PCI20402

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Specifying Control Valves For Severe Service Applications

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Instrumentation File Reference: PCI20402

For additional information on this subject, contact E.W. Reah on 875-0426

Engineering Encyclopedia

Instrumentation Specifying Control Valves for Severe Service Applications

Contents

Pages

SELECTING CONTROL VALVE MATERIALS FOR CORROSIVE FLUID APPLICATIONS.....................................................................................1 Corrosion And Its Consequences..............................................................1 Basic Corrosion Mechanisms........................................................1 Common Forms Of Corrosion.......................................................2 Quantifying Corrosion Intensity..................................................36 Consequences Of Corrosion........................................................36 Corrosive Service Flags And Typical Corrosive Applications ...............37 Flags For Corrosive Fluid Applications ......................................37 Common Corrosive Fluid Applications.......................................38 Critical Control Valve Specification Considerations ..............................41 Selection Of Appropriate Valve Types .......................................41 Material Considerations ..............................................................42 Importance Of Specifying Specific Material Grades ..................48 Significance Of An Accurate Fluid Description..........................48 Significance Of Providing Accurate Service Conditions ............53 Resources For Control Valve Selection..................................................53 SAES-L-008................................................................................53 Vendor’s Corrosion Guidelines...................................................53 SELECTING CONTROL VALVES FOR EROSIVE FLUID APPLICATIONS................................................................................................55 Erosion And Its Consequences ...............................................................55 Common Forms Of Erosion ........................................................55 Quantifying Erosion Intensity .....................................................55 Consequences Of Erosion ...........................................................57 Saudi Aramco DeskTop Standards

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Erosive Service Flags And Typical Erosive Fluid Applications.............58 Flags For Erosive Fluid Applications..........................................58 Common Erosive Fluid Applications ..........................................61 Critical Control Valve Specification Considerations ..............................62 Control Valve Styles ...................................................................62 Materials Selection Considerations .............................................80 Sizing Issues................................................................................71 Information Sources................................................................................97 SELECTING CONTROL VALVE OPTIONS FOR HIGHTEMPERATURE FLUID APPLICATIONS .....................................................98 High Temperature Applications And Their Consequences.....................98 Categories Of High Temperature Applications ...........................98 Common Applications.................................................................98 Consequences Of High Temperature Fluids On Incompatible Components.........................................................100 Consequences Of Thermal Cycling Applications .......................79 High Temperature Service Flags ..........................................................107 Saudi Aramco Definition Of High Temperature .......................107 Thermal Cycling Flags ..............................................................107 Critical Control Valve Specification Considerations ............................108 Valve Design Considerations ....................................................108 Material Temperature Ratings...................................................109 Extended Bonnets For Packing Protection................................120 Achieving Tight Shutoff At Elevated Temperatures .................121 SELECTING AND SIZING CONTROL VALVES FOR CAVITATING FLUID APPLICATIONS.................................................................................123 Cavitation And Its Consequences .........................................................123

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The Cavitation Phenomenon .....................................................123 Cavitation Versus Other Flowstream Phenomenon...................124 Common Forms Of Cavitation ..................................................125 Consequences Of Cavitation .....................................................127 Predicting The Potential For Cavitation................................................130 Saudi Aramco And Manufacturer’s System Cavitation Indices .......................................................................................130 Subjective Factors For Analyzing The Potential For Cavitation Damage....................................................................133 Cavitation Service Flags And Typical Cavitating Applications ...........143 Flags For Cavitating Fluid Applications ...................................143 Specific Applications ................................................................143 Anti-Cavitation Valve Technology.......................................................144 General Anti-Cavitation Valve And Trim Design Strategies ...................................................................................144 Specific Anti-Cavitation Valve And Trim Designs...................147 Custom Valves ..........................................................................156 Control Valve Selection Considerations ...............................................159 Performance Objective: Cavitation Damage Control Versus Cavitation Prevention....................................................159 Manufacturers Control Valve Selection Procedures .................159 Valve Performance Contingency Requirements .......................161 Sensitivity To Accurate Data ....................................................161 Importance Of Defining Worst Case Cavitating Conditions .................................................................................164 Cavitation In Combination With Other Severe Conditions.......164 Anti-Cavitation Trim And Flashing Applications .....................164 Non-Valve Methods Of Reducing The Potential For Cavitation ..................................................................................165 ISA System Indices From ISA-dRP75.23.................................167 Saudi Aramco DeskTop Standards

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SELECTING AND SIZING CONTROL VALVES FOR FLASHING FLUID APPLICATIONS.................................................................................172 Flashing And Its Consequences............................................................172 Review Of Flashing Phenomenon.............................................172 Common Forms Of Flashing.....................................................174 Quantifying Flashing.................................................................178 Consequences Of Flashing ........................................................179 Flashing Service Flags And Typical Flashing Applications .................179 Flags For Flashing Fluid Applications ......................................179 Typical Flashing Fluid Applications .........................................179 Critical Control Valve Selection Considerations ..................................180 Basic Control Valve Selection Criteria .....................................180 Erosion Resistant Control Valve Types.....................................180 Materials Of Construction .........................................................183 System Design Considerations..................................................183 Valve Sizing Procedures ...........................................................184 Flashing In Combination With Particle Erosion Or Corrosion...................................................................................185 Importance Of Accurate Data....................................................185 SELECTING AND SIZING CONTROL VALVES TO ATTENUATE AERODYNAMIC CONTROL VALVE NOISE..............................................186 Sources Of Control Valve Noise...........................................................186 Types Of Control Valve Noise..................................................186 Mechanics Of Aerodynamic Noise Generation And Transmission .............................................................................189 Quantifying Noise Intensity..................................................................190 Measurement Parameters ..........................................................190 Measurement Units And Scales ................................................190

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Measurement Techniques..........................................................193 Consequences Of Control Valve Noise.....................................198 Flags For Excessive Noise And Common Noise Applications.............201 SPL> 90 dBA For A Standard Valve ........................................201 Outlet Velocity Greater Than 0.3 Mach ....................................202 P1/P2 > 5 For Dry Gas And Superheated Steam Services ........202 SPL > Limits That Are Established By Saudi Aramco Engineering Standards ..............................................................202 Specific Applications ................................................................202 Predicting Control Valve Noise ............................................................202 Introduction...............................................................................202 Influences On Noise Generation And Transmission .................203 Noise Prediction Equations .......................................................204 Control Valve Options For Attenuating Control Valve Noise ..............209 Source Treatments Vs. Path Treatments ...................................209 Valve Style Versus Noise Attenuation ......................................210 Body Options For Globe And Angle Valves.............................210 Noise Abatement Trim Design Strategies .................................210 Commonly Available Noise Abatement Valve Options............215 Characterizing Noise Abatement Trim......................................219 Common Selection Problems And Specification Errors.......................220 Absence Of Industry Standards For Noise Prediction Equations...................................................................................220 Specifier's Failure To Identify Worst Case Service Conditions .................................................................................221 WORK AID 1: FLUID COMPATIBILITY INFORMATION THAT IS USED TO SELECT CONTROL VALVES FOR CORROSIVE FLUID APPLICATIONS..............................................................................................222 Work Aid 1A: NACE Compliant Materials Of Construction ...............222 Saudi Aramco DeskTop Standards

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Work Aid 1B: Recommended Materials Of Construction For Seawater And Brine Services ...............................................................237 Work Aid 1C: Valve And Material Selection Guidelines For Amine (DGA) Letdown Applications...................................................242 WORK AID 2: HIERARCHICAL LISTINGS OF EROSION RESISTANT VALVE STYLES AND CONSTRUCTION MATERIALS ...................................................................................................243 Work Aid 2A: Hierarchy Of Erosion Resistant Valve Styles That Is Used To Select Control Valves For Erosive Fluid Applications.......243 Work Aid 2B: Hierarchies Of Erosion Resistant Body And Trim Materials That Are Used To Select Control Valves For Erosive Fluid Applications ................................................................................257 Hierarchy Of Erosion Resistant Body Materials .......................257 Hierarchy Of Erosion Resistant Trim Materials ........................260 WORK AID 3: PROCEDURES THAT ARE USED TO SELECT CONTROL VALVE OPTIONS FOR HIGH TEMPERATURE FLUID APPLICATIONS..............................................................................................269 Body And Bonnet Material Selection .......................................269 Trim Material Selection.............................................................269 Gasket Material Selection .........................................................269 Packing Material Selection........................................................269 Bonnet Type Selection ..............................................................269 Thermal Cycling Considerations...............................................269 WORK AID 4: PROCEDURES THAT ARE USED TO SELECT AND SIZE CONTROL VALVES FOR CAVITATING FLUID APPLICATIONS..............................................................................................245 Work Aid 4A: Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Control Component’s Inc. Sizing Software..............................................................................245 Preliminary Entries....................................................................245

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Entering Fluid Properties And Service Conditions ...................245 Design Information ...................................................................245 Change Menu ............................................................................245 Calculation Results....................................................................271 Work Aid 4B: Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Valtek’s Sizing Software .......271 Preliminary Entries....................................................................247 Project Identification .................................................................247 Valve Selection .........................................................................247 Valve Sizing ..............................................................................247 Work Aid 4C: Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Fisher Control’s Sizing Program ................................................................................................273 Preliminary Entries....................................................................273 Setting Options..........................................................................273 Data Entry And Sizing Calculations..........................................273 WORK AID 5: GUIDELINES FOR VALVE STYLE AND MATERIAL SELECTION AND PROCEDURES THAT ARE USED TO SIZE CONTROL VALVES FOR FLASHING FLUID APPLICATIONS..............................................................................................251 Work Aid 5A: Procedures That Are Used To Size Control Valves For Flashing Fluid Applications ...............................................251 Work Aid 5B: Guidelines For Valve Style And Material Selection That Are Used To Select Control Valves For Flashing Fluid Applications ................................................................................251 Valve Style Selection Guidelines ..............................................251 Body and Trim Material Selection Guidelines ..........................251 WORK AID 6: PROCEDURES THAT ARE USED TO SELECT AND SIZE CONTROL VALVES TO ATTENUATE AERODYNAMIC CONTROL VALVE NOISE ............................................................................253

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Work Aid 6A: Procedures That Are Used To Select And Size Noise Attenuating Control Valves With The Fisher Sizing Program ................................................................................................253 Preliminary Entries....................................................................253 Setting Options..........................................................................253 Data Entry And Sizing Calculations..........................................253 Work Aid 6B: Procedures That Are Used To Select And Size Noise Attenuating Control Valves With Control Components Sizing Software.....................................................................................255 Preliminary Entries....................................................................255 Entering Fluid Properties And Service Conditions ...................255 Design Information ...................................................................255 Change Menu ............................................................................283 Calculation Results....................................................................283 GLOSSARY.....................................................................................................284 ADDENDUM...................................................................................................289

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Selecting Control Valve Materials For Corrosive Fluid Applications Corrosion And Its Consequences Basic Corrosion Mechanisms Electrochemical Action - Most forms of corrosion can be viewed as an electrochemical

reaction. The basic mechanics of the electrochemical reaction are illustrated in Figure 1. Use Word 6.0c or later to

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Figure 1 Basic Corrosion Process When zinc is placed in dilute hydrochloric acid as shown in the above Figure, a vigorous reaction occurs; hydrogen gas is evolved and the zinc dissolves, forming a solution of zinc chloride; i.e.: Zn + 2 H+ → Zn+2 + H2 Deterioration (corrosion) of the zinc occurs at the area where the electrons leave the metal. This area is referred to as theanode and it is where damage is observed. The area to which the electrons migrate is thecathode. The cathode is a protected area and it is not subject to corrosion damage. In some corrosion reactions, the oxidation reaction occurs uniformly on the surface of the affected metal. In other cases, the corrosive reactions occur only at specific areas. The differences in the location of the electrochemical reaction

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provide a basis for categorizing the various forms of corrosion that will be discussed throughout this section. Passivation - When sufficient oxygen is available, some metals develop a protective oxide film or passive layer. The passive layer often adds considerably to the corrosion-resistance of the metal. Passivation requires the presence of oxidizing agents. The effectiveness of the passive layer depends upon the oxidizing power of the solution as shown in Figure 2. When the initial solution oxidizing power is low, the rate of corrosion increases as a direct function of the solution oxidizing power. At some point, the metal undergoes a transition from the active to the passive region. At this point, a dramatic reduction in the corrosion rate is observed. Further increases in the solution oxidizing power ultimately cause the material to lose its corrosion-resistance. Many of the alloys that are used in control valve assemblies achieve their corrosion resistance from the phenomenon of passivation. Use Word 6.0c or later to

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Figure 2 Passivation As A Function Of Solution Oxidizing Power Common Forms Of Corrosion Uniform attack , also known as general corrosion, occurs when the metal is

uniformly dissolved by the environment. Refer to Figure 3. As a result of Saudi Aramco DeskTop Standards

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uniform attack, the metal becomes thinner and thinner and eventually fails. During this type of corrosion, the corrosion product may form a protective layer on the metal surface; e.g., rust on iron or the passive layer that forms on some stainless steels. The protective layer may slow corrosion, or, the corrosion product may also be attacked (dissolved) by the corrosive media. Uniform attack can be prevented through the selection of corrosion resistant materials, through the use of protective coatings, or by adding corrosion inhibitors to the process fluid. Uniform attack, or general corrosion, is not of great concern from a technical viewpoint because fluid/material compatibility issues can be established by immersing a particular metal specimen in a particular fluid and measuring the material loss. The results of such tests are well documented and can be used to develop material/environment compatibility guidelines. Other, more localized forms of corrosion that will be discussed in this section present a greater challenge to the valve specifier. Use Word 6.0c or later to

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Figure 3 General Corrosion Pitting Corrosion - Pitting is an extremely localized attack that results in holes in

the metal as shown in Figure 4. Pitting is often difficult to detect because of the small size of the pits, and because the pits are often covered with corrosion products. Pitting is generally associated with the presence of chlorides in the flow stream. Carbon steels are not generally subject to pitting; however, the conditions that lead to pitting do generally result in unacceptable levels of Saudi Aramco DeskTop Standards

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general corrosion of carbon and alloy steels. Ironically, many stainless steels that are selected for their general corrosion resistance are particularly susceptible to pitting. To improve stainless steel’s resistance to pitting, increasing amounts of chromium, nickel, and/or molybdenum are included in the stainless steel; e.g., the 300 series stainless steels. In these alloys, the chromium and/or molybdenum combine with oxygen at the material surface to form a tough, adherent oxide layer that is resistant to attack in many environments. The passive layer may be damaged or removed by extremely high velocity flows or by direct chemical attack. A damaged protective oxide layer may reform (repassivate) if sufficient oxygen is available. If the film does not immediately reform (repassivate), pitting may occur. The initial attack is followed by penetration of the corrosive fluid into the metal. Use Word 6.0c or later to

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Figure 4 Pitting Corrosion

Crevice Corrosion is similar to pitting corrosion but it is observed in areas where

access to oxygen is restricted; e.g., in crevices, at gasket surfaces, and under deposits or biological organisms. Refer to Figure 5. When access to oxygen is restricted, the passive layer is either nonexistent or weak. Because reduced flow rates limit the available oxygen, low flow rates can also promote crevice corrosion. Reduced flow rates also increase the potential for scaling and fouling which results in oxygen-restricted areas where repassivation cannot occur. Crevice corrosion is likely to occur in any application where pitting is

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anticipated; however, crevice corrosion is likely to begin earlier and to produce more dramatic damage than pitting corrosion. Use Word 6.0c or later to

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Figure 5 Crevice Corrosion

Erosion-Corrosion - The passive layer can be damaged by high velocity flows or

by the impingement of erosive particles on material surfaces. If the passive layer becomes eroded, the base material is exposed directly to the environment and the rate of corrosion may increase. Refer to Figure 6. To produce a tougher, more adherent passive layer that can resist erosion, materials that include increased amounts of chromium and molybdenum are specified. It is generally acknowledged that molybdenum is more influential than chromium in increasing erosion-corrosion resistance. Another solution to erosion-corrosion is to select materials that do not depend upon the passive layer for corrosion protection; for example, the Monels (N04400 and N05500) and Hastelloy B2 (N10665) do not include chromium and do not depend upon the passive oxide layer for corrosion resistance.

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Use Word 6.0c or later to

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Figure 6 Erosion Corrosion

Intergranular Corrosion is a form of corrosion in which the material along the

grain boundaries of the metal is removed. Refer to Figure 7. Intergranular corrosion begins with a phenomena known as sensitization. Sensitization is a process in which exposure to high temperature causes corrosion resistant alloys to precipitate out of the material matrix, leaving a zone at the grain boundary that is not protected from corrosion attack. For example, in some stainless steels (primarily the 300 series) that have been subjected to high heat from welding, chromium carbides may precipitate at the grain boundaries thus depleting the chromium from the immediately adjacent material. In a corrosive environment, the area of the grain boundary that has been depleted of chromium is susceptible to attack by the corrosive atmosphere. The corrosion that results is known as intergranular corrosion.

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Figure 7 Intergranular Corrosion

Galvanic Corrosion - When two dissimilar metals are immersed in a conductive

solution, an electron flow may be established between the two. The standard carbon-zinc battery that is shown in Figure 8 is a familiar example of electron flow between two dissimilar metals. If electron flow is established, galvanic corrosion of the less corrosion-resistant material (the anode) will occur.

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Use Word 6.0c or later to

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Figure 8 Carbon Zinc Battery As An Example Of Electron Flow Between Two Metals

As a general rule of thumb, the potential for galvanic corrosion increases as the separation between the two metals on agalvanic series increases. A galvanic series is a list of metals that is ordered according to the relative magnitude of the electrical potential that each produces when paired with a reference electrode. Refer to Figure 9. The metals near the top of the list are considered “noble”, or cathodic (less likely to give up electrons and therefore less susceptible to corrosion). The metals at the bottom of the list are considered to be active or anodic (they are more likely to give up electrons and therefore more susceptible to corrosion). Electron flow can be established between two metals either through direct contact or through an electrolyte (a conductive medium). The process fluid can serve as an electrolyte. When a circuit is completed between two metals that are close together in the galvanic series, the potential for corrosion is small or non-existent. As the separation of two paired metals on the chart increases, the potential for electron flow and corrosion increases dramatically.

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Instrumentation Specifying Control Valves for Severe Service Applications Platinum

Gold Graphite Titanium Silver Chlorimet 3 Hastelloy C 316 stainless steel (passi ve) 304 stainless steel (passi ve) Inconel Nickel Monel Bronzes Copper Brasses Hastelloy B Inconel (active) Nickel (active) Tin Lead 316 stainless steel (active )

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Instrumentation Specifying Control Valves for Severe Service Applications 304 stainless steel (active ) Cadmium Aluminum Zinc Magnesium

Figure 9 Galvanic Series Of Common Materials In Seawater Use Word 6.0c or later to

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Figure 10

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Zinc Washers As Sacrificial Elements To Protect Stainless Steel Valve Stems

SCC , or stress corrosion cracking, occurs when certain alloys are exposed to

specific environments and the affected component is subjected to tensile stress. Tensile stress is present in virtually all components. Tensile stress may be the result of process pressure that is exerted on a valve component, misalignment of piping, thermal expansion, and from the residual stress of cold work, welding, or heat treatments. Examples of alloy-environment pairs that are susceptible to SCC are listed in Figure 11. The concentration of the environment, the operating temperature, and the operating pressures impact the extent of SCC. In sulfide stress cracking (SSC), the corrosive action is most intense at ambient temperatures because at low temperatures the diffusion process is slowed, and, at elevated temperatures the diffusion rate is so fast that a critical concentration is never reached. Withchloride stress cracking, which is commonly encountered in deep, sour wells and in seawater and brine applications, SCC occurs at temperatures above 130 degrees F. The steps that are taken to prevent sulfide stress cracking are embodied in a guideline titled NACE MR0175 that will be discussed later. Use Word 6.0c or later to

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Envir

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Chlorid

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Hard

Hydrog

Hard

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Figure 11 Mechanics Of Stress Corrosion Cracking

The susceptibility of a material to SCC is related to its hardness level. Hardness is a physical property that relates the resistance of a material to penetration or indentation. In metals, hardness is usually measured in the laboratory by loading an indenter into a material and measuring either the depth or the surface area of the indentation. Several test procedures and scales of hardness have been established. A popular scale is the Rockwell C scale, which is abbreviated as HRC (Hardness Rockwell C). The range for the Rockwell C scale is from HRC 20 to HRC 60. For reference, hardness levels of some common materials are listed in Figure 12.

