EPRI Start Up, Lay Up & Shut Down Guidelines for Chemist

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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORKSPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) NAMED BELOW,NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS REPORT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISREPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS REPORT.

ORGANIZATION(S) THAT PREPARED THIS REPORT

Electric Power Research Institute

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box23205, Pleasant Hill, CA 94523, (510) 934-4212.

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

Copyright © 1998 Electric Power Research Institute, Inc. All rights reserved.



iii

CITATIONS

This report was prepared by

EPRI3412 Hillview AvenuePalo Alto, CA 94403

This report describes research sponsored by EPRI. It is a corporate document thatshould be cited in the literature in the following manner:

Cycling, Startup, Shutdown, and Layup Fossil Plant Cycle Chemistry Guidelines for Operatorsand Chemists, EPRI, Palo Alto, CA, 1998.TR-107754.





v

REPORT SUMMARY

The purity of water and steam is central to ensuring fossil plant component availability

and reliability. This report will assist utilities in developing cycle chemistry guidelines

for all transient operation and shutdown.

Background

EPRI has published four operating guidelines for phosphate treatment, all-volatiletreatment, oxygenated treatment, and caustic treatment. These guidelines encompass

five drum boiler water treatments and three feedwater choices that can provide the

optimum cycle chemistry for each unit. A similar, consistent approach was needed for

startup, shutdown, and layup. Improper shutdown of a unit can lead to pitting, which

is a precursor to major corrosion fatigue and stress corrosion damage in the turbine. It

can also lead to the development of nonprotective oxides on copper alloys in the

feedwater.

ObjectiveTo provide comprehensive guidelines for cycle chemistry during startup, shutdown,

and layup of fossil plants; to provide optimum procedures for the boiler, superheater,

reheater, turbine, and feedwater heaters.

ApproachEPRI developed an initial skeleton of the guidelines that provided the basis for a series

of working group meetings with members of the EPRI Fossil Plant Cycle Chemistry

Group (FPCCG). Following these meetings, EPRI and five of its cycle chemistry

consultants developed a draft document and circulated it to the 40 members of the

FPCCG for review and comment.

ResultsThis guideline provides the final link needed for comprehensive coverage of cycle

chemistry in fossil plants. It provides specific procedures and advice during cycling,

shutdown, startup, and layup for each of the boiler and feedwater treatments and



vi

covers all major water and steam touched surfaces. The guideline is applicable to drum

boiler units above 600 psi (4.1MPA), once-through subcritical and supercritical boiler

units, units with and without condensate polishers, all-ferrous and mixed metallurgy

feedwater systems, and superheaters, reheaters and turbines.

EPRI Perspective

While most utilities can meet EPRI cycle chemistry guideline limits, a large number of

problem areas have been identified that relate to poor transient (startup/shutdown)

operation and improper layup procedures. Two such important mechanisms are pitting

in unprotected reheaters, which can lead to multiple reheater leaks. and pits on low

pressure turbine blade/disk surfaces in the phase transition zone. A very low

percentage of utilities currently provide shutdown protection to boilers, feedwater

heaters, and turbines. This document will provide the important interfaces between

plant operation, plant shutdown, and transient conditions.

TR-107754

Interest Categories

Fossil steam plant O&M cost reductionFossil steam plant performance optimizationApplied science and technology

KeywordsPower plant availability

Water chemistry

Cycling

Startup

Shutdown

Layup





EPRI L icensed M aterial

viii

Section 4 provides information on layup and shutdown considerations common to mostunits: wet and dry layup, and dehumidification for all the major power plantcomponents.

Sections 5-8 deal with specific procedures for cycling, shutdown, startup, and layup for

phosphate treatments, all-volatile treatment, oxygenated treatment and caustictreatment respectively.




ix

ACKNOWLEDGMENTS

The authors of these guidelines:

R. B. Dooley, EPRIA. Aschoff, EPRI ConsultantM. Ball, EPRI ConsultantA. Bursik, EPRI ConsultantO. Jonas, EPRI Consultant of Jonas Inc.

andF. Pocock, EPRI Consultant

acknowledge that the two earlier drafts of this guideline were reviewed by the 41members of the Fossil Plant Cycle Chemistry Group (FPCCG). The authors furtheracknowledge the contributions from the following members of the FPCCG:

B. Conlin ESKOMD. Goldstrohm Salt River ProjectA. Howell New Century EnergiesD. E. Hubbard American Electric Power

A. Lindberg Commonwealth Edison J. Matthews Duke PowerV. Mrasek Public Service Company of OklahomaK. J. Shields Sheppard D. Powell AssociatesW. Urion Connectiv

