1. EPRI. Cycle Chemistry Guidelines for Fossil Plants All-Volatile Treatment Revision 1

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment Revision 1

SED R I A L

LICE

N

M AT E

WARNING: Please read the License Agreement on the back cover before removing the Wrapping Material.

Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

Technical Report

Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment Revision 1 1004187

Final Report, November 2002

EPRI Project Manager R. B. Dooley

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI

ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax). Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2002 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was prepared by EPRI 3412 Hillview Avenue Palo Alto, California 94304 Authors R. B. Dooley K. Shields A. Aschoff M. Ball A. Bursik This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment: Revision 1, EPRI, Palo Alto, CA: 2002. 1004187.

iii

REPORT SUMMARY

The purity of water and steam is central to ensuring fossil plant component availability and reliability. These revised guidelines on all-volatile treatment for drum and once-through units will help utilities reduce corrosion and deposition, and thereby achieve significant operation and maintenance cost reductions and greater unit availability. Background Over the last 10 years, EPRI has conducted numerous research activities to understand the many facets of fossil plant cycle chemistry. Included are corrosion in all-ferrous and mixed-metallurgy feedwater systems; copper in the fossil plant; volatility and partitioning of salts, oxides, and contaminants between water and steam; solubility of salts, oxides, and contaminants in water and steam; and corrosion of blades and disks in the phase transition zone of the low pressure steam turbine. EPRI’s first version of the all-volatile treatment (AVT) guidelines was published in 1996 (TR-105041). A review of the AVT guidelines was required to ensure utilities have access to the latest information and control philosophies. Objective To provide guidance for effective economical control of corrosion and deposition in drum and once-through units using AVT. Approach EPRI developed an initial skeleton of the AVT guidelines to include all pertinent research results. This was used as the basis for a meeting of the EPRI guidelines team. Following this meeting, the team developed a draft document, which was circulated to 75 members of EPRI’s Boiler and Turbine Steam and Cycle Chemistry Target for review and comment. Results The revised guidelines include a number of new features and control philosophies, including the following: •

There are now two distinctly different all-volatile treatments defined by the potential of the feedwater. Those feedwater systems having all-ferrous materials and using an oxidizing (O) treatment (no reducing agent) will operate on AVT(O). Those systems having mixedmetallurgy (copper) materials and operating with a reducing (R) agent will operate with AVT(R).

•

A separate set of target values and action levels to protect the steam turbine and the boiler are included. In previous EPRI guidelines, the boiler water limits were derived from the steam limits. v

•

New guideline values for air in-leakage and a level of 10 ppb oxygen in the condensate have been introduced to provide optimum performance for both AVT(O) and AVT(R).

•

Oxidation-reduction potential (ORP) is now a core parameter at the deaerator inlet for mixedmetallurgy cycles using AVT(R).

•

A new pH range (9.0-9.3) has been introduced to provide optimum protection for mixedmetallurgy cycles using AVT(R).

The AVT guidelines are applicable to baseload, startup, cycling, and peaking operation, and provide corrective actions to be taken when guideline limits are exceeded. EPRI Perspective These revised AVT guidelines will help utilities achieve plant-specific goals in the areas of availability, reliability, and performance. This revision now becomes a part of a suite of 11 key fossil plant guidelines, which should be employed by every fossil plant. EPRI has developed four guidelines for the five fossil plant boiler treatments and three feedwater treatments—all-volatile treatment (1004187), phosphate treatment (TR-103665), oxygenated treatment (TR-102285), and caustic treatment (TR-104007). Other guidelines address the selection and optimization of boiler water and feedwater (TR-105040), controlling flow-accelerated corrosion (TR-108859), startup, shutdown, and layup (TR-107754), chemical cleaning (1003994), condensate polishing (TR104422), makeup water treatment (TR-113692), and copper in fossil plants (1000457). In the near future, EPRI will revise the guidelines for phosphate, caustic, and oxygenated treatments based on the latest research results from the Boiler and Turbine Steam and Cycle Chemistry Target. Keywords Power Plant Availability Water Chemistry and Steam Boilers Turbines Corrosion Boiler Tube Failures

vi

EPRI Licensed Material

ABSTRACT In April 1996, EPRI published the first All-Volatile Treatment (AVT) Cycle Chemistry Guideline for Fossil Plants (TR-105041). This report is a unified, specific and comprehensive guideline for coal-, oil-, and gas-fired units. It provided the guidance necessary to complement a program for effective and economical control of corrosion and deposition within the fossil plant. This document was the first iteration from EPRI’s Interim Cycle Chemistry Guidelines published in 1986. Over the last ten years EPRI has conducted research to address the deficiencies in understanding of cycle chemistry in the following areas: •

corrosion and flow-accelerated corrosion (FAC) in all-ferrous feedwater systems

•

corrosion in mixed-metallurgy feedwater systems

•

copper in a fossil plant

•

volatility and partitioning of the major salts, oxides and contaminants in the cycle between water and steam

•

solubility of the major salts, oxides and contaminants in water and steam

•

corrosion of blades and disks in the phase transition zone (PTZ) of the low pressure steam turbine

The results from these studies have been used to revise the original AVT Guidelines. These revised guidelines now provide the guidance needed for effective and economical control of corrosion and deposition in drum and once-through units of various designs using AVT. For drum units, for the first time in EPRI Guidelines, specific targets and action levels have been derived for boiler water and steam to protect the boiler and steam turbine. New guidelines have been developed for air in-leakage and for oxygen in the condensate. Oxidation-reduction potential (ORP) has been introduced as a core parameter to help protect copper alloys in the feedwater. This will assist the new pH limits for mixed-metallurgy feedwater systems in minimizing the corrosion of copper alloys. The revised AVT Guidelines are applicable to baseload, cycling and peaking operation. Corrective actions are also provided when guideline limits are exceeded.

vii

EPRI Licensed Material

ACKNOWLEDGMENTS The authors of these guidelines R. B. Dooley K. J. Shields A. Aschoff M. Ball A. Bursik

EPRI EPRI Consultant Consultant Consultant

acknowledge the assistance provided by D. Palmer and M. Gruszkiewicz from Oak Ridge National Laboratory in providing many analyses on the volatility/partitioning of compounds between boiler water and steam. The draft guideline was reviewed by 75 members of the Boiler and Turbine Steam and Cycle Chemistry Target. The authors particularly acknowledge the contributions from the following: S. Donner T. Gilchrist R. Pate D. Reynolds M. Smith G. Verib

Consumers Energy Tri-State G&T Southern Company Dynegy AmerenUE First Energy

ix

EPRI Licensed Material

CONTENTS

1 INTRODUCTION ....................................................................................................................1-1 1.1 THE EPRI CYCLE CHEMISTRY PROGRAM ................................................................1-1 1.1.1

Program Goals and Objectives ...........................................................................1-2

1.1.2

Program Philosophy............................................................................................1-3

1.1.3

Key Cycle Chemistry Guidelines.........................................................................1-4

1.1.4

Program Vision and Future Plans .......................................................................1-5

1.2 RESEARCH SUPPORTING REVISION OF THE CYCLE CHEMISTRY GUIDELINES.........................................................................................................................1-6 1.2.1

Chemical Environment and Liquid Films in the Phase Transition Zone (PTZ)....1-6

1.2.2

Corrosion Processes in the PTZ of Steam Turbines...........................................1-8

1.2.3

Volatility and Solubility of Impurities in Steam.....................................................1-9

1.2.4

Copper Corrosion and Transport in Fossil Cycles ............................................1-11

1.3 HOW TO USE THESE GUIDELINES ..........................................................................1-12 1.4 REFERENCES.............................................................................................................1-13 2 SELECTION AND OPTIMIZATION OF FEEDWATER AND BOILER WATER.....................2-1 2.1 CHEMICALLY-INFLUENCED PROBLEMS, AND THE CONTINUUM OF TREATMENTS ......................................................................................................................2-1 2.2 SELECTION AND OPTIMIZATION OF FEEDWATER TREATMENT ...........................2-4 2.2.1 Introduction and Types of Feedwater Treatment....................................................2-4 2.2.2 All-ferrous Feedwater Systems Optimization .........................................................2-6 Optimization of All-Ferrous Feedwater Chemistry .......................................................2-9 Step 1—Review Normal or Current Feedwater Treatment .....................................2-9 Step 2—Monitoring Baseline on Current Feedwater Treatment ...........................2-11 Step 3—Evaluate Reducing Agent Requirements ................................................2-11 Step 4—Monitoring with New Feedwater Treatment ............................................2-12 Steps 5 and 6—Consider Converting to OT..........................................................2-12 Step 7—Continue to Optimize the Feedwater Treatment .....................................2-12

xi

EPRI Licensed Material

Step 8—Operation and Continuing Monitoring .....................................................2-12 2.2.3 Mixed-Metallurgy Feedwater Systems Optimization ............................................2-13 Optimization of Mixed-Metallurgy Feedwater Chemistry ...........................................2-16 Step 1—Review of Water Chemistry, Operation, and Experience........................2-17 Step 2—Baseline Monitoring.................................................................................2-18 Step 3—Water Chemistry Optimization ................................................................2-19 Step 4—Design and Material Changes.................................................................2-20 Step 5—Operation ................................................................................................2-20 Step 7—Normal Operation and Monitoring ...........................................................2-20 Step 8—Continual Check of Chemistry.................................................................2-21 Step 9—Longterm Plans .......................................................................................2-21 2.3 SELECTION AND OPTIMIZATION OF DRUM BOILER WATER TREATMENT .........2-21 Step 1—Review Normal or Current Treatment .....................................................2-23 Step 2—Continue Use of Current Treatment ........................................................2-24 Step 3—Base-Line Monitoring on Current Chemistry ...........................................2-25 Step 4—Initial Evaluation of Boiler Water Treatment Options ..............................2-25 Step 5—Consider Changing to EPT, AVT or CT ..................................................2-26 Step 6—Is There a Condensate Polisher in the Unit Cycle? ................................2-26 Step 7—Convert to AVT or OT .............................................................................2-27 Step 8—Boiler Chemical Clean.............................................................................2-27 Step 10—Need to Determine the Likelihood and Frequency of Cycle Contaminant Events..............................................................................................2-27 Steps 11, 13 and 15—Convert to EPT, PT, or CT ................................................2-27 Steps 9, 12, 14, and 16—Develop Specific Unit Chemistry Guidelines ................2-28 Steps 17 to 19—Monitor to Compare with Baseline Monitoring and to Optimize Treatment Selected................................................................................2-29 Step 20—Normal Operation..................................................................................2-29 2.4 REFERENCES.............................................................................................................2-29 3 PHILOSOPHY FOR GUIDELINE, AND RATIONALE FOR SAMPLE POINTS, ACTION LEVELS AND TARGET VALUES ..............................................................................3-1 3.1 INTRODUCTION............................................................................................................3-1 3.2 BACKGROUND DERIVATION PHILOSOPHY FOR PREVIOUS EPRI GUIDELINES.........................................................................................................................3-3 3.2.1 Illustration and Examples of Boiler Water Limits from Previous EPRI Guidelines.........................................................................................................................3-6

xii

EPRI Licensed Material

Total Carryover ............................................................................................................3-6 Illustration ....................................................................................................................3-7 3.3 BACKGROUND TO OVERALL DERIVATION OF PHILOSOPHY FOR REVISION 1 OF AVT GUIDELINES......................................................................................................3-12 3.3.1 Derivation of Philosophy for Steam and Boiler Water Limits in Revision 1 of EPRI AVT Guidelines .....................................................................................................3-12 3.4 RATIONALE FOR SAMPLE POINTS ..........................................................................3-22 Reheat Steam/Superheated Steam................................................................................3-24 Saturated Steam (Drum Boilers Only) ............................................................................3-24 Boiler Water (Drum Boilers Only) ...................................................................................3-24 Economizer Inlet and Attemperation Water....................................................................3-25 Deaerator Inlet................................................................................................................3-25 Deaerator Outlet .............................................................................................................3-25 Condensate Polisher Effluent (if Applicable) ..................................................................3-26 Condensate Pump Discharge.........................................................................................3-26 Condenser Leak Detection Trays and/or Hotwell Zones (if Applicable) .........................3-26 Makeup Treatment System Effluent ...............................................................................3-26 Condensate Storage Tank Effluent ................................................................................3-27 Air Removal System Exhaust .........................................................................................3-27 3.5 TROUBLESHOOTING, COMMISSIONING AND CORE PARAMETERS ...................3-27 3.6 RATIONALE FOR ACTION LEVELS ...........................................................................3-29 3.7 RATIONALE FOR TARGET VALUES..........................................................................3-30 Feedwater and Condensate ...........................................................................................3-30 3.8 RATIONALE FOR TARGET VALUES OF INDIVIDUAL PARAMETERS.....................3-31 3.8.1 Sodium .................................................................................................................3-31 Sodium Target Value in Reheat and Saturated Steam..............................................3-32 Sodium Target Value in Boiler Water.........................................................................3-32 Sodium Target Value in Condensate and Makeup Water..........................................3-32 Sodium Target Value in Condensate Storage Tank Effluent (Aluminum Tanks Only) ..........................................................................................................................3-32 3.8.2 Chloride ................................................................................................................3-32 Chloride Target Value in Steam.................................................................................3-33 Chloride Target Value in Boiler Water .......................................................................3-33 Chloride Target Value in Makeup Water....................................................................3-33 3.8.3 Sulfate ..................................................................................................................3-34

xiii

EPRI Licensed Material

Sulfate Target Value in Steam ...................................................................................3-34 Sulfate Target Value in Boiler Water..........................................................................3-34 Sulfate Target Value in Makeup Water ......................................................................3-35 3.8.4 Silica .....................................................................................................................3-35 Silica Target Value in Steam .....................................................................................3-35 Silica Target Value in Boiler Water ............................................................................3-35 Silica Target Value in Condensate and Makeup Water .............................................3-36 3.8.5 Dissolved Oxygen.................................................................................................3-36 Dissolved Oxygen Target Value in Economizer Inlet.................................................3-36 Dissolved Oxygen Target Value in Drum Boiler Water ..............................................3-37 Dissolved Oxygen Target Value at Deaerator Outlet.................................................3-37 Dissolved Oxygen Target Value at Condensate Pump Discharge ............................3-37 3.8.6 Oxidizing-Reducing Potential (ORP) ....................................................................3-37 3.8.7 Reducing Agents (Hydrazine or Alternates) .........................................................3-37 3.8.8 pH ..........................................................................................................................3-38 pH Control..................................................................................................................3-38 pH Target Values in Boiler Water ..............................................................................3-38 pH Target Values in Feedwater .................................................................................3-38 3.8.9 Ammonia ..............................................................................................................3-39 Ammonia Target Values ............................................................................................3-39 3.8.10 Specific Conductivity ..........................................................................................3-39 Specific Conductivity Target Values at Economizer Inlet...........................................3-39 Specific Conductivity Target Values in Makeup Water ..............................................3-39 3.8.11 Cation Conductivity.............................................................................................3-39 Cation Conductivity Target Values at Economizer Inlet.............................................3-40 Cation Conductivity in Boiler Water ...........................................................................3-40 Cation Conductivity Limit in Steam ............................................................................3-40 3.8.12 Total Organic Carbon (TOC) ..............................................................................3-41 Steam, Condensate, and Makeup TOC Target Values .............................................3-41 3.8.13 Iron and Copper..................................................................................................3-41 Total Iron and Copper Limits......................................................................................3-41 3.8.14 Air In-Leakage ....................................................................................................3-42 Air In-Leakage Limit ...................................................................................................3-42 3.9 GUIDELINE CUSTOMIZATION AND OPTIMIZATION ...........................................3-42