M

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Figure 12 Hardness Levels Of Some Common Materials

Figure 13 illustrates the relative time to failure (in hours) of bolting materials with varying hardness levels. Because of the relationship of hardness levels and SSC, the hardness of valve construction materials must be less than allowable hardness levels that have been determined by test and evaluation. Use Word 6.0c or later to

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Figure 13 Effects Of Material Hardness On Failure Caused By SCC

NACE MR0175 - The National Association of Corrosion Engineers (NACE) has

issued Standard MR0175 that specifies the proper materials, the heat-treating conditions, and the strength levels that are required to provide good service life in sour gas and oil environments.

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Figure 14 lists some of the NACE approved materials, hardness information, and pertinent remarks. Note that the maximum hardness that is allowed under the NACE guidelines depends on the material type.

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Figure 14 Common NACE Approved Materials NACE MR0175 does not address elastomer and polymer materials. However, the importance of these materials for critical sealing must be considered. User experience has shown that nitrile, neoprene, and PTFE can be applied within their normal respective temperature ranges. Because hardness is a required property for a spring, the NACE MR0175 specifications for maximum material hardness make it difficult to manufacture NACE compliant springs. Most manufacturers offer a limited number of material options when NACE compliant springs are required. To solve the problem of spring selection of control valve packing arrangements, jam-style packings that do not require a spring are typically specified. According to NACE MR0175, NACE compliant external bolting must be specified whenever the bolting will be deprived of contact with the atmosphere. External bolting includes the bonnet-to-body bolting, packing flange bolting, and line flange bolting. Conditions that deprive the bolting of access to the environment include the use of insulation, flange protectors, or burial of the valve. Quantifying Corrosion Intensity Because there is no standard "corrosion coefficient" upon which to base valve and material selection decisions, corrosion intensity is generally discussed in subjective terms. For example, corrosion intensity is often expressed as mild, moderate, or extreme. In advanced corrosion engineering studies, other parameters such as the millimeters of material lost per some unit of time provide a more precise index of a material’s corrosion resistance to specific fluids. Consequences Of Corrosion Body Damage - Figure 15 illustrates that corrosion damage to control valve bodies can range from thinning of the body wall to loss of gasket surface integrity to total failure of the body. Trim Damage - Figure 15 also illustrates several different forms of trim damage. Any loss of material at seating surfaces will degrade the ability of the valve to shutoff. Material removal may also enlarge plug-to-cage and stem-to-bushing clearances thereby allowing vibration of the valve plug and progressive damage to the plug and seat. Crevice corrosion may be observed on gasket surfaces, on the portion of the valve stem that is in the packing bore, and at other stagnant areas within the valve. SCC and intergranular corrosion are generally seen as small leaks before they result in catastrophic failures.

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Figure 15 Common Forms Of Corrosion Damage Corrosive Service Flags And Typical Corrosive Applications Flags For Corrosive Fluid Applications Flags, or clues that a particular process is corrosive can come from a variety of sources. Several flags are discussed below. Specific Applications And Fluids - Seasoned valve specifiers learn from

experience that certain applications involve corrosive fluids. For example, all sour hydrocarbons, sour water, seawater, amine stripping processes, and boiler feedwater control are universally recognized as applications that can present significant corrosion challenges. Construction Materials Of Related Equipment - If the piping, pumps, block valves,

instruments, and other equipment in the control loop are made of corrosion resistant materials, the specifier has received a clear signal that the process fluid is corrosive. Corrosion Guides And Compatibility Charts - Specifiers may consult various

corrosion guides and material compatibility charts to determine if a specific fluid will present the potential for corrosion with standard valve materials. Saudi Aramco DeskTop Standards

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Common Corrosive Fluid Applications Sour Services (NACE) - Many crude oils and natural gasses contain hydrogen

sulfide; therefore, sulfide stress cracking is very common in most gas and oil producing operations. In gas and oil production, SSC and SCC may be encountered in any application until all the sulfides and the chlorides have been removed. Refer to Figure 16. Use Word 6.0c or later to

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Figure 16 Sour And Sweet Processes In Gas And Oil Production

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As discussed previously, the susceptibility of a material to any form of SCC is related to its hardness level. Some valve manufacturers have established standard policies and practices that ensure compliance with NACE guidelines whenever a valve is specified for sour service. For example, the following summarizes the procedures that are followed by Fisher Controls. •

Carbon steel bodies and bonnets are heat treated to 22 HRC maximum, and they are post-weld heat-treated.

•

Hardened martensitic stainless steels are not used.

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Control valve packing sets are jam style only (springless or externally liveloaded).

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Valve stems are made from Nitronic 50 when higher strength is required.

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Primary trim materials are S31600 and Alloy 6.

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No machining operations that cause work hardening of the materials are performed in the manufacturing process.

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Platings and coatings are appliedover NACE approved base metals, and the coatings are not intended to provide corrosion protection.

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Bolting in Class III material is standard when the bolting is not exposed to the sour atmosphere. Bolting in Class I and Class II material is available when bolting is buried, insulated, or otherwise exposed to H 2S.

Most valve manufacturers offer specific valve constructions and/or trim options that comply with the NACE guidelines. Refer to Table 7 in Bulletin 51.1:ES (Fisher Catalog 71) and note the standard trim options that are NACE approved. Seawater - Seawater is commonly injected for recovery purposes. Regardless

of the application, seawater can present difficult and complex problems for the materials specifier. The concerns for corrosion include the following: •

General corrosion will be observed in many carbon and low alloy steels.

•

Pitting and crevice corrosion is common in many stainless steels.

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•

Above 160 degrees F, chloride stress corrosion cracking may develop. Chloride SCC disallows the use of any 300 series or 400 series stainless steel.

•

Because the seawater that is used in secondary recovery oilfield operations is often high in NaCl, CO2, and H2S, the NACE guidelines must be observed.

Brine - Brines are commonly used as refrigerants or low temperature heat

exchange media. In these recirculating applications, brines are often treated with corrosion inhibitors. At temperatures below 160 degrees F, corrosion inhibitors may prevent corrosion of carbon steel bodies and bonnets and 300 series stainless steel trim components. At temperatures above 160 degrees F, chloride SCC will occur and materials should be selected accordingly. High Pressure DGA - A popular process for removing acid gasses from natural gas involves stripping the gas with an amine such as diglycolamine (DGA) or diethanolamine (DEA). DGA stripping is common in Saudi Aramco operations. As shown in Figure 17, lean liquid amine enters the top of the tower and it flows downward across the trays. As the acid-rich gas flows upward, the amine absorbs H2S and CO2 from the natural gas. Clean gas exits the top of the tower and acid-rich amine leaves the bottom of the absorption tower. The rich amine passes through a letdown valve into a flash tank where a portion of the of the absorbed gasses flash out of the liquid. Following the flash tank, the amine passes through a regeneration process. The regenerated amine is reused in the process.

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Use Word 6.0c or later to

view Macintosh picture.

Figure 17 Typical Amine (DGA) Adsorption Process

During amine letdown, large amounts of entrained gas will come out of solution. The process, which is referred to as “outgassing”, results in twophase flow. One phase is the liquid amine and the other phase is the gaseous CO2 and/or H2S that has flashed out of the amine solution. The amine itself does not pose a corrosion problem; however, wet CO 2 can result in the formation of carbonic acid which is highly corrosive to carbon steel (not to stainless steel). Also, the presence of H2S results in the potential for stress corrosion cracking. Materials selection for sour service is not changed by the presence of CO2 and the NACE guidelines should be followed. Critical Control Valve Specification Considerations Selection Of Appropriate Valve Types When selecting valves for corrosive fluid applications, specifiers may select either standard alloy valves or they may choose to evaluate lined valves. Lined Valves - Lined valves are made from an inexpensive base metal such as carbon steel to which a non-metallic coating, cladding, or lining is applied in order to achieve corrosion resistance. Control valve body liners and trim coatings of polyvinyl chloride, rubber, PTFE, and other elastomers are

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common. All liners have limitations in terms of temperature ratings, permeability, vibration sensitivity, susceptibility to mechanical damage, thermal degradation, and adherence of the lining or coating to the base metal. Generally speaking, lined valves are selected only for highly corrosive fluids under the conditions of low pressure, low pressure drop, and low to moderate temperatures. Alloy Valves - Because of the flow, pressure, and temperature conditions that

are characteristic of most Saudi Aramco operations, standard control valve constructions with corrosion-resistant material options are generally preferred over lined valves. The common limiting factors are the availability of the desired material options for the selected valve, and the high cost of increasingly corrosion resistant alloys. For example, an alloy valve that is resistant to sour seawater may cost 3 to 4 times as much as the same valve with standard material options. (WCB body and bonnet and 316 stainless steel trim). Material Considerations To provide good performance and long life, control valve components (bodies, bonnets, trim, packing, and gaskets) must be selected of materials that are resistant to the prevailing environment. The available materials are numerous and the subject of proper material selection can easily become a career study. Fortunately, Saudi Aramco standards and vendor supplied suggestions are available to assist the specifier. It is useful to acquire a fundamental understanding of the corrosion resistance of some of the popular material options. For the discussion that follows, refer to the item in the Addendum that is titled “Composition, Characteristics, And Typical Uses For Common Control Valve Materials”. Stainless Steels - Stainless steels are the most commonly selected materials for

corrosion service applications. The broad range of materials that are commonly referred to as stainless steels (SST’s) can be segmented according to their basic structure and according to the alloying elements that are included in the composition of the material. The popular stainless steels for valve components are the martensitics, austenitics, precipitation hardened stainless steels, and duplex stainless steels. •

Martensitic stainless steels (the 400 series stainless steels) were the first stainless steels to be developed. Engineers and metallurgists soon noted that the addition of 12 percent chromium imparted greatly improved corrosion and oxidation resistance to steel. The improved corrosion resistance results from the chromium that produces a protective passive layer. Compared to other types of stainless steel, martensitics have a relatively high carbon content. The addition of carbon to a stainless steel

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results in increased hardness and an increased susceptibility to sensitization and SCC. Martensitic stainless steels can achieve great strength and hardness through various heat-treatments. •

Austenitic stainless steels (the 300 series stainless steels) typically provide increased corrosion resistance as compared to the martensitics. Type 304 stainless steel is sometimes referred to as an “18-8” because its composition includes 18 percent chromium and 8 percent nickel. The increase in chromium content results in a better chromium oxide passive later than a 400 series stainless steel. If the chromium oxide passive layer becomes damaged and pitting attack occurs, the nickel content provides increased resistance to further penetration. Because chromium increases resistance to oxidizing environments and nickel increases resistance to reducing environments, the austenitic stainless steels are resistant to a broader range of environments than the martensitic stainless steels. The 316 stainless steels also include Molybdenum which makes the passive layer tougher and more adherent, thereby increasing the material’s resistance to pitting in reducing environments. Austenitic stainless steels are hardened by cold work but they cannot be hardened by heat treatments.

•

Super-Austenitic stainless steels are those that include increased alloy content. One of the most popular super austenitic stainless steels is Avesta 254 SMO. It should be pointed out that Avesta is the name of a Swedish steel company that manufactures many alloys and “Avesta” is not a specific alloy designation. For clarity when describing this alloy, specific material designations are most appropriate. Super austenitics are sometimes referred to as the “6 Mo’s”, referring to the nominal 6 percent molybdenum content of the material. The increased chromium content provides a tougher, more adherent passive layer, the increase in nickel provides increased pitting resistance in reducing environments, and the increased molybdenum and addition of nitrogen increase the resistance to pitting by chlorides.

•

Precipitation-Hardened stainless steels can be heat treated to achieve great strength and high hardness levels. 17-4PH is the material that is most commonly used for control valve components. This material is commonly heat treated to a variety of conditions. The H900 condition is the hardest and the strongest and has been popular for many valve trim components; however, because of the exceptional hardness, material in the H900 condition is susceptible to stress corrosion cracking. To minimize SCC problems, the H1075 condition has become popular.

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Procedure

Heat to 900 degrees F for 1 hour, air cool

Heat to 1,025 degrees F for 4 hours, air cool

Heat to 1,075 degrees F for 4 hours, air cool

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Heat to 1,100 degrees F for 4 hours, air cool

Heat to 1,150 degrees F for 4 hours, air cool

Heat to 1,150 degrees F for 4 hours, air cool to ambient, reheat to 1,150 degrees F for 4 hours, air cool

Heat to 1,400 degrees F for 4 hours, air cool to ambient, reheat to 1,150 degrees F for 4 hours, air cool

Figure 18 Common Heat Treatment Procedures

•

Duplex stainless steels are becoming increasingly popular because of their superior resistance to general corrosion and SCC in both sour and chloride environments. In addition, the yield strength of the duplex stainless steels is approximately double that of the annealed austenitic stainless steels. A major consideration is that the common duplex stainless steels are NACE

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approved in the wrought form only; only one duplex (Z6CNDU20.08M) is currently NACE approved in the cast form. Nickel Base Alloys - A broad range of nickel base alloys are available for

specific corrosive environments. In general, these alloys are known as the Monels, the Inconels, and the Hastelloys; however, it is imprecise and potentially dangerous to specify a nickel alloy without giving the full description. For example, instead of describing an alloy as “Hastelloy”, the specifier should give the complete description; e.g., N06022 (Hastelloy C22 in the wrought form). •

Nickel-Copper Alloys - The first nickel-copper alloys were known as the “Monels”. Nickel-copper alloys are the industry standards for dry chlorine and hydrogen chloride gasses, hydrofluoric acid, and oxygen. They are also selected for brine and sea water applications where chloride stress corrosion cracking of S31600 is a problem. N04400 (alloy 400) is the most common grade and it is often used as a soft gasket material. N05500 (alloy K500) is a high strength, age hardenable grade and is routinely used as a high-strength shaft and stem material.

•

Nickel-Chromium Alloys - The nickel-chromium alloys were originally marketed under the Inco tradename Inconel 600 (N06600 or alloy 600). Because alloy 600 does not include Molybdenum, its corrosion resistance is poor in comparison to many other nickel-based alloys.

•

Nickel-Iron-Chromium Alloys- The most common alloy in this group is known as Carpenter 20, Alloy 20, or alloy 20Cb-3. Alloy 20 is the industry standard for sulfuric acid below 160 degrees F and it is often selected for chloride environments such as brine and sea water.

•

Nickel-Molybdenum Alloys- The most popular alloy in this family is known as Hastelloy B2 (N10665). N10665 has excellent resistance in all concentrations and temperatures of hydrochloric acid; however, if ferric or cupric ions are present, severe attack will occur. N10665 is also compatible with hydrogen chloride, sulfuric acid, acetic acid, and phosphoric aid.

Cobalt Base Alloys - The most common cobalt base alloy is often referred to as

Alloy 6 or Stellite. The correct designations are R30006 for castings, CoCr-A for rod and powder (the materials that are used to apply hardfacings), and alloy 6B for wrought forms. Alloy 6 has good corrosion resistance in a variety of environments but it is inferior to most corrosion resistant nickel base alloys. Alloy 6 performs well whenever S31600 is acceptable and it is compatible with Saudi Aramco DeskTop Standards

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steam, sea water, and brine applications. The various forms of Alloy 6 may be used when, in addition to corrosion resistance, erosion resistance is required. Elastomers - Elastomers that are compatible with corrosive service applications are generally not difficult to identify. The most common elastomer components are packing rings and valve plug seal rings. Standard PTFE and PTFE-based materials are compatible with most corrosive fluids. Coatings and Platings - Coatings and platings may be applied to base metals to

improve either the corrosion resistance or the erosion resistance of the base metal. Corrosion resistant coatings are generally applied to carbon and low alloy steels or to aluminum. Wear resistant coating are generally applied to prevent wear, or to prevent galling when a single corrosion resistant base metal is selected for components that are in a sliding wear application; e.g., a stainless steel plug and a stainless steel cage. •

Hard chromium platings are deposited by an electroplating process that requires an aqueous solution, electrodes, and an applied current. Chromium platings can exhibit very high hardness (equivalent to 70 HRC). However, all hard chromium platings include small cracks that allow corrosive fluids to contact the base metal. Accordingly, the plating offers little or no corrosion resistance and the base metal must be compatible with the environment. Chromium is compatible with steam, boiler feedwater, and dry gasses. Hard chrome plating is often applied to actuator diaphragm rods, cages, and other components that require wear resistant surfaces at temperatures up to 600 degrees F.

•

Electrolyzing is a proprietary method of applying a hard, chrome coating. The chromium deposit that results from the Electrolyzing process is referred to as a “coating” rather than a “plating”. While the traditional chrome plating process and the Electrolyzing process are similar, the coating displays improved performance. The coating retains good wear resistance and hardness at temperatures up to 1100 degrees F while the upper temperature limit for traditional hard chrome plating is 600 degrees F. Chrome coatings also provide superior galling resistance compared to traditional hard chromium plating.

•

Electroless Nickel Coating (ENC)is applied in much the same way as other platings except that electrodes and an applied current are not used. The coating is very homogenous with no crystalline structure; it is actually a metallic glass. ENC deposits are more uniform than conventional platings and they will uniformly cover small holes. Traditional platings cannot

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uniformly cover small holes in the base metal’s surface. ENC is applied to plugs and cages in order to improve wear resistance and to prevent galling. Its corrosion resistance is similar to that of 316 stainless steel. •

Nitriding is a process in which the base metal is heat treated (in a furnace) in the presence of a specific chemical atmosphere. During the process, nitrogen ties up the chromium in the alloy to form chromium nitrides and other compounds at or near the surface. This layer improves the surface hardness and overall wear properties of the treated material. However, because chromium is the primary element that produces the excellent corrosion resistance of stainless steels; all forms of nitriding severely degrade the corrosion resistance of the stainless steels. Because of the excellent wear resistance and the poor corrosion resistance that results from nitriding, nitriding is typically specified for components that will be exposed to fluids such as steam, boiler feedwater, organic solvents, and dry gasses such as nitrogen, argon, and methane.

Importance Of Specifying Specific Material Grades Failure to specify a specific material grade is a common error in valve specification. For example, a specification for “stainless steel” trim would be satisfied by any of a large number of 300 and 400 series stainless steels that have widely differing characteristics. Another example of an incomplete material specification is when a material is specified simply as “Hastelloy”. There are more than 25 different grades of Hastelloy and each has unique properties. A more complete specification would include the specific grade; e.g., Hastelloy B2, Hastelloy C276, Hastelloy G30, and so forth. Significance Of An Accurate Fluid Description Problems in valve specification are often traced to an incomplete or imprecise description of the process fluid. Figure 19 lists some common fluid descriptions and notes concerning the nature of the problems that can result.

Comment

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Rich DGA

includ es H2S and CO2, is highly corro sive, and is subje ct to NAC E guidel ines.