During the preparation of these guidelines two Target 51 member utilities alsoprovided extensive documentation on their layup experiences which arecomplementary to the procedures in the guidelines:

Iberdrola SpainEcogen Energy Australia

This report was word processed by Lorrain Sargent of Pacific Publications, and all thefigures were drawn by Marilyn Winans of the EPRI Graphics Office.






xi

CONTENTS

1 INTRODUCTION ................................................................................................................. 1-1

1.1 OVERVIEW OF THE EPRI FOSSIL PLANT CYCLE CHEMISTRY PROGRAM.......... 1-1

Volatility of Salts in Steam Cycles................................................................................... 1-2

Phosphate Chemistry/Hideout/Corrosion ........................................................................ 1-4

Deposition and Chemical Cleaning ................................................................................. 1-6Steam, Chemistry and Corrosion in the Phase Transition Zone (PTZ)............................ 1-6

1.2 EPRI FOSSIL PLANT GUIDELINES AND MANAGEMENT APPROACHES FORCYCLE CHEMISTRY.......................................................................................................... 1-7

1.3 NEED AND DEVELOPMENT FOR CYCLING/SHUTDOWN/STARTUP/LAYUPGUIDELINES...................................................................................................................... 1-9

1.4 OBJECTIVES OF THESE GUIDELINES ................................................................... 1-10

1.5 SCOPE OF THESE GUIDELINES............................................................................. 1-11

1.6 REFERENCES .......................................................................................................... 1-11

2 METALLURGICAL, DESIGN, AND OPERATING CONSIDERATIONS.............................. 2-1

2.1 INTRODUCTION.......................................................................................................... 2-1

Impurity Generation, Transport, and Corrosion Effects................................................... 2-3

Steam Cycle Materials and Their Properties................................................................... 2-8

Material Properties .......................................................................................................... 2-9

2.2 STEAM CYCLE COMPONENT CORROSION AND DEPOSITS ................................. 2-9

Basics of Material Corrosion ........................................................................................... 2-9

Cycle Component Damage Mechanisms ...................................................................... 2-17

2.3 PREBOILER SYSTEMS - ALL FERROUS VS. MIXED METALLURGY .................... 2-21

All-Ferrous Feedwater Systems(3, 4)

................................................................................ 2-22

Mixed Metallurgy Feedwater Systems (Copper Containing)(3, 4, 22, 28)

................................ 2-24

Copper Transport .......................................................................................................... 2-25

2.4 PRIORITIES FOR TRANSIENT OPERATION........................................................... 2-26




xii

2.5 EFFECTS OF STEAM CYCLE DESIGN AND OPERATION ..................................... 2-29

Drum Boiler vs. Once-through Boiler Units(7-12)

............................................................... 2-30

Sliding Pressure Operation(17,39-42)

................................................................................... 2-30

Boiler Concerns(15-17,25,39,40,43-46).............................................................................................. 2-30

Turbine(18,42,49,50)................................................................................................................... 2-33

Turbine Bypass Systems(43,49.50)

.......................................................................................... 2-38

Feedwater System Cleanup Loops(14,45,46,52,53,54) ................................................................... 2-39

Condensate Filtering and Polishing(15,45,54,67)........................................................................ 2-41

Air Inleakage and Deaeration(15,45,53-61)

................................................................................. 2-41

Condenser Deaeration .................................................................................................. 2-47

2.6 ALTERNATIVE WATER TREATMENT CHEMICALS(3,63,64) ......................................... 2-47

2.7 REFERENCES AND BIBLIOGRAPHY FOR SECTION 2 .......................................... 2-49

3 GENERAL ASPECTS COMMON TO MOST UNITS ........................................................... 3-1

3.1 DEFINITIONS .............................................................................................................. 3-1

Cycling ............................................................................................................................ 3-1

Duration of Shutdown...................................................................................................... 3-1

Forced Shutdown............................................................................................................ 3-2

System Failure but no Equipment Failure ................................................................... 3-3

Major Equipment Failure ............................................................................................. 3-3

3.2 USE OF POLISHERS AND CONDENSATE FILTRATION .......................................... 3-3

Condensate Polishing and/or Filtration(1b)

........................................................................ 3-3

Makeup Water Treatment................................................................................................ 3-3

3.3 MONITORING IMPORTANCE AND REQUIREMENTS............................................... 3-4

Sampling and Monitoring ................................................................................................ 3-4

Sampling Problems ......................................................................................................... 3-4