xiv

EPRI Licensed Material

3.10 REFERENCES ......................................................................................................3-43 4 CYCLES WITH DRUM BOILERS ON ALL-VOLATILE TREATMENT ..................................4-1 4.1 INTRODUCTION............................................................................................................4-1 4.2 AVT GUIDANCE ............................................................................................................4-2 4.3 TARGET VALUES..........................................................................................................4-2 4.4 TARGET VALUES FOR PLANTS WITHOUT REHEAT...............................................4-12 4.5 NORMAL OPERATION FOR DRUM UNITS ON AVT .................................................4-12 4.5.1 Cycle Makeup.......................................................................................................4-12 4.5.2 Condenser Leakage .............................................................................................4-13 4.5.3 Chemical Feeds—Mixed-Metallurgy Cycles.........................................................4-13 4.5.4 Chemical Feeds—All-Ferrous Systems................................................................4-15 4.5.5 Monitoring and Corrective Actions........................................................................4-15 4.6 REFERENCES.............................................................................................................4-16 5 CYCLES WITH ONCE-THROUGH BOILERS ON ALL-VOLATILE TREATMENT (AVT) .........................................................................................................................................5-1 5.1 INTRODUCTION............................................................................................................5-1 5.1.1 All-ferrous metallurgy systems ...............................................................................5-1 5.1.2 Mixed-metallurgy systems ......................................................................................5-2 5.2 AVT GUIDANCE ............................................................................................................5-4 5.3 TARGET VALUES..........................................................................................................5-4 5.4 NORMAL OPERATION FOR ONCE•THROUGH UNITS ON AVT ................................5-8 5.4.1 All-ferrous feedwater systems ................................................................................5-8 5.4.2 Mixed-metallurgy systems ......................................................................................5-8 5.4.3 Monitoring and Corrective Actions..........................................................................5-9 5.5 REACTIONS TO CONTAMINANTS IN THE CYCLE.....................................................5-9 5.6 REFERENCES.............................................................................................................5-10 6 AVT CHEMISTRY CONTROL AND CORRECTIVE ACTIONS .............................................6-1 6.1 CHEMISTRY CONTROL................................................................................................6-1 6.2 CORRECTIVE ACTIONS...............................................................................................6-3 6.3 INFLUENCE OF TREATMENT CHEMICAL PURITY ON CYCLE CHEMISTRY.........6-17 A OXYGEN REMOVAL TECHNIQUES FOR TREATED MAKEUP ........................................ A-1 A.1 VACUUM DEAERATION .............................................................................................. A-1

xv

EPRI Licensed Material

A.2 CATALYTIC OXYGEN REMOVAL ............................................................................... A-3 A.3 MEMBRANE SYSTEMS ............................................................................................... A-4 A.4 MAKEUP WATER STORAGE ...................................................................................... A-6 A.5 REFERENCES ............................................................................................................. A-8 B OXIDATION-REDUCTION POTENTIAL (ORP) ................................................................... B-1 B.1 INTRODUCTION .......................................................................................................... B-1 B.2 OVERVIEW OF ORP AND CORROSION MONITORING TECHNOLOGY ................. B-1 B.3 HARDWARE, CALIBRATION AND MAINTENANCE ................................................... B-3 B. 3.1 Voltmeter Selection .............................................................................................. B-3 B.3.2 Reference Electrodes............................................................................................ B-3 B.3.3 Corrosion Potential Probe ..................................................................................... B-4 B.3.4 ORP Probe............................................................................................................ B-4 B.4 REFERENCES ............................................................................................................. B-7 C AIR IN-LEAKAGE MONITORING AND CONTROL ............................................................. C-1 C.1 CYCLE AIR IN-LEAKAGE ............................................................................................ C-1 C.2 ROTAMETERS............................................................................................................. C-1 C.3 MULTISENSOR PROBE .............................................................................................. C-2 C.4 HELIUM AND SULFUR HEXAFLOURIDE METHODS ................................................ C-5 C.5 UTILITY AIR IN-LEAKAGE PROGRAM ....................................................................... C-8 C.6 REFERENCES ............................................................................................................. C-9 D BENCHMARKING A UTILITY’S CHEMISTRY ORGANIZATION ........................................ D-1 INTRODUCTION.................................................................................................................. D-1 ASSESSING THE CYCLE CHEMISTRY ORGANIZATION OF A UTILITY ......................... D-1 E CYCLE CHEMISTRY DATA QUALITY ................................................................................ E-1 E.1 INTRODUCTION .......................................................................................................... E-1 E.2 SAMPLING, SAMPLE CONDITIONING AND INSTRUMENTATION ........................... E-1 E.2.1 Factors Affecting Sampling ................................................................................... E-2 E.2.2 Potential Problems in the Design and Operation of Sampling Systems................ E-3 E.2.3 Sample Collection ................................................................................................. E-4 E.2.4 Sample Tubing ...................................................................................................... E-5 Pumps, Valves, and Fittings ....................................................................................... E-6 Sample Water Recovery and Drains........................................................................... E-6

xvi

EPRI Licensed Material

E.2.5 Sample Conditioning ............................................................................................. E-6 Temperature Regulation ............................................................................................. E-6 Flow Rate and Pressure Regulation ........................................................................... E-7 Sample Filters ............................................................................................................. E-8 Sample Panels............................................................................................................ E-8 E.2.6 Grab Sampling Procedures...................................................................................... E-11 Grab Samples ........................................................................................................... E-11 Sample Containers ................................................................................................... E-11 Sample Collection and Preservation......................................................................... E-12 E.2.7 Corrosion Product Sampling ............................................................................... E-12 E.3 ON-LINE MONITORING TECHNIQUES .................................................................... E-12 E.3.1 Important Considerations for Selection of Proper Monitoring Method ................ E-13 E.3.2 Analyzer Calibration Techniques ........................................................................ E-14 E.3.3 Analyzer Operation and Maintenance................................................................. E-14 E.4 DATA COLLECTION, INTERPRETATION, AND MANAGEMENT............................. E-14 E.4.1 Data Collection.................................................................................................... E-15 E.4.2 Automatic Data Collection and Storage .............................................................. E-15 E.4.3 Manual Storage of Chemistry Data ..................................................................... E-15 E.4.4 Data Analysis and Interpretation ......................................................................... E-16 E.4.5 Roles of Plant Personnel in Water Chemistry Data Collection, Interpretation and Management........................................................................................................... E-16 E.4.6 Expert Systems for Water Chemistry Management ............................................ E-17 E.5 VALIDATION OF CHEMISTRY DATA........................................................................ E-18 E.5.1 Precision, Accuracy, Bias and Drift ..................................................................... E-20 E.5.2 QA/QC for Sampling Systems............................................................................. E-23 E.5.3 QA/QC for On-Line Instruments.......................................................................... E-24 Instrument Calibration and Standardization.............................................................. E-24 Validation of On-Line Monitoring Instruments........................................................... E-25 Synthesizing Standard Samples ............................................................................... E-26 Potential Problems in Preparation of Standards ....................................................... E-26 E.5.4 Charting of Chemistry Data to Track Instrument Performance ........................... E-27 E.5.5 Interlaboratory Assessment of Grab Sample Analysis Methods ......................... E-30 E.5.6 EPRI Experience With Chemistry Data Validation .............................................. E-31 Performance Determination ...................................................................................... E-31 Acceptance Limits..................................................................................................... E-32

xvii

EPRI Licensed Material

Use of Acceptance Limits in QC ............................................................................... E-32 E.6 SUMMARY.................................................................................................................. E-33 E.7 REFERENCES ........................................................................................................... E-35 F UNIT SHUTDOWN, LAYUP, STARTUP, CYCLING AND PEAKING....................................F-1 F.1 SHUTDOWN ..................................................................................................................F-1 F.2 LAYUP ...........................................................................................................................F-4 F.2.1 Influence of Cycle Metallurgy on Layup .................................................................F-4 All-ferrous metallurgy systems.....................................................................................F-4 Mixed metallurgy systems............................................................................................F-5 F.2.2 Short-term Layup....................................................................................................F-5 F.2.3 Intermediate and Longterm Layup .........................................................................F-5 F.2.4 Maintenance Outage ..............................................................................................F-6 F.3 STARTUP ......................................................................................................................F-6 F.3.1 Corrosion Product Filters........................................................................................F-6 F.3.2 Units With Drum Boilers .........................................................................................F-8 F.3.3 Units With Once Through Boilers .........................................................................F-12 Cold Startup ...............................................................................................................F-12 Hot Startup.................................................................................................................F-13 F.4 ROAD MAP FOR SHUTDOWN AND LAYUP..............................................................F-13 Step 1 - Short-Term Layup....................................................................................F-13 Step 2 - Intermediate and Longterm Layup Common to Dry and Wet Layup .......F-15 Step 3 - Dry Air Layup...........................................................................................F-15 Step 4 - Dry Layup with Nitrogen ..........................................................................F-15 Step 5 - Wet Layup: Traditional Method (Boiler and Feedwater Heaters) ............F-16 Step 6 - Wet Layup: Low Oxygen Scavenger Method (Boiler and Feedwater Heaters) ................................................................................................................F-16 Step 7 - Wet Layup (Balance of Cycle) .................................................................F-16 Step 8 - Very Long Storage .......................................................................................F-17 Step 9 - Maintenance ............................................................................................F-17 F.5 CYCLING AND PEAKING............................................................................................F-17 F.6 REFERENCES.............................................................................................................F-19 G EPRI FOSSIL PLANT CYCLE CHEMISTRY REPORTS, GUIDELINES AND CONFERENCE PROCEEDINGS ............................................................................................. G-1 CONFERENCE PROCEEDINGS......................................................................................... G-5

xviii

EPRI Licensed Material

LIST OF FIGURES Figure 1-1 Overall Philosophy of EPRI’s Cycle Chemistry Program..........................................1-4 Figure 1-2 Schematic of the Processes in PTZ. The extra arrows pointing in to the “Deposits” indicate that deposition can occur by other methods........................................1-7 Figure 1-3 Model for the Development of Corrosion Processes in the PTZ of Steam Turbines .............................................................................................................................1-8 Figure 1-4 Partitioning Constants for Common Salts, Oxides and Acids.................................1-10 Figure 1-5 pH Dependence of the Cu Release Rates for Al-Brass, 90Cu-10Ni and Admiralty Brass After Treatment in Non-Degassed Feedwater at ORP +100 mV and –300 mV* * OX = Oxidizing, RED = Reducing .................................................................1-11 Figure 2-1 Summary of Possible Boiler Water and Feedwater for Fossil Plants as a Function of Equipment and Equipment Capability(1) .........................................................2-4 Figure 2-2 Schematic Representation of Oxide Formed on Ferrous Feedwater Surfaces During Operation with Reducing AVT ................................................................................2-6 Figure 2-3 Change in Oxidizing Reducing Potential (ORP) and Feedwater Iron Levels (Fe) at the Economizer Inlet when Hydrazine (N2H4) is Gradually Reduced on a 600MW Drum Unit with an All-Ferrous Feedwater System(8). ..........................................2-7 Figure 2-4 Schematic Representation of Oxide Formed on Iron-Based Feedwater Surfaces During Operation with Oxidizing AVT and OT.....................................................2-9 Figure 2-5 Road Map for Optimizing Feedwater Treatment for All-Ferrous Feedwater Systems ...........................................................................................................................2-10 Figure 2-6 Surface Images of Admiralty Brass After Treatment at 95°C (203°F), pH 9 and ORP = +100 mV (a,c,e,g) and ORP = -350 mV (b,d,f,h) for 8 (a,b), 24 (c,d), 100 (e,f) and 200 (g,h) hours. The surface oxide under reducing conditions is protective Cu2O. The rougher surface oxide under oxidizing conditions is CuO.(10) ......................2-14 Figure 2-7 Road Map for Optimizing Feedwater Treatment for Mixed-Metallurgy Systems ....2-17 Figure 2-8 Road Map for Optimizing Boiler Water Treatment for Drum Boilers.......................2-22 Figure 3-1 Typical Locations of Impurity Ingress, Corrosion and Deposition in a Drum Cycle ..................................................................................................................................3-2 Figure 3-2 Representative Drum Boiler Mechanical Carryover..................................................3-4 Figure 3-3 Distribution Ratios for Common Boiler Water Contaminants (This ray diagram was originally developed by N. A. Styrikovich and O. I. Martynova.(5,6))..........................3-5 Figure 3-4 All-Volatile Treatment: Drum Boiler Water Chloride vs. Operating Pressure. Calculated using mechanical and vaporous carryover for a limit of 3 ppb Chloride in Steam. It must be noted that these are the old limits used in the previous AVT Guidelines(1)......................................................................................................................3-9

xix

EPRI Licensed Material

Figure 3-5 All-Volatile Treatment: Drum Boiler Water Chloride vs. Operating Pressure. Calculated using mechanical and vaporous carryover for a limit of 2 ppb Chloride in Steam...............................................................................................................................3-10 Figure 3-6 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs. Operating Pressure...........................................................................................................................3-11 Figure 3-7 Partitioning Constants - Neutral Species ................................................................3-14 Figure 3-8 Partitioning Constants - 1:1 Compounds................................................................3-15 Figure 3-9 Partitioning Constants - 1:2 Compounds................................................................3-16 Figure 3-10 Model Predictions (bold lines) for Boiler Water Sodium as a Function of Pressure to Ensure that Less than 2 ppb Sodium is in Steam. The non bold lines are the predictions using the previous approach (mechanical carryover and ray diagram) for 2 ppb Na in steam. a) pH 9–9.6 with no mechanical carryover b) no volatility, only mechanical carryover using Figure 3-2 c) pH 9–9.6 mechanical carryover using Figure 3-2 d) pH above 8 with Cl and SO4 from 0–3 ppm......................3-17 Figure 3-11 Model Predictions (bold lines) for Boiler Water Chloride as a Function of Pressure to Ensure that Less than 2 ppb Chloride is in Steam. The non-bold lines are the predictions using the previous approach (mechanical carryover and ray diagram) for 2 ppb Cl in steam (same as Figure 3-5) a) pH 9.6, no mechanical carryover b) pH 9.0, no mechanical carryover c) no volatility, only mechanical carryover d) pH 9.6 with mechanical carryover from Figure 3-2 e) pH 9.0 with mechanical carryover from Figure 3-2 f) pH above 8, ammonia, sulfate from 0–3 ppm ..................................................................................................................................3-18 Figure 3-12 Model Predictions (bold lines) for Boiler Water Sulfate as a Function of Pressure to Ensure that Less than 2 ppb Sulfate is in Steam. The non-bold lines are the predictions using the previous approach (mechanical carryover and ray diagram) for 2 ppb sulfate in steam. a) pH 9–9.6 no mechanical carryover b) no volatility, only mechanical carryover (Figure 3-2) c) pH 9–9.6 with mechanical carryover (Figure 3-2) ......................................................................................................3-19 Figure 3-13 Model Predictions (bold lines) for Boiler Water Silica as a Function of Pressure to Ensure that Less than 10 ppb SiO2 is in Steam. Non-bold lines are the predictions using the previous approach (mechanical carryover and ray diagram) for 10 ppb of silica in steam. a) no volatility, mechanical carryover from Figure 3-2 b) pH 9–9.6, no mechanical carryover c) pH 9–9.6, mechanical carryover from Figure 3-2 ....................................................................................................................................3-20 Figure 4-1 Cycle Chemistry Diagram for a Drum Unit on All-Volatile Treatment, AVT(O). All-ferrous metallurgy in feedwater system. Operating with an oxidizing environment (no reducing agent) ............................................................................................................4-3 Figure 4-2 Cycle Chemistry Diagram for a Drum Unit on All-Volatile Treatment, AVT(R). All-ferrous and mixed-metallurgy feedwater systems. Operating with a reducing environment (reducing agent added) .................................................................................4-4 Figure 4-3 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs. Operating Pressure.............................................................................................................................4-7 Figure 4-4 All-Volatile Treatment: Drum Boiler Water Sodium vs. Operating Pressure.............4-8 Figure 4-5 All-Volatile Treatment: Drum Boiler Water Chloride vs. Operating Pressure............4-9 Figure 4-6 All-Volatile Treatment: Drum Boiler Water Sulfate vs. Operating Pressure............4-10 Figure 4-7 All-Volatile Treatment: Drum Boiler Water Silica vs. Operating Pressure ..............4-11