Lean DGA does not requir e NAC E compl iant mater ials. DGA, DEA, and MDA are

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Instrumentation Specifying Control Valves for Severe Service Applications used in hydro carbo n proce ssing opera tions. Thes e fluids are often assoc iated with SCC and erosio n probl ems. Alloy 6 (Stelli te) is often select ed becau se of its corro sion and erosio n resist ance. Amines such as hydra zine in boiler feedw ater applic ations attack

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Instrumentation Specifying Control Valves for Severe Service Applications and corro de Alloy 6. 440C or S440 04 stainl ess steel trim is often select ed for this applic ation.

Temperature and conce ntrati on have a prono unced affect on corro sion intens ity and mater ial select ion.

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Figure 19 Common Problems With Inaccurate Descriptions Of Fluids

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Significance Of Providing Accurate Service Conditions The corrosion resistance of a particular alloy to a particular fluid is often a strong function of the concentration and temperature of the fluid. For example, the alloys that are commonly selected to resist corrosion by hydrofluoric acid are show in Figure 20. Note that at high concentrations and low to moderate temperatures, a WCB cast steel body is acceptable and that at reduced concentrations, more exotic alloys are required to provide the needed corrosion resistance. Failure to provide accurate service data for this fluid could easily lead to the selection of totally incompatible materials. Use Word 6.0c or later to

view Macintosh picture.

Figure 20 Material Compatibility Chart For Hydrofluoric Acid Resources For Control Valve Selection SAES-L-008 The Materials Appendix that is included in SAES-L-008 provides a great deal of material compatibility information to the valve specifier. Compared to other compatibility tables and chars, the Materials Index in SAES-L-008 is unique because it includes (1) a listing of compounds that are particularly germane to Saudi Aramco operations and, (2) some temperature and concentration information. Vendor’s Corrosion Guidelines Most valve manufacturers also provide fluid compatibility information in the form of charts, tables, applications guides, and so forth. The limitations of most general compatibility charts is that they do not include information that relates to temperature, concentration, or wearSaudi Aramco DeskTop Standards

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couple compatibility. In addition, it must be emphasized that valve manufacturers do not recommend materials, but rather suggest material specifications. Most manufacturers believe that the user is more knowledgeable of the process than a vendor.

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Selecting Control Valves For Erosive Fluid Applications Erosion And Its Consequences While corrosion is an electrochemical action, erosion - in its simplest form - is mechanical damage that results in the gradual destruction of a material. Common Forms Of Erosion Erosion damage results from the impingement of solid particles, liquids, or liquid droplets on the target material. The various forms of erosion are shown in Figure 21 and they are discussed below. Use Word 6.0c or later to

view Macintosh picture.

Figure 21 Common Forms Of Erosion

Particle Erosion results when solid particles such as fines, soot, sand, dirt, or

scale impinge on material surfaces. Flashing Erosion results when a liquid falls below its vapor pressure and some

portion of the liquid vaporizes. The velocity of the vapor phase can increase significantly. The liquid particles, driven by the high-velocity vapor, can impact valve components and result in significant erosion damage. Erosion/Corrosion - While it is convenient to discuss erosion as an independent

phenomena, erosion generally occurs simultaneously with corrosion. Each independent phenomenon accelerates the other. Quantifying Erosion Intensity Saudi Aramco DeskTop Standards

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There is no erosion coefficient, no industry standard, and no scientific means for predicting the occurrence or intensity of erosion damage. However, a better understanding of the potential for erosion can be gained by evaluating the parameters that increase the potential for erosion in a given application. These parameters are shown in Figure 22. The fluid parameters that influence the potential for erosion damage include the size of the particles, the sharpness of the particles, the volume ratio of particles in the fluid stream, the angle of particle impingement on a material surface, and the fluid velocity. The relative susceptibility of a specific material to erosion damage is a strong function of the material’s mechanical properties (hardness, toughness, etc.) and, in many instances, the corrosion resistance of the affected material. Use Word 6.0c or later to

view Macintosh picture.

Figure 22 Influences On Erosion Intensity

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Consequences Of Erosion The possible consequences of erosion in a control valve are shown in Figure 23 and they are discussed below. Use Word 6.0c or later to

view Macintosh picture.

Figure 23 Typical Erosion Damage In Control Valves And Piping

Valve Body Damage - Erosive flows commonly result in a thinning of the valve

body casting in the area immediately below the valve port. The loss of pressure-retaining capability and the total failure of the valve body are potential results of this type of erosion. Trim Damage - The erosion of seat rings, valve plugs, cages, guide bushings,

and stems are often the first steps in a progressive failure. For example, the loss of material at seating surfaces generally results seat leakage and high velocity clearance flows. High velocity flows may cause a type of erosion known as wiredraw; a highly localized form of erosion occurs when small, highvelocity jets cut fine slices or slots into the affected components. Any high velocity clearance flows at the seat can cause accelerated erosion - even disintegration - of the seat and plug. Abrasion and wear of the guiding mechanism (either the cage or the guide bushing) can result in lateral plug instability and vibration of the valve plug and stem. Lateral stem movement can Saudi Aramco DeskTop Standards

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cause packing to wear and fail. High frequency vibration of the plug is also a cause of valve stem breakage. Valve Plugging And Sticking - If grit, fines, or other forms of solid particles

become lodged between sliding contact surfaces such as cages and plugs or shafts and bearings, the valve may seize altogether. Piping Erosion Damage - When fluids leave the control valve at high velocity, erosion of downstream piping may also occur.

Erosive Service Flags And Typical Erosive Fluid Applications Flags For Erosive Fluid Applications Because erosion is a strong function of fluid velocity, many of the flags that indicate the potential for erosion are based on valve outlet velocity. The fluid velocities at which the potential for erosion damage should be given additional engineering attention are listed in Figure 24 and they are discussed below. Velocity F la g F o r E r o si o n

V > 0.3 ( 6 0 D ) m e t e r Saudi Aramco DeskTop Standards

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s / s e c o n d , o r V > (60D ) f e e t/ s e c o n d Experienc e

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e n ti a l V > 0.15 M a c h , o r P 1 / P 2 > 3 Figure 24 Fluid Velocities That Indicate Significant Potential For Erosion Damage

Velocity Limits For Clean Fluids - Liquids, even clean liquids, can be seen as

presenting the potential for erosion damage when the fluid velocity is greater than 0.3 (60-D) meters/second or (60-D) feet second, where D is the nominal pipeline diameter. The velocity limits for clean liquids are based on the following concerns: •

Liquids, even those that are described as clean, are rarely truly “clean"; i.e., they include some dirt, scale, or other particulate.

•

High velocity liquids accelerate the removal of protective passive layers thereby hastening the erosion/corrosion process.

•

High velocity liquids are prone to flashing. The liquid droplets that form during flashing can impinge on critical valve surfaces

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The high-velocity flow of clean gasses and vapors are generally not thought of as presenting a significant potential for erosion damage. Velocity Limits For Fluids With Entrained Particles - If a fluid includes fines, scale,

sand, or other particles, erosion damage can occur at relatively low velocities. For gasses and vapors with entrained particles, the potential for erosion should be considered whenever the fluid velocity is greater than 0.15 Mach, or whenever the ratio of P1/P2 >3. For liquid flows, there is no specific flag in terms of velocity or pressure conditions; experience is the guide. Common Erosive Fluid Applications Any Application Near The Wellhead - Any valve application near the wellhead

should be evaluated for erosion because of the sand, dirt, and other particles that are commonly present in crude oils and natural gasses. High pressure gas wells, because of the high velocity flows that are encountered, should always receive additional attention in order to assess the potential for erosion. Separators - The dump valves on separators are nearly always subjected to erosive flows because of the sand, gravel, dirt, and other solids that are present in the crude oil that is being processed.

Fluidized Cat Cracking - Catalytic cracking processes typically include several control valves that must be compatible with erosive fluids. The feed valve may be subjected to impurities that passed through the initial separation process. Other valves in the process are required to resist erosion from coke (a fine, gritty form of carbon) and catalyst fines (which are generally very hard and very sharp). Wet Steam - In applications that involve saturated steam, liquid droplets that are

transported at high velocity can impinge upon critical valve surfaces and cause significant erosion damage.

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Critical Control Valve Specification Considerations When selecting control valves for erosive service applications, specifiers typically consider: • Valve styles that have been uniquely designed to direct the erosive flowstream away from critical valve surfaces. • Materials of construction with the mechanical properties that are needed to resist erosion damage. • Sizing issues. Control Valve Styles Guiding Methods - If the fluid contains solid particles, cage-guided valves may be a poor choice because of the abrasion to guiding surfaces and the plug binding that can occur if particulates becomes wedged between the plug and the cage. To reduce friction and prevent plug binding, post-guided valves are typically preferred. In some valve designs, the guide bushing is located in an area that is separated from the main flow stream in order to prevent particulates from damaging the guide bushing. Refer to Figure 25. Use Word 6.0c or later to

view Macintosh picture.

Figure 25 Post-Guiding Versus Cage Guiding For Erosive Fluid Applications Flow Geometry - As previously illustrated, the standard flow-down globe body

construction is particularly susceptible to erosion damage because of the tortuous path and the numerous opportunities for particle impingement on critical surfaces. In contrast, valve designs that provide a straight-through or Saudi Aramco DeskTop Standards

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line-of-sight flow path generally provide increased protection against erosion damage. For example, an angle body construction provides a flow path with a single turn and minimizes the potential for erosion damage. Line-of-sight rotary valves also minimize the potential for particle impingement. Figure 26 shows the differences in the flow path of standard globe style bodies and angle bodies. Erosion resistant trim materials such as tungsten carbide and ceramics are commonly available for valves of this type.

Use Word 6.0c or later to

view Macintosh picture.

Figure 26 Globe Versus Angle Bodies For Erosive Fluid Applications ANSI Class Shutoff - When a control valve that is selected for an erosive fluid

application must shut off, ANSI Class V shutoff should be selected. The tight shutoff specification will help to minimize high-velocity leakage across the seat and the increased potential for erosion. In some applications, it may be Saudi Aramco DeskTop Standards

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advisable to achieve system shutoff with the use of a separate block valve. If the control valve is not used to attain shutoff, then the damage that can result when solids are trapped between the plug and seat can also be avoided. Thick Seals Vs. Thin Seals - Because erosion manifests itself as the wearing

away of material, the life of vulnerable components can be extended if they are robust and massive rather than thin and fragile. For example, the thin seals of a standard ball segment valve or a high-performance butterfly valve will not tolerate erosive fluids nearly as well as the massive seal of the eccentric rotary plug valve that is shown in Figure 27. In addition, the plug and seat ring of the eccentric plug valve are typically available in a variety of erosion resistant materials including tungsten carbide and ceramics. Use Word 6.0c or later to

view Macintosh picture.

Figure 27 Thick Seals Of An Eccentric Rotary Plug Style Valve

Special Valve Constructions - For particularly erosive fluids and for other difficult

applications, special valve constructions may be considered. The valve that is shown in Figure 28 is a sweep flow, venturi outlet style valve that is available from many manufacturers. The valve performs well in highly erosive applications and in coking applications. To protect critical surfaces from coke Saudi Aramco DeskTop Standards

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buildup, the valve stem and the plug guiding surfaces are protected from the fluid stream. The flow path around the plug - referred to as sweep flow - also helps to prevent the accumulation of coke deposits. The enlarged outlet reduces the outlet velocity and aids in minimizing flashing and erosion damage. The valve includes provisions for injecting warm oil to ensure adequate lubrication and to prevent the build up of coke. Steam may also be injected to help prevent highly viscous fluids such as furnace bottoms from clogging the valve. For highly erosive flows, the plug tip and seat ring are available in tungsten carbide and ceramics.

Use Word 6.0c or later to

view Macintosh picture.

Figure 28 Sweep Flow Valve Design

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Hierarchy Of Erosion Resistant Valve Styles - To provide a summary of the

preceding discussion, Figure 29 lists a hierarchy of erosion resistant valve types and options.

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Valve

Comment S t y l e

Cageg u i d e d v a l v e s

Concern f o r t h e p l u g b i n d i n g i n t h e c a g e .

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S u i t a b l e w h e n t h e v o l u m e r a t i o o f p a r t i c u l a Saudi Aramco DeskTop Standards

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t e i s v e r y l o w o r w h e n t h e e r o s i v e m e d i a i Saudi Aramco DeskTop Standards

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s a f l a s h i n g l i q u i d ( w i t h n o s o l i d s ) . Cageg u i Saudi Aramco DeskTop Standards

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d e d a n g l e

y d a m a g e .

v a l v e s Postg u i d e d v a l v e s

Reduces p l u g b i n d i n g . D e s i g n s w i t

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h p r o t e c t e d b u s h i n g s o f f e r i n c r e a s e d p r o t e Saudi Aramco DeskTop Standards

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c t i o n . Post-

Angle G u i d e d

b o d y m i n i m i z e s

A n g l e V a l v e s

b o d y d a m a g e .

Post-

Liner G u i d e

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d A n g l e V a l v e s w i t h h a r d e n e d , r e p l a c e a b l e

e s e r o s i o n t o v a l v e o u t l e t a n d s e r v e s a s a

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s a c r i f i c i a l m e m b e r .

u t l e t l i n e r

Eccentric R o t a r y P l u g V a l v e s

Straight t h r o u g h f l o w p a t h m i n i

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m i z e s i m p i n g e m e n t o n c r i t i c a l p a r t s . R a t i n g s

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t o A N S I C l a s s 6 0 0 . Sweep

Very F l o w ( V e n t u r i S t y l e ) A n g

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l e

e p

V a l v e s

f l o w d e s i g n d i r e c t s f l o w a w a y f r o m c r i t

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i c a l s u r f a c e s . R a t i n g s t o A N S I C l a s s 9 0 0 . Figure 29 Hierarchy Of Erosion Resistant Valve Styles Saudi Aramco DeskTop Standards

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Materials Selection Considerations For erosive fluid applications, the general guideline for material selection is to specify materials with sufficient hardness or toughness to provide extended life in erosive fluid applications. Material Selection For Valve Bodies - When erosion of a standard, cast carbon

steel body such as WCC or WCB is anticipated, specifiers may consider the selection of a variety of increasingly erosion resistant alloys. Figure 30 lists a number of alloys and their relative resistance to erosion damage. The body material specification can also be affected by the flow geometry of the selected valve; i.e., if the flow is directed away from the body (as in an angle valve or an eccentric rotary plug valve), a standard WCB body may provide long life. In contrast, the direct impingement of the same erosive fluid on the body of a standard globe may require the selection of a WC9 (chromium-molybdenum) body in order to provide satisfactory valve life.

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Remarks

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A s t a n d a r d m a t e r i a l . M a y b e s e l e c t e d f o r

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Much g r e a t e r e r o s i o n r e s i s t a n c e t h a n c a r b o n

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Superior e r o s i o n a n d c o r r o s i o n r e s i s t a n c e .

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Figure 30 Hierarchy Of Erosion Resistant Valve Body Materials

Material Selection For Valve Trim - Because trim components are “wetted”

components, they are exposed to the high velocity fluid stream. Consequently trim for erosive service applications is always selected in erosion resistant alloys. Figure 31 lists a hierarchy of erosion resistant construction materials.

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c o r r o s i o n r e s i s t a n c e b u t , i n i t s b a s i c f o r m , o f f e r s

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h e a t t r e a t e d t o H R C 3 8 . G o o d e r o s i o n r e s i s t a n c e

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h e a t t r e a t e d u s i n g H 1 0 7 5 ( H R C 3 2 ) f o r s t a n d a r d

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h a r d e n e d t o 5 6 6 0 H R C . V e r y h a r d a n d e r o s i o n

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e r o s i o n a n d w e a r r e s i s t a n c e ; h o w e v e r , t h e b i n d e r s

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Figure 31 Hierarchy Of Erosion Resistant Valve Trim Materials

Problems With Focusing On Material Hardness Only - A common misconception is

that erosion resistance is a function of materialhardness only. Hardness is defined as a material’s resistance to penetration or indention. In metals, hardness is measured by loading an indenter into the metal and measuring either the depth or the of penetration or the surface area of the indentation. Hardness may provide an approximation of the relative erosion resistance of one material compared to another in the same alloy family. However, different families of metals may achieve their erosion-resistant properties in different ways. Mechanical properties such astoughness can have significant impact on the erosion resistance of a particular alloy. Although the property of “toughness” is difficult to precisely define, it can be viewed as the opposite of brittleness. For example, a tough metal that is subject to a heavy impact will deform before it breaks whereas a less tough material will break before deforming. The cobalt based Alloy 6 (Stellite) is well known for its erosion resistant qualities even though its hardness is well below the hardness of most erosionresistant stainless steels. Alloy 6 derives its erosion resistance from its toughness. The unusual process by which Alloy 6 achieves its toughness is shown in Figure 32 and it is summarized below. 1.

In its as-manufactured state, Alloy 6 has a specific crystalline structure.

2.

Following impact, the crystalline structure of the alloy 6 material actually changes from one basic form to another. The new crystalline structure displays a much higher resistance to strain fracture than the material’s native structure. As a result, the material’s resistance to erosion actually increases during impact.

3.

Under static conditions (following an impact), the alloy reverts to its initial, as-manufactured crystalline structure.

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Figure 32 Structural Changes In Alloy 6 That Improve Erosion Resistance Erosion Resistance and NACE MR0175 - When the potential for stress corrosion

cracking and erosion are both present, the specifier is confronted with selecting materials according to two seemingly contradictory guidelines: (1) selecting materials with sufficient hardness or toughness to withstand the erosive elements and (2) selecting materials with hardness levels that sufficiently low to satisfy the NACE MR0175 guidelines. Fortunately, many material options are available that meet both requirements. Type S41000 stainless steel is reasonably hard and it is NACE compliant if its hardness is limited to HRC 22. 316 stainless steel with Alloy 6 hardfacing provides a very popular solution that is superior to S41000 when both erosion resistance and NACE compliance must be achieved. Erosion In Combination With Other Severe-Conditions - Erosion in combination

with other severe conditions such as corrosion, cavitation, and high temperatures can further increase the complexity of the control valve and material selection process. To clearly identify all requirements, the fluid properties and service conditions must be closely evaluated. Sizing Issues Valve sizing can have a significant impact on the life of a control valve in an erosive fluid application. The sizing issues typically relate to velocity control.

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Valve Body Size Selection - Because erosion intensity is a strong function of fluid

velocity, valve body sizes should be selected that will not significantly increase the fluid velocity. For example, valve body sizes that are smaller than the pipeline size should be avoided. Valve Trim - Oversized valves can present many problems and they are

particularly troublesome in erosive fluid applications. If the trim is oversized, then the valve will throttle near the seat resulting in high velocity erosive flows across flow-controlling surfaces. To prevent these flows, extra efforts should be made to ensure that the valve trim is not oversized. Information Sources When selecting materials for erosive fluid applications, specifiers may apply information that is included in manufacturer’s specification bulletins and in other resources such as the charts previously shown in Figures 30 and 31. In addition, specifiers may draw upon the expertise of vendors and peers who have experience in equivalent or similar applications.

