier between surfaces that slide or roll past each other. Depending on equipment design and function, lubricants function within three commonly recognized regimes: hydrodynamic, mixed-film, and boundary lubrication (Fig. 8) [23]. Hydrodynamic lubrication is often the dominant lubrication regime under conditions of moderate temperatures and loads. According to the ASM Handbook on Friction, Wear and Lubrication Technology, [24] hydrodynamic lubrication is "a system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film that has sufficient pressure to separate the surfaces." In this regime, viscosity is the most important fluid characteristic because it, in combination with sliding speed, contact geometry and load, determines the thickness of the lubricating film, and determines whether or not the surfaces will contact each other. Fluid viscosity plays an important role in hydraulic applications. A hydraulic fluid that is too low in viscosity will cause low volumetric efficiency, fluid overheating, and increased pump wear. A hydraulic fluid that is too high in viscosity will cause poor mechanical efficiency, difficulty in starting, and wear due to insufficient fluid flow [25]. Since viscosity is a function of fluid temperature, the temperature operating window (TOW) for a particular viscosity grade of hydraulic fluid is a function of temperature. Figure 9 depicts

the TOW for straight grade mineral oil based hydraulic fluids. The viscosity grade indicated in the TOW corresponds to ASTM D 2422, Classification of Industrial Fluid Lubricants by Viscosity System. For example, ISO 32 hydraulic oil generally will provide satisfactory performance in a temperature window of - 8 to 64°C. There are several methods for measuring the viscosity of hydraulic fluid. The most widely utilized method is the ASTM D 445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. In this test, the time is measured for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer at a closely controlled temperature. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer. Based upon D 2442 and ISO 3448, the standard temperature for measuring hydraulic fluid viscosity is 40°C [26]. Typically, the viscosity of a hydraulic fluid is 15-68 mm^/s (centistokes) at 40°C. ASTM D 446, Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers, describes more than 15 types of viscometers that may be employed in performing a D 445 viscosity test. With the exception of invert-emulsion type fluids, hydraulic fluids are generally transparent. Consequently, a tube suitable for transparent liquids such as the popular CannonFenske viscometer may be used. For opaque liquids, a reverse-flow tube is required because it is difficult to see the meniscus as the fluid flows by the timing marks on a standard viscometer. Cannon-Fenske tubes for viscosity measurement of transparent and opaque liquids are depicted in Fig. 10.

1 MIXFn FN M

LUBRICA-•|ON B OUNDARY LIJBRICATION 0.1

c o o c g> o

0.01

»^ O

o 0.001

0.001

0.01

359

0.1

hULL-hlLM LUBRICATION 1 1

10

Sommerfeld number, (rjA// P) x 10"^ FIG. 8—Stribeck Curve of coefficient of friction versus Sommerfeld Number (S), where S = r}N/P. N shaft speed; P, average pressure between shaft and bearing due to applied load; 7), lubricant viscosity.

360 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 100

212

90

94 — 194

80

o

ABRASIVE WEAR

TOTAL WEAR

— ^ •

ELECTROLYTE (WATER)

361

CORROSIVE WEAR WEAR

WEAR DEBRI S

DFRRI.q

FIG. 11—Synergistic view of pump wear process. Fatigue, adhesive, and corrosive wear can be triggered Independently. Resulting wear debris generation leads to abrasive wear.

362

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

ameter in 4-ball wear tests from 0.72 m m to 0.42 m m at 40kg [31]. These results are tjpical of a mineral oil based antiwear hydraulic fluid where average scar diameters of less than 0.50 m m are the norm (P. W. Michael, unpublished data). While four-ball tests are effective in screening antiwear additive response, they do not directly correlate with p u m p tests [32]. This is in part due to the fact that loads in the fourball tests are constant and do not pulsate in the same way that a hydraulic p u m p does as sliding surfaces transition from high pressure to low pressure regions of the pump. In an effort to enhance the correlation between the four-ball test and full-scale p u m p eveJuations Penn State University has performed investigations involving sequential four-ball wear tests. In the sequential four-ball test, wear scars are evaluated at 10 and 40 kg and 600 r p m and the diameter of the scar is measured after the fluid has been replaced by white oil in order to measure the durability of the antiwear film [33,34]. This method yields better correlation with vane p u m p tests. The FZG Test is another bench test used for screening hydraulic fluids. FZG test equipment consists of two gear sets arranged in a foursquare configuration (Fig. 13). The FZG p r o c e d u r e is described in ASTM D 5182 S t a n d a r d Test Method for Evaluating the Scuffing (Scoring) Load Capacity of Oils. In this test, pre-examined gears are immersed in 1600

mL of oil that is heated to 90C (194°F). The test gear set is r u n in the test fluid for 15 min at successively increasing loads until the failure criteria is reached. According to the ASTM procedure, failure criteria are reached when the summed total width of scuffing wear damage from all 16 teeth is estimated to equal or exceed one gear tooth width. In DIN 51524, Part 2, a m a x i m u m weight loss of 0.27 mg/kW h for antiwear hydraulic oil is specified as well as a m i n i m u m damage stage of 10. While Reichel reported a correlation between FZG Test results and hydraulic fluid performance in vane pumps, correlation with piston p u m p performance has proven difficult to establish [35]. The most widely referenced vane p u m p wear test for hydraulic fluids is ASTM D2882, Standard Test Method for Indicating the Wear Characteristics of Petroleum and NonPetroleum Hydraulic Fluids in a Constant Volume Vane Pump (Vickers 104C). In this test, a hydraulic fluid is circulated through a rotary vane p u m p for 100 h at a p u m p speed of 1200 r/min and a p u m p outlet pressure of 2000 psi. The fluid temperature is controlled to 150°F at the p u m p inlet for most fluids. Petroleum based fluids with a viscosity greater than 46 mm'^/s and some synthetic fluids must be evaluated at 175°F. At the end of the test, the total cam ring and vane weight losses are measured and reported. Based upon ASTM

Drive gear case

Test gears with long addenda

FIG. 13—The Neimann (FZG) Four-Square Gear Test Rig.

CHAPTER D 6158, Standard Specification for Mineral Hydraulic Oils, less than 50 mg of total wear is expected from properly formulated petroleum based antiwear hydraulic oil. For invertemulsion type fluids, higher wear rates in the 100-200 mg range are c o m m o n while water glycol fluids routinely generate less than 50 mg wear in the D 2882 test. While the D2882 test is a popular benchmark for evaluating hydraulic fluids, this method is not without its problems. First of all, Vickers has discontinued production of the V104C p u m p . This will ultimately necessitate the use of substitute hardware or abandonment of the test procedure. Second, rotor and bushing failures are common in the first few hours of the test. This may be due to the fact that the p u m p was originally designed for a m a x i m u m pressure of 1000 psig. Fluid performance in the V104C p u m p is evaluated at 1000 psi using the ASTM D 2271, Standard Test Method for Preliminary Examination of Hydraulic Fluids (Wear Test). In this procedure, the p u m p stand is operated for 1000 h, which provides an extended evaluation of p u m p wear behavior under normal operating conditions. Xie et al. provide a detailed discussion of the D 2882 Test Method in the Handbook of Hydraulic Fluid Technology [36]. For higher pressure a n d mobile applications Vickers prefers their 35VQ25 vane p u m p for screening hydraulic fluid wear performance (Table 5). In the 35VQ25 test, three 50-hour tests are conducted on the same charge of test oil. For each 50-hour test a new p u m p cartridge is used. The test rig is operated at 3000 psi and 200°F with a p u m p speed of 2400 rpm. Vickers limits the amount of wear on each test kit to 90 mg: 75 mg ring, 15 mg vanes. In addition there must be no sign of scuffing on the cam ring. The Denison T6C vane p u m p test is a variable pressure vane p u m p test. In this test, a Denison T6CSH 020 p u m p cycles between 7 b a r (—100 psi) and 250 bar (—3600 psi) at onesecond intervals for 300 h [37]. The p u m p speed is nominally 1700 r/min a n d fluid t e m p e r a t u r e is m a i n t a i n e d at 80°C (176°F) for mineral oil based fluids a n d 45°C (113°F) for those based on water. The test is r u n in two 305-hour sequences. Each 305-hour test consists of a 5-hour break-in period followed by 300 h of high pressure cycling. After the first 305-hour test, the p u m p cartridge is removed for inspection and a new cartridge is installed for the second sequence. The second 305-hour sequence is r u n with 1% distilled water added to the fluid. The first stage of the T6C test serves as an aging mechanism and increases the susceptibility of the fluid to the deleterious effects of water contamination. After the second 305-hour sequence the p u m p cartridge is again removed for inspection. As with the 35VQ25 test, weight loss of cam ring and vanes, vane tip profile, and visual appearance of all components are all reported. In addition, a wet filterability test is performed on the fluid to determine if water contamination will lead to filter blinding. (See the Filterability section for a discussion of filterability tests.) Although the V104C and 35VQ25 vane p u m p tests have served the industry well for many years, these tests are not sufficient to screen hydraulic fluids that will be used in highpressure piston p u m p s applications [38]. Thus, piston p u m p tests have been to qualify the antiwear capabilities of hydraulic fluids. Komatsu, Rexroth, and S u n d s t r a n d piston p u m p tests are described below. K o m a t s u developed a piston p u m p test to evaluate

13: HYDRAULIC

FLUIDS

363

biodegradable vegetable oil based hydraulic fluids [39]. This test is based on a Komatsu HPV35+35 twin-piston p u m p using cycled pressure test conditions. In this test p u m p efficiency change, wear and surface roughness, formation of lacquer and varnish, a n d hydraulic oil deterioration are evaluated. Rexroth has proposed a three-stage piston p u m p test based on the Brueninghaus A4VSO piston p u m p [40]. Stage one is conducted at the m a x i m u m operating pressure and temperature and at the m i n i m u m viscosity specified for the fluid being tested. The test duration is 250 h at which time the p u m p is dismantled and inspected. The second stage of the test is pulsed pressure test at the m a x i m u m displacement of the p u m p . This stage is operated for one million cycles. When this stage is complete, the p u m p is dismantled and inspected. The third stage is a variable displacement stage at maximum pressure, maximum temperature, and m i n i m u m fluid viscosity. The test duration is 280 h at which time the p u m p is dismantled and inspected again. The final pass/fail assessment is made with reference to a standard damage catalog. The Sundstrand Water Stability Test Procedure test originally employed a Sundstrand Series 22 piston p u m p at a constant pressure [41]. Currently, this test procedure is conducted using a Sundstrand Series 90 piston p u m p with a 55-cc displacement. The objective of the test is to determine the effect of water contamination on mineral oil hydraulic performance and yellow metal corrosion. However, other fluids, including water-containing fluids such as HFB and HFC fluids, may also be evaluated using this test. The test duration is 225 h, at which time it is disassembled and inspected for wear, corrosion, and cavitation. If the flow degradation is equal to or greater than 10%, the test is considered to be a "fail." Antiwear

and Extreme

Pressure

(EP)

Additives

Antiwear and EP additives prevent wear of metal surfaces by forming a protective chemical film between moving parts. These additives have traditionally been labeled as antiwear or extreme pressure (EP), depending on the mechanism of protection. Antiwear additives are generally considered to form protective films that adsorb on the metal surface and function effectively under relatively mild conditions of load and t e m p e r a t u r e . Extreme pressure additives form protective films by reacting with the metal surfaces at localized high temperatures to form low shear strength films that are relatively insoluble in the bulk oil. In either case, tribological contact is between the surface films rather than the metals. Various types of chemistry are employed in the prevention of wear in hydraulic applications. Typical compounds include zinc dialkyldithiophosphates (ZDTP), tricresylphosphates (TCP), sulfur compounds, amine phosphates, dithiocarbamates, and other chlorinated, phosphorus/sulfur, and molybdenum compounds. Water Content a n d Hydrolytic Stability In many hydraulic systems, the lubricant is susceptible to contamination with water. Contamination with water can lead to a host of problems including loss of lubricity, corrosion, additive degradation, and filter plugging. Consequently, machine builders and equipment users often attempt to limit the amount of water that enters their hydraulic systems. At the same time, fluid formulators endeavor to manufacture

364 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK TABLE 5—Machine builder specifications for antiwear hydraulic oil. Properties Method(s) ISOVG Kinematic Viscosity, cSt D445 0°C max., calc. D 5133 40°Cmax. 40°C min. 100°Cmin. Flash Point °C min. D92 Fire Point, C min. D 92 Pour Point, °C, max D97 Color, max D 1500 ISO Contam. Code, max ISO 4406 Density @15°C D1298 TAN, mg KOH/g, max D664/ D974 Rust Test A D665 A Rust Test B (Salt Water) 0665 B Cu Rating (3 hr, 100°C), max. D130 TOST Oxidation, Hours to 2.0 ,^^, TAN "*^ Air Release @ SOX, minutes Q » J ~ , (max) Foam tendency/statxiity D892 Seq 1 max Seq II max Seq III max DemulsilJility @ 54°C D1401 FZG Fail Stage D 5182 Change in Hardness NBR1,168hrs@100°C Change in Volume (%) NBR1,168hrs@100°C Viscosity Index, min D 2270 Aniline Point C min. D 611 CM Thermal Stabtity D 2070 A Viscosity Change, % max TAN Variation, % max * Comparative IR Scan Sludge, mg/100 ml max Cu metal removed, mg/200 ml, max. Copper rod appearance, rating (max.) Steel deposits, mg/200 ml, max Steel metal removed, mg/200 ml, max Steel rod appearance, rating (max) Oxidation (1000 h) D4310 AN, mgKOH/g max Total sludge, mg max. Copper, mg max Iron, mg max Hydrolytic Stability D 2619 Copper wt loss, mg/cm^ max Water layer TAN. mgKOH max V104 C Pump mg wear, max D 2882 Vickers 35 VQ 25 Pump Test Vane Wear, mg max Rir^ Wear, mg max Denison P-46 (100 h) DenisonTBC, vane wear TP-30283 Cam ling wear Denison Fnterability Test, sec TP 02100 Dry, max Wet, max

Denison HF-0

Vickers

Requirements

Requirements

6 M LS-2

Cincinnati iWachine .

P68 32

P70 46

P69 68

35.2 28.8

50.6 41.4

74.8 61.2

188 215

196 218

196 218

2

3

3

LH-02 22

LH-03 32

LH-04 46

LH-06 68

300 24.2 19.8 4.1 175

420 35.2 28.8 5 190

780 50.6 41.4 6.1 190

1400 74.8 61.2 7.8 195

-21

-18

-15

-12

10

10

19/16/13 0.84 - 0.90 Pses

Pass Pass

Pass 5

5

Timeto40/40«)(O/W/E)

-10 -/O -10

90 100



Oto-7

Oto-7

Oto-6

OtolS

0to12

0to12

OtolO

Record < 25 mg. /100ml >

100 10 Report

Oto-8

< 25 mg. /100ml >



2 200 50 SO (1) 0.2 4.0

I-2S6-S SO M-2950^ 15 75

Satisfactory Satisfactoiy

600 2xdry

(1) Rqmnt. Sut>ject to Denison discretion (based on other pump/fietd history) (2) D 2882 mn at 79,4C (higher temp.) for ISO 68 and higher grade.

< 50 >

(2) no smear, scratch, etc < 0.01 > No distress < 2 X dry >

CHAPTER 13: HYDRAULIC FLUIDS hydraulic fluids that resist chemical degradation or hydrolysis in the presence of water and heat. Several ASTM methods are used to monitor water content of hydraulic fluids as well as their ability to resist hydrolytic degradation. Distillation, Centrifuge and Karl Fisher Titration Tests In ASTM D 95, Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation, the material to be tested is diluted with a water-immiscible solvent such as toluene and heated under reflux conditions. The resulting distillate is condensed and separated in a trap. The amount of water present in the sample is determined by observing the volume of water settled in the graduated section of the trap. Centrifuge tests such as ASTM D 96, Standard Test Method for Water and Sediment in Crude Oil by Centrifuge Method, can also be used for un-emulsified or insoluble water contamination in fluids. While distillation and centrifuge methods provide reasonably accurate results for samples that contain free water contamination, these methods are generally not sensitive enough for hydraulic applications. A more accurate method for quantifying water in hydraulic fluid is the Karl Fischer test (ASTM D 1744, Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent) [42]. In this test, the fluid is dispersed in a solvent such as methanol and titrated with standard Karl Fisher reagent to an electrometric endpoint (Fig. 14). The endpoint of the titration, at which free iodine is liberated.

365

may be registered either potentiometricly or by color indication. Although this method has the capability to be more accurate than distillation or centrifuge techniques, the Karl Fisher Test is susceptible to chemical interference. Calcium sulfonate, magnesium sulfonate, ZDTP and other oil additives react with iodine and have been known to interfere with the titration [43]. Hydrolytic Stability Testing Hydrolytic stability refers to the lubricant's resistance to chemical interactions with water that result in undesirable changes to fluid properties. Certain chemical components may react with water to decompose or form undesirable byproducts of hydrolysis. Heat and catalysts such as copper can accelerate the process of hydrolysis. Hydrolytically unstable oils form insoluble contaminants and acidic compounds that create hydraulic system malfunctions similar to those produced by oxidation and thermal degradation of fluids. Furthermore, antiwear additives and corrosion preventatives that are susceptible to hydrolysis are likely to lose their ability to perform their critical functions in the presence of heat and water. ASTM D 2619, Standard Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) is used to measure this fluid property. In this test, 75 g of fluid and 25 g of water are sealed in a beverage bottle with a copper strip. The test bottie is rotated in an oven for 48 h at 93°C (200°F). At the end of the test, the oil and water layers are separated

FIG. 14—The Karl Fisher apparatus (a) titrant solution, (b) burette, (c) titration cell with electrode, (d) solvent, (e) waste.

366

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3 7 ; FUELS AND LUBRICANTS

HANDBOOK

and insolubles are weighed. Viscosity a n d acid numbers are also determined. Based upon the Denison HF-0 specification (see Table 5 for details of this specification), t h e weight change of t h e copper specimen should b e less t h a n 0.20 mg/cm^ a n d the water layer acidity should be less than 4.0 mg KOH. Since exposure to water can be expected throughout the life of a fluid, hydrolytic stability is a n important design characteristic of hydraulic fluids. In genereJ, there are no additives specifically used to improve hydrolytic stability. Instead, hydrolytic stability is achieved by appropriate selection of stable components that maintain effectiveness even in t h e presence of water. Hydrolytic stability is also a key factor in the wet filterability behavior of hydraulic oils (see the Filterability section) [44]. Demulsibility Demulsibilty is the term used to describe a fluid's ability to separate from water. As discussed above in the Water Content and Hydrolytic Stability section, water contamination of the hydraulic oil may lead to various problems that adversely affect both fluid a n d equipment durability. Thus, it is desirable for hydraulic oil and water to separate as quickly as possible. In many industricJ applications, water is drained from the hydraulic oil reservoirs as it separates and settles on the bottom. For fluids with poor demulsibility, the separation is either very slow or unlikely to occur to any significant degree. Demulsibility

Testing

levels, entrained air is visible to the h u m a n eye as larger bubbles and can cause the oil to become cloudy. Uncontrolled air contamination results in a n u m b e r of undesirable consequences. Entrained air increases the compressibility of the fluid and can adversely affect its response to hydraulic control mechanisms or devices, especially in high-pressure systems. Dissolved or entrained air expands into larger bubbles as its solubility in the fluid decreases as a result of exposure to vacuum conditions at the p u m p inlet. This leads to noise and cavitation, which is the dynamic process of gas cavity growth a n d collapse in a liquid [47]. Several studies of this p h e n o m e n o n have suggested theoretical m e c h a n i s m s a n d documented experimental evidence of wear a n d increased oxidation due to cavitation [48]. Foaming is very much rooted in the fundamentcJ problem of air contamination and consequently, results in many of the same negative effects of air entrainment. It is characterized by the formation of a mass of relatively large bubbles on the surface of the fluid and is usually brought about by turbulent return of oil to the reservoir or migration of entrained air to the surface. It is desirable to have fluids with a low tendency to form foam in the first place a n d have the foam collapse quickly once formed. For effective foam control, the rate of foam collapse must be faster t h a n the rate at which entrained air migrates to the surface to form the foam. Otherwise, the foam layer will continue to increase and air may eventually be re-dispersed in the bulk fluid [49]. In severe cases, oil that produces a significant amount of foam may bubble out of hydraulic reservoir breathers, creating a fluid spill.

The speed at which water is separated from oil and the tendency of an oil to form a cuff of emulsified oil at the interface between the oil and water phases may be measured by ASTM D 1401, S t a n d a r d Test Method for W a t e r Separability of Petroleum Oils and Synthetic Fluids. In this test, a 40 ml sample of oil a n d 40 ml of distilled water are stirred for 5 min at 54°C (130°F) in a graduated cylinder. The time required for the emulsion to separate into water a n d oil phases is recorded. An oil with good demulsibility will completely separate in 30 m i n or less without a "cuff' of emulsified oil between the phases [45].

Air entrainment has increasingly become a concern due to a trend toward smaller reservoir sizes. Shorter fluid residence times therefore dictate use of hydraulic fluids with improved air release characteristics for the reasons discussed above. Several studies have shown that fluid viscosity is a critical factor influencing air release properties. Within a given class of fluids, higher viscosity and lower oil temperatures translate into slower air release characteristics. While different classes of base fluids have demonstrated unique air release advantages, there has been little success in identifying additives that improve air release properties of a base fluid.

Demulsifiers

Foam and Aeration

Demulsifiers are chemicals used to alter the surface tension at the oil/water interface to accelerate separation. T3rpical demulsifiers include alkylphenol ethers, low molecular weight synthetic sulfonates, and polyoxyalkylate resins.

Because of the importance of properly managing air contamination in hydraulic fluids, there are a n u m b e r of standardized test methods for evaluating this feature of fluid performance. The foaming tendency a n d stability of oil may be measured by ASTM D 892, Standard Test Method for Foaming Characteristics of Lubricating Oils. In this test, an oil sample is equilibrated at 24°C (75°F). Air is bubbled through oil for 5 min, and then the oil is allowed to settle for 10 minutes. The volume of foam is measured at the end of both periods. The test is repeated at 93.5°C (200°F) and again at 24°C (75°F) after the foam breaks. Various levels of foaming tendency are permitted by industry standards, but stable foam is generally not tolerated [50,51]. Not only must a hydraulic fluid resist the tendency to form stable foam, it also must allow air to rapidly rise and separate from the fluid. The Waring blender test is one test method that may be used to measure the air release properties of fluids [52]. In ASTM D 3519, Standard Test Method for Foam in Aqueous Media (Blender Test), 200 ml of the fluid is stirred

Aeration a n d Foam Under normal conditions there is always air present in a hydraulic fluid. By volume, it is present at about 7-9% at room temperature a n d atmospheric pressure [46]. In this state, it is not visible to the h u m a n eye and thus referred to as dissolved air. Higher temperatures and/or lower pressures (such as vacu u m conditions) lead to lower dissolved air levels. (See chapter on compressor lubricants for detailed discussion on gas solubility and methods of measuring gas solubility.) Fluid circulation through hydraulic systems and reservoirs may cause mecheinical introduction of air into hydraulic fluids, particularly if reservoir size or design does not allow sufficient residence time for air separation to occur. At elevated

Tests

CHAPTER at an agitation rate of 4000 to 13000 r p m for 30 s. The meixim u m total height at zero time, at 5 m i n a n d 10 m i n is recorded in order to assess the foaming and aeration tendency of a fluid under high shear conditions. Air release properties of a hydraulic fluid may also be quantified by IP 313, DIN 51381 or ASTM D 3427, Standard Test Method for Air Release Properties of Petroleum Oils. In these tests, the time in minutes for finely dispersed air in oil to decrease to 0.2% under standard test conditions is measured using a density balance. Air release times and specifications typically vary with oil viscosity. Defoamants Antifoam additives, generally referred to as defoamers or defoamants, are materials that destabilize the liquid film that surrounds air bubbles. The most commonly used defoamants are silicone polymers (particularly polydimethylsiloxanes), which function as finely dispersed marginally soluble liquid particles. Since silicon defoamants have very low surface tensions, they tend to accumulate at air/oil interfaces. When the larger bubbles rise to the surface and join other bubbles to form foam with only very thin films separating them, silicone defoamants cause these films to rupture, thus accelerating collapse of the foam. While silicone defoamants reduce the foaming tendency of a fluid, they may also tend to increase air entrainment (Fig. 15) [53]. Besides affecting air entrainment in hydraulic fluids, silicone defoamants tend to have poor filterability and storage stability due to their marginal solubility in oil. Non-silicone defoamants are increasingly used to address these disadvantages. Polyalkylacrylate additives are the most common class of non-silicone defoamants recognized in the industry. Although they do not possess the disadvantages of the silicone types, these polyalkylacrylates must be used at higher concentrations to deliver equivalent performance.

13: HYDRAULIC

It is widely recognized that beyond proper fluid selection, good fluid maintenance is the key to reliability and durability of hydraulic equipment. Fluid maintenance is closely linked to fluid cleanliness and filtration. Filtration devices, therefore, are critical tools for maintaining hydraulic fluids and system components. Hydraulic fluid "filterability" is concerned mainly with the appropriate flow characteristics of the fluid through filter media. For proper operation, the fluid should readily flow with m i n i m u m pressure drop across the filter and with negligable depletion of additives. The viscosity and chemistry of the lubricant will affect filterability. Therefore, filter size and materials should be compatible with the circulating fluid. The drive to increase hydraulic system reliability through the use of fine filtration magnifies the importance of this performance parameter. Filterability

Tests

Due to the likelihood of water contamination in many hydraulic systems and its potential impact on fluids, most of the filterability tests are designed to r u n dry and wet (with water added). Hydraulic fluid filterability tests generally consist of filtering a specified quantity of fluid t h r o u g h a standard medium while monitoring changes in flow rate (Table 6). The results are tj^pically reported in terms of a ratio between flow rates with and without water added to the fluid. This approach attempts to account for changes in filterability behavior independent of viscosity. In Denison TP 02100 the time required for complete flow of a standard volume of fluid through a specified filter is evaluated. In the Pall Filterability Test the differential pressure across a specified filter assembly is monitored over the duration of the test and cin appropriate limit is established to discriminate between fluids with good and poor filterability behavior. While key equipment

OIL WITH SILICONES

O O eg <

h-

SETTLING OR "TRANQUIL PHASE"

BLOWING OR TURBULENT PHASE"

367

Filterability

AIR RELEASED DURING BLOWING PHASE

VOLUME OF AIR BLOW IN

FLUIDS

>

»

TIME FIG. 15—Impact of silicone defoamer on foaming tendency and air release. Silicone defoamer decreases the tendency of the oil to generate foam while increasing the tendency of the fluid to retain air below its surface.

368

MANUAL

37: FUELS AND LUBRICANT

Method

TABLE 61—Filterability tests. AFNOR Pall" 0.8jU.M 0.2 70 h 70°C

Medium pore size Percent water added Aging time Temperature

HANDBOOK

Denison

3/LiM 1.0 24 h 70°C

1.2 ^M 2.0 None 25°C "Parkhurst, H., Pall Filterability Index Test for Paper Machine Oils, SLS Report No. 5669, April 1995.

builders and industrial manufacturers may require fluids to meet certain filterability criteria as measured by these tests, global hydraulic oil specifications (i.e. ASTM D 6158, ISO 11158, DIN 51524) have not yet incorporated these procedures. Filterablility

Additives

From a formulation standpoint, identifying and replacing additives with potential filterability problems (i.e., filter material incompatibility, gel-forming tendency, hydrolytic instability, etc.) has been the primary method of improving fluid filterability. Recently, dispersants have been identified that enhance filterability by preventing agglomeration of insoluble species present in the fluid. These dispersants are typically alkyphenol-based or alkyl succinimide polymers of varying molecular weights.

Corrosion Protection Chemical contaminants and corrosive by-products of fluid degradation can cause surface attack of metallic hydraulic system components. Ferrous metal corrosion in a hydraulic system is most often caused by water contamination, while copper and its alloys are susceptible to attack by the products of high temperature fluid degradation. Rusting of ferrous metal is an electrochemical reaction that occurs between the parent metal and the thin oxide layer on the metal surface formed as a result of exposure to the atmosphere [20]. Rust, which is hydrated iron oxide, compromises the integrity of the metaJ surface and adversely affects other important fluid properties w h e n it contaminates the bulk fluid. Ferrous metal corrosion protection in hydraulic systems is usually accomplished by incorporating surface-active additives such as rust inhibitors. There are several ASTM methods for evaluating the corrosion inhibition properties of hydraulic fluids. Corrosion

and Rust

Testing

The ability of fluids to prevent rusting of ferrous parts due to water c o n t a m i n a t i o n m a y be m e a s u r e d by ASTM D 665, Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. In Part A of this test, 10% distilled water is added to oil that has been heated to 60°C (140°F). Round steel rods are polished to remove their oxide coating and immersed in the oil. The oil-water mixture is continuously stirred to avoid separation while the temperature is maintained at 60°C. At the end of 24 h the specimens are inspected for rust (Fig. 16). In Part B of the method, the same procedure is used, except synthetic seawater is substituted for distilled water. As described in Part B, synthetic seawater is made by the addition

of sodium chloride, magnesium chloride, calcium chloride, and several other ionic compounds to distilled water. Part B is particularly pertinent to maritime hydraulic fluid applications where seawater, rather t h a n fresh water or condensation, is a likely source of contamination. The standard test method for measuring vapor phase corrosion inhibition of hydraulic fluids is ASTM D 5534, Test Method for Vapor-Phase Rust-Preventing Characteristics of Hydraulic Fluids. In this test, a steel specimen is attached to the cover of an ASTM D 3603 test apparatus that contains hydraulic fluid maintained at a temperature of 60°C (140°F). ASTM D 3603 is the Horizontal Disk Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water. The specimen is then exposed to water and hydraulic fluid vapors for a period of 6 h. At the end of this time, the specimen is inspected for evidence of corrosion and results are reported on a pass-fail basis. The ASTM D 5534 test is particularly relevant for water-glycol and invert-emulsion hydraulic fluids because corrosion of the underside of reservoir covers has been observed in systems that use these fluids. Accelerated corrosion can also occur when dissimilar metals are in electrical contact in the presence of an electrolyte (i.e., conductive solution). This corrosion mechanism, known as galvanic corrosion, has been found to be particularly relevant for certain biodegradable oils [54], The ability of a fluid to prevent galvanic corrosion may be measured by ASTM D 6547, Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple. In this test, a brass clip is fitted to the oil coated surface of a steel disk. The bi-metallic (brass/steel) couple is then stored in 50% relative humidity for ten days. At the end of the ten-day period, the surfaces are inspected for evidence of staining like that depicted in Fig. 17. The steel disks are rated on a pass-fail basis. Sulfur containing additives such as zinc dithiophosphate, sulfurized olefins, organic polysulfides, and carbamates may be used as antiwear and extreme pressure additives in hydraulic fluids [55]. Depending u p o n the chemical activity of these sulfur compounds, hydraulic fluids exhibit varying degrees of corrosiveness to copper when activated by high temperatures. ASTM D 2070, Standard Test Method for Thermal Stability of Hydraulic Oils is one of the most effective methods for predicting the corrosiveness of a hydraulic fluid to copper and its alloys. The ASTM D-2070 test measures the aggressiveness of chemical constituents in the fluid toward yellow metals when aged under high temperature conditions. (See the section on High Temperature Oxidation Tests) In some cases, such as when a hydraulic fluid is contaminated with sulfur containing metalworking fluid, the fluid may exhibit corrosivity to copper without requiring thermal degradation. The standard test method for measuring the copper corrosion properties of oil is ASTM D 130, Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test. In this test, a polished copper strip is immersed in oil and heated for a predefined period of time. At the end of the test, the copper strip's appearance is compared to a standard. The rating system used for the D 130 test appears in Table 7. The rating system is on a scale of one to four. The higher the copper strip rating, the greater the degree of copper corrosion. Color standards are also available from ASTM for rating copper strips [56].

CHAPTER 13: HYDRAULIC FLUIDS

369

FIG. 16—ASTM D 665 passing vs. failing rod.

FIG. 17—Galvanic corrosion: staining on test specimen by vegetable oil.

Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators Corrosion Inhibitors, Rust Inhibitors, and Metal Passivators are designed to prevent deterioration of metal surfaces that are in contact with the lubricant. Corrosion inhibitors are polar molecules that are surface active. They adsorb on the metal surface and inhibit the electrochemical reaction that produces rust. Some hydraulic fluids, particularly those used in applications that require enhanced fire resistance, are for-

mulated with water. Such fluids have entirely different corrosion inhibition requirements. For instance, water glycol hydraulic fluids must prevent corrosion in the vapor phase above the liquid due to evaporation. Thus they are formulated with vapor phase corrosion inhibitors such as morpholine. Typical classes of rust inhibitors include metallic sulfonates, amine phosphates, simple fatty acids, and succinic acid esters. Triazoles, or derivatives thereof, are commonly used metal passivators.

370

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

TABLE 7—Copper strip classifications. Rating

Designation

la

Slight tarnish

lb 2a 2b 2c

Slight tarnish Moderate tarnish Moderate tarnish Moderate tarnish

2d 2e 3a 3b

Moderate tarnish Moderate tarnish Dark tarnish Dark tarnish

4a

Corrosion

4b 4c

Corrosion Corrosion

Light orange, almost the same as freshly polished strip Dark orange Claret red Lavender Multicolored with lavender blue or silver overlaid on claret red Silvery Brassy or gold Magenta overcast on brassy strip Multicolored with red and green showing (peacock), but no gray Transparent black, dark gray or brown with a trace of peacock Graphite or lusterless black Glossy or jet black

Seal Compatibility Very critical to the successful operation of a hydraulic system is the ability to prevent leakage and accidents that are a result of failed seals. Leaks can lead to contamination, loss of pressure, loss of lubricating fluid, and environmental damage depending on the severity of the spill. In extreme temperature and pressure operations, sudden failure of seeds may have life threatening consequences, considering the potential for explosions, fires, etc. [57]. Hydraulic fluids and elastomeric seals are composed of complex chemical components that can interact as they come into contact. Depending on the chemistries involved, time, t e m p e r a t u r e , a n d mechanical stresses cause fluid interactions with the seal material, resulting in swelling or shrinkage of the elastomer compound. It is desirable to select seal materials that exhibit minimal change in hardness, volume, tensile strength etc. in service. Slight swelling of seals is preferable to shrinkage as indicated in Table 8. This is because a reduction in seal volume may result in leakage of fluid due to failure of the seal to fill the gland that retains it in place. Seal Compatibility

TABLE 8—Recommended property change limits for determining compatibility of elastomer seals for industrial hydraulic fluid applications.

Description

Testing

In general, industry recognized seed compatibility tests entail exposure of the elastomer material to the test fluid for a specified duration and at a standard temperature u n d e r static conditions. Familiar industry seal compatibility tests include ISO 7619, ISO 6072, DIN 53 538, and ASTM D 6546-00, Standard Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications. Other major organizations such as ASTM and SAE also have related specifications for sealing devices. Due to variations in elastomer chemistry, it is necessary to perform compatibility tests on the specific materials being used. While most standard tests measure changes in hardness, stress/strain properties, and volume changes after exposure to the test fluid, translation of these results to a practical application m a y be difficult, since geometry and mechanical conditions of the targeted application profoundly impact the elastomer. It is therefore recommended that seal materials be tested u n d e r conditions that closely simulate the actual application [58].

Maximum Volume Swell,

Time in Hours 24 70 100 250 500 1000

Seal Swell

Maximum Vol. Shrinkage,

%

%

Hardness Change, Shore A Points

15 15 15 15 20 20

-3 -3 -3 -4 -4 -5

±7 ±7 ±8 ±8 ±10 ±10

Maximum Tensile Strength Change, % -20 -20 -20 -20 -25 -30

Agents

These chemicals react with the elastomeric materials to cause slight swelling or softening to counteract the typical effects of temperature and mechanical stress. Seal swell agents are typically used with base fluids having very low aromatic content. Aromatic derivatives or phosphate esters are typically used to enhance the seal swell characteristics of a fluid. Coolant Separability Hydraulic systems used in machine tool operations are susceptible to contamination by aqueous cutting fluids, which contain components with poor oxidation resistance, high deposit forming tendency, and/or high corrosivity. In metcdworking applications, the hydraulic fluid may be considered a contaminant of the cutting fluid that alters its effectiveness in metal removal operations. Regardless of the perspective, a mix of these two categories of fluids is undesirable, especially if they have not been designed to be compatible. In this case, compatibility is defined as the ability of either fluid to complement, enhsince, or at least have no impact on the performance of the other w h e n mixed. The lubricant's ability to readily separate from coolants is highly desirable in most cases. However, the variety and complexity of coolant chemistries makes it difficult to ensure good separability of the hydraulic oil from all metalworking fluids [59]. There are generally n o additives specifically designed to improve coolant separability, since coolant chemistries vary so widely. The t3?pical approach is to formulate a lubricant to have good demulsibility (water separability) and then test its compatibility with specific coolants with which it is expected to come into contact. Coolant

Separability

Testing

A standard industry test method for assessing lubricant compatibility with coolants has not yet been established. However, some Icirge industrial manufacturers and lubricant suppliers do have in-house test procedures designed to simulate oil contamination by a low percentage of coolant, as well as coolant contamination by a low percentage of oil (typically referred to as tramp oil). In general, these procedures consist of mixing the lubricant with the coolant at a specified ratio a n d t e m p e r a t u r e for a s t a n d a r d duration. The fluid container, t5^ically a graduated cylinder, is then allowed to sit while the degree of separation between the coolant and the lubricant is observed at specific time intervals. Properties such as additive leaching and foam stability may also be ob-

CHAPTER served. Rapid separation, implying absence of a stable emulsion or cuff (the layer between way oil and coolant) at the interface, is very desirable (Fig. 18). Shear Stability Mobile hydraulic equipment such as excavators, farm tractors, cranes, and timber harvesters frequently are required to operate under extreme high and low temperature conditions. To accommodate wide-ranging environmental conditions, hydraulic fluids with enhanced viscosity - temperature properties are often employed. These fluids t3^ically contain viscosity index improving polymers that thicken oil at high temperatures, while having little impact u p o n their low temperature fluidity. Viscosity index (VI) is a common means for expressing the variation of viscosity with temperature. The viscosity index of an oil is calculated from the measured viscosity of the fluid at 40 and lOOX using ASTM Method D 2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C. A high VI indicates less relative change in viscosity for a given change in temperature. Vl-improved oils are commonly referred to as multigrade oils, because they meet both the low temperature requirements of low viscosity oils and the high temperature requirements of higher viscosity oils. Conceptually, an SAE

Good

13: HYDRAULIC

FLUIDS

371

lOW-30 multigrade oil consists of a lOW base oil and sufficient polymer to thicken the oil at 100°C to a viscosity equal to that of an SAE 30 weight oil (Fig. 19). Viscosity Index Improvers are typically subjected to mechanical degradation due to shearing of the molecules in high stress areas such as between gear teeth in gear pumps and vane-ring interface in vane p u m p s . High pressures generated in hydraulic systems subject fluids to shear rates up to 10^ s~' [60]. Not only does hydraulic shear cause fluid temperature to rise in a hydraulic system, but shear may bring about permanent viscosity loss in hydraulic fluids [61]. Permanent viscosity loss results from mechanical scission of polymer molecules in multigrade hydraulic fluids and often occurs after a relatively short period of time (

-12

-12

-21

-18

-15

-12

-15 -21

-8 -18

-2 -15

4 -12

2 1000

< Report > < Report > < Pass> 2 1000

2 1000

2 1000

5

5

10

13

30

30

Oto-7

Oto-6



< Report > < Report > < Report >

< Report > < Report >

-

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

5

5

10

13

5

5

10

10



30 10

2



30 10

30 10

30 10

40

-

40 10

40 10

60 10

< Report >

Oto-8

Oto-7

Oto-7

Oto-6

< Report > < Report >

0to15

0to12

0to12

OtolO

2 < Report >

2

HFAE

Oil-in-water emulsions containing typically >80% water Chemical solutions in water containing typically >80% water Water-in-oil emulsions containing approximately 45% water Water-polymer solutions containing approximately 45% of water Synthetic fluids containing no water and consisting of phosphate esters Synthetic fluids containing no water and of other compositions

< 150 in exterxled test>

Commercial Descriptions

Soluble oils High water based fluids Invert emulsions Water-glycols Phosphate esters Polyol esters

30

30

r288 hrs. f 000) Oto-8 Oto-7 Oto15

0to12

0to12

OtolO

< Report > < Report >

25 5

25 5

25 5

200

200 < Report > 50

200

200

50

50

2

•: Report >

Classification

HFDU

HM (Antiwear) 32 46

74.8 61.2 180 168

Symbol

HFDR

22

50.6 41.4 180 168

TABLE 12—ISO designations for fire resistant hydraulic fluids.

HFC

68

35.2 28.8 160 148

Fire resistant hydraulic fluids are used in the basic metals industry, die casting, military, and foundry applications. They may be found in any application where a ruptured hydraulic line presents a potential fire hazard. Fire resistant hydraulic fluids are formulated with materials that have a lower BTU content than mineral oils, such as polyol esters, phosphate esters, and water-glycol solutions. As a result, they b u m with less heat generation than mineral hydraulic oils. As with mineral hydraulic fluids, the International Organization for Standardization has established a classification system for fire resistant fluids based upon composition. Table 12 provides a list

HFB

ASTM D6158

24.2 19.8 140 128

Fire Resistant Fluids

HFAS

Requirements 32 46

50

of the ISO designations for fire resistant hydraulic fluids [69]. While power transmission, heat transfer, and lubrication are essential requirements for all types of hydraulic fluids, it is sometimes necessary to compromise these properties to accommodate a critical fluid characteristic. This is especially true of fire resistant hydraulic fluids. Fire resistant fluids differ from mineral hydraulic fluids in density, compatibility, and lubricating properties. As a result, hydraulic systems are often modified when utilizing a fire resistant fluid. To optimize the performance of fire resistant fluids, the National Fluid Power Association and ISO have published guides for their use [70,71]. These NFFA and ISO documents detail the operational characteristics of fire resistant fluids and provide suggestions for storage, use, and handling of these fluids. Table 13 provides a comparison of the properties of common fire resistant hydraulic fluids. HFA HFA fluids contain greater than 80% water. These products are sometimes referred to as 95:5 fluids, because 5% concentrations are commonly employed. The ISO 6743-4 classification divides HFA into two sub-categories: HFAE and HFAS. HFAE fluids are oil-in-water emulsions. HFAS fluids are chemical solutions or blends of selected additives in water. Typically these products are sold as concentrates and diluted prior to use in service. Because of the high vapor pressure of water, the m a x i m u m recommended bulk fluid temperature for HFA fluids is 50°C [72]. The antiwear properties of these fluids are inferior to mineral hydraulic fluids because the vis-

376

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

TABLE 13—Comparison of c o m m o n fire resistant fluid properties. Property ISO Designation Heat of Combustion" Autoignition Temp, "F* Maximum" Temperature Vapor Pressure, m b a r Specific Gravity Viscosity @ 40°C, cSt Water Content Vane p u m p rating" Compatible Seals

Antiwear Hyd. Oil

Invert Emulsion

Water Glycol

Phosphate Ester

Polyol Ester

HM 29.1 kJ/g 650 150°F 0.001 @ 50°C 0.85-0.88 32-68 0.05%

HF-B 16.3 kJ/g 830 120°F NA 0.91-0.93 80-100 43%

HF-C 5.3 kJ/g 830 120°F 80 @ 50°C 1.05-1.10 40 43%

HF-DR 19.0 kJ/g 1100-H 150°F < 1 @ 150°C 1.02-1.16 22-100 0.05%

HF-DU 21.1 kJ/g 750" 150°F NA 0.91-0.96 46-68 0.1%

100% Buna-N, Viton

33% Nitroxyl, Buna-N

67% Buna-N

67% Butyl, EPR

100% Viton, Buna-N

"Roberts and Brooks Flammability Data, NFPA T2.13.8-1997, a calculated estimate was used for HFDU.

cosity of HFA fluids is comparable to water, approximately 1 cSt. Performance is satisfactory with HFA fluids when suitable components are used but is apt to be poor if used in conventional hydraulic systems. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid a n d c o m p o n e n t suppliers w h e n utilizing HFA fluids. HFB HFB fluids are water-in-oil emulsions consisting of p e t r o l e u m oil, emulsifiers, selected additives, and water. They are commonly referred to as invert emulsions. In an invert emulsion the oil phase, which provides lubricity and rust protection, encapsulates the water phase, which provides fire resistance. The water content of an HFB fluid is normally in the 4 3 - 4 5 % range (w/w). When water content of these fluids drops below 38% due to evaporation, the fire resistance of the invert-emulsion deteriorates. Maintenance of invert emulsions is complicated by the fact that when these fluids lose water through evaporation, a high-shear mixing device is normally necessary for proper addition of make-up. The viscosity properties of invert emulsions are u n u s u a l in that evaporation of water results in a viscosity decrease. Several ASTM methods have been developed specifically for invert emulsion hydraulic fluids. ASTM D 3709, Standard Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions, is used to evaluate the freeze-thaw stability of invert emulsions. ASTM D 3707, Standard Test Method for Storage Stability of Waterin-Oil Emulsions by the Oven Test Method is used to determine if the emulsion has a propensity to separate after 48 h at 85°C. As with HFA fluids, special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and c o m p o n e n t suppliers w h e n utilizing HFB fluids. HFC HFC Fluids are solutions of water, glycols, additives, and thickening agents. They are commonly referred to as waterglycol hydraulic fluids. Typically, water-glycol fluids are formulated with diethylene glycol or propylene glycol and a polyalkylene glycol based thickening agent [73]. The low molecular weight glycol reduces the vapor pressure of tlje

fluid (relative to water) while high molecular weight polyalkylene glycol acts as a thickening agent, much like a viscosity index improver. This combination thickeners and glycols enhance the lubricating properties of a water-glycol and reduces the propensity of the fluid toward cavitation erosion. Nonetheless, operating temperatures for water-glycols are limited to a maximum of 50°C because of the effect of temperature on vapor pressure [74]. Water glycol fluids are highly alkaline due to the presence of amine based corrosion inhibitors. As a result, these fluids can attack zinc, cadmium, magnesium, and non-anodized aluminum, forming sticky or gummy residues. Consequently, these metals should be avoided when selecting system components. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFC fluids. HFD HFD Fluids are non-water containing fire resistant fluids. The first edition of International Standard ISO 6743-4 classification (1982) divided HFD into four sub-categories: HFDR, HFDS, HFDT, and HFDU. In 1999 the standard was revised, deleting the HFDS and HFDT fluids from the classification system. HFDS and HFDT fluids are no longer commercially viable because they were based upon chlorinated materials such as polychlorinated biphenyls (PCBs) or other chlorinated aromatic compounds. Environmental concerns associated with chlorinated hydrocarbons led to withdrawal of these products from the market. On the other hand, HFDR and HFDU fluids continue to be widely used in a variety of commercial and military hydraulic applications. HFDR fluids are composed of phosphate esters. The majority of phosphate ester type hydraulic fluids used in industrial applications are based upon triaryl phosphate [75]. Trialkyl and mixed alkylaryl phosphate esters are used in aviation because of their lower density [76]. Phosphate esters are difficult to ignite because they are non-volatile and chemically stable. The stability of p h o s p h a t e esters is demonstrated by the fact that they do not propagate a flame in the Standard Test Method for Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids (ASTM D 5306-92). The principal reason they do not propagate a flame is that the chemical reactions that take place during

CHAPTER combustion of phosphate esters are endothermic. Thus, phosphate esters generate less heat when burned relative to other HFD fluids. In addition, because their Are resistance is not dependent upon the presence of water or mist suppressing additives, the fire resistance of HFDR fluids does not degrade in service. HFDR fluids have been used in hydraulic applications for more than forty years and are known for excellent inherent lubricating properties [77]. In fact, aryl phosphate esters serve as antiwear additives in mineral oil based hydraulic fluids [78]. However, phosphate esters have a steep viscosity temperature curve, which makes their temperature operating window rather narrow [79]. Hydrolysis is the most c o m m o n form of degradation in HFDR fluid, and can occur in the presence of a small amount of water and heat. When hydrolysis takes place, phosphate esters break down into their constituent acids and alcohols. Due to the frequent presence of water in hydraulic applications, the sensitivity of phosphate esters to water has limited their use and significantly reduced their service life. Phosphate esters are compatible with all common metals except aluminum. Phosphate esters do not "wet" the surface of aluminum and thus aluminum should not be used in tribological contacts such as bearings [80]. Phosphate esters should never be added to systems containing mineral oil or water-based fire resistant fluids. Not only are these materials chemically incompatible with each other, in all probability preexisting gaskets, seals, hoses, and coatings are also incompatible. Special precautions also are required in the selection of filter construction materials and plumbing of p u m p inlets. Thus, it is necessary to work closely with fluid and component suppliers when utilizing HFD fluids. HFDU fluids typically are composed of polyol esters although other materials such as polyalkylene glycols are included in the HFDU category. Trimethylol propane oleate, neopentyl glycol oleate, and pentaerythritol esters are the most c o m m o n of the synthetic polyol esters. Triglycerides derived from soybeans, sunflower, and rapeseed plants are naturally occurring polyol esters that also are used in HFDU fluids. Polyol esters derive their fire resistance from a combination of factors. First, polyol esters have a relatively high flash, fire, and autoignition point. Second, they b u m with less energy than oil because of the presence of oxygen in the molecule. And finally, polyol ester fire resistant fluids employ antimist additives that enhance their spray-flammability resistance [81]. Depending upon the shear stability of the polymer, the fire resistance of the fluid may deteriorate in service. Like phosphate esters, polyol esters have excellent lubricating properties but are prone to hydrolysis in the presence of water [82]. In addition, they are vulnerable to oxidation because of unsaturation irl the fatty acid portion of the ester. These factors tend to limit their service life relative to mineral oils. Most common metals used in hydraulic applications are compatible with polyol ester hydraulic fluids, with the exception of lead, zinc, and cadmium. Unlike other fire resistant fluids, polyol esters performance is satisfactory with comm o n filter construction materials and system designs. Thus it is relatively easy to convert a hydraulic system that operates on mineral oil based hydraulic fluids to HFDU fluids.

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Environmentally Acceptable Hydraulic Fluids Environmentally acceptable hydraulic fluids have found their way into hydraulic applications where there is risk of fluid leaks and spills entering the environment (especially waterways) affecting aquatic and terrestrial life. Some examples of these niche markets include forestry, construction, locks a n d d a m s , heavy-duty lawn equipment, a m u s e m e n t parks/entertainment industry, offshore drilling, and maritime. Most environmentally acceptable hydraulic fluids exhibit two key environmental characteristics: virtual nontoxicity to aquatic life a n d aerobic biodegradability. Organizations such as the Organization for Economic Co-operation and Development (OECD), the Co-ordinating E u r o p e a n Council (CEC), and the U.S. Environmental Protection Agency (EPA) have developed standard test methods to determine the toxicity a n d biodegradability of substances. More recently ASTM has developed a Guide for Assessing Biodegradability of Hydraulic Fluids (ASTM D 6006) and a Classification of Hydraulic Fluids for Environmental Impact (ASTM D 6046) based on the above organizations' methods. Utilizing the methodology from these organizations, standard classifications and performance requirements for environmental fluids have also been established by the International Organization for Standardization (ISO) and regional environmental organizations t h a t a w a r d Eco Labels (i.e., German Blue Angel, Nordic Swan, Japanese EcoMark). ISO environmental hydraulic fluid classifications are described in Table 14. HETG Type HETG fluids are based on naturally occurring vegetable oils or triglyceride esters. Without the addition of a thickener, vegetable oils are limited to a narrow viscosity range between ISO 32 and 46. While HETG fluids biodegrade rapidly, have excellent natureil lubricity and have a natural VI in excess of 200, they are unsuitable for use at high and low temperature extremes. This is because they tend to gel at low temperatures and oxidize at high temperatures. The practical temperature limits for uses HETG fluids is —25°F to 165°F. HEES Type H E E S fluids are based on unsaturated to fully saturated synthetic esters. Common ester chemistries utilized for hydraulic fluids consist of TMP oleates, neopentylglycols, pentaerythritol esters, adipate esters, and complex esters. The synthetic esters provide better performance over HETG t5T3e hydraulic fluids with wider operating temperature ranges, broad range of ISO viscosity grades, and better oxidation stability while still maintaining biodegradability.

TABLE 14—ISO environmental hydraulic fluid classifications. Symbol

Classification

Commercial Designation

HETG HEES

Vegetable oil types Sjmthetic ester types

HEPG HEPR

Polyglycol types Polyalphaolefln types

Vegetable oils and natural esters Polyol esters, neopentylglycols, syntiietic adipate esters Polyglycols Polyalphaolefins (PAO) or synthetic hydrocarbons (SHC)

378 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK HEPG Type HEPG fluids are polyethyleneglycols (PEG), which possess good oxidation stability and low temperature flow characteristics. At molecular weights of up to 600-800, HEPG type fluids are ecotoxicologically harmless and readily biodegradable (>90% in 21 days) [83]. Some disadvantages of this class of fluids include miscibility with water, incompatibility with mineral oils, and aggressiveness toward some common t5^es of elastomer seal materials. HEPR HEPR type fluids are polyalphaolefins (PAO) or synthesized hydrocarbon (SHC) base fluids, which have significantly better viscometric properties over a wider range of temperatures than mineral base fluids with the same standard viscosity classification. Some low viscosity PAOs have shown acceptable primary biodegradability, though not as rapid as vegetable or synthetic ester base fluids (Fig. 22). Additional advantages claimed for synthetic lubricants over comparable petroleum-based fluids include improved thermal and oxidative stability, superior volatility characteristics, and preferred frictional properties.

Another challenge that comes with the various hydraulic applications is that of developing test methods that are truly representative of performance in actual systems. Bench-top tests are to be used as logical indicators of a fluid's response to expected conditions of temperature, pressure, contamination, etc. A significantly higher number of variables concurrently influence the fluid more than any single bench test can simulate. Therefore, standards and specifications consist of multiple bench tests as well as more realistic full-scale test stands that use actual pumps in typical hydraulic circuits. Test methods will continue to evolve as more sophisticated techniques are developed to predict field performance of hydraulic fluids.

ASTM STANDARDS No. D 92 D 95 D 96 D 97 D 130

CONCLUSIONS A well formulated hydraulic oil consists of a properly selected base fluid and the appropriate balance of additives, optimized to provide the best possible overall performance required for the targeted application. The versatility of hydraulics makes fluid power advantageous in a wide variety of industrial and mobile applications. With this versatility comes the challenge of developing fluids that function appropriately in a wide range of conditions, even as environmental health and safety requirements become more and more stringent. New fluid technologies continue to emerge to meet these challenges.

D 287 D 445 D 446 D 471 D 664

Title Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Water in Petroleum Products and Bituminous Materials by Distillation Test Method for Water and Sediment in Crude Oil by Centrifuge Method Test Method for Pour Point of Petroleum Products Test Method for Determination of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Test Method for Rubber Property-Effect of Liquids Test Method for Acid Number of Petroleum Products by Potentiometric Titration

I •

Polypropylene glycols

I Mininnum Maximum

Mineral oils

Hydro-treated mineral oils Polyethylene glycols —1 Vegetable oils

§

Synthetic esters 20

40

60

80

100%

FIG. 22—Chart comparing primary biodegradation of base fluids by CEC method.

CHAPTER D 665 D 892 D 943 D 974 D 1298

D 1401 D 1744 D 2070 D 2270 D 2271 D 2272 D 2422 D 2619 D 2717 D 2766 D 2783 D 2882

D 2983

D 3339 D 3427 D 3519 D 3603

D 3707 D 3709

D 4172 D 4310 D 4684

D 5133

D 5182

Test Method of Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water Test Method for Foaming Characteristics of Lubricating Oils Test Method for Oxidation Characteristics of Inhibited Mineral Oils Test Method for Acid and Base Number by ColorIndicator Titration Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum Products by Hydrometer Method Test Method for Water Separability of Petroleum Oils and Synthetic Fluids Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent Test Method for Thermal Stability of Hydraulic Oils Practice for Calculating Viscosity Index from Kinematic Viscosity at 40°C and 100°C Test Method for Preliminary Examination of Hydraulic Fluids (Wear Test) Test Method for Oxidation Stability of Steam Turbine Oils by Rotating B o m b Classification of Industrial Fluid Lubricants by Viscosity System Test Method for Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) Test Method for Thermal Conductivity of Liquids Test Method for Specific Heat of Liquids and Solids Test Method for Measurement of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method) Test Method for Indicating the Wear Characteristics of Petroleum a n d Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump Test Method for Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Test Method for Acid N u m b e r of Petroleum Products by Semi-Micro Color Indicator Titration Test Method for Air Release Properties of Petroleum Oils Test Method for Foam in Aqueous Media (Blender Test) Test Method for Rust-Preventing Characteristics of Steam Turbine Oils in the Presence of Water (Horizontal Disk Method) Test Method for Storage StabiHty of Water-in-Oil Emulsions by the Oven Test Method Test Method for Stability of Water-in-Oil Emulsions Under Low to Ambient Temperature Cycling Conditions Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method) Test method for Determination of the Sludging and Corrosion Tendencies of Inhibited Mineral Oils Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperatures Test Method for Low temperature. Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature Scanning Technique Test Method for Evaluating the Scuffing Load Ca-

D 5306 D 5534 D 5621 D 6006 D 6046 D 6080 D 6158 D 6278

D 6351 D 6546

D 6547

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379

pacity of Oils (FZG Visual Method) Standard Test Method for Linear Flame Propagation Rate of Lubricating Oils and Hydraulic Fluids Test Method for Vapor-Phase Rust-Preventing Characterisitics of Hydraulic Fluids Test method for Sonic Shear Stability of Hydraulic Fluid Guide for Assessing Biodegradability of Hydraulic Fluids Classification of Hydraulic Fluids for Environmental Impact Practice for Defining the Viscosity Characteristics of Hydraulic Fluids Specification for Mineral Hydraulic Oils Test Methods for Shear Stability of Polymer Containing Fluids Using a European Diesel Injector Apparatus Test Method for Determination of Low Temperature Fluidity and Appearance of Hydraulic Fluids Test Methods for and Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications Test Method for Corrosiveness of a Lubricating Fluid to a Bi-Metallic Couple

OTHER STANDARDS AFNOR NF E48-690: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Mineral Oils AFNOR NF E48-691: Hydraulic Fluid Power. Fluids. Measurement of Filtrability of Minerals Oils in the Presence of Water ANSI/(NFPA) S t a n d a r d T2.13.7R1-1996: Hydraulic Fluid Power - Petroleum Fluids - Prediction of Bulk Moduli ISO 6743/4 Part 4: Family H (Hydraulic Systems), Lubricants, Industrial Oils and Related Products (Class L ) : Classification Part 4: Family H (Hydraulic Systems) ISO 12922: Lubricants, Industrial Oils, and Related Products (Class L)—Family H (Hydraulic systems)—Specifications for categories HFAE, HFAS, HFB, HFC, HFDR and HFDU ISO/DIS 15380: Lubricants, Industrial Fluids and Related Procedures (Class L), Family H (Hydraulic Systems)-Specifications for Catagories HETG, HEPG, HEES and HEPR

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[66]

Piston Pumps and Vane Pumps in Severe Duty Applications, Denison Hydraulics, Marysville, OH, 1995. Claxon, P. D., "Aeration of Petroleum Based Steam Turbine Oils," Tribology, Februaiy 1972, pp. 8-13. Hatton, D. R., "Some Practical Aspects of Turbine Lubrication," The Canadian Lubrication Journal, Vol. 4, No. 1, 1984, p. 4. Rhee, I., Velz, C , and Von Bemewitz, K., Evaluation of Environmentally Acceptable Hydraulic Fluids, Technical Report No. 13640, U.S. Army Tank-Automotive Command, Research Development and Engineering Center, Warren, MI, March 1995. Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook 10th ed., Vol. 18, Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, pp. 98-112. Available from ASTM International Headquarters, 100 B a r r Harbor Drive, PO Box C700, West Conshohocken, PA. F a i n m a n , M. Z. and Hiltner, L. G., "Compatibility of Elastomeric Seals and Fluids in Hydraulic Systems," Lubrication Engineering, Vol. 37, May 1980, pp. 132-137. Ashby, D. M., "O-ring Specifications - Some Important Considerations," Hydraulics & Pneumatics, August 1999, pp. 53, 54, 90. Leslie, R. L. Sculthorpe, H. J., "Hydraulic Fluids Compatible with Metalworking Fluids," Lubrication Engineering, Vol. 28, May 1970, pp. 165-167. Carpsjo, C , Proceedings of International Symposium on Performance Testing of Hydraulic Fluids, Institute of Petroleum, London, 3-6 Oct 1978. Stambaugh, R. L. a n d K o p k o , R. J. "Behavior of Non-Newtonian Lubricants in High Shear Rate Applications," SAE Transactions 82, Paper 730487, Society of Automotive Engineers, Warrendale, PA, 1973. Stambaugh, R. L., Kopko, R. J., and Roland, T. F., "Hydraulic P u m p Performance - A Basis for Fluid Viscosity Classification," SAE International Off Highway Congress, Milwaukee, WI, Paper 901633, Society of Automotive Engineers, Warrendale, PA, 1990. Hydraulic Fluids Information, Denison Corp., Marysville, OH, 1995, p. 2. Rizvi, S. Q. A., "Lubricant Additives and Their Functions," ASM Handbook Volume 18 Friction, Lubrication, and Wear Technology, ASM International, Materials Park, OH, 1992, p. 106. Sharma, S. K., Snyder Jr., C. E., Gschwender, L. J., Lang J. C , and Schreiber, B. F., "Stuck Servovalves in Aircraft Hydraulic Systems," Lubrication Engineering, Vol. 55, No. 7, July 1999. Michael, P. W. and Webb, S., "Future Fluids - What's Coming Down the Hydraulic Line," OEM Off-Highway, January 1998, pp. 34-36.