xx

EPRI Licensed Material

Figure 4-8.................................................................................................................................4-13 Figure 5-1 Theoretical Relationship Between Specific Conductivity and pH for Ammonia Solutions ............................................................................................................................5-3 Figure 5-2 Cycle Chemistry Diagram for a Once-Through Unit on All-VolatileTreatment. All-ferrous metallurgy in feedwater system. Operating with an oxidizing environment (no reducing agent). ...........................................................................................................5-5 Figure 5-3 Cycle Chemistry Diagram for a Once-Through Unit on All-Volatile Treatment. Mixed-metallurgy feedwater systems. Operating with a reducing environment (reducing agent added) ......................................................................................................5-6 Figure 5-4 Ammonia Concentration vs. pH for Various Carbon Dioxide Concentrations. Note: the top curve represents 4 ppm CO2 and the bottom curve represents zero CO2. Source: from unpublished data derived from volatility code discussed in Section 3.3.1 ......................................................................................................................5-7 Figure A-1 Vacuum Degasifier................................................................................................. A-2 Figure A-2 Components of a GTM Unit ................................................................................... A-4 Figure A-3 GTM System Process Flow Diagram (Note: 1 gal = 3.785 litres)........................... A-5 Figure A-4 GTM System Process Flow Diagram (Note: 1 gal = 3.785 litres)........................... A-6 Figure A-5 A Storage Tank Nitrogen System and Sparging Elements(7) ................................ A-8 Figure C-1 Multisensor Probe .................................................................................................. C-3 Figure C-2 Multisensor Probe Instrument Schematic .............................................................. C-4 Figure C-3 Flow Schematic for SF6 Analyzer System ............................................................. C-6 Figure C-4 Schematic Diagram of SF6 Sampling System ....................................................... C-7 Figure E-1 Head Cup for Constant Sample Flow (pressure in psi at discharge equals the head, H inches of water, divided by 27.6) ......................................................................... E-8 Figure E-2 Sampling System Configuration Used During RP2712-3 Project(2) .................... E-10 Figure E-3 Normal Distribution Curve for Random Data........................................................ E-20 Figure E-4 The Relationship Between True Value, Measured Value, and Bias. The bias in the example shown is negative, but a positive bias is also possible. .......................... E-21 Figure E-5 Random and Systematic Errors ........................................................................... E-23 Figure E-6 Laboratory Quality Control Charts........................................................................ E-28 Figure E-7 Cation Conductivity Control Chart, RR%, CC = 0.5 µS/cm .................................. E-29 Figure E-8 Cation Conductivity Control Chart, RSD%, CC = 0.5 µS/cm................................ E-29 Figure F-1 Copper Concentration at Virginia Power’s Chesterfield Unit 6 Before and After a Filter(3) ...................................................................................................................F-7 Figure F-2 Boiler Water Copper Concentration/Drum Boiler Pressure Control Curves Developed at Miami Fort Station(7) .................................................................................F-10 Figure F-3 Drum Boiler Water Copper vs. Operating Pressure ..............................................F-11 Figure F-4 Road Map to Develop Shutdown and Layup Guidelines Common to Most Units .................................................................................................................................F-14

xxi

EPRI Licensed Material

LIST OF TABLES Table 1-1 Key Cycle Chemistry Guidelines* ..............................................................................1-5 Table 2-1 Percentage of organizations experiencing chemically influenced BTF. Results (3) from survey of 93 organizations in 2001 (Figures in parentheses indicate survey results from 1997). .............................................................................................................2-2 Table 2-2 Major Unit Transport and Deposition Problem Areas for Units with All-Ferrous and Mixed-Metallurgy Feedwater Systems ........................................................................2-5 Table 2-3 Feedwater Limits for All-Ferrous Systems .................................................................2-8 Table 2-4 Feedwater Limits for Mixed-Metallurgy Systems .....................................................2-15 Table 3-1 EPRI’s Core Monitoring Parameters and/or Minimum Level of Continuous Instruments for All Units Operating on AVT .....................................................................3-23 Table 6-1 Possible Causes of Chemistry Excursions ................................................................6-4 Table 6-2 Condensate System – Corrective Actions .................................................................6-5 Table 6-3 Feedwater System – Corrective Actions....................................................................6-6 Table 6-4 Boiler Water - Corrective Actions (Drum Boilers only) ...............................................6-7 Table 6-5 Steam Circuit - Corrective Actions.............................................................................6-7 Table 6-6a Makeup System Malfunction....................................................................................6-8 Table 6-6b Makeup Regenerant - NaOH, H2SO4 or HCl Leaking into the Cycle......................6-9 Table 6-7 Condenser Tube Leak .............................................................................................6-10 Table 6-8a Condensate Polisher Malfunction/Exhausted ........................................................6-10 Table 6-8b Condensate Polisher – Caustic or Acid Regenerant Leakage...............................6-11 Table 6-9 Excessive or Insufficient Concentration of Reducing Agent ....................................6-12 Table 6-10 Corrosion and/or Flow-Accelerated Corrosion in the Feedwater ...........................6-13 Table 6-11 High Boiler Carryover (Drum Boilers only).............................................................6-14 Table 6-12 Impurity Introduction into Turbine by Attemperating Sprays ..................................6-15 Table 6-13 Air In-leakage to Hotwell........................................................................................6-15 Table 6-14 Colloidal Silica and Iron through the Makeup System ...........................................6-16 Table 6-15 Boiler Layup Problems...........................................................................................6-17 Table B-1 To Convert ORP or Corrosion Potential Values Measured Using Reference Electrode #1 to Values on Reference Electrode #2 Scale, Add the Indicated Conversion Factor to the Measured Potential ................................................................... B-5 Table B-2 Expected ORP Values for Reference Quinhydrone Solutions at pH 4 and pH 7 .... B-6 Table C-1 Examples of a Five-Probe Air In-leakage Measurement System............................ C-4 Table C-2 MSP Probe Indications for Various Probe Positions ............................................... C-5

xxiii

EPRI Licensed Material

Table E-1 Summary of Criteria for Sampling ........................................................................... E-5 Table E-2 Working Definitions of QA/QC Terms for Fossil Plant Chemistry Data Validation ........................................................................................................................ E-19 Table E-3 Typical Sampling Errors ........................................................................................ E-24 Table E-4 Continuous Instrument Acceptance Limits, 95% Confidence Interval (Based on Seven Replicates) ...................................................................................................... E-33 Table F-1 Design features of copper filter(4) ............................................................................F-9 Table F-2 Comparison Of Copper Values in Blowdown (Figures F-2 and F-3) ......................F-12 Table F-3 Maximum Annual Exposure to Contaminant Conditions for Cycling and Peaking Operation ...........................................................................................................F-17

xxiv

EPRI Licensed Material

1 INTRODUCTION

1.1 THE EPRI CYCLE CHEMISTRY PROGRAM Availability and reliability are of paramount importance to the overall economic performance and profitability of fossil plant unit operations. Industry statistics have demonstrated the negative impacts of improper water chemistry on unit availability and reliability, as a consequence of chemistry-related failures and associated unscheduled outages. Plant assessments have shown how deficient chemistry practices reduce the efficiency and performance of fossil plant components in contact with water and steam. Further, non-optimum chemistry conditions can shorten the useful service life of fossil plant components, requiring that replacement projects begin sooner than normally required. In recognition of these issues, the EPRI Cycle Chemistry Program was established in 1984. Initial efforts and activities addressed the most obvious and apparent needs within the fossil plant industry. The EPRI response to these needs included: •

Improving the recognition and understanding of the impacts on fossil plant equipment caused by deficient chemistry practices

•

Critically appraising the science of water and steam chemistry, and identifying specific data needs and other deficiencies

•

Establishing industry guidelines for cycle chemistry in all varieties of fossil plant units

•

Through open communications, conferences and collaborative research efforts, creating a worldwide network of cycle chemistry specialists, allowing appraisal of the science and technology on a global basis

•

Preparing publications and other products intended to facilitate technology transfer to fossil plants, designed to simplify application of good chemistry practices

Over the nearly 20 years the program has been in existence, the understanding of chemistry influenced damage and the effects of deposits on unit performance has increased substantially. Unfortunately, there are still cases where the causes of damage and performance degradation are not properly identified, resulting in situations where the role of chemistry goes unrecognized, or damage and performance losses not involving chemistry are incorrectly determined to be chemistry related. There are also many cases where the optimum cycle chemistry has not been selected and continually validated, or where inadequate instrumentation was responsible for allowing units to operate with gross contamination. Deposits can also impair performance and have been experienced in many areas of the steam-water cycle. Chemistry influenced component damage in fossil plant units is widespread and includes the following mechanisms. 1-1

EPRI Licensed Material Introduction

•

Condenser tubes (steam side damage): stress corrosion cracking, pitting, condensate grooving

•

Condenser structure: flow-accelerated corrosion of steam side shell, supports, headers and piping

•

Deaerators: flow-accelerated corrosion, pitting, corrosion fatigue, and stress corrosion cracking

•

Feedwater heaters and associated piping: general corrosion and pitting, corrosion fatigue, flow-accelerated corrosion, stress corrosion cracking, and deposits

•

Economizer tubes: Pitting, flow-accelerated corrosion and corrosion fatigue

•

Boiler tubes: hydrogen damage, acid phosphate corrosion, caustic gouging, corrosion fatigue, pitting, and deposit induced overheating

•

Superheaters and reheaters: pitting corrosion, stress corrosion cracking and corrosion fatigue

•

Turbines: Corrosion fatigue, erosion and corrosion, stress corrosion cracking, crevice corrosion, pitting, and deposits (reducing efficiency and capacity)

It should be noted that some of these damage mechanisms were unknown at the inception of the program. Others were not readily distinguished from superficially similar damage mechanisms, including some that are not influenced by chemistry. In other cases, the extent of components that were vulnerable to the damage mechanism was not fully appreciated. Today, there is a very good understanding of damage mechanisms, including the influence of chemistry on many of them. Permanent solutions, based on identification of the responsible root cause and initiation of action to “kill the mechanism” are generally available. Deficient chemistry is either a root cause or significant influencing factor in all chemistry influenced damage mechanisms. Initial interim chemistry guidelines were issued in 1986.(1) Subsequent research findings, field experience with the interim guidelines, and worldwide cycle chemistry practices justified updates and revision activities. As a result, individual guidelines for phosphate, all-volatile, and oxygenated treatments were issued in the 1990s.(2-4) Additionally, a document describing favorable international experience with caustic treatment of drum boilers was published.(5) This report represents the first in a series of “third generation” EPRI cycle chemistry guidelines for fossil plants which will be published between 2002 and 2005. 1.1.1 Program Goals and Objectives The overall objectives of the program are to provide guidelines, technology and training materials, which together will assist in avoiding the major damage and failure mechanisms in fossil plants. By implementation of improved water chemistry, the following goals, which have been set for the EPRI program, are attainable by virtually all fossil plant units: •

No boiler tube failures related to cycle chemistry

•

No turbine problems involving the cycle chemistry, specifically: –

no corrosion fatigue in low pressure turbine components 1-2

EPRI Licensed Material Introduction

•

•

•

–

no stress corrosion cracking in disks

–

minimum deposits (no availability losses or performance concerns)

Optimized feedwater treatment to: –

eliminate serious flow-accelerated corrosion failures

–

minimize iron and copper transport (each to less than 2ppb in the feedwater)

Operational guidelines for all unit designs and operating conditions –

selected to protect boiler and turbine

–

customized for each unit

Simple and reliable chemistry instrumentation and control –

minimum (“core”) levels of instrumentation for all units and treatments

–

continual chemistry surveillance, control and alarms for all units

•

Optimized procedures for unit shutdown and layup

•

Eliminate unneeded chemical cleanings

•

–

appraise need to clean

–

establish effective approaches and procedures

Optimum managerial approach and support for cycle chemistry –

training of staff

–

benchmarking assessments of plant chemistry programs

–

value and risk-based management tools for assessment of cycle chemistry improvements

There are already a number of world class utility organizations that enjoy the benefits of operating without chemistry-related boiler and turbine failures, with minimal rates of corrosion product transport, requiring few (if any) chemical cleanings, etc. Many others are working with EPRI to improve their chemistry programs and making measurable progress, with commensurate changes in unit availability and performance. 1.1.2 Program Philosophy The overall philosophy of EPRI’s Cycle Chemistry Program for Fossil Plants is shown in Figure 1-1. Various projects, including state-of-knowledge assessments, technology appraisals, research and development programs, and relevant non-technical investigations are performed to improve the overall understanding of the science of water treatment technology and how to optimally apply it to working fossil plant units.

1-3

EPRI Licensed Material Introduction

Figure 1-1 Overall Philosophy of EPRI’s Cycle Chemistry Program

The results of these projects serve as critical input to development and products for use by plant personnel. As indicated in the figure, the main products consist of cycle chemistry guidelines, a cycle chemistry advisor (ChemExpert), and various training programs that ensure proper technology transfer to plant staff for optimal understanding and application. By following this approach, it has been possible to continually refine the understanding of the underlying science while also making appropriate changes in the products utilized by plant personnel. 1.1.3 Key Cycle Chemistry Guidelines In all, there are 11 essential cycle chemistry guideline documents that should be available for use by all utility personnel responsible for fossil plant water chemistry. Included are four operating guidelines,(2-5) four selection, process and transient guidelines,(6-9) and three cycle support (10-12) Table 1-1 indicates the subject matter of these guideline publications, the year of guidelines. publication, and the year in which publication of revised and updated guidelines is planned. As shown in Figure 1-1, the chemistry guideline documents are integral to the content of the training programs and other tools developed for operations, maintenance, technical, and management personnel. The updated guidelines are the initial conduit through which the findings of research and development efforts are transferred to the plants. However, the important new concepts introduced in the guidelines are subsequently integrated into training program materials and future versions of the expert system code (ChemExpert).