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SELECTING CONTROL VALVE OPTIONS FOR high-temperature FLUID Applications High Temperature Applications And Their Consequences Categories Of High Temperature Applications High temperature Applications are those in which the normal operating

temperature is sustained above a specific temperature limit. Refer to Figure 33. Thermal Cycling Applications are those in which the operating temperature

repeatedly rises and falls over a wide range of temperatures. Refer to Figure 33. Use Word 6.0c or later to

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Figure 33 High Temperature Application Vs. A Thermal Cycling Application

Common Applications High Temperature Applications - Control valves that are used to control flu gasses in furnace applications, control valves that are used to control feedstocks in various refinery operations, and control valves in high-pressure steam generation systems are all subjected to sustained high temperatures. Thermal Cycling Application - A common example of a control valve that is

subjected to thermal cycling is the valve that is used to perform the sootblowing process in a fossil-fuel boiler. The boiler tubes are delicate and cannot Saudi Aramco DeskTop Standards

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tolerate hot spots that would develop if soot were allowed to build up on the tubes. In many boiler systems, the tubes are cleaned by periodically directing high pressure steam at the tubes. The steam is controlled by a control valve that is operated at predetermined intervals, often several times a day. Another example of a thermal cycling application is the desiccant dehydration process that is used to remove moisture from many natural gasses. Refer to Figure 34. Desiccant dehydration towers use trays filled with a solid desiccant (a substance that attracts moisture). In operation, inlet gas enters into the top of the tower, the desiccant removes moisture from the gas as the gas passes downward through the tower, and dry gas exits the bottom of the tower. Over time, the desiccant becomes saturated and will not hold any more moisture. To regenerate the desiccant, the tower is heated as shown in the middle tower of Figure 34. During the heating cycle, valves A and C are closed and valves B and D control the flow of hot (400 to 500 degree F) gas upward through the tower. When the desiccant is dry, cool gas (120 degrees F) is introduced into the bottom of the tower as shown in the third tower in Figure 34. To enable continuous operation, many desiccant dehydration units include three towers. The adsorption, heating, and cooling cycle may be repeated several times a day in each tower. Refer to Figure 34 and note that the temperature of the gas that passes through valves B and D cycles between 120 degrees F and 400 to 500 degrees F. As a result, valves B and D are in a thermal cycling application.

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Figure 34 Thermal Cycling Application: Dry Desiccant Adsorption Process

Consequences Of High Temperature Fluids On Incompatible Components General Effects Of Elevated Temperature On Materials - Most metal alloys are

metastable, meaning that during the manufacture and subsequent working of the alloy component, a unique but unnatural and unstable structure is purposely developed in the alloy. The unique structure of each alloy imparts the alloy’s mechanical properties such as strength, ductility, toughness, etc. When an alloy is subjected to elevated temperatures, it tends to transform to its stable or natural structure. Examples of the effect of elevated temperatures on some materials are as follows:

•

S17400 and similar precipitation hardenable materials become brittle.

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•

Cold worked 300 series stainless steels lose the effects of cold work.

•

Duplex steels become brittle.

Graphitization Of Carbon Steel - Carbon steels possess a two-phase

microstructure that includes ferrite (pure iron) and iron carbides. At temperatures above 800 degrees F, the carbides decompose into iron and graphite flakes during a process that is known asgraphitization. Use Word 6.0c or later to

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Figure 35 Graphitization In Carbon Steels

Sensitization Of Stainless Steels - Because carbon improves a materials strength

at elevated temperatures, it is often desirable to select materials with a high carbon content. However, the addition of carbon increases the potential for sensitization. Refer to Figure 36. Recall that sensitization is a process in which exposure to high temperature causes corrosion resistant alloys to precipitate out of the material matrix, leaving a zone at the grain boundary that is not protected from corrosion attack. In a corrosive environment, the area of the grain boundary that has been depleted of chromium is susceptible to attack by the corrosive atmosphere. The corrosion that results is known as intergranular corrosion.

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Figure 36 Sensitization And Intergranular Corrosion That Result From Exposure To High Temperatures

Creep - When exposed to stress such as an increase in fluid pressure or an

increase in a loading force, most alloy components strain (deform) in proportion to the amount of stress. When the stress is relieved, the component reverts to its initial shape. The ability of a material to return to its initial form after being exposed to stress is known aselasticity. In high-temperature environments, the elasticity of a material can be affected by the phenomenon of creep. The effects of creep are illustrated in Figure 37. At temperatures that are sufficiently high, the amount of strain (deformation) may slowly increase over time and the strain may become permanent. The main effects of creep in control valves are the long-term loss of bolting forces, loss of gasket forces, and the deformation of trim parts.

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Figure 37 Effect Of Creep On The Elasticity Of A Material

Effects Of Mismatched Expansion Coefficients - When metallic materials are

heated, they expand in a predictable and repeatable manner. Each alloy has its own characteristic thermal expansion vs. temperature curve. In general, materials with similar chemical compositions have similar thermal expansion properties. The carbon steels, alloy steels, and 400 series stainless steels have fairly low thermal expansion coefficients whereas the 300 series stainless steels have very high expansion rates. At elevated temperatures, differential thermal expansion coefficients of the trim components and the body and bonnet can cause different types of problems. Figure 38 illustrates two scenarios. If the body and bonnet expands more than the cage and seat ring as shown in view B, gasket unloading will occur and the fluid will leak across the gasket surfaces. If the cage and seat ring expand more than the body and bonnet as shown in view C, the cage and/or seat ring may be damaged by crushing and the gaskets may be damaged by overloading.

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Figure 38 Effects Of Mismatched Thermal Expansion Coefficients

Gasket Failures - Gaskets that are exposed to temperatures that are greater

than the gasket material rating may become brittle and lose their ability to deform, thereby preventing them from sealing against their mating surfaces. Any such failure can result in fluid leaks erosion damage. Packing Failures - When standard PTFE packing materials are exposed to

temperatures that are above the packing material’s temperature rating as shown in Figure 39, the PTFE pacing rings may deform, they may sublimate, and/or they may begin to flow and extrude out of the packing bore as the valve stem strokes. PTFE-based packing arrangements may display all of these behaviors at temperatures above 450 degrees F.

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Figure 39 Effects Of Elevated Temperature On PTFE Packing

Consequences Of Thermal Cycling Applications Thermal Fatigue - As a hot fluid is introduced into a control valve, the component

surfaces that are in contact with the fluid are the first to respond to the increase in temperature. While the outermost surfaces of the components are attempting to expand, the material that is just behind the outermost surfaces remains cool and resists expansion. During each heating and cooling cycle, a stress gradient occurs in the components. The gradient can cause a form of thermal fatigue that, in extreme cases, results in cracking. Failures that result from thermal fatigue are rare; however, if an application frequently cycles across an extreme range of temperatures, specifiers should be alert to the potential for this form of damage. Gasket Unloading - During thermal cycling, the bonnet-to-body bolting may

repeatedly load and unload the gaskets in the control valve assembly as shown in Figure 40. Continuous loading and unloading of the gaskets can cause the gaskets to take a set (lose their elasticity). If the gaskets lose their elasticity and fail to seal, leaks can result in high-velocity erosive flows. Such flows are generally the starting point for a progressive failure of the control valve.

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Figure 40 Thermal Cycling And Gasket Failure Loosening Of Threaded Components - It has long been known that temperature cycling

has the tendency to loosen threaded components. Refer To Figure 41. In control valves, thermal cycling applications have been known to loosen threaded seat rings, threaded bonnet assemblies, and bonnet-to-body bolting.

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Figure 41 Effects Of High Temperature On Threaded Joints

High Temperature Service Flags Saudi Aramco Definition Of High Temperature Within Saudi Aramco, the definition of a high temperature application is based on the upper operating temperature limit of PTFE. According to Section 4.1.5 of SAES -J-700, the upper temperature limit of PTFE is 400 degrees F. Therefore, within Saudi Aramco, the definition of a high temperature application is any application with an operating temperature that is greater than 400 degrees F. Refer to Figure 42. Thermal Cycling Flags Thermal cycling flags are not defined by Saudi Aramco but are instead defined by valve manufacturers. Refer to Figure 42. The temperature at which thermal cycling is considered to be a problem can vary with each different valve construction; however, whenever an application repeatedly cycles over a range of 300 degrees F, problems from thermal cycling can be anticipated.

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Figure 42 High Temperature Vs. Thermal Cycling Flags Critical Control Valve Specification Considerations Valve Design Considerations Seat Ring Retention - Screwed-in seat rings are popular for many general service applications because they do not require loading from a cage component to ensure a tight fit in the valve body. However, screwed-in seat rings are generally not selected for thermal cycling applications because of the tendency of the seat ring to loosen. Screwed-in seat rings may be used successfully when the seat ring is tack welded into the valve body (see Figure 43) or if the seat ring is held firmly in place by an indexing lug on a cage or cage-like component. Bonnet-To-Body Attachment - Many small, high-pressure valves are designed

with threaded bonnet-to-body connections. Such constructions should be avoided for high temperature and thermal cycling applications unless options are available to tack weld the bonnet to the body as shown in Figure 43.

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Figure 43 Tack Welding Of Bonnets And Seat Rings For High-Temperature And Thermal Cycling Applications

Material Temperature Ratings Refer to Figure 44 for the discussion that follows. Body And Bonnet Materials - Refer to Figure 44 for the discussion that follows.

Carbon steel bodies such as WCC and WCB are commonly compatible with temperatures up to a maximum of 800 degrees F. Above this limit, the phenomenon of graphitization can occur. Between 800 degrees F and approximately 1050 degrees F, alloy steels that include additional amounts of chromium and/or molybdenum may be selected. The addition of chromium and/or molybdenum enhances the alloy’s resistance to tempering and graphitization at elevated temperatures. Grades C5 and WC9 are common. The WC9 material provides better castings and it is easier to weld. For increased high-temperature compatibility and/or for increased pressure retaining capability, alloys with still more chromium and molybdenum are specified. CF8M stainless steel (the cast version of S31600) is commonly used for temperatures up to 1500 degrees F. The pressure and temperature limits for body and bonnet materials are listed in the ANSI/ASME pressure/temperature tables and may also be listed in control valve specification bulletins. Saudi Aramco DeskTop Standards

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M

Upper T e m p e r a t u r e L i m i t

C

800

d e g r e e s F

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Approxima t e l y 1 0 5 0 d e g r e e s F

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Figure 44 Upper Temperature Limits Of Common Valve Body And Bonnet Materials

Trim Materials - The trim material options that are available for high-temperature

applications vary according to the valve manufacturer. In addition, the specific Saudi Aramco DeskTop Standards

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temperature rating for a complete trim package depends on several factors, including: •

the valve type

•

the valve body and bonnet material

•

the valve size

•

the pressure drop

•

other materials in the trim package

Because of the number of variables that must be considered, manufacturers establish charts, tables, and other selection methods for selecting a preengineered trim package for specific temperature and pressure conditions. As an example, Figure 45 illustrates the temperature and pressure drop limits of various trim options in a Cr-Mo steel body. Each trim option number refers to specific materials of construction for the plug, cage, and seat ring. Note also the cautions and selection guidance that is listed in the section below the chart. 1400 1200 1000 PRESSURE DROP, PSI

37H 2

800

1, 3

600 400

3H

200

2

0 -200 0

400

800

1200

-20 FLUID TEMPERATURE, DEGREES F1100 WITH CLASS 600 1 WC9 OR C5 CHROME MOLY STEEL BODY 1 DO NOT EXCEED THE MAXIMUM PRESSURE AND TEMPERATURE FOR THE CLASS RATING OF THE BODY MATERIAL USED,1EVEN THOUGH THE TRIMS SHOWN MAY HAVE HIGHER CAPABILITIES 2 BE ESPECIALLY CAREFUL TO SPECIFY SERVICE TEMPERATURE IF TRIM 3 OR 37 IS SELECTED 1 AS DIFFERENT THERMAL EXPANSION RATES REQUIRE SPECIAL PLUG CLEARANCES. SPECIFY TRIM 37H FOR TEMPERATURES ABOVE 4510 DEGREES F. SPECIFY TRIM 3H FOR TEMPERATURES ABOVE 800 DEGREES F. 3

TRIM 29 MAY BE USED UP TO 1440 PSI WITH CLEAN, DRY GAS. 1

4 USE TRIM 27 INSTEAD OF TRIM 29 FOR NONLUBRICATING FLUIDS SUCH 1 AS SUPERHEATED STEAM OR DRY GASSES BETWEEN 300 AND 600 DEGREES F. FIG74

Figure 45 Saudi Aramco DeskTop Standards

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Pressure/Temperature Ratings For Various Trim Material Options

Gasket Materials - Most control valves include two different types of gaskets;

spiral wound gaskets and flat sheet gaskets. The characteristics and temperature ratings of several common gasket materials are listed in Figure 46. •

Spiral Wound Gasket Options - A spiral wound gasket is made of a metal alloy that is formed into a V-shape and then wound into a spiral form. During the manufacture of the gasket, a filler is inserted between each coil of the V-shaped material. Of the options that are listed in Figure 46 below, Inconel is the strongest alloy material, it has the highest temperature rating, and it will maintain its spring properties longer than the other options. As a result, the Inconel/graphite gasket is typically recommended for thermal cycling applications.

•

Flat Sheet Gasket Options - A standard material for flat sheet gaskets is a composition material. Options such as PTFE coated Monel provide corrosion resistance, but at reduced temperature ratings, as shown in the table below.

The selection of a suitable gasket material is based on the following: •

The temperature rating of the gasket material.

•

Whether or not thermal cycling will occur.

•

The corrosion resistance of the gasket material. Standard

Optional M a t e r i a l

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Figure 46 Common Gasket Materials

Bolting Materials - As mentioned previously, each different alloy has a different

thermal expansion vs. temperature curve. However, different alloys in the same family generally have similar thermal expansion and contraction characteristics. Accordingly, the general guidelines for bolting material selection are: •

If possible, select steel bolting (for example, B7 or B16) for alloys steel bodies and bonnets.

•

If possible, select stainless steel bolting (for example, 316 or 304 stainless steel) for stainless steel bodies.

•

Whenever non-standard bolting is considered or the above guidelines cannot be followed, investigate the need for pressure and/or temperature derating to compensate for the differential in thermal expansion coefficients, differences in bolting strength, and other influences.

Packing Materials - Packing material selection is based upon the temperature at

the packing bore. The temperature at the packing bore is often considerably less than the temperature of the process fluid, especially if the valve is insulated below the packing bore or if an extended-height (extension) bonnet is specified. For temperatures below 400 degrees F, PTFE base packing arrangements are compatible with most fluids. Above 400 degrees, packing arrangements that are based on graphite materials are the industry standard. Graphite materials are compatible with a wide range of fluids; however, graphite base packing arrangements should not be selected for hot oxidizing acids (nitric acid and sulfuric acid) or for oxygen services that operate above 700 degrees F. Extended Bonnets For Packing Protection An extended bonnet locates the packing at an increased distance from the process fluid, thereby reducing the influence of the process fluid on the packing temperature. Refer to Figure 47. Section 4.1.5 of SAES-J-700 requires the selection of extended bonnets or the selection of special packing materials for applications in which the fluid temperature is greater than 450 degrees F.

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Figure 47 Extended Bonnets That Are Used At Temperatures Above 450 Degrees F

Achieving Tight Shutoff At Elevated Temperatures Metal Seats - ANSI Class VI shutoff is typically achieved with the use of soft-

seated valve constructions. However, Saudi Aramco standards define an upper temperature limit of 400 degrees F for PTFE and many other materials that are included in soft-seating arrangements. Therefore, at temperatures above 400 degrees F, ANSI Class V shutoff or better is generally achieved by specifying an unbalanced valve construction with metal-to-metal seats that have been precision lapped to achieve the shutoff specification. High Temperature Seal Rings For Balanced Valves - To achieve ANSI Class V or better shutoff with a balanced valve construction in a high-temperature environment, many manufacturers offer special high-temperature PTFE seal

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ring designs. Refer to Figure 48 and note the following features of a soft seal arrangement that is rated for temperatures up to 600 degrees F. •

The PTFE “omni seal” is pressure loaded to improve seal performance.

•

The PTFE seal includes a spring which helps to maintain a seal between the plug and cage at elevated temperatures where the PTFE material loses its elasticity.

•

The PTFE material includes a high percentage of carbon and graphite to improve its high-temperature performance.

•

An anti-extrusion ring prevents any of the hot and potentially flowing PTFE material from extruding out of the seal area. Use Word 6.0c or later to

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Figure 48 High-Temperature Balanced Plug Seal Configuration

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SELECT ing AND Siz ing CONTROL VALVEs FOR Cavitating FLUID Applications Cavitation And Its Consequences The Cavitation Phenomenon Vapor Cavity Formation and Collapse - When, in a liquid flow, the fluid pressure falls below the fluid’s vapor pressure, the fluid begins to vaporize; i.e., vapor bubbles form in the flow stream. In a control valve, the onset of vaporization often occurs near the vena contracta, as shown in Figure 49. If the downstream pressure P2 increases to a value that is greater than the fluid’s vapor pressure, the bubbles collapse and the fluid is cavitating. Use Word 6.0c or later to

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Figure 49 Cavitation

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Cavitation Versus Other Flowstream Phenomenon Cavitation Vs. Flashing - Up to the point where the decrease in the local

pressure causes bubbles to form in the fluid stream, flashing and cavitation are similar phenomenon. In a flashing fluid, however, the downstream pressure 2P is below the vapor pressure of the liquid and the bubbles that form near the vena contracta remain in the fluid stream as shown in Figure 50. Flashing will be discussed later in this Module. Use Word 6.0c or later to

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Figure 50 Cavitation Vs. Flashing

Cavitation Vs. Outgassing - When a fluid includes dissolved gasses and the fluid

is subject to pressure reduction or to agitation (both of which occur as the fluid flows through a control valve), the dissolved gas may come out of solution in a process that is known as outgassing. Refer to Figure 51. Outgassing differs from cavitation and flashing in that it is not a thermodynamic event and it occurs independently of the values of the fluid’s vapor pressure and the pressure at the vena contracta. In addition, the bubbles that form as a result of outgassing may remain in the downstream flow regardless of the value of 2P. An increase in pressure andtime may both be required to force the gas bubbles back into solution.

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Figure 51 Cavitation Vs. Outgassing

Common Forms Of Cavitation Hard Cavitation Vs. Soft Cavitation The term “hard” cavitation is used to describe the worst-case scenario in terms of the potential for cavitation damage. Hard cavitation implies that there are no circumstances or conditions present in the application that will have a mitigating effect on the intensity of the cavitation or the potential for cavitation damage. Cold water is the classic example of a fluid that will exhibit hard cavitation. The phrase “soft cavitation” is used to describe any application in which either the fluid properties or the service conditions serve to lessen the potential for cavitation damage. For example, the cavitation that occurs in a multi-species fluid such as a hydrocarbon mixture may be less likely to cause significant cavitation damage because the mixture includes components with several different vapor pressures. As the local fluid pressure is reduced, not all of the components will vaporize, and the components that remain in the liquid form may cushion the collapse of the vapor cavities. In addition, fluids that are

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viscous and outgassing fluids may provide a cushioning effect on vapor cavity implosions. Refer to Figure 52. Use Word 6.0c or later to

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Figure 52 Hard Vs. Soft Cavitation

Specifiers typically view the cavitation that occurs in crude oil as “soft cavitation”. In a crude oil flow, the cavitation damage that occurs as a result of vapor cavity implosion may not present as great a concern as the noise and vibration that occurs. As hydrocarbon liquids become more refined (less viscous and closer to a single species fluid), the damage from vapor cavity implosions becomes a major concern. Incipient Vs. Full Blown Cavitation Specifiers will often encounter the term “incipient” cavitation. The term “incipient” cavitation defines the point at which the first vapor cavities form in the fluid stream. On a plot of flow (Q) versus the square root of the pressure drop that is shown in Figure 53, this point is observed as the first deviation of the actual flow plot from the plot of predicted flow. Incipient cavitation occurs Saudi Aramco DeskTop Standards

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when the local fluid pressure first dips below the fluid’s vapor pressure. Damage may or may not occur at this point. At increased pressure drops, more and more bubbles form and collapse in the fluid stream. At the condition of fully choked flow, the cavitation that occurs is often described as “fully blown cavitation” or as “choked flow cavitation”. These terms indicate there is a substantial potential for cavitation damage; however, they are highly subjective and they provide little real guidance to the valve specifier. Use Word 6.0c or later to

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Figure 53 Incipient Vs. Choked Flow Cavitation

Consequences Of Cavitation Valve And Piping Damage - If the vapor bubbles that are formed during the

cavitation cycle implode on or near fluid boundaries such as valve components and pipe walls, high-velocity microjets and sonic waves can result in rapid and catastrophic damage to the components as shown in Figure 54.