3.4 MAJOR CHEMICAL TRANSIENT................................................................................ 3-5

Chemical Transients and Equipment Failures................................................................. 3-6

3.5 MINIMIZATION OF AIR IN-LEAKAGE......................................................................... 3-7

3.6 CORRECTIVE ACTIONS............................................................................................. 3-8

3.7 HOW TO USE THE PRESENT EPRI GUIDELINES FOR CYCLINGOPERATION....................................................................................................................... 3-8

Drum Units ...................................................................................................................... 3-9

Once-Through Units...................................................................................................... 3-10




xiii

3.8 OPERATING PROCEDURES.................................................................................... 3-10

3.9 ENVIRONMENTAL CONSIDERATIONS ................................................................... 3-10

3.10 REFERENCES ........................................................................................................ 3-11

4 SHUTDOWN AND LAYUP CONSIDERATIONS COMMON TO MOST UNITS................... 4-14.1 INTRODUCTION.......................................................................................................... 4-1

4.2 LAYUP PRACTICES.................................................................................................... 4-2

Short-term vs. Longterm Layup....................................................................................... 4-3

4.3 WET LAYUP (12-15) ......................................................................................................... 4-8

4.4 DRY LAYUP USING DEHUMIDIFIED AIR................................................................... 4-9

4.5 FEEDWATER HEATERS, CONDENSER, REHEATER AND TURBINE.................... 4-14

Turbine.......................................................................................................................... 4-15

Feedwater Side of Condensers and Feedwater Heaters .............................................. 4-16Shell Side Feedwater Heaters....................................................................................... 4-16

Superheater .................................................................................................................. 4-16

Deaerator and Storage Tank......................................................................................... 4-17

4.6 LAYUP MONITORING............................................................................................... 4-17

4.7 ENVIRONMENTAL CONSIDERATIONS ................................................................... 4-17

4.8 ROAD MAP FOR SHUTDOWN AND LAYUP ............................................................ 4-18

4.9 REFERENCES .......................................................................................................... 4-22

5 PHOSPHATE TREATED DRUM UNITS.............................................................................. 5-1

5.1 INTRODUCTION.......................................................................................................... 5-1

5.2 CURRENT NORMAL OPERATING GUIDELINES....................................................... 5-2

5.3 STARTUP PROCEDURES........................................................................................ 5-10

Road Map ..................................................................................................................... 5-11

5.4 CYCLING AND PEAKING UNITS.............................................................................. 5-12

5.5 SHUTDOWN PROCEDURES.................................................................................... 5-12

Road Map ..................................................................................................................... 5-14

5.6 MIXED METALLURGY SYSTEMS ............................................................................ 5-16

5.7 CORRECTIVE ACTIONS........................................................................................... 5-17

5.8 LAYUP....................................................................................................................... 5-17

5.9 REFERENCES .......................................................................................................... 5-18

6 ALL-VOLATILE TREATMENT ............................................................................................ 6-1




xiv

6.1 INTRODUCTION.......................................................................................................... 6-1

6.2 ONCE-THROUGH UNITS............................................................................................ 6-2

Current Guidelines .......................................................................................................... 6-2

Startup ............................................................................................................................ 6-2

Shutdown........................................................................................................................ 6-7

Cycling and Peaking ..................................................................................................... 6-10

Layup ............................................................................................................................ 6-11

6.3 DRUM BOILERS WITH ALL-FERROUS FEEDWATER HEATING SYSTEMS.......... 6-11

Current Guidelines ........................................................................................................ 6-11

Startup .......................................................................................................................... 6-18

Shutdown...................................................................................................................... 6-22


Layup ............................................................................................................................ 6-26

6.4 DRUM UNITS WITH MIXED METALLURGY FEEDWATER HEATING SYSTEMS... 6-27

Current Guidelines ........................................................................................................ 6-28

Startup .......................................................................................................................... 6-28

Shutdown...................................................................................................................... 6-32


Layup ............................................................................................................................ 6-36

6.5 REFERENCES .......................................................................................................... 6-36

7 OXYGENATED TREATMENT............................................................................................. 7-1

7.1 INTRODUCTION.......................................................................................................... 7-1

7.2 ALL-FERROUS CYCLES WITH ONCE-THROUGH BOILERS.................................... 7-2

Current Normal Operating Guidelines ............................................................................. 7-2

Startup Procedures ......................................................................................................... 7-4

Shutdown Procedures..................................................................................................... 7-7

Short-Term Shutdown. ................................................................................................ 7-8

Longterm Shutdown.................................................................................................... 7-9

Emergency Shutdown. .............................................................................................. 7-10