1-4

EPRI Licensed Material Introduction Table 1-1 Key Cycle Chemistry Guidelines* Guideline Type/Subject

Year Published

Planned Updates

•Phosphate Treatment (EPT/PT)

1994

2003

•All-Volatile Treatment (AVT)

1996

2003

•Oxygenated Treatment (OT)

1994

2004

•Caustic Treatment (CT)

1994

2004/5

Operating Guidelines

Selection, Process and Transient Guidelines •Selection and Optimization

1994

•Flow-Accelerated Corrosion

1997

•Cycling/Startup/Shutdown/Layup

1998

•Control of Copper in Fossil Plants

2000

Cycle Support Guidelines •Makeup (Revision 1)

1999

•Chemical Cleaning (Revision 2)

2001

•Condensate Polishing

1996

2005

*See Appendix G for further information on these and other publications.

1.1.4 Program Vision and Future Plans Utilities desiring optimum benefits from these and future cycle chemistry guidelines publications will derive the best results as follows: •

Perform initial benchmarking assessments of existing cycle chemistry and boiler tube failure reduction programs to establish worldwide rankings for each unit and to identify areas of deficiency. (EPRI’s approach to Cycle Chemistry Benchmarking is included as Appendix D.)

•

Organizations desiring optimized chemistry should arrange for Boiler Tube Failure Reduction/Cycle Chemistry Improvement Program (BTFR/CCIP) training to familiarize staff with: a) the controllable aspects of the key cycle chemistry program guidelines, b) the importance of formalized, management supported, BTFR/CCIP Programs, and c) the importance of establishing customized chemistry treatment programs based on the guidelines. Use of a Corporate Mandate document provides the needed support from management.

•

Use EPRI ChemExpert to provide minute-by-minute surveillance of the chemistry, early warning of chemistry excursions, direction to response actions which will minimize or 1-5

EPRI Licensed Material Introduction

prevent damage to equipment. This program will also eventually allow assessment of the risks of continued operation with out-of-specification chemistry until normal chemistry can be restored, as well as the value of program improvements •

Perform follow-up benchmarking assessments to track progress in improving plant chemistry programs and identify any areas where the staff’s understanding of the guidelines would benefit from review of training materials

Feedback obtained from utility personnel working with these products is an important means by which EPRI’s Cycle Chemistry Program is continually upgraded.

1.2 RESEARCH SUPPORTING REVISION OF THE CYCLE CHEMISTRY GUIDELINES EPRI research conducted over the last ten years has resolved prior knowledge deficiencies in several critical areas. These include: •

Chemical environment and liquid films in the Phase Transition Zone (PTZ) region of the low pressure steam turbine

•

Corrosion processes in the PTZ of steam turbines

•

Volatility and solubility of impurities in steam and water

•

Copper corrosion and transport in fossil plant steam-water cycles

The influence of research in these areas on the development of guidelines follows in subsequent sections. Sections 2 and 3 provide further details on how the research findings influence selection of treatments and the rationale for these new AVT chemistry guidelines. 1.2.1 Chemical Environment and Liquid Films in the Phase Transition Zone (PTZ) The risk of chemistry related turbine damage is greatest within the so-called phase transition zone (PTZ), where corrosion of low pressure (LP) blades and disks results in substantial availability losses with commensurate cost impacts on the affected fossil generating units. EPRI has sponsored a series of research projects intended to understand the environment in the PTZ in detail, to improve the understanding of the corrosion damage processes in the PTZ and the (13-16) effects of the chemistry on these processes. As a result of 10 years of research and international collaborative work with 23 organizations, Figure 1-2 schematically shows the important processes that take place in the PTZ. The impurities, oxides and ions in the superheated steam act as centers for the heterogeneous nucleation of the first drops of moisture sometimes called the “first” or “early” condensate. These drops concentrate the impurities (for chloride and sulfate up to 200 times) and are now known to be electrically charged. However, it is very important to note that there is no oxygen within these droplets, even for units operating on oxygenated treatment (up to 400 ppb oxygen). The droplets can impinge on the turbine surfaces (blades and disks) and give rise to liquid films on the surfaces. The properties of these liquid films have been extensively studied in EPRI (17,18) The concentration of impurities in the liquid film is at least ten times higher than in research. 1-6

EPRI Licensed Material Introduction

the droplets and the pH can drop down to below 7. These liquid films are important because they provide the dynamic environment for the PTZ corrosion mechanisms. It has also been shown that those liquid films have a potential and a high conductivity. As these liquid films flow off the blade surfaces they generate relatively large droplets compared with the early condensate droplets. Both are charged as they flow into the exhaust hood at an approximate moisture level of around 8%.

Figure 1-2 Schematic of the Processes in PTZ. The extra arrows pointing in to the “Deposits” indicate that deposition can occur by other methods.

The EPRI research has also determined that the unit cycle chemistry has a major effect on the properties of the liquid films. Parallel work has also addressed deposition of salts, oxides and impurities onto the PTZ surfaces. Some of the main conclusions from this large body of research, which directly influences the development of the revised EPRI AVT Guidelines include: •

It appears that less deposition and thus more reliable operation in the steam turbine PTZ will be accomplished by reducing the current AVT steam limits for Na, Cl, and SO4 to below 2 ppb, although there is significant transport and deposition of steam impurities in units with concentrations of sodium and chloride in steam of less than 0.1 ppb.

•

The concentration of impurities by drying of liquid films and moisture droplets on surfaces, where the surface temperature is above the steam saturation temperature can be significant. Liquid films have been detected on blade surfaces above the saturation line.

•

Pitting can be initiated in relatively pure steam on typical turbine materials in cycling drum boiler units that do not apply any shutdown protection to the turbine.

•

The cycle chemistry plays a major influence on the chemical composition of the early condensate and of the liquid film on blades. 1-7

EPRI Licensed Material Introduction

•

Low volatility salts and octydecylamine (ODA) in steam affect the condensation process, particularly at lower steam expansion rates such as those in turbines.

The relation of these conclusions to cycle chemistry treatment and steam purity is clear, and these “third generation” chemistry guidelines have included these results in deriving some of the revised limits. 1.2.2 Corrosion Processes in the PTZ of Steam Turbines

Loading

Chemistry

These extensive new findings on the environment and liquid films in the PTZ have led to a new model of how corrosion processes (corrosion fatigue (CF) and stress corrosion cracking(SCC)) take place. Figure 1-3 shows the model. When the turbine is operating, the chemical environment in susceptible locations in the PTZ consists of dynamic liquid films and deposition of salts, oxides and impurities. There is no oxygen in the liquid films. However, when the turbine shuts down, most organizations do not provide any protective environment, and so the deposits become moist once the surfaces cool down, and cause passivity breakdown and pits to form. Repetition of the operating/shutdown environments eventually leads to microcracks. Only when the turbine is operating is the loading (cyclic or steady state) sufficient to drive the microcracks into corrosion fatigue or stress corrosion cracks. Only when the unit is operating are liquid films present, which provide the environment for the cracks to propagate. Thus the main ramifications for the development of these new guidelines are the importance of higher purity steam (reduction of the limits for sodium, chloride and sulfate) and of protecting the turbine during shutdown.

Turbine Operation

Shutdown

Deposits and Liquid Films (No O2)

Deposits, Oxygen and Moisture

Pitting Crevices

Pitting Microcracks

Cyclic

CF

Steady State

SCC

No Loading

Figure 1-3 Model for the Development of Corrosion Processes in the PTZ of Steam Turbines

1-8

EPRI Licensed Material Introduction

Ultimately, research results in this area will allow development of a computer model of the LP turbine that can be used to appraise the potential for corrosion fatigue and stress corrosion cracking. This model will include predictive capabilities to assess the time to failure once precursor conditions (pits, crevices and microcracks) are determined to be present. The improved model will eventually be integrated into other EPRI software such as BLADE and ChemExpert. 1.2.3 Volatility and Solubility of Impurities in Steam More than a decade of research(19-25) has shown that volatility predictions based on the ray diagram are not accurate in defining volatile carryover in drum boilers of fossil plant units. In some cases, the values suggested by the ray diagram are off by as much as two orders of magnitude. Volatility experiments conducted to reevaluate impurities of interest resulted in a much better understanding of this subject and the complexities involved in properly applying this science to fossil plants. All of the results from 10 years of EPRI research are shown in Figure 1-4. Key findings of the volatility experiments and subsequent data evaluations may be summarized as follows. •

Impurities may carry over as neutral salts, as 1:1 ionic compounds or as 1:2 ionic compounds. Separate diagrams for each of these groups are shown in Section 3.

•

Chloride exhibits relatively high volatility as hydrochloric acid (HCl), minor volatility as ammonium chloride (NH4Cl), and only minimal volatility as sodium chloride (NaCl).

•

Sodium hydroxide (NaOH) has a volatility comparable to that of NaCl and introduction of these species to the steam path from the boiler is strongly dependent on mechanical carryover rather than volatile carryover.

•

Under oxidizing conditions, volatile transport of sulfur species to the steam occurs mainly as sulfuric acid (H2SO4), however, under reducing conditions, volatile carryover of sulfur dioxide, (SO2), a neutral compound, can introduce sulfur to the steam. Also, carryover of ammonium sulfate (NH4HSO4) will be present in steam at high pH under AVT chemistry conditions.

•

Phosphoric acid (H3PO4) is considered a neutral compound as it does not ionize appreciably at boiler operating temperatures, yet volatile carryover was determined to be quite low. Transport of phosphates to the steam is essentially the result of mechanical carryover. Trisodium phosphate (Na3PO4), is, for practical purposes, non-volatile.

•

Organic acids such as formic acid (HCOOH) and acetic acid (CH3COOH) are neutral compounds under fossil plant operating conditions and are very volatile, however, any organic salts in steam would exhibit lower volatility and could lead to stripping of organic acids from LP steam into the early condensate.

•

Volatile carryover of copper from boiler water to the steam occurs mainly as cupric hydroxide (CU(OH)2) and cuprous hydroxide (CuOH); both species exhibit very high volatility across the whole range of temperatures and pressures experienced in a fossil plant from startup to full operation.

•

Silica volatilization characteristics are quite consistent with the findings of other researchers. 1-9

EPRI Licensed Material Introduction

•

Carryover predictions for operating fossil plant units with drum boilers require use of a thermodynamic model to make an appraisal of the boiler water components as a system; a model has been developed that allows prediction of saturated steam chemistry and the chemistry of the early condensate – initial testing shows that component volatility is pH dependent and that mechanical carryover rate has a significant impact on steam purity.

The model is currently under development but has been used in Section 3 to validate the derivation of boiler water limits from the steam limits. The final version of the code will be incorporated within the EPRI ChemExpert software. It is envisioned that doing so will allow utilities to customize their boiler water limits and action levels for individual drum boiler units and to predict changes in steam purity and early condensate chemistry with variations in the boiler water chemistry.

Figure 1-4 Partitioning Constants for Common Salts, Oxides and Acids

1-10

EPRI Licensed Material Introduction

1.2.4 Copper Corrosion and Transport in Fossil Cycles Under Program Copper, EPRI sought to establish an improved understanding of copper corrosion, transport and deposition in fossil plant cycles, leading to new guidelines for control of mixed metallurgy cycles.(9,24,26-29) It has been shown that the corrosivity of individual copper alloys is dependent primarily on oxidation-reduction potential (ORP), and also on pH, as shown in Figure 1-5. Ramifications here for the revised guidelines are that the minimum in corrosion for copper alloys occurs in the pH range 9.0-9.3 under reducing conditions. Also, the solubility of cupric oxide (CuO) in steam, about 1-2 ppb under fossil plant conditions, was found to be generally consistent with prior research, while the solubility of cuprous oxide (Cu2O), about 23.5 ppb, was determined to be somewhat greater than predicted by other investigators. In boiler water, the copper oxides exhibit increasing solubility as a function of temperature up to around 200oC (392oF). At typical boiler operating temperatures however, the solubilities appear to be independent of temperature, about 2 ppb for CuO, and about 6 ppb for Cu2O. Boiler pH (ammonia concentration) has considerable influence on copper oxide solubility at high temperatures. These results have been used to develop revised boiler water copper limits. Because of the solubility of the copper oxides in boiler water and the high volatility of the oxides to steam, it is of paramount importance that reducing conditions exist during any shutdown periods to minimize the transport of copper to steam.

Figure 1-5 pH Dependence of the Cu Release Rates for Al-Brass, 90Cu-10Ni and Admiralty Brass After Treatment in Non-Degassed Feedwater at ORP +100 mV and –300 mV* * OX = Oxidizing, RED = Reducing

1-11

EPRI Licensed Material Introduction

1.3 HOW TO USE THESE GUIDELINES This introductory section of the guidelines has presented the EPRI Cycle Chemistry Program objectives and achievable goals, and has shown how recent research findings provide the direction for the revisions to the previous AVT guidelines. These chemistry guidelines are applicable to fossil units with drum and once-through boilers operating with AVT. As with prior versions, the guidelines represent the best practices for those units that should utilize AVT. This in no way implies that the guidelines, as presented, can be immediately applied to individual units. Additional guidance is provided to assist users of the guidelines to develop an optimized cycle chemistry program with customized limits and action levels. Further, it is important to understand that AVT is not an acceptable treatment for all units, and that the suitability and applicability of the cycle chemistry treatment approaches to a given unit may change over time. As described in Section 2, it is important to verify the suitability of AVT for use in a specific unit prior to initial operation with AVT and whenever significant changes in unit design and operation occur. In the case of existing units operated on AVT for more than five years, users of these guidelines are strongly urged to use the road maps and supporting discussions of Section 2 to verify that the present AVT program is still optimum for the unit in question because the latest research findings have influenced the applicability of the available feedwater and boiler water treatment options. Section 3 of the guidelines presents and discusses the new rationale used to develop these guidelines to establish appropriate target values and action levels. This section is most important (a “must read”) as it will be noted that the approach to be followed is completely new and different from previous EPRI guidelines, and differs with the system metallurgy. The Section 3 presentation also defines the preferred sample points, chemical feed points and required on-line instrumentation for all situations to which AVT is the desired cycle chemistry. Sections 4 and 5 cover the revised AVT guidelines for drum boiler and once-through boiler units, respectively. As in prior guidelines, cycle diagrams are used to summarize the sample collection and chemical addition point, the basic (generic and uncustomized) chemistry target values and action levels, and the monitoring requirements. As explained in each of these sections, the instrumentation requirements now differ depending on whether an oxidizing or reducing chemistry is employed (AVT(O) or (AVT(R)). Guidance is given on how the guidelines should be used to establish unit-specific target values and action levels. Section 4 on drum boiler units includes a discussion of chemistry data requirements for using EPRI ChemExpert software. Also presented are discussions of best practices for unit shutdown, layup, startup and cycling service as well as makeup water requirements and the role of chemical cleaning in optimized AVT programs. Section 6 addresses other aspects of optimized AVT programs. Primary attention is focused on prompt identification of chemistry excursions by plant personnel, accurate analysis of the cause or causes of the deviations from normal chemistry limits, and provision of corrective actions consistent with this analysis. In addition, this section considers the purity of treatment chemicals to be applied to units operated on AVT. Sections 1-6 are followed by seven appendices, providing further details on important topics related to AVT usage. Appendix A describes ways to remove dissolved oxygen from treated makeup water, which is very beneficial when filling units for startup. Techniques for monitoring 1-12

EPRI Licensed Material Introduction

feedwater ORP are presented in Appendix B. Condenser air in-leakage monitoring and control, necessary to ensure attainment of target values for dissolved oxygen in condensate, are presented in Appendix C. Appendix D provides information on the EPRI approach to cycle chemistry benchmarking. The importance of proper sampling and analysis practices as they relate to chemistry program management is reviewed in Appendix E. The contributions of staff training to the overall success of the program are also covered. Appendix F covers unit shutdown, layup, startup, cycling and peaking. As indicated in Table 1-1, there are 11 key cycle chemistry guideline documents that all personnel within the organizations supporting the EPRI Cycle Chemistry Program should have. However, these publications represent only a small part of the products created since program inception in 1984. Appendix G provides a listing of all currently available guidelines, technical reports, and conference proceedings.