[67] Lubrizol Ready Reference for Lubricant and Fuel Performance, The Lubrizol Corporation, Wickliffe, OH, 1998, p. 111. [68] Colver, R., Chek-Chart Publications, Simon & Schuster, NY, 1995, p. 45. [69] ISO 6743-4: Lubricants, Industrial Oils and Related Products (Class L) - Classification - Part 4: Family H (hydraulic systems). International Organization for Standardization, Geneva, 1999. [70] NFPA/T2.13.1: R3-1997, R e c o m m e n d e d Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids In Industrial Systems, NFPA, Milwaukee, WI, 1997. [71] ISO 7745: Hydraulic Fluid Power - Fire Resistant (FR) Fluids Guidelines for Use, International Organization for Standardization, Geneva, 1999. [72] Vickers Guide to Alternative Fluids, Eaton Corp., Southfield, MI, November 1992. [73] Totten, G. E. and Sun, Y., "Water-Glycol Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 917-982. [74] NFPA/T2.13.1: R3-1997, Recommended Practice - Hydraulic Fluid Power - Use of Fire Resistant Fluids in Industrial Systems, NFPA, Milwaukee, WI, 1997. [75] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," in Handbook of Hydraulic Fluid Technology G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027. [76] Parker O-Ring H a n d b o o k Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18. [77] O'Connor, J. and Boyd, J., Standard Handbook of Lubrication Engineering, McGraw-Hill, NY, 1968, pp. 2-16. [78] MIL-H-5606G Military Specification: Hydraulic Fluid, Petroleum Base, Aircraft, Missile and Ordinance, NATO code n u m b e r H-515, U.S. Department of Defense, Fort Belvoir, VA, 1994. [79] Parker O-Ring H a n d b o o k Y2000 Edition, Parker Hannifin Corp., Cleveland, OH, 1999, pp. 3-18. [80] Phillips, W. D., "Phosphate Ester Hydraulic Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 1025-1027. [81] Gere, R. A. and Hazelton, R. A., "Polyol Ester Fluids," Handbook of Hydraulic Fluid Technology, G. E. Totten, Ed., Marcel Dekker, NY, 2000, pp. 983-1022. [82] Hohn, B. R., Michaelis, K., and Dobereiner, R., "Load Cjirrying Capacity of Fast Biodegradable Gear Lubricants," Lubrication Engineering, 1999, Vol. 55, p. 37. [83] Bartz, W. J., Environmentally Acceptable Hydraulic Fluids, Technische Akademie Esslingen, Ostfildern, Germany.

MNL37-EB/Jun. 2003

Compressor Lubricants Desh Garg, ^ George E. Totten, ^ and Glenn M. Webster^

Charle's Law states that at a constant pressure, the volume of a gas increases in proportion to the temperature:

COMPRESSORS ARE VITALLY IMPORTANT IN MANY INDUSTRIAL TECH-

NOLOGIES. For example, compressors are used in nearly every industry including steel, automotive, petroleum, mining, food, gas production, and storage and energy conversion [1]. The purpose of the compressor lubricant is to reduce friction and wear of the working parts of a compressor such as bearings, gears, and pistons; reduce internal leakage; and if the oil is compressed in the compression zone, to provide heat transfer to reduce the temperature of the gas being compressed [2]. In addition, a properly formulated compressor lubricant should provide corrosion protection and be sufficiently stable to minimize the potential for deposit formation on hot surfaces within the system [3]. The following topics will be discussed in this chapter: a basic tutorial on gas compression, the classification, operation and lubrication of typical gas compressors, lubricant types and classifications, solubility of common gases, and a review of recommended compressor lubricant testing. The chapter will not discuss refrigeration compressors applications. See Chapter 15.

II V2

T2

In addition, if the temperature of a gas increases as the pressure increases when the volume is held constant then Amonton's Law states that: Pi_ P2

T2

For these calculations, all temperatures are in reference to absolute zero. Therefore, if temperature in degrees Fahrenheit (°F) is used, absolute temperature in degrees Rankine (°R) is calculated from: "Rankine = °F 4- 460 Similarly, if the temperature is in degrees Celsius (°C), the absolute temperature in degrees Kelvin is calculated from: "Kelvin = "C + 273

DISCUSSION

Charle's Law and Boyle's Law are combined to form the wellknown Ideal Gas Law which states:

Gas Laws

PxV, P2V2 Ty T2 Avogadro's Law states that the equal volumes of gases at the same temperature and pressure contain the same number of molecules:

The objective of this section is to provide a basic background of the behavior of gases with respect to pressure, temperature, and volume. For example, Boyle's Law states that at a constant temperature, the product of a pressure and volume of a gas is constant:

PV = nRT

PiFi = P2V2 When performing these calculations, the reference value used to determine the pressure must be indicated. If the reference value is a vacuum, then the pressure is absolute {Pa) However, if the reference level is atmospheric pressure {Patm), then it is called gauge {Pg) pressure. They are related by:

Where n is the number of moles, R is the so-called gas constant that is selected to be consistent with the units of temperature, pressure, and volume used in the calculation (see Table 1). Dalton's Law states that the total pressure (Py) of a mixture is the sum of the partial pressures of the constituent gases (a,b,c, ) in the mixture: PT = Pa + Pb + Pc+

Absolute pressures must be used for the gas law relationships to be discussed here.

Similarly, the total volume of a gas is equal to the sum of the partial volumes of the constituent gases (Amagat's Law): VT=Va + Vb+V,+ If the temperature of a gas decreases or if the pressure increases sufficiently, the gas will undergo a change of state to a liquid. Further decreases in temperature or increases in pressure will convert the liquid into a solid. If the tempera-

' Desh Garg Consulting, 14 Carlson Terrace, Flshkill, NY 12524. ^G.E. Totten & Associates, LLC, P.O. Box 30108, Seattle, WA 98103. 3 63 Rockledge Rd., Hartsdale, NY 10530.

383 Copyright'

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384 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK ture is increased indefinitely, a point will be reached where the gas can no longer be liquified by increasing pressure. The highest temperature at which a gas can be liquified by increasing pressure is called the critical temperature of the gas. The pressure required to liquefy a gas at the critical temperature is called the critical pressure of the gas. A summary of physical constants for selected gases is provided in Table 2. When pressure, temperature, and volume variation of a gas follows the ideal gas law, it is referred to as an ideal gas. However, as the pressure increases, the behavior of a gas deviates from that predicted by the ideal gas law. This is due to the compressibility of the gas and is accounted for in the ideal gas calculation by using the compressibility factor (Z); PiV, TiZ,

PV curve as shown in Fig. 2 [4]. For isothermal compression, temperature is held constant during compression by removal of the heat of compression, and the work performed corresponds to: PrVr = P2V2

Adiahatic (isoentropic) compression occurs when there is no heat added or removed during compression: PiVf

Where k is the ratio of specific heats. Comparison of the work required for an isothermal process (ADEF) and an adiahatic process (ABEF) shows that less work is required for an isothermal process. However, an isothermal process is impossible to achieve, although compressors are designed for as much heat removal as possible. Similarly, adiahatic compression is also impossible since

P2V2 T2Z2

Values for the compressibility factor are obtained from reference charts called general compressibility charts such as those available in Ref. 4.

t

The Gas Compression Cycle PV Curves The product of pressure (P) X volume (V) is work. Work is equal to force X distance where pressure corresponds to force and volume corresponds to distance. The horizontal line shown in the PV curve of Fig. 1 corresponds to the distance, for example, the distance a piston moves and the vertical line corresponds to force on the cylinder of a piston, for example. The area under the curve is P X y and is equal to the work performed during the cycle.

Pd

I'".'"• .--' - V'. -'13

-J,-;

IJJ Isothermal versus Adiahatic Operation Theoretically, there are two ways that a positive displacement compressor can be operated; either isothermally or adiabatically. These modes of operation are illustrated using a

WORK

-h ;:v.

'"•^"/t-i-'i-r'

Ps

TABLE 1—Ideal gas law constants (R). 8.3143 8.3143 1.9872 82.054

P2V'2

I

VOLUME INCREASES

X l O V gs/deg mole joules/d eg mole cal/deg mole cc atm/deg mole

FIG. 1—Illustration of a PV diagram that d o e s not include clearance volume.

TABLE 2 --Physical constants of natural gas components.

Gas Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Hexane Carbon Dioxide Hydrogen Sulfide Nitrogen Oxygen

Chemical Formula CH4 C2H6 CjHg C4H10 C4H10 C5H12 C5H12

CeHn CO2 H2S N2 O2

Critical Temperature (°R) 344 550 666 735 766 830 846 915 548 673 227 278

Critical Pressure (psia) 673 708 617 529 531 483 489 440 1073 1306 492 732

Molecular Weight (g/mole) 16 30 44 58 58 72 72 86 44 34 28 32

Density @ 60°F, 14.7 psia Specific Gravity Air= 1.0 Lbs/ft^ 0.554 1.038 1.523 2.007 2.007 2.491 2.491 2.975 1.519 1.176 0.967 1.105

0.0424 0.0799 0.1180 0.1578 0.1581 0.190 0.190 0.227 0.1166 0.0897 0.0738 0.0843

Specific Heat @ 60°F, 14.7 psia Cp Cv BTU/lb/°F BTU/lb/°F 0.527 0.410 0.388 0.387 0.387 0.383 0.388 0.386 0.199 0.238 0.248 0.219

0.403 0.344 0.343 0.353 0.353 0.355 0.361 0.363 0.154 0.180 0.177 0.157

Cp/Cv 1.308 1.192 1.131 1.097 1.097 1.078 1.076 1.063 1.293 1.325 1.400 1.346

CHAPTER 14: COMPRESSOR ^E

AB - ADIABATIC AC - POLYTROPIC AD - ISOTHERMAL . \

[4]:

THEORETICAL NO CLEARANCE

Tjv = 100 - CiR^'") - 1)

0} W Ul DC

where R is the compression ratio which is defined as the absolute discharge pressure divided by the absolute inlet pressure of a compressor and C is the cylinder clearance (%). This is a theoretical value and equation must be modified to account for inefficiencies such as internal leakage, gas friction, pressure drops through valves, etc. This is done by introducing the factor "L."

Q.

F

..A

VOLUME

—•

FIG. 2—PV diagram illustrating theoretical compression cycles.

some heat is always added or emitted. Actual compression is referred to a polytropic cycle: Where n is experimentally determined for each type of compressor and usually not equal to k. Thermodynamically, isothermal and adiabatic processes are reversible but polytropic processes are irreversible, steady-state processes. The value of n may also be calculated from: (n-l)/n

Ti

385

Leeikage past valves and piston rings, Slight increase of gas volume due to heat rise from the warm cylinder. Theoretical volumetric efficiency (TJV) is calculated from

D CB

UJ

LUBRICANTS

7)^ = 100 - C(i?^* - 1) - L The value of L is variable depending on the compressor, lubricant and the gas. For a moderate pressure air compressor with a petroleum oil lubricant, the value of L may be approximately 5%. Power Requirement To properly size a compressor and its components, it is necessciry to determine the amount of power required to drive the system. To do this, it is necessary to determine the amount of brake horsepower required to compress a given volume of gas from the incoming inlet pressure to the desired discharge pressure [5]. Brake horsepower is defined as the ideal isoentropic (theoretical) horsepower plus any fluid (valve, fluid flow, and other leakage) or mechanical friction losses [5]. Theoretical horsepower may be calculated from:

Pi

Gas Compression Cycle Consider the situation where a gas is compressed in a piston cylinder from the inlet pressure, Ps, to the discharge pressure, Pd, along the lone 1-2 in Fig. 3a. Since it is impossible to discharge all of the gas due to the volume of space not covered by the piston stroke, there will be a residual value referred to as clearance volume. This is typically the area between the cylinder and the head of the piston illustrated in Fig. 3a. Typically, clearance volumes range from 4-20%. Figure 3fo illustrates the completion of the compression stroke along the path 2-3. When the piston reaches point 3, the discharge valve closes and the piston undergoes the expansion stroke 3-4 (Fig. 3c) until the pressure drops below the inlet pressure at point 4. At point 4, the inlet valve opens and the gas fills the cylinder as shown in Fig. 3>d and the process is repeated. Volumetric Efficiency Piston displacement represented by the line 5-1 on the PV curve shown in Fig, 4. The actual capacity is less than that represented by the piston displacement, line 4-1. The ratio of the actual capacity to the total displacement is referred to as the volumetric efficiency. The volumetric efficiency is always less than that that derived theoretically because: • The re-expansion of the gas trapped in the cylinder clearances, • Entrance losses due to the pressure drop at the inlet.

(B)

FIG. 3—The compression cycle. A. Compression Strol £2(1 - C„ Hzn + nH20

Poly(ethylene) waxes may be produced by the industrial polymerization of ethylene using high or low pressure ethylene polymerization technology [10], or as thermal decomposition products of the polyethylene polymers. The molecular weights Emd melting points of the synthetic waxes as compared with the Fischer-Tropsch waxes are listed in Table 4. The market stability of pricing a n d availability of insect and vegetable waxes is affected by climate conditions a n d natural disasters. With the advent of the petroleum industry, the waxes from mineral and synthetic sources surpassed the annual production of the combined total of the other two wax categories. Waxes from insect and vegetable sources are mixtures of long chain fatty acids, esters of aliphatic alcohols, and hydrocarbons. Waxes from mineral origins are chemi-

' Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607-7000. ^ CITGO Petroleum Corporation, Highway 108 South, P.O. Box 1578, Lake Charles, LA 70602. ^ 63 Rocklege Rd., Hartsdale, NY 10530.

525 Copyright'

2003 by A S I M International

(1)

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526

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

TABLE 1—Compositional analysis of beeswax. Component

TABLE 3—Products derived from t h e Fischer-Tropsch process. Approx. Typical Yield (wt. %)

Amount in wt. %

Monoesters, CisHjiCOOCsoHsi; C25H51COOC30H61 55-65 Diesters, triesters, hydroxy diesters 8-12 Free fatty acids, C23COOH-C31COOH 9.5-10.5 Free fatty alcohols, C34OH-C36OH 1-2 Hydroxy-monoesters, Ci4H29CH(OH)COOC26H6i 8-10 Hydrocarbons", C25H52-C31H64 12-15 Moisture and mineral impurities 1-2 "Hydrocarbons most commonly found in beeswax include nonacosane (C29H60) and nentriacontane (C31H64).

Product Paraffins (i.e., methane, ethane, propane, and butane) Olefins (i.e., methylene, ethylene, propylene, and butylene) Gasoline (Cs-Cn) Diesel (C12-C18) Ci9 to C23 Medium Wax (C24-C35) Hard Wax (>C35) Water soluble non-acid chemicals Water soluble acids

7.2 5.6 18.0 14.0 7.0 20.0 25.0 3.0 0.2

TABLE 2—Chemical composition of carnauba and candelilla wax. Carnauba (wt.

Component

Monoesters Fatty alcohols Free fatty acids Hydrocarbons" Resins Moisture and inorganic residue

%)

Candelilla (wt. %)

83-88% 2-3 3-4 1.5-3.0 4-6 0.5-1

28-30% 2-3 7-9 49-57 4-6 2-3

"Hydrocarbons commonly found in carnauba and candelilla wax are principally hentriacontane (C31H64) and tritriacontane (C33H68).

TABLE 4—Comparison of Fischer-Tropsch waxes with other synthetic waxes. Type of Wax Fischer-Tropsch wax Low Pressure polyethylene wax High Pressure polyethylene wax Pyrolysis" waxes

iWolecular Weight

Melting Points, °C

500-1200 900-3000

85-110 90-125

500-4000

85-130

1000-3000

90-130

"Pyrolysis waxes are derived from thermo-cracking of polyethylene.

cally inert and are primarily composed of straight chain (paraffinic) hydrocarbons. Petroleum wax may vary compositionally over a wide range of molecular weight, up to hydrocarbon chain lengths of approximately C50-C60. It is typically a solid at room temperature and is derived from relatively high boiling petroleum fractions during the refining process. Petroleum waxes are a class of mineral waxes that are naturally occurring in various fractions of crude petroleum. They have a wide range of applications that include: coating of drinking cups; an adhesives additive; production of candles and rubber; as components of hot melts, inks, and coatings for paper; and they can be used in asphalt, caulks, and binders. This chapter will provide a review of petroleum waxes including history, production, types, chemical composition, molecular structure, and property testing.

POWER PLANT

COAL

SIEAM 1

GASmCAHOM 4—

''

OXYGEN PLANT

AIR

AE GON N, nrr^f

RAW CAS FDBinCAIION

CO, + H,S-*

OSOTE PflTAR NAPHTHA

PURE GAS

FT PROCESS AI.COHOLS KErroms

LA.

F T WATER WORK-UP

WATER

SEPARATION

FHACnOHAIION

GASES NAI>^HA

CRYOGENIC SEPARATION

WATER

praancATioN

WAX

DISCUSSION Classification of Crude Oils and Chemical Structure of Ingredients

Cj/Ci

CH« EIHYL0IE PLANT

REFORMING

on.

rt:o,

PHACnONAnON OUCOMEBISAHON

••OILS

STEAM O2

CHj

CjHj

FIG. 1—Generalized Sasol Plant for hydrocarbon synthesis by the Fischer-Tropsch Process.

Petroleum crude oil, commonly referred to as crude oil, is a complex mixture of hundreds of compounds including solids, liquids, and gases that are separated by the refining process. Solid components at room temperature include asphalt / bitumen and inorganics. Liquids of increasing viscosity vary from gasoline, kerosene, diesel oil, and light and heavy lubricating stock oils. Also included are the major components of natural gas, which include methane, ethane, propane, and butane [11]. An elemental einalysis of crude oil shows that it consists of primarily two elements: hydrogen (11-14%) and carbon

CHAPTER

19: PETROLEUM

WAXES

527

TABLE 5 --Crude oil content. Crude Type

Solvent Neutral Oil

Base Oil

Wax Content

Sulfur and Nitrogen

Asphalt

API Gravity"

ASTM Test Method

Paraffinic base Naphthenic base Intermediate base Asphaltic base

Yes No No No

Yes Yes Yes Yes

'/^y i i

i

(4)

bm = 1352y«3'Ay + X>''^« ) r ^rri =

'^yAi

For the RM equation there is another alternative in extending it to mixtures by replacing Tc and Pc with Tcm and Pcm as given below:

(b)

.

•

».

..

FIG. 14—(a) An atomic force microscope image of wax "trees" growth in a lowering temperature solidification of wax from solution. Varying the temperature gradient causes a transition between the growth of wax plates and growth of a tree-like structure with regular branches; (b) An atomic force microscope image of banded growth of wax due to addition of crystallization inhibitors (courtesy of Prof. J.L. Hutter).

RM-2: T,m =

{^yiyiTl^pMy^yiyiT,i,pX

Rfn = YI,yiyiR% i J

be superior in some respects to the earlier ones. The RedhchKwong (RK) equation that is a modification of the van der Waals equation, was a considerable improvement over other equations of relatively simple forms at the time of its introduction. In the Soave-Redlich-Kwong (SRK) equation, the temperature-dependent term of a/T"-^ of the RK equation is replaced by a function denoted by a that depends on the acentric factor of the compound and temperature. The PengRobinson (PR) equation is another cubic equation of state involving acentric factor. Riazi and Mansoori [33] modified the parameter h of the RK equation by introducing a function, denoted by '?, that depends o n the refractive index of the compound. They showed that the resulting equation is quite accurate in the prediction of hydrocarbon densities. MohsenNia et al. [34] proposed that the 3M equation in which the repulsive part of the RK equation is modified based on the statistical mechanics improved the thermodynamic predictions appreciably. This equation is shown to be more accurate for heavy hydrocarbon phase behavior calculation than most of the other equations of state. RK and 3M equations are two-constant-parameter equations of state, while the RM,

These equations of state can be used to calculate properties of Wcix, its components, i.e., vapor pressure, and molar volumes of liquid at saturated-, sub-cooled and supercriticEd-conditions as well as the solubility of wax in supercritical solvents. To perform phase equilibrium and other saturated property calculations for WEIX in liquid and vapor states, we need to perform equality of pressures and fugacity calculations [32]. The fugacity coefficient of a component of the wax in a mixture (100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 -2100

4.0 50.0 36.3 39.5 32.4 19.2 15.8 11.0 18.5 33.2 51.4 81.7 89.9 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 -650

19.1 17.2 11.9 8.3 7.0 9.9 16.5 20.7 28.9 33.3 37.4 37.7 31.7 28.8 30.3 23.4 29.3 23.9 30.2 30.4 33.0 69.1 79.4 57.9 82.5 75.8 61.5 56.9 35.4

0.8 0.7 3.0 3.0 5.6 0.8 3.1 2.4 2.7 2.1 3.1 6.6 2.9 3.3 5.8 4.0 5.6 6.2 9.2 10.4 8.8 18.6 24.7 33.9 42.5 44.5 48.9 51.8 12.7

0.5 2.9 2.6 1.9 1.9 1.5 1.9 1.2 1.2 1.5 1.2 4.4 0.4 0.5 2.5 0.6 0.9 1.6 3.0 3.6 2.4 1.2 1.5 1.7 2.7 2.7 3.2 3.9 2.0

47 84 114 101 130 91 88 80 87 82 86 27 40 27 42 27 45 43 44 42 42 21 22 23 12 12 12 12 1483

a b,c b,d b,e b,f b,h b,h b,h b,h c,h b,g,h b b,g,h b b,g,h b b,g,h b b,g b,g b,g b,g b,g b,g b b b b

Overall " Angus et al.. 1979. '' Frenkel et al. , 1997a. •= Goodwin, 1974. '' Goodwin and Roder, 1976. " Goodwin and Haynes, 1982. ' H a y n e s and Goodwin, 1976. ^ Morgan and Kobayashi, 1994. '• Salerno et al. , 1986.

this figure, the 3M and RM equations aire capable of predicting supercritical solubilities accurately. In all these cases the unlike-interaction parameter,fcy,is best fitted to experimental data. Table 15 shows the interaction parameters of various equations of state for a number of systems at various temperatures along with the AAD%. According to this table, the 3M equation of state gives the least value of AAD%. Differential Scanning Calorimetry When a solid is heated, it may absorb heat resulting in a temperature increase or a structural change (phase transition) such as a solid to liquid or a transition from one crystalline form to another. These transitions may be endothermic (absorb heat) or exothermic (emit heat) depending on the thermal process that is occurring. These thermal processes may be quantitatively measured by differential scanning calorimetry (DSC). DSC analysis is performed by heating two small sample pans, one containing the material being analyzed and the other empty and used as a reference. The analysis concept is that the two sample pans are maintained at a very small temperature difference (± 0.01°C). Each pan is heated with two heaters; a main heater and an auxiliary heater. After begin-

ning the experiment by supplying heat with the main heaters, while heating the temperature difference (AT) between the sample and reference pans is sensed using a thermopile (set of thermocouples) which produces a small (0-5/xV) off-setting voltage. The auxiliary heater is then used to heat the sample pan to keep the off-balance voltage close to zero. The instrument displays the differential power (AP) between the two pans as a function of temperature. The area under the peak of differential power (AP) versus temperature (T) provides an experimental measure of the energy or total enthalpy change (AH) of the entire process [39,40]. As described in ASTM Test Method D 4419, the melting point can readily be determined by DSC analysis, as can heat of fusion, which is also an important characterization parameter for waxes. Heat of fusion is defined as the increase in enthalpy accompanying the conversion of one mole, or a unit mass, of a solid to a liquid at its melting point at constant pressure and temperature [43]. The heat of fusion (AATf) is obtained from the melting transition peak illustrated in Fig. 16, by measuring the total area under the peak that is proportional to the heat flow per mass of material. Heat flow is the heat emitted per second, therefore the area under the peak is given in units of (heat • temperature • time"*) for the mass of the sample used. As a result the area per unit mass (APUM)

540

MANUAL 37: FUELS AND LUBRICANTS o

/—tr-s-S^*-" °

-5 -6 ^-7

j 1

0-9 -11 -12

HANDBOOK

/ / RK •

Pr •4

o

/--rrQjQ.oo

-5 -6

Ss -n -12 -13

•

PR

•

f

„M-~-.

Pr

2

Pr

FIG. 15—Solubility of n-tritriacontane (n-CssHes) in supercritical carbon dioxide at 308 K as predicted by various equations of state and compared with the experimental solubility data [32].

of the sample will be APUM

Heat X Temperature Ttime X Mass

Q J^ ~0M

(12)

Typically, the actual units of \Hf&re (joules • Kelvin • seco n d s " ' • g r a m s " ' ) . Typically, the APUM is divided by the heating rate (K/s) of the DCS experiment used to collect the data. This will simplify the expression to yield the specific heat of melting: Q.T APUM Heating Rate

e.M

X e

Q_

M

(13)

Since the mass of the sample that was analyzed is known, it is then multiplied by the heat emitted/gram of sample to 5deld the amount of heat given off (Q) during the melting process. (14) %XM =M M Figure 16 illustrates the DSC traces for three different petroleum waxes; one for each wax type - paraffin (Fig. 16a), intermediate (Fig. 16^), and microwax (Fig. 16c). The DSC

trace shown by Fig. 17 demonstrates the decrease in crystallinity as the melting point of the wax increases. The thermal analysis procedure for this work was started at - 50°C for o p t i m u m crystallization of the wax. The wax sample was heated at a controlled rate to +150°C. The point at which there is a deflection in the base line is the temperature that the wax begins to melt. The point at which the peak scan returned to the base line is the temperature the wax sample is completely melted. The peak area represents the amount of energy used to melt the wax sample and is calculated as described above. In addition, an estimate on the expected melting point can be distinguished. The experienced technologist could tell by looking at the shape of a DSC trace if the wax is a paraffin, intermediate, or microwax. Paraffin waxes typically exhibit sharp peaks as shown in Fig. 16a, DSC peak shapes for intermediate waxes are less sheirp as shown in Fig. \6b, and microwaxes exhibit even less sharp peaks, typically like the peak shown in Fig. 16c. It should be noted that there is a characteristic small transition peak in the DSC trace for a macrocrystalline paraffinic wax as illustrated in Fig. 16fl. The transition that is indicated is a solid-solid phase change (orthorhombic to hexagonal

TABLE 15 —Interaction parameter (ki2) of some systems. System C2H6 - M-C28H58 C2H6 - n-C29H6o C2H6 - M-C30H62 C2H6 - K-C32H66

C2H6 - ra-C33H68

CO2 - n-C28H58

C 0 2 - M-C29H60 CO2 - «-C3oH62 CO2 - n-C32H66

CO2 - n-CjsHfts

AAD%

kyi

T [K]

P [bar]

RK

3M

RM-2

308.2 308.2 308.2 313.2 308.2 313.2 318.2 319.2 308.2 313.2 318.2 307.2 308.2 313.2 318.2 318.6 323.4 325.2 308.2 318.2 308.2 318.2 308.2 318.2 328.2 308.2 318.2 328.2

56-240 65-240 66-200 66-136 66-240 66-200 80-240 80-136 65-240 65-202 65-240 123-181 80-240 90-275 100-250 119-284 125-327 121-284 100-240 100-240 90-250 105-250 120-240 140-240 140-240 120-240 140-240 140-240

-0.4638 -0.4146 -0.4777 -0.4738 -0.5124 -0.5011 -0.5248 -0.4872 -0.4632 -0.4459 -0.4506 -0.3458 -0.3161 -0.2910 -0.2915 -0.3067 -0.2973 -0.2946 -0.2751 -0.1961 -0.3254 -0.3125 -0.4140 -0.3913 -0.3777 -0.3461 -0.3384 -0.3057

-0.2099 -0.1618 -0.2066 -0.2137 -0.2259 -0.2264 -0.2438 -0.2241 -0.1933 -0.1845 -0.1918 -0.0936 -0.0901 -0.0835 -0.0867 -0.0859 -0.0869 -0.0867 -0.0530 -0.0540 -0.1141 -0.1197 -0.1500 -0.1462 -0.1345 -0.1051 -0.1043 -0.0990

0.0807 0.1215 0.1025 0.0901 0.1020 0.1050 0.0966 0.1087 0.1100 0.1433 0.1433 0.2487 0.2477 0.2532 0.2504 0.2531 0.2552 0.2540 0.2782 0.2789 0.2481 0.2439 0.2448 0.2483 0.2596 0.2825 0.2832 0.2878

PR

SRK

RM-1

RK

3M

RM-2

PR

-0.0553 -0.0131 -0.0571 -0.0517 -0.0707 -0.0658 -0.0762 -0.0565 -0.0286 -0.0240 -0.0203 0.0110 0.0296 0.0365 0.0359 0.0347 0.0385 0.0321 0.0645 0.0818 0.0327 0.0273 -0.0162 -0.0035 0.0044 0.0262 0.0280 0.0428

-0.0189 0.0260 -0.0206 -0.0139 -0.0297 -0.0263 -0.0347 -0.0157 0.0137 0.0193 0.0228 0.0507 0.0708 0.0765 0.0746 0.0736 0.0764 0.0690 0.1075 0.1451 0.0779 0.0700 0.0316 0.0426 0.0477 0.0754 0.0872 0.0876

-0.1283 -0.0701 -0.1121 -0.1233 -0.1214 -0.1131 -0.1142 -0.1219 -0.0779 -0.0580 -0.0530 0.0211 0.0194 0.0286 0.0232 0.0278 0.0314 0.0287 0.0670 0.0672 0.0084 -0.0005 -0.0139 -0.0092 0.0104 0.0496 0.0494 0.0557

46.2 53.1 25.0 13.9 46. 24.4 45.2 23.6 50.2 39.6 45.7 51.0 52.3 46.7 64.7 53.7 62.5 64.2 71.8 81.2 69.6 67.5 59.5 67.3 57.5 68.4 65.0 67.4 60.0

27.0 29.9 22.8 30.8 43.0 34.5 17.7 37.4 34.4 21.5 24.2 7.5 18.3 25.0 13.4 8.2 5.8 7.3 21.5 22.2 17.1 8.3 8,1 6.7 9.2 25.0 22.9 18.5 20.3

14.4 23.9 57.2 29.7 56.2 40.2 22.8 39.3 50.7 28.9 28.0 34.4 35.0 44.5 31.0 33.2 28,9 22,6 12,9 6,9 28,8 28.7 24.1 22.3 27.1 11,3 4,8 3,5 28,3

38, 48. 22. 18. 41. 20. 32. 28. 43. 25. 42. 45. 49. 39. 47. 45. 53. 45. 67. 76. 66. 57. 54. 55. 40. 64. 61. 60. 46.

Overall " Kalaga and Trebble, 1997. ' Moradinia and Teja, 1986. " Suleiman and Eckert, 1995 •' Moradinia and Teja, 1988. " McHugh et al., 1984 ^Reverchon et a ., 1993. ^ Chandler et al., 1996.

S5.0

sao 7SJ>

fata ^

TOO

2\S»C 55.syc SISJ 1316.99 2IB.99

sg s ^*' &o1 *"35,BJM Z5JI

"T—

I soo

-2SJ)

TSJ»

IOOLO

12M

1500

Tenq>a-^iire(.Specimen

ViU.:.v.M>'.'-.-..!.-.vA-.X!-'.J-; L

disruption occurs across 50% of the waxed p a p e r surface when the test strips are separated. The temperature at which the first film disruption occurs on the waxed paper when the test strips are separated is the wax picking point. Test Method D 1465 is used to d e t e r m i n e the t e m p e r a t u r e at which two strips of wax-coated p a p e r will adhere to each other. Surface disruption of wax coatings at relatively low a m b i e n t t e m p e r a t u r e s is a performance problem for low melting point waxes. If the surface of a waxed p a p e r is blocked together, then surface gloss and barrier properties will be altered. Two strips of wax-coated paper are placed on a calibrated t e m p e r a t u r e gradient plate for 17 h and removed, cooled, and peeled apart to determine the block point temperature. Figure 29 illustrates a Type A and a Type B blocking plate used for these measurements. Coefficient of Kinetic Friction - Test Method D 2534—A coated surface under load is pulled at a uniform rate over a second coated surface. This is d o n e experimentally by preparing a "sled" with a weight and then pulling it over the surface to be tested using a horizontal plane and pulley assembly. The force required to move the load is measured, and the coefficient of kinetic friction (/j-k) is calculated as follows: fH, = A/B

rf

J ^JJ

^^*/J

FIG. 28—Diagram of relative positions of essential elements of Glossmeter used in Standard Test Method D 1834.

Total Wax Content - Test Method D 3344—Many of the functional properties of a wax-treated paperboEird are dependent on the amount of wax that is present. Test Method D 3344 determines the total amount of wax in a sample of wax-treated corrugated paperboard by extraction. It is applicable to specimens that have been waxed by either impregnation (saturation) operations or coating operations, or combinations of the two. Weight of Wax Applied During Coating - Test Method D 3708—Test Method D 3708 is used to determine the weight of a hot melt coating applied to corrugated board by curtain coating. This method is intended for use as a routine process control in the plant. The a m o u n t of wax applied is determined by attaching a folded sheet of paper to production corrugated board, running the combination through the curtain coater, and subsequently determining the applied weight of wax on the sheet of paper. Blocking Point - Test Method D 1465—The blocking point of a wax is defined as the lowest temperature at which a film

(15)

Where A = the average scale reading from the electronic load cell-type tension tester for 150 m m (6 in) of uniform sliding and B = sled weight (g). The value obtained is related to the slip property of the wax coating. High slip property values may not be desirable for many commercial articles that have been coated with petroleum wax. Abrasion Resistance - Test Method D 3234—This test method is designed to help predict the resistance in change of gloss that coatings may be subject to during the normal handling of coated paper and paperboard products. Abrasion resistance is the resistance to change in gloss when that coating has been subjected to an abrading action by an external object. Test Method D 3234 is conducted by dropping 60 g of sand on a very small area of a coating under fixed conditions. The abrasion resistance test apparatus is illustrated in Fig. 30. Gloss is measured with a 20° specular glossmeter illustrated in Fig. 28 before and after the abrading action by the falling sand. Hot Tack - Test Method D 3706—Hot tack is defined as the cohesive strength during the cooling stage before solidification of a heat seal bond formed by a wax-polymer blend. Flexible packaging materials are formed into finished packages by joining surfaces with heat sealed bonds. The bonding process is performed o n high-speed packaging lines and the application pressure used to hold the surfaces together is released before the bond has completely solidified. The wax-polymer blend must have enough hot tack while still in a molten stage to hold the sealed areas together until the blend has cooled. In Test Method D 3706, flexible packaging specimens are heat-sealed together over a series of temperatures and dwell times. Immediately after each seal is formed and before it has started to cool, a force tending to separate the specimens is applied by a calibrated spring. If the hot tack of the blend is strong enough, the seal remains closed until it has solidified; if not, the seal separates. Thus each spring force and test condition either passes or fails. The pattern of pass/fail results is plotted to the blend characteristics.

CHAPTER 19: PETROLEUM WAXES 553 $1 nun K 30S KHX Itey ttDck mtil Medkt 2S mm » 2B RHR (1" s 1").

K S I S MDI

( r « i s r K »*i

7C2 MR iVty Iwit.

13 MM ^ l / r i h (AM. T«m4l lor 13 mm d / T }

Type A Blocking Plates

Into (Mb tWRi tmiiiimd

Type B Blocking Plate FIG. 29—The two types of blocking plates used In Standard Test Method D-1465 to measure the blocking point of wax.

554 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 500 ml Separatory Funnel

60 g of Sand Stopcock

Size I H

Stem Cut Off from Separator/ Funnel U.S. Standard Sieve No. 12

^ " ^ 2 B mm (I") I.D.

Specimen

For tscating Exact Position for Mailing Glossmeter Readings

For Dropping 60 g of Sand

FIG. 30—Apparatus for measuring abrasion resistance of wax coatings in Standard Test lUlethod D 3234.

Acknowledgments The authors thank Dr. George Totten for his helpful advice and guidance in the preparation of this chapter and Dr. Sony Oyekan, Dr. Chen-Hwa Chiu, Dr. Sang J. Park, and Mr. Adrian D'Sousa for their technical assistance.

D 721 D 937 D 938 D 1168 D 1298

ASTM STANDARDS D 1223 No. D 87 D 97 D 127 D 156 D 287 D 445 D 612

Title Test Method for Melting Point of Petroleum Wax Test Method for Pour Point of Petroleum Products Test Method for Drop Melting Point of Petroleum Wax Including Petrolatum Test Method for Color, Saybolt, of Petroleum Products Test Method for Gravity, API, of Crude Petroleum and Petroleum Products (Hydrometer Method) Test Method for Kinematic Viscosity of Transparent and Opaque Liquids Test Method for Carbonizable Substances in Paraffin Wax

D 1321 D 1465 D 1500 D 1832 D 1833 D 1834 D 2423

Test Method for Oil Content of Petroleum Waxes Test Method for Cone Penetration of Petrolatum Test Method for Congealing Point of Petroleum Wcixes, including Petrolatum Test Method for Hydrocarbon Waxes Used for Electrical Insulation Test Method for Density, Relative (Specific Gravity) or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Test Method for Specular Gloss of Paper and Paperboard Test Method for Needle Penetration of Petroleum Waxes Test Method for Blocking and Picking Points of Petroleum Wax Test Method for Color, ASTM, of Petroleum Products (ASTM Color Scale) Test Method for Peroxide Number of Petroleum Wax Test Method for Odor of Petroleum Wax Test Method for 20° Specular Gloss of Wax Paper Test Method for Surface Wax on Waxed Coated Paper

CHAPTER 19: PETROLEUM WAXES 555 D 2500 D 2534 D 2669 D 2895 D 3234 D 3235 D 3236 D 3 344 D 3451 D 3521 D 3522

D 3 706 D 3708 D4419

D 5442 E 1 E 260 E 355 E 473 E 537

Test Method for Cloud Point of Petroleum Products Test Method for Coefficient of Kinetic Friction for Wax Coating Test Method for Apparent Viscosity of Petroleum Waxes Compounded with Additives (Hot Melts) Test Method for Gloss Retention of Waxed Paper and Paperboard after Storage at 40° C (104° F) Test Method for Abrasion Resistance of Petroleum Wax Coatings Test Method for Solvent Extractables in Petroleum Waxes Test Method for Apparent Viscosity of Hot Melt Adhesives and Coatings Materials Test Method for Total Wax Content of Corrugated Paperbocird Standard Practices for Testing Polymeric Powders and Powder Coatings Test Method for Surface Wax Coating on Corrugated Board Test Method for Applied Wax Coating and Impregnating (Saturating) Wax in Corrugated Board Facing Test Method for Hot Tack of Wax-Polymer Blends by Flat Spring Test Test Method for Weight of Wax Applied During Curtain Coating Operation Test Method for Transition Temperatures of Petroleum Waxes by Differential Scanning CaJorimetry Test Method for Analysis of Petroleum Waxes by Gas Chromatography Specification for ASTM Thermometers Practice for Packed Column Gas Chromatography Practice for Gas Chromatography Terms a n d Relationships S t a n d a r d Terminology Relating to Thermal Analysis Test Method for Assessing the Thermal Stability of Chemicals by Methods of Thermal Analysis

OTHER STANDARDS No. BS 4633 & 4634

BS 4695 DIN 53175

DIN 53181

Title Method for the determination of crystallizing point. Method for the determination of melting point and/or melting range Method for d e t e r m i n a t i o n of melting point of petroleum wax (cooling curve) Binders for paints, varnishes and similar coating materials; determination of the solidification point (titer) of fatty acids (method according to Dalican) Binders for paints, varnishes and similar coating materials; determination of the melting interval of resins by the capillary method

ISO 1392 ISO 2207 ISO 3016 ISO 3841 JIS K 00-64 JIS K 00-65 NFT60-114 NFT20-051

Determination of crystallizing point— General method Petroleum waxes—Determination of congealing point Petroleum products—Determination of pour point Method for determination of melting point of petroleum wax (cooling curve) Testing methods for melting points of chemical products Test methods for freezing point of chemical products Petroleum products—Melting point of paraffins Chemical products for industrial use. Determination of melting point. Method for the determination of crystallizing point (freezing point).

REFERENCES 1] Hackett, W. J., Maintenance Chemical Specialties, Chemical Publishing Co., Inc., NY, 1972. 2] Warth, A. H., Chemistry and Technology of Waxes, Reinhold Publishing Corp., NY, 1956. 3] Bennet, H., Industrial Waxes, Vol. 1, Chemical Publishing Company, Inc., NY, 1963. 4] Puleo, S. L., "Beeswax," Cosmetics and Toiletries, Vol. 102, Allured Publishing CompEiny, Inc., Chicago, 1987. 5] Warth, A. H., Chemistry and Technology of Waxes, Reinhold Publishing Corp., NY, 1956. 6] Letcher, C. S., "Waxes," Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 24, 3'''' ed., 1984, pp. 466-481. 7] Dry, M. E., "Sasol's Fischer-Tropsch Experience," Hydrocarbon Processing, August, 1982, pp. 121-124. 8] Erchak, Jr., M., "Process for the Oxidation of High Molecular Weight Aliphatic Waxes and Product 880kb, U. S. Patent 2,504,400, Washington DC, April 18, 1950. 9] Haggin, J., "Fischer-Tropsch: New Life for Old Technology," Chemical and Engineering News, October 1981, pp. 22-32. 0] Caraculacu, A., Vasile, C, Caraculacu, G., "Polyethylene Waxes, Structure, and Thermal Characteristics," Acta Polymerica, Vol. 35, No. 2, 1984, pp. 130-134. 1] Brooks, B. T., Boord, C. E., Kurtz, S. S., and Schmerling, L., The Chemistry of Petroleum Hydrocarbons, Vol. 1, Reinhold Publishing Corp., NY, 1954. 2] Gruse, W. A., Chemical Technology of Petroleum, 2°'' ed., McGraw-Hill Company, NY, 1942. 3] Mazee, W. M., "Petroleum Waxes," Modem Petroleum Technology, 4"" ed., 1973, pp. 782-803. 4] Vasquez, D. and Mansoori, G. A., "Identification and Measurement of Petroleum Precipitates," Journal of Petroleum Science and Engineering, Vol. 26, Nos. 1-4, 2000, pp. 49-56. 5] Misra. S., Baruah, S., and Singh, K., Paraffin Problems in Crude Oil Production and Transportation: A Review, SPE Production and Facilities, Society of Petroleum Engineers, Richardson, TX, Feb. 1995, pp. 50-54. 6] Holder, G. A. and Winkler, J., "Wax Crystsillization from Distillate Fuels," Journal of the Institute of Petroleum, Vol. 51, No. 499, 1965, pp. 228-243. 7] Mansoori, G. A. and Canfield, F. B., "Variational Approach to Melting," Journal of Chemical Physics, Vol. 51, No. 11, 1969, pp. 4967-4972.

556 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [18] Pourgheysar, P., Mansoori, G. A., and Modarress, H., "A SingleTheory Approach to the Prediction of Solid-Liquid and Liquid-Vapor Phase Transitions," Journal of Chemical Physics, Vol. 105, No. 2 1 , 1996, pp. 9580-9587. [19] Park, S. J. and Mansoori, G. A., "Aggregation and Deposition of Heavy Organics in Petroleum Crudes," International Journal of Energy Sources, Vol. 10, 1988, pp. 109-125. [20] Branco, V. A. M., Mansoori, G. A., De Almeida Xavier, L. C , Park, S. J., and Manafi, H., "Asphaltene Flocculation and Collapse from Petroleum Fluids," Journal of Petroleum Science and Engineering, Vol. 32, 2001, pp. 217-230. [21] Svendsen, J. A., "Mathematical Modeling of Wax Deposition in Oil Pipeline Systems," AIChE Journal, Vol. 39, No. 8, 1993, pp. 1377-1388. [22] Brown, T. S., Nielsen, V. G., and Erickson, D. D., "Measurement and Prediction of the Kinetics of Paraffin Deposition," Journal of Petroleum Technology, April 1995, p p . 328-329. [23] Noll, L., "Treating Paraffin Deposits in Producing Oil Wells," Topical Report NIPPER-551 (DE92001010), Bartelsville Project Office, U.S. Department of Energy, Bartelsville, OK, 1992. [24] Sanchez, J. H. P. and Mansoori, G. A., "In Situ Remediation of Heavy Organic Deposits Using Aromatic Solvents," Paper # 38966, Proceedings of the 68th Annual SPE Western Regional Meeting, Bakersfield, CA, May 1998. [25] Paraffin Products: Properties, Technology, Applications, G. Y. Mozes, Ed., Elsevier, NY, 1982. [26] Murad, K. M., Lai, M., Agarwal, R. K., and Bhattachaiyya, K. K., "Improve Quality of Wax by Hydrofinishing," Petroleum Hydrocarbons, Vol. 7, No. 2, 1972, pp. 144-7. [27] Ferris, S. W., "Characterization of Petroleum Waxes Tappi," TAPPI Special Technical Association Publication No. 2, 1963, pp. 1-19. [28] Himran, S., Suwono, A., smd Mansoori, G. A., "Characterization of Alkanes And Paraffin Wcixes for Application as Phase Change Energy Storage Medium," £nergySoMrce5, Vol. 16,1994, pp. 117-128. [29] Humphries, W. F., Performance of Finned Thermal Capacitors, NASA TND-7690, Washington, D.C., 1974. [30] Haji-Sheikh, A., Eftekhar, J. and Lou, D. Y. S., "Some Thermophysical Properties Of Paraffin Wax as a Thermal Storage Medium," Progress in Astronatics and Aeronautics 86, 1983, pp. 241-253. [31] Du, P. C. and Mansoori, G. A., "Phase Equilibrium of Multicomponent Mixtures: Continuous Mixture Gibbs Free Energy

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40] [41] [42]

[43]

Minimization and Phase Rule," Chemical Engineering Communication, Vol. 54, 1987, pp. 139-148. Hartono, R., Mansoori, G. A., and Suwono, A., "Prediction of Molar Volumes, Vapor Pressures and Supercritical Solubilities of Alkanes by Equations of State," Chemical Engineering Communications, Vol. 173, 1999. pp. 23-42. Riazi, M. R. and Mansoori, G. A. "Simple Equation of State Accurately Predicts Hydrocarbon Densities," Oil & Gas Journal, 1993, pp. 108-111. Mohsen-Nia, M., Modarress, H., and Mansoori, G. A., "A Simple Cubic Equation of State for Hydrocarbons and Other Compounds," SPE Paper No. 26667, Proceedings of the 1993 Annual SPE Meeting, Society of Petroleum Engineers, Richardson, TX, 1993. Nikitin E. D., Pavlov, P. A., and Bessanova, N. V., "Critical Constants of n-Alkanes with from 17 to 24 Carbon Atoms," Journal of Chemical Thermodynamics, Vol. 26, 1994, p p . 177-182. Frenkel, M., Gadalla, N. M., Hall, K. R., Hong, X., and Marsh, K. N., TRC Thermodynamic Tables-Hydrocarbon; Non-Hydrocarbon, R. C. Wilhoit, Ed., Thermodynamic Research Center, The Texas A & M University System, College Station, TX, 1997. Twu, C. H., "An Internally Consistent Correlation for Predicting the Critical Properties and Molecular Weights of Petroleum and Coal-Tar Liquids," Fluid Phase Equlibrium, Vol. 16, 1984, pp. 137-150. Edalat, M., Mansoori, G. A., and Bozar-Jomehri, R. B., "Vapor Pressure of Hydrocarbons, Generalized Equation," Encyclopedia of Chemical Processing and Design - 61, Marcel Dekker, Inc., NY, 1997, pp. 362-365. Letoffe, J. M., Claudy, P., Garcin, M., and Voile, J. L., "Evaluation of Crystallized Fractions of Crude Oils by Differential Scanning Calorimetry, Correlation With Gas Chromatography," Fuel, Vol. 74, No. 1, 1995, pp. 92-5. Braun, R., "Limits in Differential Thermoanalysis of Wsixes," Fette Seifen Anstrichm, Vol. 82, No. 2, 1980, pp. 76-81. Handbook on Antioxidants and Antiozonants, Goodyear Chemicals, Akron, OH, 1977. Wink, W. A., Delevanti, C. H., and Van den Akker, J. A., Instrumentation Studies LXXVII, Study on Gloss I, A Goniophotometric Study of High Gloss Papers, TAPPI, Technical Association of the Pulp and Paper Industry, Vol. 35, December 1953, p. 163A. Tomsic, J., Dictionary of Materials and Testing, 2"^ ed., SAE International, Warrendale, PA, 2000, p. 205.

MNL37-EB/Jun. 2003

Lubricating Greases Thomas M. Verdura, ^ Glen Brunette, ^ and Rajesh Shah~

tremely small, uniformly dispersed, and capable of forming a relatively stable, gel-like structure with the liquid lubricant. Greases are distinct from lubricating pastes that can appear grease-like. Pastes are mostly solids; generally about 70-95% solids, but sometimes merely wetted solids. The solid thickener concentrations of greases range from about 3-30%, tjrpically about 10%. Also, for pastes, affinity between the solid and liquid phases is not essential; neither is it necessary that a stable, gel-like structure be formed. The manufacturing methods also differ. Pastes are simply solid-liquid mixtures formed using low-shear mixing. Grease manufacturing requires considerably m o r e processing, usually including synthesizing the thickener in the fluid. A strict time, temperature, a n d mixing profile must be followed to properly synthesize the thickener. Next, thorough mixing and blending in the desired additives at the proper time and temperature has to be done prior to the final finishing a n d processing. Included in the finishing a n d process steps is homogenization, where t h e grease is passed through a mill to disperse the thickener and additives, or deaerating, to remove entrained air or both where needed.

T H E ESSENTIAL FUNCTION OF ANY LUBRICANT is to prolong the life

and increase the efficiency of mechanical devices by reducing friction and wear. Secondary functions include heat dissipation, corrosion protection, power transmission, and contaminant removal. Generally, fluid lubricants are difficult to retain at t h e point of application a n d m u s t be replenished frequently. If, however, a fluid lubricant is thickened, its retention is improved, a n d lubrication intervals c a n b e extended. A lubricating grease is simply a lubricating fluid which has been gelled with a thickening agent so that the lubricant can be retained more readily in the required area. This is not to say that the thickener does not play a part in the lubrication. Depending on the type of thickener being used and the lubricating regime, some thickeners will contribute in the lubrication. Lubricating greases have a n u m b e r of advantages over lubricating fluids. Some of these are: • Dripping and spattering are nearly eliminated • Less frequent applications are required • Greases are easier to handle • Less expensive seals can be used • Greases can form a seal in many cases and keep out contaminants • They adhere better to surfaces • They reduce noise and vibration • Some grease remains even when relubrication is neglected Grease was previously defined as a gelled lubricating fluid. Although this simplistic definition conveys the general concept of a grease, a more extensive discussion is required to provide a fuller understanding of just what constitutes a lubricating grease. A lubricating grease is a semi-fluid to solid product of a dispersion of a thickener in a liquid lubricant. Additives, either liquid or solid, Eire usually included to improve grease properties or performance. By definition, grease is a lubricant. It is also essentially a two-phase system—a liquid-phase lubricant into which a solid-phase finely divided thickener is uniformly dispersed. The liquid is immobilized by the thickener dispersion that must remain relatively stable with respect to time and usage. At operating temperatures, thickeners are insoluble or, at most, only slightly soluble in the liquid lubricant. There must be some affinity between the solid thickener and the liquid lubricant in order to form a stable, gel-like structure. The thickener can be constituted of fibers (such as various metallic soaps), or plates or spheres (such as certain non-soap thickeners). The essential requirements are that the particles be ex-

' Retired, General Motors NAO Research and Development, Warren, MI. ^ CITGO Petroleum Corporation, Oklahoma City, OK. ^ Koehler Instrument Company Inc., Bohemia, NY.

COMPOSITION Greases are composed of a lubricating fluid, a thickener, and usually performance enhancing additives. A complete discussion of the many variations of grease formulations and manufacturing process would require far more space than allotted for this work. Extensive discussion on grease chemistry and manufacturing process has been published [1-4]. Therefore, only a n overview of the most common grease types and methods has been presented. The lubricating fluids t h a t c a n b e thickened to form greases vary widely in composition and properties and are a n extremely important component of the grease. Lubricating fluids can account for as m u c h as 95% of a grease. By iar, the largest volume of greases in use today consists of those made with petroleum oils thickened with soaps. Many types of petroleum oils are used, including naphthenic, paraffinic, hydrocracked, and hydrogenated. The viscosity of the oil used also varies. Oil blends with a viscosity between an ISO 100 and 220 are most common, however, for specialized greases, the oil viscosity can be lighter or much heavier. In addition to petroleum oils, other lubricating fluids such as vegetable oils, silicones, synthetic hydrocarbons, and others can be used. Of the synthetic fluids used in grease manufacturing the most common type is poly(alpha)olefin (PAO). Because it is more expensive, its usage is small compared t o petroleum oil. Greases made with PAO as the lubricating fluid can provide good performance over a wide-temperature range. Many products have been used as thickeners for grease. Soaps were the first thickeners used and still have the widest

557 Copyright'

2003 by A S I M International

www.astm.org

558 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

S Lithium soaps 0 Calcium soaps H Aluminum soaps H Sodium and other soaps g^o m Clay & other non-soaps B Polyurea FIG. 1—Worldwide grease production by thickener type (1999 NLGI Survey). application (see Fig. 1). Some of the other thickeners that have been employed include polymers, clays, silica gel, and pigments. Soaps are present in greases in the form of fibers. The structure and size of these fibers, i.e., thickness and length, depend on the metallic moiety and the conditions u n d e r which they are formed. In general, soap fibers can vary from about l-100^lm in length with about a 10 t o l length-to-diameter ratio. Large, coarse fibers do not absorb fluids as well as fine, closely knit fibers. Thus, a higher percentage of thickener is required for coarse fiber soaps to make greases having the same consistency as those made with fine fiber soaps. Non-soap thickeners are generally smaller, even colloidal, and have either a spheroidal or plate like structure. Soap thickeners are formed by neutralization reaction of an acid and base to form a salt and water as a byproduct. When the acid is a fatty acid, its salt is called a soap, and the reaction is Ccilled saponification. If the acidic component has a narrow range of moleculcir weight, as in fatty acids, a simple soap is made, e.g., lithium stearate. Reacting the metallic base with two dissimilar acids of widely different molecular weight will form a complex soap, e.g., calcium stearate acetate. Mixed-base greases consist of a mixture of two or more different thickener systems, such as sodium-calcium or alum i n u m complex-clay. The natural fatty materials used for soap formation can be of animed or vegetable origin. Beef tallow and its derivatives are the major source of animal fatty materials. The reaction product with beef tallow derivatives Eind lithium hydroxide is lithium stearate soap. Castor oil derivatives are the m a i n source of vegetable oil fatty materials. Because the acids derived from castor oil have a hydroxyl group (OH) on the 12''' carbon of the acid molecule, the reaction product with castor oil derivatives and lithium hydroxide is lithium 12-hydroxystearate soap. Although the resulting soap is named according to the acid most prevalent, the natural occurring fatty acids are actually mixtures of similar acids (in the form of a glyceride) with slightly different molecular weights. The type of fatty materials used affects the properties of the soap and the grease. Greases made from castor oil derivatives Eire more work stable and have higher dropping points than greases made from beef tallow derivatives due to the hydrogen bonding at the hydroxyl group. The use of mixtures or blends of fatty materials can obtain improved cost, oil separation, worked stability, or other properties.

The most common basic components used to neutralize the fatty acids are hydroxides of lithium, calcium, and sodium. The resulting greases are named according to the basic component and the acidic component used to form the soap, e.g., calcium stearate grease, lithium 12-hydroxystearate grease, etc. Grease Selection When selecting a grease for a particular application, one should first determine the type and viscosity of the lubricating fluid required because the fluid has a major influence in grease lubrication. The properties of greases are determined by the characteristics of the thickener system, by the viscosity and type of the fluid component, and by the performance additives used. Similar criteria as is used for selecting industrials oils can be used when determining the type and viscosity of the lubricating fluid needed in a grease for a particular application. In general, heavily loaded and slow speed applications require higher oil viscosity Eind lightly loaded high speed applications require lower oil viscosity. E a c h thickener system has its own unique characteristics. Additional discussion of the properties of the various soap type greases will be provided in subsequent paragraphs.

GENERAL CHARACTERISTICS Describing the general characteristics of a grease based on the thickener type is difficult because the base oil components and the performance additives used effect the characteristics of the grease as m u c h or more than the thickener type. Therefore, assigning characteristics to a grease based on thickener t5^e has limited value, however, some characteristics inherent to the thickener tjfpe are discussed below. The reader should be aware that some of the weaknesses of a thickener type can be improved and some of the strengths of a thickener type can be degraded with the inclusion of performance additives. Aluminum Soap Greases Aluminum soap greases are usually made with preformed soap unlike most other thickeners that Eire made in situ. Alum i n u m distearate is the m o s t c o m m o n conventional alum i n u m soap used to make grease. It is dissolved in hot oil in a mixing grease kettle, and the hot mixture is poured into pans to gel and cool. The cooling rate affects the final consistency. The final product is a smooth, transparent grease with a tendency to thin when worked but with excellent water resistance. Aluminum soap greases are used as thread and way lubricants. The poor work stability of aluminum-soap greases is used to advantage in electrically conducting conveyers of electroplating systems; the grease thins down during use, allowing good electrical contact in the track rollers while still providing good lubrication.