Shutdown as a Result of a Serious Chemistry Excursion.......................................... 7-10

Cycling and Peaking Operation(3-7)

................................................................................. 7-10

Layup Practices............................................................................................................. 7-11

7.3 ALL-FERROUS CYCLES WITH DRUM BOILERS..................................................... 7-12




xv

Current Normal Operating Guidelines ........................................................................... 7-12

Startup Procedures ....................................................................................................... 7-14

Shutdown Procedures................................................................................................... 7-19

Short-Term Shutdown. .............................................................................................. 7-19

Longterm Shutdown.................................................................................................. 7-20

Emergency Shutdown. .............................................................................................. 7-21

Shutdown as a Result of a Serious Chemistry Excursion.......................................... 7-22

Cycling and Peaking Operation..................................................................................... 7-23

Layup Practices............................................................................................................. 7-23

7.4 REFERENCES .......................................................................................................... 7-24

8 CAUSTIC TREATMENT FOR DRUM BOILERS ................................................................. 8-1

8.1 INTRODUCTION.......................................................................................................... 8-18.2 ALL-FERROUS FEEDWATER HEATING SYSTEMS.................................................. 8-2

Current Guidance Document........................................................................................... 8-2

Startup ............................................................................................................................ 8-2

Shutdown........................................................................................................................ 8-5

Cycling and Peaking ....................................................................................................... 8-5

Layup .............................................................................................................................. 8-7

8.3 MIXED METALLURGY FEEDWATER HEATING SYSTEMS ...................................... 8-7

Current Guidelines .......................................................................................................... 8-9

Startup ............................................................................................................................ 8-9

Shutdown...................................................................................................................... 8-11


Layup ............................................................................................................................ 8-12

8.4 REFERENCES .......................................................................................................... 8-12






xvii

LIST OF FIGURES

Figure 1-1 Partitioning Constants KD for Common Boiler Water Salts, Acids and BasesRepresented by Mathematical Functions of the Reciprocal of Temperature inKelvin up to the Critical Temperature of Water, Tc.......................................................... 1-4

Figure 2-1 Three supports for reliable cycling operation........................................................ 2-2

Figure 2-2 Typical water chemistry and corrosion effects of layup, startup and cyclingfor a drum boiler cycle..................................................................................................... 2-4

Figure 2-3 Sources of contaminants enhanced by cycling operation and examples ofengineering solutions. ..................................................................................................... 2-5

Figure 2-4 Mollier diagram for a fossil cycle........................................................................... 2-6

Figure 2-5 Potential - pH diagram for carbon steel in 300 °C water(19)

.................................. 2-11

Figure 2-6 Corrosion of mild steel and solubility of magnetite at 300°C, showingcorrosion rate laws

(15,20.21)................................................................................................ 2-12

Figure 2-7 Potential - pH diagram for copper in ammonia solutions at 25°C(22)

.................... 2-13

Figure 2-8 Average copper release as a function of pH....................................................... 2-14

Figure 2-9 Effect of pH on steady state release rates for 90Cu/10Ni and 70Cu/30Niexposed to ammonia solutions containing 8-21 µg/kg oxygen, flowing at ~1ft/s

(0.3m/s) and at a temperature of 35° - 38°C

(22)

.............................................................. 2-15Figure 2-10 Corrosion fatigue diagram for NiCrMoV LP turbine disk and rotor steel18 ........ 2-16

Figure 2-11 Stress corrosion of NiCrMoV disk steel vs. yield strength for "good" waterand steam(18) .................................................................................................................. 2-17

Figure 2-12 The effect of pH on iron and copper concentration at the economizer inlet.Source: D. Frey, Mechanics of Corrosion Product Formation and Transport(14) ............ 2-22

Figure 2-13 Calculated and measured HP rotor temperatures - startup .............................. 2-34

Figure 2-14 Turbine fatigue index vs. temperature change and time(51)

............................... 2-37

Figure 2-15 Turbine valves for partial arc and full arc admission......................................... 2-37

Figure 2-16 Rotor thermal stress as a function of time with sliding pressure. Initial

throttle to metal temperature difference = +50°F, throttle temperature ramp =200°F/h, loading rate = 2% per minute.......................................................................... 2-37

Figure 2-17 Turbine bypass system..................................................................................... 2-39

Figure 2-18 Condensate/feedwater cleanup loops(46)

........................................................... 2-40

Figure 2-19 Expected dissolved oxygen at the deaerator outlet vs. load for tray andspray deaerators ........................................................................................................... 2-43

Figure 2-20 Condenser deaerating capacity with and without retrofitted devices(62)