1.4 REFERENCES 1. Interim Consensus Guidelines on Fossil Plant Water Chemistry, EPRI, Palo Alto, CA: June 1986. CS-4629. 2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Treatment for Drum Units, EPRI, Palo Alto, CA: December 1994. TR-103665. 3. Cycle Chemistry Guidelines for Fossil Plants: All Volatile Treatment, EPRI, Palo Alto, CA: April 1996. TR-105041. 4. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment, EPRI, Palo Alto, CA: December 1994. TR-102285. 5. Sodium Hydroxide for Conditioning the Boiler Water of Drum-Type Boilers, EPRI, Palo Alto, CA: January 1995. TR-104007. 6. Selection and Optimization of Boiler Water and Feedwater Treatments for Fossil Plants, EPRI, Palo Alto, CA: March 1997. TR-105040. 7. Guidelines for Controlling Flow-Accelerated Corrosion in Fossil Plants, EPRI, Palo Alto, CA: November 1997. TR-108859. 8. Cycling, Startup, Shutdown and Lay-up Fossil Plant Cycle Chemistry Guidelines for Operators and Chemists, EPRI, Palo Alto, CA: August 1998. TR-107754. 9. Guidelines for Copper in Fossil Plants, EPRI, Palo Alto, CA: November 2000. 1000457. 10. Revised Guidelines for Makeup Water Treatment, EPRI, Palo Alto, CA: October 1999. TR-113692. 11. Guidelines for Chemical Cleaning of Conventional Fossil Plant Equipment, EPRI, Palo Alto, CA: November 2001. 1003994.

1-13

EPRI Licensed Material Introduction

12. Condensate Polishing Guidelines, EPRI, Palo Alto, CA: September 1996. TR-104422. 13. Steam, Chemistry and Corrosion in the Phase Transition Zone of Steam Turbines, Volume 1: Key Results, Summary, and Interpretation, EPRI, Palo Alto, CA: February 1999. TR-108184-V1. 14. Steam, Chemistry and Corrosion in the Phase Transition Zone of Steam Turbines, Volume 2: Part 1: Individual Contributions of Participants, EPRI, Palo Alto, CA: February 1999. TR-108184-V2P1. 15. Steam, Chemistry and Corrosion in the Phase Transition Zone of Steam Turbines, Volume 2: Part 2: Individual Contributions of Participants, EPRI, Palo Alto, CA: February 1999. TR-108184-V2P1. 16. Turbine Steam, Chemistry and Corrosion: Generation of Early Liquid Films in Turbines, EPRI, Palo Alto, CA: September 1999. TR-113090. 17. Turbine Steam, Chemistry and Corrosion: Experimental Turbine Tests, EPRI, Palo Alto, CA: September 1997. TR-108185. 18. Corrosion of Low Pressure Steam Turbine Components, EPRI, Palo Alto, CA: November 2000. 1000557. 19. Behavior of Ammonium Salts in Steam Cycles, EPRI, Palo Alto, CA: December 1993. TR-102377. 20. Assessment of the Ray Diagram, EPRI, Palo Alto, CA: August 1996. TR-106017. 21. Volatility of Aqueous Sodium Hydroxide, Bisulfate and Sulfate, EPRI, Palo Alto, CA: February 1999. TR-105801. 22. Vapor-Liquid Partitioning of Sulfuric Acid and Ammonium Sulfate, EPRI, Palo Alto, CA: February 1999. TR-112359. 23. Volatility of Aqueous Acetic Acid, Formic Acid, and Sodium Acetate, EPRI, Palo Alto, CA: July 2000. TR-113089. 24. Behavior of Aqueous Electrolytes in Steam Cycles: The Solubility and Volatility of Cupric Oxide, EPRI, Palo Alto, CA: November 2000. 1000455. 25. The Volatility of Impurities in Steam/Water Cycles, EPRI, Palo Alto, CA: September 2001. 1001042. 26. State-of-Knowledge of Copper in Fossil Plant Cycles, EPRI, Palo Alto, CA: September 1997. TR-108460. 27. Corrosion of Cu-Ni-Zn Alloys in Water-Ammonia Power Plant Environments: Development of High Temperature Potential-pH (Pourbaix) Diagrams, EPRI, Palo Alto, CA: November 1999. TR-113697. 1-14

EPRI Licensed Material Introduction

28. Copper Alloy Corrosion in High Purity Feedwater, EPRI, Palo Alto, CA: November 2000. 1000456. 29. Influence of Water Chemistry on Copper Alloy Corrosion in High Purity Feedwater, EPRI, Palo Alto, CA: October 2001. 1004586.

1-15

EPRI Licensed Material

2 SELECTION AND OPTIMIZATION OF FEEDWATER AND BOILER WATER

2.1 CHEMICALLY-INFLUENCED PROBLEMS, AND THE CONTINUUM OF TREATMENTS The unit chemistry selection and continuing optimization of feedwater and boiler water treatments are of paramount importance to the performance of a fossil plant. Often the chemistry for both is decided during initial unit design, and then “fine-tuned” during decommissioning or early operation. Historically, the science of chemical treatments has changed over the last 20 years of the EPRI Cycle Chemistry Program, and many organizations have indeed changed treatments. Some examples of this include: •

a gradual progression from coordinated phosphate to congruent phosphate to tri-sodium phosphate treatments (EPT and PT)

•

conversion from AVT to a phosphate treatment

•

conversion from AVT to OT, and

•

for all-ferrous feedwater systems, conversion from a reducing AVT environment (using reducing agents) to either an oxidizing AVT environment (without reducing agents) or OT.

However, it is very rare than an organization conducts a comprehensive monitoring campaign when such changes are made. Most often it is accomplished quickly, and often without a chemical clean, which is especially important if a boiler water treatment is changed. Also, and often more importantly, is the fact that as the unit gets older, it becomes more prone to contaminant ingress from increasing condenser leaks and air in-leakage. The majority of organizations do not continually (at least on a six months frequency) check whether the treatment is still optimized or even adequate for the changed conditions. Most often, when asked, organizations indicate “we operate with this boiler water (or feedwater) treatment because we always operated this way.” This situation is not satisfactory. Within this overall scenario is included the fact that very often organizations do not change contaminant limits when boiler water chemistry is changed. The classic most recent example is (1) the change from congruent phosphate treatment (CPT) to equilibrium phosphate treatment. This change from CPT to EPT was desperately needed by the industry in the early 1990s to prevent phosphate hideout, hideout return, and acid phosphate corrosion. The publication of the (2) EPRI Phosphate Guidelines has reversed these trends. However, on changing from CPT to EPT, there also needs to be a change of boiler water control limits, especially for chloride. For 2-1

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

example for a 2500 psi (17.2 MPa) boiler, the chloride limit with EPT is about 25 ppb, whereas for PT it is about 1 ppm. Thus it is not surprising that there has been a drastic increase around the world in hydrogen damage tube failures in EPT units. Overall this is part of a very disturbing trend of an increasing number of chemically influenced failures. Table 2-1 indicates the percentage of organizations experiencing cycle chemistry influenced BTF. Here it has to be said that not only those organizations with the poorest BTF statistics and poorest cycle chemistry performance have experienced hydrogen damage; in some cases it has occurred in organizations that have been benchmarked in the “World Class” category. Table 2-1 Percentage of organizations experiencing chemically influenced BTF. Results from survey of 93 organizations in 2001(3) (Figures in parentheses indicate survey results from 1997). Organizations having Chemically Influenced BTF

81%

(61%)

•

Hydrogen Damage

57%

(37%)

•

Acid Phosphate Corrosion

25%

(17%)

•

Corrosion Fatigue

45%

(43%)

•

Pitting

40%

(7%)

•

Stress Corrosion Cracking (SCC)

28%

(18%)

•

Caustic Gouging

11%

(11%)

Over the last two years EPRI has benchmarked a large number of organizations based on five key fossil plant cycle chemistry factors.(1) The benchmarking process is provided in Appendix D. The process provides a ranking for a unit/plant/organization from “World Class” to “Below Average.” World Class essentially means that a unit has no chemically influenced failures or performance losses, has a full complement of cycle chemistry instruments (see Table 3-1) which are alarmed for operators, has very few chemical cleans based on optimum choice of feedwater treatment, and has minimum cycle losses (increased makeup addition) due to blowdown. EPRI also conducts a boiler tube failure reduction program/cycle chemistry improvement program (BTFRP/CCIP), and over the period 1999 to 2002 has provided initial training and review to over 50 organizations worldwide. Some interim results from these two activities of benchmarking and BTFRP/CCIP have recently been collated into an assessment of the (1) importance of cycle chemistry on fossil plant performance. It is clear from this assessment that the most reliable and best performing units in the world operate on OT or AVT(O), have condensate polishing, all-ferrous feedwater systems, do not use reducing agents, have tight condensers, and excellent air in-leakage control. These organizations also take time to ensure that the boiler and feedwater treatments match the equipment and the equipment capability.

2-2

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

It is, of course, appreciated that while all units to be built in the future should adhere to these guiding principles to achieve performance requirements, not all currently operating units can instantly change to this spectrum. This again emphasizes the need for continuing and continual assessment of a unit’s cycle chemistry, which is the main focus of this section. There are five possible choices for drum boiler water treatment, as discussed in Section 2.3: AVT, OT, EPT, PT and CT. There are three possible choices for feedwater treatment, as discussed in Section 2.2: AVT(O), AVT(R) and OT. The chosen treatments need to match the unit, unit materials (particularly feedwater), cooling water, and possible contaminant ingress. The recent performance results mentioned above have led to a diagram, which simplifies the process of selection, or at least acts as a first-cut analysis. EPRI calls this the “Continuum of Treatments.” Figure 2-1 attempts to provide an overview of the Continuum. The width of the wedge or funnel, in which the continuum of treatments is located, is proportional to the possible level of contamination in the cycle, and to the ability of any of the five boiler water treatments to neutralize or buffer any contaminants. As the point of the funnel is approached the level of possible contaminants becomes less and the more likely the plant will have very good feedwater control. Such a plant will have a condensate polisher and/or a very tight condenser. As the open end of the funnel is approached, plants might have no condensate polisher, seawater or high TDS water from a cooling lake or a cooling tower. The better the air in-leakage, the nearer to the point will the plant be positioned. The vertical dotted line represents an arbitrary distinction between units with all-ferrous and mixed metallurgy feedwater systems. Clearly there is a relationship with the boiler water treatments: OT and AVT(O) are only applicable to units with all-ferrous feedwater systems, but AVT(R), EPT, PT and CT are applicable to both all-ferrous and mixed-metallurgy. In terms of cycle purity, moving towards the point of the funnel requires lower feedwater contaminants. For example, the continuum for cation conductivity at the economizer inlet might be: for OT (< 0.15µS/cm), for AVT(O) and AVT(R) (< 0.2 µS/cm), with PT and CT (< 0.3 µS/cm), and with EPT positioned approximately at AVT requirements. In terms of performance, the best results occur using OT and AVT(O), whereas operating with PT and CT (and sometimes EPT) there are operational problems such as hideout and hideout return. There clearly has to be benefit in an organization changing the chemistry treatments and maybe the unit equipment (adding a polisher, changing out copper feedwater heaters). This is (13) the focus of EPRI’s program on assessing the value to cycle chemistry.

2-3

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

Figure 2-1 Summary of Possible Boiler Water and Feedwater for Fossil Plants as a Function of Equipment and Equipment Capability(1)

The primary purpose of this section of the new Guidelines is to provide the needed advice, guidance, and road maps for selection and optimization of boiler water and feedwater. The process can, and should, be used each and every time a change in treatment is contemplated, and also used for continual assessment of boiler water and feedwater. (4) This process, which was developed by EPRI in 1997, is even more important today because of the deteriorating statistics of chemically influenced failures and performance losses. For full consideration of this topic, the reader is strongly recommended to review the complete original document.(4)

2.2 SELECTION AND OPTIMIZATION OF FEEDWATER TREATMENT 2.2.1 Introduction and Types of Feedwater Treatment For all-ferrous feedwater systems (no copper alloys in the feedwater, but there can be copperbased condenser tubing), the generation and transport of corrosion products (magnetite, hematite and ferric oxide hydrate) occurs mainly due to corrosion and flow-accelerated corrosion of low 2-4

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

pressure and high pressure feedwater heaters, deaerators, economizer inlet tubing and piping, feedwater piping and drain lines. For mixed-metallurgy systems, the generation and transport of corrosion products (cupric and cuprous oxide) occurs mainly due to corrosion of any low pressure and high pressure feedwater heater tubes manufactured from copper alloys. Some possible consequences of these corrosion problems are outlined in Table 2-2. Table 2-2 Major Unit Transport and Deposition Problem Areas for Units with All-Ferrous and MixedMetallurgy Feedwater Systems All-Ferrous Systems •

•

Mixed-Metallurgy Systems

Waterwall pressure drop due to ripple magnetite deposits on waterwalls of oncethrough units

• HP turbine copper deposits • Primary superheater copper deposits • Copper deposits in HP heaters

Flow-accelerated corrosion

• Pump strainer/screen copper deposits All Feedwater Systems •

Boiler deposits and increased boiler pressure drop

•

At least five boiler tube failure mechanisms

•

Frequent need to chemical clean

•

Boiler feedpump fouling

•

Orifice fouling

Thus feedwater chemistry is critical to the overall reliability of fossil plants. Corrosion products are generated here, flow around the cycle, deposit in various areas acting as initiating centers for most of the major failure mechanisms, and often need removing by chemical cleaning. There are three distinctly different feedwater treatments: •

Reducing all-volatile, AVT(R), which uses ammonia and a reducing agent. Here the oxidation-reduction potential, ORP, should be in the range –300 to –350mV [Ag/AgCl/sat, KCl], which is necessary to protect mixed-metallurgy feedwater systems.

•

Oxidizing all-volatile, AVT(O), where the reducing agent has been eliminated. Here the ORP will be around 0 mV and could be positive.

•

Oxygenated treatment (OT) where oxygen and ammonia are used. Here the ORP will be around +100 to +150mV.

Most operators can easily meet the feedwater requirements for cation conductivity (less than 0.2 mS/cm). However, it is clear that this is not sufficient by itself, and major efforts need to be applied to the generation and transport of corrosion products. Very achievable by the best units operating on AVT(R) is less than 2 ppb of iron and/or copper, for units operating with AVT(O) the iron levels can be around 1 ppb, and for units operating with OT iron levels should be around 0.5 ppb.