Calcium Soap Greases The Ccirliest known greases were made with calcium soaps. Greases thickened with hydrated calcium soaps (usually cal-

CHAPTER 20: LUBRICATING cium stearate) are water resistant, work stable, and inexpensive. The name hydrated calcium comes from the fact that a small amount of water (about 0.1% wt.) is required to stabilize the grease. The water associates with the calcium soap and provides the molecular cohesion needed for efficient thickening. These calcium greases are also known as conventioneJ calcium, calcium cup, or lime soap greases. The greatest shortcoming of a hydrated calcium soap grease is its low dropping point, typically about 95°C (200°F). This limits conventioneJ calcium soaps to use in only low temperature applications. Anhydrous calcium soaps (usually calcium 12-hydroxystearate) are not as temperature limited because they do not need water to hold the soap together due to the stabilizing effect of the hydroxyl group (OH). With a dropping point of about 150°C (300°F), they are somewhat more temperature resistant. Anhydrous calcium greases have the same advantages as hydrated calcium greases, but are slightly more expensive cind do not run the risk of drying out and softening. They can be used as multi-purpose greases within their temperature limitations. Historically, the greatest usage for Einhydrous calcium grease has been in a low temperature grease made with a low-viscosity base oil designed to meet earlier versions of the military specification MIL-G-10924 for applications where operation over a wide range of climatic conditions is essential.

GREASES

559

thermal limitations of the oils and additives they contain. Aluminum complex greases have excellent water tolerance characteristics as well as high-temperature resistance, consequently, they are widely used in steel mills and other wet applications. The soap used for aluminum complex grease is usually eJuminum benzoyl stearoyl hydroxide. This soap can be formed by reacting aluminum isopropylate with stearic acid, benzoic acid, and water. To limit or avoid the isopropyl alcohol byproduct, other aluminum compounds can be used. Compatibility with other greases can be a problem with aluminum complex greases. Calcium Complex Soap Greases Calcium complex greases Eire usually made by reacting acetic acid and a fatty acid with lime. The resulting grease has inherent EP (extreme pressure) properties and provides good friction and wear performance. Thickening efficiency is not very good with this soap, therefore, a relative high soap concentration is required. As a result, the low-temperature performance is not as good as other complex soap greases, and they tend to harden in long-term storage or under pressure in lubrication systems. Used within its limitations, calcium complex greases are cost effective but are not compatible with most other grease types. Lithimn Complex Soap Greases

Sodium Soap Greases Sodium (soda) soap greases have higher dropping points [about 175''C (350°F)] than calcium greases and form a very fibrous grease. They Eire not water resistant and emulsify in the presence of water, yet, they have inherent rust protection properties. The soap can be made by reacting sodium hydroxide cind tallow, therefore, is a very inexpensive thickener. In earlier times, when calcium and sodium greases made up most of the market, these greases were the most temperature resistant. They are still used in moderately high temperature applications, such as electric motor bearings. Sodium soap greases are not normally compatible with other greases. Lithium Soap Greases Lithium greases were the first so-called multipurpose greases. They provide both good water resistance, similar to calcium soap greases, and even higher-temperature properties than sodium soap greases. The soap is usually manufactured in a portion of the lubricating oil using lithium hydroxide and a fatty material of either animal or vegetable origin. Lithium 12-hydroxystearate greases have dropping points of about 190°C (375°F). They also have good work stability; that is, they do not soften much when worked. Lithium greases have good over-all performance and are cost effective. In North America more lithium grease is made them all other tj^es combined. Aluminum Complex Soap Greases Complex soap greases are noted for their high-dropping points [230°C (450°F) and higher], although most are not recommended for use up to the dropping point due to

Lithium complex grease performance is generally like that of lithium greases except dropping points are about 50°C higher. Lithium complex is a misnomer used to describe high dropping point lithium greases. Because, lithium is mono-valent (mccining it can only react with one acid per ion) it cannot form a traditional complex soap where two or more acids are reacted with one basic ion. However, there are severed components that can be used to enhance the molecular interactions of the soap molecules and increase the dropping point enough to call the resulting grease a "lithium complex." The most common method is by forming a lithium salt of a dibasic acid (usually azelaic or sebacic) in situ with lithium 12-hydroxystearate soap. Lithium complex greases provide good low-temperature performance and excellent high-temperature life performance in tapered roller bearings. It is the most populcir of the complex greases and has wide-spread application. Polyurea Greases Polyurea greases are similar to the complex soap greases with respect to high-temperature performance; dropping points are about 245°C (475°F). The reactants (isocyanates and amines) are hazardous but the resulting thickener is considered quite safe. Polyurea greases can be made by various methods resulting in various strengths and weakness. Most have good oxidation resistance, water resistance, pumpability, and high temperature performemce. Wesiknesses generally associated with poljoirea greases include poor shear stability, poor storage stability, and incompatibility with other greases, however, some of these weaknesses have been overcome with more recent formulations [5]. Although used in all types of bearings, they have proved especially useful for the

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lubrication of sealed-for-life, ball bearings used in electric motor bearings where superior oxidation resistance and high-temperature life is essential. Organo-Clay Greases Clay thickened greases are made with a modified clay design to have some affinity for the lubricating oil. The process can be as easy as mixing the modified clay thickener with the lubricating oil and providing enough shear to disperse the thickener. They have been referred to as non-melting greases because they tend to decompose before reaching their dropping point. They usually have poor work stability properties as compared to soap greases. Because many of the common additives used in grease will soften clay thickened greases, they are more difficult to inhibit with extreme pressure additives. Although water resistant, they can be susceptible to severe degradation from other contaminants, such as brine. Low-temperature performance could be considered satisfactory, but many clay greases are formulated with high-viscosity base oils for high-temperature applications; such greases will have poor low-temperature properties. Clay greases are not normally compatible with other greases.

CLASSIFICATION AND SPECIFICATIONS Over the past six decades, efforts to develop a grease classification system were frustrated by seemingly insurmountable difficulties. Until recently, the American grease industry had not developed grease specifications for industry-wide applications. However, in 1989, after about 15 years of development, ASTM D4950, Standard Classification and Specification for Automotive Service Grease, was published. Development of this standard was a cooperative effort by ASTM International, NLGI (National Lubricating Grease Institute), and SAE (Society of Automotive Engineers). It is the first and only cooperatively-developed grease specification accepted by American industry. Development of this standard was made possible only because several grease performance tests were developed specifically for automotive applications. These performance tests (D 3527, D 4170, D 4289, D 4290, and D 4693) are discussed subsequently. D 4950 classifies automotive service greases into two chassis grease and three wheel bearing grease categories based on the performance needs of several service conditions. A guide to the requirements of these categories is given in Table 1. The NLGI has established a licensing procedure for greases qualifying for ASTM D 4950, categories GC and LB. In conjunction with licensing, three certification symbols can be used only with the highest-performance categories of chassis and wheel bearing greases. NLGI specifically prohibits the use of the symbols with lesser-performance grease categories. Industry response to the licensing system has grown over the last decade and containers of grease bearing the certification symbols are commonly available in the aftermarket, and their availability is expected to expand. Beginning with the 1992 model year, most U.S. automakers began recommending the use of NLGI Service Greases GC, LB, and GCLB for scheduled maintenance of chassis and wheel bearings of passenger cars and light-duty trucks.

TABLE 1—Guide to requirements for grease categories (ASTM D 4950). Test

Description

LA

LB

GA

GB

GC

D-217 0-566" D-1264 D-1742 D-1743 D-2266 D-2596 D-3527 D-4170 D-4289 D-4290 D-4693

Penetration Dropping Point Water Washout Oil Separation Rust Protection 4 Ball Wear 4 Ball EP High Temperature; Life Fretting Wear Elastomer Compatibility Leakage Low Temperature Torque

/ /

/ /

/ •

• / • / • / /

/ / / / / / / •

• / •

/ / /

•

/

/ / / / / / /

/

'^D-2265 may be substituted.

TEST METHODS AND THEIR SIGNIFICANCE Greases are used for particular lubrication applications because of their intrinsic properties. Users and producers alike need a common means to describe the properties required for grease performance for particular applications. Test methods are devised to describe the requisite properties. When the usefulness of some test becomes known throughout the industry, it is developed, through cooperative effort, into an industry standard such as an ASTM standard. The standard tests used to determine the properties of petroleum oils Eire commonly applied to grease base stocks, as well. Few among these are kinematic viscosity (D 445), flash and fire points (D 92), pour point (D 97), and aniline point (D 611), etc. The significance of these tests are discussed elsewhere throughout this book. There are no standard tests for the evaluation of grease thickeners, per se, because most thickeners (soaps) are formed in situ and are not used independently of the grease. Consequently, there is little need for standard tests to evaluate neat thickeners. However, work is currently in progress, including an ASTM Round Robin using the Penn State Microoxidation test, to study oxidation characteristics of greases, which helps lend limited insight to the evaluation of the thicker system. ASTM and IP (Institute of Petroleum) have developed and standardized a number of tests to describe the properties and performance characteristics of lubricating greases. Because these tests are conducted in laboratories under well-defined conditions, they are used primarily as screening tests. Some of the grease tests do indicate how a grease might perform in service, but direct correlation between laboratory and field performance is often unattainable because the tests never precisely duplicate service conditions. Consistency Consistency can be defined as the degree to which a plastic material, such as a lubricating grease, resists deformation under an applied force. It is a measure of the firmness or rigidity of the thickener structure of the grease. The standard method for measuring grease consistency is the penetration test. Consistency is reported in terms of ASTM cone penetration, NLGI Consistency Number, or apparent viscosity. Cone

CHAPTER 20: LUBRICATING penetrations and NLGI number are discussed in the following paragraphs. Apparent viscosity is included in the Shear Stability section. Cone Penetration ASTM D 217 (IPSO) Test Methods for Cone Penetration of Lubricating Grease is the universal standard for the determination of the penetration of a normal grease sample. (In this context, normal greases are those that are neither too soft nor too hard to be measured by this method.) In this method, a double-tapered cone of prescribed construction sinks under its own weight into a sample of grease at 25°C (77°F) for 5 s. The depth of penetration, measured in tenths of a millimeter, is the penetration value. (It is common practice to omit the units when reporting or specifying penetration vEilues.) Firm greases have low-penetration values, whereas soft greases have high-penetration values. Full-scale penetration tests require about 500 g (1 lb) of sample. The penetration number for smaller samples of grease can be determined by using ASTM D 1403 (IP310), Test Methods for Cone Penetration of Lubrication Grease Using One-Quarter and One-Half Scale Cone Equipment. The one-quarter and one-half scale tests require about 5 and 50 g samples, respectively. As might be expected, the reducedscale tests are not as precise as the full-scale tests. Reducedscale penetration values are not normally used as such; instead, equations are given to convert the reduced-scale penetrations to equivalent full-scale values. Recent advances in instrumentation, such as automatic and digital Penetrometers, have helped improve the precision of this test method. The following paragraphs describe the several types of penetration measurements. Unworked Penetration—This value is obtained when a penetration measurement is made on a grease transferred from the original container to the standard grease worker cup, with only a minimum amount of disturbance. This result is not always reliable, because the amount of disturbance cannot be controlled or repeated exactly. This measurement may be significant to indicate consistency variances in transferring grease from container to equipment. Worked Penetration—Worked penetration is the standard penetration measurement for a grease. It is measured after a grease has been worked for 60 double strokes in the standard grease worker. This method is more reliable than unworked penetration, because the disturbance of the grease is standardized by the prescribed working process. A significant difference between unworked and worked penetration can indicate poor shear stability—and indicate a need for further evaluation by prolonged working or roll test (see the Shear Stability section). Prolonged Worked Penetration—This value is obtained after a sample has been worked for a prolonged period in the grease worker, i.e., 10000, 50000, 100000, etc., double strokes. After prolonged working, the sample and worker are brought back to penetration test temperature 25°C (77°F) in 1.5 h. It is then worked for 60 double strokes and the penetration is measured. This test is significant because it can indicate the degree of shear stability of a grease. Block Penetration—If a grease is firm enough to hold its shape, it is not transferred to a worker cup container for test.

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Instead, the penetration is determined on three faces of a freshly-cut, 50-mm (2-in.) cube of grease. Undisturbed Penetration—Undisturbed penetration is that measured on a grease sample in a container as received, without any disturbance. This measurement was formerly a requirement in D 217, but because of the uncertainty of repeatable sample handling, it now is included merely as an information item. Such measurements can be used for consistency control in grease manufacture, and to assess the degree to which a grease develops false body or set with prolonged storage. NLGI Consistency Numbers—On the basis of worked penetrations, the NLGI has standardized a numerical scale as a means of classifying greases in accordance with their worked consistency. This scale is shown in Table 2 in order of increasing hardness. The majority of greases used in automotive and industrial applications fall in the range of NLGI No. 1 to NLGI No. 3. Consistency Stability The consistency of a grease may change with its history. Some greases may harden with age; others may change due to wide fluctuations in temperature. Evaluation of these changes needs to be on an individual basis. That is, the test grease needs to be subjected to controlled aging or temperature fluctuations, with penetration measurements taken periodically. The consistency of greases may also change in service due to changes in the size and dispersion of thickener particles resulting from mechanical shearing. The ability of a grease to resist changes in consistency during mechanical working is referred to as consistency stability, shear stability, work stability, or mechanical stability. Two test methods have been standardized to evaluate the stability of a grease resulting from two degrees of low-shear working. Prolonged Worked Penetration & Low Temperature Penetration ASTM D 217 (IP50), described previously, is used before and after prolonged working in a grease worker to determine the change in grease consistency. Because shear rates are low, evaluation of shear stability by prolonged working is time consuming (working 100000 strokes takes nearly 28 h). Mechanical grease workers with cut off timers help make this a little more manageable task, however, this test is not extensively used anymore.

TABLE 2—NLGI consistency classification. NLGI Consistency No.

ASTM Worked Penetration at 25°C

000 00 0 1 2 3 4 5 6

445-475 400-430 355-385 310-340 265-295 220-250 175-205 130-160 085-115

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Roll

Stability

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Results can be obtained more quickly with the roll stability test, which operates at somewhat higher shear conditions. Roll stability is determined by ASTM D 1831, Test Method for Roll Stability of Lubricating Grease. It is used in conjunction with the reduced-scale penetration test [D1403 (IP310)]. After a worked penetration has been measured on a grease sample, 50g of the worked grease is transferred into a horizontally-mounted cylinder containing a 5-kg (11-lb) steel roller. The cylinder is rotated at 165 r p m for 2 h. The inner roller rolls over the grease, working it during the test. After the test, the penetration of the grease is once again measured by D 1403, and the change between before and after penetrations gives a n indication of shear stability. Recent trend (last few years) has been to run the roll stability tests at elevated temperatures, often even above 100°C, instead of the conventional room temperature tests, to help evaluate the roll stability of greases at a higher temperature. Roll stability tests are widely used in specifications. Test results are significant insofar as they can show a directional change in consistency that could occur during service. No accurate correlation between roll-test results and actual service performance has been established. In both shear stability tests, the change in consistency is reported as either the absolute change in penetration values or the percent change as outlined in ASTM D 1831. If absolute values are to be reported, the quarter-scale values are first converted to full-scale values.

Flow Properties The consistency of a grease is a critical p a r a m e t e r which helps define the ability of a grease to perform under given operating conditions. Consistency, as measured by penetration, is affected by temperature. But the penetration test is not suitable for determining the minor, yet sometimes significant, changes in consistency as the grease approaches temperatures at which phase changes in the thickener occur. Penetration is basically a flow measurement; in addition, there are other flow-measurement tests that can be utilized to evaluate this property at other conditions. Flow Properties

at High

Temperatures

ASTM Test Method D 3232, Measurement of Flow Properties of Lubricating Greases at High Temperatures, can be used to evaluate flow properties of lubricating greases under hight e m p e r a t u r e , low-shear conditions. Using this method, a grease sample is packed into a n annular channel in an alum i n u m block. The packed block is placed on a hot plate capable of attciining temperatures in excess of 315°C (600°F) at a heating rate of 5 ± r C (10±2°F)/min. A special trident probe spindle, attached to a Brookfield-RVF viscometer, is lowered into the grease sample, and the hot plate is turned on. Simultaneously, the spindle is rotated at a constant 20 rpm, and torque measurements are read from the viscometer every minute. Readings are continued until the reading drops below 0.5 on the viscometer scale or until the maximum sample temperature of interest is attained. With these data and a n appropriate conversion equation, a plot of apparent viscosity versus temperature is prepared.

The trident probe test gives information not obtained with the dropping point test. The dropping point test (to be described subsequently) determines the temperature at which the first drop of material drops from the orifice of the standard cup. That first drop may be "melted" grease or leeiked fluid. For example, if a grease becomes so fluid that it drops from the cup, the trident probe test will show a low viscosity at the same temperature. In another case at the same temperature, where it is only oil that drops from the cup, the trident probe will show a higher viscosity than the first case, indicating that the grease is still firm. The results of this test provide a n indication of the flow properties of a grease between room a n d elevated temperatures. Although the test does not give actual flow rates, as in a pipeline, it provides a means for obtaining some indication of this property. This test is used more for grease development, rather than for specifications. Apparent

Viscosity

Grease is by nature a non-Newtonian material. It is characterized by the fact that flow is not initiated until stress is applied. Increases in shear stress or pressure produce disproportionate increases in flow. The term apparent viscosity is used to describe the observed viscosity of greases; it is measured in Poises in D 1092, Test for AppEirent Viscosity of Lubricating Greases. Since apparent viscosity varies with both temperature and shear rate, the specific temperature and shear rate must be reported along with the measured viscosity. In this test, a sample of grease is forced through a capillary tube by a floating piston actuated by a hydraulic system using a two-speed gear p u m p . From the predetermined flow rate cind the force developed in the system, the apparent viscosity is CcJculated using the classic Poiseuille equation. A series of eight capillaries and two p u m p speeds provide 16 shear rates for the determination of apparent viscosities. The results are expressed in a log-log graph of apparent viscosity as a function of shear rate at a constant temperature, or apparent viscosity at a constant shear rate as a function of temperature. This apparatus also has been used to measure the pressure drops of greases u n d e r steady-flow conditions at constant temperature. Such information can be used to estimate the pressure drop or required pipe diameters in distribution systems. Also, apparent viscosity data are useful for evaluating the ease of handling or pumping at specified temperatures of dispensing systems; it is often used to evaluate pumpability at low temperatures. (The NLGI can provide a group of charts that relate pressure drop, apparent viscosity, shear rate, and pipe-flow data.) Apparent viscosity also is used to provide an indication of the directional value of starting and running torques of grease-lubricated mechanisms. Specifications may include limiting values of apparent viscosity for greases to be used at low temperature. Currently an ASTM Round Robin is being conducted to rewrite the U.S. Steel Grease Mobility test method under the aegis of ASTM. The Low Temperature grease mobility test method utilizes only one capillary tube, maintained at various temperatures. The amount of grease collected in a specific time (gms/minute) is measured and used as an indication of the mobility of the grease. The test is easier to conduct

CHAPTER t h a n ASTM D 1092, and uses axi apparatus similEir to the apparent viscosity test method. Low-Temperature

Torque

(Ball

Bearings)

Greases designed for low-temperature applications must not stiffen or offer excessive resistance to rotation. However, greases harden emd become more viscous as the temperature is lowered. Sometimes the grease can become so rigid in the bearing t h a t excessive torque is required for rotation. In extreme cases, the grease can solidify to the point of bearing lock-up. Two standard test methods are available to measure bearing torque at low temperatures. Both operate on the same principle—the restraining force or torque is measured while grease-lubricated test bearings are r u n at low speed. The tests differ in the size and types of bearings, the intended applications, and types of greases (i.e., in viscosity characteristics). ASTM D 1478, Test Method for Low-Temperature Torque for Ball Bearing Greases, measures the starting and running torque of lubricating greases packed in small ball bearings at temperatures as low as —54°C (—65°F). In this procedure, fully packed bearings are installed on a spindle that can be rotated at 1 rpm. The assembly is inserted in a cold box. The outer race is connected by a cord assembly to a spring scale, which measures the restraining force. When the m o t o r is started, the initial, peak restraining force is recorded. After running for 10 min, the restraining force is recorded again. The force values are multiplied by the length of the lever arm, and the products are reported as the starting and running torque in gram-centimeters (g-cm). [Because the cgs (centimeter-gram-second) metric unit, g-cm, is nearly universally used in grease specifications, it is the standard unit of torque measurement for this test; some newer specifications require the SI torque unit, N-m (Newton-meter).] In the past several years, advances in data acquisition techniques have helped in making these test methods easier to r u n and have improved precision and reliability. Because this method was developed using greases with extremely low-torque cheiracteristics at - 5 4 ° C (-65°F) it may not be applicable to other greases, speeds, or temperatures. If a machine has significantly more power available than is actually required, torque is not an important consideration. On the other hand, it can be very i m p o r t a n t in low-powered equipment. This test is significant because it provides a means of comparing the low-temperature torque effects of widely different greases. It is useful in the selection of greases for low-powered mechanisms, such as instrument bearings used in aerospace applications. The suitability of this method for other applications requiring different loads, speeds, and temperatures should be determined on an individual basis. Usually, test conditions are substantially different from those found in service, so test results may not correlate with actUcJ service performance. Low-Temperature

Torque

(Tapered Roller

Bearings)

For applications using larger bearings or greater loads, D 4693 Test Method for Low-temperature Torque of GreaseLubricated Wheel Bearings, is better suited than D 1478. D 4693 can be used to predict the performance of greases in automotive wheel bearings operating at low temperatures. It will differentiate among greases having distinctly

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different low-temperature characteristics. It is used for specification purposes, a n d it is one of the automotive grease performance tests required by D 4950. It is not known to correlate with other types of service. It should be noted that greases having such characteristics that permit torque evaluations by either D 1478 or D 4693 will not give the same values in the two test methods (even when converted to the same torque units) because the apparatus and test bearings are different. D 4693 determines the extent to which a grease retards the rotation of a specially-manufactured, tapered roller bearing assembly. The test unit is a model device that closely duplicates an automotive wheel bearing assembly. Additionally, it employs a spring-loading m e c h a n i s m to improve test repeatability; and it is not intended to simulate any service-load condition. Although the test assembly cind torque-measuring transducer are necessarily inside the cold-test chamber, the drive mechanism can be either inside or outside. In this test, a sample of test grease is stirred and worked, and a specified a m o u n t is packed into the two test bearings. The test assembly is heated to mitigate the effects of grease history; it is then cooled at a specified rate to —40°C (—40°F). A drive m e c h a n i s m rotates the spindle at 1 rpm, a n d the torque required to prevent rotation of the h u b is measured by a strain gage load cell at precisely 60 s after start of rotation. The results are recorded on a strip chart or using a data acquisition system that can be displayed and stored via computer. (Starting torque was found to be less repeatable, as well as a redundant measurement, and is not determined in this test.) Heat Resistance Heat affects greases in several ways. As the temperature increases, greases soften and flow more readily (see description of D 3232); oxidation rate increases (see description of D 942); oil evaporation increases (see descriptions of D 972 and D 2595); the thickener melts or loses its ability to retain oil (see descriptions of D 566 and D 2595). Sometimes these phen o m e n a are all involved simultaneously (see descriptions of D 3336, D 3337, and D 3527). Dropping

Point

The dropping point of a grease is the temperature at which it passes from a semi-solid to a liquid. Although a thickener can have a definite melting point, the resulting grease does not. Rather, the thickener loses its ability to function as a grease thickener as the temperature is increased. As the temperature is raised, the grease softens to the extent that it loses its selfsupporting characteristic, the structure collapses, and the grease flows under its own weight. When this phenomenon takes place in a standard cup under standard conditions, it is called the dropping point. Two similar procedures are used to determine the dropping point of grease. In both methods, a prescribed layer of grease is coated on the inner surface of a small cup whose sides slope toward a hole in the bottom. With ASTM D 566 (IP132), Test Method for Dropping Point of Lubricating Grease, the sample is heated at a prescribed rate until a liquid drop falls from the cup. In ASTM D 2265, Test Method for Dropping Point of Lubricating Grease Over Wide Tempera-

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ture Range, the sample is introduced into a preheated environment so that the heating rate is controlled more uniformly. In both tests, the difference in temperature between the grease in the cup and the environment are taken into account when calculating the dropping point of the grease. Some greases containing non-soap thickeners may not separate oil or melt. Cooperative testing indicates that dropping points by Methods D 566 and D 2265 generally agree up to about 260°C (500°F). In cases where results differ, there is no known significance. The dropping point is useful (1) in establishing bench marks for quedity control, (2) as an aid in identifying the type of thickener used in a grease, and (3) as an indication of the maximum temperature to which a grease can be exposed without complete liquefaction or excessive oil separation. (For simple soap greases, the maximum usable temperature is about 20-30°C lower than the dropping point. For complex soap grease, the maximum usable temperature is limited by the base oil characteristics, and under dynamic conditions seldom exceeds 175°C. Some complex soap greases will tolerate intermittent operation at higher temperatures, and although they may liquefy somewhat, their structure will reform as the temperature is reduced.) Greases normally do not perform satisfactorily at temperatures near or above the dropping point; other factors are involved. High-temperature performance can depend on the application method and frequency, whether a softened grease is retained at the point of application by proper seals, and whether the high temperature is continuous or intermittent. High temperature stability and evaporation properties of the grease also can affect performance. Dropping point is most useful as a quality-control tool. Unless correlation has been established, dropping point has no direct bearing on service performance. Performance at high temperature would be better evaluated with one of the performance-type tests or by actual experience. Evaporation Loss Exposure of a grease to high temperatures can cause evaporation of some of the liquid lubricant, thus causing the remaining grease to become drier and stiffer or leading to other undesirable changes in the grease structure. Greases containing low-viscosity oils for good low-temperature performance may be susceptible to evaporation losses at higher temperatures. Evaporation also can cause problems where vapors may be hazardous or combustible, or interfere with operations. In most applications, even high-temperature applications, evaporation is not a serious problem because of effective sealing. However, when it is necessary to evaluate evaporation loss, two ASTM test methods are available. Evaporation Loss of Greases and Oils D 972, Test Method for Evaporation Loss of Lubricating Greases and Oils, determines mass evaporative losses from greases or oils at any temperature in the range of 100-150°C (210-300°F). A weighed sample of lubricant is placed in an evaporation cell in an oil bath at the desired test temperature. Heated air at a specified flow rate is passed over the sample surface for 22 h, after which, the loss in sample mass is determined.

Evaporation Loss Over Wide Temperature Range D 2595, Test Method for Evaporation Loss of Lubricating Greases over Wide Temperature Range, augments D 972, which is limited to 150°C (300°F), and was developed because of higher service temperatures. D 2595 can be used to determine the loss of volatile materials from a grease over a temperature range of 93-316°C (200-600°F). This test uses the same sample cup as D 972, but the rest of the apparatus is markedly different. It uses an aluminum block heater, instead of an oil bath, to achieve much higher temperatures. The other test conditions, i.e., air flow rate and test duration, remain the same. Laboratories equipped with D 2595 do not need D 972; the results will be similar, but not necessarily identical. Within their respective temperature constraints, both tests can be used to compare evaporation losses of greases intended for similar service. All other factors being equal, greases having the least evaporative losses will probably perform longer in high-temperature service. Results of these evaporation tests may not be representative of volatilization that can occur in service. Oil Separation (Static Bleed Test) Nearly all greases will separate some oil during storage, but they differ markedly in the amounts that are liberated. If a grease separates too much oil, the grease could harden to the extent that lubrication will be affected. Opinions differ on whether or not lubrication depends on oil bleeding; greases that do not separate some oil during operation can be noisy in service. However, excessive liberation of free oil during storage is to be avoided. Oil can be released from a grease at varying rates depending on the gel structure, the nature and viscosity of the lubricating fluid, and the applied pressure and temperature. ASTM D 1742, Test Method for Oil Separation from Lubricating Greases During Storage, is used to determine the tendency of lubricating greases to separate oil when stored at 25°C (77°F) at an applied air pressure of 1.72kN/m^ (0.25psi). It gives an indication of the oil retention characteristics of lubricating greases stored in both normally-filled and partiallyfilled containers. This test is not suitable for use with greases softer than NLGI No. 1 consistency because of a tendency for the grease to seep through the screen. The test is useful because the results correlate directly with oil separation, which occurs in 16-kg (35-lb) containers of grease stored at room temperature. Storage in other containers gives similar results. This test should not be used to predict the oil separation of grease under dynamic service conditions. (See Standards Under Development, Cone Test, for a description of an elevated-temperature, static bleed test.) Oil Separation (Centrifuge Test) ASTM D 4425 describes a procedure for determining the tendency of lubricating grease to separate oil when subjected to high centrifugal forces. The results can be related to grease performance in shaft couplings, universal joints, and rolling element thrust bearings subjected to large or prolonged centrifugal forces. Results correlate well with actual service performance. In this test, pairs of centrifuge tubes are charged with test grease and placed in a high-speed centrifuge. The grease sam-

CHAPTER 20: LUBRICATING pies are subjected to a centrifugal force equivalent to a relative centrifugal acceleration, G value, of 36000 at 50°C. (The units for the G value are awkward and not used, but they have acceleration dimensions of length/time'^.) The normal test duration is 24 h, but it can be extended to 48 or 96 h. At these specified time intervals, the centrifuge is stopped, and the amount of separated oil is measured and the volume percent calculated. The resistance-to-separation index, called the K36 value, is reported as the volume percent of separated oil/total test hours (both actual values are reported as a fraction; the fraction is not to be reduced). Leakage from Wheel Bearings There are two tests to evaluate leakage of grease from wheel bearings at high temperatures. The older test, D 1263, Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases, utilizes a modified automotive front hub assembly (1940s vintage design and bearings). The two bearings are packed with a specified amount of test grease, and an additional 85g is distributed in the hub. The assembly is run at 660 rpm for 6 h at 104°C (220°F). After the test, the amount of grease that leaked into the hubcap and collector is weighed. The bearings are washed and examined for varnish, gum, and lacquer-like materieJ. ASTM Test D 1263 provides a means to differentiate among grease products with distinctly different leakage characteristics. In addition, skilled operators can observe significant changes in other grease characteristics that may have occurred during the test. However, these observations are subject to differences in personal judgment and cannot be used for quantitative ratings. The test does not distinguish between wheel bearing greases having similar or borderline leakage. Accelerated Leakage from Wheel Bearings ASTM D 4290, Test Method for Determining Leakage Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions, uses the same principle of operation as does D 1263. It is, however, a more modem test that uses a model front wheel-hub-spindle assembly employing current production, tapered roller bearings. The test apparatus is identical to that used in D 3527 (however, the test conditions are markedly different). In D 4290, the test temperature, 160°C (320°F), is significantly higher, and the duration, 20 h, is much longer than that of D 1263. D 4290 was developed to evaluate wheel bearing leakage of greases intended for use in vehicles equipped with disk brakes, which involve much higher operating temperatures. It is used in grease specifications for such applications, and it is one of the performance tests required by D 4950. This test is not known to correlate with any other type of service. Oxidation Stability Bomb Oxidation Test The Standard Test Method for Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method, D 942 (IP142), determines the resistance of lubricating greases to oxidation when stored statically in an oxygen atmosphere in a sealed system at an elevated temperature. In this test, five glass dishes are filled with 4g of grease, each, for a total of 20g. These dishes are then racked and sealed in a bomb, which is

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then pressurized to 758kPa/cm^ (llOpsi) with oxygen. The bomb is heated in a bath at 99°C (210°F) to accelerate oxidation. The amount of oxygen absorbed by the grease is recorded in terms of pressure drop over a period of 100 h and, in some cases, 500 or 1000 h. The pressure drop is a net result of the consumption of oxygen by oxidation of the grease and the gain in pressure due to any gases or volatile by-products released from the grease. Care must be exercised in the interpretation of data derived from the oxidation bomb test. Additives incorporated into the grease can produce misleading results because they can also react with oxygen. As an example, sodium nitrite is sometimes added to grease to serve as a rust inhibitor. In the oxidation bomb test, this material consumes oxygen to form sodium nitrate. In this instance, the drop in pressure is not indicative of the amount of oxidation of the grease alone. Also, greases containing excess carbonate can release CEirbon dioxide gas whose vapor pressure will tend to offset the pressure decrease due to oxygen absorption. The bomb oxidation test was originally designed to predict shelf storage life of greases in prepacked bearings. Whatever its original intent, experience has shown little correlation with the stability of grease films in bearings or on other parts. It predicts neither the stability of greases stored in containers for long periods nor those used under dynamic conditions. Its primary usefulness is for quality control to indicate batchto-batch uniformity. It can be used to estimate relative oxidation resistance of greases of the same type, but it should not be used to compare greases of different types. Although widely used for specification purposes, it is important to note that D 942 has been severely criticized for its potential for misleading results and for having no relation to oxidation in service. There are no standard, dynamic oxidation tests. For dynamic tests that are influenced to a greater or lesser extent by the oxidation resistance of the grease, see descriptions of D 3336, D 3337, and D 3527. PDSC Oxidation Test A number of grease test methods have recently been developed by the ASTM grease committee, one of them being the PDSC (pressure differential scanning calorimetry) technique for evaluating oxidation stability. In this test, a few mg of grease is placed in a sample pan in a bomb, which is pressurized to 3.5MPa (500psi) with oxygen and regulated at that pressure until an exothermic reaction occurs. From a plot of heat as a function of test time, the oxidation induction time (called extrapolated onset time) is determined. This method of evaluating oxidation stability has two significant advantages over D 942: 1) It is considerably faster, generally less than an hour, vs. 100 h or more, and 2) unlike D942, this method is not subject to false values from greases that give off carbon dioxide or other gases during heating. The disadvantages are that the apparatus is expensive, and like D942 the results are not known to correlate with service performance. Another test being developed is the thin film microoxidation test, which has been shown in literature to provide a good indication of the oxidation stability of a grease sample. Greases in Ball Bearings at Elevated Temperatures ASTM D 3336, Test Method for Performance Characteristics of Lubricating Greases in Ball Bearings at Elevated Temper-

566

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atures, is used to evaluate the performance characteristics of lubricating greases in ball bearings operating u n d e r light loads at high speeds cind elevated temperatures for extended periods. Correlation with actual field service cannot be assumed. This method has been criticized for having two test spindles qualified that purportedly do not necessarily give the same results. In this test, the lubricating grease is evaluated in a 20-mm, SAE No.204, heat resistant, steel ball bearing rotated at 10000 r p m under light loads of 22-67N (5-151bf) at a specified elevated temperature u p to 370°C (700°F). The test is r u n on a specified, test-temperature-dependent, operating cycle until lubrication failure or completion of a specified time. (Unless automatic controls Eire used, a 72-h weekend shutdown is required.) With superior greases, tests can last u p to several thousand hours. Multiple tests need to be r u n because the results follow Weibull, rather than normal, distributions. Greases in Small

Bearings

The computer and aircraft industries have used miniature bearings for many years. As the trend toward miniaturization increased in other industries, a suitable test was needed to evaluate lubricating greases in small bearings. ASTM D 3337, Test Method for Evaluation of Greases in Small Bearings, was developed to serve this purpose. Although this test is not the equivalent of long-term service-performance tests, it can be used to predict relative grease life at high temperature in a reasonable test period. Also, this test can measure running torque at both one r p m and 12000 r p m if this property is significcmt for the intended application. The method will not differentiate among greases of closely related characteristics. ASTM D 3337 determines grease life and torque in a small (6.35mm bore) R-4 ball bearing. In this test the bearing is run at 12000 r p m with a 2.2N (1/2-lbf) radial and a 22N (51bf) axicJ load. While a test temperature of 250°C (or 500°F) may be typiccJly specified, the equipment is capable of testing u p to 315°C (600°F) if high-temperature bearings are used.

Extreme Pressure and Wear A lubricant functions by separating bearing surfaces. If the separation was always complete, parts would never wear. However, the integrity of the lubricant film cannot be maintained u n d e r all conditions, and contact occurs to varying degrees. Such contact depends on operating conditions (such as load and speed), lubricant properties (such as fluid viscosity and grease consistency), and lubricant chemistry (such as the presence of wear inhibitors and extreme pressure additives). Several tests are available for evaluating the antiwear and load-carrying properties of greases. Extreme

Grease

Life

With the advent of automotive disk brakes in the 1960s, then c u r r e n t test m e t h o d s proved inadequate for evaluating greases for this high-temperature application. A specific, correlating test method was needed, and after several yeeirs of development, one was standardized. ASTM D 3527, Standard Test Method for Life Performance of Automotive Wheel Bearing Greases, evaluates grease life in a tapered roller, wheel bccirings in a model, front wheel assembly r u n at 1000 rpm, under a thrust load of 11 IN, at 160°C, using a cycle of 20 h on and 4 h off. The test apparatus is the same as that of D 4290, b u t the operating conditions a n d measured peirameters are quite different. Motor torque is monitored, and the test is terminated at a calculated, preset torque vEilue. Grease life is indicated by the n u m b e r of "on" hours (or n u m b e r of cycles) to failure. This is a severe test; the results are influenced by a combination of grease properties, such as oxidation stability, shear stability, and volatility. As with D 3336, multiple tests need to be r u n because the results follow Weibull distributions. This test method is used for specification purposes and is required by D 4950.

Timken

Method

ASTM D 2509, Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method), can be used to determine the load carrying capacity of a grease at high loads. Non-stemdard techniques have been devised to measure wear at lighter loads, but they are not discussed. In the Timken test, a tapered roller bearing cup is rotated against a stationary, hardened steel block. Both parts are lubricated with the test grease prior to starting the test. During the test, the p a r t s are continuously lubricated with fresh grease by means of a feed mechanism. Using a lever system with a ten-fold mechanical advantage, fixed weights apply a force to the block in line contact with the rotating cup. Loads are applied step-wise until lubrication failure occurs, as evidenced by inspection for scoring or welding. The "OK Value" is the m a x i m u m load the lubricant film will withstand without rupturing and causing scoring in the contact zone after a 10-min. run. This test is a rapid method that can be used to differentiate between greases having low, medium, or high levels of extreme pressure properties. It is widely used for specification purposes; however, the results may not correlate with service performance. Extreme

Wheel Bearing

Pressure

Pressure

Four-Ball

Test

ASTM D 2596, Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Four-Ball Method), is another test used to determine the load-carrying properties of lubricating greases. With this procedure two evaluations are made: (1) the Load-Wear Index (formerly called Mean-Hertz Load), and (2) the Weld Point. This test was developed to evaluate the extreme pressure and antiweld properties of a lubricant. The tester is operated with one steel bcdl rotated under load against three like bsJls held stationary in the form of a cradle. The grease under test covers the contact area of the four balls. Loads u p to 800 kgf (7845N, 17601bf) can be applied to the balls to achieve unit pressures u p to 6.9 X lO^kPa (1 000 000 psi). The procedure involves the running of a series of 10-second tests over a range of increasing loads until welding occurs. During a test, scars are formed in the surfaces of the three stationary balls. The diameter of the scar depends on the load, speed, test duration, and lubricant. The scars are measured under a microscope having a calibrated grid. From the scetr measurements, the Load-Wear Index is calculated. The lowest load at which the rotating ball seizes and then welds to the stationary balls is called the weld point; it indicates

CHAPTER that the load carrying capacity of the grease has been exceeded. The significance of this test is that it is a rapid method that can be used to differentiate a m o n g greases having low, medium, or high levels of extreme pressure properties. It is widely used for specification purposes, b u t the results may not correlate with service performance. Because of their poor lubricity, some lubricating greases containing a silicone or a halogenated silicone fluid component are not suitable for testing by this test method. Wear Preventive

Characteristics

of

Grease

ASTM D 2266, Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) is used to determine the wear-preventive characteristics of greases in sliding steel-on-steel applications. This test does not distinguish between EP and non-EP greases. As in D 2509, a four-ball configuration is used, but there are few other similarities as the apparatus and operating conditions are conspicuously different. Wear prevention qualities are evaluated from the diameters of the wear scars that occur on the stationary balls during the test. The test is significant because it can be used to determine the relative wear-preventing properties under the test conditions, so it is useful for grease development. D 2266 is widely used in grease specifications, but its actual usefulness is suspect because of the following limitations. If test conditions are changed, the relative ratings may change. 1. Wecir characteristics are not predicted for metal combinations other than AISI (American Iron and Steel Institute) E52100 steel unless non-stcindard balls of other materials are used. 2. No differentiation can be made between extreme pressure and non-extreme pressure greases. 3. No correlation can be inferred between the results of the test and field service unless such correlation has been established. Fretting

Wear

Fretting wear is a form of attritive wear caused by vibratory or oscillatory motion of small amplitude. It is characterized by the removal of finely-divided particles from the rubbing surfaces. Air can cause immediate loccil oxidation of the wear particles produced by fretting wear, and moisture can hydrate the oxidation product. In the case of ferrous metals, the oxidized wear debris is abrasive iron oxide (Fe203) having the appearance of rust, which gives rise to the nearlysynonymous terms, fretting corrosion and friction oxidation. Fretting is a serious problem in industry. If severe enough, it can cause destructive vibrations, premature failures, and pcirts seizure. No grease can give total protection if fretting conditions exist, but greases vary significcintly in their ability to mitigate fretting wear. ASTM D 4170, Test Method for Fretting Wear Protection by Lubricating Grease, evaluates grease performance in a proprietary test machine (Fafnir Friction Oxidation Tester) which oscillates two grease-lubricated, ball thrust bearings, u n d e r specified conditions of load, speed, and angle. Fretting wear is determined by measuring the mass loss of the bearing races (the balls Eind retainers are not included).

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A related, but somewhat different, phenomenon often accompanies fretting wear. False brinelling is localized fretting wear that occurs when the rolling elements of a bearing vibrate or oscillate with small amplitude while pressed against the bearing race. The mechanism proceeds in stages: 1) asperities weld, are torn apart, and the wear debris which is subsequently formed is oxidized; 2) due to the small-amplitude motion, the oxidized detritus cannot readily escape, and being abrasive, the oxidized wear debris accelerates the wear. As a result, wear depressions are formed in the bearing race. These depressions are often polished and appear similar to the Brinell depressions obtained with static overloading, hence the term, false brinelling. D 4170 cannot distinguish between false brinelling and fretting wear. If false brinelling does occur, it is included in the determination of fretting wear when bearing race mass losses are measured. This test correlates with the fretting performance of greases in wheel bearings of passenger cars shipped long distances. D 4170 also has been used to predict grease performance in automobile drivelines. It is used for specification purposes and is one of the performance tests required by D 4950. Oscillating

Motion

There is another wear test involving oscillatory motion, namely, ASTM D 3704, Test Method for Wear Preventive Properties of Lubricating Greases Using the Block on Ring Test Machine in Oscillating Motion. Ring and block parts, similar to those of the Timken Tester (D 2509), are operated u n d e r varying conditions of load, speed, oscillation angle, time, temperature, and specimen surface finish and hardness to simulate service conditions. This test can distinguish among greases of low, medium, and high wear preventive properties and can be used for grease development. The user should determine whether test results correlate with service performance or results from other bench test machines. Oscillating

Wear (SRV)

Test

A high load, high frequency, low amplitude, high speed reversal test, using the SRV (Schwingung, Reibung, Verschleiss) a p p a r a t u s simulates high-speed vibrational or start-stop motions that occur in many mechanisms. Two procedures, using a ball-on-disk configuration, have been developed: [ASTM D 5706 (EP Test) and ASTM D 5707 (Wear)] one to measure wear-protection qualities and coefficient of friction, and the other to measure the ability of a grease to carry loads under extreme pressure. (This apparatus can use other configurations to suit different applications.) Both procedures have been correlated with grease performance in automotive driveline mechanisms and are used in grease specifications for these applications. These procedures can be used to evaluate lubricants and materials for other applications of similar motion. Corrosion Copper

Corrosion

Lubricated parts that contain copper alloys, such as copper or brass electrical components or bronze gears and bearings, m a y be susceptible to the corrosive effects of formulated greases. For example, such corrosion can cause high resis-

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tance in electrical contacts, or premature bearing failure from chemical attack. D 4048, Test Method for Detection of Copper Corrosion from Lubricating Grease by the Copper Strip Tarnish Test, is the grease analog of the more familiar D 130 used to evaluate oils. In this test, a prepared copper strip is totally immersed in test grease and heated at specified conditions, usually 100°C (210°F) for 24 h. At the end of the test period, the strip is removed, washed, and compared with the ASTM Copper Strip Corrosion Standards. Although this method is used in specifications, the user must establish correlation between test results cind actual service performance. This test does not determine the ability of a grease to inhibit copper corrosion caused by factors other than the grease itself; neither does it determine the stability of grease in the presence of copper. The sole determination is the chemical staining of copper by lubricating grease. Rust Prevention Greases must not be corrosive to metals they contact and should not develop corrosion tendencies with aging or oxidation. A method for assessing rust prevention by greases is ASTM D 1743, Test Method for Corrosion Preventive Properties of Lubricating Greases. In this method, a tapered roller bearing is packed with grease, and following a short run-in period, dipped into distilled water and stored above the water in 100% relative humidity at 52°C (125°C) for 48 h. The bearing is then cleaned and examined for corrosion. Either a Pass or Fail result is reported. The significance of this test is that it indicates those greases capable of preventing rust and corrosion in static or storage conditions. This test is widely used in grease specifications. The correlation with service conditions, particularly under static conditions, is considered to be quite good. Accelerated Corrosion Tests Two test methods were recently worked on to evaluate the corrosion protection properties of greases under severe conditions. These are: 1) a version of D 1743 using synthetic sea water now called ASTM D 5969, and 2) two procedures of the IP220/DIN51802 dynamic rust test [commonly known as the EMCOR test (ASTM D 6138)], one using distilled water and another using salt water. Effect of Water Contamination by water can affect greases and grease performance in several ways. Corrosion or rust protection, previously discussed, is one. Other effects include change in consistency, texture, or adhesiveness. An emulsion can be formed, which will probably be an inferior lubricant, or it could be washed away. Attempts to standardize means of evaluating these effects have had mixed success. Two standard tests do exist, however: the water washout test and the water spray-off test. Water Washout The ability of a grease to resist washout under conditions where water may splash or impinge directly on a bearing is an important property in the maintenance of a satisfactory lu-

bricating film. ASTM D 1264, Test for Water Washout Characteristics of Lubricating Greases, evaluates the resistance of a lubricating grease in a bearing to washout by water. This test method uses a standardized bearing, available from ASTM. It is a 204K Conrad-type, ball bearing equipped with shields but without seals. The bearing is packed with 4g of test grease then rotated at 600 rpm while a jet of water, at either 38°C (100°F) or 79°C (175°F), impinges on the bearing housing for 1 h. The bearing is then dried, and the percent grease loss by weight is determined. This test method serves only as a relative measure of the resistance of a grease to water washout. It should not be considered the equivalent of a service evaluation unless such correlation has been established. Test results are affected by grease texture and consistency. Test precision is poor, especially with soft greases. Although widely used, this test can give misleading results. Even comparative results between similar greases may not predict the relative performance of the two greases in actual service. Water Spray-Off ASTM D 4049, Test Method for Resistance of Lubricating Grease to Water Spray, is used to evaluate the ability of a grease to adhere to a metal panel when subjected to direct water spray. Test results correlate directly with operations involving direct water impingement, such as steel mill roll neck bearing service and certain automotive body hardware applications. In this test, a 0.79 mm (1/32 in.) film of test grease is uniformly coated onto a stainless steel panel; then water, at 38°C (100°F), is sprayed directly on the panel for 5 min. The spray is controlled by specified spray nozzle, pump, and plumbing. After the spraying period, the panel is dried, weighed, and the percentage of grease spray-off is determined. Miscellaneous Contamination ASTM D 1404, Test Method for Estimation of Deleterious Pcirticles in Lubricating Grease, defines a deleterious particle as one which will scratch a polished plastic surface. The test is applicable to all greases, even those containing fillers. In fact, it can be used to test fillers, such as graphite, if they are dispersed into a grease (or petrolatum) that is known to be free of deleterious particles. It can be used also to test other semi-solid or viscous-liquid substances. With this method, the test material is placed between two clean, highly polished acrylic plastic plates held rigidly and parallel to each other in metal holders. The assembly is pressed together by squeezing the grease into a thin layer between the plastic plates. Any solid particles in the grease larger than the distance of separation of the plates and harder than the plastic will become embedded in the opposing plastic surfaces. The apparatus is so constructed that one of the plates can be rotated about 30° with respect to the other while the whole assembly is under pressure. This will cause the embedded particles to form characteristic arc-shaped scratches in one or both plates. The relative number of such solid particles is estimated by counting the total number of arc-shaped scratches on the two plates.

CHAPTER The test has significcince because it is a rapid means for estimating the n u m b e r of deleterious particles in a lubricating grease. However, a particle that is abrasive to acrylic plastic m a y not be abrasive to steel or other bearing materials. Therefore, the results of this test do not imply performance in field service. Elastomer

Compatibility

Nearly all grease-lubricated mechanisms have elastomeric seeds to retain lubricant zmd exclude contaminants. In order for these seals to function properly, the grease must be compatible with the rubber-like elastomer seal. ASTM D 4289, Test Method for Compatibility of Lubricating Grease with Elastomers, is a simple total immersion test designed to evaluate the compatibility of grease with elastomer specimens cut from standard sheets. It also can be used as a guide to evaluate compatibility of greases with rubber products not in standard sheet form. Unlike other standard compatibility tests, which are designed to evaluate elastomers in standard fluids, the emphasis of D 4289 is the evaluation of the greases. Elastomer specimens are cut from s t a n d a r d ASTM sheets (D 3182) a n d immersed in test grease for 70 h at either 100 or ISO'C. Compatibility is evaluated by determining the changes in volume and DurometerA hardness (D 2240). (Volume is determined by the water displacement method, D 471.) The volume and hardness change values determined in this test do not duplicate the changes that occur in rubber seals in actual service conditions. However, they can be correlated in many instances. For example, the volume-change values correlated very well (r^ = 0.99) with those that occurred in a vehicle test. Because of wide variations in grease and elastomer formulations and service conditions, correlations between this test and particular applications should be determined on an individual basis. This method provides for optional testing with two Reference Elastomers to evaluate relative compatibility. The results CcUi be used to judge a service characteristic of lubricating greases; in this respect, the test m e t h o d is useful for specification purposes. ASTM D 4950 requires testing with Reference Elastomer CR (polychloroprene) cind Reference Elastomer NBR-L (acrylonitrile-butadiene). Compatibility Mixing of two different grease types often occurs when a mechanism is service lubricated with a type of grease different from that already in the bearing. If the two greases are incompatible, the likelihood is that lubrication will be inadequate and/or the lubrication life will be greatly shortened. The problem of incompatible grease mixtures has long been known. Foreknowledge of the chemistry of the greases is not often reliable in predicting compatibility. Compatibility needs to be judged on a case-by-case basis. In light of this need, a standard practice was developed Eind is now our new ASTM method ASTM D 6185. There are several non-standard means worthy of consideration, however. One such practice involves the preparation of three binary mixtures in concentrations of 10:90, 50:50, and 90:10 mass ratios. These three mixtures and the two neat greases are then tested for dropping point (D 566 or D 2265),

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shear stability (either by prolonged working or by the roll test, D 1831), oil separation (D 1742), and storage stability (cheinge in consistency after prolonged storage, such as one to six m o n t h s ) . Other tests, s u c h as D 3527 (life performance), D 4290 (leakage), water spray-off (D 4049), etc. EJSO should be r u n if the application warremts such. Incompatibility is indicated if any mixture tests worse t h a n the poorer of the neat greases. The repeatability of the test methods must be tciken into consideration when making such determinations. Static Bleed (Cone)

Test

Federal Test Method (FTM) 791C, Method 321.3, static bleed test (also known as the cone test), was developed as ASTM Standard D 6184. In this method, a 10-g sample of grease is placed in a 60-mesh wire cone, which is then suspended in a covered bejiker and placed in a n oven at 100°C (212°F) for 30 h (some specifications may require different temperatures or durations). The amount of oil that bleeds from the grease collects in the bejiker is weighed, and the percentage of separated oil is calculated. Some specifications require the cone and residual grease to be weighed also, and the additional weight loss is reported as evaporation.

Chemical Analysis D 128, Test Methods for Analysis of Lubricating Grease, is the premier standard for grease analysis. This procedure gives flow diagrams and details for the analysis of conventioneJ greases, i.e., those made of soap-thickened petroleum oils. The constituents that can be determined are soap, unsaponifiable matter (petroleum oil, etc), water, free alkalinity, free fatty acid, fat, glycerin, a n d insolubles. A supplementary test method is provided for application to greases that cannot be analyzed by conventional methods because of the presence of nonpetroleum oils or nonsoap thickeners. These test procedures can be used to identify and estimate the a m o u n t of some of the constituents of lubricating greases. The methods are applicable to many, but not all, greases. Composition should not be considered as having a direct bearing on service performance unless such correlation has been established. D 128 references several useful, but nonstandard, methods that can be used for grease analysis. Infrzired Spectroscopic (IR) analysis is commonly used for research purposes, quality control, and specification purposes. But, a n IR method has not been standardized because the technique frequently must be adapted to the specific grease being analyzed. Although D 128 is the general analytical method for greases, t h e r e are other test m e t h o d s for specific constituents. These include the following: • D 95—Test Method for Water in Petroleum Products and Bituminous Materials by Distillation • D 129—Test Method for Sulfur in Petroleum Products (General B o m b Method) • D 808—Test Method for Chlorine in New a n d Used Petroleum Products (Bomb Method) • D 1317—Test Method for Chlorine in New and Used Lubricants (Sodium Alcoholate Method) • D 3340—^Test Method for Lithium and Sodium in Lubricating Greases by Flame Photometer.

570

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

DISCONTINUED STANDARDS Severed standards have been discontinued since the 5th edition of the Manual on Significance of Tests for Petroleum Products. Unless otherwise noted, the reason for discontinuance of the following test methods was lack of interest, i.e., the standcirds were no longer being used or supported by the industry: • D 1262—Test Method for Lead in New and Used Greases (Discontinued 1991) • D 1402—Test Method for Effect of Copper on Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method (Discontinued 1985) • D 1741—Test Method for Functional Life of Ball Bearing Greases (Discontinued 1991) • D 3428—Test Method for Torque Stability, Wear, and Brine Sensitivity Evaluation of Ball Joint Greases (Discontinued 1990). (D 3428 was withdrawn because the ball joint used as the test piece was no longer available and a suitable substitute was not found.)

STANDARDS UNDER DEVELOPMENT Ignition Test Because grease fires in steel mills are a common occurrence, there is a need for a standard method for evaluating the ignition and flammability characteristics of greases used in such applications. Techniques for evaluating these cheiracteristics are being developed. The procedure most likely to be evaluated in round robin testing consists of placing a shaped sample of grease on a metal ramp; a methenamine tablet is inserted in the grease and ignited. If the grease burns, the characteristics of the fire (time to grease or oil ignition, bum time, fire size, self-extinguishing, etc.) are compared with referenced descriptions and assigned a rating.

ISO STANDARDS In 1987, ISO (International Standards Organization) published ISO 6743-9, which is an international classification system for grease. About 75% of commercial greases can be described by this standard. This standard classifies lubricating greases according to the operating conditions of the use— unlike most other ISO product standards, which are classified according to specific end-use. The nature of greases allows a specific grease to be used in many applications. This makes it impractical to classify greases by end-use, and a properties description is a reasoned alternative. Consequently, users are advised to use ISO 6743-9 to define the requisite grease properties, but are they cautioned not to rely solely on the standard for grease selection for a particular application. Rather, users are advised to consult with the grease supplier, as well. In this classification system, each grease will have one symbol only. This symbol should correspond to the most severe conditions of temperature, water contamination, and load in which the grease can be used. Grease products are designated in a uniform manner, with each character having its own sig-

nificance. The line call-out code will have the following format: ISO-L-XSiS2S3S4N where: ISO L X Si 52 53 54 N

= = = = = = = =

identification of the standards organization designator for class (lubricants) designator for family (grease) symbol for Lower Operating Temperature symbol for Upper Operating Temperature symbol for Water Contamination symbol for EP NLGI consistency number

Symbols (letters) are used to designate the four operating conditions. The Lower Operating Temperature is defined by symbols representing requirements at 0, -10, -20, —30, —40, and 180°C. Nine symbols are used to define the effects of water, which include both water contamination and antirust requirements. Only two symbols are used for the EP or load carrying requirement. Limits have been tentatively established for the operating conditions. But, full implementation of IS06743-9 depends on the development of ISO standard test methods. National standards must be converted to ISO format and approved by the ISO community. The national standards proposed to ISO are as follows: Lower Operating Temperature Upper Operating Temperature Water Effect

EP (load carrying capacity)

NFT60-171 (France), a lowtemperature penetration test. ASTM D 566 and D 3336 ASTM D 1264 for Water Contamination and DIN 51802 (Germany)/ IP220 (United Kingdom) for Rust Protection ASTM D 2596

ASTM STANDARDS* No. D 217 D 566 D 942 D 972 D 1092

Title Test Methods for Cone Penetration of Lubricating Grease Test Method for Dropping Point of Lubricating Grease Test Method for Oxidation Stability of Lubricating Greases by the Oxygen Bomb Method Test Method for Evaporation Loss of Lubricating Greases and Oils Test Method for Apparent Viscosity of Lubricating Greases

' Analytical standards not included.

CHAPTER D 1263 D 1264 D 1403

D 1404 D 1478 D 1742 D 1743 D 1831 D 2265 D 2266 D 2509

D 2595 D 2596

D 3232 D 3336

D 3337 D 3527 D 3704

Test Method for Leakage Tendencies of Automotive Wheel Bearing Greases Test Method for Water Washout Characteristics of Lubricating Greases Test Methods for Cone Penetration of Lubricating Grease Using One-Quarter and One-Half Scale Cone Equipment Test Method for Estimation of Deleterious Particles in Lubricating Grease Test Method for Low-Temperature Torque of Ball Bearing Greases Test Method for Oil Separation from Lubricating Grease During Storage Test Method for Corrosion Preventive Properties of Lubricating Greases Test Method for Roll Stability of Lubricating Grease Test Method for Dropping Point of Lubricating Grease Over Wide Temperature Range Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method) Test Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method) Test Method for Evaporation Loss of Lubricating Greases Over Wide-Temperature Range Test Method for Measurement of Extreme-Pressure Properties of Lubricating Greases (Four-Ball Method) Test Method for Flow Properties of Lubricating Greases at High Temperatures Test Method for Performance Characteristics of Lubricating Greases in Ball Bearings at Elevated Temperatures Test Method for Evaluation of Greases in Small Bearings Test Method for Life Performance of Automotive Wheel Bearing Grease Test Method for Wear Preventive Properties of Lu-

D 4048

D 4049 D 4170 D 4289 D 4290

D 4425 D 4693 D 4950 D 5483

D 5706

D 5707

D 5969

D 6138

D 6184 D 6185

20: LUBRICATING

GREASES

bricating Grease Using the (Falex) Block on Ring Test Machine in Oscillating Motion Test Method for Detection of Copper Corrosion from Lubricating Grease by the Copper Strip Teirnish Test Test Method for the Resistance of Lubricating Grease to Water Spray Test Method for Fretting Wear Protection by Lubricating Greases Test Method for Compatibility of Lubricating Grease with Elastomers Test Method for Determining the Leakage Tendencies of Automotive Wheel Bearing Grease Under Accelerated Conditions Test Method for Oil Separation from Lubricating Grease by Centrifuging (Koppers Method) Test Method for Low-Temperature Torque of Grease-Lubricated Wheel Bearings Classification and Specification for Automotive Service Greases Test Method for Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a HighFrequency, Linear-Oscillation (SRC) Test Machine. Test Method for Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV) Test Machine. Test Method for Corrosion Preventive Properties of Lubricating Greases in the Presence of Dilute SjTithetic Sea Water Environments Test Method for Determination of Corrosion Prevention Properties of Lubricating Greases Under Dynamic Wet Conditions (EMCOR Test) Test Method for Oil Separation from Lubricating Grease (Conical Sieve Method) Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases

OTHER STANDARDS^ Standard No. ISO

DIN

2176 2137

51801 51804/1-2

132 50

51805 51802 51811 51803

220 112 5

51809/1-2 51813

37/137/139 134

51807/2 51807/1 51808 51817

215

2160

51814 51815

IP

FTM791b

COST

Characteristic/Property

102 132

1421 311

6793 5346

135

40001.2 5309.4

5757

Determination of Dropping Point Determination of Cone Penetration 1/1 - 1/2 - 1/4 Cone Determination of Flow Pressure Corrosion Preventing Properties Corrosive Effects on Copper Determination of Ash of Greases (Incl. Sulfate) Neutralization Number Content of Solid Foreign Matters Effect of Water Water Washout Test Static Test Oxidation Stability Oil Separation

NF-T 60

M 07-037

1461 6474

133/112

142 121 M 07-38

571

3005.3

6370

3453 321/2

5734 7142 1631

Content of Base Oil and Soap Content of Li/Na/Ca by Atomic

Absorption Spectroscopy (continues)

572 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Standard No. ISO

DIN

IP

FTM 791 b

NF-T 60

COST

Characteristic/Property

9270

Determination of Li/Na by Flame Photometer Determination of Solids (Graphite or M0S2) Determination of Particle Size of Solid Lubricants Density Evaporation Loss Content of Water

199 51831

3720/22

51832 59 183 74

3733

9566 1044 2077

113

51816/1 51816/2

139 51350-4/5

6503.2

239 326

331.2/333.1

51806 168 266 (51821/2) (51821/1) 186 R868 1817

53505 53521

3104 3016 2592 2977

51562 51597 51376 51775/51787 51820 E

3603.3 71 15 36 2

Pumpability Properties SheU-DeLimon Rheometer Decompression Characteristics Apparent Viscosity Roll Stability Extreme Pressure Properties Shell-Four-Ball Test Timken Test Mechanical Dynamic Testing SKF-R2F Roller Bearing Performance Churning FAG-FE 8 (EP Greases) FAG-FE 9 Wheel Bearing Leakage Low Temperature Torque Elastomer Compatibility Hardness Change (Shore A) Volume Change Tests on Base Oil Viscosity Pour Point Flash Point Aniline Point Infrared-Analysis

^ Reprinted with permission from The Lubrizol Corporation, Wickliffe, OH.