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Figure 54 Cavitation Damage That Results From Imploding Vapor Cavities

Vibration - In many liquid flows, vibration of the valve and piping is as great a

concern as the potential for damage from the implosion of vapor cavities. Figure 55 shows a representative plot of valve and pipeline vibration versus the value of sigma (σ = P1-Pv/P1-P2). Following the occurrence of incipient cavitation, the intensity of the vibrations increases rapidlyas the value of sigma decreases. Cavitation has been known to cause vibrations of sufficient intensity to break welded joints and damage pipeline supports.

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Figure 55 Valve And Pipeline Vibration Versus The Value Of Sigma

Hydrodynamic Noise - Cavitation may also be accompanied by moderate to high levels of hydrodynamic noise. However, the intensity of cavitation that is required to generate objectionable levels of hydrodynamic noise is generally sufficient to cause rapid and catastrophic damage to the valve and piping. As a result, the concern for damage from vapor cavity implosion and from pipeline

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vibration is generally a far greater concern than the concern for high levels of hydrodynamic noise. Predicting The Potential For Cavitation Saudi Aramco And Manufacturer’s System Cavitation Indices Saudi Aramco Index Ksa - Saudi Aramco makes use of the cavitation index K sa. Refer to Figure 56. Ksa is defined as Ksa = P1-P2/P1-Pv. As the value of P1-P2 approaches the value of P1-Pv, the pressure dip that occurs at the vena contracta is more likely to drop below the value P v; hence, an increasing value of Ksa indicates an increased potential for cavitation. Values of K sa that are greater than approximately 0.75 indicate a substantial potential for cavitation and cavitation damage. A Ksa value of 0.99 signals the maximum potential for cavitation and cavitation damage. If the value of K sa is 1.0 or greater, P2 is less than Pv and the fluid is flashing. Fisher A r and Mokveld K cs - Fisher Controls and Mokveld each use a cavitation

index that is identical to the Saudi Aramco index K sa. However, Fisher uses the term Ar instead of Ksa and Mokveld uses the term Kcs instead of Ksa. Use Word 6.0c or later to

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Figure 56 Cavitation Indices K sa, A r, and K cs

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Valtek Sigma - Valtek uses the cavitation indexsigma. Values of σ that

approach 0 signal an increasing potential for cavitation. A sigma value of 0 or less indicates flashing conditions. The significant relationship is the pressure differential between P2 and Pv. Refer to Figure 57. Use Word 6.0c or later to

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Figure 57 Valtek Cavitation Index σ ∆Pchoked (Masoneilan) - Some manufacturers (including Masoneilan and others)

evaluate the system potential for cavitation by calculating the value of∆Pchoked (∆Pcriticial in Masoneilan nomenclature). Note that a valve must be initially selected in order to obtain a value of FL. If the actual pressure drop is greater than the allowable pressure drop, the flow is choked and, if P 2>Pv, the flow is also assumed to be cavitating. The problem of associating cavitation with the choked flow pressure drop is that the calculated value of∆Pchoked predicts the choked flow flow rate only; it does not predict the precise ∆P at which choked flow will occur nor does it provide any clear indication of cavitation intensity. As shown in Figure 58, incipient cavitation is likely to occur at pressure drops that are lower than the choked flow pressure drop.

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Figure 58 ∆Pallow (∆ ∆Pchoked or ∆Pcritical ) As An Indicator Of Cavitation Ci (CCI) - Although cavitation is a function of pressure conditions (P vc < Pv and P2 > Pv) some manufacturers, including Control Components Incorporated,

prefer to evaluate the velocity conditions rather than pressure conditions that will cause cavitation to occur. Because of the pressure/velocity relationships that are defined by Bernoulli's theorem, the relative tendency of a system to cavitate can be expressed in terms of fluid velocity as well as in terms of fluid pressure. The cavitation index that is used by CCI is C i. Ci = 9724 (P-Pv)/ V2 where: Ci

Cavitation index

9724

A constant

P

The fluid pressure at any point in the valve, psia

Pv

The fluid vapor pressure, psia fluid density, lbm/ft3

V

Fluid velocity at the point where P is measured, ft/sec

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If, as shown in Figure 59, the computed value of C i is 1.0 or less, the system will cavitate. In essence, this means that the fluid pressure P at the point that is being examined will be less than the fluid’s vapor pressure and cavitation will occur. Use Word 6.0c or later to

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Figure 59 Control Components Cavitation Index C i Because the values of V and P are not readily known, the index C i is not easily or quickly determined.

Subjective Factors For Analyzing The Potential For Cavitation Damage In addition to the empirical methods that predict the occurrence of cavitation, several subjective factors can be evaluated in order to assess the relative potential for cavitation related problems. These factors are discussed below and they are listed in Figure 60. Fluid Viscosity- As previously mentioned, highly viscous fluids such as heavy crude oils can lessen the effects of cavitation. Viscous fluids have two effects on cavitation:

1. Viscous fluids impede the nucleation and growth of vapor cavities 2. Viscous fluids help to cushion the collapse of the vapor cavities. Dissolved Gas Volume - If the liquid flow includes a large volume of entrained

(dissolved) gas that comes out of solutions as flow passes through the valve, Saudi Aramco DeskTop Standards

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the presence of the vapor in the fluid stream may help to cushion the collapse of the vapor bubbles. Fluid Composition - If a liquid flow consists of a mixture of substances with

widely varying vapor pressures (which is often the case with hydrocarbon liquids), the classical “single fluid” model for cavitation does not apply. With fluid mixtures, fluid vaporization may occur over a range of pressures as opposed to the single vaporization pressure of a single-species fluid. The net impact is generally a reduction in the intensity of cavitation related problems. Duty Cycle - If a valve will only be subjected to severe cavitating conditions for

short periods of time, e.g., at startup, shutdown, or during rare transients, the valve may be able to provide long life and good performance even though cavitation does occasionally occur. In some applications where the occurrence of cavitation is rare and occurs for short periods of time, the selection of hardened trim materials may be sufficient to resist cavitation damage. Pressure Scale Effects - The potential for cavitation is not absolutely defined by

indices such as Ksa, Ar, or σ. Laboratory tests indicate that the potential for cavitation damage increases as the upstream pressure increases. Size Scale Effects - Investigators have determined that the potential for

cavitation and cavitation damage increases as the valve size increases. The size scale effect is also independent of the popular cavitation indices such as Ksa, Ar, or σ.

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Figure 60 Subjective Factors For Analyzing The Potential For Cavitation Damage

Cavitation Service Flags And Typical Cavitating Applications Flags For Cavitating Fluid Applications Each unique application must be studied carefully in order to determine the potential for cavitation. However, a general rule of thumb is that any application with a value of Ksa that is greater than or equal to 0.8 should be closely examined to determine the potential for cavitation and cavitation related problems. Specific Applications Many applications, because of the nature of the fluid properties and/or the service conditions, are universally recognized as cavitating applications. Several such applications are discussed below. Saudi Aramco DeskTop Standards

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Boiler Feedwater - A boiler feedwater control valve is susceptible to cavitation

because the fluid is a single-species, the feedwater control system operates at elevated temperatures (with the effect of reducing the vapor pressure), and the valve operates at very high pressure drops. Tanker Loadout − A tanker loadout application is susceptible to cavitation

because, for economy, the control valves are typically high efficiency types. At the beginning of the loading cycle, there is large pressure drop across the valve because P2 approaches atmospheric pressure until the level in the tanker increases. This application presents the potential for mild and periodic cavitation as opposed to the constant and severe cavitation that is inherent in other services. Pump Bypass Or Recirculation Valve - The recirculating valve or bypass valve on

a pump typically controls a high pressure drop, low flow stream. The high pressure drops create a significant potential for cavitation, especially for singlespecies fluids such as water. Water Injection - For secondary recovery operations, high pressure water that

often includes brine, sour liquids, and sand is pumped, at high pressure, into the reservoir. Because the pressure drops across the valve are often very large, cavitation is a natural result. In addition, salt can cause chloride stress cracking, the sour liquids can cause sulfide stress cracking, and any particles such as sand can cause rapid erosion. The combination of cavitation, corrosion, and erosion can dramatically shorten valve life unless the specifier selects appropriate anti-cavitation valve designs that are made of corrosion and erosion resistant materials. Anti-Cavitation Valve Technology General Anti-Cavitation Valve And Trim Design Strategies Low Recovery Trim Designs - The most common design strategy that is used to

prevent the occurrence of cavitation is the selection of low recovery valves and trim. The objective is to maintain the fluid pressure at the vena contracta at a pressure that is greater than the fluid’s vapor pressure. As shown in Figure 61, the pressure dip at the valve vena contracta is not nearly as large as it is in a high recovery trim. As the recovery coefficient (F L or Km) approaches a value of 1.0, the pressure dip becomes smaller and smaller. If F L or Km = 1.0 there is no pressure recovery, Pvc will remain above Pv, and, if P2 > Pv, the fluid will not vaporize.

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Figure 61 The Potential For Cavitation As A Function Of Pressure Recovery

Pressure Drop Staging - In order to maintain Pvc above Pv, most anti-cavitation

valve trims employ a pressure drop staging strategy. Pressure drop staging involves directing the fluid through a series of several small restrictions, or stages, as opposed to directing the flow through a single large restriction. Each successive restriction dissipates a certain amount of the available energy and presents a lower inlet pressure to the next stage. As shown in Figure 62, the pressure dip that occurs at each stage is much smaller than the pressure dip that would result from a single large restriction.

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Figure 62 Pressure Drop Staging

Damage Resistant Materials Of Construction- Although the recovery

characteristics of a valve or trim may determine the pressure conditions under which cavitation occurs, the recovery characteristic of a device does not necessarily predict the occurrence of cavitationdamage. Cavitation damage is influenced to a large degree by the ability of the selected trim materials to resist cavitation damage. The material properties that provide the greatest resistance to cavitation damage are hardness and toughness. As a general guideline, materials that provide resistance to cavitation damage include - in order of increasing resistance to damage - 316 stainless steel, 440C stainless steel, 17-4 stainless steel, tungsten carbide, and Stellite (Alloy 6). Figure 63 illustrates an application in which mild cavitation will be expected because there is some fluid vaporization. However, if the valve materials are sufficiently cavitation resistant, cavitation damage may not occur.

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Figure 63 Preventing Cavitation Damage With Damage Resistant Materials Of Construction

Specific Anti-Cavitation Valve And Trim Designs Straight Through Holes, Radial Flow Designs - The cage that is shown in Figure

64 includes multiple, straight-through holes. The holes serve several functions. •

In a flow-down configuration, the holes direct the collapsing vapor cavities to the center of the cage. The flowstream loses some energy as the individual flow streams impinge upon one another. In addition, vapor cavity collapse is likely to occur in the center of the cage rather than near critical boundary surfaces.

•

The holes separate the large free jet into many small flow streams and the total flow stream energy is divided into many small energy sources. By breaking the single, large flow stream into many small streams, the frequency of the noise that is generated is shifted upward. At higher

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frequencies, valve and pipeline vibration are less likely to produce significant problems. This trim style is useful for treating low levels of cavitation. For large valves (> 12 to 16 inches) and for large pressure drops (>300-400 psid), some manufacturers have successfully minimized low level, low frequency vibration problems by installing this trim in a flow-up configuration. Use Word 6.0c or later to

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Figure 64 Single Stage Cavitation And Noise Control

Multi-Stage, Parallel Hole, Radial Flow Design - This method of pressure staging is

incorporated in many manufacturers trim designs. Figure 65 shows an example of Fisher Controls’ Cavitrol III trim. The geometry of the holes is specially designed to provide effective pressure staging while maintaining maximum flow capacity. Trims are available to provide one, two, three, or four stages of pressure reduction.

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Figure 65 Fisher Controls Cavitrol III Trim

Multi Stage, Offset Hole, Radial Flow Design - This design is incorporated in

Valtek’s ChannelStream trim. The trim is essentially a cartridge that is made of several concentric cylinders. As shown in Figure 66, the flow travels first through the holes in the outer cylinder and it then enters a channel that is machined into the second cylinder. This flow path is repeated in successive stages to provide up to six stages of pressure reduction.

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Figure 66 Flow Concept Of The Valtek ChannelStream Trim

Stacked Plate, Tortuous Path, Radial Flow Designs - The “stacked plate” or

tortuous path approach to pressure reduction is employed in the Valtek TigerTooth trim and in the CCI Drag trim. In Valtek’s Tiger-Tooth trim (see Figure 67), concentric grooves (or teeth) are machined on both sides of a series of circular stacked disks. Flow passes from the center of the disc in a radial, wave-like manner. The numerous turns in the flow path provide the staged pressure reduction that is desired. Trim is available with up to seven stages of pressure reduction.

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Figure 67 Flow Path Through The Valtek Tiger Tooth Trim

CCI’s DRAG trim also includes a number of plates. Each plate includes multiple flow passages and each passage includes a number of right-angle turns as shown in Figure 68.

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Figure 68 Control Components Incorporated Drag Trim

Axial Flow, MultiStep Design - This trim design is the basis of Masoneilan’s VRT

(Variable Resistance Trim) product. As shown in Figure 69, the flow is directed upward and parallel to the axis of the valve plug and stem. The trim is made up of a number of plates that are drilled or machined to create a flow path that includes many turns or stages. When the valve plug is throttling near the seat, the flow is forced through a maximum number of stages. As the valve plug approaches the open position, the flow is directed through fewer and fewer stages. As a result, this trim is most suitable for applications where the pressure drop decreases with increasing flow; i.e., the potential for cavitation and cavitation related problems decreases at the normal and maximum flow rate. In constant pressure drop applications where cavitation could occur at any or every point in valve travel, this trim may not provide the required cavitation protection.

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Figure 69 Masoneilan VRT Trim Concept

Notched Plug, Axial Flow Design - This trim design is the basis for Masoneilan’s

“Lincoln Log” trim that is shown in Figure 70. In this axial flow design, the plug and cage are machined to form several throttling surfaces, or stages, along the length of the plug. As the valve is stroked, each stage throttles in unison and the pressure drop is divided among each of the stages. Because the flow passages in the Lincoln Log trim are larger than the flow passages in most other anti-cavitation trim designs, the Lincoln Log trim is especially tolerant of dirty fluids.

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Figure 70 Flow Pattern Of The Masoneilan Lincoln Log Trim

Combination Axial And Radial Flow Design - Fisher Controls Cav IV trim is an

axial flow design that includes a drilled-hole, radial flow cage element for each axial stage. As shown in Figure 71, the flow is directed downward through the valve. After the flow passes each axial stage, the flow is directed through a drilled hole cage. The advantage of this design is that the large number of stages can eliminate cavitation in highly demanding applications.

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Axial Flow Path Through The Valve

Flow

Drilled Hole, Radial Flow Cages

Cav4

Figure 71 Combination Axial Flow With Radial Flow Cage Elements

Brute Strength Approach To Cavitation Damage Control - Some applications

present challenges that cannot be met by sophisticated anti-cavitation valve technology. For example, when a fluid is extremely erosive, is alternately flashing and cavitating, and includes large particles that are not compatible with the small flow passages of most anti-cavitation trim designs, specifiers may select a “brute strength” approach to damage control. Figure 72 shows a sweep flow, angle body valve with tungsten carbide trim and a hardened outlet liner. This particular valve design has provided long valve life in difficult applications where other, more sophisticated approaches have failed.

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Figure 72 A “Brute Strength” Approach To Cavitation Damage Prevention

Custom Valves Characterized Anti-Cavitation Trim - Most standard anti-cavitation trims produce

an approximately linear inherent flow characteristic. In applications where multiple stages of pressure reduction are required at low flow conditions only, a standard multi-stage trim can unnecessarily reduce the maximum capacity of Saudi Aramco DeskTop Standards

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the trim and require the selection of a very large valve size. When the pressure drop and the potential for cavitation related problems decrease at the maximum flow condition, most anti-cavitation trims can be characterized as shown in Figure 73. In a characterized anti-cavitation trim, the trim includes the number of stages that are required to prevent cavitation when the valve is throttling near the seat and the pressure drop is at maximum. At mid travel positions where the pressure drop decreases, the number of stages is reduced. When the valve is fully open (or nearly so) and the pressure drop is at its minimum value, the number of stages may be further reduced or, if there is no potential for cavitation, the trim may include straight-through flow passages. Although Figure 73 illustrates the means by which a drilled-hole cage is characterized, nearly all trim designs can be custom characterized. Use Word 6.0c or later to

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Figure 73 Decreasing Pressure Drop Application And Characterized Anti-Cavitation Cage

Super Severe Service Custom Valves - Many manufacturers have the capability to

design and manufacture super-special valves for difficult, super-severe service applications. Super-special valves are unique, one-of-a-kind designs that are designed specially designed for unique and especially demanding applications. For example, Figure 74 shows a custom valve that was designed for use as a Saudi Aramco DeskTop Standards

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liquid level control valve on a high pressure separator. The valve was designed to provide cavitation protection, corrosion resistance, and erosion resistance. The initial flow direction is flow down through a drilled hole cage. The upper cage provides one stage of cavitation protection and forces the flashing and outgassing to occur in the void between the upper and lower plugs. The lower cage provide the benefits of a flow-up orientation; i.e., the flow is broken into several smaller jets to prevent valve plug instability that can result from flashing and outgassing. Although custom valves have a high first cost, they may be the most economical solution when “standard” valves do not provide satisfactory performance or valve life. Use Word 6.0c or later to

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Figure 74 Super-Severe Service Valve For A Cavitating, Erosive, Corrosive, And Outgassing Fluid

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Control Valve Selection Considerations Performance Objective: Cavitation Damage Control Versus Cavitation Prevention When selecting valves for cavitating applications, one must determine whether the objective is to select trim that will totally eliminate the potential for cavitation (by preventing bubble formation) or to eliminate the potential for cavitation damage. Cavitation Prevention - For a majority of applications, trims are available that will

totally eliminate cavitation by preventing the formation of bubbles in the flow stream. In high pressure and high pressure drop applications, the prevention of cavitation may require a large number of stages which in turns leads to larger and larger body sizes and more costly valves. For critical applications that are constantly operated at severe conditions, the selection of valves and valve trim that will totally eliminate cavitation may be the most cost-effective solution over time. Prevention Of Cavitation Damage - In applications where the potential for

cavitation damage only occurs at system startup, system shutdown, or during operating transients, it may be more economical to: 1.

Select a trim design that will prevent fluid vaporization during normal operating conditions.

2.

Select materials of construction that will resist cavitationdamage during startup, shutdown, and other periods of operating transients.