2-5

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

2.2.2 All-ferrous Feedwater Systems Optimization All of the three feedwater treatments mentioned above in Section 2.2 are possible for use in allferrous feedwater systems. The basis of either of the AVT treatments is an elevated pH in all cycle streams. The common alkalizing agent is ammonia. Originally, and up to the late 1980s, the ammonia dosing was always combined with a reducing agent feed, such as hydrazine. This treatment is now termed AVT(R), which indicates that the oxygen level at the condensate pump discharge (CPD) is low enough (< 10 ppb) (minimum air in-leakage) that a reducing agent can be added to the cycle to produce a reducing environment with ORP < 0 mV. Figure 2-2 illustrates the oxide formation (magnetite, Fe3O4) which will be formed on all the ferrous surfaces throughout the feedwater system. The dissolution of Fe3O4 into the feedwater flow is dependent on the ORP. The more reducing is the feedwater the greater is the dissolution and thus the higher is the amount of iron corrosion products measured at the economizer inlet. Flowaccelerated corrosion (FAC) occurs by exactly the same mechanism, which is accelerated at (5-7) locations where the flow hydrodynamics are elevated. Under reducing conditions that produce FAC or normal corrosion, organizations are not able to meet the guideline requirements of less than 2 ppb iron in the final feedwater at the economizer inlet.

Figure 2-2 Schematic Representation of Oxide Formed on Ferrous Feedwater Surfaces During Operation with Reducing AVT

Investigations performed since the late 1980s, and hundreds of unit operating experiences have indicated that eliminating the reducing agent feed minimizes the corrosion product generation.(8) This treatment is now termed AVT(O). It also requires the air in-leakage be minimized to produce oxygen levels at the CPD of less than 10 ppb. Figure 2-3 shows a very typical example in a 600 MW drum unit with an all-ferrous feedwater system. As the reducing agent (hydrazine) was reduced over a period of 90 days, the ORP increased from about –350mV to around +100mV in the oxidizing range, and the iron levels reduced markedly. There was no change in the feedwater oxygen level. 2-6

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

Figure 2-3 Change in Oxidizing Reducing Potential (ORP) and Feedwater Iron Levels (Fe) at the Economizer Inlet when Hydrazine (N2H4) is Gradually Reduced on a 600MW Drum Unit with an All-Ferrous Feedwater System(8).

2-7

EPRI Licensed Material Selection and Optimization of Feedwater and Boiler Water

The feedwater key parameters for AVT(R) and AVT(O) are summarized in Table 2-3. Table 2-3 Feedwater Limits for All-Ferrous Systems Parameter

AVT(O)

AVT(R)

OT

pH

9.2–9.6

9.2–9.6

D 9–9.6 O 8–8.5

< 0.2

< 0.2

< 0.15

Fe (ppb)

< 2 ( 20

Sodium, ppb

C

£3

£6

£ 12

> 12

Parameter

LP heaters

HP heaters

£3

T

£3

Sulfate, ppb

T

£3

Silica, ppb

C

£ 10

Specific conductivity, mS/cma

C

£ 0.1

T or W £ 300

Condensate Storage Tank Effluent

Boiler

Sample

N

C

Chloride, ppb

Total organic carbon, ppb

Makeup treatment system

Deaerator

Sample

Sodium, ppb

Condensate storage tank

Attemperation (See economizer inlet)

Target

Parameter

Condenser

Economizer Inlet (EI) and Attemperation Water Target

C or S

Cation conductivity a or sodium

Blowdown

Parameter

Sample

Condenser Leak Detection Trays or Hotwell Zones (If applicable)

Boiler Water (Blowdown or Downcomer) Parameter

Target

Condensate polisher

Target

N (aluminum tanks only)

Sample

Sodium, ppb

£ 10

W

Condensate Pump Discharge (CPD)

Consistent with pH

Deaerator Outlet Parameter

Target

Oxygen, ppb

Sample

N

T

< 10

Condensate Polisher Effluent (If applicable) (CPE)

* * Maximum Annual Exposure to Contaminant Conditions

Target

Sample

N

1

2

3

Sodium, ppb

C

£3

£6

£ 12

> 12

Cation conductivity, µS/cm a

C

£ 0.2

> 0.2

—

—

Silica, ppb

C

£ 10

> 10

—

Parameter

Target

Parameter

Ammonia

—

Sample

N

1

2

3

C

£6

>6

—

—

C

£3

£6

£ 12

> 12

C

£ 0.3

> 0.3

—

—

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Plants with polisher Plants without polisher Plants with polisher Plants without polisher

*

Sodium, ppb

*

Cation conductivity, µS/cm a

Total organic carbon, ppb

T

£ 200

> 200

—

—

*

Oxygen, ppb

C

£ 10

£ 20

> 20

—

Cumulative Hours per Year

Targets

Base Load

Cycling

—

—

1 (Action Level 1)

336 ( 2 weeks)

672 (4 weeks)

2 (Action Level 2)

48 (2 days)

96 (4 days)

3 (Action Level 3)

8

16

Immediate Shutdown

1

2

N (Normal)

Legend

Footnotes

Sample Frequency

Target Values

C S D W T

N 1 2 3

= continuous = grab, once per shift = grab, once per day = grab, once per week = troubleshooting and commissioning

= Normal = Action Level 1 = Action Level 2 = Action Level 3

a = Conductivity and pH measured at 25° C b = See curves of maximum allowable concentration versus pressure c = Must be determined d = See curves of allowable cation conductivity vs pressure

Sample and Chemical Feed Identification

*

+

= “Core” parameter alarmed in Control Room = “Core” parameter

= Continuous sample = Chemical feed

Figure 4-1 Cycle Chemistry Diagram for a Drum Unit on All-Volatile Treatment, AVT(O). All-ferrous metallurgy in feedwater system. Operating with an oxidizing environment (no reducing agent)

4-3

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Reheat Steam Target

Parameter

* *

Saturated Steam Target

N

1

2

3

C

£2

£4

£8

>8

Cation conductivity, mS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Silica, ppb

T

£ 10

£ 20

£ 40

> 40

Chloride, ppb

T

£2

£4

£8

>8

Sodium

T

Sulfate, ppb

T

£2

£4

£8

>8

Silica

T

Total organic carbon, ppb

W

£ 100

> 100

—

—

Carryover

T

Specific conductivitya

T

—

—

—

—

Parameter

+

Sample

Sodium, ppb

Sample

Air Removal System Exhaust Parameter

+

Air inleakage, scfm

Sample

N

1

2

3

Immediate Shutdown

Sodium

C or S

b

b

b

b

—

Chloride

T

b

b

b

b

—

Sulfate

T

b

b

b

b

—

Silica

T

b

b

b

b

—

*

pHa Oxygen, ppb

C

c

—

—

—

9.6 < 9.0 > 9.3

—

—

C

Cation conductivity, mS/cma

C

Iron, ppb

T

£2

>2

—

—

Copper, ppb

T

£2 1-10

>2 £ 15

— £ 20

— > 20

£5

£ 10

£ 20

> 20

£3

£6

£ 12

> 12

Sodium, ppb

C C C

£ 0.4

LP heaters

£ 0.8

Sample

N

Sodium, ppb

C

£3 £3

Chloride, ppb

T

Sulfate, ppb

T

£3

Silica, ppb

C

£ 10

Specific conductivity, mS/cma

C

£ 0.1

T or W £ 300

Total organic carbon, ppb

Condensate Storage Tank Effluent Parameter

HP heaters

Target

Parameter

* Makeup treatment system

Condensate polisher

Target

N (aluminum tanks only)

Sample

Sodium, ppb

£ 10

W

Condensate Pump Discharge (CPD) Deaerator Outlet

Specific conductivitya

Mixed Fe-Cu

C

Condensate storage tank

Deaerator

Consistent with pH

£ 0.2

Makeup Treatment System Effluent

Sample

Boiler

D

All ferrous

LP turbine

Attemperation (See economizer inlet)

Ammonia

Oxygen, ppb

Target

Condenser

Economizer Inlet (EI) and Attemperation Water Target Sample

C or S

Cation conductivity a or sodium

Blowdown

Parameter

Sample

Condenser Leak Detection Trays or Hotwell Zones (If applicable)

Boiler Water (BW) (Blowdown or Downcomer) Parameter

Target

Parameter

> 0.8

Target

Sample

N

T

< 10

Oxygen, ppb

Condensate Polisher Effluent (CPE) (If applicable)

Deaerator Inlet (DAI) Parameter

Target

Parameter Sample

N

S

Consistent with ORP

Reducing agent, ppb

*

ORP, mV

C

* *

-300 to -350

Maximum Annual Exposure to Contaminant Conditions

Target

Target

Sample

N

1

2

3

C

£6

>6

—

—

C

£3

£6

£ 12

> 12

C

£ 0.3

> 0.3

—

—

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Total organic carbon, ppb

T

£ 200

> 200

—

—

Oxygen, ppb

C

£ 10

£ 20

> 20

—

Parameter

Ammonia and reducing agent

Sample

N

1

2

3

Sodium, ppb

C

£3

£6

£ 12

> 12

Cation conductivity, µS/cm a

C

£ 0.2

> 0.2

—

—

Silica, ppb

C

£ 10

> 10

—

—

*

Sodium, ppb

*

Cation conductivity, µS/cm a

*

Plants with polisher Plants without polisher Plants with polisher Plants without polisher

Cumulative Hours per Year Targets N (Normal)

Base Load

Cycling

—

—

1 (Action Level 1)

336 ( 2 weeks)

672 (4 weeks)

2 (Action Level 2)

48 (2 days)

96 (4 days)

3 (Action Level 3)

8

16

Immediate Shutdown

1

2

Legend

Footnotes

Sample Frequency

Target Values

C S D W T

N 1 2 3

a = Conductivity and pH measured at 25° C b = See curves of maximum allowable concentration versus pressure c = Must be determined d = See curves of allowable cation conductivity vs pressure

= continuous = grab, once per shift = grab, once per day = grab, once per week = troubleshooting and commissioning

= Normal = Action Level 1 = Action Level 2 = Action Level 3

Sample and Chemical Feed Identification

*

+

= “Core” parameter alarmed in Control Room = “Core” parameter = Continuous sample = Chemical feed

Figure 4-2 Cycle Chemistry Diagram for a Drum Unit on All-Volatile Treatment, AVT(R). All-ferrous and mixed-metallurgy feedwater systems. Operating with a reducing environment (reducing agent added)

4-4

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

Figure 4-2 features the use of ORP instrumentation at the deaerator inlet to ensure that reducing conditions are maintained at all times. While ORP is not specified as a core parameter for Figure 4-1 (oxidizing regime), its inclusion might help to ensure the desired magnitude of the oxidizing state for the all-ferrous feedwater cycle. Figures 4-1 and 4-2 list target values (N) and from one to three action levels for a compendium of sampling points throughout the heat cycle. The steam values are more restrictive than featured in previous Guidelines, in order to maintain enhanced turbine protection, with steam limits of 2 ppb sodium, chloride and sulfate versus 3 ppb in previous editions. The limit for silica in the steam remains the same at 10 ppb. Research has indicated that, for mixed-metallurgy systems (with strict air in-leakage and oxygen control, the addition of a reducing agent, and with reducing conditions), the pH range at the economizer inlet can be increased from 8.8 to 9.1 (previously) to 9.0 to 9.3. This increase in pH will reduce the transport of iron in the cycle, while minimizing the copper bearing materials from corrosion. It needs to be recognized that the ORP needs to remain in the reducing regime for these mixed-metallurgy systems. Simply changing the pH will not suffice. There are a number of features to keep in mind when adapting Figures 4-1 and 4-2 to an individual plant. Particular emphasis has been placed in the development of these new guideline cycle diagrams to make control of a unit as simple as possible, and to minimize repetitive grab sampling.

•

The Core Parameters (Table 3-1) are indicated by an asterisk (*). These instruments should be monitored continuously and on-line. These have a “C” for “continuous” in the sample column.

•

There are a number of other parameters, which have also been designated by a “C” as they provide useful confirmation of unit condition, but are not regarded as “Core” instruments. These parameters are: silica and sodium in makeup, specific conductivity at the economizer inlet, and boiler water sodium.

•

There are a number of other parameters on these diagrams, which have been newly designated in this guideline as “T” for troubleshooting. It is only necessary to monitor these if one of the continuous “Core” instruments provides an indication that the parameter is out of its “normal” guideline value. In most cases, this will be accomplished by grab sample; however, in some cases an organization might have decided to install a continuous analyzer. In the case of iron and copper at the economizer inlet, it should be recognized that in a well operated plant it should not be necessary to monitor these parameters more frequently than once or twice a year; and then only to confirm the feedwater regime remains optimized.

Other points of interest on Figures 4-1 and 4-2 are:

•

Dissolved oxygen at the CPD should be controlled to less than 10 ppb. This is a much better control for the feedwater than air in-leakage into the condenser.

•

A limit for copper at the economizer inlet remains on Figure 4-1 for those units with copper based condenser tubing.

•

Reducing agent (hydrazine preferred) feed in Figure 4-2 is a requirement, whereas in Figure 4-1 it is not required nor is it necessary if the unit is to be operated with AVT(O). For both AVT(R) and AVT(O) rigorous air in-leakage control must be maintained to ensure oxygen levels at the CPD of less than 10 ppb. 4-5

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

As discussed in Section 3 (Table 3-1), a new suite of core parameters has been developed for Figures 4-1 and 4-2. The following core parameters are applicable in these revised AVT Guidelines:

•

Makeup treatment system effluent specific conductivity is required to detect malfunction of the makeup treatment system.

•

Sodium and cation conductivity at the condensate pump discharge to provide cycle contamination control from condenser leaks and makeup malfunctions.

•

Dissolved oxygen at the condensate pump discharge to ensure the optimum feedwater potential can be controlled. It also provides an indication of air in-leakage into the cycle.

•

Deaerator inlet ORP (Figure 4-2)- required to ensure reducing conditions are maintained to minimize copper transport.

•

Economizer inlet sodium (or condensate polisher effluent sodium) on units with a condensate polisher is required to prevent condensate polisher malfunction and to guard against cycle contamination.

•

Economizer inlet dissolved oxygen and cation conductivity.

•

Boiler water pH and cation conductivity. Cation conductivity is the main control for boiler water contaminants, and thus, corrosion.

•

Saturated steam carryover is required to be determined on a routine/ periodic basis (every six months) to prevent excessive contamination of the turbine caused by boiler drum internal separation failure, foaming, high boiler water levels, etc.

•

Air removal system exhaust air in-leakage is required to prevent serious corrosion via excess cycle dissolved oxygen and carbon dioxide.