REFERENCES [6] [1] Polishuk, A. T., A Brief History of Lubricating Grease, Llewellyn & McKane, Inc., Wilkes-Barre, PA, 1998. [2] Boner, C. J., Manufacture and Application of Lubricating Grease, Reinhold PubUshing Corp., NY, 1954. [3] Boner, C. J., Modem Lubricating Greases, Scientific Publications (G.B.) Ltd, Broseley, Shropshire, England, 1976. [4] Lubricating Grease Guide, 4 * ed.. National Lubricating Grease Institute, Kansas City, MO, 1996. [5] Ward, C. E., "Polyurea Greases," National Lubricating Grease

[8]

[9] [10]

Institute (NLGI) Grease Education Advanced Course, Kansas City, MO, 1998. "Lubricating Greases," Ch. 9, Manual on Significance of Tests for Petroleum Products, 5th ed., G. V. Dyroff, Ed., ASTM International, West Conshohocken, PA, 1989. Federal Test Method Standard No. 79IC: Lubricants, Liquid Fuels, and Related Products: Methods of Testing, Available from Global Engineering Documents, Irvine, CA, Sept. 30, 1986. "Index of the NLGI Spokesman," Compact Disk, National Lubricating Grease Institute, Kansas City, MO, 2000. Annual Book of ASTM Standards, Volumes 05.01, 05.02, 05.03, ASTM International, West Conshohocken, PA.

MNL37-EB/Jun. 2003

Mineral Oil Heat Transfer Fluids John Fuhr, ^ Jim Oetinger, ^ George E. Totten,^ and Glenn M. Webster^

The focus of this chapter will be on mineral oil derived heat tremsfer fluids. This discussion will include a basic overview of the heat transfer coefficient as a fluid chciracterization parameter followed by a discussion of fluid chemistry and the impact on properties. An overview of various test procedures used in the selection and maintenance of mineral oil heat transfer fluids will be provided. System maintenance, operation, and design will be discussed toward the closing.

BECAUSE OF COST AND POTENTIAL FOULING PROBLEMS, indirect

process heating designs are usually preferred over direct, fuel-fired heating or individual electrically heated units [2]. For indirect process heating, heat transfer is usually accomplished by steam or by using a h e a t transfer fluid (HTF). Examples of HTFs include: petroleum based mineral oils, glycols, silicones a n d various S3Tithetic fluids such as alkyleated aromatics, terphenyls, a n d mixtures of bi and diphenyls and their oxides. Selected physical properties for a n u m b e r of illustrative tj^pes of heat transfer fluids are provided in Table 1. These data show that there are notable differences in the physical properties of different classes of different fluids within a class.

DISCUSSION Heat Transfer Coefficient

Steam is one of the most economical, since it is generated easily and it possesses excellent heat transfer properties, due to its relatively high heat of vaporization and high thermal conductivity. However, steam suffers frora a n u m b e r of disadvantages such as corrosion and the fact that it must be used in high-pressure equipment. It is often most suitable for temperatures of ,

ec

/

sMN

/'

iO+ ^ RCOO • + M"+ + H^ 2 RCOOH M"^/M8 5.5 2.5

92 1

tests, including Test Method D 445, is discussed in detail in Chapter 32, Flow Properties and Shear Stability and will not be discussed further here. Open Cup and Closed Cup Flash Points (ASTM D 92 and D 93) Because the majority of systems operate safely above the Flash Point of the fluid, periodic determination is usually unnecessary. A reduction in flash point does indicate the presence of low boiling components, which result from thermal cracking. A rapid decrease in flash point may indicate that severe overheating has occurred. The presence of volatile components can be determined by ASTM D 92, Cleveland Open Cup Flash Point and/or ASTM D 93 Pensky-Martens Closed Cup Flash Point Test. As determined by these methods, the flash point is the lowest temperature, corrected to barometric pressure of 101.3kPa (760 mm Hg), where the application of an ignition source causes the vapors of the fluid being tested to ignite, either in an open cup (D 92) or closed cup (D 93). These test procedures are described in detail in Chapter 25, Volatility. Water Content (ASTM D 95 and D 1744) Heat trsinsfer systems operating at temperatures of 120°C or greater must, for reasons of safety, be dry, because destructive high pressures are generated when water enters the high temperature sections of the system. If water is not removed, the vapor will cause pump cavitation and possible fluid discharge through the expansion tank vent Test Method D 95 is a classic distillation procedure for water content. It is conducted by heating the fluid under reflux with a water immiscible solvent that codistills with water in the fluid. Condensed water and solvent are continuously separated in a trap, the water settles in the graduated section of the trap. The amount of water is reported as % by volume of the original sample. However, distillation is only suitable for relative high levels of water content. In most well-maintained heat transfer systems, the actual water content is lower than that necessary for distillation. A more suitable procedure for determination of water content is Test Method D 1744, a Karl Fischer method where the sample is titrated to an electrometric end point [19].

583

Safety Mineral oil heat transfer fluids are hydrocarbons, and therefore are combustible with the associated safety risks. However, mineral oil based heat transfer systems have been in safe operation for over 100 years. System Leakage Unlike hydraulic systems, mineral oil heat transfer systems are not generally pressurized. The leaks that do occur are found mostly in threaded fittings, joints, valves, and pumps— the fluid will slowly weep rather than gush or spray. Leaking heat transfer fluid will tjrpically smoke rather than bum—even at temperatures in excess of their flash and fire points. However, hot mineral oil fluid vapors can also be highly flammable if allowed to accumulate in a poorly ventilated area. Insulation Fires All organic heat transfer fluids are capable of spontaneous combustion when leaks occur into porous insulation (such as fiberglass or calcium silicate). The exact mechanism of the autoignition has not been established. One possibility is that partial oxidative decomposition occurs in the insulation (similar to the way heat is generated in a pile of oily rags or wood chips). The heat produced adds to the system heat and ultimately exceeds the autoignition temperature of the fluid. An alternate mechanism is that the autoignition temperature of the fluid decreases as it degrades in the insulation. In either case, if air enters the insulation at this point and contacts the degraded fluid, spontaneous combustion will occur. Automatic Shutdown Controls Although rare, fires have occurred in heaters when fluid circulation is interrupted (due to pump failure or blockage) and the high temperature shutdown fails. In this situation, the fluid temperature rapidly increases above the autoignition temperature while the increasing thermal stress eventually causes either the heater tube or housing to fail. All systems should incorporate automatic shutdown controls for high fluid outlet temperature and loss or reduction of fluid flow through heater. Design and Construction Like any other industrial system, the proper design and installation of heat transfer fluid systems is critical to their smooth functioning and extended operating life. In this section, an overview of recommended practices to minimize the potential of system fires will be provided. Component Selection In designing and constructing a thermal oil system, attention must be paid to the selection of appropriate components. If care is not taken, poor operation, system failure, and fires can result. In this section, an overview of various system design considerations for mineral oil heat transfer fluids will be provided. Piping Schedule 40 seamless carbon steel piping is recommended. Schedule 80 piping is recommended for threaded installations up to 1 in. Threaded fittings are not recommended for

584

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

piping greater than 2 in. Pipe should be free of mill scale, wedding flux, quench oils, and lacquers. The use of copper should be minimized. Flanges and

Fittings

Flanges and fittings must be 300 lb forged steel, Yu in. raised face. Studs and nuts should be continuous threaded, alloy steel with heavy hex nuts. Valves should be 300 lb cast or forged steel, or nodular (ductile) iron with steel or stainless steel trim. For o p t i m u m service, bellows may be considered. The use of cast iron in thermal oil systems is not recommended. Suggested gasket and packing materials include: • Flange gaskets—Spiral wound graphite filled or filled PTFE (maximum temperature 450°F) • Valve stem packing—Rings of die formed graphite foil • Pump packing—End (nonextrusion) rings of braided carbon yeim • Mechanical Seals—Carbon versus silicon or tungsten carbide for continuous service. Silicon Ccirbide versus tungsten carbide for intermittent service. Elastomers Fluoroelastomer r e c o m m e n d e d to 450°F, perfluoroelastomers recommended to 600°F. Compatibility of other elastomers should be evaluated using tests such as ASTM D 471. Insulation Heat loss should not exceed 80 btu/ft at operating temperature. Nonporous (closed cell foam glass type) insulation is recommended. Porous insulation(such as glass fiber and calcium silicate) can be used on straight piping runs where leakage is unlikely. In such installations, nonporous insulation should be installed around leak prone areas such as valve and instrument taps and extended 18 in. m i n i m u m on either side. Weep holes should be drilled in the b o t t o m of insulation around veJves. If possible, flanges should be left uninsulated. Metal covers with weep holes can be installed for personnel protection. Pumps P u m p s should be cast carbon steel. Positive displacement p u m p s should be of the "gear within a gear" design. Canned or magnetic drive p u m p s are typically not required since fugitive emissions regulations do not apply for mineral oils. It is recommended that the alignment be rechecked after the system is operating at temperature. Flex hose should be installed on the inlet and outlet. Pressure

Gauges

Y-strainers One-quarter to Ys in. opening is recommended. A 60 mesh element should be instcJled for start-up only. Flow

Protection

Many systems utilize a pressure differential switch to provide a method of shutting the system down when fluid flows drop below set limits. Some systems are equipped with flowmeters in addition to the pressure differential switches. However, since flowmeters can fail in the open position, they are typically not recommended for use as a flow switch. Installation During installation or construction, four areas should be addressed: • system cleanliness • component orientation • system tightness • allowance for thermal expansion and contraction System

Cleanliness

Care must be taken to assure that the system is clean and dry. Both the "hard" and "soft" contamination must be removed as the system is being assembled. Hard contamination such as mill scale, weld spatter/slag, and dirt can cause premature failure of p u m p and vzJve seals. A startup strainer should be monitored continuously during initial system circulation. Soft contamination such as quench oil, welding flux, and protective lacquer coatings can potentially dissolve in the fluid. Although this would be expected to exhibit only a minimal effect in most cases, such contaminants may be carried through the heater where they may degrade at much lower temperatures than the fluid itself, and can cause fouling of heater surfaces. Component

Orientation

Expzinsion tanks should be located far enough away and piped so that the temperature is n o greater than 1 SOT for vented systems. Design the expansion tank for twice the expansion volu m e of the system where the tcink is % full cold and M full when the system is at operating temperature. Valves should be m o u n t e d system sideward so that leakage from the stem or from bonnet gasketing drips away from the piping. System

Tightness

The system should be chcirged with an inert gas once construction is completed. Not only will corrosion be prevented, but the system can be pressure-tested using simple soap-bubble detection methods at potential leak points.

Pressure gauges should be rated to 100 psi, 650°F (343°C) at a temperature range of 300-600°F; Thermometers should be calibrated according to ASTM E l to provide accurate readings in this range [6].

ASTM STANDARDS

Expansion

No. D 91

Joints

For expansion joints, it is recommended that the HTF system be designed for an expansion growth of 4 in. per 100 ft, mini m u m . Both loops and joint expansion devices are acceptable. Either m u s t be high-temperature rated and must be considered part of the piping system.

D 92 D 93

Title Test Method for Precipitation Number of Lubricating Oils Test Method for Flash and Fire Points by Cleveland Open Cup Test Method for Flash Point by Pensky-Martens Closed Cup Tester

CHAPTER D 97 D 130

D 189 D 445 D 471 D 524 D 611

D 664 D 877 D 893 D 1160 D 1169 D 1298

D 1319 D 1500 D 1744 D 1747 D 2007

D 2140 D 2425 D 2501 D 2502

D 2549

D 2717 D 2766 D 2786

D 2887 D 3238

D 3239

Test Method for Pour Point of Petroleum Products Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Test Method for Conradson Carbon Residue of Petroleum Products Kinematic Viscosity of T r a n s p a r e n t a n d Opaque Liquids (The Calculation of Dynamic Viscosity) Test Method for Rubber Property-Effect of Liquids Test Method for Ramsbottom Carbon Residue of Petroleum Products Test Method for Aniline Point and Mixed Aniline Point of Petroleum Products a n d H y d r o c a r b o n Solvents Test Method for Acid N u m b e r of Petroleum Products by Potentiometric Titration Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes Test Method for Insolubles in Lubricating Oils Test Method for Distillation of Petroleum Products at Reduced Pressure Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids Test Method for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Test Method for ASTM Color of Petroleum Products (ASTM Color Scale) Test Method for Determination of Water in Liquid Petroleum Products by ICarl Fischer Reagent Test Method for Refractive Index of Viscous Materials Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method Carbon-Type Composition of Insulating Oils of Petroleum Origin Hydrocarbon Types in Middle Distillates by Mass Spectrometry Calculation of Viscosity-Gravity Constant (VGC) of Petroleum Oils Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils F r o m Viscosity Measurements Separation of Representative Aromatics a n d N o n a r o m a t i c s Fractions of High-Boiling Oils by Elution Chromatography Test Method for Thermal Conductivity of Liquids Test Method for Specific Heat of Liquids and Solids Hydrocarbon T5rpes Analysis of Gas-Oil Saturates Fractions by High Ionizing Voltage Mass Spectrometry Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography Test Method for Calculation of Carbon Distribution and Structured Group Analysis of Petroleum Oils by the n-d-m Method Aromatic Types Anedysis of Gas-Oil Aromatic Fractions by High Ionizing Voltage Mass Spectrometry

21: MINERAL D 3524 D 3525 D 4291 D 4530 D 4808

D 5186

D 5291 D 5292

D 5372 D 6546

D 6743 E 659 G4

OIL HEAT TRANSFER

FLUIDS

585

Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography Gasoline Diluent in Used Gasoline Engine Oils by Gas Chromatography Trace Ethylene Glycol in Used Engine Oil Test Method for Determination of Carbon Residue (Micro Method) Hydrogen Content of Light Distillates, Middle Distillates, Gas Oils, and Residua by Low-Resolution Nuclear Magnetic Resonance Spectroscopy Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Aromatic Ccirbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy Guide for Evaluation of Hydrocarbon Heat Transfer Fluids Stcindard Test Methods for a n d Suggested Limits for Determining Compatibility of Elastomer Seals for Industrial Hydraulic Fluid Applications Test Method for Thermal Stability of Organic Heat TrEinsfer Fluids Test Method for Autoignition Temperature of liquid Chemicals Method for Conducting Corrosion Coupon Tests in Plant Equipment

OTHER STANDARDS ISO 6743-12: 1989 (E) International Standard, "Lubricants; Industrial Oils and Related Products (Class L)"-Classi£ication-Part 12: Family Q (Heat Transfer Fluids)

REFERENCES [1] Guffey, G. E., "Sizing Up Heat Transfer Fluids and Heaters," Chemical Engineenng, Vol. 104, No. 10, 1997, pp. 126-131. [2] Green, R. L., Larsen, A. H., and Pauls, A. C, "The Heat Transfer Fluid Spectrum," Chemical Engineering, Vol. 96, No. 2, 1989, pp. 90-98. [3] Green, R. L. and Morris, R. C, "Heat Transfer Fluids-Too Easy to Overlook," Chemical Engineering, Vol. 102, No. 4, 1995, pp. 88-92. [4] Seider, E. N. and Tate, G. E., "Heat Transfer and Pressure Drop," Industrial Engineering and Chemistry, Vol. 28, 1936, pp. 1429-1436; b. Kern, D. Q., "Chapter 6-Counterflow: Double Pipe Exchangers," Process Heat Transfer, McGraw Hill Inc., NY, 1950, p. 103. [5] ASTM E 1, "Specification for ASTM Thermometers," Annual Book of ASTM Standards, Vol. 14.03. [6] Anon. "Product Review: Oil Refining and Lubricant Base Stocks," Industrial Lubricatoin and Tribology, 1997, Vol. 49, No. 4, pp. 181-188. [7] Singh, H., "Characterization of Lube Oil Base Stock—Approach and Significance," Advances Production & Application of Lubricant Base Stocks, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 303-310.

586 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [8] Hoo, G. H. and Lewis, E., "Base Oil Effects on Additives Used to Formulate Lubricants," Adv. Prod. Appl. Lube Base Stocks, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 326-333. [9] Prince, R. J., "Base Oils from Petroleum," Chemistry and Technology Lubricants, R. M. Mortimer a n d S. T. Orszulik, Eds., Blackie, Glasgow, 1992, pp. 1-31. [10] Yoshida, T., Watanabe, H., and Igarashi, J., "Pro-Oxidant Properties of Basic Nitrogen Components in Base Oil," Proceedings of the 11''' International Colloquium of Industrial and Automotive Luhrication-Vol. 1, W. J. Bartz, Ed., Technische Academie Esslingen, Esslingen, 1998, pp. 433-444. [11] Adhvatyu, A. and Singh, I. D., "FT-NMR and FT-IR Applications in Lubricant Distillation and Base Stock Characterization," Tribotest Journal, Vol. 3, No. 1, 1996, pp. 89-95. [12] Singh, H. and Singh, I. D., "Use of Aromaticity to Estimate Base Oil Properties," Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 288-294. [13] Al-Bamwan, M., "Base Stocks Properties/Characteristics, Additive Response and Their Interrelationship," Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, Proceedings, International Symposium, H. Singh, P. Rao, and T. S. R. Tata, Eds., McGraw-Hill, New Delhi, 1994, pp. 303-310. [14] Al-Sammerrai, D., "Study of Thermal Stabilities of Some Heat Transfer Oils," Journal of Thermal Analysis, 1985, Vol. 30, No. 4, pp. 163-110. [15] Jones, C , "Properties of Hydraulic Fluids," Mechanical World Engineering, January 1964, pp. 3-5. [16] Farris, J. A., "Extending Hydraulic Fluid Life by Water and Silt Removal," Field Service Report 52, Industrial Hydraulics Division, Pall Corporation, Glen Cove, NY. [17] Godfrey, D. a n d Herguth, W. R., "Physical and Chemical Properties of Industricil Mineral Oils Affecting Lubrication-Part 4," Lubrication Engineering, Vol. 51, No. 12, 1995, pp. 977-979. [18] Adhvaryu, A., Pandey, D. C , and Singh, L. D., "Effect of Composition on the Degradation Behavior on the Decomposition of

[19] [20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

Base Oil," Symposium on Worldwide Prospectives on the Manufacture, Characterization, and Application of Lubricant Base Oils, Division of Petroleum Chemistry, Inc., 213 National Meeting of the American Chem. Society, 1997, pp. 227-228. Fuchs, H. C. G., "Understand Thermal Ansdysis Techniques," Chemical Engineering Progress, Vol. 93, No. 12, 1997, pp. 39-44. Kern, D. Q., Process Heat Transfer, McGraw-Hill Book Company, NY, 1950, pp. 99, 103. "Chapter 3-Viscous Oils," Manual on Hydrocarbon Analysis, 6'*^ Edition, A. W. Drews, Ed., 1998, ASTM International, West Conshohocken, PA, p p . 25-30. B a r m a n , B. N., "Hydrocarbon-Type Analysis of Base Oils and Other Heavy Distillates by Thin Layer Chromatography with Flame-Ionization Deection and by the Clay-Gel Method," Journal of Chromatograpic Science, Vol. 34, No. 5, 1996, pp. 219-225. Sassiat, P., Machtalere, G., Hui, F., Kolodziejczyk, H., and Rosset, R., "Liquid Chromatographic Determination of Base Oil Composition and Content in Lubricating Oils Containing Dispersants of the Polybuteneylsuccinimide Type," Analytical Chimica Acta, Vol. 306, No. 1, 1995, pp. 73-79. Kagdiyal, V., Joseph, M., Sastry, M. I. S., Satapathy, S., Basu, B., Jain, S. K., et al., "Estimation of Polycyclic Aromatic Hydrocarbons of Base Oils by HPLC and by UV Spectroscopic Technique: A Comparison," Proceedings of the Advances Production & Application of Lubricant Base Stocks, Proceedings International Symposium, H. Singh and T. S. R. Prasada Rao, Eds., Tata McGraw-Hill, New Delhi, India, 1994, pp. 295-302. Jain, M. C , Bansal, V., Jain, S. K., Srivastava, S. P., and Bhatnagar, A. K., "The Role of Thermal and High Temperature Gas Chromatographic Techniques in the Characterization of Base Oils Blends," Proceedings Adv. Prod. Lube Base Stocks, H. Singh and T. S. R. Prasada Rao, Eds., Tata McGraw-Hill, New Delhi, India, 1994, pp. 272-279. Powell, J. R. and Compton, D. A. C. "Automated FT-IR Spectrometry for Hydrocarbon-Based Engine Oils," Lube Engineering, Vol. 49, No. 3, 1993, pp. 233-239. Anon., "Characterizing Base Oils," Lubrizol Newsline, Lubrizol Corporation, Wickliffe, OH, December 1996, pp. 5.

MNL37-EB/Jun. 2003

Non-Lubricating Process Fluids: Steel Quenching Technology Bozidar Liscic, ^ Hans M. Tensi, ^ George E. Totten, ^ and Glenn M. Webster^

THIS CHAPTER WILL FOCUS ON QUENCHING TECHNOLOGY FOR STEEL

HEAT TREATING APPLICATIONS. Quenching is the process of cooling metal parts to achieve the desired microstructure, hardness, strength or toughness. Quenching can produce both desirable and undesirable residual stresses and distortion in addition to cracking. Steel, for example, is heated to the austenitizing t e m p e r a t u r e , that t e m p e r a t u r e where the austenite m i c r o s t r u c t u r e is formed. To obtain o p t i m u m hardness, strength, and toughness, the maximum amount of martensite transformation microstructure is desired. The primary function of the quenching medium is to control the rate of heat transfer from the surface to optimize the microstructure while minimizing undesirable features such as cracking and distortion [1]. The selection of a quenching medium is dependent on the composition of the alloy, the desired final microstructure and the surface to volume ratio of the part. The most c o m m o n quenchant media are usually liquids or gasses. Liquid quenchants include: mineral oils, water, water that may contain salt or caustic additives, and aqueous polymer solutions. The most c o m m o n gasses, which may or may not be pressurized, include nitrogen, helium, and argon. Various aspects of quenching technology will be discussed in this chapter. This includes: hardenability, fundamentals of quench processing, description of c o m m o n quench media, cooling curve analysis, and quench bath maintenance. Included in this discussion is the current status of international standards development for quenchant characterization. This discussion will be limited to steel quenching technology. Quenching of non-ferrous alloys are not discussed here although the basic surface cooling principles involved are the same.

DISCUSSION Steel Transformation Properties such as hardness, strength, ductility, and toughness are dependent on the microstructure and grain size. The first step in the process is to heat the steel to its austenitizing temperature. The steel is then cooled rapidly to avoid the formation of ferrite and maximize the formation of martensite, which is a relatively hard transformation product, to achieve the desired as-quenched hardness. ' University of Zagreb, Zagreb, Croatia. ^ Technical University of Munich, Munich, Germany. ^ G. E. Totten & Associates, Inc., LLC, Seattle, WA.

The most common transformation products that may be formed in quench-hardenable steels from austenite in order of formation with decreasing cooling rate: martensite, bainite, peeirlite (which is a mixture of ferrite and cementite), and pearlite/ferrite. Each of these microstructures provides a unique combination of properties, and especially the relationship between ferrite and cementite in pearlite—depending on the carbon content and the cooling velocity—strongly influences the mechanical properties. These transformation products have been described by J.R. Davis (in order of their formation with increasing cooling velocity) [2]: 1. Austenite—A microstructural phase characterized by a face-centered cubic iron (gamma iron) crystallographic structure. It is the desired solid solution microstructure produced prior to hardening. An austenite microstructure is illustrated in Fig. lA [2]. 2. Ferrite—^A near carbon-free solid solution of one or more elements in a body-centered, cubic arrangement in which alpha iron is the solvent. Fully ferritic steels are only obtained when the carbon content is very low. Ferritic grain boundaries, as illustrated in Fig. IB [2] is the most obvious microstructural feature 3a. Pearlite—A microstructural phase characterized by its body-centered crystallographic structure which is a metastable lamellcir aggregate of ferrite and cementite (or with an extremely low cooling rate, a mixture of globular cementite in ferrite) whose microstructure is shown in Fig. 1C [2] resulting from the transformation of austenite at temperatures above the bainite range. 3b. Cementite—Brittle compound of iron and carbon, which is known as iron carbide with the approximate chemical formula FesC and is characterized by an orthorombic crystal structure. When it occurs as a phase in steel, the chemical composition will be affected by the presence of m a n g a n e s e a n d other carbide-forming elements. The highest cementite contents are observed in white cast irons (Fig. ID) [2]. 4. Bainite—^A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite transformation temperature, but above the start of martensite transformation (Ms). Upper bainite is an aggregate that contains parallel lathshape units of ferrite and carbides and produces a feathery appearance in optical microscopy, as shown in Fig. IE [2]. It is formed above approximately 350°C (660°F). Lower bainite exhibits an acicular appearance similar to t e m p e r e d martensite, as shown in Fig. I F [2] a n d is formed below approximately 350°C (660°F).

587 Copyright'

2003 by A S I M International

www.astm.org

588 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

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(g),(h) FIG. 1—Illustrations of microstructural transformation products: a) Equiaxed austenite grains and annealing twins in an austenitic stainless steel. 250X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 20, p. 26. b) Low-carbon ferritic sheet steel etched to reveal ferrite grain boundaries-. 100X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 25, p. 30. c) Pearlite structure in high-carbon steel (Fe-0.5C). The cementite lamellae are white; the ferrite is dark: 500X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 26, p. 30. d) White cast iron contains massive amounts of cementite (white) and pearlite (dark): 500X; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 64, p. 66. e) Upper bainite in 4360 steel; ASM Materials and Engineering Dictionary, ASM International, Materials Park, OH 44073-0002, Fig. 173, p.156. f) Lower bainite (dark plates) in 4150 steel; Reprinted from Ref. 3, p. 67 by courtesy of Marcel Dekker, Inc. g) Microstructure of lath martensite; 500X; Reprinted from Ref. 3, p. 68 by courtesy of Marcel Dekker, Inc. h) Microstructure of plate martensite; light shading is retained austenite; 500X; Reprinted from Ref. 3, p. 69 by courtesy of Marcel Dekker, Inc. i) Microstructure of tempered martensite; 500X; ASM Materials and Engineering Dictionary, ASM international. Materials Park, OH 44073-0002, Fig. 365, p. 308.

(f)

CHAPTER

22: NON-LUBRICATING

PROCESS

5. Martensite—A generic term for microstructures formed by a diffusionless phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite in steel is characterized by its body-centered tetragonal crysteJlographic structure. The a m o u n t transformation from austenite to martensite depends on the cooling rate and on the lowest temperature attained since there is a distinct t e m p e r a t u r e where martensitic transformation begins (Ms) and ends (Mf). Three microstructural forms of martensite are: lath (Fig. IG [3]), plate (Fig. I H [3]) a n d t e m p e r e d (Fig. 11[3]) martensite. The formation of these products and the proportions of each are dependent on the austenitization time (because of increasing solution of elements in austenite with increasing time), the time and temperature cooling history of the particulcir alloy, and composition of the alloy. The transformation products formed are typicEilly illustrated with the use of transformation diagrams, which show the temperature-time dependence of the microstructure formation process for the alloy being studied. Two of the most commonly used transformation diagrams are TTT (time-temperature-transformation) a n d CCT (continuous cooling transformation) diagrams. TTT

Diagrams

TTT diagrams, which are also called isothermal transformation (IT) diagrams, are developed by heating small samples of steel to the t e m p e r a t u r e where austenite transformation structure is completely formed i.e., austenitizing temperature, and then rapidly cooling to a temperature intermediate between the austenitizing and the Ms temperature, and then holding for a fixed period of time until the transformation is complete, at which point the trcinsformation products are determined. This is done repeatedly until a TTT diagram is constructed such as that shown for a n unalloyed steel (AISI 1045) in Fig. 2A. TTT diagrams can only be read along the isotherms.

FLUIDS: CCT

STEEL

QUENCHING

TECHNOLOGY

589

Diagrams

Alternatively, seimples of a given steel may be continuously cooled at different specified rates and the p r o p o r t i o n of tremsformation products formed after cooling to various temperatures intermediate between the austenitizing temperature and the Ms determined to construct a CCT diagram, such as the one shown for an unalloyed carbon steel (AISI 1045) in Fig. 2B. CCT curves provide data on the temperatures for each phase trzmsformation, the amount of transformation product obtained for a given cooling rate with time, and the cooling rate necessary to obtain mEirtensite. The critical cooling rate is dictated by the time required to avoid formation of pearlite for the particular steel being quenched. As a general rule, a q u e n c h a n t m u s t produce a cooling rate equivalent to, or faster than, that indicated by the "nose" of the pearlite transformation curve in order to maximize mEirtensite transformation product. CCT diagrams can only be read along the curves of different cooling rates. Caution: Although it is becoming increasingly common to see cooling curves (temperature-time profiles) for different cooling media (quenchants) such as oil, water, air, and others superimposed on either TTT or CCT diagrams, this is not a rigorously correct practice and various errors are introduced into such analysis due to the inherently different kinetics of cooling used to obtain the TTT or CCT diagrams versus the quenchants being represented. A continuous cooling curve can be superimposed on a CCT, but not on a TTT diagram. Hardenability Hardenahility has been defined as the ability of a ferrous material to develop hardness to a given depth after being austenitized and quenched. This general definition comprises two subdefinitions, the first of which is the ability to achieve a certain hardness [4]. The ability to achieve a certain hcirdness level is associated with the highest attainable hardness, which depends on the carbon content of the steel and more

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FIG. 2—a) Time-Temperature-Transformation (TTT) diagram of an unalloyed steel containing 0.5% carbon; b) ContinuousCooling-Transformation diagram of an unalloyed steel containing 0.6% carbon.

(b)

590

MANUAL

3 7: FUELS

AND LUBRICANTS

HANDBOOK

specifically on the amount of carbon dissolved in the austenite after austenitizing. This is illustrated by considering the problem of hardening of high-strength, high-carbon steels. The higher the concentration of dissolved c a r b o n in the austenitic phase, the greater the increase in mechanical strength after rapid cooling and transformation of the austenite in the metastable martensite phase. Martensitic steels typically exhibit increasing hardness and strength with increasing carbon content, as shown in Fig. 3, but they also exhibit relatively low ductility. However, with increasing carbon concentration, martensitic transformation from austenite becomes more difficult, resulting in a greater tendency for retained austenite and correspondingly lower strength. The second subdefinition of hardenability refers to the hardness distribution within a cross section from the surface to the core under specified quenching conditions. It depends on the carbon content, which is interstitially dissolved in austenite and the a m o u n t of alloying elements substitutionally dissolved in the austenite during austenitization. Therefore, as Fig. 3 shows, carbon concentrations in excess of 0.6% do not yield correspondingly greater strength [7]. Also, increasing carbon content influences the Mf temperature relative to Ms during rapid cooling as shown in Fig. 4 [8]. In this figure, it is evident that for steels with carbon content above 0.6%, the transformation of austenite to martensite will be incomplete if the cooling process is stopped at 0°C or higher. The depth of hardening depends on the following factors: • Size cmd shape of the cross section • Heirdenability of the materiEd • Quenching conditions The cross section shape exhibits a significant influence on heat extraction during quenching and therefore, on the hardening depth. Heat extraction is dependent on the surface area exposed to the quenchant. Bars of rectangular shape achieve

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FIG. 4—Influence of the carbon content in steels on the temperature of the start of martensite formation (Ms) and the end of martensite formation (Mf).

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H-D X)

Since the Biot number is dimensionless, this expression means that the Grossmann value, H, is inversely proportional to the b a r diameter. This method of numerically analyzing the quenching process presumes that heat transfer is a steady state, linear (Newtonian) cooling process. However, this is seldom the case and almost never the case in vaporizable quenchants such as oil, water, and aqueous polymers. Therefore, a significant error exists in the basic assumption of the method. Another difficulty is the determination of the //-value for a cross section size other than one experimentally measured. In fact, //-values depend on cross section size. Values of H do not account for specific quenching characteristics such as composition, oil viscosity, or temperature of the quenching bath. Tables of //-values do not specify the agitation rate of the quenchant either uniformly or precisely (see Table 2). Therefore, although //-values are commonly used, more current and improved procedures ought to be used when possible. For example, cooling curve analyses a n d the various methods of cooling curve interpretation that have been reported [1,5] are all significant improvements over the use of Grossmann Hardenability factors.

Q u e n c h i n g F i m d a m e n t a l s a n d C o o l i n g Curve Analysis Steel Wetting

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Kinetics

Hardening of steels (so cedled Martensitic- or Bainitic- Hardening) requires preheating (austenitizing) of the steel to temperatures in the range of 750-1100°C, from which the steel is quenched (cooled) in a defined way to obtain the desired mechanical properties such as hardness a n d yield strength. Most liquid vaporizable quenchants used for this process exhibit boiling temperatures between 100 and 300°C at atmospheric pressure. When parts are quenched in these fluids, the wetting of the surface is usually time dependant, which influences the cooling process and the achievable hardness. G. J. Leidenfrost described the wetting process about 250 years ago [10]. The Leidenfrost Temperature is defined as the surface temperature where the vapor film collapses and the surface is wetted by the liquid. Literature describes temperature-values for this event for water at atmospheric pressure between 150 and 300°C [11-14]. It is apparent that the Leidenfrost Temperature is influenced by a variety of factors, part of which cannot be quantified precisely even today.

594 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK For a nonsteady state cooling process, the surface temperature at all parts of the workpiece is not equal to the Leidenfrost Temperature. When the vapor blanket (or film boiling) collapses, wetting begins by nucleate boiling due to the influence of lateral heat conduction (relative to the surface) [15]. This is due to the simultaneous presence of various heat transfer conditions during vapor blanket cooling (or film boiling [FB]), nucleate boiling [NB], and convective heat transfer [CONV] with significantly varying heat transfer coefficients apB (100 to 250 W/irn^K)); aNB (10 to 20 kW/{m^K)y, and ttcoNv (ca. 700 W/(m'^K)). Figure 10 schematically illustrates the different cooling phases on a metcJ surface during an immersion cooling process with the so-called "wetting front," w, (separating the "film boiling phase" and the "nucleate boiling phase") and the change of the heat transfer coefficients, a, along the surface coordinate, z, (mantle line). In most cases during immersion cooling, the wetting front ascends the cooling surface with a significant velocity, v, whereas during film cooling the wetting front descends in the fluid direction [13,16]. An example of wetting heated cylindrical and prismatic specimens which are submerged in water is shown in Fig. 1 l a and fo [13,17]. Because of the different wetting phases on the metal surface (and the enormous differences of their values of apB,ttNB,and acoNv). the time dependant temperature distribution within the metal specimens will also be influenced by the velocity and geometry of the wetting front (for example, circle or parabolic-like) as well as geometry of the quenched part. Figure 12 illustrates different types of wetting behavior under different conditions [17]. By changing a quenching parameter, for example the chemical composition of the fluid, the period of wetting (tw) can be reduced over more than one order of magnitude. The time interval, t^^, when the wetting front appears—usually at the lower end of the specimen—^up to the time the wetting front has moved across the entire specimen surface; sometimes two wetting fronts appear as illustrated in Fig. l i d . In addition to explosive-like wetting (fw ~ 0), a foam may appear in the fluid neeir the specimen

Immersion cooling Wetting front»

surface, which will depress the heat flux from the specimen into the fluid. Factors Influencing

Film

Boiling

To quantitatively define the change of the wetting behavior, for example, to ascertain the cooling process or to develop or aneJyze quenching fluids, the measurement of the electrical conductance between the submerged sample and a counter electrode is helpful [13]. During the film boiling phase, the hot metal is largely insulated by the vapor film surrounding the metal and conductance between the metal a n d the counter electrode is low. When the vapor blanket (film boiling) ruptures on the metal surface, localized wetting begins and conductance increases. The increase in conductance of the wetted metal is proportional to the amount of the metal surface wetted by the quenchant. When the metal surface is completely wetted, conductance is at its highest value. Figure 13a schematically illustrates a normal electrical conductance, (G) increase corresponding to the percentage of the wetted surface (compare Fig. 11A or 1 IB). Three other possibilities of wetting are also shown. Figure I3b shows a rapid rewetting process (or "explosive" wetting) similar to that shown in Fig. 12. Figure 13c illustrates rapid wetting followed by insulation by bubbles adhering to the metal surface and Fig. I3d illustrates rapid wetting with repeated new formation of film boiling, a process which occurs with many aqueous polymer quenchant solutions. In all four diagrams, the temperature, Tc, is shown, which is measured in the center of the probes. The time, ts, characterizes the time (and the corresponding temperature value from the Tc (t) slope, the temperature Ts) when wetting begins. This shows that temperature measurements in the center of probes provide poor information about the real quenching process that is insufficient to adequately characterize the hardening process. This is also illustrated in Fig. 14 [19] by T(t) slopes, measured in the center and the surface of cylindrical probes, quenched in water with t^v + 0, and in an aqueous poljTner solution with a short ty^ time. To obtain a better definition of the wetting kinematic, the starting time, ts, of wetting, the finishing time, tf, of wetting and the difference between tf and ts as the wetting time fi^ should be used. The effect of vEiriation of these parameters on the quenching process is summarized in Table 3.

Film boiling

Impact of the Wetting Process o n Cooling Behavior

Nucleate boiling Convective beat transfer Heal transfer coefficient at

Film cooling Film of liquid Convection boiling

Fluid drops Wetting front ir

FIG. 10—Wetting behavior and cliange of heat transfer coefficient a along the surface [13,16].

An illustration of the influence of a wetting process occurring over a long time (a so-called "non-NEWTONIAN wetting") on the temperature - time cooling curves, measured near the surface at different distances from the lower end of the probe, is illustrated in Fig. 15 [17]. The wetting front requires about 18 s to arrive at a height of Z = 80 m m taken from the discontinuities of the curve "0" (ca. Is) and curve "80" (ca. 19s). If there is an explosion-like wetting of the probe (a so called "NEWTONIAN wetting"), these five t e m p e r a t u r e slopes cire congruent. If the temperature was measured (like usucJ) in the center of the probe, the large differences in wetting behavior would not be observed. The temperature distribution within the probe during quenching (indicated by isotherms at different times after

CHAPTER 22: NON-LUBRICATING

(a)

(b)

(c)

4.0 s

3.8 s

4.3 s

7.0 s

5.7 s

8.3 s

PROCESS FLUIDS: STEEL QUENCHING

TECHNOLOGY

595

10 s

6.9 s

12,3

3^2 s

4,92 s

5,97 s

7,38 s

(d)

FIG. 11—Process of transition between tlie three cooling phases—film boiling (FB), nucleate boiling (NB) and convective cooling (CONV) during immersion cooling of CrNi -steel specimens with a cylindrical geometry 25 mm dia x 100 mm), a. Wetting process of a cylindrical CrNi-steel specimen being quenched from 850°C into water at 30°C with an agitation rate of 0.3 m/s b. prismatic geometry (15 x 15 x 45 mm) in water of 60°C without forced convection; Immersion temperature of 860°C, c. CrNi-steel probe (25 mm dia. x 100 mm) in oil at 60°C without agitation, d. hollow cylinder (60 od X 30 id X 60 mm long) quenched into oil at 60° and no agitation. Note: there are two wetting fronts.

beginning the quenching process), having a "non-NEWTONIAN wetting" is shown in Fig. 16a. where there is a great difference relative to that of the "NEWTONIAN wetting" (Fig. 166). In the second case, the temperature gradient, T, is radial whereas in the case 'a it is axial. Of course the hardness distribution in the probes must be extremely different. In the case of "a," a strong axial hardness distribution (accompanied by a very low radial hardness distribution); in the case of "b," a very strong radial hardness gradient is observed.

Cooling Curve Data Acquistion and Analysis Data Acquistion and Analysis The need to acquire sufficient data to adequately define a cooling curve for subsequent analysis has long been recognized. Special data acquisition devices, including hardware such as oscillographs, were used for work reported by Jominy [20], French [21], and others [21,22]. However, this equipment is difficult to calibrate, which has inhibited widespread use of cooling curve analysis. Currently, sufficient data ac-

596

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

quisition rates can be achieved with personal computers equipped with analog-to-digital (A/D) converter boards. Although computer hardware is available, there are no published guidelines for selecting the proper data acquisition rate, which varies with probe alloy, size, and quench severity. Perhaps the best method for selecting the required acquisition rate is to determine it experimentally. This can be done by repeatedly quenching a probe in cold (25°C, or 77°F) water, one of the more severe quenchants, and collecting data at

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/3

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40 0

10

15

20

Time in s

Time in s

FIG. 14—Comparison of cooling curves measured at different positions in a cylindrical CrNi-steel probe (25 dia. x 100 mm) during slow wetting (water) and sudden wetting (aqueous polymer solution) at (a) center and (b) close to the probe surface at three indicated heights (1,2, and 3).

TABLE 3"—Effect of fluid and metal property variation on quench severity.

6.35 s

Effect on Property Variation Increasing, 4- =Decreasing)

7.65 s

(t ==

FIG. 12—Transition from film boiling (FB) to nucleate boiling (NB) during immersion of cylindrical silver specimen (15 mm dia x 45 mm) quenched from 850°C into a 10% aqueous polymer quenchant solution at 25°C without agitation [18] In comparison with the tw-values of Fig. 13 a and b, the wetting time is extremely short [18].

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I

618 MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

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FIG. 56—liiustration of cooling curve performance of a severely degraded aqueous polymer quenchant compared to water and a fresh solution at the same concentration, bath temperature, and agitation.

CHAPTER

22: NON-LUBRICATING

PROCESS

ing curve analysis methodology that has been successfully used in the heat treating shop will be described. In addition to illustrating the overall utility of modeling and simulation of production quenching conditions, this example also illustrated advanced techniques for investigating quenching behavior. Temperature

Gradient

Quenchant

Analysis

Background—Liscic designed a system for practical measurement, recording, and evaluation of quenching (cooling) intensity u n d e r w o r k s h o p conditions, which expresses quenching intensity by a continuous change in relevant therm o d j n a m i c functions during the entire quenching process. This approach should be contrasted with the Grossmann Hfactor concept, which expresses quenching intensity with a single value and which was shown earlier to be of limited value in quantitatively represented quench severity when agitation is used. All of the different cooling curve analysis methods discussed thus far utilize relatively small, usually 12.5 m m or 0.5 in diameter, round bar probes with a single thermocouple placed at the geometric center. Such probes, while excellent for quality control purposes, are of limited value for use under workshop conditions. The reasons include: • Because of its relatively small mass and low heat capacity, these small probes will cool in about 10-30 s whereas an actual workpiece of 50 m m (2.0 in) diameter will require 500-600 s to cool below 200°C (392°F) in the center of the workpiece when quenched into an unagitated quench oil. Therefore, to adequately model actual quench processes under production conditions, a probe of similar mass and dimensions is necessary. • The actual heat transfer coefficient during quenching of actual production parts may be simulated using a small cylindrical probe. However, the heat transfer coefficient during nucleate boiling is heavily dependent on bar diameter [92]. The magnitude of this dependence increases with decreasing bar diameter below 50 m m (2.0 in). The dependence is less pronounced for bar diameters greater than 50 m m (2.0 in). Therefore, for the same quenching conditions, the heat transfer coefficient on the surface of a small diameter cylinder is quite different than that expected on the surface of most production parts with diameters > 5 0 m m (2.0 in). Important criteria for a cooling curve analysis system to be utilized to model quench processes under workshop conditions should be applicable to: 1) a wide variety of quenchant media including: water, brine, aqueous polymer solutions, salt baths, quench oils, fluidized beds, and gas quenchants; 2) a wide variety of quenching conditions including: different bath temperatures, agitation rates, and fluid pressures; and 3) all quenching techniques including: direct i m m e r s i o n quenching, interrupted quenching, martempering, austempering, and spray quenching. The method to be reported here provides for recording of thermodynamic functions during each test to enable the user to ancdyze the peirticular quenching process of interest and quenching conditions, to evaluate quenching intensity, and to compare it with previously performed tests in other facilities under different conditions. To do this, the user will establish a database of quenching intensities of different systems within the production facility. This database will provide the user with input data for subsequent computer simulation of

FLUIDS:

STEEL

QUENCHING

TECHNOLOGY

623

the quenching process to determine optimal quenchant and quenching conditions for every part being produced. In addition, for optimal simulation, it is important that the m e a s u r e m e n t m e t h o d be sufficiently sensitive to reflect changes in each of the important quenching peirameters (specific character of the quenchant, quenchant bath temperature, in addition to mode and degree of agitation). This criterion is addressed by recording the temperature-time history (cooling curve) at particular points within the probe used for analysis. This requires the measurement of transient temperatures within a solid body when high thermal gradients are involved, which requires that the following inherent effects be considered: • Damping Effect—Changes in surface t e m p e r a t u r e are damped in magnitude when sensed inside the body compared to their magnitude at the surface. • Lagging Effect—Changes in the surface temperature are sensed within a finite time after they occur at the surface. The greater the distance of the temperature measurement point from the surface, the greater the damping and lagging effects. • Response Time—When working with thermocouples, it is important to consider another effect that is inherent with every thermocouple-response time, which is the time necessary to reach 63.2% of its total signal output when the thermal junction is subjected to a step change in temperature [93]. The time constant of a sheathed, grounded thermocouple of 1.5 m m (0.062 in) outer diameter is 1.5 s. To reach 99% of its full signal output when subjected to a step change in temperature, the time constant must be multiplied by 5, which makes a 7.5 s delay. Thermocouples with an even greater diameter will exhibit an even greater time constant and delay in response time. Theoretical Principles—Because of all of the above described requirements, effects, and limitations, instead of recording only one cooling curve at the center of a small cylindrical test probe (as in laboratory tests) the heat flux density at the surface of the quench probe has been selected as the main feature in measuring, recording, and eveduating quenching intensity. This is because changes of the h e a t flux density during the quenching process best represent the dynamics of heat extraction. The method itself, known in the literature as the Temperature Gradient Method, is based on the known physical rule that heat flux at the surface of a body is directly proportional to the temperature gradient at the surface multiplied by the thermal conductivity of the material of the body being cooled: q = \

dT dx

Where: q is the heat flux density (W/m^), i.e., the quantity of heat transferred through a surface unit per unit time, A is thermal conductivity of the body material (W/mK), and dT/dx is the temperature gradient inside the body at the body surface, perpendicular to it (K/m). Hardware—The essential feature of the method being described here is the LISCIC/NANMAC quench probe." It is "* The Llscic-Nanmac probe is manufactured by the NANMAC Corporation, Framingham, MA.

624

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

constructed from AISI Type 304 stainless steel, which is 50 mm (2.0 in) diameter X 200 mm ( 4.0 in ) length, instrumented with three thermocouples placed at the half-length cross section as shown in Fig. 57. One thermocouple measures the actual surface temperature of the probe {T„), another measures the temperature at a point 1.5 mm (0.06 in.) below the surface (TO, and the third one measures the temperature at the center of the cross section. The thermocouples inside the body are standard sheath-tjrpe thermocouples. The thermocouple at the surface is of special design (U.S. Patent 2,829185), which allows continuous measurement of the true surface temperature of a solid body without any damping or lagging effects, in real time, because of its extremely fast response time of about 10 /AS

/ixm (0.0036 in), is placed between a split-tapered insert and pressed into the thermowell (body). The thermal junction is formed by grinding and polishing across the sensing tip. The mica insulation between the two dissimilar ribbons is so thin that metallic whiskers of one ribbon element bridges across the mica to the other ribbon element, and makes hundreds of microscopic friction welded junctions, which are parallel to one another, thus forming one composite measuring junction. The microscopic burrs of the metal from the thermowell (housing) bridge the thin layers of mica, thus electrically grounding the thermal junction to the thermowell at the sensing tip. The metal of the thermowell becomes the third intermediate element, and since the temperature is the same on both sides of the thermal junction, the EMF produced by the secondary junctions on both sides of the main thermal junction cancel each other out, leaving the EMF of the main thermal junction as the only observed EMF (Law of Intermediate Metals in Thermocouples). Any subsequent erosion of the surface of the thermowell (body) simply forms new junctions while removing the old junctions, hence its name "Self Renewing Thermocouple." Because of its unique characteristics, this is the best type of thermal junction to be used for heat transfer calculations because it registers all of the phenomena occurring at the surface in real time. The temperatures recorded at the surface iT„) and at 1.5 mm below the surface of the quench probe (Ti) permit the

(10-5 s)

There are two important requirements for a thermocouple used for measuring surface temperature: 1. Its thermal junction should be two-dimensional (instead of the usually encountered three dimensional geometry) 2. It should be flush with the surface The unique details of the sensing tip of this thermocouple are as follows. In the vicinity of the hot measuring junction, the round thermocouple wires are flattened into ribbons of about 38 /xm (0.0015 in) thickness. These ribbon elements are electrically insulated from each other and from the thermowell by sheets of mica insulation of about 5 /xm (0.0002 in) thickness. This "sandwich" of ribbon elements and mica insulations, having a total thickness of about 91

Handle(optional i

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FIG. 57—The LISCIC/NANMAC quench probe.

fiberglass insulated leads # 20 gage X 36." Standard connector attached. (3-required-one shown for c l a r i t y ) .

CHAPTER

22: NON-LUBRICATING

PROCESS

calculation of the temperature gradient within this surface layer at each m o m e n t throughout the quenching process. The role of the center thermocouple (Tc) is to indicate the timing for heat extration from the core of the probe and to provide a continuous measurement of the temperature difference between the surface and the core, which is essential for the calculation of thermal stresses. Specific features of the LISCIC/NANMAC quench probe are: 1. The size of the probe and its mass ensures sufficient heat capacity and radially symmetric heat flow in the cross sectioned plane where the thermocouples are located. 2. The probe is constructed from austenitic stainless steel and does not undergo microstructural change upon heating or quenching, n o r does it evolve or absorb heat because of microstructural transformations. 3. The surface condition of the probe can be maintained for each test by polishing the sensing tip of the surface thermocouple before each measurement. 4. Extremely fast response times of the surface thermocouple (10~^ s), and the absence of any damping or lagging effects allow any transient temperature at the surface to be measured and recorded exactly and in real time. 5. The heat transfer coefficient at the probe's surface, because of its sufficiently large diameter, may be used in computer simulation of actual production parts. W h e n the quenching intensity is to be determined, the probe is heated to 850°C (1562°F) in a suitable furnace, then transferred quickly to the quenching bath and immersed vertically. The probe is connected to a data acquisition system including a personal computer. The data acquisition card contains three A/D converters and amplifiers enabling digital recording of all three thermocouple signal outputs. Software—In addition to cooling curve data, the cooling curve analysis p r o g r a m (TGQAS - T e m p e r a t u r e Gradient Quenching Analysis System) described here permits advanced computational modeling of production quenching systems. The TGQAS consists of three modules: 1. Module I: zTEMP-GRAD (Temperature Gradient Method)— In each test, three cooling curves are obtained: r „ for the surface of the probe, Ti for the point 1.5 m m (0.06 in) below the surface, and Tc for the center of the probe. TypiccJ cooling temperature-time profiles for each of these points is illustrated in Fig. 58. The temperature gradient between Ti and Tn is calculated from these cooling curves by multiplying the corresponding data by the temperature-dependent thermal conductivity, and the heat flux density versus time, q = fit) (see Fig. 58b) and the heat flux density versus surface temperature, q = f{T„) (see Fig. 58c) are calculated. Calculation of the differences between each thermocouple location versus time, AT — f(t), provides the functions illustrated in Fig. 59d. Calculation of the integral under the heat flux density curve (which represent the a m o u n t of heat extracted) from the beginning of immersion until a predetermined time, provides the functions shown in Fig. 59e. For heat extracted from the probe, the curve designated by "(" (i.e., for the surface layer of 1.5 m m thickness) is valid. In each point where thermocouples are located, the cooling rate curves versus surface temperature: dT/dt = fiT„) are calculated as shown in Fig. 59f.

FLUIDS:

STEEL

QUENCHING

TECHNOLOGY

625

Calculated functions, graphically represented in Fig 58 and Fig. 59 permit comparison of the actual quenching intensity among different quenchants, quenching conditions, and techniques. Based on these thermodynamic functions, each quenching test m a y be evaluated with respect to: depth of hardening, (when comparing two quenching processes), thermal stresses, a n d possible superposition of structural transformation stresses that will occur during a particular quenching process and delayed quenching, i.e., whether continuous or discontinuous cooling rates are occurring (with consequences on hardness distribution on the cross section after hardening). These thermodynamic functions also provide the basis for automatic control of quenching intensity during the quenching process. 2. Module II: HEAT-TRANSF (Calculation of Heat Transfer Coefficient and Cooling Curves)—The function of this module is the calculation of the temperature distribution in the cross section of round bars. It is based on the numerical method of control volumes where the heat conduction in the radial direction is solved as a one-dimensional problem. The software consists of two m a i n subroutines. The first subroutine utilizes the measured surface temperature as an input parameter to calculate the temperature distribution over the probe's cross section versus time and the heat transfer coefficient between the surface and the quenchant versus time and versus the surface temperature as illustrated in Fig. 60a and Fig. (sOb. The second subroutine utilizes the calculated heat transfer coefficient for a particular quenching test as input parameter, which permits the simulation of quenching cylindrical workpieces of varying diameters u n d e r the conditions of each quenching test that are stored in the users database of quenching intensities. Physical properties of the workpiece can be selected for the desired steel grade. The cooling curve at any point within the cross section can be calculated as illustrated in Fig. 60c along with corresponding heat transfer coefficient versus time and versus the surface temperature. 3. Module III: CCT-DIAGR (Prediction of Microstructure and Hardness after Quenching)—This module is used to predict the microstructure and hardness after quenching of cylindrical workpieces of different diameters. It contains a n open data file of CCT (Continuous Cooling Transformation) diagrams in which the user may store u p to 60 CCT diagrams of his own choice. This program enables the user to superimpose every calculated cooling curve on the CCT diagram of the desired steel. From the superimposed cooling curves (shown on the CRT monitor during analysis) the u s e r can select the percentage of microstructural phases transformed and the hardness value at the selected point within the cross section after hardening as illustrated in Fig. 61. For a cross section of the selected diameter, cooling curves are calculated at three or five characteristic points (surface, 3/4R, 1/2R, 1/4R and center), using the HEAT-TRANSF module. The CCT-DIAGR module enables the user to d e t e r m i n e hardness values at these points, which will permit prediction of the hardness distribution curve. Note 17: In the case of delayed quenching, where a discontinuous change of cooling rates occur, the prediction of microstructural transformations and hardness

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628 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK 2000

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CH2) Methylene C-H asym./gym. stretch Methylene C-H bend Methylene - (CH2)-rocklng (n>3) Cyclohexane ring vibrations Methyne (>CH-) Methyne C-H stretch Methyne C-H bend Skeletal C-C vibrations Special methyl (-CHj) frequencies Methoxy methyl ether O-CH3 C-H stretch Methylamino, N-CH3, C-H stretch

FIG. 22—Saturated aliphatic (alkane/alkyl) group frequencies. Courtesy Coates Consulting.

Olefinic (alkene) Group Frequencies Origin 0=0

C-H

Group frequency wavenumber (cm') 1680-1620 1625 1600 3095-3075 +3040-3010 3095-3075 3040-3010

0-H

1420-1410 1310-1290

C-H

995-985+915-890 895-885

C-H

970-960 700 (broad)

Assignment Alkenyl 0 = 0 stretch Aryl-substituted C=C Conjugated 0 = 0 Tenninal (vinyl) C-H stretch Pendant (vinylldene) C-H stretch Medial, cis- or frans-C-H stretch Vinyl C-H in-plane bend Vinylidene C-H in plane bend Vinyl C-H out-of-plane bend Vinylidene 0-H out-of-plane bend trans 0-H out-of-plane bend cis C-H out-of-plane bend

bonds around the aromatic ring is defined by the structure of the bands in the spectrum. Other important bcuids for aromatic ring vibrations are positioned around 1600 and 1500 c m ~ \ which are exhibited as pairs with some splitting. The nature and structure of these two bands are largely influenced by the position and nature of substituents on the ring [45]. On the surface, the interpretation of halogenated comp o u n d s contained in infrared spectra is functionally very simple. While not always true, the polar nature of the group consisting of a single atom linked to carbon produces a dis-

ANALYSIS

669

tinctive spectral contribution. T5T3ically, a unique group frequency associated with halogen-carbon stretching is assigned to the C—^X bond (Fig. 24). If more than one halogen is present, the identification of the group frequency is somewhat more complex. It is largely influenced by whether the halogens are on the same or different CcU"bon atoms, and if different, their relative proximity is important. Relating to alcohols and hydroxy c o m p o u n d s , the 0—H stretch is probably one of the most pronounced and characteristic of all the infrared group frequencies. There is typiCcdly a high degree of association coming from hydrogen bonding with other hydroxy groups. And, in cases, these may come from hydroxy groups from within the same molecule (intramolecular bonding). Alternatively, they may associate with nearby molecules (intermolecular bonding). Collectively, the effects of hydrogen bonding result in the production of a well-defined but broad band cuid the lowering of mean absorption frequency. This is exhibited in compounds such as carboxylic acids, which produce strong hydrogen bonding. See Fig. 25 for alcohol a n d hydroxy c o m p o u n d group frequencies [45]. Because alcohols exist as three distinct classes, primary, secondcury, and tertiary, they are identified by the extent of carbon substitution on the central hydroxy-substituted carbon. The infrared characterization of these alcohols is reflected in the position of the OH stretch absorption but also by other absorptions including the C—O— stretching frequency. These can be observed in the primary and secondary alcohols shown in the spectra shown in Fig. 26. Ethers are somewhat related to edcohol and hydroxy compounds where the hydrogen of the hydroxy group is substituted by an aromatic (aryl) or aliphatic (alkyl) molecular fragment. Otherwise, the overall appearance of an ether spectrum is sharply different from any associated alcohol due to the impact of the hydrogen bonding on the hydroxy group [45]. In amines, the terms primary, secondary, and tertisiry are used to describe the substituted nitrogen as opposed to carbon as with alcohols. As with alcohols, these structural differences are significant and distinctly influence the infrared

Aliphatic Organohalogen Compound Group Frequencies Origin

FIG. 23—Olefinic (alkene) group frequencies. Courtesy Coates Consulting.