For example, it may be more economical to select a smaller two-stage trim that is made of 316 stainless steel with Alloy 6 hardfacing than a larger valve with a four-stage trim that is made of a less damage resistant material such as a standard 410 or 416 stainless steel. Manufacturers Control Valve Selection Procedures Various techniques have been developed by valve manufacturers to evaluate system conditions and select anti-cavitation control valves. Although each manufacturer’s methods and techniques are different, most methods involve two major steps. Step 1. Assessment of the system potential for cavitation with the use of a system cavitation index. Step 2. Selection a valve with a valve cavitation index that is appropriate for the value of the system cavitation index. Several manufacturer’s methods for valve selection are discussed below.

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Fisher Controls - Fisher Controls’ method for anti-cavitation valve and trim

selection is a two-step process. 1.

Calculate the system cavitation index. The system cavitation index is the application ratio, or Ar, where: Ar = P1-P2/P1-Pv

2.

Select a valve with a Kc rating that is greater than the value of Ar.

The Kc values that are published by Fisher Controls are designed to guide one to the selection of a specific valve and trim that will prevent cavitation related problems; i.e., damage from vapor cavity implosion, excessive noise, or excessive valve and piping vibration. The Kc values are based on the recovery coefficient of the valve as well as experiential factors that also take into account the materials of construction, the valve size, and the pressure drop. Valtek - To select a Valtek anti-cavitation valve and trim, one also performs a

two-step procedure. 1.

Calculate the system cavitation index,σoperating, where: σoperating = P2-Pv/P1-P2

2.

Select a valve with a σmin rating that is less than the value of σoperating.

The σmin values that are published by Valtek are designed to guide one to an estimated valve size only. Valtek’s literature indicates that final sizing must be performed by factory personnel who will account for pressure scaling effects, size scaling effects, trim exit velocity, and other factors. Masoneilan - To select an appropriate Masoneilan anti-cavitation valve and trim,

an initially selected valve is evaluated in terms of its pressure recovery coefficient and the calculated value of∆Pcritical (∆Pallow or ∆Pchoked) versus the value of the actual pressure drop ∆ ( Pactual). 1.

Calculate the value of ∆Pcrit, where: ∆Pcrit = Cf2(∆Ps) Cf = FL ∆Ps = P1 - FfPv

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Note: The value of

Pcrit is the same as the choked flow pressure drop that is calculated with the use of the standard ISA liquid flow sizing equations. 2.

Compare the value of ∆Pcrit with the actual ∆P. If the value of ∆Pactual is less than the value of ∆Pcrit, the selected valve is satisfactory. If the value of ∆Pcrit is less than the value of∆Pactual, then a valve with a higher Cf (FL) should be selected.

CCI - The basic criteria for selection of a particular CCI Drag trim is the

selection of a trim that will limit fluid velocity at the trim exit to a value that is less than 100 feet per second. Because the indices and calculations that are used to calculate fluid velocities throughout the valve are somewhat complex, most specifiers make use of CCI’s valve selection and sizing software in order to select an appropriate valve and trim. Valve Performance Contingency Requirements Changes In Service Conditions - Specifiers should always allow for the possibility

that the valve will be operated at pressure drops that are higher than those that are specified on the ISS or on the process and piping drawings. In addition, the system may be operated at elevated fluid temperatures which will cause an increase in the value of Pv and an increase in the potential for cavitation. In order to minimize the potential for cavitation related problems when service conditions change, specifiers should always specify a valve with an extra margin of cavitation resistance. For example, if the value of K sa for a given application is 0.85, then a valve with a Kc of approximately 0.9 should be considered. Size Scale Effects - Control valve manufacturers often interpolate the cavitation

indices for large valves on the basis of research that has been performed on smaller valves. Because of size scale effects (larger valves often cavitate more readily and more intensely than smaller valves of the same design), the valve cavitation index for a large valve may be somewhat overrated. Within Saudi Aramco, it has been observed that manufacturers often ignore or miscalculate the effects of valve size on cavitation damage resistance. Therefore, when large anti-cavitation valves and trim are being selected, the specifier should allow for an additional margin of cavitation protection; i.e., the specifier should select a valve with a higher Kc, lower σmin, etc. than is indicated by the normal calculations. Generally speaking, size scale effects should be considered for all valves that are larger than 6 inches unless size scale affects have been fully considered by the manufacturer. Sensitivity To Accurate Data Saudi Aramco DeskTop Standards

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Importance Of Accurate Fluid Properties - When specifying anti-cavitation valves,

the specifier must make a concerted effort to secure accurate fluid properties. For example, the calculations that are used to predict cavitation are highly dependent upon the value that is given for the fluid’s vapor pressure. Figure 75 shows that if an incorrect value is given for the vapor pressure, one may determine that an application is flashing when it is actually cavitating, or vice versa. Use Word 6.0c or later to

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Figure 75 Fluid Behavior Versus The Value Of P v Importance of Accurate Service Conditions - The need to secure accurate service

conditions is illustrated in Figure 76 which shows a plot of vibration as a function of P1-P2/P2-Pv. Note the rapid increase in vibration that follows the onset of incipient cavitation. If the pressure drop is understated or overstated in this range, then cavitation intensity cannot be accurately predicted.

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Figure 76 Vibration Intensity As A Function Of P 1-P 2/P2-P v

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Importance Of Defining Worst Case Cavitating Conditions When specifying valves for cavitating applications, the specifier should make a concerted effort to identify the worst-case conditions for cavitation. Some of the conditions that help to define the “worst-case” scenario are described below. Startup And Shutdown - During system startup and shutdown, the system is

often operated at low flow conditions for some period of time. In many applications, low flow operation is accompanied by high pressure drops that increase the potential for cavitation. To protect against cavitation damage during system startup and shutdown, the service conditions should be clearly identified and considered during the selection process. Changes In Operating Conditions - Specifiers should remain alert to the

possibility of changes in operating conditions. For example, if the system is likely to be operated at an elevated temperature, with a higher inlet pressure, or with a reduced outlet pressure, an allowance for additional cavitation damage prevention should be made during valve selection. Reduced Throughput - If it can be anticipated that the system will be operated at

reduced capacity (extreme turndown), the reduced capacity service conditions should be evaluated during the valve selection process and a valve should be selected that will prevent cavitation related problems. Cavitation In Combination With Other Severe Conditions Cavitation in combination with other severe service conditions such as corrosion and/or erosion can quickly compound the rate and intensity of cavitation damage. When fluids are erosive or corrosive, specifiers must give special attention to the materials of construction that are selected. Anti-Cavitation Trim And Flashing Applications In some instances, a fluid may be cavitating at the normal flow condition and the maximum flow condition while it is flashing at a low flow condition. Two general guidelines help to guide the specifier under this circumstance. 1. Of the two phenomenon, cavitation is by far more damaging than flashing; therefore, cavitation must be treated with an appropriate anti-cavitation trim. 2. In multi-stage, anti-cavitation trims, the flashing damage is most likely to occur between the trim stages (inter-stage flashing). Therefore, the following guidelines apply. a.

If possible, select single stage anti-cavitation trim.

b.

If multi-stage anti-cavitation trim must be selected to prevent cavitation related problems, select materials of construction that are highly

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resistant to damage from vapor cavity implosion and from flashing erosion; i.e., 316 stainless steel with Alloy 6 hardfacing, solid Alloy 6, tungsten carbide, etc. Non-Valve Methods Of Reducing The Potential For Cavitation During the valve selection process, the specifier should remain alert to means of minimizing cavitation related problems other than valve selection. Two such possibilities are discussed below. System Design - In some applications, a change in valve placement can help to

minimize cavitation related problems. For example, moving a feed valve from a mid-line position to a tank mounted position can reduce the potential for cavitation damage to the valve and piping. By mounting the valve on or near the tank as shown in Figure 77, the vapor cavities will implode inside the vessel where they will not cause damage to valve parts or pipe walls.

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Figure 77 Minimizing Cavitation Damage Through Valve Placement

Valve Elevation - A change in the elevation of a valve can also have a significant

impact on the potential for cavitation and cavitation related problems. For example, Figure 78 shows the difference in the values of P 1 and P2 of a distillation column feed valve when the valve is located near the top of the column (Installation A) and when the valve is located near the bottom of the column (Installation B). When the valve is located near the bottom of the vessel, P1 is increased because there is less friction loss and less head loss. P2 is also increased because of the additional head at the valve outlet. Both pressure conditions serve to decrease the potential for cavitation.

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Figure 78 Minimizing Cavitation Damage By Changing The Elevation Of The Valve

ISA System Indices From ISA-dRP75.23 Standardization - Given the broad range of methods for predicting cavitation and

for rating the cavitation resistance of a particular control valve, there is considerable confusion and controversy concerning the preferred methods for system assessment and for assigning valve indices. In an effort to standardize system assessment and valve selection procedures, ISA subcommittees have prepared a draft recommended practiceISA-dRP75.23 Considerations For Evaluating Control Valve Cavitation . This standard provides a recommended methodology for evaluating the potential for cavitation and for rating the cavitation resistance of control valves. Saudi Aramco DeskTop Standards

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System Cavitation Parameter σsystem - In order to evaluate the potential for

cavitation in a given system, the ISA recommends the parameterσsystem where σsystem = (P1-Pv)/(P1-P2). Using this analysis, the potential for cavitation increases as σ approaches 1.0. Refer to Figure 79. As the value ofσsystem increases from 1.0 to approximately 17, the potential for cavitation decreases. σsystem is related to the Saudi Aramco index Ksa as follows: σsystem = 1/Ksa. The parameter σ as used by the ISA is not the same as theσ parameter that is used by Valtek. Use Word 6.0c or later to

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Figure 79 ISA System Cavitation Index σsystem σ: Universal Control Valve Cavitation Index - The parameter σsystem only quantifies the service conditions. By itself, the value ofσsystem does not convey any information about the performance of a particular valve in a particular application. In order to gain utility from the parameterσsystem, the ISA recommended practice describes a methodology in which the behavior of a specific valve can be predicted as a result of the value ofσsystem. The ISA recommended practice suggests that manufacturers test their valves under standard test conditions and assign several valve performance indices. Several indices are illustrated in Figure 80 and they are discussed below. Saudi Aramco DeskTop Standards

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Figure 80 ISA-dRP75.23 Control Valve Cavitation Indices

•

σI - The coefficient of incipient cavitation is the value of (P 1-Pv)/(P1-P2) at which cavitation can first be detected. This coefficient can be determined with noise or vibration measurements as shown in Figure 80.

•

σc - The coefficient of constant cavitation is the value of (P 1-Pv)/(P1-P2) at which mild, steady cavitation occurs. Damage is not usually associated with this level of cavitation. This coefficient can be determined with noise or vibration measurements as shown in Figure 80.

•

σid - The coefficient of incipient cavitation damage is the value of (P 1Pv)/(P1-P2) at which the onset of cavitation damage occurs. This value

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cannot be determined from noise or vibration measurements; it must be evaluated in the laboratory. •

σch - The coefficient for choking cavitation is the value of (P 1-Pv)/(P1-P2) that causes choking in the valve. The maximum levels of noise, vibration, and material damage have been observed to occur at or just prior to this condition. The value of σch may be estimated with the following: σ ch =

P1 − Pv FL 2 (P1 − FF Pv )

•

σmv - The coefficient of maximum vibration can be determined by identifying the value of (P1-Pv)/(P1-P2) at which the maximum vibration or noise occurs on a plot such as the one shown in Figure 80.

•

σmr - The manufacturer’s recommended minimum limit σ( mr) is an operational limit that is supplied by the valve manufacturer. The determination of this value may be based on laboratory analysis, experience with specific applications, or an understanding of specific valve features.

σ Parameters That Are Used During Valve Selection - The specifier may select a

specific valve on the basis of any of the aboveσ parameters. For example, if the value of σsystem is 2.5 and the specifier’s objective is to limit cavitation to the level of constant cavitation, the specifier would select a valve with aσc of 2.5 or less. The decision of which parameter to use during the selection of a particular valve is largely subjective and may depend upon many factors such as valve style, percentage of valve travel, duty cycle, location, desired life, and past experience. According to the draft recommended practice, “the valve manufacturer should be consulted in this matter.” Scale Effects - ISA-dRP75.23 includes provisions for calculating size scale

effects (SSE) and pressure scale effects (PSE) for a particular control valve. The calculations are explained in the draft. Piping Factors - Upstream pipe reducers and downstream expansions cause a

variation in cavitation levels and in sizing coefficients. To account for these effects, ISA-dRP75.23 defines a mathematical procedure for evaluating the effects of reducers and expanders (swages) on the performance of a particular valve. Future Application Of ISA -dRP75.23 - When manufacturers fully endorse and

comply with the recommended practiceISA-dRP75.23, and when Saudi Aramco DeskTop Standards

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manufacturers test and publish the various values ofσ that describe valve performance in cavitating applications, the specifier will have a massive amount of unbiased data on which to base his valve selection decisions. However, considerable time will be required for manufacturers to test their products and to publish the results. Immediate Application Of ISA-dRP75.23 - Until valve manufacturers complete comprehensive testing of their products and until the results of the tests are published, the only data that is likely to be available is a listing ofσmr values. The σmr values that will be initially published will likely to be translated from existing cavitation indices.

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SELECTING AND SIZING CONTROL VALVES FOR FLASHING FLUID Applications Flashing And Its Consequences Review Of Flashing Phenomenon Flashing Compared To Cavitation - When, in a liquid flow, the fluid pressure falls

below the fluid’s vapor pressure, the fluid begins to vaporize; i.e., vapor bubbles form in the flow stream. In a control valve, the onset of vaporization often occurs near the vena contracta, as shown in Figure 81. If the downstream pressure (P2) increases to a value that is greater than the fluid’s vapor pressure (Pv), the bubbles collapse and the fluid is cavitating. If the downstream pressure P2 is less than the fluid’s vapor pressure, the bubbles, or vapor cavities, remain in the fluid stream and the fluid is flashing. It is important to note that flashing occurs only as a function of the values of P v and P2; i.e., flashing is independent of the inlet pressure P1 and the vena contracta pressure Pvc. Use Word 6.0c or later to

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Figure 81 Flashing Phenomena That Occurs When P 2 90 dBA For A Standard Valve Whenever any standard control valve with standard trim and with standard downstream piping generates an SPL that is in excess of 90 dBA, the specifier must evaluate the application in terms of excessive noise generation.

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Outlet Velocity Greater Than 0.3 Mach A valve outlet velocity that is greater than 0.3 Mach indicates the potential for excessive noise generation. 0.3 Mach is the approximate boundary at which the noise that is propagated into the downstream piping becomes greater than the noise that is generated in any valve, including those valves with quiet trim options. P1/P2 > 5 For Dry Gas And Superheated Steam Services Significant potential for excessive noise generation exists whenever the pressure ratio (P1/P2) in psia of dry gas and superheated steam is 5.0 or greater. Pressure ratios of 5.0 or greater can create the high outlet velocities that are associated with high levels of aerodynamic noise. SPL > Limits That Are Established By Saudi Aramco Engineering Standards Saudi Aramco Engineering Standards SAES-J-700 and SAES-A-105 that were previously discussed define absolute SPL limits for various applications and conditions. Specific Applications In addition to the flags that are described above, many services are known, through experience, to present the potential for excessive levels of aerodynamic noise. Control valve applications that commonly present high potential for noise generation include: •

Compressor bypass valves.

•

Atmospheric vent valves.

•

Gas injection valves.

Predicting Control Valve Noise Introduction The SPL of a valve that is being considered for a given application can be predicted with the use of various noise prediction equations that have been developed by valve manufacturers, by standard organizations such as the ISA and the IEC, and by academia.

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Influences On Noise Generation And Transmission The common noise prediction methods are based on broadly varying approaches to acoustic theory, experience, and laboratory research. Most techniques could be made more precise if more complete system and operating conditions could be made available. However, the common prediction techniques are reasonably accurate in view of the information that is commonly available to the specifier. The parameters that are typically included in prediction methods are discussed below. Pressure Drop ( ∆ P) - The pressure drop across the valve is a representation of

the total energy that is available to be converted into sound energy. Flow Rate - Mass flow, in conjunction with other factors, helps to quantify the

total stream power that can be converted to noise. Pressure Drop Ratio ( ∆ P/P1) - The pressure drop ratio serves to account for fluid

velocity. The impact on velocity is as follows:

Impact On Fluid Velocity ≈

1 1 − ∆P / P1

Downstream Pressure P 2 - The downstream pressure influences the fluid density

and therefore the fluid velocity at the valve outlet. Downstream pressure also influences the degree of coupling that occurs.

Valve Acoustic Efficiency Factors - Acoustic efficiency is a measure of how much

of the total flow stream energy will be converted to sound (the ratio of the stream power that is converted into sound to the stream power). Acoustic efficiency is a complex function of the valve’s pressure recovery coefficient, the number and size of flow passages, other valve design factors, and the relative pressures at the inlet, vena contracta, and the valve outlet. Downstream Pipe Size - The size of the downstream piping has a direct effect on

fluid velocity in the downstream system. Downstream Pipe Schedule - The mass and the acoustic characteristics of the

downstream piping influence the degree of acoustic coupling and the transmission losses that occur at the pipewall. Valve Noise Peak Frequency Vs. Pipe Coincident Frequency - As the peak

frequency of the valve generated noise approaches the pipe coincident frequency (the natural resonant frequency of the pipe), a greater degree of coupling occurs and more noise is transmitted to the environment. As the frequency of the valve generated noise moves away from the pipe coincident Saudi Aramco DeskTop Standards

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frequency, coupling is less complete and less noise is transmitted to the environment. Fluid Temperature - The fluid temperature influences fluid density and therefore

fluid velocity. Distance From Source - The measured SPL decreases as the point of

measurement is moved further from the source. Noise Prediction Equations Differences In Nomenclature - The equations that have been developed by each

valve manufacturer and the equations that are endorsed in ISA/IEC standards do not necessarily include terms for each of the influences previously described. Many noise prediction techniques account for multiple influences with a single term and minor influences are sometimes ignored. In addition, manufacturers often use entirely unique approaches that involve proprietary coefficients that prevent the user from "reverse-engineering" the equations. As a result, each manufacturer's prediction techniques are designed to provide accurate results for that manufacturer's valves only. Application - Generally speaking, several different methods may be used to

apply the noise prediction techniques that have been developed by valve manufacturers. Methods include: •

Direct calculation with the use of appropriate equations.

•

Graphical methods in which one refers to a series of charts or tables to determine the values of the various components of the total noise level; e.g., one may determine the SPL that is associated with the pressure drop, with the pressure ratio, with the downstream piping, etc. and, then, sum all the components.

•

Sizing software that calculates SPL levels at the same time the valve sizing equations are solved.

Of the three methods, the software approach is by far the most time-efficient and the most preferred. Fisher Noise Prediction Equations - Fisher Controls’ noise prediction equation is

as follows: LpA = DLpADP+DLpACg+DLpADP/P1+DLpAK+DLpAP2 + ∆LpAM2 Saudi Aramco DeskTop Standards

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Where: LpA

Sound pressure level (dBA)

DLpA∆P

Base SPL as function of DP

DLpACg DLpA∆P/P1

The correction for the required Cg The correction for the pressure drop ratio DP/P 1abs

DLpAK

The correction for the pipe size and schedule.