The core parameters shown in Figures 4-1and 4-2 are the minimum required for routine chemistry monitoring and provide basic input data for the EPRI computer program “ChemExpert”. The basic requirement for every unit is that a minimum level of instruments should be installed that can uniquely identify any problem on the unit. A series of curves have been developed which present smooth, continuous relationships of boiler water target values as a function of boiler pressure. These curves have been designed to provide boiler corrosion protection, but as indicated in Section 3.3.1 they have been validated to ensure they are below the boiler water contaminant values which would cause the steam values to be exceeded. Thus these curves provide protection to the boiler and the turbine. The following boiler water limits are presented:

•

cation conductivity versus operating pressure (Figure 4-3)

•

sodium versus operating pressure (Figure 4-4)

•

chloride versus operating pressure (Figure 4-5)

•

sulfate versus operating pressure (Figure 4-6)

•

silica versus operating pressure (Figure 4-7) 4-6

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Drum Pressure (MPa) 100 90 80 70 60

4.8

6.2

7.6

9.0

10.3

11.7

13.1

14.5

15.8

17.2

18.6 19.6

2300

2500

2700 2850

50 40 Action level 3

Cation Conductivity - µS/cm

30 20

Action level 2 Action level 1

10 9 8 7 6 5

Normal

4 3 2

1 600 700

900

1100

1300

1500 1700 1900 2100 Drum Pressure (psia)

Figure 4-3 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs. Operating Pressure

4-7

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Drum Pressure (MPa) 4.8

6.2

7.6

9.0

10.3

11.7

13.1

14.5

15.8

17.2

18.6 19.6

2500

2700 2850

10 9 8 7 6 5 4 Action level 3 3

Sodium (ppm Na)

2 Action level 2 Action level 1 1.0 0.9 0.8 0.7 0.6 0.5

Normal

0.4 0.3

0.2

0.1 600 700

900

1100

1300

1500 1700 1900 Pressure (psia)

2100

2300

Figure 4-4 All-Volatile Treatment: Drum Boiler Water Sodium vs. Operating Pressure

4-8

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Drum Pressure (MPa) 3.0

4.8

6.2

7.6

9.0

10.3

11.7

13.1

14.5

15.8

17.2

18.6 19.6

2500

2700 2850

2.0 1.5

1.0 0.9 0.8 0.7 0.6

Action level 3

0.5 0.4

Chloride (ppm Cl)

0.3

Action level 2

0.2 Action Level1 0.10 0.09 0.08 0.07 0.06

Normal

0.05 0.04 0.03

0.02

0.01 600 700

900

1100

1300

1500 1700 1900 Pressure (psia)

2100

2300

Figure 4-5 All-Volatile Treatment: Drum Boiler Water Chloride vs. Operating Pressure

4-9

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Drum Pressure (MPa) 3.0

4.8

6.2

7.6

9.0

10.3

11.7

13.1

14.5

15.8

17.2

18.6 19.6

2.0 1.5 1.0 0.9 0.8 0.7 0.6

Action level 3

0.5 0.4

Action level 2

Sulfate (ppm SO4)

0.3 Action level 1 0.2

Normal 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03

0.02

0.01 600 700

900

1100

1300

1500 1700 1900 Pressure (psia)

2100

2300

2500

2700 2850

Figure 4-6 All-Volatile Treatment: Drum Boiler Water Sulfate vs. Operating Pressure

4-10

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment Drum Pressure (MPa) 20

4.8

6.2

7.6

9.0

10.3

11.7

13.1

14.5

15.8

17.2

18.6 19.6

15 10 9 8 7 6 5 4 3 Action level 3

Silica (ppm SiO2)

2

Action level 2 Action level 1

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 Normal 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 600 700

900

1100

1300

1500 1700 1900 2100 Drum Pressure (psia)

2300

2500

2700 2850

Figure 4-7 All-Volatile Treatment: Drum Boiler Water Silica vs. Operating Pressure

As discussed in Section 3.3.1, cation conductivity should be regarded as the main control for a drum boiler operating on AVT. Thus Figure 4-3 should be the primary boiler water control curve. Maintaining the boiler water cation conductivity within these limits will provide boiler corrosion protection. These curves include the effects of chloride and sulfate. It should be noted that chloride and sulfate are designated as “T” parameters on the cycle chemistry diagrams 4-11

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

(Figures 4-1 and 4-2), so EPRI has provided the boiler water curves for chloride and sulfate (Figures 4-5 and 4-6). These are derived from the cation conductivity curves; sulfate represents a lesser risk than chloride in initiating corrosion, so the concentration of sulfate has been taken as twice that of chloride. These two sets of curves are provided so that an organization can check chloride and sulfate levels in the boiler in cases when the cation conductivity levels are exceeded. Alternatively, at the choice of the organization, the chloride curves can be used directly if the organization has a continuous chloride analyzer in the case of maybe seawater cooling. The curves for sodium (Figure 4-4) are based on limiting the excess of sodium over chloride and sulfate, and thus on the amount of sodium hydroxide in the boiler water. The curves for silica (Figure 4-7) are based on the need to limit the amount of silica carried over into the steam and are based on the partitioning data. Silica is also designated a “T” parameter. Copies of Figures 4-1 to 4-3, modified if necessary to reflect specific unit characteristics and/or experience, could be included in the plant operating procedures and prominently displayed in the control room, water and steam sample room, and chemistry laboratory. Copies could also be displayed at the makeup treatment system control panel and the condensate polisher control panel (if applicable).

4.4 TARGET VALUES FOR PLANTS WITHOUT REHEAT The sample points and core parameters shown in Figures 4-1 and 4-2 are the same for plants without reheat as for those with reheat. However, the target values and action levels are less restrictive for non-reheat plants, generally by a factor of 2 times the corresponding reheat values. This same factor of 2 applies to the boiler pressure versus concentration curves, Figures 4-3 through 4-7.

4.5 NORMAL OPERATION FOR DRUM UNITS ON AVT 4.5.1 Cycle Makeup Makeup water to the cycle is an important source of oxygen, carbon dioxide and chemical contamination, thus affecting adversely the levels of iron and copper contamination. Older makeup systems may not provide the necessary quality of water, which generally requires a final polishing stage, usually a mixed bed demineralizer. Contaminants such as chlorides, sulfates, caustic and acids can emanate from makeup treatment systems. Such contamination may cause an increase in copper pickup in the cycle . The effect of oxygen ingress with the makeup water is shown in Figure 4-8.

4-12

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

Figure 4-8

The following are results of a comprehensive utility survey(4) by EPRI related to cycle water makeup:

•

40% put makeup directly into the cycle

•

66% put makeup into vented storage tanks

Serious consideration must be given to removing oxygen from the makeup and condensate water and storing the makeup water unvented to the atmosphere if copper and iron are to be controlled to minimal levels. Oxygen removal can be achieved in a number of ways. This topic is fully discussed in Appendix A. 4.5.2 Condenser Leakage Condenser leakage will introduce chemical contamination and oxygen, which will adversely impact the control of iron and copper in the cycle as well as having other deleterious effects. Condenser leakage must be continuously monitored by sodium and cation conductivity. 4.5.3 Chemical Feeds—Mixed-Metallurgy Cycles Maintaining a reducing environment in the condensate is essential if feedwater copper corrosion transport is to be controlled to an acceptable level. The availability of a deaerating condenser 4-13

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

and deaerating heater will have a positive effect on oxygen control. However, it will still be necessary to feed a reducing agent to ensure a negative ORP at the most desirable level of –350 mV at the deaerator inlet. The most desirable and recommended reducing agent is hydrazine. The reducing agent should be fed at the condensate pump discharge, or, if polishers are installed, at the outlet of the polishers. It is not satisfactory to add the reducing agent after the deaerator. It is not an acceptable or effective practice to increase the reducing agent to levels above 30 ppb to counteract increasing levels of air in-leakage. In the LP feedwater system (up to the deaerator) the reaction rate between a reducing agent and oxygen is very low. The optimum practice is to monitor ORP at the LP heater outlet (deaerator inlet) to ensure that reducing conditions are maintained. If air in-leakage is allowed to be excessive with oxygen levels at the condensate pump discharge greater than 10 ppb, then it is not possible to add sufficient reducing agent to make the low pressure feedwater reducing (at deaerator inlet). A number of alternate reducing agents have been introduced in the past several years and are in use at some power stations. The main concern with the use of these alternate chemicals is their thermal decomposition ( breakdown) products, which usually include organic acids and carbon dioxide. The use of these chemicals must be carefully considered and the breakdown products carefully evaluated before use in the cycle is allowed. Cation conductivity typically will be elevated above the normal target value of 0.2 µS/cm around the cycle when using alternate treatments. The cycle pH should be controlled in the range of 9.0 to 9.3 using ammonia. Ammonia should be fed at the same location as with hydrazine. Separate lines are preferred. Reagent grade chemicals are recommended to avoid any unknown contamination from chemical additions. A number of alkaline organic chemicals have been utilized to control pH. The same admonishments apply to these chemicals as to hydrazine and alternatives, as previously discussed. However, it should be remembered that the overriding philosophy of the EPRI guidelines is that organic amines and organic reducing agents are not needed, required or recommended in the fossil plant cycle. The following target values at the deaerator inlet and the economizer inlet are required for proper copper control:

•

Reducing agent (hydrazine or alternate) - to maintain an ORP of between -300 and -350mV at the deaerator inlet (use minimum amounts of reducing agent to achieve the desired ORP)

•

Oxygen- deaerator inlet ≤ 10ppb, economizer inlet ≤ 5ppb

•

pH- 9.0 to 9.3

•

Cation conductivity- ≤ 0.2 µS/cm

Maintenance of these parameters at the above levels should produce the desirable feedwater copper level of 2 ppb or less.

4-14

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

4.5.4 Chemical Feeds—All-Ferrous Systems While it is not required or recommended (assuming adequate oxygen and air in-leakage control) to maintain a reducing atmosphere, oxygen and carbon dioxide should still be rigidly controlled by reducing air in-leakage and by minimizing dissolved oxygen in the makeup water. Many operators have found that reducing agent feed is not required if these practices are followed. By eliminating reducing agent feed while also minimizing air in-leakage and dissolved oxygen in the makeup, it has been found in numerous instances that iron transport was effectively minimized, and levels of around 1 ppb at the economizer inlet are easily achievable. All-ferrous systems can operate at the higher pH levels of 9.2 to 9.6, further minimizing iron transport. Reagent grade chemicals are recommended to avoid any unknown contamination from chemical additions. 4.5.5 Monitoring and Corrective Actions A comprehensive monitoring program should be established, based upon the use of continuous monitors where available, supplemented by grab samples, if necessary as outlined in Section 4.3. Comprehensive monitoring protocols are given in several EPRI Guidelines such as the Cycling, Startup, Shutdown and Layup Fossil Plant Cycle Chemistry Guidelines for Operators and (5) Chemists . The cycle diagrams in that publication are based upon the applicable boiler water chemistry in use (phosphate, AVT or caustic). An important new parameter for copper control in mixed-metallurgy cycles in this revision of the AVT Guidelines, is ORP. The ORP must be in the reducing range to control copper adequately, as previously described. It is especially important that ORP is monitored in the LP feedwater at the deaerator inlet, and not at the economizer inlet. Conditions could be reducing in the HP feedwater (at the economizer inlet), but oxidizing in the LP feedwater due to excessive air inleakage or oxygen saturated makeup additions. Many utilities have reported problems with the use of ORP sensors and variable ORP readings. It is important that these instruments are calibrated and subjected to frequent QA. Please see Appendix B for comprehensive details. Deviations from established target values must be vigorously investigated and brought rapidly under control if copper and iron are to be minimized in the cycle.

4-15

EPRI Licensed Material Cycles With Drum Boilers on All-Volatile Treatment

4.6 REFERENCES 1. R.B. Dooley, J. Mathews, R. Pate and J. Taylor, “Optimum Chemistry for ‘All-Ferrous’ th Feedwater Systems: Why Use An Oxygen Scavenger?”, 55 Annual Meeting, International Water Conference, Pittsburgh, PA, Oct. 31-Nov. 2, 1994. 2. Guidelines for Chemical Cleaning of Conventional Fossil Plant Equipment. EPRI, Palo Alto, CA: November 2001. 1003994. 3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: December 1994. TR-102285. 4. “Survey of 63 Utilities”. Fifth International Conference on Fossil Plant Cycle Chemistry. EPRI Proceedings. Edited by B. Dooley and J. Matthews TR-108459. December 1997. 5. Cycling, Startup, and Lay-up Fossil Plant Cycle Chemistry Guidelines for Operators and Chemists. EPRI, Palo Alto, CA.: August 1998. TR-107754.

4-16

EPRI Licensed Material

5 CYCLES WITH ONCE-THROUGH BOILERS ON ALLVOLATILE TREATMENT (AVT)

5.1 INTRODUCTION The operating philosophy for once•through steam generating units recognizes that all soluble feedwater contaminants have to dissolve in the superheated exiting steam and be within the allowable turbine inlet steam purity limits. The corrosion products transported to the steam, generator from the pre-boiler system that would be available for deposition in the lower radiant furnace, have to be maintained at a concentration level low enough to avoid any necessity for, or to provide economical operating periods between, chemical cleans to remove these deposits before they cause equipment damage. (9)

Therefore, the feedwater treatment has to be volatile, either AVT (these guidelines) or OT. The alkalizing agent used has to meet the requirement of being completely volatile and not being thermally decomposed at exiting superheated steam temperatures in excess of 538 °C (1000 °F) at both subcritical and supercritical pressures. The only volatile chemical found to meet these requirements is ammonia (NH3) that is applied as ammonium hydroxide (NH4OH). In selecting the optimum cycle chemistry, the materials used for condenser tubing and in feedwater systems have to be respected. Application of copper or copper alloys for condenser or feedwater heater tubing requires particular consideration (copper species transport into the steam generator and have solubility in steam). To optimize control and transport of feedwater contaminants, feedwater systems should be constructed of iron-based materials. Full flow condensate polishing systems utilizing either "deep-bed" and/or "powdered-resin" systems are utilized at the condenser discharge or in the low pressure feedwater system at temperatures below the decomposition point of anion exchange resins (140°F, 60°C). Some systems are designed with pre-coat or cartridge filters ahead of the condensate polishers. 5.1.1 All-ferrous metallurgy systems Feedwater systems having carbon steel heaters and piping are required to operate at a pH of 9.29.6 (at 25°C or 77°F) to minimize flow-accelerated corrosion. In absence of carbon dioxide, adjusting this pH requires the addition of 500-2200 ppb of ammonium hydroxide as NH3 (Figure 5-1). Additional application of a reducing agent such as hydrazine is not advised for units with all-ferrous metallurgy. The oxidizing-reducing potential, ORP, will be just above 0 mV when operating without any reducing agent, and in this way will minimize flow-accelerated (1,2) corrosion. 5-1

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

There are still a few organizations operating units with once-through boilers and all-ferrous metallurgy using a reducing agent such as hydrazine (or an alternative). This type of AVT does not represent the state-of-the-art treatment and should be converted to oxidizing AVT (AVT(O)) or OT. In units with copper alloys employed for condenser tubing (arsenical copper, aluminum bronze, brass, 90-10 and 70-30 copper-nickel alloys) copper corrosion in the air removal section of the condenser or in crevices between the tube and the tube support plate may require a reduction of the upper boundary of the pH operating range (e.g., from 9.6 to 9.4). When the cycle pH is relatively high, the polishers—for economic and regenerant waste disposal reasons—should, if possible be operated normally in the ammonium form and/or beyond the ammonia breakthrough for the deep bed cation resins. Many utilities maintain stand•by resin beds in the hydrogen form to use during periods of condenser leakage. Powdered resins (having low ion exchange capacity) are normally operated in the ammonium form. Ammonium form operation reduces the efficiency of mixed bed ion exchangers especially for sodium removal from the condensate, although continuing to efficiently remove both iron and copper corrosion products from the condenser (if copper containing condensers are utilized). This guideline does not address oxygenated treatment, which solves many of these problems and (3) is covered by a separate Oxygenated Treatment Guideline, which will be revised in 2004. 5.1.2 Mixed-metallurgy systems Cycles with once-through boilers and mixed-metallurgy in the feedwater system are not very common. Nevertheless, some information dealing with this untypical configuration is appropriate. Feedwater systems with feedwater heater tubing made of copper or copper alloys are required to operate at a pH of 9.0-9.3 (at 25°C or 77°F) and under reducing conditions (the optimum ORP should be between –300 and –350 mV) to minimize the copper corrosion and transport of copper oxides into the boiler.(4) Adjusting this pH requires the addition of 250-700 ppb of ammonium hydroxide as NH3 (Figure 5-1). Controlling air in-leakage and the feed of aqueous solution of a reducing agent such as hydrazine in front of the feedwater systems is required for ensuring the required reducing conditions in the entire feedwater system.(4) The reader is referenced to Section 4 in this Guideline where mixed-metallurgy systems are extensively covered. The cycle pH is lower than in cycles with all-ferrous metallurgy. This makes polisher operation in the H/OH cycle more practical.