24: HYDROCARBON

Group frequency wavenumber (cm"')

C-F

1150-1000

C-CI

800-700

C-Br

700-600

0-1

600-500

Assignment

Aliphatic fluoro compounds, C-F stretch Aliphatic chtoro compounds, C-CI stretch Aliphatic bromo compounds, C-Br stretch Aliphatic iodo compounds, C-l stretch

Note that the ranges quoted serve as a guide only; the actual ranges are influenced by carbon chain length, the actual number of halogen substituents, and the molecular conformations present.

FIG. 24—Aliphatic organohalogen compound group frequencies. Courtesy Coates Consulting.

670

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

Alcohol and Hydroxy Compound Group Frequencies Group frequency Assignment Origin wavenumber (cm-1) 0-H

0-H

0-H C-0

3570-3200 (broad) 3400-3200 3550-3450 3570-3600 3645-3630 3635-3620 3620-3540 3640-3530' 1350-1260 1410-1310 720-590 -1050' -1100° -1150' -1200'

Hydroxy group, H-bonded OH stretcti Normal "polymeric" OH stretch Dimeric OH stretch Internally bonded OH stretch Nonbonded hydroxy group, OH stretch Primary alcohol, OH stretch Secondary alcohol, OH stretch Tertiary alcohol, OH stretch Phenols, OH stretch Primary or secondary, OH in-plane bend Phenol or tertiary alcohol, OH bend Alcohol, OH out-of-piane bend Primary alcohol, C-0 stretch Secondary alcohol, C-0 stretch Tertiary alcohol, C-0 stretch Phenol, C-0 stretch

'Frequency influenced by nature and position of other ring substituents. "Approximate center of range for tlie group frequency. Courtesy Coates Consulting.

FIG. 25—Alcohol and hydroxy compound group frequencies. Courtesy Coates Consulting.

I I I I I I I I I I I I I o o o o o o o

o

o

Wavenumber (cm') 100

o N-H

-3450

>N-H

3490-3430

=N-H

3350-3320

>N-H C-N

1650-1550 1190-1130

C-N

1210-1150

C-N

1340-1250

C-N

1350-1280

C-N

1360-1310

N-H

Assignment

Primary amino Aliphatic primary amine, NH stretch Aromatic primary amine, NH stretch Primary amine, NH bend Primary amine, CN stretch Secondary amino Aliphatic secondary amine, NH stretch Aromatic secondary amine, NH stretch Heterocyclic amine, NH stretch Imino compounds, NH stretch Secondary amino, NH bend Secondary amine, CN stretch Tertiary amino Tertiary amine, CN stretch Aromatic amino Aromatic primary amine, CN stretch Aromatic secondary amine, CN stretch Aromatic tertiary amine, CN stretch

FIG. 27—Amine and amino compound group frequencies. Courtesy Coates Consulting.

Example Carbonyl Compound Group Frequencies Group frequency (cm-1) 1610-1550/1420-1300 1680-1630 1690-1675/(1650-1600)' 1725-1700 1725-1705 1740-1725/(2800-2700)' 1750-1725 1735 1760-1740 1815-1770 1820-1775 1850-1800/1790-1740 1870-1820/1800-1775 2100-1800

Functional group Carboxylate (carboxyllc acid salt) Amide Quinone or conjugated ketone Carboxyllc add Ketone Aldehydle Ester SIx-membered ring lactone Alkyl carbonate Add (acyl) hallde Aryl carbonate Open-chain add anhydride Five-membered ring anhydride Transition metal carbonyls

' Lower frequency band is from the conjugated double bond. "Higher frequency band characteristic of aldehydes. associated with the termlhal aldehydic C-H stretch.

FIG. 28—Carbonyl compound group frequencies. Courtesy Coates Consulting.

ANALYSIS

671

bricating oils. Essentially any compound that forms covalent bonds within a molecular ion fragment will produce a characteristic absorption spectrum with unique group frequencies. The metal complexes and chemical fragments associated with heteroxy groups such as nitrates, sulfates, phosphates, silicates, etc. and transition metal carbonyl compounds have already been generally discussed as related to the salts of carboxyllc acids, amino, and ammonium compounds [45]. ASTM Petroleum Products and Lubricants IR Test Standards w i d e r Subcommittee D02.04 Aromatics in Finished Gasoline by GC-FTIR: ASTM D 5986 This method can be used for determining aromatic content in gasolines that contain oxygenates such as alcohols emd ethers as additives. It can be used for both, and does not interfere with benzene and other aromatics by this method. The sample is injected through a cool on-column injector into a gas chromatograph equipped with a methylsilicone WCOT column interfaced to a FT-IR instrument. Benzene/Toluene in Gasoline by Infrared (Ir) Spectroscopy: ASTM D 4053 A gasoline sample is examined by infrared spectroscopy and following a correction for interference is compared with calibration blends of known benzene concentration. Benzene/Toluene in Engine Fuels using Mid-IR Spectroscopy: ASTM D 6277 A beam of infrared light is imaged through a liquid sample cell onto a detector, and the detector response is determined. Wavelengths of the spectrum that correlate highly with benzene or interferences are selected for analysis using selective bandpass filters or mathematically by selecting areas of the whole spectrum. Methyl Tert-Butyl Ether in Gasoline by Infrared Spectroscopy: ASTM D 5845 This infrared method measures MTBE and other oxygenates in the concentration ranges from about 0.1 to about 20 mass percent. A sample of gasoline is analyzed by infrared spectroscopy. ASTM STANDARDS No. Title D 1319 Hydrocarbon Types by Fluorescent Indicator Adsorption D 1840 Naphthalene Hydrocarbons in Aviation Turbine Fuels by Ultraviolet (UV) Spectrophotometry D 2427 Hydrocarbon Types in Gasoline by Gas Chromatography D 2789 Hydrocarbon Types in Gasoline by Mass Spectrometry D 2887 Boiling Range Distribution of Petroleum Fractions by Gas Chromatography D 3524 Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography

672 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK D 3525

Gasoline Diluent in Used Engine Oils Gas Chromatography Method D 3606 Benzene/Toluene in Gasoline by Gas Chromatography D 3 710 Boiling Range Distribution of Gasoline Fractions by Gas Chromatography D 4053 Benzene/Toluene in Gasoline by Infrared (IR) Spectroscopy D 4291 Ethylene Glycol in Used Engine Oil D 4420 Aromatics in Finished Gasoline by Gas Chromatography D 4815 Methyl Tert-Butyl Ether in Gasoline by Gas Chromatography D 5186 Aromatics and Polynuclear Aromatics in Diesel and Aviation Turbine Fuels by SFC D 5480 Engine Oil Volatility by Gas Chromatography D 5501 Ethanol Content in Denatured Fuel Ethanol by Gas Chromatography D 5580 Aromatics in Finished Gasoline by Gas Chromatography D 5599 Oxygenates in Gasoline by Gas Chromatography D 5623 Sulfrir Determination by GC-Sulfrir Detector D 5769 Aromatics in Gasoline by Gas ChromatographyMass Spectrometry (GC-MS) D 5845 Methyl Tert-Butyl Ether in Gasoline by Infrared Spectroscopy D 5986 Aromatics in Finished Gasoline by GC-FTIR D 6277 Benzene/Toluene in Engine Fuels Using Mid-IR Spectroscopy D 6293 Oxygenates O-PONA Hydrocarbons in Fuels by Gas Chromatography D 6296 Olefins in Engine Fuels by Gas Chromatography D 6352 Boiling Range Distribution of Petroleum Distillates by Gas Chromatography D 6379 Hydrocarbon Types Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates D 6417 Estimation of Engine Oil Volatility by Capillary Gas Chromatography ASTM Petroleum Products and Lubricants N M R Test Standards under Subcommittee D02.04 Aromatics in Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance (HR-NMR)

OTHER STANDARDS IP 156

Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption

REFERENCES [1] Oehler, U., NMR-A Short Course, University of Guelph, Ontario, Canada, http://www.chembio.uogueIph.ca/driguana/NMR/TOC. HTM, Jan. 2001. [2] Silverstein, R., Bassler, G., and Morrill, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John WUey and Sons, Inc., NY, 1991, pp. 166-201. [3] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 802-808.

[4] NMR Spectral Archive, National Institute of Advanced Industrial Science and Technology. Tsukuba, Ibaraki, Japsin. SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/. [5] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technlp, Paris, 2000, p. 21. [6] Bouquet, M., "Determination of Hydrogen Content of Petroleum Products Using Low Resolution Pulsed NMR Spectrometry," Fuel, Vol. 64, 1985, pp. 226-228. [7] Gauthier, S. and Quignard, A., "Accurate Determination of Hydrogen Content in Petroleum Products by Low Resolution," '// NMR Revue IFF. Vol. 50, No. 2, 1995, pp. 249-282. [8] Willis, J., Lubrication Fundamentals, Marcel-Dekker, Inc., NY, 1980, pp. 1-25. [9] ASTM D 5292-93: Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1993. [10] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition. McGraw-Hill, Inc., NY, 1996, pp. 831-864. [11] Norris, T. A., "Chromatography II," Lubrication, Vol. 65, No. 2, 1979, pp. 13-24. [12] ASTM D 5307: Determination of Boiling Range Distribution of Crude Petroleum by Gas Chromatography, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. [13] Altgelt, K. and Gouw, T., Chromatography in Petroleum Analysis, Marcel-Dekker, Inc., NY, 1979, pp. 41-73. [14] Troyer, D. and Fitch, J., Oil Analysis Basics, Nona Corporation, Tulsa, OK, 1999. [15] Fitch, J., "Oil and Water Don't Mix," Practicing Oil Analysis Magazine, July/August 2001, p. 20. [16] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing Editions Technip, Paris, p. 53. [17] "Gas Chromatography with Atomic Emission Detector for Turbine Engine Lubricant Analysis," JOAP Conference Proceedings, University of Dayton Research Institute, Dayton, OH, 1994, pp. 485^96. [18] Norris, T. A., "Chromatography II," Lubrication, Vol. 65, No. 2, 1979, pp. 13-24. [19] Li, Z., "Separation Techniques with Liquid Chromatography," The FRH Journal, 1984, pp. 69-76. [20] Stevenson, R., "Rapid Separation of Petroleum Fuels by Hydrocarbon Type," Journal of Chromatographic Science, Vol. 9, 1971, pp. 257-262. [21] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 31. [22] "La Chromatographic d'Exclusion sur Gel," Journal of Dubois Analysis 16, Vol. 3, No. LWVI-LXXIII, 1988. [23] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-HUl, Inc., NY, 1996, pp. 865-896. [24] API 1509, Engine Oil Licensing and Certification System, American Petroleum Institute, Washington, DC, 1996. [25] ASTM Test Standard D 2007: Characteristic Groups in Rubber Extender and Processing Oils and Other Petroleum-Derived Oils by the Clay-Gel Absorption Chromatographic Method, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. [26] RR: D02-1388, ASTM International, West Conshohocken, PA, 1996. [27] Shugar, G. and BaUinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 753-759. [28] Silverstein, R., Bassler, G., and MorriU, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John Wiley and Sons, Inc., NY, 1991, pp. 289-314. [29] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 41-58.

CHAPTER 24: HYDROCARBON ANALYSIS [30] Determination of Aromatic Hydrocarbons in Lubricating Oil Fractions by Far Ultraviolet Absorption Spectroscopy, R. A. Burdett Molecular Spectroscopy, Institute of Petroleum, George Sell, London, 1954, pp. 30-41. [31] Haas, J., et al., "A Simple Analytical Test and a Formula to Predict the Potential for Dermal Carcinogenicity From Petroleum Oils," American Industrial Hygiene Association Journal, Vol. 48, No. 11, 1987, pp. 935-940. [32] Ogan, K., BCatz, E., and Slavin, W., "Determination of Polycyclic Aromatic Hydroceirbons in Aqueous Samples by Reverse Phase Liquid Chromatography," Analytical Chemistry, Vol. 51, No. 8, 1979, pp. 1315-1320. [33] MS Spectral Archive, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/. [34] Norris, T. A., "Chromatography II," Lubrication, Vol. 68, No. 3, 1979, pp. 25-40. [35] Silverstein, R., Bassler, G., and Morrill, T., Spectrometric Identification of Organic Compounds, Fifth Edition, John Wiley and Sons, Inc., NY, 1991, pp. 3-39. [36] Denis, J., Briant, J., and Hipeaux, J., Lubricant Properties Analysis and Testing, Editions Technip, Paris, 2000, pp. 24-28. [37] ASTM D 2786-91: S t a n d a r d Test Method for H y d r o c a r b o n Types Analysis of Gas-Oil S a t u r a t e s Fractions by High

[38]

[39]

[40]

[41] [42]

[43] [44]

[45]

673

Ionizing Voltage Mass Spectrometry, Annual Book of ASTM Standards, ASTM I n t e r n a t i o n a l , West Conshohocken, PA, 1991. Status of Application of Mass Spectrometry to Heavy Oil Analysis, Advances in Mass Spectrometry, AMSPA, Waldron, 1986, pp. 175-191. ASTM D 2425-93: S t a n d a r d Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1993. Coates, P. and Setti, L., Oils, Lubricants and Petroleum Products, Characterization by Infrared Spectra, Marcel-Dekker, Inc., NY, 1985. Archer, E. D., "Infrared Analysis I," Lubrication, Vol. 55, 1969, pp. 13-32. Shugar, G. and Ballinger, J., Chemical Technicians' Ready Reference Handbook, Fourth Edition, McGraw-Hill, Inc., NY, 1996, pp. 761-775. Anonymous, Chapter 10.9, Infrared Analyzers. ASTM E 168: Standard Practices for General Techniques of Infrared Quantitative Analysis, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 1999. Coates, J., Infrared Spectra, A Practical Approach to the Interpretation of Coates Consulting, Marcel Dekker, NY, 1985.

MNL37-EB/Jun. 2003

Volatility Rey G. Montemayor^

DISCUSSION

VOLATILITY, IN ITS SIMPLEST DEFINITION, IS THE TENDENCY OF A

LIQUID TO CHANGE INTO VAPOR. For fuels, lubricants, and other petroleum products, this tendency is measured in a variety of ways. Volatility parameters are related to the perform a n c e characteristics and/or safety of these materials. Among the various ways of determining the volatility properties of materials are: distillation, rate of evaporation measurement, flash point test, and vapor pressure determination. Distillation determines the temperatures required to evaporate known portions of the material, as well as the temperatures at which distillation begins and ends. Distillation also determines the boiling range of the materials. A volatility property particularly important in solvents and coating materials is the rate of evaporation. The flash point of a liquid is the lowest temperature, corrected for bcirometric pressure, at which application of an ignition source causes the vapor above the specimen to ignite. Vapor pressure is the force per unit area exerted on the walls of a closed container by the vaporized portion of the liquid material in the container. The significance of the different volatility properties varies from one material to another. For crude oil, distillation data are critical assay information. For solvents distillation, flash point and evaporation rate are important parameters. For motor and aviation gasoline, distillation and vapor pressure data are paramount since these properties are related to the performance characteristics of these materials. Distillation characteristics of diesel and other nonaviation fuels exert a great influence on their performance. Flash points and fire points for distillates and residual fuels, as well as lubricants, are significant from the perspective of safety in handling, storage, and transportation of these materials. This chapter will deal with the different test methods dealing with the volatility characteristics of fuels and lubricants. A summary of the test methods, scope, significance, precision, and results will be given. Other internationcd standards dealing with volatility properties determination and their corresponding ASTM standards will be referenced. Distillation for materials other than crude oils is discussed under the section on Distillation. Flash point eind fire point determinations are discussed in the Flammability section. Distillation test methods applicable to crude oils are dealt with in the Crude Distillation section. Vapor pressure measurements are covered in the Vapor Pressure section.

' Imperial Oil Ltd., Products and Chemicals Division, Sarnia, Ontario Canada

Distillation Distillation parameters are importcuit volatility characteristics of motor and other automotive spark-ignition fuels, aviation gasoline, aviation turbine fuels, diesel and other nonaviation gas turbine fuels, solvents, and other petroleum products. Gasolines and gasoline blends are used in a variety of engines operating u n d e r various atmospheric and mechanical conditions. In order to provide satisfactory performance, gasoline must have the optimum distillation characteristics. Gasoline that vaporizes too readily in pumps, fuel lines, and carburetors will cause decreased fuel flow to the engine resulting in rough engine operation or stoppage. If the gasoline does not vaporize easily, difficulty in start-up, poor warm-up and acceleration, as well as unequal distribution of fuel to the combustion cylinders may result. The distillation temperatures of various petroleum products can be determined at atmospheric pressure using ASTM D 86, Standard Test Method for Distillation of Petroleum Products [1] or at reduced pressure using ASTM D 1160, Standard Test Method for Distillation of Petroleum Products at Reduced Pressure [2]. The 10%, 50%, and 90% (recovered) distillation points are important control points in the production of fuel blends. In the mid-1980s the use of automatic distillation equipment gained popularity because of increased ability to control the rate of distillation, emd efficiency in operation. Figure 1 [3] shows the classical manucJ D86 distillation set-up using a gas burner. Figure 2 [4] shows the manual D86 distillation apparatus assembly using electric heater. The use of an electric heater improved the ability to control the rate of distillation. However, it was still difficult to m a i n t a i n the 4 - 5 mL/min distillation rate required by the method. The advent of automatic distillation apparatus allowed the distillation parameters specified in the method to be controlled accurately without operator intervention. The distillation flask and the receiving cylinder in an automatic D86 distillation unit are essentially the same as the manual unit. Although electric heaters eire used, the use of microprocessors to control the rate of distillation provided precise temperature control and allowed conformance to the method requirements. Platinum resistance temperature probes or thermocouples replaced the mercuiy-in-glass thermometers to allow unattended operation. Automatic level followers using optical sensors removed the necessity of manually observing and measuring recovered distillation volumes. Data obtained by automatic distillation units are very similar to those obtained by manual instruments. However, the data are not statisti-

675 Copyright'

2003 by A S I M International

www.astm.org

676

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

Thermometer Distilling Flask

Bath Cover

Heat Resistant Boards

Air Vents SupportFIG. 1—Manual D 86 distillation unit with gas burner.

cally equivalent. Data are available for users to compare manual and automatic D 86 results (Tables lA and IB) [5,6]. Similar improvements have occurred with D 1160.The relative bias between manual and automatic D 1160 are given in Table 2 [7]. ASTM D 86, Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure This test method is under the jurisdiction of ASTM Committee D-02 on Petroleum and Lubricants, and is the direct responsibility of Subcommittee D02.08 on Volatility. This standard was originally published in 1921. The latest edition is D 86-OOa and incorporates a major rewrite of the standard, which was started in 1996. This test method covers the distillation at atmospheric pressure of natureil gasolines, motor gasolines, aviation gasolines, aviation turbine fuels, special boiling point spirits, naphthas, white spirit, kerosines, gas oils, distillate fuel oils, and similar petroleum products, utilizing either manual or automatic equipment. In a D86 distillation, a 100 mL specimen is distilled under prescribed conditions that are appropriate to its nature. Table 3 [8] shows the distillation group characteristics of various materials tested to provide D86 distillation data. Table 4 [9] and Table 5 [10] indicate the various test parameters required for each distillation group. Systematic observation of temperature readings and volumes of condensate are made, and from these data, the results of the test are calculated and reported. The basic method of determining the boiling range of a petroleum product by performing a simple batch distillation has been in use as long as the petroleum industry has existed. It is one of the oldest test methods under the jurisdiction of ASTM Committee D 02, dating from the time when it was still referred to as the Engler distillation. Since the test method

has been in use for such an extended period, a tremendous amount of historical databases exist for estimating end-use sensitivity on products and processes. The distillation characteristics of hydrocarbons often have an important effect on their safety and performance, especially in the case of fuels and solvents. Volatility is the major determinant of the tendency of a hydrocarbon to produce potentially explosive vapors. It is also critically important for both automotive and aviation gasolines, affecting starting, warm-up, and tendency to vapor lock at high operating temperature or at high altitude, or both. The presence of high boiling point components in these and other fuels can significantly affect the degree of solid combustion deposit formation. Distillation limits are often included in petroleum product specifications, in commercial contract agreements, process refinery/control applications, and for compliance to regulatory requirements. The precision of manual and automatic D86 distillation for group 1 materials is given in Table 6 [11]; Table 7 [12] for groups 2,3,and 4 using manual distillation; and Table 8 [13] for groups 2,3, and 4 using automatic distillation. Generally, the distillation temperatures corrected for barometric pressure, corresponding to IBP, 5%, 10%, 20%, 30%, 40%, 50%), 60%), 70%), 80%, 90%, 95%, and FBP are reported. The initial boiling point (IBP) is the corrected temperature reading that is observed at the instant the first drop of condensate falls from the lower end of the condenser tube. The 5-95% distillation temperature is the corrected temperature reading corresponding to each 5-95% volume distilled or recovered. The final boiling point (FBP) or end point (EP) is the maximum corrected temperature reading obtained during the test. Sometimes, the dry point (DP) is required. DP is the corrected temperature reading that is observed or detected at the instant the last drop of liquid evaporates from the lowest point in the distillation flask. The corrections to be applied to the observed temperature readings are obtained by means of the Sydney Young equation given in Eqs 1,2, and 3, as appropriate, or by the use of Table 9 [14]: Cc = 0.0009 (101.3 - Pk) (273 -I- t^)

(1)

G = 0.00012 (760 - P) (273 + tc)

(2)

Cf = 0.00012 (760 - P) (460 + tf)

(3)

where: Cc and Cf = corrections to be added algebraically to the temperature reading in Celsius or Fahrenheit tc = observed temperature reading in Celsius tf = observed temperature reading in Fahrenheit Pk — barometric pressure prevailing at the time of the test in kPa P = barometric pressure prevailing at the time of the test in mm Hg. After applying the corrections, the corrected temperature readings are rounded and reported to the nearest 0.5°C (1.0°F) or 0.1 °C (0.2°F), as appropriate to the apparatus used. However, other calculated values can be reported from D86 distillation data, such as percent recovery and percentages evaporated at prescribed temperature readings. Typical D86 results for motor gasoline, reformulated gasoline, diesel, and jet fuel are given in Table 10 [15], Table 11 [16], Table 12 [17], and Table 13 [18].

CHAPTER 25: VOLATILITY

25 min.

Length of part ' bath approx. 390

Front View

NOTE 1—^Legend: l-Condenser bath 2-Bath cover 3-Bath temperature sensor 4-Bath overflow 5-Bath drain 6-Condenser tube 7-Shield 8-Viewing window 9a-Voifage regulator 9b-Voltmeter or ammeter 9o-Power switch 9d-Power light indicator 10-Vent

11-Distillation flask 12-Temperature sensor 13-Flasl< support board 14-Flasl« support platform 15-GrDund connection 16-Electric heater 17-Knob for adjusting level of support platform 18-Power source cord 19-Reoeiver cylinder 20-Receiver cooling bath 21-Receiver cover

FIG. 2—Manual D 86 distillation unit with electric heater.

677

TABLE lA—Relative bias between manual and automatic D86 distillation. Sample

IBP

5%

10%

20%

30%

40%

50%

60%

70%

80%

90%

95%

FBP

Sample

1 2 3 4 5 6 7 8 9 10 11 12 13* 14*

+ 1.1 (+0.9) +0.7 +0.3 +0.5 + 1.2 +0.3 +0.3 + 1.7 + 1.5 +0.9 + 1.0 +0.3 +0.5

+ 1.9 (0.0) + 1.4 +0.6 + 1.3 + 1.2 +0.8 +0.5 +2.0 + 11.5 + 1.1 (+2.4)" +0.3 +0.4

+2.2 +0.8 + 1.6 +0.8 + 1.3 + 1.6 +0.8 +0.7 + 1.8 + 1.2 + 1.2 +2.3 +0.4 +0.7

+ 1.6 +0.5 + 1.0 +0.8 + 1.3 + 1.2 +0.7 +0.6 + 1.5 +0.7 +0.8 + 1.2 +0.3 +0.5

+ 1.4 +0.4 +0.8 0.3 + 1.2 + 1.2 +0.8 +0.7 + 1.5 +0.4 +0.7 + 1.2 +0.2 +0.8

+0.7 +0.6 +0.6 0.7 + 1.0 + 1.1 +0.8 + 1.2 + 1.5 +0.6 +0.6 + 1.2 +0.9 + 1.1

+0.8 +0.2 +0.3 +0.6 +0.9 +0.8 + 1.0 + 1.2 + 1.2 +0.9 + 1.1 + 1.2 + 1.4 + 1.7

+0.7 +0.1 +0.1 +0.8 +0.6 + 1.1 + 1.5 + 1.1 +0.9 + 1.0 + 1.0 +0.9 + 1.0 + 1.7

+0.7 +0.1 +0.2 + 1.1 +0.8 + 1.2 + 1.6 + 1.3 + 1.3 + 1.4 +0.4 + 1.1 +0.1 + 1.0

+0.1 +0.4 +0.9 + 1.2 + 1.0 +0.2 + 1.6 + 1.9 +0.6 + 1.9 +0.5 +0.2 + 1.1 +0.5

+0.4 (+4.7)" +0.5 +0.8 +0.4 -0.1 + 1.5 + 1.1 -0.4 +0.9 -0.4 -0.7 + 1.2 +0.3

0.7 ( + 1.3)" +0.1 +0.5 +0.4 +0.2 + 1.7 + 1.2 +0.4 +0.1 +0.1 (-0.8) + 1.0 0.0

-0.4 (-1.2)° -0.8 -0.9 -0.9 -0.3 -0.7 -0.8 -1.2 -2.1 -0.8 -0.9 -1.2 -0.8

1 2 3 4 5 6 7 8 9 10 11 12 13* 14*

''Points between parentheses have not been included in the precision analysis. 'Gasoline-alcohol blends. NOTE—1. Data reported are based on averages of ASTM and IP data. 2. Fourteen samples of gasoline were analyzed in 26 laboratories. 3. The bias reported below is (average of automated results)—(average of manual results). TABLE IB—Relative bias between manual and automatic d 86 distillation, various samples [6] (currently under review in ASTM Subcommittee D02.08). Summary of Average Relative Bias in °C (auto—manual) ASTM Interlaboratory Crosscheck Data from 1994 to 1998 Sample Jet A Diesel Mogas Refor

12 14 13 36

#Lab A

#Lab M

99 129 76 86

26 43 17 8

IBP -0.7 -1.9 -1.1 -1.1

5% 1.9 2.7 0.2 0.7

10%

20%

30%

40%

50%

60%

70%

80%

90%

1.1 2.1 -0.1 0.3

0.8 1.2 -0.5 0.2

0.6 1.2 0.1 -0.1

0.7 1.1 -0.7 -0.1

0.7 1.0 -0.3 -0.2

0.8 0.7 -0.3 0.4

0.9 0.7 -0.4 0.5

0.8 0.6 -0.5 0.1

0.6 0.5 -1.9 -0.5

60%

70%

80%

1.4 1.2 -0.5 0.8

1.6 1.2 -0.7 0.9

1.5 1.1 -0.9 0.2

90% 1.0 0.8 -3.5 -0.9

95% 0.3 0.3 -3.4 -2.1

FBP -0.6 0.4 -1.0 0.4

95%

FBP -1.1 0.8 -1.7 -0.7

Summary of Average Relative Bias in °F (auto—manual) ASTM Interlaboratory Crosscheck Data from 1994 to 1998 Sample

N

#Lab A

#Lab M

IBP

5%

Jet A Diesel Mogas Refor

12 14 13 36

99 129 76 86

26 43 17 8

-1.3 -3.4 -1.9 -2.0

3.3 4.9 0.3 1.4

10% 2.1 3.8 -0.2 0.6

20%

30%

40%

1.3 2.2 -0.9 0.3

1,1 2.1 0.1 -0.2

1.2 2.0 -1.2 -0.2

50%) 1.3 1.9 -0.5 -0.4

0.5 0.5 -5.2 -3.8

TABLE 2—Relative bias between m a n u a l and automatic D 1160 [7]. 1 mm Hg Pressure (All AET Values in °C) Sample 2

Sample 1 Boiling Point IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP

Manual

Automatic 225.1 268.2 288.9 321.0 347.1 370.1 392.1 416.0 440.8 472.1 518.5 547.5

239.7 270.1 290.1 324.0 349.5 373.4 395.7 420.0 443.5 472.5 514.1 544.8

Manual

Automatic 342.6 371.3 380.7 388.7 394.4 400.9 407.2 414.0 422.6 433.1 452.6 493.9

338.4 373.5 380.7 388.4 394.3 400.0 405.9 412.8 421.7 433.1 452.0 488.1

Sample 3 Manual

Automatic 321.3 363.7 378.7 397.5 412.1 426.6 439.9 453.2 467.7 486.0 511.5 547.4

330.8 364.4 379.0 397.2 411.8 425.9 439.6 452.4 467.8 485.6 514.1 538.7

10 m m Hg Pressure (All AET Values in °C) Sample 1

Sample 2

Sample 3

Boiling Point

Automatic

Manual

Automatic

Manual

Automatic

Manual

IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP

203.2 252.3 274.7 313.5 340.6 363.1 385.6 408.4 433.0 461.3 507.5 538.5

199.6 254.5 280.7 316.5 342.2 366.4 388.9 411.5 436.5 465.5 506.9 536.3

343.0 370.2 376.9 383.3 391.0 397.0 402.9 409.1 419.4 430.3 450.5 492.8

342.7 370.3 378.1 384.8 391.1 396.9 403.3 410.2 419.0 430.9 451.2 482.7

319.0 360.7 374.6 392.7 408.0 422.4 436.4 450.7 465.3 483.3 509.1 544.0

319.0 359.9 374.2 392.5 407.4 421.2 434.1 448.1 462.9 480.3 504.8 536.3

CHAPTER 25: VOLATILITY

679

TABLE 3—Group characteristics. Group 0 Sample characteristics Distillate tj^pe

Group 1

Group 2

Group 3

Group 4

a65.5 >9.5

(mm Hg) T 35 50 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 IK 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 2^ 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350

0 109.0 114.9 120.8 126.7 132.6 138.5 144.3 150.2 156.0 161.9 167.7 173.6 179.4 185.2 191.0 196.8 202.6 208.3 214.1 219.9 225.6 231.4 237.1 242.8 248.5 254.3 260.0 2K.7 271.3 277.0 282.7 288.4 294.0 299.7 305.3 310.9 316.5 322.2 327.8 333.4 338.9 344.5 350.1 355.7 361.2 366.8 372.3 377.8 383.4 388.9 394.4 399.9 405.4 410.9 416.4 421.8 427.3 432.7 438.2 443.6 449.1 454.5 459.9 465.3

0.5 109.6 115.5 121.4 127.3 133.2 139.0 144.9 150.8 156.6 162.5 168.3 174.1 180.0 185.8 191.6 197.4 203.1 208.9 214.7 220.4 226.2 231.9 237.7 243.4 249.1 254.8 260.5 266.2 271.9 277.6 283.3 288.9 294.6 300.2 305.9 311.5 317.1 322.7 328.3 333.9 339.5 345.1 350.7 356.2 361.8 367.3 372.9 378.4 383.9 389.4 394.9 400.4 405.9 411.4 416.9 422.4 427.8 433.3 438.7 444.2 449.6 455.0 460.4 4^.8

1 110.2 116.1 122.0 127.9 133.7 139.6 145.5 151.4 157.2 163.1 168:9 174.7 180.5 186.3 192.1 197.9 203.7 209.5 215.3 221.0 226.8 232.5 238.2 244.0 249.7 255.4 261.1 266.8 272.5 278.2 283.8 289.5 295.1 300.8 306.4 312.0 317.7 323.3 328.9 334.5 340.1 345.6 351.2 356.8 362.3 367.9 373.4 378.9 384.5 390.0 395.5 401.0 406.5 412.0 417.4 422.9 428.4 433.8 439.3 444.7 450.1 «5.6 461.0 466.4

1.5 110.7 116.7 122.6 128.5 134.3 140.2 146.1 151.9 157.8 163.6 169.5 175.3 181.1 186.9 192.7 198.5 204.3 210.1 215.8 221.6 227.3 233.1 238.8 244.5 250.3 256.0 261.7 267.4 273.0 278.7 284.4 290.0 295.7 301.3 307.0 312.6 318.2 323.8 329.4 335.0 340.6 346.2 351.8 357.3 362.9 368.4 374.0 379.5 385.0 390.5 396.0 401.5 407.0 412.5 418.0 423.5 428.9 434.4 439.8 445.3 450.7 A56A 461.5 466.9

2 111.3 117.2 123.1 129.0 134.9 140.8 146.7 152.5 158.4 164.2 1701 175.9 181.7 187.5 193.3 199.1 204.9 210.7 216.4 222.2 227.9 233.7 239.4 245.1 250.8 256.5 262.2 267.9 273.6 279.3 285.0 290.6 296.3 301.9^ 307.5 313.2 318.8 324.4 330.0 335.6 341.2 346.8 352.3 357.9 363.4 369.0 374.5 380.0 385.6 391.1 396.6 402.1 407.6 413.1 418.5 424.0 429.5 434.9 440.4 445.8 451.2 456.7 462.1 467.5

2.5 111.9 117.8 123.7 129.6 135.5 141.4 147.3 153.1 159.0 164.8 170.6 176.5 182.3 188.1 193.9 199.7 205.5 211.2 217.0 222.7 228.5 234.2 240.0 245.7 251.4 257.1 262.8 268.5 274.2 279.9 285.5 291.2 296.8 302.5 308.1 313.7 319.3 325.0 330.6 336.1 341.7 347.3 352.9 358.4 364.0 369.5 375.1 380.6 386.1 391.6 397.1 4(^.6 408.1 413.6 419.1 424.6 430.0 435.5 440.9 446.3 451.8 457.2 482.6 468.0

3 112.5 118.4 124.3 130.2 136.1 142,0 147.8 153,7 159,5 165,4 171,2 177.0 182.9 188,7 194,5 200,3 206,0 211.8 217.6 223.3 229.1 234.8 240.5 246.3 252.0 257.7 263.4 269.1 274.8 280.4 286.1 291.7 297.4 303.0 308.7 314.3 319.9 325.5 331.1 336.7 342.3 347.9 353.4 359.0 364.5 370.1 375.6 381.2 386.7 392.2 397.7 403.2 408.7 414.2 419.6 425.1 430.6 436.0 441.5 446.9 452.3 457.7 463.1 468.6

3.5 113.1 119.0 124.9 130.8 136.7 142.6 148.4 154.3 160.1 166.0 17f.8 177.6 183.4 189.2 195.0 200.8 206.6 212.4 218.1 223.9 229.6 235.4 241.1 246,8 252.5 258.3 263.d 269.6 275.3 281.0 286.7 292.3 298.0 303.6 309.2 314.9 320.5 326.1 331.7 337.3 342,8 348,4 354,0 359,5 365.1 370.6 376.2 381.7 387.2 392.7 398.2 403.7 409.2 414.7 420.2 425.6 431.1 436.6 442.0 447.4 452.9 458.3 463.7 469.1

4 113.7 119.6 125.5 131.4 137.3 143.1 149.0 154.9 160.7 166.6 172.4 178.2 184.0 189.8 195.6 201.4 207.2 213.0 218.7 224.5 230.2 236.0 241.7 247.4 253.1 258.8 264.5 270.2 275.9 281.6 287.2 292.9 298.5 304.2 309.8 315.4 321.0 326.6 332.2 337.8 343.4 349.0 354.5 360.1 3K.7 371.2 376.7 382.3 387.8 393.3 398.8 404.3 409.8 415.3 420.7 426.2 431.6 437.1 442.5 448.0 453.4 458.8 464.2 4^.6

4.5 114.3 1202 126.1 132.0 137.9 143.7 149.6 155.5 161.3 167.1 173.0 178.8 184.6 190.4 196.2 202.0 207.8 213.5 219.3 225.0 230.8 236.5 242.3 248.0 253.7 259.4 265.1 270.8 276.5 282.1 287.8 293.4 299.1 304.7 310.4 316.0 321.6 327.2 332.8 338.4 344.0 349.5 355.1 360.7 366.2 371.8 377.3 382.8 388.3 393.8 399.3 404.8 410.3 415.8 421.3 426,7 432,2 437,6 443,1 448,5 453,9 459,4 464.8 470,2

CHAPTER 25: VOLATILITY TABLE 20—Precision of manual D 1160. Repeatability Pressure

689

Reproducibility

0.13kPa(l mmHg)

1.3 kPa (10 mmHg)

0.13 kPa(l mmHg)

1.3 kPa (10 mmHg)

17 3.3

15 7.1

56 31

49 27

IBP FBP Volume Recovered

5-50%

60-90%

5-50%

60-90%

5-50%

60-90%.

5-50%

60-90%

C/V% 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0

2.4 2.9 3.2 3.4 3.6 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.8 4.9 5.0 5.0 5.1 5.1 5.2 5.2 5.3 5.3 5.4 5.4 5.5 5.5

2.5 3.0 3.3 3.5 3.7 3.9 4.0 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.8 4.9 5.0 5.1 5.1 5.2 5.2 5.3 5.4 5.4 5.5 5.5 5.6 5.6 5.7 5.7

1.9 2.4 2.8 3.1 3.3 3.6 3.8 3.9 4.1 4.3 4.4 4.5 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.0 6.1 6.2 6.3

2.0 2.5 2.9 3.2 3.5 3.7 3.9 4.1 4.3 4.4 4.6 4.7 4.8 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 5.3 6.3 6.4 6.5

6.5 10 13 16 16 21 23 25 27 29 30 32 34 35 37 38 40 41 43 44 46 47 48 50 51 52 54 55 56 57

3.9 6.0 7.6 9.4 11 12 13 15 16 17 18 19 20 23 22 23 24 25 25 26 27 28 29 30 30 31 32 33 33 34

7.0 9.3 11 12 14 15 16 16 17 18 19 19 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 27 28 28

5.4 7.2 8.5 9.8 11 11 12 13 13 14 15 15 16 16 16 17 17 18 18 19 19 19 20 20 20 21 21 21 22 22

TABLE 21—Precision of automatic D 1160 distillation in °C [7]. 1 mm Hg Pressure Boiling Point IBP 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% FBP

Repeatability 15.9 8.0 5.5 3.8 3.8 5.2 4.7 6.5 4.4 7.3 6.5 9.5

10 mm Hg Pressure

Reproducibility 29.9 17.5 16 12.2 12 9.6 8.8 9.5 11.1 13.1 24.8 37.4

Repeatability

Reproducibility 35.4 11.1 8.0 10.4 7.1 8.6 10.2 12.8 11.3 14.1 25.5 19.7

16.7 6.8 3.7 4.3 3.2 2.9 3.1 3.7 3.4 3.8 5.0 6.4

TABLE 22—Cross reference of international distillation standards relative to ASTM distillation test methods. ASTM Designation U.S. D86 D 1160 D2892 D5236

Distillation Pressure Atmospheric Vacuum Vacuum Vacuum

ISO

IP

BS

AFNOR

DIN

Europe 3405 6616 8708

U.K. 123

U.K. 7392

France M07-002

Germany 51 751 51 356 51567 51 567

FTM

JIS

791-1001

Japan K2254 K2258

690

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

flammable liquids as those having flash points below 37.8°C and combustible liquids as those having flash points between 37.8°C and 93.3°C using specified ASTM flash point test methods. In 1990, the U.S. decided to align its definition of flammable and combustible material with the United Nation's definition, and now defines [23] flammable liquid as having a flash point below 60.5°C and a combustible liquid with flash point between 60.5°C and 93.3°C. Gasoline and aviation fuels are obviously flammable liquids. Flash point specifications have been established for aviation turbine fuels, kerosines, diesel fuels, fuel oils, hydrocarbon solvents, lubricants, and other petroleum products. There are a number of ASTM flash point test methods: D 56, Flash Point by Tag Closed Tester [24]; D 92, Flash and Fire Points by Cleveland Open Cup [25]; D 93, Flash Point by Pensky-Martens Closed Cup Tester [26]; D 1310, Flash Point and Fire Points of Liquids by Tag Open Cup Apparatus [27]; D 3278, Flash Point by Setaflash Closed Cup Apparatus [28]; D 3828, Flash Point by Small Scale Closed Tester [29]; and D 3941, Flash Point by the Equilibrium Method With a Closed Cup Apparatus [30]. A new flash point method, D 6450, Flash Point by Continuously Closed Cup (CCCFP) Tester [31] that was recently approved as an ASTM standard is included in this work. Table 23 gives the comparison of the different ASTM test methods of determining the flash point of a material. These flash point test methods all require a given specimen size, a prescribed rate of heating, temperature measuring device, introduction of a heating source at specific stage during the test, some mechanism of detecting the flash point, and barometric pressure correction. In manual flash point equipment, the ignition source is a gas test flame, and the mechanism of detecting the flash point is the visual observation of a flame that instantaneously propagates itself over the entire surface of the fluid. When the ignition source is a gas flame, the application of the test flame may cause a blue halo or an enlarged flame prior to the actual flash point. Such a phenomenon is not a flash and should be ignored. In the automated equipment, the specimen size and equipment dimensions are the same as those of the manual apparatus. However, the rate of heating, temperature measurements, and detection of the flash point are done automatically according to the meinual procedure requirements. Resistance

temperature probes or thermocouples are used to measure the temperature of the specimen during the test, and changes in the ionization current, thermal conductivity, or pressure are used to detect the occurrence of the flash point. Table 24 gives a summary of the applicable scope, temperature range, repeatability, and reproducibility for these various flash point methods. All reported flash point values are corrected for the ambient barometric pressure at the time of the test. The observed flash point is corrected for barometric pressure by using the equations [32]: Corrected flash point = C + 0.25 (101.3 - p)

(7)

= C + 0.033 (760 - P)

(8)

where: p = ambient barometric pressure in kPa P - ambient barometric pressure in mm Hg. Historically, the proper operation of flash point testers was verified by determining the flash point of 1,4-dimethyl benzene (p-xylene). However, due to its toxicity and its relatively low flash point (27°C), eiltemative flash point verification fluids were studied. The results of an interlaboratory study conducted in 1993 by the ASTM S-15 Coordinating Committee on Flash Point in cooperation with the NationsJ Institute of Standards and Technology led to the establishment of consensus reference flash point values for n-decane, n-undecane, n-tetradecane, and n-hexadecane to be used as flash point verification fluids for the different flash point test methods. Table 25 summarizes the method specific flash points of these reference standards [33,34]. The study also attempted to determine the relative bias of the different flash point test method as shown in Table 26 [33]. However, the report of the study stressed that the observed relative bias among the different flash point test methods are only applicable to the pure hydrocctrbon liquids used in the study, and may not be applicable to mixtures. The observed D 56/D 93 of 0.97 for n-decane and 0.98 for n-undecane was not unexpected since the rate of heating for D 56 is much slower than D 93 thus allowing thermal equilibrium between the bulk of the specimen and the vapors above it. Open cup flash point methods like D 92 are expected to give higher flash points than closed cup flash point methods. This is because sufficient volatile vapor

TABLE 23—Comparison of the test parameters of different ASTMflashpoint test methods. ASTM Designation D56 D92 D 93 Free. A

Cleveland Pensky-Martens

Open Closed

70 mL 75 mL

slice

Stirring Rate N/A N/A N/A 90-120 r p m

D 93 Proc. B

Pensky-Martens

Closed

75 mL

>110X

250 r p m

D 1310 D3278 D3828 D3941

Tag Setaflash Small Scale Equilibrium Method Continuously Closed Cup

Open Closed Closed Closed Closed Closed

50 mL 2 mL 2mL 50 mL 75 m L ImL

All All All All All All

D6450

Apparatus Tag

Cup Type Closed

Sample Size

Expected Flash Point

50 mL



0.00

z

a. 3 OT V) 10000 Ui

0.15

O

UJ

V

"5 15000

1X If) 3

20000

a. z 5000

_-^\-

0

-0.05

-5000 90

105 120 135 150 165 180 195 210 225 240 255 270 285

CRANK ANGLE [degrees; TDC=180] FIG. 2—Heat release rate diagram-small premixed burn phase.

O

H U lU -5 Z

CHAPTER 27: DIESEL FUEL COMBUSTION that occur during the fuel mixing and combustion processes. A simplistic view of the diesel diffusion burning process involves a relatively thin flame front formed at the interface between the fuel and air. In this view, the fuel is on one side of the flame and air on the other. The initial soot formation reactions include thermal break down (pyrolysis) of the fuel before it enters the flame. Under these conditions, the soot precursor reactions (leading to greater soot formation potential) favor high temperature and long residence times. More soot precursors are formed if the mixing rates are slow and the temperatures are high. Higher temperatures also lead to higher soot oxidation rates. The level of PM emissions from a given diesel engine is the result of the difference between the formation rate and the oxidation rate. In general, higher temperatures tend to lead to lower soot and PM emissions, due primarily to the effect of the increased temperature on the PM oxidation rate. The diesel engine designer faces a conflict between PM and NOx emissions. High temperatures are desirable for PM emission control, but they also lead to high NOx emissions. It is this trade-off that constitutes one of the primary concerns of modem diesel engine design. In general, current practice is to reduce the combustion temperatures to the highest level possible consistent with achievement of the NOx emissions standard. The PM emissions are controlled through engine combustion chamber and injection system design to prevent interactions of the fuel and combustion chamber wall. In addition, higher injection pressures are used to produce higher mixing rates that lead to lower residence times in the pyrolysis zone and thus lower soot formation rates. The effects of fuel properties and composition on soot formation were studied extensively by Naegeli and Moses [5]. They concluded that the primary controUing fuel property is the hydrogen content. This is demonstrated in Fig. 3 where the soot emissions from a diffusion flame are plotted versus the fuel hydrogen to carbon ratio. Fuel hydrogen and carbon contents are determined in the laboratory using ASTM D 5291. It is interesting to note that the fuel hydrogen content is the primary fuel variable affecting both NOx and PM. High hydrogen content (as H atoms) fuels have lower adiabatic flame temperatures and thus lower NOx formation tendencies. The NOx relationship to fuel hydrogen to carbon ratio is presented in Fig. 4, where the adiabatic flame temperatures and the predicted NOx emissions (Zeldovich Mechanism) are plotted versus fuel hydrogen to carbon ratio [6]. Higher hydrogen content fuels also tend to form less soot, and thus lower PM emissions.

1.5

1.75

1.85

Fuel H/C Ratio

FIG. 3—Soot concentration versus fuel H/C ratio.

2

CHARACTERISTICS

3

4

721

5

H/C (Atomic Ratio)

FIG. 4—Adiabatic flame temperature and NOx versus fuel hydrogen to carbon ratio.

D I E S E L F U E L I G N I T I O N QUALITY Current Practice (Cetane Number) Diesel fuel specifications currently include only two specified properties that are related directly to combustion. Heat of Combustion, and cetane number. Heat of Combustion is a fundamental property of the fuel, based on the Heat of Formation of the individual molecules. Its determination is fairly straightforward following ASTM D 240, which employs a bomb calorimeter. The procedure consists of completely burning a weighed sample of the unknown fuel in a combustion bomb that is contained in a controlled temperature bath. The Heat of Combustion is determined by measuring the temperature increase of the bath. Cetane number, on the other hand, as determined following ASTM D 613, is not a fundamental property of the fuel. Cetane number is a defined parameter designed to provide an indication of the ignition quality of diesel engine fuels. Higher cetane number means that the fuel has better ignition qUcJity than fuels with lower cetane numbers. Cetane number is determined in an engine test in which the ignition quality is rated versus those of blends of two reference fuels. The history and method of ASTM D 613 and cetane numbers are described in the following paragraphs. In 1932, Boerlage and Broeze [7] proposed that the ignition quality of a fuel be based on a comparison of its ignition delay time in a diesel engine to that of a blend of two reference fuels. They developed the "cetene scale" in which a fuel was assigned a "cetene number." The reference fuels were two pure hydrocarbons, cetene (C16H32) and mesitylene. Cetene burned readily in conventional engine, while mesitylene did not bum at all.

722 MANUAL 37: FUELS AND LUBRICANTS

nil

HANDBOOK

11 u

7//////1 1

PRE-CHAMBER

w/rr/rrz/rm

FIG. 5—Cross section of the CFR engine.

In 1935, ASTM adopted this form of diesel fuel rating system using hexadecane (C16H34) and alpha-methylnaphthalene ( d iH]o) as the reference fuels. The former was assigned a cetane number of 100, while the latter was given a cetane number of 0 [8]. In 1962, ASTM added heptamethylnonane, (C16H34) to the cetane scale as an intermediate, low ignition quality fuel with a defined cetane number of 15 [9]. The standard apparatus for determining and comparing ignition delay times is a Coordinated Fuels Research (CFR) diesel engine developed by the Waukesha Motor Company. It is a one-cylinder, four-stroke cycle engine with a cylindrical prechamber chamber design (see Fig. 5) and a compression ratio capability ranging from 6:1 to 28:1. The ASTM D 613 test procedure consists of running the test fuel at specified conditions of speed, load, and intake temperature. The injection timing is adjusted so that the start of injection is 13° Before Top Dead Center (BTDC). The compression ratio is adjusted until ignition occurs at TDC. The test fuel is then replaced with blends of the reference fuels until one is determined to have a slightly higher compression ratio and one with a slightly lower compression ratio than the test fuel. The cetane number of the test fuel is determined by linear interpolation of the cetane numbers of the reference fuels. As an example, typical U.S. diesel fuel has a cetane number of 45. The 45 cetane number reference fuel is a blend consisting of 64.7 vol.% hexadecane and 35.3 vol.% heptamethylnonane . Unfortunately, a number of problems are associated with using the CFR engine for evaluating ignition delay time and thus cetane number. It has been criticized for a variety of significant shortcomings. The primary complaint is a failure of cetane number to consistently provide an accurate measurement of ignition quality. The specific shortcomings of the current cetane procedure were extensively discussed during a CRC-hosted workshop, Diesel Fuel Combustion Performance, held in Atlanta, Georgia in 1984 [10]. It appears that the main

problem with the cetane procedure is related to the fact that neither the engine nor the test conditions are representative of current engine design or typical operating conditions. The current procedure, developed over 50 years ago, involves the use of the Waukesha engine. The prechamber has a movable end plate, which is used to change the volume of the prechamber and, thus, the compression ratio, as shown in Fig. 5. The specified operating conditions of the test are equivalent to a high-speed idle test, with the speed set at 900 rpm and the fuel flow set at 13 ml/min (equivalent to an air/fuel ratio of approximately 30:1). One of the more frequent complaints is the fact that the CFR engine design is not representative of current diesel engines. The engine design includes a cylindrical precombustion chamber. The fuel injector is located in one end of the prechamber and the other end is moveable and is used to vary the compression ratio. The motion of the prechamber end plate not only changes the compression ratio, but also changes the distance from the injection nozzle to the combustion chamber wall. Modern diesel engines, both lightduty and heavy-duty, utilize direct in-cylinder fuel injection. These engines have mixture preparation, surface condition, combustion chamber geometry, and thermodynamic condition effects that are significantly different than the prechamber CFR engine. This means the current cetane test engine and test method expose the fuel to temperature history, airfuel ratio, and surface effects that are different than those encountered in most actual engines in the field. Another complaint is that the test condition used in the test method is effectively a moderate load at very low speed. The test speed is 900 rpm and the air-fuel ratio is 30:1. This speed is much higher than cranking speed and thus does not represent cold start conditions. The speed is also much lower than normal operating speeds in most high-speed diesel engines, so that it is not representative of hot running conditions, and thus it is not a good indicator of the hot running characteristics of the fuel. The moderate load, represented by the 30:1 air/fuel ratio, does not incorporate intake pressures above ambient (not boosted) so that compression pressure histories are different than in modem turbocharged diesel engines. Some of the other reported problems are related to the cost and time required for the measurement [11]. Other problems are associated with the repeatability and reproducibility [11-13]. Accurate and repeatable determination of the cetane number of alternative and ignition-improved fuels, and fuels at the lower end of the cetane scale, are all problematic [12-15]. Many researchers are even questioning the validity of the cetane scale as an indicator of ignition quality [9,12,14,16-18]. The current test method has poor repeatability and reproducibility. LeBreton [12] conducted tests on the repeatability of the CFR engine results and found the standard deviation for the data to be 0.8 cetane numbers. However, the sample only contained fuels with cetane numbers between 45 and 50. Glavinceveski et al. [13] reported repeatability results for a set of 48 fuels as being 1.57 cetane numbers. The test resuks for these same fuels, performed in a number of engines, was calculated to be 4 cetane numbers. Once again, the majority of the fuels had cetane number ratings between 40 and 50, and none had cetane number below 37. Glavinceveski et al. [13] showed that the scatter of the cetane number rating

CHAPTER

2 7: DIESEL

method increases dramatically as the rating drops below 40 cetane number. The standard test method (ASTM D 613) is reported to have a repeatability of ± 1 CN. Indritz [19] examined data for a large n u m b e r of reference and test fuels, tested in a n u m b e r of test engines. The results of his comparisons are presented in Fig. 6, where the cetane numbers are plotted versus compression ratio. Two observations can be made regarding the results. First, it is clear that there is not a universal relationship between the cetctne n u m b e r and the compression ratio. It is likely that the lack of a universal relationship between cetcine n u m b e r a n d compression ratio results from day-to-day and engine-to-engine differences in the combustion. Injection nozzle performance, combustion chamber deposits, variation in compression ratio calibration, a n d variations in the injection p u m p pressure and calibration all contribute to the differences. Second, the variation in the measurements is on the order of +/— 5 CN. An error of 5 CN Ccin mean the difference between starting or not starting in cold weather, depending u p o n the specifics of the engine design. Much of the research into ignition quality has centered on how the ignition delay time a n d cetane n u m b e r relate to the conditions under which the tests are conducted. Yu et al. [20], in experiments performed in a specifically designed research engine, conducted duplicate tests in which combustion occurred every other cycle. They found that the peak of the difference in the pressure traces of the fired a n d unfired cycles increased with decreasing ignition delay time. Tsao et cd. [21] found that delay times increased with decreasing air temperature and engine speed. Hardenberg and Hase [22] reported the delay times decreased with increasing compression pressure a n d air temperature. Finally, Parker et al. [23] and Walsh and Cheng [24] showed the ignition delay time can be decreased by increasing the fuel temperature at injection. Other studies of the ignition delay time have focused on differentiating the physical and chemical aspects of the ignition delay time. The physical aspects include the atomization, mixing rates, a n d vaporization rates of the fuel. These physical aspects are related to the fuel properties of viscosity, density, and distillation characteristics. The chemical aspects relate mostly to the chemical composition of the fuel and the corresponding impacts on the chemical kinetics of the thermal decomposition and free radical generation mechanisms. 820

•80

880

WOO

t040

1080

CAtCULATB) ISENTROPIC TEMPERATURE - K

fS

FUEL COMBUSTION

CHARACTERISTICS

723

Elliott [25] noticed the ignition delay time decreased with lower fuel/air ratio. In a more comprehensive study of physiCcil delay time, Wakil et al. [26] drew the same conclusion and showed that it was caused by the relative spacing of the fuel droplets. They also found that physical delay times increased with droplet size a n d fuel boiling point due to vaporization characteristics. Rao a n d Lefebvre [27] also determined that the physical delay time is always a significant pEirt of ignition delay. In a study of chemiccd delay times, Chang et al. [28] found that the rate of reaction of high-cetane-number fuels increases faster after ignition t h a n t h a t of low-cetanen u m b e r fuels. Cox a n d Cole [29] studied the chemical kinetics involved in chemical delay time and demonstrated a large increase in delay time with decreasing oxygen concentration of the air/fuel mixture. There have been a n u m b e r of studies concerning the effects on cetane number caused by blending alternative fuels with diesel fuel ASTM #2 (DF-2). Saeed a n d Henein [30] found that the addition of 10 vol% ethanol in diesel fuel caused only a slight decrease in cetane number. However, they noted a drastic decrease in cetane n u m b e r as the amount of ethanol in t h e blend increased from 20-70 vol%. Henein a n d Fragoulis [9] studied the effects of blending a number of alternative fuels with DF-2. They found that blends containing indolene, unleaded gasoline, and No. 6 fuel oil each produced a drastic d r o p i n cetane n u m b e r . Blends of DF-2 with m e d i u m n a p h t h a a n d Jet A fuel p r o d u c e d very small decreases in cetane number, while a blend with No. 4 fuel oil caused the cetcine n u m b e r to increase slightly. Dabovisek and Savery [31] concluded that the ignition delay a n d therefore cetane n u m b e r of a blend of two fuels is controlled by the component with the greatest autoignition resistance, or lowest cetane number. Needham a n d Doyle [14] determined cetane number for synthetic and alternative fuels. They conducted studies of the cetane number of blends containing naphtha, sunflower oil, sunflower oil ester, shale oil, coal synthetic liquids, and tar sands. They found that ASTM D 613 for the vegetable oil Eind the blends of naphtha and methanol with DF-2 did not accurately predict the ignition delay. Finally, Siebers [18] conducted constant volume combustion b o m b tests on blends of naphtha Etnd coal derived liquids with DF-2, and on a degummed sunflower oil, a sunflower oil monoester, and methanol to determine how the delay times varied with temperature in the bomb. All fuels behaved similarly t o reference fuels w i t h the exception of methanol, for which the delay time increased dramaticcdly as the temperature decreased, presumably due to the unique chemical structure of methanol.

•

P r o p o s e d Alternatives to ASTM D 6 1 3

I

Due to the issues discussed above, including the time Eind expense required to conduct D 613 cetane n u m b e r determinations, a n u m b e r of edtemative methods have been proposed. The proposed methods have included numerous correlations with other physical a n d chemical properties, constantvolume combustion b o m b based methods, and correlative techniques based on NMR and FTIR analysis of the test fuel. Hardenberg and Hase [22] tried to use activation energy as an indicator of cetane number. Collins a n d Unzelman [32] used API gravity a n d mid-point temperature. Klopfenstein

R • Rvtaranca FtMl 0 " Unknown Furt

"^n^^ -1HU_^, •I 12

13

M

W

It

17

COMPRESSION RATIO

FIG. 6—Scatter plot of Cetane measurements.