DLpAP2

The correction for valve outlet pressure, P2 (psig) ∆LpAM2

Correction to be used only when the valve outlet velocity is higher than the recommended outlet velocity

Valtek Noise Prediction Equations - Valtek’s noise prediction equation is as

follows: SPL = Vs + Ps + Es + Ts + Gs + As Where: SPL

Sound pressure level (dBA)

Vs

flow factor - 6.95 Ln (Cv) + 4.8

Ps

pressure factor - 9.03 Ln (P1) +17.2

Es

pressure ratio factor - 30 Log (∆P/P1) + 25.24

Ts

temperature correction factor - -7.68 Log (T1) + 20.78

Gs

gas property correction factor - 7.26 Log (Mw) - 11

As

pipewall attenuation factor - from Valtek Engineering Bltn. 3 Masoneilan Noise Prediction Equations - The equation that Masoneilan publishes

in the literature that address noise prediction is as follows: SL = 10log [28CvCfP1P2D2ηT /t3] + SLg

Where: SL

Sound pressure level, dBA

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28

Units constant

Cv

Actual required flow coefficient

Cf

Critical flow factor (same as FL)

P1

Upstream pressure, psia

P2

Downstream pressure, psia

D2

Downstream nominal pipe diameter, inches

η

Acoustical efficiency factor, dimensionless; determined graphically from a chart.

T

Absolute temperature, degrees R

t

Pipe wall thickness, inches

SLg

Gas property factor, dBA; dimensionless; determined graphically from a chart.

CCI Noise Prediction Equations - The equations that Masoneilan publishes in the

literature that address noise prediction are as follows: Trim Noise (SPLt) SPLt = dBw + dBp1 + dB 2 - dBp2 - dBnt - dBd2 - dBt2 - dBr + 63 - A Pipe Noise (SPLp) Inlet: SPLp1 = dBw + dB 1 + dBd1 - dBt1 - dBr - 104 - A Outlet: SPLp2 = dBw + dB 2 + dBd2 - dBt2 - dBr - 104 - A Where: SPLt

Total trim noise in dBA

dBw

function of mass flow rate

dBp1

function of inlet pressure

dBρ2

function of outlet fluid density

dBp2

function of outlet pressure

dBd1

function of the inlet pipe I.D.

dBnt

function of number of turns in the disk

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dBd2

function of the downstream pipeI.D.

dBt2

function of the downstream pipewall thickness

dBr

function of near field distance

63

units constant

104

units constant

A

thermal lagging; 5 dBA for steam and 0 dBA for no thermal lagging.

ISA Noise Prediction Equations - ISA Standard S75.17-1989 and Part 8, Section

3 of IEC 65B/231/DIS describe a very thorough method for predicting the SPL of a standard valve. The ISA/IEC noise prediction method requires one to manually solve up to forty or more equations in order to calculate an estimated SPL value and the method appliesonly to standard valves (those that do not include quiet trim options). The technique involves the following major steps: 1. Determination of a regime. Every application will fall into a regime (Regime I through Regime V) depending on the pressure conditions. 2. Determination of the acoustic efficiency factor for the regime. The acoustic efficiency factor, η, is a measure of the flow stream energy that can be converted to sound energy. 3. Calculation of the sound power level. The sound power level, W a, is calculated with the use of the acoustic efficiency factor and a corrected value that indicates the stream power. 4. Calculation of the valve internal sound pressure level, pLi. Lpi is calculated as a log function of a constant, the sound power (previously calculated), the mass density, the speed of sound under downstream conditions, and the inside diameter of the downstream piping. 5. The transmission loss is calculated as a function of the pipe coincident frequency and the peak generated frequency of the control valve noise. 6. The final sound pressure level is calculated as a function of the internal sound pressure level that is corrected for transmission loss. Because it is mathematically intensive, because its use is strictly limited to outlet velocities that are equal to or less than 0.3 mach, and because it cannot be used to predict the SPL of noise-abatement valves, specifiers rarely make use of the ISA/IEC equations. Specifiers do, on occasion, use the ISA/IEC Saudi Aramco DeskTop Standards

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equations to determine if the results that are obtained with a manufacturers method are reasonably accurate.

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Control Valve Options For Attenuating Control Valve Noise Source Treatments Vs. Path Treatments The methods that are used to attenuate aerodynamic control valve noise can be categorized as either source treatments or path treatments. Common source and path treatments are shown in Figure 107 and they are discussed below. Use Word 6.0c or later to

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Figure 107 Source Versus Path Treatments Of Aerodynamic Noise Source Treatments - The term source treatment refers to the measures that are

taken to actually reduce the amount of noise that is generated by the valve. Source treatments address the cause of the noise, rather than the symptom. As Illustrated in Figure 107, the most common source treatment is the selection of special control valve trim that is designed to reduce the level of noise that is generated in the valve and propagated through the downstream piping. Because source treatments address the problem rather than the symptom, Saudi Aramco typically prefers source treatments over path treatments. Path Treatments - The term path treatment refers to any measure that is taken to prevent the noise that is generated within the valve and the piping from reaching the environment. As shown in Figure 107, common examples of path treatments include heavy-walled pipe, pipeline insulation, and equipment that is inserted into the pipeline that reduces the intensity of the sound that reaches the environment. Saudi Aramco DeskTop Standards

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Valve Style Versus Noise Attenuation Different valve styles (globe, angle, ball, butterfly) of the same capacity will produce broadly differing levels of aerodynamic noise. When noise attenuation in a throttling application is an objective, the selection of a globe or angle style valve is generally recommended for the following reasons: • Low efficiency valves (those with high Km or FL values) tend to limit the maximum fluid velocity to a greater extent than high efficiency (ball and butterfly) valves. • A broad range of special noise-abatement trims are routinely available for globe and angle style valves whereas only a few noise-abatement options are available for rotaryshaft (ball and butterfly) valves. Body Options For Globe And Angle Valves Valve bodies with enlarged flow areas and with expanded outlet connections are often used to limit aerodynamic noise by reducing velocities. Refer to Figure 108. Enlarged bodies are typically identified with a nomenclature such as “an 8x6 body”, indicating an 8-inch body that includes 6-inch trim. A valve that is described as having a 6x8x8 body will have a 6inch inlet, an 8-inch nominal body size, and an 8-inch outlet connection. Use Word 6.0c or later to

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Figure 108 Example Of An Expanded Flow Areas And Expanded Outlet Connection Noise Abatement Trim Design Strategies Saudi Aramco DeskTop Standards

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Reducing Sound Power Levels - Most noise-abatement valve trims are designed to separate the flowing fluid into many small flow streams. The division of the large free jet into many small fluid streams reduces the scale of the shock cell shear and the intensity of the consequent noise. The strategy of breaking the fluid stream into several small streams is effective because of the relationships between port area, sound power, and the sound pressure level. These relationships are illustrated in Figure 109. Use Word 6.0c or later to

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Flow

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WA ∝

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WB ∝ 8

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2 x WB,

LpA(A)

LpA(B)

Figure 109 Capacity, Sound Power, And LpA For Single Port And Multi-Port Valve Trim Minimizing Shock Cell Interaction - In order to preserve the benefits of breaking

the free jet (the flow stream that enters the valve) into many small streams, the individual streams must not be allowed to recombine after exiting the trim. Figure 110 shows that the streams grow and recombine as the pressure drop ratio increases. If the streams recombine after exiting the trim, the noise levels will increase. Figure 110 also shows two methods of preserving stream separation. •

The use of smaller passages in the trim.

•

Increased separation of the individual passages.

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Figure 110 Shock Cell Interaction Shifting Frequencies - Another benefit of multiple-passage trims is that the

frequency of the sound that is generated by each small hole is much higher than the frequency of the sound that would be generated by a single large passage. The frequency is often shifted to a frequency that is much greater than the pipe coincident frequency; therefore, much of total noise that is generated in the valve does not couple to the pipewall and it is not radiated to the environment. Refer to Figure 111.

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Figure 111 Valve Noise Frequency Versus The SPL That Is Transmitted To The Environment Commonly Available Noise Abatement Valve Options Slotted Cages - Many manufacturers offer noise abatement trim that is based on a slotted cage design. Figure 112 shows a typical slotted cage that is similar to the Fisher Whisper Trim I design. The slots separate the fluid stream and reduce the amount of flow turbulence, thereby reducing the level of noise that is generated as the fluid flows through the cage passages.

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Figure 112 Slotted Cage Parallel Hole, Radial Flow Trims - Noise-abatement cages that are based on a

parallel hole, radial flow design are commonly available. Valtek’s MegaStream trim is shown in Figure 113 and Fisher Control’s Whisper Trim III cage design is shown in Figure 114. To ensure that the small flow streams remain separated as the fluid exits the cage, trim is typically available with various hole sizes and hole spacing dimensions. In addition, the flow may be directed through several stages of drilled hole components.

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Figure 113 Valtek’s MegaStream Cage Design

Use Word 6.0c or later to

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Figure 114 Fisher Control’s Whisper Trim III Cage Design Tortuous Path Trims - The prime objective of noise-abatement trims that are

based on tortuous path designs is to introduce frictional losses that will reduce the velocity of the fluid as it passes through the trim. Control Components’ Drag trim is shown in Figure 115 and Valtek’s TigerTooth trim is shown in Figure 116. Saudi Aramco DeskTop Standards

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Figure 115 CCI’s Drag Trim Design

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Figure 116 Valtek’s TigerTooth Trim Axial Flow Trims - The axial flow valve and trim design that is shown in Figure 117 includes a multiple-step plug and seat design. Because of the relatively Saudi Aramco DeskTop Standards

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large flow passages through the trim, this trim design is well suited to the control of gasses that include entrained solids. This design is unique to Masoneilan’s 77000 series LO-DB product. Use Word 6.0c or later to

view Macintosh picture.

Figure 117 Masoneilan’s 77000 Series LO-DB Trim Design Characterizing Noise Abatement Trim In many applications, excessive noise may be generated at one flow condition, moderate noise may be generated at another flow condition, and, entirely acceptable levels of noise may be generated at yet another flow condition. If a cage with small holes and wide hole spacing is selected to ensure adequate noise attenuation at the worst-case flow condition, a very large valve size may be required to achieve the needed flow capacity. A characterized cage is often a viable option to the selection of a larger valve. As shown in Figure 118, characterization is accomplished by designing a cage that provides the appropriate balance of noise attenuation and flow capacity that is needed over the rated travel of the valve. For example, if considerable noise attenuation is needed at the minimum flow condition, small, widely spaced holes may be located near the seat. If only moderate noise attenuation is needed at mid-travel positions, larger holes with a wider spacing may provide the needed noise attenuation while providing additional flow capacity. In some applications, the chief concern at the maximum flow condition is flow capacity rather than

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noise attenuation. To achieve the needed capacity, the holes in the upper portion of the cage may be large and closely spaced. Characterized cages are designed for specific application requirements. To enable the design of an optimally characterized cage, specifiers must provide the valve manufacturer with the pressure and flow conditions for as many operating points as possible Use Word 6.0c or later to

view Macintosh picture.

Figure 118 Characterized Noise Abatement Cage Design Common Selection Problems And Specification Errors Absence Of Industry Standards For Noise Prediction Equations The prediction and abatement of control valve noise is an area for which there are few universally accepted standards. As a result, specifiers must remain aware of the ramifications of the broadly varying methods that are used to predict aerodynamic noise and of the valve vendor’s interests in winning bid awards on the basis of low cost. Vendor Tendency To Under-Predict Noise - Although most manufacturer’s noise

prediction techniques are based on sound engineering principles and up-todate acoustic theories, it is only logical that any method of noise prediction would include an accuracy limit of at least plus or minus 5 dBA. However, it is in the manufacturer’s best interest to be as optimistic as possible; i.e., manufacturers may take advantage of rounding, push the performance limits of specific valves, employ prediction techniques other than their published techniques, or calculate the SPL at working distances rather than at standard distances.

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Limited User Ability To Evaluate Submitted Bids - Most noise prediction techniques include some factors for which the derivation is known only to the manufacturer. As a result, there is no common basis for evaluating all the various prediction methods or the amount of noise attenuation that is provided by various noise abatement trim options. Confidence in a particular manufacturer’s claims can only be gained through experience.

Specifier's Failure To Identify Worst Case Service Conditions In order for vendors to submit bids for products that will provide the desired noise attenuation, specifiers must provide complete and accurate data and they must fully document the worst-case service conditions. Worst-Case Scenarios - To ensure proper performance, specifiers must define all

worst case scenarios. Worst-case scenarios for noise generation include the conditions that occur during startup, during shutdown, during emergency situations, and during periods of increased or decreased throughput. Changes In Service Conditions Because of changes in process design, changes in daily throughput, or changes in fluid composition, the valve SPL can change dramatically. Whenever possible, specifiers should anticipate such changes in operating conditions and provide all pertinent data.

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Work Aid 1:

Fluid Compatibility Information That Is Used To S elect Control Valves For Corrosive Fluid Applications

Work Aid 1A: NACE Compliant Materials Of Construction Remarks/ A p p l i c a t i o n s

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p o s t w e l d h e a t t r e a t m e n t

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Instrumentation Specifying Control Valves for Severe Service Applications Cast form

i s n o t N A C E a p p r o v e d ; t h e r e f o r e , b o d i e s m u s t b e f o r g e d

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h a r d f a c e d w i t h A l l o y 6 f o r i n c r e a s e d d u r a b i l i t y

Excellent

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Instrumentation Specifying Control Valves for Severe Service Applications Moderate

i n c r e a s e i n h a r d n e s s o v e r S 3 1 6 0 0 , b u t l e s s

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Highly

e r o s i o n r e s i s t a n t s e a t r i n g s

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a p p r o v e d b u t l o w s t r e n g t h . M a y r e q u i r e l a r g e r s t e m

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s t r o n g e r t h a n S 3 1 6 0 0 .

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C l a s s I I I B o l t i n g i s e x p o s e d t o a t m o s p h e r e a n d

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C l a s s I I B o l t i n g i s e x p o s e d t o H 2 S b e c a u s e

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m a t e r i a l f o r p r e s s u r e r e g u l a t o r s p r i n g s

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Instrumentation Specifying Control Valves for Severe Service Applications Belleville

s p r i n g s i n e x t e r n a l l y l o a d e d p a c k i n g d e s i g n s

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Work Aid 1B:

Recommended Materials Of Construction For Seawater And Brine Services

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Work Aid 1C:

Valve And Material Selection Guidelines For Amine (DGA) Letdown Applications

Refer to the following: • The compatibility table (Table I) that is located in SAES-L-008. • Fisher Controls PS Sheet 59:042(A) Application Guideline - Rich Amine Letdown Valve (located in the Addendum of This Module).

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Work Aid 2:

Hierarchical Listings Of Erosion Resistant Valve Styles And Construction Materials

Work Aid 2A:

Hierarchy Of Erosion Resistant Valve Styles That Is Used To Select Control Valves For Erosive Fluid Applications Comme

Valve S t y l e Cageg u i d e d v a l v e s

n t

Potential f o r t h e p l u g b i n d i n g i n t h e

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c a g e . S u i t a b l e w h e n t h e v o l u m e r a t i o o f p a r Saudi Aramco DeskTop Standards

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. Cage-

Angle g u i d e d

b o d y r e d u c e s

a n g l e v a l v e s

d a m a g e . Post

Postg u i d e d v a l v e Saudi Aramco DeskTop Standards

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s

c e s p l u g b i n d i n g . D e s i g n s w i t h p r o t e c t e d b u

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s h i n g s o f f e r i n c r e a s e d p r o t e c t i o n . Post-

Angle G u i d e d

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i n i m i z e s

A n g l e V a l v e s

b o d y d a m a g e .

Post-

Liner G u i d e d A n g l e V a l v e s

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v a l v e

w i t h h a r d e n e d , r e p l a c e a b l e

o u t l e t a n d

o u t l e t l i n e r

p i p i n g .

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t a r y P l u g V a l v e s

r o u g h f l o w p a t h m i n i m i z e s i m p i n g e m e n t o n c r

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0 0 . Sweep

Very F l o w ( V e n t u r i S t y l e ) A n g l e V a l v e s

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a t i n g s t o A N S I C l a s s 9 0 0 .

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Work Aid 2B:

Hierarchies Of Erosion Resistant Body And Trim Materials That Are Used To Select Control Valves For Erosive Fluid Applications Hierarchy Of Erosion Resistant Body Materials

Remarks

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Instrumentation Specifying Control Valves for Severe Service Applications A standard m a t e r i a l . M a y b e s e l e c t e d f o r m i l d l y e r o s i v e

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g r e a t e r e r o s i o n r e s i s t a n c e t h a n c a r b o n s t e e l

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e r o s i o n r e s i s t a n c e i n f l a s h i n g a p p l i c a t i o n s

Hierarchy Of Erosion Resistant Trim Materials

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Good corrosion resis tanc e but, in its basi c form , offer s little erosi on resis tanc e.

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Instrumentation Specifying Control Valves for Severe Service Applications Typically heattreat ed to HRC 38. Goo d erosi on resis tanc e but lack s gene ral corr osio n resis tanc e.

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Instrumentation Specifying Control Valves for Severe Service Applications Typically heattreat ed usin g H10 75 (HR C 32) for stan dard servi ce and with H11 50 (HR C 33) for NAC E. Goo d stren gth, hard ness , and erosi on resis tanc e.

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Instrumentation Specifying Control Valves for Severe Service Applications Hardfacing on plug tips, plug guidi ng surfa ces, and seat rings provi des exce llent resis tanc e to erosi on and corr osio n.

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hard ened to 5660 HRC . Very hard and erosi on resis tant in noncorr osiv e appli catio ns. such as boile r feed wate r and stea m. Very susc eptib le to SCC .

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Instrumentation Specifying Control Valves for Severe Service Applications Very tough mate rial with supe rior erosi on resis tanc e. Corr odes rapid ly in the pres ence of som e boile r feed wate r corr osio n inhib itors (hyd razin es).

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erosi on and wear resis tanc e; how ever, the bind ers that hold the tung sten carbi de are susc eptib le to corr osio n in som e appli catio ns inclu ding hydr azin etreat ed boile r feed wate r and amm onia.

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Instrumentation Specifying Control Valves for Severe Service Applications Unequaled erosi on resis tanc e with good corr osio n resis tanc e; sele cted for extre mely erosi ve appli catio ns.

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Work Aid 3:

Procedures That Are Used To Select Control Valve Options For High Temperature Fluid Applications

Body And Bonnet Material Selection Refer to any source of ANSI pressure/temperature ratings for bodies and bonnets. One source is Fisher Specification Bulletin 59.1:021. Trim Material Selection Refer to the appropriate specification bulletin and locate the manufacturers recommendations for trim packages that are compatible with the body and bonnet material. Select trim that is rated for the maximum operating temperature. Also ensure that the trim will provide the necessary resistance to corrosion and erosion. Gasket Material Selection Refer to the manufacturers temperature ratings for both flat sheet gaskets and spiral wound gaskets. Packing Material Selection Refer to Section 4.1.5 of SAES-J-700 for packing material guidelines. Standard PTFE is to be selected for temperatures up to 400 degrees F. Above 400 degrees F, graphite packing materials are to be selected. Bonnet Type Selection Refer to Section 4.1.5 of SAES-J-700 for bonnet selection guidelines. At temperatures above 400 degrees F, extended bonnets are to be considered. Thermal Cycling Considerations Observe all notes in the manufacturer’s product literature. In general, the following are to be avoided:

•

Threaded bonnets

•

Threaded seat rings

In addition, the materials of construction of spiral wound gaskets should be closely evaluated.