5-2

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

Figure 5-1 Theoretical Relationship Between Specific Conductivity and pH for Ammonia Solutions

5-3

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

5.2 AVT GUIDANCE Cycle chemistry target values and action levels are presented in this section for each monitoring point for units with once-through boilers on all-volatile treatment. A target or normal value and as many as three action levels are given for each parameter at each monitoring point. This section is intended to be used by operating staff and management personnel. Similarly designed once-through boilers may behave differently in response to impurity ingress and feedwater corrosion product ingress because of differences in pre-boiler systems, balance-ofplant designs and materials choices, differences in makeup system design and operation, variable effectiveness of air-removal equipment, and variable avoidance of cooling water in-leakage control. Minimizing deposition of pre-boiler corrosion product oxides in the waterwalls of once-through (5) boilers is critical and it plays a key part in a number of boiler tube failure (BTF) mechanisms especially in overheating and for circumferential cracking mechanisms. In the long term, the accumulated deposits must be removed by "timely" chemical cleaning to avoid and/or reduce the (6) incidence of these boiler tube failures. This should be the basis of the optimization of AVT and is, of course, one of the reasons to consider conversion of the unit to oxygenated treatment (OT). The first step is to use the road map approach described in Section 2 to optimize the feedwater treatment for all possible operating circumstances.

5.3 TARGET VALUES The sample points, monitoring parameters, target values, and action levels were established for those plants having once-through boilers. The following figures show the target values and action levels for each sample point and monitoring parameter:

•

Cycle chemistry diagram (Figure 5-2) for all-ferrous systems operating with oxidizing conditions, AVT(O).

•

Cycle chemistry diagram (Figure 5-3) for mixed-metallurgy systems operating with reducing conditions, AVT(R).

•

pH versus ammonia at different carbon dioxide levels (Figure 5-4).

5-4

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT) Reheat Steam Target

Sample

N

1

2

3

Sodium, ppb

C

£2

£4

£8

>8

Cation conductivity, mS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Silica, ppb

T

£ 10

£ 20

£ 40

> 40

Chloride, ppb

T

£2

£4

£8

>8

Sulfate, ppb

T

£2

£4

£8

>8

Total organic carbon, ppb

T

£ 100

> 100

—

—

Specific conductivitya

T

—

—

—

—

Parameter

* *

Air Removal System Exhaust Parameter

+

Target

Air inleakage, scfm

Sample C or S

Condenser Leak Detection Trays or Hotwell Zones (If applicable) Target Parameter Cation conductivitya or sodium

IP turbine

HP turbine

LP turbine

Makeup Treatment System Effluent

Sample C

Condensate storage tank

*

Condenser Economizer Inlet (EI) and Attemperation Water Target Sample

Parameter pHa

All-ferrous metallurgy

Ammonia

Specific conductivitya

* *

C D

N

1

2

3

9.2-9.6

< 9.2 > 9.6

—

—

Deaerator

C

Cation conductivity, mS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Iron, ppb

T

£2

>2

—

—

Copper, ppb

T

£2

>2

—

—

Oxygen, ppb

C

< 10

> 10

—

—

Parameter

LP

HP heaters

N

C

£3 £3

Chloride, ppb

T

Sulfate, ppb

T

£3

Silica, ppb

C

£ 10

Specific conductivity, mS/cma

C

£ 0.1

T or W £ 300

Condensate Storage Tank Effluent

Boiler

Consistent with pH

Sample

Sodium, ppb

Total organic carbon, ppb

Makeup treatment system

Attemperat

Target

Parameter

Condensate polisher

Target

Sodium, ppb

Sample

N (aluminum tanks only)

W

£ 10

Condensate Pump Discharge (CPD) Deaerator Outlet Parameter

Target

Sample

N

1

T

< 20

£ 40

Oxygen, ppb

* *

Condensate Polisher Effluent (CPE) Parameter

* *

Target

Sample

Target

Parameter

Ammonia

N

1

2

3

Sodium, ppb

C

£3

£6

£ 12

> 12

Cation conductivity, µS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Silica, ppb

C

£ 10

> 10

—

—

*

Sample

N

1

Sodium, ppb

C

£3

>3

Cation conductivity, µS/cma

C

£ 0.3

³0.3

Total organic carbon, ppb

T

£ 200

> 200

Oxygen, ppb

C

£ 10

£ 20

Maximum Annual Exposure to Contaminant Conditions Cumulative Hours per Year Base Load

Cycling

—

—

1 (Action Level 1)

336 ( 2 weeks)

672 (4 weeks)

2 (Action Level 2)

48 (2 days)

96 (4 days)

3 (Action Level 3)

8

16

Immediate Shutdown

1

2

Targets N (Normal)

Legend

Footnotes

Sample Frequency

Target Values

C = continuous S = grab, once per shift D = grab, once per day W = grab, once per week T = troubleshooting and commissioning

N = Normal 1 = Action Level 1 2 = Action Level 2 3 = Action Level 3

a = Conductivity and pH measured at 25° C

Sample and Chemical Feed Identification

*

+

= “Core” parameter alarmed in Control Room = “Core” parameter = Continuous sample = Chemical feed

Figure 5-2 Cycle Chemistry Diagram for a Once-Through Unit on All-VolatileTreatment. All-ferrous metallurgy in feedwater system. Operating with an oxidizing environment (no reducing agent).

5-5

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT) Reheat Steam Target

Parameter

* *

Sample

N

1

2

3

Sodium, ppb

C

£2

£4

£8

>8

Cation conductivity, mS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Silica, ppb

T

£ 10

£ 20

£ 40

> 40

Chloride, ppb

T

£2

£4

£8

>8

Sulfate, ppb

T

£2

£4

£8

>8

T

£ 100

> 100

—

—

T

—

—

—

—

Total organic carbon, ppb Specific conductivity

a

Air Removal System Exhaust Parameter

+

Target

Air inleakage, scfm

Sample C or S

Condenser Leak Detection Trays or Hotwell Zones (If applicable) Target Parameter Cation conductivity or sodium

IP turbine

HP turbine

LP turbine

Makeup Treatment System Effluent

Sample a

C

*

Condenser Target

Parameter pHa

Sample

N

1

2

3

C

9.0-9.3

< 9.0 > 9.3

—

—

mixed- metallurgy

Ammonia

Specific conductivitya

* *

D

Cation conductivity, mS/cma

C

Iron, ppb

T

Deaerator Boiler

Consistent with pH

C

Makeup treatment system

Attemperat

£ 0.2

£ 0.4

£ 0.8

> 0.8

£2

>2

—

—

Copper, ppb

T

£2

>2

—

—

Oxygen, ppb

C

20

LP

Condensate polisher

£3

Chloride, ppb

T

£3

Sulfate, ppb

T

£3

Silica, ppb

C

£ 10

Specific conductivity, mS/cma

C

£ 0.1

T or W £ 300

Condensate Storage Tank Effluent Target

Sodium, ppb

Sample

N (aluminum tanks only)

W

£ 10

Condensate Pump Discharge (CPD)

Deaerator Outlet Parameter

Target

Oxygen, ppb

Sample

N

T

< 10

Parameter

Sample

N

Reducing agent, ppb

S

Consistent with ORP

ORP, mV

C

-300 to -350

Parameter

*

Target

* *

Condensate Polisher Effluent (CPE)

* *

Target

Sample

N

Target

Sample

N

1

Sodium, ppb

C

£3

>3

Cation conductivity, µS/cma

C

£ 0.3

³ 0.3

Total organic carbon, ppb

T

£ 200

> 200

Oxygen, ppb

C

£ 10

£ 20

Parameter

Ammonia and Reducing agent

Deaerator Inlet (DAI)

Maximum Annual Exposure to Contaminant Conditions

N

C

Total organic carbon, ppb

Parameter

HP heaters

Sample

Sodium, ppb

Condensate storage tank

Economizer Inlet (EI) and Attemperation Water

Target

Parameter

1

2

3

Sodium, ppb

C

£3

£6

£ 12

> 12

Cation conductivity, µS/cma

C

£ 0.2

£ 0.4

£ 0.8

> 0.8

Silica, ppb

C

£ 10

> 10

—

—

*

Cumulative Hours per Year

Targets N (Normal)

Base Load

Cycling

—

—

1 (Action Level 1)

336 ( 2 weeks)

672 (4 weeks)

2 (Action Level 2)

48 (2 days)

96 (4 days)

3 (Action Level 3)

8

16

Immediate Shutdown

1

2

Legend

Footnotes

Sample Frequency

Target Values

a = Conductivity and pH measured at 25° C

C = continuous S = grab, once per shift D = grab, once per day W = grab, once per week T = troubleshooting and commissioning

N = Normal 1 = Action Level 1 2 = Action Level 2 3 = Action Level 3

Sample and Chemical Feed Identification

*

+

= “Core” parameter alarmed in Control Room = “Core” parameter

= Continuous sample = Chemical feed

Figure 5-3 Cycle Chemistry Diagram for a Once-Through Unit on All-Volatile Treatment. Mixed-metallurgy feedwater systems. Operating with a reducing environment (reducing agent added)

5-6

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

10 9 8 7 6 5 4

Ammonia (ppm NH3)

3 2

1 0.9 0.8 0.7 0.6 0.5

CO2 (ppm) 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.20 0.10 0.05 0.00

0.4 0.3 0.2

0.1 8.8

9.0

9.2

9.4

9.6

9.8

10.0

pH at 25 °C Figure 5-4 Ammonia Concentration vs. pH for Various Carbon Dioxide Concentrations. Note: the top curve represents 4 ppm CO2 and the bottom curve represents zero CO2. Source: from unpublished data derived from volatility code discussed in Section 3.3.1

5-7

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

There are a number of features to keep in mind when adapting Figures 5-2 and 5-3 to an individual plant. Particular emphasis has been placed in the development of these new guideline cycle diagrams to make control of a unit as simple as possible, and to minimize repetitive grab sampling.

•

The Core Parameters (Table 3-1) are indicated by an asterisk (*). These instruments should be monitored continuously and on-line. These have a “C” for “continuous” in the sample column.

•

There are a number of other parameters, which have also been designated by a “C” as they provide useful confirmation of unit condition, but are not regarded as “Core” instruments. These parameters are: silica and sodium in makeup, and specific conductivity at the economizer inlet.

•

There are a number of other parameters on these diagrams, which have been newly designated in this guideline as “T” for troubleshooting. It is only necessary to monitor these if one of the continuous “Core” instruments provides an indication that the parameter is out of its “normal” guideline value. In most cases, this will be accomplished by grab sample; however, in some cases an organization might have decided to install a continuous analyzer. In the case of iron and copper at the economizer inlet, it should be recognized that in a well operated plant it should not be necessary to monitor these parameters more frequently than once or twice a year; and then only to confirm the feedwater regime remains optimized.

A copy of Figure 5-2, modified if necessary to reflect specific unit characteristics and/or experience, could be included in the plant operating procedures and prominently displayed in the control room, water and steam sample room, and chemistry laboratory. Copies could also be displayed at the makeup treatment system control panel and the condensate polisher control panel (if applicable).

5.4 NORMAL OPERATION FOR ONCE•THROUGH UNITS ON AVT 5.4.1 All-ferrous feedwater systems Normal AVT(O) chemistry control includes the injection of ammonium hydroxide (NH4OH) for pH control. The pH control range for systems utilizing carbon steel is 9.2•9.6 (77°F, 25°C). This requires 500•2200 ppb of ammonia as NH3 (see Figure 5•1) in the absence of carbon dioxide. Reagent grade chemicals are recommended to avoid any unknown contamination from chemical additions. 5.4.2 Mixed-metallurgy systems Normal AVT(R) chemistry control includes the injection of ammonium hydroxide (NH4OH) for pH control and of a reducing agent such as hydrazine for ORP control. The pH control range for mixed-metallurgy systems is 9.0-9.3 (77°F, 25°C). This requires 250-700 ppb of ammonia as NH3 (see Figure 5-1) in the absence of carbon dioxide. Dosing of hydrazine into the condensate upstream of feedwater heaters in association with good air in-leakage control and less than 10 ppb of oxygen at the condensate pump ensures a reducing ORP in the range of –300 to –350 mV 5-8

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

at the deaerator inlet. Here, too, reagent grade chemicals are recommended to avoid any unknown contamination from chemical additions. 5.4.3 Monitoring and Corrective Actions A comprehensive monitoring program should be established, based upon the use of continuous monitors where available, supplemented by grab samples, only if necessary as outlined in Section 5.3. Comprehensive monitoring protocols are given in several EPRI Guidelines such as the Cycling, Startup, Shutdown and Layup Fossil Plant Cycle Chemistry Guidelines for Operators and Chemists.(7) An important new parameter for copper control in mixed-metallurgy cycles in this revision of the AVT Guidelines is ORP. The ORP must be in the reducing range to control copper adequately, as previously described. It is especially important that ORP is monitored in the LP feedwater at the deaerator inlet, and not just at the economizer inlet. Conditions could be reducing in the HP feedwater (at the economizer inlet), but oxidizing in the LP feedwater due to excessive air inleakage or oxygen saturated makeup additions. Some organizations have reported problems with the use of ORP sensors and variable ORP readings. It is important that these instruments are calibrated and subjected to frequent QA. Please see Appendix B for comprehensive details. Deviations from established target values must be vigorously investigated and brought rapidly under control if copper and iron are to be minimized in the cycle. This is covered in Section 6 of these guidelines.

5.5 REACTIONS TO CONTAMINANTS IN THE CYCLE Air in-leakage is a concern as it can affect condensate polisher anion capacity due to carbon dioxide removal and, of course, affects condensate pump discharge dissolved oxygen concentrations. This is particularly important in cycles with mixed-metallurgy because operating with oxygen levels greater than 10 ppb at the condensate pump discharge will jeopardize the ability to have reducing conditions in the LP feedwater system. Air in-leakage is also important for units operating with oxidizing AVT(O) and OT. Condenser cooling water in-leakage, makeup water contamination, condensate storage tank contamination and improper condensate polisher regeneration can also be sources of contaminants. Three typical examples are:

•

chloride or sulfate excursions due to condenser in-leakage

•

caustic or acid contamination from makeup (or condensate polisher) regenerant or

•

silica from extraneous sources

Satisfactory operation of once-through steam generators and the associated turbines requires that the cation conductivity of condensate (downstream polishers), feedwater, and steam is less than 0.2 µS/cm. 5-9

EPRI Licensed Material Cycles With Once-Through Boilers on All-Volatile Treatment (AVT)

This revision of the AVT Guideline suggests that maximum normal operation levels for final feedwater in cycles with all-ferrous metallurgy should be: iron