724

MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

[33] correlated density and mid-boiling point temperature with cetane number, while Murphy [34] included percent hydrogen. Glavincevski et al. [13] tried to use aromatics to predict cetane number, and Steere [35] developed a correlation between cetane number, aniline point, viscosity, density, and D 86 distillation. Cetane index (ASTM D 976) involves correlations that are modified periodically, and that incorporate a number of different properties. While all of the correlations provide reasonable predictions of the CN of fuels that are similcir to those used to develop the correlations, they are not reliable for prediction of cetane numbers of fuels that are different than the fuels used to develop the coirelations. Typically, these correlations must be modified periodically to accommodate changes in feedstock source and refinery processing technology. In addition, the accuracy of the correlation is limited to the precision of ASTM D 613 test method and the number of engine tests used to obtain an average cetane number for an individual fuel sample. Nuclear magnetic resonance (NMR) has been used to predict cetane number [10,36,37]. Bailey et al. [37] used the relative quantities of methine and methylene hydrogen to predict the cetane numbers of non-aromatics. They also developed a model for aromatic hydrocarbon fuels, based on the relative quantity of alpha hydrogen and relative squared sum of alkyl hydrogen along with the methine and methylene hydrogen. It was reported that the deviations from perfect correlation with D 613 cetane number were most likely due to scatter in the D 613 results. Fodor [38] reported some promising results using Fourier Transform Infrared (FTIR) analysis of the fuel. In these techniques the fuel sample is analyzed using FTIR and the resulting spectra are used to develop correlations with the cetane number. While the techniques are generally very fast and require very small fuel samples, the methods are based on correlations and are unstable when used outside the range of properties/specifications of the fuels used to develop the correlation. Again, as stated above, the accuracy of the correlation is related to the accuracy and number of ASTM D 613 engine tests. Another significant issue with the current cetane rating scale is related to a concern over whether cetane number provides an accurate and representative indication of the ignition quality of all diesel fuels. Ignition quality in a diesel engine is important for cold start, for engine noise, and can be related to both NOx and PM. Tavacha and Cliffe [17] noted that cetane number is a measure of the ignition temperature, not ignition quality. Needham and Doyle [14] found that ignition delay is not the controlling factor in determining overall performance. They recommended development of a new rating method. Sieber's [18] results showed that cetane number does not provide an accurate measure of ignition quality of fuels whose ignition delay dependence on temperature, or compression ratio, or the type of ignition (single two stage) differs from the reference fuels. Oilman et al. [39-40] studied the effects of fuel properties on engine emissions. The tests, performed in a 1988 Detroit Diesel Series 60 engine calibrated to near 1991 emissions levels, indicated that both the NOx and the PM were related to the cetane number. The Series 60 is a direct injection, turbocharged, intercooled diesel engine. The CFR engine is an indirect injection, naturally aspirated engine. Recent results

by Ryan et al. [41], however, indicate that cetane number is not related to emissions from modem heavy-duty diesel engines operating at standard ambient conditions. Analysis of the heat release rates indicate that the combustion process in these modem diesel engines is dominated by diffusion burning, with the combustion rate controlled by the rate of fuel injection. Under these circumstances, cetane number, as a measure of the ignition delay time, is not important for typical diesel fuels because the start of combustion is controlled by the conditions in the engine and the design of the injection system. Injection system designs that allow for control of the rate of fuel injection can reduce the amount of fuel injected during the ignition delay time, and thus reduce the dependence on the ignition quality of the fuel, as discussed previously. It is clear that the current procedure for rating the ignition quality of diesel fuel is no longer adequate. First, it probably does not represent the sensitivity of current and future engines as indicated by the work of Ryan et al. [41], and second, because it is not as repeatable or reproducible as it needs to be for use in current emd older engines. If the results of Indritz [19] are accepted, a variation of +/— 5 cetane numbers is possible. Using the results of Ullman et al. [39,40] a 5 cetane number variation can produce as much as a 4% difference in the NOx emissions and as much as a 13% variation in the PM emissions from older and current engines. Ryan et al. [42-45] developed a constant volume combustion bomb based technique (CVCA) for rating the ignition quality of fuels for diesel engines. The primary objective of these efforts was to develop a technique that is based on measurement of the ignition delay time, a fundamental combustion property of the fuel. The goal was to eliminate the engine sensitivity inherent in the ASTM D 613 technique. A great deal of effort was devoted to developing a technique based on a calibration using the defining reference fuel rather than a correlation. Several hundred fuels were examined using this technique. The test fuels included petroleum based diesel fuels, various refinery products, coal slurries, various oxygenated fuels, blends of various fuels and fuel components, a wide range of cetane improver additives, and a large number of alternative fuels. The apparatus and technique, now called the IQT (Ignition Quality Tester), has been further refined and developed by Allard et al. (45-47) and is currently being examined for approval as an ASTM test method. Based upon the above discussion, it appears that the engine-based technique for diesel-fuel ignition quality rating is not acceptable for the numerous reasons listed. It is also clear that the other noncombustion-based techniques (FTIR, NMR, Property Correlation) are also not acceptable because they involve correlations that are valid only for the type of fuels used to develop the correlations. It appears that the best approach is one in which a fundamental combustion property, such as ignition delay measured in a well-defined experiment, is used as the basis for the rating. It appeeirs that it is also desirable to use a technique that is based on a calibration rather than one based on a correlation. CVCA/IQT Method As indicated above, the IQT method is based on the use of the CVCA technique developed by Ryan et al. [42-45]. This tech-

CHAPTER 27: DIESEL FUEL COMBUSTION nique was originally suggested by Hum et al. [ 15] and Yu et al. [14]. In this method, the fuel sample is injected into a constantvolume combustion chamber, which contains air at a pre-selected elevated temperature and pressure. The constant-volume combustion chamber is equipped with a pressure transducer at one end and an inward opening pintle nozzle at the other end. A proximeter is installed on the injection nozzle for recording the start of injection. The injection nozzle is supplied with fuel from a pneumatically driven, single-plunger fuel injection pump. The initial conditions in the bomb, the quantity of fuel injected, and the fuel injection characteristics are all precisely controlled. Figure 7 is a schematic showing the internal geometry of the constant-volume combustion chamber. Figure 8 is a photograph of the IQT.

CHARACTERISTICS

725

The test method consists of setting the initial conditions in the bomb, followed by injection of the fuel sample and recording of the pressure history during injection, ignition, and combustion. The needle lift trace, using the output of the proximeter on the injection nozzle, is used to document the start of injection. The needle lift data and the pressure variation in the vessel are recorded and used to determine the time from the start of injection to the start of combustion. This time has been defined as the ignition delay time. Figure 9 is a pressure trace showing actual data generated during a test in the IQT. As can be seen in the figure, the pressure in the bomb initially drops due to evaporative cooling of the injected fuel. The pressure then rises rapidly due to combustion. The igni-

Inlet Tc Chamber Surface

Tc High Temperature Policeman

Injection Nozzle Body

Tc Nozzle Tip

Exhaust

FIG. 7—Schematic of the bomb geometry used in the IQT.

3

4

5

6

7

9

10

Ignition Delay (mS) FIG. 8—Photograph of the IQT.

FIG. 9—Pressure versus time in the IQT during ignition and combustion.

726

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

tion delay time is precisely defined by the initial rise of the needle lift trace and the point at which the pressure recovers back to the initial pressure. The IQT is a calibrated technique, meaning that the cetane number-ignition delay time relationship is defined by calibration with a large n u m b e r of reference fuel blends. Initially, the calibration curves were developed using blends of the secondctry reference fuels, but recent results [48] indicate that the use of ASTM D 613 National Exchange Group (NEG) checks fuels provide for a more consistent and cost effective calibration. Figure 10 is a plot showing the calibration curve

for the IQT, where the cetane n u m b e r is plotted versus the ignition delay time. The test method consists of charging the fuel reservoir on the injection p u m p with the unknown fuel. This requires approximately 50 mL sample of fuel to accomplish both the system flush as well as fuel charge for testing. The system is then set for initiation a n d the test sequence is automatically started. The sequence consists of venting the vessel and pressurizing to the initial pressure. After a short stabilization time the test fuel is injected and the pressure and needle lift histories are recorded and used to define the ignition delay

N-cetane: 100 CN

E 3 O

c

ffi

O

Heptamethylnonane: 15 CN 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Ignition Delay (mS) FIG. 10—IQT calibration curve [48].

70 \ r • A

65 0)

iqtvscl-613 IQTCNvsColH

^

Col22vsd613

• — ^

Col17vsCol16 Col 25 vs Col 24

•

Ji 60

E 3

^•k.

Z c

55

0)

50

O

•

•

•

w^

CO

(D 45

a ^"^^

40 35 35

40

45

50

55

60

IQT Predicted Cetane Number FIG. 11—Results of recent round robin testing of the IQT.

65

70

CHAPTER 27: DIESEL FUEL COMBUSTION CHARACTERISTICS time. The vessel is then vented and the process repeated. This sequence is repeated 32 times for each fuel test. The entire test is accomplished in approximately 15 min. Upon completion of the 32 injections, the average ignition delay time is used to automatically determine the derived cetane n u m b e r from a software based ignition delay/derived cetane n u m b e r model. All IQT systems used the same model for determination of the cetane number. The software averaging and the resulting statistical data provide a very good indication of the quality of the data a n d the health of the system. Routine operation of the system also involves periodic, repeated tests using a check fuel to determine the validity of the calibration and the long-term health of the system. Figure 11 is a plot showing the results of a small-scale round robin test in which three different IQT units were compared to each other and to well-documented ASTM D 613 data. The results indicate that the IQT provides a very repeatable and reliable rating of the fuels. Similar comparisons with a wide rcuige of fuels indicate that the IQT also works very well for additized fuels and alternative fuels.

ASTM STANDARDS No. D86 D93 D130

D240 D445

D482 D524 D613 D975 D976 D1319

D1796

D2500 D2622

D5291

E659

Title Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure Standard Test Method for Flash-Pont by PenskyMartens Closed Cup Tester Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip tarnish Test Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Test Method for Ash from Petroleum Products S t a n d a r d Test Method for R a m s b o t t o m Carbon Residue of Petroleum Products Standard Test Method for Cetane Number of Diesel Fuel Qil Standard Specification for Diesel Fuel Oils Standard Test Methods for Calculated Cetane Index of Distillate Fuels Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption Standard Test Method for Water and Sediment in Fuel Oils by the Centrifuge Method (Laboratory Procedure) Standard Test Method for Cloud point of Petroleum Products Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry Standard Test Methods for Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants Standard Test Method for Autoignition Temperature of Liquid Chemicals

727

REFERENCES [1] Dec, J. and Espey, C, "Chemlluminescence Imaging of Autoignition in a DI Diesel Engine," SAE Paper 982685, Society of Automotive Engineers, Warrendale, PA, 1998. [2] Bowman, C. T., "Kinetics of Pollutant Formation and Destruction in Combustion," Progress in Energy Combustion Science, Vol. 1, 1975, pp. 3 3 ^ 5 . [3] Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, NY, 1988. [4] "Air Quality Criteria for Particulate Matter," U.S. EPA, EPA 600/p-99/002aB, March 2001. [5] Naegeli, D. W. and Moses, C. A., "Effects of Fuel Properties on soot Formation in Turbine Combustion," SAE Paper 781026, Society of Automotive Engineers, March 1978. [6] Ryan, T. W., Ill, "Emissions Performance of Fischer Tropsch Diesel Fuel," presented at the Intertech Gas to Liquids Conference, 17-19 May 1999, San Antonio, TX. [7] Boerlage, G. D. and Broeze, J. J., "Ignition Quality of Diesel Fuels as Expressed in Cetane Numbers," SAE Journal, Vol. 27, 1932, pp. 283-293. [8] Schweitzer, P. H., "Methods of Rating Diesel Fuels," Chemical Reviews, Vol. 22, 1938. [9] Henein, N. A., Fragoulis, A. N., and Luo, L., "Correlations Between Physical Properties and Autoignition Parameters of Alternate Fuels," SAE Paper 850266, Society of Automotive Engineers, Warrendale, PA, 1985. [10] Diesel Fuel Combustion Performance Workshop, CRC, Atlanta, GA, 1984. [11] Gulder, O. L., Glavincevski, B., and Burton, G. F., "Ignition Quality Rating Methods for Diesel Fuels-A Critical Appraisal," SAE SP, Qct. 1985. [12] LeBreton, M. D., "Repeatability Test on the CFR Cetane Engine," SAE 841340, Society of Automotive Engineers, Warrendale, PA, 1984. [13] Glavincevski, B., Gulder, O. L., and Gardner, L., "Cetane Number Estimation of Diesel Fuels from Carbon Type Structural Composition," SAE Paper 841341, Society of Automotive Engineers, Warrendale, PA, 1984. [14] Needham, J. R. and Doyle, D. M., "The Combustion and Ignition Quality of Alternative Fuels in Light Duty Diesels," SAE Paper 852102, Society of Automotive Engineers, Warrendale, PA, 1985. [15] Hum, R. W. and Hughes, K. J., "Combustion Characteristics of Diesel Fuels as Measured in a Constant-Volume Bomb," SAE Transactions, Vol. 6, No. 1, p. 24. [16] Hardenberg, H. O. and Ehnert, E. R., "Ignition Quality Determination Problems with Alternative Fuels for Compression Ignition Engines," SAE Paper 811212, Society of Automotive Engineers, Wartendale, PA, 1981. [17] Tavacha, J. W. and Cliffe, J. O., "The Effects of Cetane Quality on the Performance of Diesel Engines," SAE Paper 821232, Society of Automotive Engineers, Warrendale, PA, 1982. [18] Siebers, D. L., "Ignition Delay Characteristics of Alternative Diesel Fuels: Implications on Cetane Number," SAE Paper 852102, Society of Automotive Engineers, Wartendale, PA, 1986. [19] Indritz, D., "What is Cetane Number," Symposium on the Chemistry of Cetane Number Improvement, ACS, Miami, FL, April, 1985. [20] Yu, T. C, Uyehara, Q. A., Meyers, P. S., Collins, R. N., and Mahadevan, K., "Physical and Chemical Ignition Delay in an Operating Diesel Engine Using the Hot-Motored Technique," SAE Transactions, Vol. 64, 1962, p. 690. [21] Tsao, K. C, Myers, P. S., and Uyehara, O. A., "Gas Temperatures During Compression in Motored and Fired Diesel Engines," SAE Transactions, Vol. 70, 1962, p. 136.

728 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [22] Hardenberg, H. O. and Hase, F. W., "An Empirical Formula for Computing the Pressure Rise Delay of a Fuel from Its Cetane Number and from the Relevant Parameters of Direct-Injection Diesel Engines," SAE 790493, Society of Automotive Engineers, Warrendale, PA, 1979. [23] Parker, T. E., Forsha, M. D., Stewart, H. E., Horn, K., Sawyer, R. F., and Oppenheim, A., "Induction Period for Ignition of Fuel Sprays at High T e m p e r a t u r e s and Pressures," SAE Paper 850087, Society of Automotive Engineers, Wsirrendale, PA, 1985. [24] Walsh, G. J. and Cheng, W. K., "Effects of Highly Heated Fuel on Diesel Combustion," SAE Paper 850088, Society of Automotive Engineers, Warrendale, PA, 1985. [25] Elliot, M. A., "Combustion of Diesel Fuel," SAE Transactions, Vol. 3, No. 3, p. 490. [26] El Wakil, M. M., Myers, P. S., and Uyehara, O. A., "Fuel Vaporization and Ignition Lag in Diesel Combustion," SAE Transactions, Vol. 64, 1956, p . 712. [27] Rao, K. V. L. and Lefebvre, A. H., "Spontaneous Ignition Delay Times of Hydrocarbon Fuel/Air Mixtures," ASME Transactions, Vol. 70, 1985. [28] Chiang, C. W., Myers, P. S., and Uyehara, O. E., "Physical and Chemical Ignition Delay in an Operating Diesel Engine Using the Hot-Motored Technique-Part II," SAS Transactions, Vol. 68, 1960, p. 562. [29] Cox, R. A. and Cole, J. A., "Chemical Aspects of the Autoignition of Hydrocarbon/Air Mixtures," Combustion and Flame, Vol. 60, 1985, p. 109. [30] Saeed, M. N. and Henein, N. A., "Ignition Delay Correlations for Neat Ethanol DF-2 Blends in a DI Diesel Engine," SAE Paper 841343,Societyof Automotive Engineers, Warrendale, PA, 1884. [31] Dobovisek, Z. and Savery, C. W., "Ignition Delay of Selected Alternative Fuels in IC Engines," SAE Paper 859225, Society of Automotive Engineers, Warrendale, PA, 1985. [32] Collins, J. M. and Unzelman, G. H., "Diesel Trends Emphasize Cetane Economics, Quality, and Prediction," presented at the API 47th Midyear Refining Meeting, May 1982. [33] Kloptenstein, W. E., "Estimation of Cetane Index for Esters of Fatty Acids," JAOCS Vol. 59, No. 12, Dec. 1982, p. 531. [34] Murphy, M. J., "An Improved Cetane Number Predictor for Alternative Fuels," SAE Paper 831746, Society of Automotive Engineers, Warrendale, PA, 1983. [35] Steere, D. E., "Development of the Canadian General Standards Board (CGSB) Cetane Index," SAE Paper 841344, Society of Automotive Engineers, Warrendale, PA, 1984.

[36] Bowden, J. N. and Frame, E. A., "Effect of Orgsinic Sulfur Compounds on Cetane Number," ACS, April, 1985. [37] Bailey, B. K., Russell, J. A., Wimer, W. W., and Buckingham, J. P., "Cetane N u m b e r Prediction Modeling," SwRI Report No. SwRI 9435, Southwest Research Institute, San Antonio, TX, 1986. [38] Fodor, G. E., "Analysis of Petroleum Products by Midband Infrared Spectroscopy," SAE Paper 941019, Society of Automotive Engineers, Warrendale, PA, 1994. [39] UUman, T., "Investigation of the Effects of Fuel Composition o n Heavy-Duty Diesel Engine Emissions," SAE Paper 892072, Society of Automotive Engineers, Warrendale, PA, 1989. [40] UUman, T., Mason, R. L., and Montalvo, D. A., "Effects of Fuel Aromatics, Cetane Number, and Cetane Improver on Emissions from a 1991 Prototype Heavy-Duty Diesel Engine," SAE Paper 9072171, Society of Automotive Engineers, Warrendale, PA, 1990. [41] Ryan, T. W., Olikara, C , Buckingham, J., and Dodge, L. G., "The Effects of Fuel Properties on Emissions from a 2.5 gm NOx Heavy-Duty Diesel Engine," SAE Paper 982491, Society of Automotive Engineers, Warrendale, PA, Oct., 1998. [42] Ryan, T. W., "Correlation of Physical and Chemical Ignition Delay to Cetane Number," SAE Paper 852103, Society of Automotive Engineers, Warrendale, PA, 1985. [43] Ryan, T. W. and Stapper, B., "Diesel Fuel Ignition Quality as Determined in a Constant Volume Combustion Bomb," SAE Paper 870586, Society of Automotive Engineers, Warrendale, PA, 1987. [44] Ryan, T. W. and Callahan, T. J., "Engine and Constant Volume B o m b Studies of Diesel Ignition and Combustion," SAE Paper 881626, Society of Automotive Engineers, Warrendale, PA, 1988. [45] Ryan, T. W., "Development of a Portable Fuel Cetane Quality Monitor," Belvoir Fuels and Lubricants Research Report No. 277, Southwest Research Institute, San Antonio, TX, 1992. [46] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)," SAE Paper 961182, Society of Automotive Engineers, Warrendale, PA, 1996. [47] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)-Part II," SAE Paper 971636, Society of Automotive Engineers, Warrendale, PA, 1997. [48] AUard, L. N., et al., "Diesel Fuel Ignition Quality as Determined in the Ignition Quality Tester (IQT)-Part III," SAE Paper 1999010, Society of Automotive Engineers, Warrendale, PA, 1999.

MNL37-EB/Jun. 2003

Engineering Sciences of Aerospace Fuels Eric M. Goodger^

RFNA red fuming nitric acid RP-1 rocket propellant narrow-cut kerosine SAE SOCIETY OF AUTOMOTIVE ENGINEERS SIT spontaneous ignition temperature S entropy Sa air specific impulse Sf fuel specific impulse s.f.e.e steady flow energy equation syngas synthetic, or synthesis, gas, (CO -I- H2) AT flame temperature rise in luminometer J"* m a x i m u m adiabatic reaction temperature at constant pressure TAFLE Thornton aviation fuel lubricity evaluation TLV threshold limit value U,u internal energy, specific internal energy UDMH unsymmetrical dimethylhydrazine UHC u n b u m t hydrocarbons V,v volume, specific volume VM molar volume VP vapour pressure W , w work transfer, specific work transfer WSIM water separation index modified P fuel density r ratio of specific heat capacities {cp/c^

NOMENCLATURE A/F air-fuel ratio o n volume basis a/f air-fuel ratio on mass basis ARP Aerospace Recommended Practice (SAE) BOCLE ball-on-cylinder lubricity evaluator C velocity of air CI/LIA corrosion inhibitor/lubricity improving additive CNG compressed natural gas CU conductivity unit (microsiemens/meter) D(X-Y) dissociation enthalpy between a particular X—Y bond E overall energy E activation energy E(X-Y) m e a n empirical dissociation enthalpy between many X-Y bonds FBP final boiling point FSII fuel system icing inhibitor H, h enthalpy, specific enthalpy \Ha standard enthalpy of atomization AHf standard enthalpy of formation AH° standard enthalpy of reaction Hi total thermochemical enthalpy at temperature T based on standard initial temperature of 298.15 K HFRR high frequency reciprocating wear rig HiTTS high temperature thermally stable lATA INTERNATIONAL AIR TRANSPORT ASSOCIATION IBP initial boiling point Id density impulse Is specific impulse JFTOT jet fuel thermal oxidation tester K partial pressure equilibrium constant K' concentration equilibrium constant LNG liquefied natural gas M molar mass, g/mol MTBE methyl tertiary butyl ether m mass, kg NG natural gas NIR near infrared spectroscopy n.f.e.e. non-flow energy equation p pressure Q, q heat transfer, specific heat transfer R gas constant Ro universal gas constant ' Managing Editor, Landfall Press, 28E Jessopp Road, Norwich, Norfolk, NR2 3QB, UK.

IN AEROSPACE APPLICATIONS, PROPULSION SETS THE MOST STRIN-

GENT requirements in terms of the levels of energy to be provided for the purpose, subject to the constraints of mass and volume available for carriage of the fuel within the vehicle. In this study, the following definitions are employed, with the maximum levels of net specific energy shown in parentheses: Conventional fuels - aviation fuel mixtures of a hydrocarbon nature invariably derived from petroleum (44 MJ/kg) High-performance fuels—hydrogen, and individual hydrocarbon materials of particularly high energy content (120 MJ/kg) Substitute high-performance fuels—materials based on non-cryogenic compounds of C, H, O, N, boron, etc. (68 MJ/kg) The performance of a bulk fuel in practice is a function of both the properties of the fuel in question and the conditions

729 Copyright'

2003 by A S I M International

www.astm.org

730 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK under which it is used. The former in turn are dependent on the nature and properties of the fuel components, whereas the latter influence the extent of intrinsic energy released and its conversion to produce vehicular thrust. This is particularly true with ramjet and rocket engine fuels since the chemiccJ behavior of the combustion products within the duct between the combustion chamber and the thrust nozzle outlet can exert an overriding influence on the level of resultant thrust. This chapter provides a concise overview of the heat input requirements of high performance engines together with the heat release available from candidate fuels. It also includes brief comment on the handling characteristics of these fuels in order to ensure that the most attractive candidates are not precluded from use by insurmountable problems within the distribution, storage, and vehicular fuel systems.

ENGINE THERMODYNAMICS The strength of the bonds between individual atoms comprising a fuel molecule represents stored chemical energy, and this energy is required to be released and transformed in some way to produce propulsive thrust, the customary chain of conversions following the energy route from chemical to heat to mechanical. These events take place in a heat engine, and the b r a n c h of engineering science that includes such heat-to-work conversions is known as Thermodynamics. Although the actual processes involved throughout a practical heat engine are quite complex, a simplified overview is adopted by providing a thermodynamic basis of gases associated with idealized conditions and processes, then incorporating the necessary effects of reality since reeil gases do not follow exactly these idealized processes. The science of thermodynamics is built on the concept of a perfect gas that follows certain laws absolutely during changes in its major properties of pressure (p), temperature (T), and volume (V) [Appendix 1]. Fortuitously, the complete range of properties of such a gas at any given state can be fixed by specifying two (unrelated) properties only. This makes it possible to represent the complete state of the gas by means of a unique point on a two-dimensional graph of one property plotted against another (e.g., p against V). Furthermore, when the state of the gas changes in etn ideal (reversible) manner, this process of change can be represented by a unique line on the graph. Even further, when a n u m b e r of different changes follow each other in such a m a n n e r as to return to the original state, they form a closed loop, which represents a cycle that could be repeated indefinitely. Careful selection of the cyclic loop processes can therefore provide a heat transfer into the gas together with a work transfer outwards, thus giving the basis of an ideal cyclic heat engine [Appendix 2]. Such a cycle, even though idccJ, is subject to the fundamental laws of thermodjTiamics which stipulate that, although energy cannot be created or destroyed, not all the heat input can be converted to work because part of the heat must be rejected at a lower temperature. Hence, the net heat input must equal the work output, but the word net must be included or implied. Ideal gas cycles conceived by sequencing various processes include the following: 1. Camot—Meiximum efficiency between given temperature limits, but insufficiently practical since work output low.

2. Stirling, & Ericsson—Camot efficiency; approximated by small-scale non-flow units employing continuous external combustion. 3. Otto—Efficiency lower t h a n C a m o t ; broadly similar to property changes in spark-ignition reciprocating piston engine. 4. Diesel—Efficiency lower than Camot; broadly similar to property changes in compression- ignition reciprocating piston engine. 5. Bray ton—Efficiency lower than Camot; broadly similar to property changes in continuous-flow gas turbine, ramjet and rocket engines. A major advantage of continuous-flow over reciprocating engines is that power is being generated continuously rather t h a n intermittently, hence the power-volume ratios are higher. The particular theoretical cycle of interest as a yardstick for assessing high-performance jet propulsion is therefore the Bra5?ton Cycle (known in Europe as the Joule cycle). This comprises em initial compression without heat rejection through the walls (i.e., "adiabatic," and in fact "isentropic" since it is also reversible), followed respectively by heat addition at constcint pressure (isobaric), work output by isentropic expansion, and finally an isobciric heat rejection returning to the initial state (Fig. 1). Thermodynamic expressions are available for the heat and work transfers applying to the various components of this cycle and, on completing an overall energy book-keeping exercise, the thermcil efficiency of the Brayton cycle appears as follows: I Ur-D/r

work output = 1 heat input

''Brayton

where rp is the pressure ratio of the cycle, equal to p^max'/'mm; x/Pl and the index y is the ratio of / specific heat capacity of gas at constant pressure \ I specific heat capacity of gas at constant volume j and equals 1.4 approximately for air.

E}4}ansion

Compression

Heat rejection

Heat addition

in Z

rejection

-•

3to4

• 4to1

FIG. 1—^The Brayton cycle shown on a non-flow basis.

CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 731 Hence, the higher the pressure ratio, the better the efficiency of heat-work conversion. AppHcations of the Braj^on cycle to gas turbine engines are shown schematically in Fig. 2. The Brayton and other ideal cycles were conceived as having the heat transfers effected across the boundary wall separating the working gas (usually air, hence the expression "air standard cycle") and the surrounding atmosphere. This is a practical possibility and is used, in fact, in Stirling and Ericsson type engines; however such heat transfers tend to take time. Since the chemical energy of a fuel cem be released readily by reaction with oxygen (as shown later under Oxidation Heat Release) combustion with atmospheric air is the most convenient and effective approach to oxidation (although not necessarily the most efficient or desirable, but electrochemical oxidation as in a fuel cell does not appear likely to suit the high-performance r e q u i r e m e n t s of aerospace propulsion). The alternative oxidants required in the case of rocket engines operating beyond the Earth's atmosphere are included in Table 1.

Heat addition

2» •

^

^

Turbine

Compressor

Heat rejection a) Closed circuit (industrial)

Heat addition

^3 •

^

^

/|

1/ Propulsive gases 1 b) Open circuit (propulsion)

FIG. 2—Schematic of gas turbine engines utilizing the Brayton cycle on a steady-flow basis.

Fortuitously, under most conditions the behavior of real gases, such as air upstream of the combustor and burnt products downstream, approximates very closely to that of perfect gases, and ideal cycles can thus be used for comparative purposes with the actual changes of events in practical engines. In practice, also, it is customary to fit a diffuser (a duct of increasing cross sectional area) upstream of the compressor so that the reduced air velocity promotes an initial part of the compression. The corresponding fitment of a nozzle (a duct of decreasing cross sectional area) downstream of the turbine reduces the pressure with a corresponding increase in exit gas velocity and thus in thrust. Once the exit velocity reaches the sonic level, (= V ( g RT) = 331.45 m/s in dry air at 0°C), the exit area chokes, with n o possibility of the effects of chEmges in conditions being transferred back to the nozzle inlet. Consequently, further acceleration of the gas requires expansion of the nozzle, as shown in Fig. 3. Thus, the key thermodjTiamic requirement of high performance fuels is an ability to b u m readily and completely within high-pressure air or other oxidant flowing at high speed, with minimal radiation so that the bulk of the released heat remains within the working gas. The next step, therefore, is to

TABLE 1—Relative performance of rocket fuels and oxidants. Merit order Formula Fuels Liquid hydrogen Hydrazine UDMH Hydyne RP-1 Liquid a m m o n i a Ethanol Liquid diborane Oxidants Liquid fluorine Liquid oxygen Nitrogen tetroxide HTP RFNA

Chlorine trifluoride

Id

LH2

N2H4 (CH3)2N2H4

Unsymmetrical dimethylhydrazine 60/40 mass mixture of UDMH and diethylenetriamine (NH2CH2CH2)2NH rocket propellant narrow-cut kerosine

LNH3 C2H50H LB2H6 LF2 L02

N204 H202 HN03

CIF4

High test peroxide with cone. H2O2 > 80% Red fuming nitric acid containing 7% or more of dissolved oxides of nitrogen

732

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

identify the candidate fuels available, examining their properties with a view to their meeting this requirement.

FUEL MOLECULAR STRUCTURE The majority of conventional fuels, and also some alternative high performance variants, are based on compounds of hydrogen and carbon (hydrocarbons). The structure of these molecules is determined largely by the valency (chemical combining power) of the two basic elements. Since hydrogen is monovalent, and carbon tetravalent, the simplest hydrocarbon s t r u c t u r e possible is clearly CH4 (Fig. 4). This is named methane, and forms the main constituent of the many natural gases (NG) found around the world, invariably incorporating small amounts of heavier hydrocarbons, nitrogen, carbon dioxide, etc. depending on the location and the geological history. As the standard domestic/industrial fuel, this is one of the exceptions of using an individual hydrocarbon fuel commercially. As a gas, it does not lend itself easily to t r a n s p o r t applications unless stored u n d e r compression (CNG) or cryogenically liquefied (LNG). Many other hydroFuel Nozzle

Diffuser

Air-

Open circuit gas turbine with Internal heat release plus diffuser and subsonic nozzle Fuel

t-—Exhaust jet

Air-

f

(1)

Note that "mole" is the n a m e of the physical quantity, whereas "mol" is the symbol. As a n example, the molar mass of methane, CH4, is derived as follows: -—Exhaust jet

Oxidant

Since the atom of hydrogen is the lightest of all the elements, its mass was taken as unity, cind the masses of all other atoms assessed relative to it. With the subsequent adoption of the C''^ isotope as the mass basis with a value of 12 exactly, adjustment to other elemental values gives a value just exceeding unity for naturally-occurring hydrogen, but approximate rounded integers generally give sufficient accuracy in practice. Hence, the molar mass (M) of a hydrocarbon CaHb for example would be given by: M = 12 a + b g/mol (not kg/mol)

Fuel Ramjet with supersonic nozzle Fuel Oxidant

t-

^

Fuel b)

The saturated hydrocarbons contain their maximum content of hydrogen and are, therefore, stable since they do not need to react to seek further hydrogen. On the other hand, the unsaturated hydrocarbons incorporate multi-bonding between adjacent carbon atoms, which imparts instability as these bonds open relatively easily to admit additional hydrogen or to permit inter-bonding of like molecules (polymerization), leading to self-contamination by the formation of gums and other deposits. Nevertheless, these unsaturated hydrocarbons do exist because although they are energetically unstable to elemental decomposition, they are reasonably stable kinetically in that the rates of reaction are very slow below SOO^C at low pressure in the absence of catalysts—comparable to a stone remaining stationary resting on a hillside rather than sliding down the slope. B u o y a n c y i n Air

Fuel

b)

Storage Stability

— > Exhaust jet

#

a)

carbon structures are feasible, most of them categorized into a n u m b e r of "series" depending on their general formulae, as shown in Table 2. The main properties of the first members of these hydrocarbon series are shown in Table 3. ICnowledge of the molecular structure of a hydrocarbon in terms of size, shape, and carbon-carbon bonding together permit prediction of a number of physical a n d chemical characteristics that transfer across to fuel in bulk, as shown in the following analysis:

Rocli

^ 40-

H-cot d

Shelldyne M Adamantines _ Pertiydrofluoranthene l-Methyipertiyfluorene icalin 4 ^ '* Cyciododecarres

Energy-dense cyclics

Normal and monocyclic hydrocarbons C^

An interesting conclusion concerns the level of specific energy available from unit masses of stoichiometric fuel-air mixtures. Although hydrogen enrichment of a fuel tends to a higher specific energy on a fuel mass basis, the stoichiometric air-fuel mass ratio also rises, consequently more air is available to share the energy output. Hence, the stoichiometric mixture values of specific energy vary little from about 2.9 MJ/kg (net). This means that, in general, whereas the fuel carrying capacity of a vehicle is largely affected by specific energy of the fuel, the performance of the engine is not.

7^

XT

Conventional petroleum fuels

Maximum Comibustion Temperature HaW

0.4

T

1

I

0.8 Density kg/L at 15°C

1.2

"7 1.6

FiG. 6—Energy densities of candidate high-performance fuels [1,2].

element contributing more calorific value to the resultant liquid hydride. Examples of such fuels include hydrides of boron, beryllium, lithium, magnesium, aluminium, and titanium, plus compounds of hydrogen, carbon, and boron as included in Table 4.

The isobaric adiabatic reaction temperature for a commercial fuel mixture cannot be calculated without knowledge of the individual chemical constituents and their proportions. However, a broad assessment can be made on the assumption of an average molecular formula in each case. The combustion temperature values included in Table 4 have been computed on the following basis: Fuel

Assumed Average Formula

Aviation gasoline Aviation kerosine Gas oil

Cl2.5H24.4 C15H27.3

C7.3H15.3

The calculated vEilues for the petroleum fuels are seen to vary little from 2025°C. On the other hand, methanol is lower

CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 741 at 1969°C, while nitromethane and pentaborane are higher at 2412 and 2527°C, respectively.

FUEL COMBUSTION PERFORMANCE IN CONTINUOUS FLOW

Smoke Tendency

Fuel Preparation and Mixing

Within fuel-rich regions of a flame, hot fuel molecules can become so agitated thermally that they crack into portions, involving the release of free atoms of carbon in the form of smoke. This is undesirable in a heat engine because: • Combustion efficiency is reduced through the loss of the calorific value of the free carbon. • Glowing particles of carbon radiate heat at a relatively high rate and thus overheat the combustor liner so reducing its life. • Tendencies to soot deposition within the combustor are increased, with the possibilities of mechanical damage due to t e m p e r a t u r e gradients a n d differential expansion in the combustor liner. • Tendencies to exhaust smoke emissions to the environment are increased. For jet fuels, ASTM D 1322, Smoke Point of Kerosine and Aviation Turbine Fuel, is based on determination of the maxi m u m height to which a diffusion flame, wick-fed from the sample, can be adjusted without smoking (Fig. 7). Being based on an early design of an illuminating oil lamp bearing no resemblance physically to an aero gas-turbine combustor, this method may appear primitive. Nevertheless, it does test the fuel u n d e r diffusion conditions where smoke is m o r e likely to be generated in the actual combustor operating at high pressure. In ASTM D 1740, Luminometer Numbers of Aviation Turbine Fuels, the luminometer n u m b e r is determined with a smoke-point type lamp operating at a fixed level of flame radiation by comparing the flame temperature rise above the inlet air level for the sample with those for two reference fuels, tetralin (an aromatic) and zsooctane (a paraffin), as follows:

Fuels b u m only in the vapor/gas phase, consequently the first stage in satisfactory combustion is to vaporize the fuel and mix it thoroughly with the oxidant. In an aero gas turbine engine, the low volatility of kerosine is such that carburation at ambient air temperatures would be too slow for a satisfactory rate of vapor generation. Vaporization at flame temperature is practicable, however, and the "walking stick" (or T-shaped double walking stick) type of vaporizer tube fed with air under relatively low pressure receives narrow streams of liquid fuel that are vaporized to an extent depending upon the fuel flow rate, and introduced directly into the flame [1].

f AT sample - \T tetralin \ Luminometer Number = 100 U j f-octane - AT tetralin I Correspondence has been found between smoke point cUid luminometer n u m b e r over a wide range of kerosines.

©

Inclined mirror for ~ viewing smoke in flame centre

^ Photocell

In most o t h e r applications to gas-turbine engines, the volatility of the fuel is increased substantially to an effective level roughly equivalent to that of a gasoline by subdividing into a large n u m b e r of very small droplets. This process of spraying, loosely termed "atomization," gives a massive increase in surface area per unit volume of fuel, and so augments the vaporization rate by several orders of magnitude. The fuel is also distributed in space so that mixing with air is effected rapidly. Immediately after a fuel droplet is injected into the flame zone, vapor will form and b u m as a premixed flame. As the velocity of the droplet falls, it will become surrounded by a diffusion flame. The proportion of premixed burning may be raised by increasing the relative velocity between the fuel and air, consequently combustion in the gas turbine chamber is dictated by the characteristics of the fuel injector as well as the geometry of the combustor liner itself. This subdivision of a film of fuel into filaments and droplets is achieved by the shearing action arising from the velocity differences between the spray elements and the air. A number of different pressure-jet injection designs are available. These practices are not standardized but are adopted in engine manufacturer's development laboratories. An alternative approach is to achieve the required shear from velocity difference by accelerating the air rather than the fuel. This gives rise to the air-assist a n d air-blast injectors where a stream of high-velocity air meets a stream of low-velocity fuel. Droplet sizes tend to be small and, being controlled by airflow, spatial distribution of fuel is largely unaffected by fuel flow rate [1].

/ Chimney

Ignition

"niemiocouple Reflecting scale

Luminometer Configuration ^®''*''"

Flame-height adjuster

,

Sample teo-ootane

J ^standard 1 1 1 U 1

i

7

i

/\ rs'A

\

\

Flame temperature rise, iT Smoke Lamp

Luminometer No = 100

/Ars-AT|.\ ^ATU-ATL;

FIG. 7—Smoke point and luminometer number. Schematics of apparatus [1].

Ignition is initiated by some form of high-energy spark or torch igniter, and the flame then sustains continuous ignition of the entering mixture, whereas in rocketry, certain fueloxidant pairs aire selected because they are hjrpergolic, i.e., ignitable on contact, which eliminates the need for separate ignition equipment. In this case, however, it is essential for spontaneous ignition to tak;e place on start-up with the minim u m of delay, otherwise a "heird start" will ensue, comparable to the mechanism of diesel knock but with much more damaging consequences. As there is no standard laboratory method, this refers to the method adopted in continuousflow energy practice to effect ignition.

742

MANUAL

37: FUELS AND LUBRICANTS

HANDBOOK

Flame Stabilization In the aero gas turbine combustor, the flame is required to be stabiHzed in a defined location against the entry stream of air over a wide range of conditions, consequently a balcince must be maintcuned at all times between the velocities of the flame and the approaching mixture. The flame velocities of most fuels in laminar flow eire in the order of 0.5 m/s only and, although increased markedly by temperature and turbulence, the velocity of the incoming mixture, emd thus the eiir, must be reduced by some form of betffle in order to achieve velocity balance. In the gas turbine chamber, the velocity of the entering air is reduced initially by diffusion. The air then goes through a process of flow reversal by being fed around the outside of the flame tube before entering through side apertures and encountering a low-pressure region in the core of an air swirl. The result is a stabilized toroidal vortex of swirling air, the inner surface of which flows upstream. Introduction of the fuel into this surface promotes a region of flame-air velocity balcince. In advanced aero gas turbine engines, a technique of premixing and prevaporization is used in order to control emissions more effectively. In rocketry, sprays of the liquid fuel Eind oxidant may be ctrranged to impinge for thorough mixing.

Air D i l u t i o n In contrast to piston engines where the working surfaces (piston crowns) are subjected to combustion temperatures for only short periods of time, the blades on a gas-turbine disk operate continuously in a high-temperature environment, and are also subjected to centrifugeJ stresses. Consequentiy, the products from the flame must be cooled sufficiently to suit the metallurgy of the turbine blades. Customarily, some 28% of the chamber air is used for combustion in the primary zone, which gives a flame temperature of about 2050°C. The remaining air is introduced progressively downstream, first as secondary Eiir to reduce the temperature of the combustion products to about 1450°C in order to offset the effects of dissociation, and then as dilution air to bring the turbine entry temperature (TET) down further to the m a x i m u m level of about 1000°C. Higher TET values of 1350°C and above are acceptable by intemeJ cooling of the nozzle guide vjines and turbine blades with air bled from the compressor. Despite fuel residence times within the chamber of a few milliseconds only, combustion efficiencies under design conditions are high, typically 99.5% or above. Even exhaust smoke that is just visible represents a combustion inefficiency of no more than 0.01%.

Turbine Entry Temperatiu-e Distribution The distribution of entry temperature over the turbine disk should be reasonably uniform in order to avoid localized

peak temperatures that would limit blade life and engine performance. The temperature traverse qucJity at the outlet of the chamber is known as the pattern factor, defined as follows: ^ mean.exit ' mean.inlet

Pattern factor =

The m e a n exit temperature values are generally obtained by taking measurements at test points located at the centroids of a n u m b e r of equal sub-cireas comprising the total exit area. Customarily, velocities are also measured at these test points, and each temperature weighted on a mass flow basis. Specific Impulse In ramjet engines, a higher flame temperature is permitted in the absence of turbine blades located within the hot propelling gas stream, whereas in rocket engines, even higher temperatures pertain in the absence of atmospheric diluents. In both cases, therefore, the extent of dissociation of the combustion products, and of any subsequent part recombination within the propelling nozzle, are usucJly sufficient to influence substantially the thrust level based on energy density alone. The performance of high speed jet engines is therefore rated in terms of the stream thrust exerted at the nozzle throat exit plcine where the velocity is sonic, using a parameter termed specific impulse since it is the impulse (thrust) based on unit flow rate of the propelling products. For ramjets, this parameter may be based on the mass flow rate of the air alone, or on either the mass of volume flow rates of the fuel alone. Thus ,. .r. . , „ Air specific impulse = bn = f t " F ma _ , .p. • 1 o Fuel specific impulse = Sf= f f T rrif

Stream thrust ~ra :~ Air mass flow rate N s/kg (relates to engine thrust) Steam thrust -=—5 5 ;— Yuei mass flow rate

N s/kg (relates to mass limitation)

„ , J .^ . , cSteam thrust Fuel density impulse = Sf^ = Fuel volume flow rate p = -y- ~ Sfpf N s /L (relates to volume limitation) where py = fuel density, kg/L. Relative vsJues of the specific impulse parameters are included in Table 5 for selected elements, non-carbon hydrides, and organometallies. These show a broad interrelationship with Ccdorific values, but eJso some significant differences. Data for representative monoreactants are shown in Table 6.

TABLE 6—Properties and performance of representative monoreactants [6-8]. Density Monoreactant UDMH Hydrazine Nitromethane HTP Tetranitromethane

' Expanding from 68 to 1 atm.

Formula

(kg/L)

Id* (N s/L)

(CH3)2N2H2 N2H4 CH3NO2 H2O2 C(N02)4

0.78 1.008 1.12 1.44 1.638

1530 1958 3126 2330 2907

(N s/kg)

Reaction Temp. *(K)

1961 1942 2491 1618 1775

1154 905 264 1278 2170

CHAPTER 28: ENGINEERING SCIENCES OF AEROSPACE FUELS 743 For the rocket engine, the performcince is simiWly based on variants of the specific impulse, as follows: p Specific impulse = Is = N s/kg, and Density impulse = /'^ = (pi/pzf'' ''^'->= T1/T2 Hence-n = 1 -(;/(>)'"-"'^

Process

It is evident that reversible processes can be arranged sequentially in a variety of ways so that they return to the initial point and comprise complete cycles. Thus, as outlined earlier, the properties do not change once the cycle is completed since the initial and final states coincide. Clearly this cyclic process encompasses a fixed area (which represents work transfer in the case of pressure-volume axes), and can be repeated indefinitely. Each individual component process is related to external transfers of energy (as heat and/or work), which are all calculable from thermodynamic theory. Consequently, suitable selection of component processes, and of their relationship within the cycle, can result in overall transfers of heat and work. Traversing the cycle diagram clockwise gives rise to the potential of the continuous conversion of heat input to work output (as in a heat engine), provided the second law of thermodynamics is followed in that such a conversion can never be complete, i.e., some of the heat input must be rejected as heat output at a level of temperature lower than the initial. The derivation of the effi-

Compression ratio r^ 10 20 L Typical S-l Typical C-l may r

maX i j .

60 OTTO

2 'o 40 ic (U

. •" -

maxrn Cut-off ratio Spark ignition piston engine Compression ignition piston engine Gas turtine engine

—r 10

"T—

20

Isothermal

r=k

Non Flow

Steady Flow °

Cp (T2 - Ti)

R (T2 - Ti)

0

R T In (p,/p2) = RTln(v2/v;)

RT\n{p,/p2) = « r i n (V2 /V,)

^ T ^ (Ti - T2)

—I

n= 1 Pol3^ropic pv° = k n = n

(7 - n) , ^ Cv j^ _ J {T,

^ , T2)

dD

SI

30

FIG. A2.2—Variation of engine cycle thermal efficiencies.

Heat Transfer

RT\n(p,/p2) RT\n(v2/vt)

20

Pressure ratio Xp

Work Transfer Isobaric p = k n = 0

75 E )_

Typical GT a S-l C-l GT

TABLE A2.1-- E n e r g y distribution in thermodynamic processes \_p v" = constant, a n d p V = i? r ] . Process

30

(T, - T2)

Isentropic p v"* = k n = -y

0

Cv {Ti - T2)

Cp {T, - T2)

Isochoric v = k

Cv {T2 - T,)

0

R {T, - T2)

n = 00

' Assuming no changes in potential (height) or kinetic (velocity) energies.

752 MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

is not, the quotient (Q/T) - which is the entropy (S) - does meet the definition of a property since it does not change over the complete cycle. A fuller discussion of these thermodynamic issues is given in Ref. 1.

APPENDIX 3

TABLE A3.1—Thermochemlcal bond enthalpies, kj/mol [1]. Bond

Enthalpy

Bond

Enthalpy

AHa.H2(g) AHa.02(g) AH,.N2(g) AHa.C2(gr) D (H—O) E (H—0)

435.4 498.2 946.2 717.2 428.7 463.1

D(H—OH) E(C—H) E(C—C) E(C=C) E(C=C) E(C—0)

497.5 414.5 347.5 615.5 812.2 351.7

Oxidation Heat Release The study of heat transfers associated with chemical reactions is termed Thermochemistry. Since the events of interest here relate to flowing, rather than non-flowing, fluids at constant pressure, changes of energy Eire expressed in terms of enthalpy r a t h e r t h a n internal energy (see the section on Calorific Values). The formation of a molecule (for example, a fuel) from its component elements can be imagined as the following twostage process: 1. Absorption of sufficient enthalpy to release the individual atoms of the c o m p o n e n t molecule into freely gaseous form, as for example; C(gr) ^ C(g), and H2(g) ^ 2 H(g)

which is the mean value for a n u m b e r of such bonds in different molecules, and in different locations within them. A simple physical analogy of the complete formation process is given by a mass descending from a depression in a hilltop; the overall change in potential energy is the net result of the climb to the lip of the depression, and the subsequent descent from it (Fig. A3.1). Representative values of AHa, D(X—^Y) and E(X—Y) are given in Table A3.1 Hence, in the methane example above, the net enthalpy released, which is described as the standcird enthalpy of formation, AHf, is given by: A//f .CH4 = lAHa - l E ( X - Y ) approximately

where (gr) represents carbon in its standard condition of graphite, and (g) represents gas. 2. Release of excess enthalpy on the combination of these free atoms on the formation of the combined molecule, as for example; C(g) + 4 H(g) -> CH4(g) methane The energy involved in the first stage of the process is designated the "enthalpy of atomization," AHa, and is invariably directed inwards. In thermochemistry, therefore, this is classed as positive since it adds to the total stock of energy of the elemental material. The second stage involves an energy release which, although considered negative on the above basis, is customarily described in the opposite sense as the "bond dissociation enthalpy," D(X—Y), representing the atomization enthalpy required to dissociate the X—^Y bonds of the completed molecule back to free atoms. For convenience, use is often made of the empirical bond enthalpy, E(X—Y),

= [AHa-Cigr) + 2 AHa-H2{g)] - 4 [ E ( C - H ) ] [717.2 + 870.8] - 1658.0

70 kJ/mol

(cf. — 74.90 kJ/mol by measurement) The term "standard" (superscript "o") indicates that the initial and final temperatures are c o m m o n at 25°C. The negative value above indicates that a net release of entheJpy occurs during formation, hence the resulting methane molecule is more stable than its parent elements. Values of AHf for other hydrocarbon molecules are included in Table 3. If now the dissociation enthalpy (but see activation energy below) is applied to a fuel molecule, together with the atomization enthalpy of its stoichiometric oxygen molecules, the total number of free atoms may rearrange themselves as oxide products, and so fall into a m u c h deeper enthalpy trough as they release their new quantities of bond dissociation enthalpy. Hence, Standard enthalpy of oxidation reaction = AH? = 2:(A//;)p -

= E(n AHf\

I(A//;?)R

- Km

AHf\

where subscripts P cmd R refer to products and reactants respectively, mi = moles of reactcmt i, and

XD (X-Y)p

(AHpP AH?

Uj = moles of product j . As an example, consider the stoichiometric oxidation of methane to gaseous CO2 and H2O given the following vaJues; A///-CH4(g) = - 7 4 . 9 0 kJ/mol

Product molecules

FIG. A3.1—Schematic of standard molar enthalpies of formation (A H,°) and of reaction (AHr°) [1].

AHf •C02(g) = - 3 9 3 . 5 2 kJ/mol, and A//°-H20(g) = - 2 4 1 . 8 3 kJ/mol Since CH4(g) + 2 02(g) = C02(g) + 2 H20(g),

CHAPTER

28: ENGINEERING

it follows that A/:/^°.CH4(g) = [1 (-393.52) + 2 (-241.83)] - [1 (-74.90) + 0]

SCIENCES

OF AEROSPACE

FUELS

753

The Arrhenius plot also gives: Gradient = E/R^ = 73 100/8.314 3 = 8792 K

= - 3 9 3 , 5 2 - 483.66 + 74.90 APPENDIX 4

= - 8 0 2 . 2 8 kJ/mol It is noteworthy that the enthalpy of formation of O2, N2, C(gr) and other elemental molecules at their standard conditions is zero since AHa and D(X—^Y) are equal and opposite in sign. It is also of interest to note that the above value is a net quemtity since the product water is in the vapor phase. (See the section on Calorific Values.) From the above discussion on enthalpies of formation, it might appear that the full complement of the dissociation bond energies would be required in order to dissociate a fuel molecule into its component free gaseous atoms of carbon and hydrogen. However, the chain nature of the ignition process implies that only one bond need be broken to start the chain. Furthermore, some molecules will have more than the average level of energy, and also both bond breaking and remaking are occurring together. All these factors result in an "activation energy" of a considerably lower level t h a n D(C—C) or D(C—H). The actual value can be determined experimentally by plotting ignition temperature (T) against delay (t) on the Arrhenius basis as follows:

Proportions of Dissociated Combustion Products At temperatures above about 1800 K, the thermal agitation of combustion products is such that they begin to dissociate back towards their reactant materials, giving reversibility to the combustion reaction, thus A + B^

combustion. dissociation

At a sustained high temperature, therefore, a condition of dynamic equilibrium exists with rates of combustion and dissociation exactly equal, so t h a t the reactant a n d product materials coexist in proportions that remain constant. These proportions can be determined by using values of parameters known as "partial pressure equilibrium constants" at the given temperature, as shown below. The rates of such reactions at selected temperatures can be determined by experiment, and are found to be proportional to the instantaneous concentration of each material raised to some power, that is, forward reaction rate = kp [A]^ [B]'', and

Reaction rate where

q t

e E Ro T

dt " ^

quantity of heat released per unit mass reaction time = Naperian base = 2.7183 = activation energy under the given conditions = universal gas constant = 8.3143 J/mol K = absolute temperature

reverse reaction rate

kR [C]'^

where

instantaneous molar concentration of material X,

[X]

••

dq Since --T- a t -\tae'^''°^:

constant e

k = rate constant for the reaction, and a, b, and c are experimentally-determined powers. Since the two rates are equal at d5Tiamic equilibrium.

[Cf

It follows that Int

In (ti/t2) = Hence, E

E

_\_

J_

Rn

Ti

T2

Ro lniti/t2) Ti

J_

T2 For n-octcine the calculations appear as follows: 8.3143 In (20/1.2)

It follows from Avogadro's law (equal volumes of all gases at the same t e m p e r a t u r e a n d pressure contain the same n u m b e r of molecules) that

^

^

598/ 8.3143 In 16.67 0.001992 - 0.001672

8.3143 X 2.8135 0.00032

= 73 100 J/mol = 73.1 kJ/mol This compares with 347.5 and 414.5 kJ/mol for initially dissociating the C—C and C—H bonds respectively, and with 208.59 for AHf.

t, = K = partial pressure equilibrium constant for the reversible reaction,

Furthermore,

—^ = -—n r. where p = total pressure. p total mol ^ ^ Thus in each case, p^ can be expressed in the following manner: Px = P, (

L\

^^502

= concentration equilibrium constant for the reversible reaction

Y + constant + constant

which is the equation of a straight line of In t against 1/T. The value of £ can then be determined by taking two points on the straight line, as follows:

U

^ = K' kR

[A]^ [Bf

1, where Uj = total moles of product present.

Since carbon oxidizes in two stages, first to CO and then to CO2, it is the final stage at the high temperature that experiences dissociation, that is CO + O.5O2 ^ CO2 Hence,

PCO2 pCO {pOzT

,, _ Kco, —

"CO2 nr>_/n+^0.5 "C0(p"02/"t)'

754 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK Similarly for the oxidation of hydrogen,

Solution for the six unknown values of n requires six equations, four of which are provided by molar balances of C, H, O, and N as for the simple non-dissociated case. The remaining two equations are then derived from the published values of Kco2 and KHJO for the temperature in question, where

H2 + 0.5 O2 ^ H2O PH2O PH2 ip02)°'

Hence,

"H2O = K, '' " " ' " "H2 (p"O2/"t)0-5

(Note; These two equilibrium constants are used for solving the dissociated general mixture case below). Since both reactions are occurring together, the half mole of oxygen produced by dissociation of the mole of carbon dioxide may be considered as the oxygen required by the mole of hydrogen. Combination of the two combustion equations then gives: CO2 + H2 ^ CO + H2O This is known as the water-gas shift reaction (not to be confused with the water-gas reaction, which is: CO -I- H2 ^ C + H2O) and leads to: Water-gas shift equilibrium constant = KWGS K,H20

PCOPHIO

KcOj Pco2 Pw.2 (Note: This equilibrium constant is used for solving the imaginary non-dissociated rich mixture case since, although n o dissociation complications arise, the distribution of the limited oxygen available to the CO and H2 has to be determined). Selected values of the partial pressure equilibrium constants are given in Table A4.1. Although at the higher temperatures dissociation proceeds further to promote radicals such as O, H, OH and NOx (that is, NO + NO2), a first approximation to realistic conditions can be made by considering dissociation restricted to CO, H2 and O2 only, giving the following combustion equation for any general mixture strength; CaHb + m (O2 + 3.76 N2)

where m = moles of 02/mole fuel, as determined from the given air-fuel ratio. Solution follows by iteration on values of ns/nt. It might be assumed that, as the products cool, the effects of dissociation would vanish, and the combustion equation then revert to the complete form in Eq 2 of Stoichiometry. But in fact, the partially-burnt products endure on cooling due to freezing of the reactions when energy is lost through contact with the walls of the containing vessel and any inert molecules present. TABLE A4.1—Partial pressure equilibrium constants for oxidation reaction, atm~°'^ [9]. 298.15

300 500 1000 1500 2000 2100 2200 2300 2400 2500 2700 3000

KcOj

1.1641 X 10"^ 575.44 X lO''^ 10.593 X lO^'' 16.634 X 10^ 207.01 X 10^ 765.60 345.94 168.27 87.097 47.753 27.543 10.351 3.0549

K,H20 •

ni n j {p Us/Ut)"

and

n2

n4 {p ns/nt)"

Computer programs exist for the determination of product concentrations, but the method of solution shown below permits derivation from first principles, typical of those o n which the computer software itself is based. 1. At given temperature, read Kco2 and KHJO from Table A4.1 1. Assume value for ns/nt, and evaluate (p ns/nt)"-^ 3. Evaluate ni/ua and n2/n4 from Kco2 and KHJO expressions respectively, using Step 2 value. 4. Evaluate Ui and ns from carbon balance, and n2 and n4 from hydrogen balance using Step 3 value. 5. Evaluate ns from oxygen balance. 6. Evaluate nj from nitrogen balance. 7. Evaluate nt = n5/(assumed ns/ut), and compare with Enj, with j = 1 to 6. 8. Repeat from Step 2 until Ut = Xnj. Example Allowing for dissociation to CO, H2, and O2 only, the stoichiometric combustion equation for methane-air at its isobaric adiabatic combustion temperature of 2247K (Appendix 5) appears as follows: CH4 + 2 (O2 + 3.76 N2) = 0.903 CO2 + 1.961 H2O + 0.097 CO + 0.038 H2 + 0.068 O2 + 7.52 N2

= ni CO2 + n2 H2O + na CO -I- n4 H2 + Uj O2 + ug N2

Temperature, K

Kco2

KH20

KWGS

11.169 X 10^^ 6.1094 X 10^' 76.913 X 10^' 11.535 X 10' 530.88 X 10^ 3.4670 X 10^ 1.6866 X 10' 874.98 480.84 277.43 167.49 68.077 22.029

9.5945 X 10"^ 10.617 X 10"^ 7.2607 X 10"' 0.6935 2.5645 4.5285 4.8754 5.1999 5.5207 5.8097 6.0810 6.5769 7.2110

Comparison of these product quantities with those for the non-dissociated case below shows clearly the influence of dissociation. CH4 + 2 (O2 + 3.76 N2) = CO2 + 2 H2O + 7.52 N2

APPENDIX 5 Calculation of M a x i m u m Reaction Temperature The method of solution is based on the concept of equating the enthalpy released by the reactants, in generating the dissociated products at the initial temperature, with that which would have been required to heat those products from the initial temperature to the final temperature J*. As in most thermochemical work, the standard initial temperature is taken as 25°C (298.15 K). Hence, [Chemical enthalpy released (negative) with reactants at 298.15 K oxidized to products at 298.15 K, that is, the standard enthalpy of reaction] is equal to [Physical enthalpy absorbed (positive) by products in heating from 298.15 K t o r * ] that is, - MIr° = (//^*jp algebraically But \Hr° = [{^H^)p - (^H^)B.'\ Thus, (H^* + A//;)p - (AHf )R = 0

CHAPTER

28: ENGINEERING

Values of AHf for the products are listed in the literature [9], together with values of the physical enthalpies absorbed when heating from 298.15 K to the different levels of T. The author has found it helpful to sum the two values shown on the left-hand side above and designate it as total thermochemical enthalpy at temperature T, ( H i ) , tabulating these sums for direct use at the temperature levels of interest. The above equation then becomes

SCIENCES

OF AEROSPACE

FUELS

755

E. Repeat from step A at new level of T (say 2300 K). F. Find T* by interpolation (or minor extrapolation) to give zero in equation of step D. (All values of n at T* can be found by similar extrapolation). Example All gaseous reaction of stoichiometric methane-air at 1 atmosphere with dissociation pattern as shown;

(AH/)R = 0

where H[* = total thermochemical enthalpy at temperature T* based on initial 298.15 K

CH4(g) + 2 (O2 + 3.76 N2) = ni CO2

Expansion gives

For methane, a = 1, and b 4. A. Select T = 2200 K B 1 Kco2 = 168.27, KHJO = 874.98 2 Assume nj/ut = 0.00519, thus (p ns/ut)"-^ = 0.07204 3 ni/nj = 0.07204 (168.27) = 11.936 n2/n4 = 0.07204 (874.98) = 61.387 4 n3 = a/(l + ni/ns) = 0.07730 ni = 1 - nj = 0.9227 n4 = (b/2)/(l + n2/n4) = 0.03206 n2 = 2 - n4 = 1.9679

Euj (Hr*)j - Emi (A//;)i = 0,

for reactants i and products j

Since AHf-Oz = Mlf-Hz = 0 due to the self-canceling of the atomization and bond dissociation enthalpies, (for example 02—i^Ha) -^ 2 O—[D(0—O)] -^ 02), it follows that the previous expression can be simplified further to Euj (fff*)j - mfuei (AH/)fuei = 0, and then to EUj ( / / r * ) j -

+ n2 H2O -t- n3 CO + n4 H2 -I- ns O2 + 7.52 N2

(A///°)fuel

= 0, since only one mole of fuel is usually considered. Since both linear and non-linear equations require solution simultaneously here, iteration is necessary on T in the non-dissociated case, and on both n and T in the dissociated case. As with product concentrations, computer programs exist for the determination of maximum reaction temperature, but derivation from first principles using the total thermochemical enthalpy concept is as shown below. A. Select some appropriate level of t e m p e r a t u r e T (say, 2200 K). B. Determine all molar product "n" values; For the non-dissociated case, from Eq 3 For the dissociated case, from the method shown in Appendix 4 C. Weight each value of n by its appropriate H[ at the selected T, from Table A5.1 D. Sum these weighted values, subtract {8Hf)iue\, and check for zero.