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Work Aid 4:

Procedures That Are Used To Select And Size Control Valves For Cavitating Fluid Applications

Work Aid 4A:

Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Control Component’s Inc. Sizing Software

Preliminary Entries 1. Change to the appropriate directory. The default is c:\cci. 2. Launch the program by typing the name of the executive file: VALSIZ. 3. When prompted to select a method of data input option, select Enter New Data. 4. When prompted for a run descriptor, enter any appropriate name. 5. When prompted to select a valve style, select the desired valve type. 6. When prompted to enter a pipe geometry factor option, select Computer To Calculate. Enter the upstream pipe size. Enter the downstream pipe size. 7. When prompted to select a nominal valve size option, select Computer To Calculate. 8. When prompted to select a noise option, select Noise Level Not Calculated. 9. When prompted to select a percent over-capacity margin, enter 10 percent. 10. When prompted, enter the number of flow conditions. Entering Fluid Properties And Service Conditions Note: The procedures in this section will be repeated for each flow condition. 1. When prompted, enter the inlet pressure and select the appropriate units. 2. When prompted, enter the outlet pressure and select the appropriate units. 3. When prompted, select the fluid type. 4. When prompted, enter the fluid temperature, select the appropriate units, and very the fluid state. 5. When prompted, select either volumetric or mass flow units. Enter the flow rate and select the units for the flow rate. Repeat items 1 through 5 immediately above for each flow condition. Design Information 1. When prompted, enter the design pressure (the shutoff pressure) and select the appropriate units. 2. When prompted, enter the design temperature (a temperature that will provide some safety margin; e.g., a temperature that is 25 percent higher than the normal operating temperature. Select the appropriate units. Change Menu The change menu displays all the information that has been entered. As the change menu screens are displayed for review, the specifier may select entries to change by placing the cursor on the entry to be changed and, then, pressing the space bar. When all of the change

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menu screens have been displayed, the items that have been selected will be listed and they can be changed at that time. Calculation Results 1. After all changes have been made, several screens display the results of the sizing calculations. 2. The first screen shows the valve Cv that is required. Other valve indices and ANSI Class body rating information is also displayed. 3. The screen that is titled “Trim Exit Velocity Analysis” lists the trim outlet velocities for various trims. The trim that will provide an outlet velocity that is less than 100 feet per second will be indicated by color coding. 4. The next several screens display the results of the calculations for each of the service conditions. The number of turns that are required to prevent cavitation damage at that flow condition are shown on each screen. 5. When prompted to view application information, select NO. 6. When prompted to select an option to proceed, select the appropriate response. Work Aid 4B:

Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Valtek’s Sizing Software

Preliminary Entries 1. Change to the appropriate directory. The default is c:\valtek. 2. Launch the program by typing the name of the executive file: QQ. 3. When prompted, select Valve Sizing. Project Identification The entries in the boxed area in the upper right corner of the screen identify the project. They are optional entries. For the purpose of these exercises, press the cursor down arrow until the cursor is on the first entry field of the boxed area on the left hand side of the screen. Valve Selection To identify the selected valve style and options, move the cursor with the use of the up arrow and the down arrow. For each entry field, a sub-menu will appear on the screen. Select the option that is desired by typing the number that precedes the option. 1. 2. 3. 4.

Valve Sizing Press either the F2 key or the Page Down key to display the valve sizing. Enter the appropriate values in all the entry fields that are highlighted. Entries are not required for the fields that are titled Required Cv. To select the fluid, move the cursor to the entry field that is titled “Fluid” and, then, press the space bar. Select the appropriate fluid from the list. Press the F3 key to calculate the valve size information. A description of the selected valve is displayed in the lower left hand corner of the screen.

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To exit the program, press the F10 key several times and follow the instructions that are given in the prompt.

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Work Aid 4C:

Procedures That Are Used To Perform Basic Selection And Sizing With The Use Of Fisher Control’s Sizing Program

Preliminary Entries 1. Change to the appropriate directory. The default is c:\fsp. 2. Launch the program by typing the name of the executive file: FSP. When the title screen appears, press the Enter key. 3. From the menu that appears, select Valve. 4. From the menu that appears, select Fisher Water. 5. From the menu that appears, select Valve Sizing and LpA. Setting Options Press the F3 key and ensure that the options are set as follows:

Solve for Cg, Cs, or Cv. LpA (SPL) OFF Cavitation Check ON Calculate SG Pipe Size/Sched Warnings ON To change an option, place the cursor on the option and press enter. When all options have been set, press the ESCAPE key to return to the program. Data Entry And Sizing Calculations 1. Enter the appropriate data in the Service Conditions portion of the screen. 2. Enter the appropriate data in the Valve Specifications portion of the screen. To determine the value of Km for the initially selected valve, locate the Fisher Catalog 10 page for the initially selected valve. Browse through the Km values that are listed and select a typical value. Enter this value. To determine the value of Kc for the initially selected trim, refer to the Help Screens by performing the following procedures: Press the F1 key twice to view an index of Help Screens.

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Press the "K" key to navigate to the topics that begin with the letter 'K'. Select "Kc Table" from the index of topics Press the PAGE DOWN key until the Help Screens for the selected valve is displayed. Determine the value of Kc. Press the Escape key to return to the sizing screen. Enter the value of Kc. 3. Press the F2 key to calculate and display the valve sizing information. 4. To enter data for the minimum flow condition and for the maximum flow conditions, the data that has been entered for the normal flow condition can be copied. To copy data from the normal flow screen to the minimum or maximum flow screens, perform the following: •

Press the ESCAPE key.

•

With the use of the left arrow key or the right arrow key, move the cursor to the condition to which values are to be copied.

•

Press and hold the ALT key, and, then, press the C key.

•

Enter the number of the flow condition that is to be copied to the selected flow condition.

•

To copy the information to the new condition and to view the calculation screen, press the ENTER key.

•

Change the sizing inputs that are different for this flow condition (P1, dP, and Q).

•

Press the F2 key to calculate the sizing information.

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Display The Calculated Results

Press the F9 key to display a table of calculated values. Ensure that the valve Kc is greater than the value of Ar. If the value of Ar is greater than the Kc of the selected valve, select a valve trim with a higher value of Kc and repeat the sizing procedures.

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Work Aid 5:

Guidelines For Valve Style And Material Selection And Procedures That Are Used To Size Control Valves For Flashing Fluid Applications

Work Aid 5A:

Procedures That Are Used To Size Control Valves For Flashing Fluid Applications

1. Ensure the values that are given as the fluid properties and the service conditions are accurate. A slight error in these values can cause an application to be erroneously interpreted as flashing, cavitating, or neither flashing or cavitating. 2. Because flashing is usually accompanied by choked flow, the valve sizing pressure drop must be limited to the lesser of the ∆Pactual or the ∆Pchoked . The equation for calculating choked flow is: ∆Pallow = FL2(P1-rcPv) where:

Work Aid 5B:

∆Pallow

the maximum pressure drop that is effective in producing flow

FL

ISA nomenclature for the control valve recovery coefficient. Fisher nomenclature is Km where Km = FL2.

P1

Upstream fluid pressure.

rc

The critical pressure ratio; 0.96-0.28(Pv/Pc) where Pv is the fluid’s vapor pressure and Pc is the fluid’s critical pressure.

Pv

The fluid’s vapor pressure. Guidelines For Valve Style And Material Selection That Are Used To Select Control Valves For Flashing Fluid Applications

Valve Style Selection Guidelines 1. For flashing fluid applications, specifiers should select control valve styles according to same guidelines that are applied to erosive flows. Refer to Work Aid 2A of this Module. 2. Because flashing tends to occur downstream of the control valve, specifiers should consider the use of outlet liners. 3. If flashing and cavitation can occur in the same valve, specifiers should avoid multistage anti-cavitation trim if possible. Interstage flashing can damage multi-stage anticavitation trim. Body and Trim Material Selection Guidelines

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1. For flashing fluid applications, specifiers should select control valve materials of construction according to same guidelines that are applied to erosive flows. Refer to Work Aid 2B of this Module. 2. Materials of construction must also be selected on the basis of their compatibility with the process fluid (corrosion resistance) and on the basis of their temperature ratings. Other Considerations

1. If outlet liners are not available, a spool piece of heavy, sacrificial piping can be installed downstream of the control valve. 2. In some instances, flashing can be avoided by changing the system design parameters. Any change in system design that will help to maintain the value of Pvc above the value of Pv should be pursued. 3. If the valve discharges to a tank or vessel, it may be possible to mount the valve directly on the tank and direct the flashing into the vessel where it will not cause damage. Specifiers should consult with system design personnel to ensure the feasibility of this approach.

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Work Aid 6:

Procedures That Are Used To Select And Size Control Valves To Attenuate Aerodynamic Control Valve Noise

Work Aid 6A:

Procedures That Are Used To Select And Size Noise Attenuating Control Valves With The Fisher Sizing Program

Preliminary Entries 1. Change to the appropriate directory. The default is c:\fsp. 2. Launch the program by typing the name of the executive file: FSP. When the title screen appears, press the Enter key. 3. From the menu that appears, select Valve. 4. From the menu that appears, select Fisher Vapor. 5. From the menu that appears, select Valve Sizing and LpA. Setting Options Press the F3 key and ensure that the options are set as follows:

Solve for Cg, Cs, or Cv. LpA (SPL) ON Pipe Size/Sched Warnings ON Diffuser: Manual Sizing To change an option, place the cursor on the option and press enter. When all options have been set, press the ESCAPE key to return to the program. 1. 2.

Data Entry And Sizing Calculations Enter the appropriate data in the Service Conditions portion of the screen. Under the heading Valve Specifications, enter an estimated value of C1 for the selected valve type. An estimated value of C1 may be determined by browsing through the C1 column on the Fisher Catalog 10 page for the selected valve and identifying a value of C1 that is typical for the type and size of the selected valve. Alternatively, many specifiers perform initial sizing with the C1 values that are listed below.

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Typical C 1 Values That Are Used For Initial Sizing

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3.

With the cursor on Valve Type, press the F4 key and select the valve type from the drop down menu.

4.

Enter the pipe size and schedule.

5. 6.

Press the F2 key to calculate the valve sizing and noise prediction information. To enter data for the minimum flow condition and for the maximum flow conditions, the data that has been entered for the normal flow condition can be copied. To copy data from the normal flow screen to the minimum or maximum flow screens, perform the following: a.

Press the ESCAPE key.

b.

With the use of the left arrow key or the right arrow key, move the cursor to the condition to which values are to be copied.

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c.

Press and hold the ALT key, and, then, press the C key.

d.

Enter the number of the flow condition that is to be copied to the selected flow condition.

e.

To copy the information to the new condition and to view the calculation screen, press the ENTER key.

f.

Change the sizing inputs that are different for this flow condition (P1, dP, and Q).

g.

Press the F2 key to calculate the sizing information.

Display The Calculated Results

Press the F9 key to display a table of calculated values. Work Aid 6B:

Procedures That Are Used To Select And Size Noise Attenuating Control Valves With Control Components Sizing Software

Preliminary Entries 1. Change to the appropriate directory. The default is c:\cci. 2. Launch the program by typing the name of the executive file: VALSIZ. 3. When prompted to select a method of data input option, select Enter New Data. 4. When prompted for a run descriptor, enter any appropriate name. 5. When prompted to select a valve style, select the desired valve type. 6. When prompted to enter a pipe geometry factor option, select Computer To Calculate. Enter the upstream pipe size. Enter the downstream pipe size. 7. When prompted to select a nominal valve size option, select Computer To Calculate. 8. When prompted to select a noise option, select User To Select Downstream Pipe. 9. When prompted to select a percent over-capacity margin, enter 10 percent. 10. When prompted, enter the number of flow conditions. Entering Fluid Properties And Service Conditions Note: The procedures in this section will be repeated for each flow condition. When prompted, each of the values that is requested. Design Information 1. When prompted, enter the design pressure (the shutoff pressure) and select the appropriate units. 2. When prompted, enter the design temperature (a temperature that will provide some safety margin; e.g., a temperature that is 25 percent higher than the normal operating temperature. Select the appropriate units.

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Change Menu The change menu displays all the information that has been entered. As the change menu screens are displayed for review, the specifier may select entries to change by placing the cursor on the entry to be changed and, then, pressing the space bar. When all of the change menu screens have been displayed, the items that have been selected will be listed and they can be changed at that time. Calculation Results 1. After all changes have been made, several screens display the results of the sizing calculations. While viewing these screens, record the pertinent data. 2. When prompted to view application information, select NO. 3. When prompted to select an option to proceed, select the appropriate response.

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GLOSSARY ∆ Pcav A-weighting aerodynamic noise ambient noise application ratio (Ar) austenitic cavitation

creep crevice corrosion Cv dB dBA decibel diffuser

dynamic unbalance

elasticity erosion

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The pressure drop at which a particular valve will become susceptible to cavitation damage. The adjustment to a sound pressure measurement that compensates for the frequency sensitivity of the human ear. The noise that is associated with high speed, turbulent gas flows. The background sound pressure level of a given environment. The ratio of the system pressure drop to the pressure differential between P1 and Pv that is used to provide an index of the susceptibility of a system to cavitate. A family of stainless steels that include 18 percent chromium and 8 percent nickel. In liquid service, the noisy and potentially damaging phenomenon that accompanies vapor bubble formation and collapse in the flowstream. Cavitation is most commonly encountered in high pressure and high pressure drop services. The loss of elasticity that occurs over time at elevated temperatures. Corrosion that occurs in areas where access to oxygen is restricted. see flow coefficient see decibel A-weighted decibel A unit that expresses the ratio of two sound pressure levels; i.e., 1 dB = 20 log10 Ps/Po, where Ps is the measured sound pressure and Po is a reference pressure. A noise abatement device that is essentially a downstream, fixed restriction, the purpose of which is to reduce the pressure drop across both the valve and the diffuser to reduce aerodynamic noise. The net force produced on the valve stem in any given open position by the fluid pressure acting on the closure member and stem within the pressure retaining boundary, with the closure member at a stated opening and with stated flowing conditions. The ability of a material to return to its initial form after being exposed to stress. The damage that results from the impingement of particles or vapor droplets on critical valve surfaces.

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erosion

erosion corrosion extension bonnet flashing flow characteristic flow coefficient

fluid frequency spectrum Hertz high-recovery valve hydrodynamic noise IEC incipient cavitation inherent flow characteristic installed flow characteristic

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The damaging effects of flashing or abrasive media impinging on component surfaces. Erosion may be forestalled with hardened materials or with valve designs that separate the flowstream from critical valve components. A form of corrosion that occurs when erosive particles erode the protective passive layer and the base material is attacked by the environment. A bonnet with a packing box that is extended above the bonnet joint of the valve body so as to maintain the temperature of the packing above or below the temperature of the process fluid. Phenomenon observed in liquid service when the pressure of the fluid falls below its vapor pressure and when it does not recover to a pressure above the vapor pressure. Indefinite term, see inherent flow characteristic and installed flow characteristic A constant (Cv), related to the geometry of a valve, for a given valve opening, that can be used to predict flow rate. See ANSI/ISA S75.01 "Control Valve Sizing Equations" and ANSI/ISA S75.02 "Control Valve Capacity Test Procedure". (The number of U.S. gallons of water at 60 degree F that will flow through a valve with a one pound per square inch pressure drop in one minute.) Substance in a liquid, gas, or vapor state. A plot of sound pressure level versus frequency. The measure of frequency, or cycles per second. A valve design that, due to streamlined internal contours and minimal flow turbulence, dissipates relatively little flow-stream energy. The noise that is associated with cavitation. It sounds like gravel flowing through the valve and associated piping. International Electrotechnical Commission The onset of cavitation, observed when the first vapor cavities begin to form in the liquid stream. The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to rated travel with constant pressure drop across the valve. The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to rated travel when the pressure drop across the valve varies as influenced by the system in which the valve is installed. 285

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intergranular corrosion ISA Kc

Km

line source low-recovery valve LpA mach number martensitic microjets Micropascal NACE noise

octave band

outgassing passive layer

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A form of corrosion that occurs along the grain boundaries of a material as a result of sensitization. Instrument Society of America. Control valve damage index that is used to describe a control valve's relative susceptibility (due to its pressure recovery characteristics and its materials of construction) to cavitation damage. The pressure recovery coefficient for a control valve. Km is determined by valve manufacturers and published in sizing catalogs. Km is used to calculate the ∆Pallow (choked flow pressure drop) for valve sizing purposes. The value of Km may also be used to predict cavitation damage. A noise source from which equal noise levels are measured on an imaginary cylinder with the line source as the axis of the imaginary cylinder. A pipeline is a typical line source. A valve design that dissipates, due to the turbulence that is created by the contours of the flow path, a considerable amount of flowstream energy. An A-weighted sound pressure level; see sound pressure. The ratio of the fluid speed to the speed of sound in the fluid at the local conditions. A family of stainless steels that includes 12 percent chromium. Microscopic, high velocity fluid streams produced as a result of vapor bubble collapse in cavitating liquids. A unit of pressure measurement for very small pressures. One micropascal is equal to 10-6 Newton/m2. National Association of Corrosion Engineers. Any sound that is considered unpleasant or unwanted. The sound that is generated by the fluid leaving the control valve is considered noise because of its intensity and because of its high-frequency, broad-band spectrum. One of the established frequency groupings in which the highest frequency in the grouping is twice the lowest (such as the band 2000 to 4000 Hertz). Frequencies are grouped so that filters can be constructed to measure the sound pressure level over the bandwidth. The action of dissolved gasses coming out of solution as a result of pressure reduction or agitation. A naturally occurring deposit of tough, adherent oxides that form on the surface of a material.

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point source Pv Pvc rebound recovery restricted trim* SCC sensitization

silencer

sound

sound intensity sound level meter sound power sound pressure source SPL SSC Saudi Aramco DeskTop Standards

A noise source from which equal noise levels are measured on an imaginary sphere, the center of which is the point source. A vent is a typical point source. The vapor pressure of a fluid. Pressure at the vena contracta. The successive collapse, regrowth, and collapse of vapor bubbles in a cavitating liquid. A relative term that describes the difference in pressure between the valve vena contracta and the downstream system. Control valve trim which has a flow are less than the full flow area for that valve. Stress corrosion cracking. A process in which exposure to high temperature causes corrosion resistant alloys to precipitate out of the material matrix, leaving a zone at the grain boundary that is not protected from corrosion attack. A device that removes acoustic energy from the flow stream. There are two methods of silencer construction. The dissipative or packed silencer removes the acoustic energy by dissipating it into heat in the sound absorbing material lining the structure. The reactive or packless silencer provides an impedance mismatch to the acoustic energy such that the acoustic energy is reflected back to the source and prevented from traveling downstream. An auditory sensation that is caused by pressure oscillations in the ambient atmosphere due to the vibration that is created in an elastic medium by a change in pressure, stress, or displacement. The average rate of sound power that is transmitted in a specified direction through a unit area. An instrument that includes a microphone, an amplifier, an output meter, and usually frequency weighting networks for the measurement of sound pressure. The measurement of total sound energy per unit of time that radiates from a source. No meters are available to directly measure sound power. The force per unit area that is caused by a sound wave. The media where vibration is created due to a change in its pressure, stress, or displacement. Sound pressure level, generally expressed in terms of dB or dBA. SPL is being replaced by the term LpA. Sulfide stress cracking 287

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thermal cycling trim trim, anti-cavitation

vapor pressure (Pv) vena contracta

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Descriptive of a process in which the operating temperature repeatedly cycles over an arbitrary but broad range of temperatures. The internal parts of a valve which are in flowing contact with the controlled fluid. Trim that is specifically designed to eliminate or reduce cavitation and cavitation damage in a control valve. Common designs stage the total pressure drop across one or several specially designed restrictions. The pressure at which a given liquid begins to vaporize, given a constant temperature. The location where the cross-sectional area of the flowstream is at its minimum size, where fluid velocity is at its maximum value, and where local fluid pressure is at its lowest value. The vena contracta normally occurs downstream of the actual physical restriction in a control valve.

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ADDENDUM Composition, Characteristics, And Typical Uses For Common Control Valve Materials Fisher Controls PS Sheet 59:042(A) - Applications Guideline - Rich Amine Letdown Valve

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