5 ns

2m - X - (ni + n2) _ 3 - 2.8906

= 0.05468

6 ng = 7.52 ^ 0.05468 ns 10.536 (assumed nj/ut) 0.00519 Suj = (10.52 + ns) = 10.575

7 n,

Since slight difference between Ut and Xuj, repeat from step B. 2 using ng/ut = 0.00518 This gives reasonable equality of 10.57, and all appropriate values of n, as follows: ni = 0.923; uj = 1.968; ns = 0.774; n4 = 0.0321; ns = 0.0547 C. Weight each n value with its appropriate Ht^ value at 2200 K e.g., for CO2, ni {Hf) = 0.923 (-289.95) = - 267.6 D. Summation of these weighted values, less formation enthalpy for fuel, gives: Snj (Hj)i - (A//;)f,ei = - 1 0 2 . 4 4 - (-74.85) = [-27.59] Since this is not zero, repeat from Step A for new temperature of 2300 K.

TABLE AS. 1—Standard-based total thermochemical enthalpy levels for• gases, kj/mol [9] [1], H[= (H^ + AH}) for Compounds

Hi = H"for Elements

Temperature, K

CO2

H2O

CO

H2

O2

N2

298.15 300 500 1000 1500 2000 2100 2200 2300 2400 2500 2700 3000

-393.52 -393.46 -385.21 -360.12 -331.81 -302.07 -296.02 -289.95 -283.85 -277.73 -271.60 -259.27 -240.66

-241.83 -241.76 -234.91 -215.85 -193.73 -169.14 -164.00 -158.79 -153.53 -148.22 -142.86 -132.01 -115.47

-110.53 -110.47 -104.60 -88.843 -71.680 -53.790 -50.154 -46.509 -42.853 -39.183 -35.505 -28.121 -16.987

0 0.054 5.883 20.686 36.267 52.932 56.379 59.860 63.371 66.915 70.492 77.718 88.743

0 0.054 6.088 22.707 40.610 59.199 62.986 66.802 70.634 74.492 78.375 86.199 98.098

0 0.054 5.912 21.460 38.405 56.141 59.748 63.371 67.007 70.651 74.312 81.659 92.738

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Similar calculations give ns/iit = 0.00785, hence; n , = 0.881; na = 1.953; na = 0.119; n4 = 0.047; nj = 0.083 Summation of weighted values gives [ + 3 1 . 6 3 ] This also is not zero, but interpolation with value - 27.59 from Step D gives r * = 2247 K Similar interpolation gives the related vEilues of the product moles: ni = 0.903; nz = 1.961; nj = 0.097; n4 = 0.038; ns = 0.068 Note For stoichiometric methane-air, T* = 2327 K, nondissociated = 2247 K, dissociated to CO, H2, and O2 only (as shown above) = 2223 K, dissociated to CO, H2, O2, H, O, OH, and NO For stoichiometric methane-oxygen, r * = 3045 K, dissociated to CO, H2, O2, H, O, OH, and NO

In the absence of the effects of gravity or motion, this energy change is located entirely within the internal energy of the system material, hence: n.f. (q — w) = and

The Energy Equation From the laws of thermodynamics, it is evident that, for a system operating in a cycle, the initial and final states are identical, and the net heat input is equal to the work output, with some of the heat input being rejected to a lower temperature sink. On a specific basis, therefore: qnet - W = ( q i n -

Qout) " W = 0

This is usucJly expressed simply as: q - w = 0 When the process is not cyclic, the initial and final states differ, and the energy equation then becomes: q — w = Ae

(A6.1)

n.f. d q z - 1W2) = M2 - Ml

This is known as the non-flow energy equation (n.f.e.e.), any change in internal energy leading to changes in temperature or phase. In the presence of effects of gravity or motion, as in a system flowing steadily through a control region, balaxices of mass cind of energy appear as shown below: Mass balance: mi = m2 = p A C = Ai Cj/vi = A2 C2/V2 where p = density of fluid A = cross-sectionzJ area of flow C = velocity of flow relative to control region, and V = specific volume of fluid = 1/p Energy balance: s.f.(iq2 — 1W2) = (hi -hi)

APPENDIX 6

AM

+ g (z2

^2

£2^

where z is the height above the Earth's surface, or some other datum. This is known as the steady-flow energy equation (s.f.e.e.), which can also be expressed in various reduced forms depending on the constancy of certain terms. One useful form concerns negligible changes in both potential and kinetic energies, applicable to horizontal flow with approximately similcir entry and exit velocities, as shown below: Reduced s.f.e.e. = s.f. dqz — 1W2) = h2 — hi

(A6. 2)

This compares directly with the non-flow case in Eq A6.1. Since this present study concerns steady flow applications to jet engines, most t h e r m o d y n a m i c considerations deal in terms of enthalpy rather thein internal energy.

MNL37-EB/Jun. 2003

Properties of Fuels, Petroleum Pitch, Petroleum Coke, and Carbon Materials Semih Eser^ and John M. Andresen^

their important properties, including standard methods of measurement and the significance of individual properties in industrial applications.

THIS CHAPTER PROVIDES AN OVERVIEW ON THE ORIGIN, PROPERTIES,

AND APPLICATIONS of fuel oils, petroleum pitch, petroleum coke, and some related carbon materials. A c o m m o n thread among these materials is the line of p r o d u c t i o n that connects petroleum refining to the manufacture of carbon materials. Bottom fractions from catalytic cracking of gas oil (FCC decant oil) or from thermal cracking of naphtha (ethylene tar) can be used as residual fuel oil, or as feedstocks for producing carbon black, petroleum pitch, and p r e m i u m petroleum coke for graphite electrodes. Petroleum pitch is, in turn, used for producing carbon fibers and carbon-carbon composite materials, and for densification of graphite electrodes used in electric-arc furnaces to recycle scrap iron and steel. The residua from vacuum distillation in petroleum refining are subjected to severe thermal cracking in coking processes to produce light and medium distillates and petroleum cokes that are used as solid fuel, or as filler for manufacturing carbon anodes for electrochemical production of aluminum. It is important to recognize that ferrous metals and light metals industries, as well as manufacturing a n u m b e r of carbon materials, are closely linked with the products of petroleum refining, including decant oils, petroleum pitch, and petroleum coke.

S p e c i f i c a t i o n s a n d A p p l i c a t i o n s o f F u e l Oils Fuel oils are burned to generate heat for different purposes ranging from home heating to raising steam in utility boilers to generate electricity. Different types of burners used in these applications under various climatic and operating conditions dictate the need for different grades of fuel oils. A standard specification by the ASTM International has divided fuel oils into five basic grades, designated as Nos. 1, 2, 4, 5, and 6 (ASTM D 396). Based on the production methods used in petroleum refining, fuel oils fall into two broad classifications: distillates and residuals. The distillates comprise overhead or distilled fractions, whereas the residuals consist of bottoms remaining from the distillation, or blends of distillates with the bottoms from distillation, visbreaking, and catalytic cracking processes [2-4]. The six grades of oils can consist of different t5^es according to the refining processes used in their production, as described in the section on Petroleum Refining [5]:

For each material, production processes, product properties, and specifications are discussed in the context of respective industrial applications. Standard methods used for characterization of these materials are identified with a focused discussion on the interpretation of the results obtained from the standard tests. Literature references are provided for further information on each topic.

1. 2. 3. 4.

FUEL OILS Petroleum refining processes generate many product streams that can be used as fuel oil either in single streams or in blends to adjust the desired properties for specific applications. A broad definition of fuel oils does occasionally include diesel fuels since they are closely related to distillate and heavy fuel oils. In this section, however, diesel fuels are not included for discussion, since they are covered separately in this manual [1]. Industrial use of fuel oil for generating heat is the principal focus in this section. Specifications and applications of fuel oils are introduced with an overview of

' Department of Energy and Geo-Environmental Engineering and The Energy Institute, respectively. College of Earth and Mineral Sciences, Pennsylvania State University, 110 Hosier Building, University Park, PA 16802.

straight-run distillate; straight-run residual (i.e., reduced crude); catalytically cracked distillate; cracked residuals from thermal or catalytic cracking, or hydrocracking; 5. blends of any of the streams listed above. Cracked oils have rather different composition and properties from those of the straight-run oils, as discussed later. In the ASTM specification. Grades No.l and No.2 are distillates and Grades No. 4-6 are usually residuals. Some heavy distillates may, however, be sold as Grade No. 4 fuel oil. Grade Nos. 4 and 5 are subdivided into light and heavy categories. Table 1 summarizes the common uses and some significant properties of the different grades of fuel oils [6]. Grades 1 and 2 are used in domestic and small industrial burners. Grade 1 is a particularly light distillate for use in the vaporizing type burners and under storage conditions that require low pour points. Grades 4-6 are used in pressure atomizing-type commercial/industrial b u r n e r s that can handle high viscosity fuels. The viscosity of these residual fuels increase with the increasing number in the grade scale such that Grade 6 fuel, also called Bunker C, requires preheating for handling and burning [6]. In general, all grades of fuel oil should be homogeneous hydrocarbon oils, free from inorganic acid, and free from ex-

757 Copyright'

2003 by A S I M International

www.astm.org

758

MANUAL

3 7: FUELS AND LUBRICANTS

HANDBOOK

TABLE 1—Common uses and some significant properties of different grades of fuel oils. Grade

Classification

No. 1

Distillate

No. 2

Distillate

No. 4 (Light)

Heavy Distillate

No.4

Heavy Distillate, Distillate/Residual blends

No. 5 (Light) & No. 5 (Heavy)

Residual

No. 6 or Bunker C

Residual

cessive amounts of solid or fibrous foreign matter. All grades containing residual components should remain uniform in normal storage and not separate by gravity into light and heavy oil components outside viscosity limits for the grades. Table 2 lists the required specifications for all fuel oil grades according to the standard designation ASTM D 396. A representative sample should be taken for testing in accordance with ASTM Practice D 4057. Modifications of limiting requirements agreed upon between the seller and the buyer should fall within limits specified for each grade, except as stated in supplementary footnotes for Table 2. The most important considerations in selecting a burner and a particular grade of oil include the volume of the oil consumed, and a match between the capabilities of the b u r n e r systems and properties of the fuel oils. The heavier grades are less expensive, but they must be handled and burned efficiently to take advantage of the lower fuel cost, especially in large volume applications. Sufficient heating in the storage tank and insulated transfer pipelines are usually necessary to ensure steady flow of heavy oil to the burner. High-viscosity oils require additional preheating at the burner for proper atomization.

I m p o r t a n t F u e l Oil P r o p e r t i e s Fuel oils are burned to generate heat. The a m o u n t of heat captured in combustion systems is the principal concern for the fuel user. Important properties of fuel oils must, therefore, relate to their combustion characteristics and performance in different types of burners and fuel handling systems. Understanding these properties helps the user to select a fuel best suited for a specific application. It is also useful to discern the relationships between different properties of fuel oils. All the properties of fuel oils identified in Table 2 are discussed below, with reference to standard measurement methods, and the significance of the measured properties. Gravity The density of petroleum oils is often expressed in terms of API gravity, a scale devised by the American Petroleum Institute and National Bureau of Standards (continued as National Institute of Standards and Technology). The API

Application Domestic and small industrial burners of the vaporizing type Atomizing type domestic and small industrial burners Pressure-atomizing type commercial/industrial burners Pressure-atomizing type commercial/industrial burners; preheating not required for handling or burning Industrial burners; preheating may be required for handling and burning Industrial burners; preheating required for handling and burning

Significant Properties Volatility and pour point Volatility and viscosity Viscosity and pour point Viscosity, flash point, and sulfur content

Viscosity, flash point, and sulfur content Viscosity, flash point, and sulfur content

gravity is inversely proportional to specific gravity—the ratio of the density of oil at 60°F to the density of water at 60°F— according to the following equation: °API = (141.5/spgr60°F/60°F) - 131.5 The API gravity is the most commonly used property to classify crude oil and refined petroleum products since the early days of petroleum industry. It has also been used to predict many other characteristics of petroleum oils. Recent variations in crude oil composition coupled with new processes in petroleum refineries have, however, diminished the usefulness of API gravity as a descriptor for other properties of petroleum oils. Two oils with the same API gravity, for example, can have many different characteristics because of large differences in composition (e.g., different combinations of paraffinic, naphthenic, and aromatic hydrocarbons). The API gravity still remains to be an important property, since it can be measured easily, and used in a number of empirical correlations for approximate estimation of other properties of petroleum fractions [7]. The API gravity of fuel oils is measured by using a standard hydrometer according to ASTM Test Method D 287 or D 1298. Because of the significant volume expansion of oils upon heating, the API gravity varies strongly with temperature. An API gravity of 12 at 60°F, for example, would correspond to an API gravity of 18 at 180°F (82°C) [4]. Therefore, the volumetric heating value of oils (Btu/gal) decreases significantly with the increasing API gravity. The reasons for this trend are discussed in the next subsection. Table 2 shows the gravity limits for the three grades of oil (>35 for No. 1; > 3 0 for No. 2; < 3 0 for No. 4 light). Although the differences in gravity between different grades may vary depending on the composition of the oils and the refining processes, typical API gravity ranges for fuel oils are as follows: No. 2: 26-39°; No. 4: 24-32°; No. 5: 16-22°; No. 6: 10-15°. As expected, API gravity decreases with the increasing grade number. The API gravity also changes according to the fuel types within a given grade. For No.2 fuel, for example, the API gravity of straight-run fuel oils range between 36 and 39°, while that of thermally, and catalytically cracked oils would fall in the range 24-28°, and 29-32°, respectively. For

CHAPTER 29: PROPERTIES

OF FUELS, PETROLEUM PITCH. PETROLEUM

COKE, AND CARBON MATERIALS

759

TABLE 2—Detailed requirements for fuel oils (ASTM D 396). Properties

Specific Gravity, 60/60°F deg API Flash point, °C (°F) min Pour point, °C (°F) msix Kinematic viscosity, mm'^/s (cSt)'' At38°C(100°F)min max At40''C(104''F)min max At 50°C (100°F) min max Saybolt Viscosity Universal at 38°C (100°F) min max Furol at 50°C (122°F) min max Distillation temperature, °C (°F) 10% point max 90% point min max Sulfur content, mass, mcix Corrosion copper strip, max Sulfated ash, % mass, max Carbon residue, 10%*; %m, max Water and sediment, % vol, max

No. 1

0.8499 35 min 38 (100) 18''(0) 1.4 2.2 1.3 2.1

No. 2

0.8762 30 min 38 (100) - 6 " (20) 2.0" 3.6 1.9 3.4

(32.6) (37.9)

No. 4 (Light)

No. 4

No. 5 (Light)

No. 5

No. 6

55 (130)

55 (130)

60 (140)

0.8762" 38 (100) -6" (20) 2.0 5.8

(32.6) (45)

55 (130) - 6 " (20) 5.8 26.4^" 5.5 24.0'

(45) (125)

>26.4 65^

>24 58^

(>125) (300)

>65 194^^ >58 168^ (42) (81)

>92 638^^

(>300) (900)

(>900) (9000)

(23) (40)

(>45) (300)

215(420)

0.5 3

282'^ (540) 338 (640) 0.5* 3

0.15

0.35

0.05

0.05

288 (550)

0.05

0.10

0.15

0.15

(0.50)"

(0.50)'

(1.00)'

(1.00)"

(2.00)"

"It is the intent of these classifications that failure to meet any requirement of a given grade does not automatically place an oil in the next grade unless in fact it meets all the requirements of the lower grade. ''In countries outside the United States other sulfur limits may apply. "Lower or higher pour points may be specified whenever required by conditions of storage or use. When pour point less than - 18°C (0°F) is specified, the mini m u m viscosity for grade No. 2 shall be L7 cSt (31 SUS) and the m i n i m u m 90% point shall be waived. '^Viscosity values in parentheses are for information only and not necessarily limiting. "The amount of water by distillation plus the sediment by extraction shall not exceed the value shown in the table. For Grade No. 6 fuel oil, the amount of sediment by extraction shall not exceed 0.5 weight %, and a deduction in quantity shall be made for all water and sediment in excess of I.O weight %. ^Where low su Ifur fuel oil is required, fuel oil falling in the viscosity range of a lowered numbered grade down to and including No. 4 may be supplied by agreement between purchaser and supplier. The viscosity range of the initial shipment shall be identified and advance notice shall be required when changing from one viscosity range to another. This notice shall be in sufficient time to permit the user to make the necessary adjustments. *This limit guarantees a m i n i m u m heating value and also prevents misrepresentation and misapplication of this product as Grade No. 2. ''Where low sulfur fuel is. Grade 6 fuel oil will be classified as low pour -H5°C (60°F) max or high pour (no max). Low pour fuel should be used unless all tanks and lines are heated.

oils from a single refinery stream, the API gravity can indicate w h e t h e r they are straight-run, or cracked oils. For blends, e.g., No. 4 oil, the concentration of different types of oils present, would, therefore, determine the API gravity of the blend in the corresponding range. In general, the API gravity of fuel oils can be qualitatively related to other properties of the oils as shown below [7]: • The higher the API gravity, the lower the viscosity and carbon residue. • The higher the API gravity, the lower the volumetric heating value (Btu/gal), and the higher the gravimetric heating value (Btu/lb). • The higher the API gravity, the lower the C/H ratio. • The higher the API gravity, the higher the rate of combustion, and the shorter the flame length. Heating

Value

Heating value is broadly defined as the amount of heat released by complete combustion of a unit quantity of fuel. Experimental measurements can be reported as total (or high).

or net (or low) heating values in Btu per gallon. The total heating value includes the latent heat of evaporation of the water vapor produced during the combustion. For determining the net heating value, the water from combustion is considered to remain in the gaseous state, and, therefore, the latent heat of evaporation is not recovered. Although no direct reference is made to heating value measurement in the ASTM classification of fuel oils (there is indirect reference through limiting API gravity. See footnote g in Table 2), heating value is a n important specification requirement. The heating value, or the heat of combustion of fuel oils, can be measured by the ASTM D 240. The net heating values of fuel oil samples can also be measured by the ASTM D 4529, D 3338, or D 4809. Depending on the specific gravity and composition of fuel oils, total heating value ranges typically between 130 000 and 160 000 Btu per gallon (36 400-44 800 kJ/liter). The net hea-ting value is usually 8400-8500 Btu per gallon (2350-2380 kJ/liter) lower t h a n the total heating value, depending on the hydrogen content of the fuel. Figure 1 shows

760

MANUAL

37: FUELS AND LUBRICANTS

.5' 160000 ..... ...-r-:. 5 155000 J^^^/^,'::™ :• .:v.;-::=:;.:.;V " 150000 oT 145000 -^sPiRiet^/.f:^*** *^ii_ ••••&• I 140000 •^'-'iftKi^> 135000 ^•[^••-T.':--^^^^ o) 130000 ~ 125000 ••••; ri^7^:;:\ S 120000 "\r^ ^ ^i ' ^ I r' f H I". ^ ' i.":—' '1 — r

"^^^^feu'

••

^

•

'

*

'

'

^

*

^

;

^

HANDBOOK

-Total Heating Value -Net Heating Value

'

0 4 8 1216202428323640 API Gravity

FIG. 1—Total and net heating values of fuel oils versus API gravity.

a plot total heating value and net heating value of selected fuel oils as a function of API gravity from the data given in reference [8] (1 Btu/gallon = 0.28 kJ/liter). It can be seen that the volumetric heating value increases with the decreasing API gravity, because of the decreasing hydrogen content of the fuels with the decreasing API gravity. In addition to measurement by standard methods and prediction from the API gravity, there are a number of empirical correlations used to calculate the heating value. Two of the most commonly used correlations are given below: Total Heating Value (Btu/lb) = 14 600C + 62 000(H-O/8) + 4000S where C, H, O, and S are the weight percentages of these elements in the fuel [8]. Net Heating Value (kJ/kg) = 55 500 - 14 400d - 320S where d is the density (kg/liter), and S is the sulfur content in wt% [9]. It should be noted that on a weight basis (e.g., Btu/lb, or kJ/kg), the heating value of a fuel oil decreases with the increasing density (or decreasing API gravity). However, this trend is reversed when reporting the heating value on a volu m e basis (e.g., Btu/gal, or kJ/liter) since the decreasing heating value is more than compensated by the decreasing volume as the density increases. For example, an oil with 14 API gravity have 290 Btu less per pound (676 kJ/kg) than a 20 API gravity oil, but one a volume basis the 14 gravity oil will have 3840 Btu more per gallon (1075 kJ/liter) than the 20 gravity oil [8]. Therefore, oils with a lower API gravity will provide more heat on a volume basis, but other properties of the fuel oil, such as viscosity, pour point, and carbon residue, may be limiting factors in selecting a given oil, as discussed below.

ture. The kinematic viscosity, expressed in mm^/s (formerly called centistoke, cSt), is determined as the product of the m e a s u r e d flow time a n d the calibration constant of the viscometer. Although the kinematic viscometers are the approved instruments, Saybolt viscometers (Universal and Furol) are also widely used for measuring viscosity. In Saybolt viscometers, the time for the flow of a given volume of liquid is measured under variable head (as opposed to the constant head in kinematic viscometers), which decreases as the volume of the oil in the tube decreases. The measured viscosity is expressed as Saybolt Seconds. The only difference between Universal and Furol viscometers is the larger orifice of the Furol tube (3.15 mm) t h a n that of the Universal (1.77 mm). The Universal is used for testing low-viscosity oils, while the Furol is used for high-viscosity oils to obtain reliable m e a s u r e m e n t s by avoiding the exceedingly long, or short, test times depending on the viscosity of oil. Most frequently used temperatures for viscosity measurements are 38°C (100°F) for Saybolt Universal and Kinematic viscometers, and 50°C (122°F). Conversion tables can be used for viscosity conversions between different values [11]. Table 3 gives the Saybolt and kinematic viscosity limits for different grades of fuel oils. The grade Nos. 5 and 6 have very broad ranges of viscosity because of the use of many different types of burners that can handle a range of viscosities. In applications, where viscosity needs to be closely controlled, oils can be purchased on a viscosity basis. Some suppliers specify their oil as, for example. No. 5-300, indicating No. 5 fuel oil with a viscosity of 300 SSU (Saybolt Seconds Universal) at 100°F (38°C). The viscosity of fuels depends strongly on temperature; the viscosity decreases sharply with the increasing temperature [11]. This strong temperature dependence is used to control the viscosity of residual fuel oils by preheating. When the viscosity of heavy oils is reduced, pumping becomes easier and better atomization is achieved for combustion. Conversely, heavy oils become viscous and extremely difficult to handle at low temperatures. In cold weather applications, the lowest operating temperatures must be considered for selecting the oil with the adequate viscosity for easy pumping and handling. In some applications, the viscosity of heavy fuel oils is controlled by blending with stocks of lower viscosity. Mixing of fuel oils with different chemical characteristics may, however, cause incompatibility problems leading to deposit and sludge formation in fuel handling systems [12]. The high viscosity of fuel oil causes the following problems [11,13]: • Difficulty in pumping from storage tank to burner; loss of p u m p suction

Viscosity The viscosity of oil measures its resistance to flow [10]. It is one of the most important properties of especially the residual fuel oils that affect handling, heating, pumping, and atomization of heavy residual fuels in combustion. The most commonly used viscosity term, kinematic viscosity, is determined by measuring the time for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer (ASTM D 445 with specifications given in ASTM D 446). The measurements must be m a d e under a reproducible driving head and at a closely controlled and known tempera-

TABLE 3—Summary of ASTM methods for the characterization of petroleum pitch properties. Property

Softening point Viscosity Solvent Fractionation Coking Value Density Ash Sulfur Content

ASTM Designation

D 3461, D 2319, D 36, D 61, D 3104 D5018 D 4746, D 2764, D 2318, D 4072, D 4312 D 4715, D 2416

D 2320, D 70, D 4892, D 71, D 2962 D2415 D 1266, D 4045, D 2622

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• Insufficient oil flow to the burner causing problems with starting and erratic combustion • Poor atomization causing inefficient combustion and dribbling of oil at the burner nozzle The low viscosity of the fuel oil causes, in turn, the following problems [11,12]: • Too much oil pumped to the burner causing incomplete combustion, resulting in smoke, carbonization of burner nozzle, and soot formation in the combustion chamber • Low heat generation because of the low heating value of low-viscosity, or high API gravity of the fuel oil • P u m p slippage, too much oil can slip the cup with rotary cup burners, resulting in poor atomization. Pour

Point

Pour point represents the temperature at which a fuel oil stops flowing. This property, measured by ASTM D 97, indicates the lowest temperature of the utility of a fuel in flow systems, and it is particularly relevant to waxy heavy oils and heavy oils that require preheating for pumping. Straight-run oils usually have higher p o u r points than cracked oils because of the higher concentrations of normal paraffins in straight-run oils. The usefulness of the pour point test in relation to residual fuel flow properties is questionable. The pour point test does not indicate what happens when an oil has a considerable head of pressure behind it w h e n it is pumped along a pipeline, or gravitated from a storage tank. Among the tests devised to assess the low-temperature flow characteristics of heavy residual fuel oils is ASTM D 3245. ASTM D 3245 is, however, a time-consuming test, and not suitable for routine control testing, along with its limitations in application to very waxy fuels. Flash

Point

As fuel oils are heated, they evaporate, and the vapors flash at a certain temperature when ignited by an external flame. This temperature is called the flash point. There are two standard ASTM tests to measure the flash point: ASTM D 92 (Cleveland), a n d ASTM D 93 (Pensky-Martens), using an open-cup, and a closed-cup flash tester, respectively. The closed-cup tester gives lower flash points because of the more effective retention of very light vapors that are blown away by the air flow in the open-cup tester. Therefore, the closed-cup test is more sensitive in detecting the small amounts of light vapors in the fuel oil samples. As shown in Table 3, desirable flash points of fuel oils range from 38°C (Grades N o . l , 2) to 60°C (Grade No.6) measured by closed-cup testers. Flash points lower than the desired values may cause fire hazard, whereas high flash points can lead to difficulties with starting, especially in a cold furnace. Distillation ASTM D 86 is used to determine the 10% and 90% points of the distillate fuel oils (Grade No. 1 and No. 2) as specified in Table 3. The distillation characteristics (volatility) of these fuels are related to their ignition and combustion properties and the tendency to form solid combustion deposits. The lower and upper limits to volatility are set to ensure safety and smooth operation in the use of these fuels. Water and Sediment (BSW)—The presence of water and sediment, also called bottom sediment and water (BSW), in

761

fuel oils can cause a n u m b e r of operational problems, including [14], • Plugging of burner tips • Erratic and unsteady combustion • Flame instabilities • Erosion of burner tips and mechaniccJ parts. The origin of BSW can be traced back to the original crude oil, contamination during refinery processes, or thermal degradation a n d oxidation reactions during storage. Usually heavy oils (No. 5 and No. 6) contain greater amounts of BSW than light oils because of the concentration of BSW of crude oil in the residual fractions due to the high specific gravity and high viscosity of the residual fuels. Light oils, such as No. 2 and No. 4, are usually clean, except for some water and small amounts of fine sediment. Table 1 shows that m a x i m u m BSW is limited to 0.05%vol for oils No. 1 and No. 2, 0.5% vol for No. 4, 1.0% vol for No. 5, and 2% vol for No. 6. Three s t a n d a r d m e t h o d s are used to determine water and bottom sediment either together (ASTM 1796 -by centrifugation with added toluene), or separately (ASTM D 95, Azeotropic distillation for water and ASTM D 473, Extraction with toluene and weighing the residue). Specific rules apply to reporting BSW, if water and sediment measurements are carried out separately (see footnote in Table 3). The occurrence of BSW in fuel oils can be reduced by careful storage, blending, and transportation practices that reduce the formation and/or dispersion of the sediments in the fuel oils [14]. Carbon

Residue

The term carbon residue is used in several different connotations related to the use of fossil fuels, including carbonaceous particles present in fuel, carbon formed on the burner tips and furnace walls because of incomplete combustion, and carbonaceous residue remaining after pjTolysis of fuels in standard tests. Only the results from standard carbon residue tests are used in fuel specifications. It is important to distinguish between the carbon deposition due to the high carbon residue of the fuel oil, and coke or soot formation as a result of poor combustion [15]. Understanding the difference between these two types of deposition helps identify the root cause of any deposition problem: fuel composition or combustion conditions [16]. Three standard methods used to determine carbon residue are ASTM D 189, Conradson Carbon Residue (CCR), ASTM D 524, Ramsbottom Carbon Residue (RCR), and ASTM D 4530, Micro Method. Most specifications are based on CCR, but correlations exist to convert between CCR and RCR test results [ASTM D 189]. Table 3 shows that carbon residue is specified only for light fuel oils, No. 1 and No. 2, because small vaporizing-pot and sleeve-t3^e burners used in domestic applications have less tolerance for carbon deposition. The carbon tests are conducted on 10% bottom residue of the fuel samples remaining after distillation. Ash Ash results from the noncombustible organic and inorganic species found in fuel oils. Most of the ash can be traced back to the constitution of the crude oil from which the fuel oils

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were derived. To a lesser extent, contamination during refining and handling may be responsible for the ash content. Most of the ash-producing materials present in crude oil (e.g., water soluble sodium and calcium chlorides, and oil soluble organometallic compounds of nickel and vanadium) tend to concentrate in the heavy products, such as residual fuel oils. Ash contents of fuel oils can be determined by weighing the n o n c o m b u s t i b l e residue after c o m b u s t i o n using ASTM D 482. Table 3 shows the permissible ash contents specified for Grades No. 4 (0.05%, 0.10% for light and heavy, respectively), and No.5 (0.15%). No specifications are given for No. 1 and 2 because ash is seldom found in distillate oils. Also, there is n o ash specification for No. 6 oils (which may contain u p to 0.2% ash [17]) since the combustion equipment designed for burning heavy residual fuels can handle relatively high ash contents. Problems encountered with high ash contents depend on the application and type of combustion operation, including contamination of products in direct firing applications (e.g., glass and ceramic industry), erosion of p u m p p a r t s and burner tips, and accumulation of ash on boiler tubes. High metal contents of heavy oils would also be responsible for producing particulate emissions from combustion with potentially significant toxicity [18]. The removal of ash from fuel oils is not practical and very costly because most ash-forming compounds are soluble in the oil. Problems with high ash contents are usually addressed by blending with low ash oils and/or by reducing the impact of ash in combustion operations with fuel additives and combustion system treatments [17]. Sulfur

Content

Sulfur is one of the most troublesome elements found in fuel oils. Sulfur is p r e s e n t in a variety of complex chemical compounds in crude oils. Since the molecular chemistry of sulfur compounds is extremely complex, sulfur specifications depend on the measurement of the total sulfur content of fuels. Because of the high-boiling points of sulfur-containing compounds, sulfur tends to concentrate in heavy residual fractions obtained from petroleum refining. The sulfur content of fuel oils generally increases with the increasing grade number. The sulfur range in each grade shows large variations because of differences in sulfur contents of crude oils, refining processes, and blending. There are several ASTM tests that are used to determine the sulfur contents of fuel oils, inclu-ding ASTM D 1266, Lamp Method (for No. 1 only), ASTM D 1552, High-Temperature Combustion, ASTM D 129, Oxygen Bomb, and ASTM D 4294, Non-Dispersive X-ray Fluorescence. The sulfur limits in fuel oil specifications shown in Table 3 designate the m a x i m u m total sulfur contents for No. 1 and No. 2 oils as 0.5%. No sulfur specification is listed for grades No. 4, 5, and 6. For many applications, however, consumers specify the maximum allowable sulfur limit to comply with the environmental regulations (e.g., Clean Air Act a n d Amendments in the U.S.) or to limit damage to materials in combustion systems. Problems associated with high sulfur contents in fuels are related principally to combustion products, namely sulfur dioxide (SO2) and sulfur trioxide (SO3), which further react

in the presence of oxygen and moisture to produce sulfurous, or sulfuric acid. These acids will cause corrosion of any exposed metal surfaces, such as boiler shell and tubes. The presence of sulfur compounds may also cause problems in various materials applications, such as glass, ceramic, and metals production [19]. Total emissions of sulfur oxides from combustion systems depend almost entirely on the sulfur content of the fuel without any significant effect of the boiler size, combustor design, or the fuel grade. Sulfur contents of fuel oils can be reduced by hydro treatment in petroleum refining [4,5] or by blending with oils having lower sulfur contents. Flue gas scrubbers are used in large installations to remove sulfur from stack gases to comply with SO2 emission limits. A particular corrosion test, ASTM D 130, Copper Strip Corrosion, is used to characterize the corrosive properties of fuels related mainly to sulfur compounds, such as dissolved hydrogen sulfide, mercaptans, active elemental sulfur, along with inorganic acids and ammonia. The presence of these compounds in fuels lead to the corrosion of copper heating lines, cooling coils, and nonferrous metal fittings. The copper strip corrosion test measures the extent of discoloration of copper strips that came into contact with fuel samples under specified conditions. Reference strips used to measure the extent of discoloration are rated from No. 1 (light-orange) to No. 4 (jet black). The No. 1 and No. 2 grades have a m a x i m u m Corrosion Strip specification of No. 3, dark tarnish. Instability

and

Incompatibility

The instability of residual fuel oils refers to the tendency of a fuel to produce deposit by itself, while incompatibility is the tendency of a fuel to produce a deposit when blended with other fuels [20]. Two oils that are each stable as single fuels may become incompatible when they are mixed. Major incompatibility problems occur when an oil with an asphaltene content of greater t h a n 3 - 5 % is blended with paraffinic oils. The incompatibility results in the formation of tar-like precipitates that cause problems in handling and combustion systems. ASTM D 4740 can be used to predict the compatibility between a residual fuel oil and a specific distillate fuel oils, such as a No. 1 or No. 2 oil. This test is also used for predicting the compatibility of residual fuels, although it may not be reliable for residual oil mixtures. The asphaltene content of residual oil is believed to affect its stability and compatibility [3,12]. ASTM D 3279 can be used to determine the asphaltene content of residual fuel oils.

Conclusions A standard classification of fuel oils according to selected specifications facilitates the selection of the right fuel oil for wide ranging applications in different combustion systems that show significant variations in size, combustor design, and process needs. A large number of ASTM methods exist to characterize many properties of fuel oils that are of interest for a particular application. Understanding the significance of these tests and careful interpretation of the test results will help troubleshoot m a n y performance problems in fuel oil handling and combustion systems.

CHAPTER 29: PROPERTIES

OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS

PETROLEUM PITCH Petroleum pitch has become an important material for a n u m b e r of industrial applications, in particular, for manufacturing high-performance c a r b o n fibers and carboncarbon composites. This section provides an overview of p r o d u c t i o n processes, applications, and properties of petroleum pitch. Production Processes and Applications Petroleum pitch is a term used for certain petroleum residues due to their resemblance to other pitch materials, such as coal tar pitch, that are thick, dark colored bituminous substances obtained from destructive distillation processes. Generally, petroleum pitch is the nonvolatile product obtained from thermal or catalytic cracking of heavy petroleum residua [4,21]. It may also be defined as the high boiling point fraction (420-520°C) obtained by vacuum distillation (0.5-1.0 m m Hg) of catalytic cracking bottoms [22], or as solvent deasphalted bottoms [23]. Due to its resemblance to coal tar pitch, the ASTM methods that are summarized in Table 3 are often used to test both materials. However, petroleum and coal tar pitches are substantially different in origin, structure, and behavior, and, therefore, a ccireful interpretation of the test results is necessary. CoaJ tar pitch is obtained from the distillation of volatile by-products, or tar, from coke ovens during the manufacture of metallurgical coke from coal. The differences in the origin and processes that lead to the production of petroleum and coal tar pitch are apparent in their chemical composition and respective industrial applications [24,25]. For instance, the carbon content of petroleum pitch is around 85-90%, which is somewhat lower than that of coal tar pitch (94-96%). This is linked to the higher hydrogen content of 4-6% for petroleum pitches, versus 2 - 3 % for coal tar pitches. Hence, petroleum pitches contain a relatively high proportion of aliphatic carbons compared to coal tar pitches. Both petroleum and coal tar pitches are predominantly aromatic, with the majority of alkyl groups being methyl. However, while a tj^ical coal tar pitch has an aromaticity of 98-99%, petroleum pitches have around 10-20% aliphatic carbon. Coal tar pitches contain relatively more condensed aromatic ring structures with a bridgehead aromatic carbon content (i.e., aromatic carbon only bound to other aromatic carbons) of 0.45-0.50. Therefore, the volatile compounds in coal tar pitches, as detected by GC-MS, are mainly three to six aromatic ring compounds, such as phenanthrene, fluoranthene, pyrene, benzo[a]pyrene, benzo[b]fluroanthene, and a n t h a n t h r e n e , while the nonvolatile compounds have a much higher condensed structure. In contrast, petroleum pitches have a rather open structure, with a bridgehead aromatic carbon content around 0.35-0.40 [26]. The volatile compounds are two-to four-ring compounds that are heavily alkylated. The non-volatile compounds are non-defined entities of larger a r o m a t i c ring structures substituted with long-chain alkyl groups and possibly some saturated rings. These differences in structure and composition explain the differences in softening points and viscosity behavior, solubility, density, and coking yields from the coal tar and petroleum pitches, as described in Properties of Petroleum Pitch.

763

The applications of different p e t r o l e u m pitches are governed by their properties, d e p e n d i n g largely on their chemical composition. ASTM D 2569 is used to determine the distillate contents of pitches. A soft pitch, which appears semi-viscous at ambient temperatures due to its low glass transition temperature (see Softening Point), will produce a relatively large amount of distillate even at low temperatures and weak vacuum (if applied). On the other hand, hard pitch will yield little distillates even at temperatures up to 360°C. Accordingly, industrial distillation of pitches is frequently used to produce carbon-precursors with high softening point and coking yields [27]. An alternative approach utilizes airblowing to alter the chemical composition of the pitches to meet the required specifications [23]. Generally, petroleum pitch is used in the production of prem i u m petroleum coke or as an impregnation pitch in the manufacture of carbon artifacts for the aluminum and steel industry [28]. The high carbon content and low mineral matter content of petroleum pitch makes it an excellent precursor for the production of high performance carbon fibers and carbon-carbon composites (see Carbon Fibers from Petroleum Pitch). Molded in sheets, carbon fibers have higher strength than steel (227 and 200 GPa, respectively) but only 1/5 of the density, 1720 a n d 7830 kg m~^, respectively [29]. These impressive properties result from the formation of a discotic liquid crystal phase, carbonaceous mesophase that produces high tensile strength and modulus. Carbonaceous mesophase is formed by alignment of disk-like molecules (see Formation of Coke Microtexture in Coking Processes for a description of carbonaceous mesophase). The a m o u n t of mesophase in a p e t r o l e u m pitch can be established following ASTM D 4616. Although most of the terminology of this method concerns coal tar pitches, the central concept about isotropic (non-mesophase) and anisotropic (mesophase) is valid for petroleum pitches. The influence of primary quinoline insolubles, described in detail in Solvent Fractionation, is minimaJ or nonexistent in petroleum pitches. Hence, mesophase is both readily developed and detected in petroleum pitches, with clear identification of mesophase spheres or anisotropic texture domains. The a m o u n t of mesophase, as found through ASTM D 4616, is generally a function of pitch condensation, heat-treatment temperature, and time. Recently, an in-situ ^H NMR technique has been developed to follow the mesophase development directly during heat treatment [30]. High performance carbon products from mesophase pitches have a wide range of applications, including lightweight components for vehicles, boats, planes and the space industry; high strength and wear parts in brakes and engine pistons, and medical artifacts (e.g., in artificial heart, bone plates and ligaments) due to their bio-compatibility with blood, soft tissue, and bones [31,32].

Properties of Petroleum Pitch The determination of various properties of petroleum pitch is crucial for the establishment of pitch consistency, which can be utilized as a strong marketing tool. Pitch buyers often dem a n d that the properties described in the Softening Point through Ash sections meet their specifications, to assure that the product they are buying fits their requirements and process line. Currently, there are two m a i n practices used for

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pitch transportation. The most frequent practice involves transporting pitch as a hquid around 180°C, but some users prefer to receive the pitch as a soHd (often referred to as pencil pitch from its extrusion into small rods). Due to the various ways of handling, some localized variation in pitch composition can occur. Hence, an important tool for establishing sample uniformity is the use of ASTM D 4296, Stand a r d Practice for Sampling Pitch. The strength of this method lies in its compatibility with both solid and liquid pitch. A proper sampling lays the foundation for valuable data collection from the following standard test methods. Softening

TABLE 4—Comparison of the five standard test methods used for determining softening point in pitches. Method

Description

D 3461

Softening point of asphaltene and pitch (Mettler cupand-ball method)—The pitch is loaded in a small metal cup with a bottom orifice and a ball is placed on top of the solid pitch. The assembly is placed in a furnace and a light beam is used to detect the temperature when the ball is penetrating the pitch (the softening point). Softening point of bitumen (ring-and-bsdl apparatus)— The pitch is loaded in a small metal ring and a ball placed on top of the pitch. The assembly is placed in a bath and the temperature when the ball is penetrating the pitch is manually detected (the softening point). Softening point of pitch (cube-in-air method): Molded cubes of pitch with center holes are placed on hooks and suspended in an oven. The temperature of the hooks when the pitch is flowing is manually detected (the softening point). Softening point of pitch (cube-in-water method): Molded cubes of pitch with center holes are placed on hooks and suspended in water. The temperature of the hooks when the pitch is flowing is manually detected (the softening point). Softening point of pitches (Mettler softening point method): The pitch is loaded in a small metal cup with a bottom orifice. The assembly is placed in a furnace and a light beam is used to detect the temperature when the pitch is flowing through the orifice (the softening point).

D 36

Point

Petroleum pitches are mixtures of condensed aromatic compounds. As a result, petroleum pitches are eutectic, i.e., soften at a lower temperature than the individual melting points of the different compounds that make u p the pitch. At ambient temperatures, the pitch has an isotropic structure and is generally characterized as a glassy solid (hardpitch). When the pitch is heated in an inert atmosphere, it has no defined melting point, but will pass through a glass transition region before it becomes an isotropic liquid. While a pure compound, e.g., anthracene, goes from a solid to liquid at 217.5°C, a mixture of similar aromatic compounds and their alkylated derivatives will exhibit a wide temperature interval (4Tg can range from 10' to over 10^°C) from the end of the solid phase to a fully liquid state. The transition from a glassy solid to a viscous substance gives rise to a physico-chemical characteristic CcJled the glass transition temperature, Tg [33]. Although there is no ASTM method yet developed, both the Tg and the temperature interval, ZlTg can be determined by different methods [34] such as ' H N M R , electron-nuclear double resonance (ENDOR) or differential scanning calorimetry (DSC). The enthalpy, /IH, and activation energy, Ea, for pitches can then be derived from the above techniques, ft has been shown that the Ea can be considered to be proportional to the mean molecular size in pitch, and can give an estimate of the degree of condensation [35]. During the transition from a glass to a liquid, the viscosity of the pitch changes drastically (see Viscosity). However, it is relatively tedious and labor intensive to determine the viscosity behavior for every single pitch. Therefore, several standard test methods, ASTM D 3461, D 36, D 2319, D 61, and D 3104, have been developed to establish when a pitch reaches a viscosity of about 10^ Pa s, generally referred to as the softening point of the pitch [36]. Table 4 lists the differences between the five standard test methods given by ASTM. D 3104, Mettler Softening Point Method or D 61, Ring and Ball Softening Point, are the most commonly used. D 3104, Mettler Softening Point Method, provides fast and accurate determination of the softening point but requires the purchase of an instrument. D 61, Ring and Ball Softening Point, is fast and inexpensive, and, therefore, it is also frequently used. The softening points can be given both in °C or °F, and sometimes they give rise to pitch nomenclature, where Ashland petroleum pitches A-170 and A-240 have softening points of 170 a n d 240°F, respectively. Furthermore, it has been reported that pitches having similar softening point can have different Tg, which can be associated to differences in viscosity as described below.

D 2319

D 61

D3104

Viscosity The determination of changes in viscosity and other rheological properties during and after the glass transition region is crucial for applications of pitch as a binder for electrodes and road aggregate. ASTM D 5018, Standard Test Method for Shear Viscosity of Coal-Tar and Petroleum Pitches, is a good tool to follow the rheological properties of pitches at temperatures of 40-100°C over its softening point, since it requires a relatively simple setup. The setup consists of a hotplate, a temperature controller, a thermometer, and a rotational viscometer. The pitch is melted into a cup on the hotplate where the rotor from the viscometer is inserted, and at different temperatures the viscosity is measured. The method is limited to 230°C and 15 000 cps (15 Pa s). Viscosity, 17, is defined as the ratio of the shear stress, T, to the rate of change of shear strain, ySR, at constant temperature and pressure, TJ = r/ySR [37]. When the viscosity of a system is only a function of temperature and pressure, and independent of the shear rate at constant temperature and pressure, the fluid can be classified as Newtonian. For nonNewtonian systems, such as polymers and most liquid crystals, the viscosity is dependent on the shear rate as well [34]. Isotropic pitches before thermal decomposition are mainly Newtonian [38]. However, non-Newtonian flow, such as Bingham behavior, has been detected where there is no flow until the shear stress exceeds a critical value called the yield stress [39]. The variations in viscosity with temperature for the petroleum pitch Ashland A-240 is compared to a coal tar pitch and its solvent fractions (see Solvent Fractionation in Fig. 2 [40]. With increasing temperature, there is a rapid decrease in viscosity during softening of the pitch. In the tem-

CHAPTER 29: PROPERTIES

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765

FIG. 2—Changes in viscosity with temperature for Ashland A-240 petroleum pitch compared with a coal tar pitch, its toluene soluble (TS) and insoluble (Tl) fraction together with various TS:TI mixtures [40]. Reprinted with permission from Elsevier Science.

TABLE 5—Nomenclature and solvents used historically in the petroleum and coal tar pitch industries. Petroleum Pitch

Coal Tar Pitch

Carboids Carbenes

Insoluble in CS2 Insoluble in CCI4, but soluble in CS2

a-resin /3-resin

Asphaltenes

Insoluble in n-pentane, but soluble in CCI4 or CeHft

y-resin Resinoid Crystalloid

perature region prior to the pitch becoming a hquid, there is a tail off in the viscosity reading, e.g., from 150-2008C for A240 petroleum pitch. ASTM D 5018 deals with the change in viscosity in this region. From the rheological measurements, the suitability of a pitch for certain applications can be established, e.g., as a binder or for impregnation purposes. In Fig. 2, the increase in the viscosity after 4508C is due to t h e r m a l induced chemical reaction in the pitch, such as coking (Coking Value), or the development of mesophase (further explained in F o r m a t i o n of Coke Microtexture in Coking Processes). In addition, rheology studies give information about elastic properties important for the thermo-forming process of fibers and composite impregnation [41]. An example of the effects of the elastic properties is the die-swell during the extrusion process in fiber spinning, which often is undesirable [42]. Solvent

Fractionation

A drawback in pitch characterization is that the nomenclature used for solvent fractionation of petroleum pitch and coal tar pitch has been different in the past [36]. The

Insoluble in quinoline (pyridine) Insoluble in toluene (benzene, dimethylformamide), but soluble in quinoline Soluble in toluene Insoluble in n-hexane (petroleum ether), but soluble in toluene Soluble in n-hexane

petroleum industry used CS2, CCI4, CsHe, and n-pentane, while the coal tar pitch producers used quinoline or pjridine, benzene or toluene, and petroleum ether or n-hexane. Table 5 lists the nomenclature associated with each solvent for the two industries. The standard test methods developed by ASTM Eire largely based on the nomenclature and solvent fractionation scheme originally developed for cocJ tar pitch that has become the common terminology in pitch fractionation [43]. The a-resin, or QI (quinoline-insoluble) content, is determined using either ASTM D 4746 or D 2318. Table 6 compares the two methods for determining the QI content. Generally, petroleum pitches contain very little or n o QI unless they have been heat-treated (see below). The 7-resin, or TS (toluene-soluble), content can be established using ASTM D 4072, D 4312, or D 2764. Table 7 compares the three methods for obtaining the TS content. From the two previous measurements the ;8-resin, or TI - QS (toluene-insoluble and quinoline-soluble), can be calculated by difference. The different resin fractions above consist of c o m p o u n d s with different molecular masses and heteroatom contents. A sim-

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TABLE 6—Comparison of the two ASTM methods for determining the QI content in pitches. Method D 4746

D 2318

Description Determination of quinoline insolubles (QI) in tar and pitch by pressure fihration; The pitch is dissolved in quinoHne at 75°C and fihered through a porcelain filtration crucible with a medium-porosity bottom at a pressure in the range of 10-30 psig using nitrogen. The sample is washed in hot quinoline until clear followed by acetone and dried. The portion of the pitch remaining in the crucible is defined as the QI fraction. Quinoline-insoluble (QI) content of tar smd pitch; The pitch is dissolved in quinoline at 75°C and digested for 20 min. The solution is filtered through a porcelain filtration crucible with a fine-porosity (7 yum) bottom using a suction filter apparatus. The sample is washed in hot quinoline until clear, followed by toluene, then acetone and dried. The portion of the pitch remaining in the crucible is defined as the QI fraction.

TABLE 7—Comparison of the three ASTM methods used for determining the TS content in pitches. Method D 4072

D 4312

D 2764

Description Toluene-insoluble (TI) content of tar and pitch: The pitch is dissolved in toluene at 95°C for 25 min and filtered through an extraction thimble using a gravimetric filtration tube. The extraction thimble is transferred to an extraction apparatus where it is further extracted for 18 h using a toluene reflux rate of 1 to 2 drops/s. The extraction thimble is dried at 105°C for 30 min and the portion of the pitch remaining in the thimble is defined as the TI fraction. Toluene-insoluble (TI) content of tar and pitch (short method): The pitch is dissolved in toluene at 95°C for 25 min and filtered through an extraction thimble using a gravimetric filtration tube. The extraction thimble is transferred to an extraction apparatus where it is further extracted for 3 h using a toluene reflux rate of 120-150 drops/min. The extraction thimble is dried at 110°C for 30 min and the portion of the pitch remaining in the thimble is defined as the TI fraction. Dimethylformamide-insoluble (DMF-I) content of tar and pitch: The pitch is dissolved in dimethylformamide at 95-100°C and digested for 30 min. The solution is filtered through a porcelain filtration crucible with a fine-porosity (7 /xm) bottom using a suction filter apparatus. The sample is washed in hot dimethylformamide until clear followed by acetone and dried at 105-110°C. The portion of the pitch remaining in the crucible is defined as the DMF-I fraction (mainly used as a rapid TI determination).

plified comparison between the three fractions could be that the y-resin (TS) is the lowest molecular weight fraction, the ;8-resin is an intermediate, while the a-resin (QI) is very high molecular weight material. These differences result in the TS having high H/C atomic ratio and low softening point, while the QI has low H/C atomic ratio and generally does not soften. The a-resin (QI) is a primary specification of pitches in the carbon industry, and Table 8 summarizes the desired content for some applications. A further sub-division of the a-resins can be made into primary, secondary QI, and extraneous im-

TABLE 8—Pitch apphcations depending on QI content [43]. Application Binder pitch Impregnating pitch Needle coke manufacture (highly anisotropic)

'-Resin (QI) content of pitcli, % 9-18 2-5 4Al + SCOa - desired reduction reaction 2) Al203(diss) +2C(anode) ^ 2Al + 3CO - generation of primary CO, low C efficiency 3) CO2 + 2C(anode) —> 2CO - uudesired consumption of carbon 4) O2 + C(anode) ^ 2C02 ; O2 + 2C(anode) ^ 2 C 0 - uudesircd consumption of carbon by a i r b u m Obviously, the first reaction is the desired reaction, whereas third and fourth reactions lead to undesired consumption of anode carbon by Boudouard and a i r b u m reactions, respectively [25]. The second reaction also produces Al but only with half of the yield obtained from the first reaction. An intermediate reactivity of carbon anodes (sponge coke and pitch coke) is, therefore, desirable to promote the desired reduction reaction and to inhibit the undesired anode consumption. In contrast to the electrochemical production of aluminum by the consumption of carbon anodes, the consumption of graphite electrodes in electrical-eirc production of steel takes place only due to the uncontrolled b u m - u p of the electrodes in air [88]. Consequently, extremely low reactivities of graphite electrodes (made u p of needle coke filler and coal tar pitch binder derived carbons subjected to graphitization heat treatment) are desired to minimize electrode consumption by a i r b u m . The reactivity of cokes in oxidizing atmospheres depends primarily on the degree of microcrystalline order (i.e., optical texture), accessible surface eirea (related to the porosity), and catalysis by inorganic impurities, such as Ni, V, Fe, K, Na, and Ca. Oxidation, or gasification of cokes is a surface reaction that occurs at active sites at elevated temperatures. Carbon atoms at the edges of graphene layers (or basal planes) or at defects on graphene layers such as vacancies and dislocations, are m u c h more reactive than the carbon atoms in basal planes. The high reactivities of edge atoms result from the dangling bonds, or unpaired sp^ electrons, at these sites that readily chemisorb oxygen. Highly anisotropic cokes, such as needle cokes, with relatively large microcrystallites found in well-developed flow domains or domain textures have low concentrations of these active edge sites, and, therefore, exhibit low reactivities. Sponge cokes, in comparison, have a lower degree of anisotropy, and higher porosity, and, therefore, have higher reactivities than needle cokes. High porosity, or high accessible surface area, increases the ease of diffusion of oxidant molecules to the active sites. Inorganic impurities, on the other hand, promote the dissociation of molecular oxygen to produce more active oxygen species that readily react with carbon atoms even in the basal planes,

producing pits by removal of carbon atoms as CO, or CO2 [86]. Increasing the heat treatment temperatures decreases the concentration of structural defects [87] and at sufficiently high temperatures (e.g., during graphitization heat treatment u p to t e m p e r a t u r e s 3300K), helps remove inorganic impurities by evaporation. The graphite electrodes have low reactivities because of low defect, or active concentration, low porosity, and low concentrations of inorganic impurities [88,89]. Thermal gravimetric analysis (TGA) can be used to measure the reactivity of coke samples in different oxidizing atmospheres (e.g., air, O2, or CO2). In these experiments, the weight of a sample placed in a microbalance is monitored and recorded continuously as a function of increasing temperature, or as a function of time at a given temperature. ASTM D 5341 Measuring Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR), can be adapted to measure the reactivity of petroleum cokes.

Conclusions Petroleum cokes have many industrial applications, ranging from generating heat (fuel coke) to manufacturing carbon products such as carbon anodes and graphite electrodes. Produced from various petroleum heavy residua or fractions from petroleum refining operations, petroleum cokes are classified into different types on the basis of feedstocks (e.g., vacuum distillation residua or FCC decant oil), coke production processes (e.g., delayed coking or fluid coking), or their appearance (e.g., sponge coke, shot coke, or needle coke). Feedstock composition, the coking process used, and coking conditions determine the structure a n d properties of petroleum cokes that are important in various applications. Among the important properties of petroleum cokes are volatile matter content, density, sulfur and metal contents, optical texture, coefficient of thermal expansion, and reactivity. Standard methods have been developed to quantitatively determine many of these properties for selecting, or producing the most desirable cokes for a given application. A fundamental understanding of the relationships between feedstock constitution and the structure and composition of petroleum cokes, and those between coke properties and coke structure is critically important for controlling the coking processes and subsequent thermal treatment operations to manufacture carbon materials with the desired properties.

CARBON MATERIALS FROM RESIDUAL FUEL OIL, PETROLEUM PITCH, AND PETROLEUM COKE As introduced previously in this chapter, residual fuel oil (mainly FCC decant oils), petroleum pitch, and petroleum coke are used as precursors for manufacturing a variety of carbon materials with very different properties and applications. In this section, four different carbon materials will be further discussed as examples to illustrate a glimpse of the diversity in microstructure and properties of the carbon materials. Four carbon materials are: carbon blacks, carbon fibers, carbon anodes, and graphite electrodes that are produced from decant oil, petroleum pitch, sponge coke, and

CHAPTER 29: PROPERTIES

OF FUELS, PETROLEUM PITCH, PETROLEUM

needle coke, respectively. Manufacturing processes, properties, and applications of these four carbon materials will be reviewed briefly with reference to their respective precursors as discussed before in this chapter. Carbon Blacks Classification, Manufacturing and Applications

Processes,

Carbon black is defined as "an industrially manufactured colloidal carbon material in the form of spheres and of their fused aggregates with sizes below 1 /u-m" [61]. They are produced by gas phase thermal decomposition of hydrocarbons using various feedstocks and processes [90-93]. Of the five major processes, Lampblack Process, The Cheinnel Process, Acetylene Process, Thermal Process, and Oil Furnace Process, the last one is the most widely used process to produce most of the carbon blacks available today [90]. These processes yield products t h a t are identified by the process names, such as lamp black, channel black, acetylene black, themiEj black, and furnace black. Each carbon black has a unique structure and a set of properties that are determined by the different manufacturing processes. Except for the Channel (or Impingement) and Acetylene processes that convert natural gas and acetylene to carbon blacks, respectively, a r o m a t i c oils are used as principal feedstocks for carbon black production. Most commonly used oils are residual fuel oils, or FCC decant oils. Carbon black oils, a distillate fraction of coal tar produced in by-product coke ovens, are also used as feedstocks in some processes. Thermal Black and Furnace Black processes that use aromatic oils as feedstocks are described below, along with the major applications of carbon blacks manufactured by these processes. Thermal blacks are produced by thermal decomposition of oils in the absence of air using a cyclic process that consists of heating and production cycles that rotate in a pair of furnaces (generators) in 2.5 min intervals [90]. The furnaces are lined with open checker brickwork that is preheated before the introduction of oil feed. Production of carbon black takes place in a heated furnace followed by a steam purge to remove the products (carbon black and byproducts including hydrogen gas). The products are sprayed with water for cooling and passed through a collection filter to separate the carbon black particles. Following the steam purge, air is passed through the furnace to bum-off the carbon black remaining in the furnace (supplemented by burning oil, if necessary) to produce heat for the next production cycle. Pairs of furnaces are used for continuous operation using S3rnchronous heating and production cycles in separate furnaces. Thermal blacks consist of larger particles (—250 to 500 n m average particle diameter) with lower degree of aggregation compared to other types of carbon black. They are used in applications that require very high volume fractions of fillers, including the production of r u b b e r a n d cross-linked polyethylene, as well as some specialty pol5rmers [90]. As opposed to intermittent production in the Thermal Black process, carbon black is produced continuously from highly aromatic oils in a combustion gas environment at high temperatures in a Furnace Process [90]. Production of furnace black takes place in a fraction of a second when the feed is injected into a flame that is established in the reactor with oil or

COKE, AND CARBON MATERIALS

779

natural gas and excess air. Immediately after carbon black production, the products are cooled with a water spray and further cooled as they pass through a heat exchanger before carbon black particles cire collected in a bag filter. Because of high gas flow rates, the carbon black particles reach the bag collector in less than a second after the feedstock oil is injected into the reactor. The size distribution of furnace black particles is controlled by the rate of cooling with water spray. For production of particles with a very small size (for high color applications), large quantities of water are needed for cooling, u p to 40:1 process water to carbon black ratio [90]. For use in rubber and plastic applications, a large fraction of furnace product is pelletized (beaded) using water to provide easy handling and less dust formation. For applications that require dry beads, such as in inks and coatings, powdered ceirbon black particles are beaded in a rotating drum where agglomerated carbon black act as nuclei to grow beads [90]. For carbon black production in the oil furnace process, the feedstock oil must be completely vaporized and pyrolyzed. If the conversion of oil to a carbonaceous solid takes place in a liquid phase, large particles of "coke" are produced. Coke particles, undesired contaminants in carbon black product, are much larger in size and very different in microstructure compared to ceirbon black particles. The formation of coke in Oil Furnace Process probably results from the presence of high-molecular-weight, aJkyl substituted PAH that contain more than five rings in polycondensed aromatic systems [58]. These compounds would tend to polymerize and go through liquid-phase carbonization to form coke. Figure 14 shows polarized-light-micrographs of some particles from a sample of "coke" produced in carbon black manufacturing. As opposed to the isotropic texture of small particles of carbon black, the "coke" particles showed anisotropic texture of pyrolytic carbon microstructures produced by gas to solid transformation taken place during liquid-phase carbonization (bottom micrograph in Fig. 14).

FIG. 14—Polarized-light micrographs of "coke" produced in carbon blacl< manufacturing.

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Many industrial applications of c a r b o n blacks rely on strength reinforcement (in elastomers), color (in inks and coatings), UV protection, and electrical conductivity (in polymers). The uses of carbon blacks in these applications derive from a n u m b e r of properties that are related to the size, morphology, and surface chemistry of carbon blacks. The meas u r e m e n t and significance of these properties for specific applications are described in the following section. Properties

of Carbon

Blacks

Four fundamental properties of carbon blacks that are important for industrial applications are particle size, structure (aggregate size and shape), porosity, and surface activity. These fundamental properties determine, to varying extents, a n u m b e r of functional properties, such as surface area, tint strength, and oil absorption, which are measured by well-established ASTM m e t h o d s to characterize carbon blacks. Table 17 lists ASTM methods used for characterization of carbon blacks. The particle d i a m e t e r (fineness) is the most i m p o r t a n t property of carbon blacks that relates to the level of elastomer reinforcement and color. Both the level of elastomer reinforcement and the degree of blackness increases with the decreasing particle size. The measurement of carbon particle size is very difficult and highly dependent on the measurement technique used. The most dependable method involves direct m e a s u r e m e n t of particle sizes a n d aggregate sizes using standard transmission electron microscopy (TEM) procedures (ASTM D 3849). Surface area measurements are also used (ASTM D 6556, D 3765, and D 1510) as indications of average particle size. All these surface area measurements are also affected by the pore size distribution, and in some instances, by surface properties of the particles. The results obtained by a commonly used standard method (ASTM D 1765) adopted in 1965 for characterization of rubber grade carbons are often inconsistent with those obtained by TEM procedures (ASTM D 3849). Because of difficulties in replicating particle size measurements even using ASTM D 3849 in different laboratories, particle size ranges reported by ASTM D 1765 have been abandoned in the ASTM designation of carbon blacks since 1996. Average nitrogen surface areas are re-

ported in ASTM D 1756 instead of particle size ranges. Using ASTM D 1765, carbon blacks are classified into different grades based on the average N2 surface area using a letter (N or S - referring to Normal, or Slow cure) and a three-digit number (e.g., N121 through N990 for rubber grades). First n u m b e r of the designation roughly corresponds to the particle size. The average N2 surface area, and the mean particle size of grade N121 are 121-150 m^lg, and 19 nm, respectively. For grade N990, the same parameters are 0-10 rr?lg, and 285 nm, respectively [90]. The term structure designates the irregular shapes of the chain- or grape-like aggregation of carbon black particles and is m e a s u r e d by absorption of dibutylphthalate (DBTA) (ASTM D 2414). The packing of chain- or grape-like particles creates internal voids t h a t absorb DBTA. Therefore, the higher the measured DBTA absorption, the higher the total level of structure, or more irregular shaped particles in the sample. The structure is a very important property of carbon blacks that affects the dispersion of carbon blacks in different mixtures. Porosity in carbon blacks is produced by steam gasification of carbon during cooling of particles after formation. A large fraction of porosity is found at the interior of the carbon black particles, because the surface layers in the concentric (onionlike) structure of carbons are more stable than the disordered internal layers. Porosity is an important property of carbon blacks used in conductivity and color applications. Carbon blacks used in elastomer reinforcement have low porosity. Surface activity is defined as the tendency of a carbon black to interact with its surroundings. Specific interactions depend on the physical and chemical characteristics of the surface a n d on the properties of the surrounding matrix. Surface composition, e.g., surface functional groups, particularly oxygen functional groups, strongly affects surface activity. Surface activity, the most difficult fundamental property to characterize, is very important in understanding and controlling the elastomer reinforcing properties and the dispersion of carbon blacks, including the rheological properties of polymer and carbon black mixtures. Other important properties of carbon blacks include sulfur content, extractable organics, ash, and sieve residue that axe.

TABLE 17—Standard test methods used for characterization of carbon blacks. Fundamental Property

Particle Size

Structure

Measured Property

ASTM Method

N2 surface area CTAB adsorption Iodine number Particle and aggregate sizes Particle size range, N2 surface area

D4820 D3765 D1510 D3849

Tint strength Dibutylphthalate (DBTA) absorption Sulfur content Extractable organics Ash

D3265 D2414

Sieve residue

D1765

D1619 D4527 D1506 D1514

Remarks

Surface area is considered as a measure of particle size, but measurements by different techniques are also sensitive to pore size distribution and surface chemistry. Particle and aggregate sizes are determined directly by transmission electron microscopy (TEM) Used for rubber grade carbons; usually not consistent with the results from D3849 especially for larger particle sizes; reporting of particles size ranges was abandoned in 1996. The smaller the particle size, the higher is the degree of blackness Higher DPTA absorption, higher the total level of structurechain-or grape-like aggregation Mostly non-reactive sulfur in aromatic heterocyles Thermally stable compounds produced in the flame extracted with toluene and analyzed by UV transmission at 325 nm Mostly Inorganics irom water, or trace metals and catalyst particles in the feedstock (FCC decant oil) Particulate contamination reported as 35 and 325 mesh residues

CHAPTER 29: PROPERTIES

OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS

measured by the standard methods hsted in Table 17. Figure 14 shows that the sieve residue may consist of coke particles produced by liquid-phase carbonization, and by pjrolytic deposition processes. Carbon Fibers from Petroleum Pitch Carbon fibers are produced from polyacrylonitrile (PAN), and pitch by melt spinning, stabilization, carbonization processes, and from hydrocarbons by vapor phase decomposition reactions catalyzed by metal catalysts [93]. Carbon fibers are used to fabricate C—C composites, or other composite materials such as ccirbon fiber reinforced plastics (CFRP) [94,95]. Petroleum pitch is also used to produce the carbonaceous matrix in the C—C composites. Unique properties of C—C composites, such as high specific strength and stiffness, high temperature strength, high corrosion resistance, good friction and wear properties, and low thermal expansion make them very desirable materials. Some important applications of C—C composites include manufacturing aircraft brakes, rocket nozzles, nose cones, and materials in the aerospace industry [95]. Currently, approximately 90% of all commercial carbon fibers are produced from PAN [94], but pitch-based fibers offer significant improvement in some fiber properties and, in some cases, reduced cost. Compared to PAN, petroleum pitch costs less and gives a higher carbon yield. Both PAN and petroleum pitch can be used to produce isotropic fibers. Inferior mechanical properties of isotropic pitch-based fibers limit their use to thermal insulation and some friction material applications. The principal advantage of petroleum pitch-based fibers is in the p r o d u c t i o n of anisotropic fibers from mesophase pitch. Mesophase pitch fibers offer m u c h more ordered structures, and, thus, much higher modulus, but lower strength properties compared to those of PAN fibers. Table 18 lists the tensile modulus, tensile strength, and density of selected PAN- and pitch-based fibers. Mesophase pitch is produced by thermal or catalytic processing of isotropic petroleum pitch. Thermal processing involves heating petroleum pitch from room temperature to 400-500°C. The solid pitch melts at temperatures between 100 and 200°C and its viscosity decreases. At temperatures greater t h a n 400°C, the viscosity of pitch melt starts to

TABLE 18—Properties of carbon fibers [94]. Fiber Pitch-based P-25 P-55 P-75 P-100 E-35 E-75 E-105 PAN-based T-300 T-2 AS-4 T-^0 HMS

Tensile Modulus, GPa

Tensile Strength, GPa

Density, g/cm^

159 379 724 724 241 517 724

1.38 1.72 2.24 2.24 2.83 3.10 3.31

1.90 2.00 2.15 2.15 2.10 2.16 2.17

231 172 231 290 345

3.24 2.24 3.64 3.25 2.21

1.79 1.80 1.78 1.83

781

increase upon pyrolysis/polymerization reactions with the attendant formation of mesophase (see Formation of Coke Microtexture in Coking Process). Strictly controlled heating is necessary to control the kinetics of mesophase formation to produce a mesophase pitch that is suitable for melt spinning process. A complete conversion of isotropic pitch to mesophase is necessary, but the resulting mesophase pitch must have a sufficiently low melting point and low viscosity to allow melt spinning. This becomes a challenge for treating pitches with a complex composition containing a wide distribution of molecular constituents with different reactivities towards mesophase formation. A pre-fractionation of such pitches may be necessary to obtain a more homogenous mixture of molecular species with comparable propensities for mesophase formation [96]. These pretreatment processes increase the cost of producing mesophase pitches suitable for carbon fiber production. Mesophase pitches produced from single compounds such as acenaphthylene, naphthalene, or methylnaphthalene by thermal, or catalytic procesess [97-99] are ideal, but expensive precursors to anisotropic carbon fibers. Continuous c a r b o n fibers are p r o d u c e d in a three-step process: spinning, stabilization (oxidation), and carbonization. To produce anisotropic carbon fibers using a melt-spin process, powdered mesophase is heated above its melting point and forced through a spinneret and wound onto a rotating reel. The m i c r o d o m a i n s of mesophase are aligned along the fiber axis as the melt passes through the spinneret. The fibers are drawn to approximately 10-/xm diameter from 100 /Am diameter at the exit of the spinneret, producing a fiber with a high degree of molecular orientation. The spun fibers need to be stabilized by oxidation to prevent melting and loss of structure during carbonization. For stabilization, fibers are oxidized at temperatures below their melting point, typically at 275-325°C, depending on the composition and dia m e t e r of the fibers [100]. The stabilized fibers are carbonized to increase their carbon content by heating to 1000-1600°C in an inert atmosphere. Carbonized fibers can be subjected to graphitization heat t r e a t m e n t to produce graphitic fibers. Mesophase carbon fibers have high modulus (>520 Gpa), high thermal and electric conductivity, and low thermal expansion coefficients. These properties cannot be achieved by PAN-based isotropic carbon fibers. In addition to being a precursor to mesophase carbon fibers, petroleum pitch is also used to produce a carbonaceous matrix in C-C composites with PAN fibers and as impregnating pitch for densification of C-C composites [100,101].

Carbon Anodes for Aluminum Production The principal function of carbon anodes in aluminum production is to provide reactant carbon for electrochemical reduction of AI2O3 to produce aluminum metal (see Reactivity). As introduced in Section 3, calcinable sponge cokes produced by delayed coking are used as fillers with coal-tar pitch binders to manufacture ceirbon anodes for aluminum production. There are two types of anodes used for industrial aluminum production: the prebaked anode and the Soderberg anode [25]. The principal difference between the manufacture of the two anodes is found in the carboniza-

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tion/baking stage. The "green" Soderberg anode paste is carbonized/baked by the heat generated by the electrolysis cell, whereas the prebaked anode, as the n a m e suggests, is carbonized and baked in a separate furnace to produce the finished anode. Manufacture of prebaked anode involves three stages: • green paste production • paste compaction • anode baking For grain paste production, grains of anode grade calcined sponge coke (55-65%) is mixed with butts (material from used anodes) a n d coke fines (15-30%) and coal-tar pitch binder (14-17%). The mixing is carried out at a temperature 50-60°C higher than the melting point of the binder pitch to ensure a sufficiently low viscosity of the binder to flow into pores and voids of the aggregates. Particle size distribution of the aggregates plays an i m p o r t a n t role in controlling the properties and the performance of the anode. Paste compaction, i.e., forming of the green paste into anode blocks, is achieved either by hot pressing, or vibratory compacting. Vibratory compacting is a preferred process for large anode blocks heavier than 700 kg [27]. The final stage of anode production is carbonization/baking to convert the thermoplastic binder pitch into coke by heat treatment. In this stage both expansion and shrinkage take place, respectively, during carbonization of the pitch binder at temperatures between 200 and 600°C, and upon subsequent heating to temperatures of 100-1250°C over several weeks. Cores (usually 2-inch diameter) taken from commercial anodes are tested for some or all of the following properties: green apparent density, baked apparent density, conversion of pitch to coke, volume change during baking, electrical resistivity, carbon dioxide/air reactivity, consumption during electrolysis, thermal expansion, thermal conductivity, compressive and flexural strength. Young's modulus, gas permeability, porosity, microstructure, and cracking resistance. Among these properties, baked apparent density and electrical resistivity are the primary characterization parameters that correlate well with many of the other variables. ASTM Section D2.5E is developing standard methods for testing laboratory anodes and cores taken from commercial anodes. The following methods are in development: sampling, electrical resistivity, C02/air reactivity, thermal conductivity, cind thermal expansion. Graphite E l e c t r o d e s for Electric-Arc F u r n a c e s High-performance graphite electrodes are required for electric-arc steel production because of extremely high temperatures, u p to 4000°C, and large temperature gradients involved in the process [102]. The function of the graphite electrodes in electric-arc furnaces is to provide high electrical current density to produce an arc between two- or three-electrodes to generate sufficient heat to melt the furnace charge of scrap iron and steel. In contrast to the use of carbon anodes in the aluminum industry, carbon consumption is not necessary in the operation, but is unavoidable under the extreme conditions present in the furnace. As in carbon anode production, graphite electrodes for the steel industry are manufactured from a green mix of petroleum coke filler and coal-tar pitch binder. There are, however, substantial differences in preparation of the green mix and subsequent processing steps com-

pared to those employed in the carbon anode production, as described in Carbon Anodes for Aluminum Production. A highly graphitizable needle coke (see Petroleum Coke) is used as filler for the graphite electrodes. The amount of binder pitch used is increased from about 15 %wt for anodes to 20 %wt for graphite electrodes. Processing steps for manufacturing graphite electrodes include mixing of needle coke with the binder pitch, extrusion, baJsing, impregnation, rebaJiing, and graphitization. After preparation of the green mix with the carefully sized needle coke particles and the binder pitch, the electrode is formed by extrusion to the desired diameter and length (up to 70 cm in diameter and 270 cm in length). An extrusion aid, such as an aliphatic, or fatty acid, is added to the mix to provide lubrication on the adapter walls during extrusion. The critical effect in this step is the alignment of the needle coke particles parallel to the adapter wall, or the working direction of the electrode [54]. Following extrusion, the green electrode is baked by heating slowly to 800-1000°C and baked for several days depending on the electrode formulation. This step converts binder pitch into solid coke and helps maintain the shape of the electrode. As the temperature increases, the pitch softens, melts, devolatilizes and goes through carbonization to form pitch coke. Rapid baking causes problems, such as expansion, distortion, and formation of pits, which would lead to quality faults in the final product [71]. Pitch impregnation steps are employed to increase the density of the electrodes by filling the open pores in the baked electrode, created by devolatilization and carbonization of the binder pitch during the baking step. Usually, petroleum pitch (see Petroleum Pitch) is used for impregnation because it is essentially free of solids (Ql-quinoline insolubles) that can form a cake on the surface of the electrode, slowing down or stopping the impregnation process. After impregnation, the electrode is rebaked to convert the impregnating pitch into coke. Depending on the desired density of the electrodes, the impregnation and rebaking cycle may be repeated several times. In the final step, the rebaked electrode is graphitized by heating to approximately 3300 K to convert the filler and pitch cokes into synthetic graphite. For graphitization heat treatment, an electric current is passed directly through the electrode to generate the heat necessary to reach the graphitization temperature. The manufactured graphite electrodes must have high mechanical strength, high electrical conductivity, low chemical reactivity, and low thermal expansion. Methods and procedures used to measure the important properties of graphite electrodes were described by Heinz [103]. Most standard methods described for characterization of needle cokes can also be used for evaluation of commercial graphite electrodes or laboratory test beirs. It should be recognized that because of the high structural anisotropy of graphite electrodes, these properties show extremely large variations depending on the direction in which the measurements are made, i.e., withgrain, parallel to the working direction, or across-grain, perpendicular to the working direction. In graphite, for example, the thermal expansion normal to the basal planes is 30 times higher than that parallel to the planes; the electriccJ conductivity is 10 000 times higher along the basal planes than that across the planes.

CHAPTER 29: PROPERTIES OF FUELS, PETROLEUM PITCH, PETROLEUM COKE, AND CARBON MATERIALS 783 Conclusions A glimpse of the fascinating diversity and versatility in structure and properties of different materials made up, essentially, of a single element, carbon, should be apparent in the four examples discussed in this section: carbon black, carbon fibers, carbon anodes, and graphite electrodes. Carbon blacks, particulate aggregates of carbon produced by vapor phase decomposition of aromatic oils, provides elastomer reinforcement and color pigments used in many industrial operations. Pitch-based carbon fibers offer very high modulus and impressive strengths that find applications in the manufacture of Carbon-Carbon composites. Carbon Carbon anodes are used for electrochemical reduction of cJuminum oxide to produce metallic aluminum. Graphite electrodes, with their excellent thermal properties along with high electrical conductivity, high strength, and low chemical reactivity, find applications in electric-arc furnaces for recycling scrap iron and steel. These materials are manufactured from petroleum- and coal-based feedstocks in well-established processes under carefully controlled conditions. Although the detailed mechanisms of how the feedstocks are converted into the final products are not well known, critical properties of these carbon materials are effectively controlled by the right selection of feedstocks and the operating conditions. Many established standard methods are used for the characterization of feedstocks and the finaJ products, and several ASTM standard methods are currently under development.

D 70 D 71 D 86 D 92 D 93 D 95 D 97 D 129 D 130 D 167 D 189 D 240 D 287 D 346

CONCLUSION Covering a vast area of petroleum-derived products ranging from fuel oils, highly aromatic feedstocks from catalytic and thermal cracking, petroleum pitch, and petroleum coke to industrial carbons and graphites, within the bounds of one chapter presents a great intellectual challenge. The coverage in this chapter has been necessarily limited to highlighting the interconnectivity between the end products in this wide array of materials in reference to production processes, property characterization using standard methods, and the significance of measured properties in relation to specific industrial applications. It is important to recognize that despite the great diversity in the nature and applications of the materials covered in this chapter, there are common threads that constitute the complex web of converting petroleum feedstocks to fuels and materials. A careful use of standard methods for classification of materieJs and quantification of important properties has contributed to the technology of converting petroleum feedstocks into desired fuels, hydrocarbons, and carbon materials, as discussed in this chapter and others.

D 396 D 445 D 446 D 473 D 482 D 524 D 1266 D 1298

D 1510 D1552 D 1756

ASTM STANDARDS

D 1765

No. D 36

D 1796

D 61

Title Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus) Standard Test Method for Softening Point of Pitches (Cube-in-Water Method)

D2318

Standard Test Method for Specific Gravity and Density of Semi-Solid Bituminous Materials (Pycnometer Method) Standard Test Method for Relative Density of Solid Pitch and Asphalt (Displacement Method) Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure Standard Test Method for Flash and Fire Points by Cleveland Open Cup Standard Test Methods for Flash-Point by Pensky-Martens Closed Cup Tester Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation Standard Test Method for Penetration of Bituminous Materials Standard Test Method for Sulfur in Petroleum Products (General Bomb Method) Standard Test Method for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test Standard Test Method for Apparent and True Specific Gravity and Porosity of Lump Coke Standard Test Method for Conradson Carbon Residue of Petroleum Products Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method) Standard Practice for Collection and Preparation of Coke Samples for Laboratory Analysis Standard Specification for Fuel Oils Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity) Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers Standard Test Method for Sediment in Crude Oils and Fuel Oils by the Extraction Method Standard Test Method for Ash from Petroleum Products Standard Test Method for Ramsbottom Carbon Residue of Petroleum Products Standard Test Method for Sulfur in Petroleum Products (Lamp Method) Standard Practice for Density, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method Standard Test Method for Carbon Black—Iodine Adsorption Number Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method) Standard Test Method for Rubber ChemicalsSolubility Standard Classification System for Carbon Blacks Used in Rubber Products Standard Test Method for Water and Sediment in Fuel Oils by the Centrifuge Method (Laboratory Procedure) Standard Test Method for Quinoline-Insoluble (QI) Content of Tar and Pitch

784

MANUAL

D2319 D 2320 D 2414 D 2415 D 2416 D 2492 D 2569 D 2622

D 2638 D 2764 D 2962 D 3104 D 3177 D 3245 D 3279 D 3338 D 3461 D 3765 D 3849

D 3997 D 4045

D 4057 D 4072 D 4239

D 4292 D 4294

D 4296 D 4312 D4421 D 4422

37: FUELS AND LUBRICANTS

HANDBOOK

S t a n d a r d Test Method for Softening Point of Pitch (Cube-in-Air Method) Standard Test Method for Density (Relative Density) of Solid Pitch (Pycnometer Method) Standard Test Method for Carbon Black—Oil Absorption Number Standard Test Method for Ash in Coal Tar and Pitch Standard Test Method for Coking Value of Tar and Pitch (Modified Conradson) S t a n d a r d Test Method for F o r m s of Sulfur in Coal Standard Test Method for Distillation of Pitch Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry Standard Test Method for Real Density of Calcined Petroleum Coke by Helium Pycnometer Standard Test Method for DimethylformamideInsoluble (DMF-I) Content of Tar and Pitch Standard Test Method for Calculating VolumeTemperature Correction For Coal-Tar Pitches S t a n d a r d Test Method for Softening Point of Pitches (Mettler Softening Point Method) Standard Test Methods for Total Sulfur in the Analysis Sample of Coal and Coke Standard Test Method for Pumpability of Industrial Fuel Oils Standard Test Method for n-Heptane Insolubles Standard Test Method for Estimation of Net Heat of Combustion of Aviation Fuels Standard Test Method for Softening Point of Asphalt and Pitch (Mettler Cup-and-Ball Method) Standard Test Method for Carbon Black—CTAB (Cetyltrimethylammonium Bromide) Surface Area S t a n d a r d Test Method for Carbon Black—Primary Aggregate Dimensions from Electron Microscope Image Analysis Standard Practice for Preparing Coke Samples for Microscopical Analysis by Reflected Light Standard Test Method for Sulfur in Petroleum Products by Hydrogenolysis and Rateometric Colorimetry S t a n d a r d Practice for Manual Sampling of Petroleum and Petroleum Products Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High Temperature Tube Furnace Combustion Methods Standard Test Method for Determination of Vibrated Bulk Density of Calcined Petroleum Coke Standard Test Method for Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy Standard Practice for Sampling Pitch Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch (Short Method) S t a n d a r d Test Method for Volatile Matter in Petroleum Coke S t a n d a r d Test Method for Ash In Analysis of Petroleum Coke

D 4529 D 4530 D 4616

D 4715 D 4740 D 4746

D 4809

D 4892 D 5003 D 5004 D 5061

D 5056 D 5018 D 5600

D 5341

D 5187

D 6374 D 6376

D 6556

Standard Test Method for Estimation of Net Heat of Combustion of Aviation Fuels Standard Test Method for Determination of Carbon Residue (Micro Method) S t a n d a r d Test Method for Microscopical Analysis by Reflected Light a n d Determination of Mesophase in a Pitch Standard Test Method for Coking Value of Tar and Pitch (Alcan) Standard Test Method for Cleanliness and Compatibility of Residual Fuels by Spot Test S t a n d a r d Test Method for Determination of Quinoline Insolubles (QI) in Tar and Pitch by Pressure Filtration Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method) Standard Test Method for Density of Solid Pitch (Helium Pycnometer Method) Standard Test Method for The Hardgrove Grindability Index (HGI) of Petroleum Coke Standard Test Method for Real Density of Calcined Petroleum Coke by Xylene Displacement Standard Test Method for Microscopical Determination of Volume Percent of Textural Components in Metallurgical Coke S t a n d a r d Test Method for Trace Metals in Petroleum Coke by Atomic Absorption S t a n d a r d Test Method for Shear Viscosity of Coal-Tar and Petroleum Pitches S t a n d a r d Test Method for Trace Metals in Petroleum Coke by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) Standard Test Method for Measuring Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR) Standard Test Method for Determination of Crystallite Size (Lc) of Calcined Petroleum Coke by XRay Diffraction S t a n d a r d Test Method for Volatile Matter in Green Petroleum Coke Quartz Crucible Procedure S t a n d a r d Test Method for Determination of Trace Metals in Petroleum Coke by Wavelength Dispersive X-Ray Fluorescence Spectroscopy Standard Test Method for Carbon Black—Total and External Surface Area by Nitrogen Adsorption

REFERENCES [1] Westbrook, S. and LeCren, R., "Automotive Diesel and NonAviation Gas Turbine Fuels," Cii. 5, Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing, G. E. Totten, R. Shah, and S. R. Westbrook, Eds., ASTM International, West Conshohocken, PA, 2003. [2] Gruse, W. A. and Stevens, D. R., Ch. 12, Chemical Technology of Petroleum, McGraw-Hill, NY, 1960. [3] For, N., Stability Properties of Petroleum Products, Israel Institute of Petroleum and Energy, Tel Aviv, Israel, 1992. [4] Gray, M. R., Upgrading Petroleum Residues and Heavy Oils, Marcel Dekker, Inc., NY, 1994. [5] Rakow, M. S., "Petroleum Oil Refining," Ch. 1, Fuels and Lubricants Handbook: Technology, Properties, Performance, and

CHAPTER

29: PROPERTIES

OF FUELS,

PETROLEUM

Testing, G. E. Totten, R. Shaw, and S. R. Westbrook, Eds., ASTM International, West Conshohocken, PA, 2003. [6] Schmidt, P. F., Ch. 4, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [7] Schmidt, P. F., Ch. 5, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [8] Schmidt, P. F., Ch. 6, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [9] Petroleum Refining 1- Crude Oil, Petroleum Products, Process Flow Sheets, J.-P. Wauquier, Ed., Gulf Publishing Company, Houston, TX, 1995, p. 237. [10] Anonymous, "Making Sense Of Fuel Oil Viscosity," Marine Engineers Review, Vol. 56, July-August, 1999. [11] Schmidt, P. F., Ch. 7, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [12] Mushrush, G. W. and Speight, J. G., Ch. 11, Petroleum Products: Instability and Incompatibility, Taylor & Francis, Washington, DC, 1995. [13] Peyton, K. B., Ch. 5, Fuel Field Manual, McGraw-Hill, NY, 1997. [14] Schmidt, P. F., Ch. 8, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [15] Walsh, P. M. and Olen K. R, "Emission of U n b u m e d Coke from Combustion of Residual Fuel-Oil in Wall-Fired Electric Utility Boilers," Journal of The Institute of Energy, Vol. 66, 1993, pp. 140-146. [16] Schmidt, P. F., Ch. 9, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [17] Schmidt, P. F., Ch. 10, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [18] Miller, C. A., Linak, W. P., King, C , and Wendt, J. O. L., Combustion Science and Technology, Vol. 134, No. 1-6, 1998, pp. 477-502. [19] Schmidt, P. F., Ch. 13, Fuel Oil Manual, Fourth Edition, Industrial Press Inc., NY, 1985. [20] Martin, C. V. G., Proceedings of the Third World Petroleum Congress, Section VII, 1951, Institute of Petroleum, London, 1952, pp. 66-75. [21] Newman, J. W., Petroleum. Derived Carbons, ACS Symposium Series No. 21, M. L. Deviney and T. M. O'Grady, Eds., American Chemical Society, Washington, DC, 1976, pp. 52-62. [22] Dickakian, G., Petroleum Derived Carbons, ACS Symposium Series No. 303, J. D. Bacha, J. W. Newman, and J. L. White, Eds., American Chemical Society, Washington, DC, 1986, p p . 118-171. [23] Newman, J. W. and Newman, K. L., Ch. 6, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. RodriguezReinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [24] Gray, R. J. and Krupinski, K. C , Ch. 7, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. RodriguezReinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [25] Zander, M., Ch. 8, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [26] Andresen, J. M., Martin, Y., Moinelo, S. R., Maroto-Valer, M. M., and Snape, C. E., "Sohd State C NMR and High Temperature Determination of Bulk Structural Properties for Mesophase-Containing Semi-Cokes Prepared from Coal Tar Pitch," Carbon, Vol. 36, 1998, pp. 1043-1050. [27] Gray, R. J. and Krupinski, K. C , Ch. 7, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, a n d F. RodriguezReinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [28] Waller, J. H., Grimes, G. W., and Matson, J. A., Petroleum Derived Carbons, ACS Symposium Series No. 303, J. D. Bacha, J. W. Newman, and J. L. White, Eds., American Chemical Society, Washington, DC, 1986, pp. 144-154.

PITCH, PETROLEUM

COKE, AND CARBON

MATERIALS

785

[29] Beetz, Jr., C. P., Schmueser, D. W., and Hansen, W., "Summary of Panel Discussion, Challenges to the Researchers of Carbon-Fibers and Composites from the Automotive and Boatbuilding Industries," Carbon, Vol. 27, 1989, pp. 767-771. [30] Andresen, J. M., Garcia, R., Maroto-Valer, M. M., Moinelo, S. R., and Snape, C. E., "Characterization of Mesophase Develo p m e n t in Pitch by High T e m p e r a t u r e In Situ H NMR," Preprints., American Chemical Society, Division of Petroleum Chemistry, Vol. 4 1 , No. 3, 1996, pp. 621-624. [31] Fitzer, E., "The Future of Carbon-Carbon Composites," Carbon, Vol. 25, 1987, pp. 163-190. [32] Burchell, T. D., "Carbon Material for Advanced Energy Applications," International Conference on Carbon, Newcastle, UK, 1996, p p . 185-188. [33] Ehrburger, P., "Glassy Properties of Coal Tar Pitch Materials," Energeia, Vol. 5, No. 3, 1994, pp. 1-3. [34] Marsh, H., Introduction to Carbon Science, Butterworth, London, 1989. [35] Lahaye, L., Ehrburger, P., Saint-Romain, J. L., and Couderc, P., "Physicochemical Characterization of Pitches by Differential Scanning Calorimetry," Fuel, Vol. 66,1987, pp. 1467-1471. [36] Rand, B., Ch. 8, Handbook of Composites, Vol. 1, W. Watt and B. V. Perov, Eds., Elsevier, NY, 1989. [37] Dealy, J. M., Rheometers for Molten Plastics, Van Nostrand Reinhold Company, NY, 1982. [38] Cheung, T., Turpin, M., and Rand, B., "Controlled Stress, Oscillatory Rheometry of Mesophase-Pitches," Carbon, Vol. 33, 1985, pp. 1673-1679. [39] Bhatia, G., Aggarwal, R. K., Chari, S. S., and Jain, G. C , "Rheological Characteristics of coal Tar and Petroleum Pitches With a n d Without Additives," Carbon, Vol. 15, 1977, pp. 219-223. [40] Bhatia, G., Fitzer, E., and Kompalik, D., "Mesophase Formation in Defined Mixtures of Coal Tar Pitch Fractions," Carbon, Vol. 24, 1986, pp. 4 8 9 ^ 9 4 . [41] Nazem, F. F., "Flow of Molten Mesophase Pitch," Carbon, Vol. 20, 1982, p p . 345-354. [42] Turpin, M., Cheung, T., and Rand, B., "Controlled Stress, Oscillatory Rheometry of a Petroleum Pitch," Carbon, Vol. 32, 1994, pp. 225-230. [43] ICremer, H. A., "Recent Development in Electrode Pitch and Coal Tar Technology," Chemistry and Industry, Vol. 27, 1982, pp. 702-713. [44] Kuo, K., Marsh, H., Eind Broughton, D., "Influence of Primary QI and Particulate Matter on Pitch Carbonizations," Fuel, Vol. 66, 1987, pp. 1544-1551. [45] Marsh, H., Latham, C. S., and Gray, E. M., "The Structure and Behaviour of QI Material in Pitch," Carbon, Vol. 23, 1985, pp. 555-570. [46] Taylor, G. H., Pennock, G. M., Fitz G. J. D., and Brunckhorst, L. F., "Influence of QI on Mesophase Structure," Carbon, Vol. 31, 1993, p p . 341-354. [47] Romovacek, G. R., "Estimating the Concentration of Secondary Quinoline Insolubles," Carbon, Vol. 24, 1986, pp. 4 1 7 ^ 2 1 . [48] Menendez, R., Granda, M, and Bermejo, J., Ch. 9, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [49] Grjotheim, K., Krohn, C , Malinovsky, M., Matiasovsky, K., and Thonstad, J., Aluminium Electrolysis: Fundamentals of the Hall-Heroult Process, Aluminium-Verlag, Diisseldorf, 1982. [50] Gary, J. H. and Handwerk, G. E., Petroleum Refining: Technology and Economics, Marcel Dekker, NY, 1994, pp. 71-99. [51] Ellis, P. T. £ind Hardin, E. E., "How Petroleum Delayed Coke Performs in a Drum," Light Metals, 1993, pp. 509-513. [52] Adams, H. A., Ch. 10, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997.

786 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK [53] Eser, S., Jenkins, R. G., Malladi, M., and Derbyshire, F. J., "Carbonization of Coker Feedstocks and Their Fractions," Carbon, Vol. 24, 1986, pp. 77-82. [54] Mochida, I., Fujimoto, K., a n d Oyama, T., Chemistry and Physics of Carbon, Vol. 24, P. A. Thrower, Ed., Marcel Dekker, NY, 1994, pp. 111-212. [56] White, J. L., "Petroleum Derived Carbons," ACS Symposium Series No. 21, M. L. Deviney, and T. M. O'Grady, Eds., American Chemical Society, Washington, DC, 1976, p. 282. [57] Oya, A., Qian, Q. Z., and Marsh, H., Fuel, Vol. 62, 1983, p. 274. [58] Eser, S., Supercarbon: Synthesis, Properties, and Applications, Vol. 147, S. Yoshimura and R. P. H. Chang, Eds., SpringerVerlag, BerUn, 1998. [59] Eser, S. and Jenkins, R. G, "Carbonization of Petroleum Feedstocks. 1. Relationships Between Chemical Constitution of the Feedstocks and Mesophase Development," Carbon 27, 1989, pp. 877-887. [60] Eser, S. and Jenkins, R. G, "Carbonization of Petroleum Feedstocks. 2. Chemical Constitution of Feedstocks Asphaltanes and Mesophase Development," Carbon 27, 1989, pp. 889-897. [61] Fitzer, E., Kochling, K.-H., and Marsh, H., "Recommended Terminology for the Description of Carbon as a Solid-(IUPAC Recommendations 1995), Pure and Applied Chemistry, Vol. 67, 1995, pp. 473-506. [62] Rumsey, J. C. V. and Pitt, G. J., "Some Techniques for the Characterization of Cokes and Graphites," Fuel, Vol. 57,1978, p. 155. [63] Patrick, J. W., Reynolds, M. J., and Shaw, F. H., "Development of Optical Anisotropy in Vitrains during Carbonization," Fuel, Vol. 52, 1973, p. 198. [64] Sanada, Y., Furuta, T., Kimura, H., and Honda, H., "Formation of Anisotropic Mesophase from Various Carbonaceous Materials in Early Stage of Carbonization," Fuel, Vol. 52, 1973, p. 143. [65] Ragan, S. and Marsh, H., "The Influence of Oxidation upon Strength and Structure of a Needle-Coke and a Coal Extract Coke," Carbon, 1983, Vol. 2 1 , No. 2, p. 157. [66] Mochida, I., Korai, Y., Nesumi, Y., and Oyama, T., 'Carbonization in a Tube Bomb. 1. Carbonization of Petroleum Residue into a Lump of Needle Coke," Industrial & Engineering Chemistry Product Development,Yo\. 25, No. 2, 1986, p. 201. [67] Martin, S. W., Petroleum Products Handbook, V. B. Guthrie, Ed., McGraw-Hill, NY, 1960, pp. 14. [68] Pysz, R. W., Hoff, S. L., and Heitz, E. A., "Terminology for the Structural Evaluation of Coke via Scanning Electron-Microscopy," Carbon, Vol. 27, 1989, p. 935. [69] Qiao, G., "Digital Image Analysis of Needle Cokes and Other Solid Carbons and Their Thermal Expansion Behavior," Ph. D. Thesis, The Pennsylvania State University, May 2000. [70] Gray, R. J. and Devanney, K. F., "Coke Carbon Forms—Microscopic Classification and Industrial Applications," International loumal of Coal Geology, Vol. 6, 1986, pp. 277-297. [71] Mantell, C. L., Carbon and Graphite Handbook, John Wiley & Sons, NY, 1968, p . 520. [72] Page, D. J., The Industrial Graphite Engineering Handbook, UCAR Carbon Company Inc., 1991. [73] Reynolds, W. N., Physical Properties of Graphite, Elsevier, London, 1968, p . 80. [74] Kingeiy, W. D., Bowen, H. K., and Uhlmann, D. R., Introduction to Ceramics, John Wiley & Sons, NY, 1976, p. 591. [75] Hutcheson, J. M. and Price, M. S. T., Proceedings of the 4 * Carbon Conference, 1960, p. 645. [76] Sutton, A. L. and Howard, V. C , Journal of Nuclear Materials, Vol. 7, 1962, p. 58. [77] Price, R., Bokros, J. C , and Koyama, K., "Thermal Expansivities and Preferred Orientation of Pyrolytic Carbon," Carbon, Vol.5, 1967, p . 423. [78] Zimmer, J. E. and White J. L., 1 3 * Biennial Conference on Carbon, Extended Abstracts, 1977, p. 318.

[79] Mochida, I., Ogawa, M., and Takeshita, K., Bulletin of the Chemical Society of Japan, Vol. 49, No. 2, 1976, p. 514. [80] Collins, F. M., "Dimensional Changes During Heat-Treatment a n d T h e r m a l Expansion of Polycrystalline Carbons a n d Graphite," Proceedings, 3''''Conference on Carbon, 1957, p. 659. [81] Matsuo, H. and Sasaki, Y., "Relation between Anisotropy Ratio of Thermal Expansion Coefficient and Bacon Anisotropy Factor," Carbon, Vol. 12, 1974, p . 351. [82] Widmann, G. and Riesen, R., Thermal Analysis, Terms, Methods, Application, Heidelberg, Germany, 1987, p. 17. [83] Hole, M., Foosnaes, T., and 0 y e , H. A., "Relationship between Thermal Expansion and Optical Texture of Petrol Coke," Light Metals, 1991, p. 575. [84] Eilertsen, J. L., Rrvik, S., Foosnaes, T., and 0ye, H. A., "An Automatic Image Analysis of Coke Texture," Carbon, Vol. 34, 1996, p. 375. [85] Heintz, E. A., "Influence of Coke Structure on the Properties of the Carbon-Graphite Artefact," Fuel, Vol. 64, 1985, p. 1192. [86] Blyholder, G. and Eyring, H., Journal of Physical Chemistry, Vol. 61, 1957, p. 682. [87] Marsh, H., Taylor, D. A., and Lander, J. R., "Kinetic Study of Gasification by Oxygen a n d Carbon Dioxide of Pure and Doped Graphitizable Carbons of Increasing Heat Treatment Temperatures," Carbon, Vol. 19, 1981, p. 375. [88] Marsh, H. and Kuo, K, Introduction to Carbon Science, H. Marsh, Ed., Butterworths, London, 1989, p. 108. [89] Gregg, S. J. and Tyson, R. F. S., "The Kinetics of Oxidation of Carbon and Graphite by Oxygen at 500°-600°," Carbon, Vol. 3, 1965, p. 39. [90] Taylor, R., Ch. 4, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [91] Donnet, J. B., Bansal, R. C , and Wang, M.-J., Carbon Black Science and Technology, Second Edition, Marcel Dekker, NY, 1993. [92] Schwob, Y., "Acetylene Black: Manufacture, Properties, and Applications," Chemistry and Physics of Carbon, P. L. Walker and P. A. Thrower, Eds., 1982, p. 109. [93] Donnet, J. B. and Bansal, R. C , Carbon Fibers, Marcel Dekker, NY, 1984. [94] Murdie, N., Ch. 14, Introduction to Carbon Technologies, H. Marsh, E. A. Heintz, and F. Rodriguez-Reinoso, Eds., Universidad de Alicante, Secretariado de Publicaciones, Spain, 1997. [95] Johnson, D. J., Introduction to Carbon Science, H. Marsh, Ed., Butterworths, London, 1989, p. 197. [96] Edie, D. D. and Diefendorf, R. J., Ch. 2, Carbon-Carbon Materials and Composites, NASA 1254, Park Ridge, NJ, 1992. [97] Singer, L. S., "The Mesophase a n d High Modulus Carbon Fibers from Pitch," Carbon, Vol. 16, 1978, pp. 409-415. [98] Yoon, S.-H., Korai, Y., Mochida, I, and Kato, I., "The Flow Properties of Mesophase Pitches Derived from Methylnapthalene and Napthalene in the Temperature-Range of Their Spinning," Carbon, Vol. 32, 1994, pp. 273-280. [99] Mochida, I., Yoon, S.-H., Korai, Y., Kanno, K., Sakai, Y., and Komatsu, M., 'Carbon-Fibers from Aromatic-Hydrocarbons," Chemtech, Vol. 25, No. 2, 1995, p p . 29-36. [100] Cranmer, J. H., Plotzken, I. G., Peebles, L. H., and Uhlmann, D. R., "Carbon Mesophase-Substrate Interactions," Carbon, Vol. 21, 1983, pp. 201-207. [101] Meyer, R. A. and Gyetvay, S. R., Petroleum Derived Carbons, ACS Symposium Series No. 303, J. D. Bacha, J. W. Newman, and J. L. White, Eds., American Chemical Society, Washington, D.C., 1986, pp. 380-394. [102] Vohler, O., von Sturm, F., and Wege, E., Vlhnann's Encyclopedia of Industrial Chemistry, A5, VCH, Heidelberg, 1986, p . 98. [103] Heinz, E. A., "The Characterization of Petroleum Coke," Carbon, Vol. 34, 1996, p. 699.

MNL37-EB/Jun. 2003

Oxidation of Lubricants and Fuels Gerald J. Cochrac^ and Syed Q. A. Rizvi^

OXIDATION OF LUBRICANTS AND OXIDATION INHIBITORS

viscosity index for Group III Oils is >120. In general. Group II, Group III, and Group IV Oils are low in aromatic structures and structures with unsaturation. Hence, they oxidize at a slower rate than Group I Oils that are high in such structures. This is because such structures more readily form hydroperoxides and peroxy radicals that are essential to the oxidation process. Synthetic basestocks (Group V Oils) have oxidation rates that vary because of varying structures. Alkylaromatics, for example, contain aromatic rings, and hence oxidize faster than ester basestocks, which in turn oxidize faster than olefin oligomers (PAOs) that belong to Group IV. Vegetable oils oxidize at a fast rate as well because of the presence of unsaturation. The oxidatively susceptible structures in various basestocks are shown in Fig. 1. The hydrogens in the immediate vicinity of a r o m a t i c rings, double bonds, and oxygen atoms are most vulnerable to oxidative attack. Such hydrogens are shown in Fig. 2 and discussed in detail below.

MODERN LUBRICANTS PRIMARILY COMPRISE PERFORMANCE ADDI-

TIVES such as dispersants, detergents, oxidation inhibitors, viscosity modifiers, a n d others, blended in a base fluid (see chapter on Additives and Additive Chemistry). Many properties of the lubricant such as viscosity, viscosity index, slipperiness (reduced friction), film-strength, p o u r point, oxidation stability, volatility, and flammability, therefore depend on the properties of both the base fluid and the additive package. However, since the base fluid in a lubricant is present in excess of 70%, its contribution towards the listed properties predominates that of the additive package. Oils used to lubricate today's equipment, by virtue of being hydrocarbon based, are susceptible to oxidation. Oxidation results in the formation of polar compounds such as aldehydes, ketones, carboxylic acids, and oxygenated polymers. These products of oxidation are either corrosive or lead to the formation of resin, deposits, and sludge, all of which can impair the proper functioning of the equipment. The resin derived from lubricant oxidation consists of oxygenated oligomeric or polymeric molecules of approximately 500-1000 molecular weight. Deposits, on the other hand, comprise materials of a m u c h higher molecular weight. Sludge is a mixture of these materials with water, oil, and other contaminants. Oxidation resistance of a base oil leirgely depends upon the structure of the hydrocarbons present. Base oils are of mineral (petroleum) origin, synthetic chemical origin, or biological origin. "While mineral oil basestocks are obtained directly from petroleum fractionation, synthetic basestocks are manufactured through transformations of primarily petroleum derived organic chemicals. Base stocks of biological origin include vegetable oils and animal fats. API (American Petroleum Institute) has established five base oil categories on the basis of percent sulfur and percent saturates. Group I contains oils that have >0.03% sulfur and < 9 0 % saturates by mass. These oils have a viscosity index range of 80-120. Group II and III Oils, on the other hand, have cr-

^

1

4

1

6

1

10

Time (days) RCOOH + Metal

-»-

Metal Salts

F I G . 5 — M e c h a n i s m of h y d r o p e r o x i d e d e c o m p o s i t i o n .

(21)

F I G . 9 — C a t a l y t i c e f f e c t of Iron c a r b o x y l a t e o n t h e rate of oxidation.

CHAPTER Oil thickening occurs mainly due to polymerization or association of certain oxidation products. A model showing oxidative and thermal degradation of lubricants is shown in Fig. 11 [2]. During the t e r m i n a t i o n stage, t h e radicals either selft e r m i n a t e or t e r m i n a t e by reacting with oxidation inhibitors (Eqs 13-16, Fig. 3). The m e c h a n i s m involving self-termination leading to the formation of harmful carbonyl compounds is shown in Eq 18 of Figs 5. Oxidation inhibitors circumvent the radical chain mechanism of the oxidation process (Eqs 9-11 and 14-16, Fig. 3). Oxidation inhibitors can be Sulfur

F u e L ^ H2SO4

(29) (30)

R,C=0

-*-

Resins

Resins + Soot + Oil + H2O Resin + Soot

(31) —*"

Sludge

Resin Coated Soot

(32) »- Deposits

FIG. 10—Formation of harmful products.

(33)

30: OXIDATION

OF LUBRICANTS

AND FUELS

791

classified as hydroperoxide decomposers and radical scavengers, depending upon the mode of their controlling action. Sulfur and phosphorus-containing inhibitors, such as sulfides, dithiocarbamates, phosphites, and dithiophosphates, structures shown in Fig. 12, act as hydroperoxide decomposers. Nitrogen and oxygen-containing inhibitors, such as arylamines and phenols, structures shown in Fig. 13, act as radical scavengers. Hydroperoxide decomposers convert chain-propagating hydroperoxides to alcohols while they themselves become oxidized to higher oxidation levels. Sulfur compounds, represented by alkylsulfides, react with hydroperoxides and are converted into sulfoxides or sulfones. Sulfoxides can decompose thermally to form other sulfurcontaining products, such as sulfonic and sulfuric acids, which themselves are hydroperoxide decomposers. Nonhindered phenols also act as hydroperoxide decomposers and, in the process, are converted to polyhydroxy compounds [1]. See Parts 1 and 2 of Fig. 14 for the mechanism. Phosphorus compounds also act as hydroperoxide decomposers. Phosphines, rarely used as inhibitors because of their toxicity, are not as effective as other phosphorus derivatives. This is because they react stoichiometrically with hydroperoxides to form phosphine oxides that lack further oxidationinhibiting ability. Alkyl phosphites are better because during

FIG. 11—High-temperature lubricant degradation model.

792 MANUAL 37: FUELS AND LUBRICANTS

HANDBOOK

Sulfides

R-S-(s)-S-R

R-S-R Monosulfide

R = Olefin or fatty ester derived

Polysulfide

DIthlocarbamates

R.

-N-C,

R = Alkyl

Zn

CH,

R

S -12

Ashless Dithiocarbamate

Zinc Dithiocarbamate

Phosphites and DIthiophosphates

RO^ /^S

R0-. RO

R0"%

H

Diall*\A I 8 B

Flowmeter

mm

0000000 i 00000001

ill

ooooooo j

>-/ ® « ®

' r '. %^''m^

B

#

'fgJMvDSC Oxygen Cylinder FIG. 43a—ASTM D 5483, Grease Oxidation by PDSC. Test unit.

200

Typical PDSC Thermal

180 120 80 40

Sample: Grease A Size: 2.00 mg Temperature: 210°C Oxygen Flow: 100 mUminutes Induction Time: 42.4 minutes

EXO

0

ENDO 42.4

-40

209.8°C

-80 J 0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 38.0 40.0 44.0 Time (min) FIG. 43b—ASTM D 5483, Grease Oxidation by PDSC. Onset temperature.

808

MANUAL 3 7: FUELS AND LUBRICANTS

HANDBOOK

Wheel Bearing Lubricant Tester (Elevation View)

• tdclfontc pick up

Inboard 8«arlng

LM&704e Cone LM&7CI0 Cup

OulboTd Baannq-

LM11949 Cone LMII9I0 Cup

FIG. 44—ASTM D 3527, Wheel Bearing Life. Test apparatus.

gasoline automotive engines. This test is run at 160°C (320°F) and utilizes a high-pressure reactor pressurized with oxygen, along with a soluble metal catalyst package, a fuel catalyst, and water. These conditions partially simulate the environment to which an oil may be subjected in a gasoline combustion engine. The test oil is mixed with an oxidized/nitrated fuel component; a mixture of soluble metal naphthenates (lead, copper, iron, manganese and tin naphthenates), and distilled water. The oil mixture is placed into an oxidation vessel charged with oxygen to a pressure of 620 kPa (90 psig). The assembled pressure reactor is placed in an oil bath held at 1608C, and is rotated axially at a speed of 100 rpm. A thin film of oil is formed within the glass container by vessel rotation, result-

ing in effective oil-oxygen contact. The test is terminated when a rapid decrease of the reactor pressure is observed. The time when the pressure begins to decrease rapidly is called the Oxygen Induction Time, and is used as a measure of the oil's oxidation stability. This method is used to evaluate oxidation stability of lubricating base oils with additives in the presence of chemistries similar to those found in gasoline engine service. This test does not constitute a substitute for engine testing, which measures wear, oxidation stability, volatility, and deposit control characteristics of engine oil lubricants (see Figs. 45 and 46). Thermo Oxidation Engine Oil Simulation Test (TEOST) (MHT-4 Protocol)—This method is not an ASTM standard; it

CHAPTER is under consideration within an ASTM technical committee, but has not received all of the approvals required to become an ASTM standard. Savant Laboratories originally developed this test in the 1980s. In 1989, Savant, with cooperation of the Chrysler Corporation, developed a procedure that correlated with turbocharger deposits. The e q u i p m e n t b e c a m e k n o w n as the Chrysler/Tannas TEOST® apparatus. This test method describes the general oxidation and depositforming characteristics of engine oils at moderately high temperatures (MHT) of 285°C, using the TEOST apparatus. Using a TEOST test apparatus, a 10-gram sample of the engine oil containing an organo-metallic catalyst (lead, iron, manganese, tin and copper naphthenates) is forced to flow past a tarred, wire-wound depositor rod held in a glass mantled casing. The rod is resistively heated to obtain a constant

30: OXIDATION

OF LUBRICANTS

Drive Unit

FIG. 45—ASTM D 4742, Thin Film Oxygen Uptake Test (TFOUT). Test apparatus.

To Pressure Recorder Ctosure

0.64 cm

^'-^"^ ti Sample Container

O-Ring Seal TFE Cover Sample Container

Aluminum Insert

7.46 cm

6.03 cm Diameter Aluminum Insert

809

temperature of 285°C (545°F) for 24 h. During this time, dry air is forced to flow through the mantle chamber at a specific rate of 10 mL/minute. At the end of the test, the depositor rod and the components of the chamber are carefully rinsed of oil residue using a volatile hydrocarbon solvent. After drying the rod, the mass of the deposits is determined. The hydrocarbon solvent rinse is filtered and weighed, and the mass of the accumulated filter deposits is determined. The mass of deposits on the rod plus the mass of deposits on the filter is the total deposit mass. The mass of deposits that have accumulated on the inside surface of the mantle are also weighed. The maximum deposit requirements for this method Eire: ILSAC GF-2/Factor Fill = 60 mg; Chrysler Service Fill Only = 30 mg; API SJ = 60 mg. Table 6 shows typical test results for six oils, plus additional GF-3 prototype, used in round robin testing.

Bomb Stem

r

AND FUELS

(Bomb I.D. Is 6.03 cm)

FIG. 46—ASTIM D 4742, Thin Film Oxygen Uptal 150°C tests (ASTM D 4683, or D 4741, or D 5481) to ensure a m i n i m u m high-shear viscosity for each of the five SAE grades. KINEMATIC VISCOSITY/TEMPERATURE RELATIONSHIP In 1921, Neil MacCoulI created a viscosity - t e m p e r a t u r e chart (Fig. 26) for sale by The Texas Company [69]. The ordinate of the chart was the double logio of the kinematic viscosity plus a constant and the abscissa was the logio of the absolute temperature. This chart was used to graphically show the relationship between viscosity a n d t e m p e r a t u r e for mineral oils. It was stated that A close study of the various values of viscosity at different temperatures reveals the fact that temperature-viscosity relations can be so expressed by a simple formula that when plotted to the proper coordinates a straight line relationship is shown ... It has been found that there is no curvature to the line for any mineral oil so far tested, until the temperature has been reduced to the point where paraffine begins to precipitate . . . [69]. The equation used in the graph was logio (logio {v + constant)) = A - B logio T where,

pears that Walther may not have been aware of the prior work by MacCouU until 1929 when advised by Herschel [73]. The constant in the above equation is required for the hightemperature, low-kinematic-viscosity part of the chart. The constant was the subject of study in the 1930s, and various values from 0.6 to 0.8 have been used. However, as reported by W. Andrew Wright [74], . . . a constant of 0.6 was used down to viscosities of 1.5 cSt. Below this viscosity, the constant was changed to 0.65 for the range from 1.5 to l.OcSt, 0.70for l.Oto 0.7 cSt, and 0.75 for 0.7 to 0.4 cSt. This problem was the subject of an intensive study by Wright resulting in the development of an improved chart at very low kinematic viscosities and high temperatures. The equation for the new charts is listed in the appendix to ASTM D 341 93. In 1974, Manning suggested a modification of the equations by Wright. These equations, shown below, are also listed in the appendix to ASTM D 341 - 93, and have been widely used, especially in the computational fitting of kinematic viscosity-temperature data by computers [75]. This relationship is expressed as follows: logio logio Z = A - B logio T

(17)

Z = V -I- 0.7 + exp(-1.47 - 1.84v - 0.51 v^)

(18)

V = [Z - 0.7] - exp(-0.7487 - 3.295 [Z - 0.7] + 0.6119 [Z - O.lf - 0.3193 [Z - 0.7]^)

^^^^

(16)

v = kinematic viscosity, mm'^/s or cSt T = absolute t e m p e r a t u r e , degrees Rankine or Kelvin A, B = coefficients determined for the liquid.

In addition to charts prepared with kinematic viscosity in units of centistokes versus temperature in degrees Fahrenheit and degrees Centigrade (Celsius), charts were also prepared with the ordinate in Saybolt Universal seconds (SUS) and Saybolt Furol seconds (SFS). The abscissa of these charts is the logarithm of the absolute temperature and could be in degrees Fahrenheit or in degrees Celsius. The "constant" in the above equation is necessary if the kinematic viscosity is less than 1. Otherwise, the term logio (logio W)) cannot be evaluated because it would require the taking of a logarithm of a negative number. According to an article in Lubrication [70], the early charts had a constant varying from 0.4 to 1.0, and 0.7 was "considered to give the best overall results." The chart (see Fig. 26) was later published in the 1927 International Critical Tables [71], along with the following paragraph: Variation of Viscosity with Temperature—If the viscosity of an oil is known at two temperatures, its viscosity at a third temperature may be obtained graphically with the aid of [Fig. 26]. When the viscosity temperature values for any oil are graphed on this chart, a straight line will be obtained for all portions of the temperature range within which the oil remains a homogeneous liquid of constant composition [69]. Copies of this chart may be obtained by addressing The Texas Company, New York City. In the meantime, C. Walther published other viscosity-temperature charts in the late 1920s. After experimenting with several different equations, Walther adapted the equations of MacCouU, using a constant of 0.8 instead of 0.7 [72]. It ap-

V = kinematic viscosity, xnm^ls (or cSt) T = temperature, K (or °R) A and B = constants A review of the MacCoull viscosity-temperature charts is given in the May and June 1950 issues oi Lubrication. Various alternative charts have been proposed by others [76,77,78,79]. Andrew Wright was chairman of the ASTM D02 Subcommittee 7 on Flow Properties for a n u m b e r of years, and his paper [74] contains information on the development of the ASTM Viscosity-Temperature charts and their use within ASTM. The MacCoull chart has been published by ASTM since 1932, starting with the standard D 341 - 32T. Some liquids show a good relationship with the equation given in the Andrade [80] paper of 1930 (and also those of S. E. Sheppard, Ejring and others). This is of the form: 77 = A e^'T

(20)

The MacCoull equation is essentially of a similar form: 77 + 0.7 = A e'^'T'^

(21)

The SUS/°F charts were abandoned many years ago with the worldwide emphasis on the use of the System Internationale (SI) units in the specifications for petroleum products. For many years a segment of these viscosity-temperature charts has been used for blending two-component stocks. The constant-temperature method of blending requires the kinematic viscosity of each component at a given temperature. Using the chart from 0-100°F as if these temperatures represented 0 and 100% of the two components respectively, the kinematic viscosity is plotted on the charts, drawing a straight line between 0 and 100% of the components. Thus, the approximate kinematic viscosity of blends could be estimated from the straight line connecting the two components

CHAPTER 32: FLOW PROPERTIES AND SHEAR STABILITY

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