............. 2-44




xviii

Figure 2-21 Cycle iron concentration during a cold startup for two layup practices(62)

.......... 2-45

Figure 2-22 Effect of air-saturated makeup water on condensate oxygen level(59)

............... 2-46

Figure 3-1 Metals Concentrations in Feedwater During Startup Operations(9,10) ..................... 3-5

Figure 4-1 Nitrogen blanketing of a drum boiler showing the nitrogen connections(12)

. .......... 4-7

Figure 4-2 Corrosion Rate of Steel Relative to Humidity of Air ............................................ 4-10Figure 4-3 Rotary Desiccant Dehumidifier

(17)........................................................................ 4-11

Figure 4-4 Block Diagram of Dehumidifier Steam/Feed Cycle(18)

.......................................... 4-12

Figure 4-5 Steamside Dehumidification Flow(19)

................................................................... 4-13

Figure 4-6 Turbine dry layup using dehumidified air(23)

......................................................... 4-16

Figure 4-7 Dry layup of 107 MW turbine showing measured values of temperature(°F/°C) and air humidity ................................................................................................. 4-16

Figure 4-8 Road Map to Develop Shutdown and Layup Guidelines Common to MostUnits.............................................................................................................................. 4-18

Figure 5-1 Older Forms of Phosphate Treatment .................................................................. 5-3

Figure 5-2 Schematic of Operating Ranges of Boiler Water on Equilibrium PhosphateTreatment (EPT), Congruent Phosphate Treatment (CPT) and PhosphateTreatment (PT)

(1). The CPT is shown to its maximum Na:PO

4 molar ratio of 2.8; the

normal operating range is below the Na:PO4 molar ratio of 2.6....................................... 5-4

Figure 5-3 Cycle Chemistry Diagram for a Drum Unit on Equilibrium PhosphateTreatment (Plants With Reheat)—Core Parameters Marked. ......................................... 5-5

Figure 5-4 Equilibrium Phosphate Treatment: Boiler Water Sodium vs. OperatingPressure (Plants With Reheat)........................................................................................ 5-6

Figure 5-5 Equilibrium Phosphate Treatment: Boiler Water Chloride vs. OperatingPressure (Plants With Reheat)........................................................................................ 5-7

Figure 5-6 Equilibrium Phosphate Treatment: Boiler Water Sulfate vs. OperatingPressure (Plants With Reheat)........................................................................................ 5-8

Figure 5-7 Equilibrium Phosphate Treatment: Boiler Water Silica vs. OperatingPressure (Plants With Reheat)........................................................................................ 5-9

Figure 5-8 Road Map for Startup of PT or EPT Units .......................................................... 5-10

Figure 5-9 Road Map for Shutdown of PT or EPT Units (This should be used inconjunction with Figure 4-8.) ......................................................................................... 5-13

Figure 6-1 Cycle Chemistry Diagram for a Once-Through Unit on All-Volatile Treatment...... 6-3

Figure 6-2 Startup of Once-through Units with All-Ferrous Feedwater Heaters..................... 6-5

Figure 6-3 AVT - Shutdown of Once-Through Units with All-Ferrous Feedwater Heaters ...... 6-8

Figure 6-4 Cycle Chemistry Diagram for a Drum Unit on All-Volatile Treatment (Plantswith Reheat).................................................................................................................. 6-12

Figure 6-5 All Volatile Treatment: Drum Boiler Water Sodium vs. Operating Pressure(Plants With Reheat) ..................................................................................................... 6-13

Figure 6-6 All-Volatile Treatment: Drum Boiler Water Chloride vs. Operating Pressure(Plants With Reheat) ..................................................................................................... 6-14




xix

Figure 6-7 All-Volatile Treatment: Drum Boiler Water Sulfate vs. Operating Pressure(Plants With Reheat) ..................................................................................................... 6-15

Figure 6-8 All-Volatile Treatment: Drum Boiler Water Silica vs. Operating Pressure(Plants With Reheat) ..................................................................................................... 6-16

Figure 6-9 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs. Operating

Pressure (Plants With Reheat)...................................................................................... 6-17Figure 6-10 AVT - Startup of Drum Boilers with All-Ferrous Feedwater Heaters ................. 6-20

Figure 6-11 AVT - Shutdown of Units with Drum Boilers with All-Ferrous and MixedMetallurgy Feedwater Heaters ...................................................................................... 6-23

Figure 6-12 AVT - Startup of Drum Boilers with Mixed Metallurgy Feedwater Heaters........ 6-30

Figure 7-1 Cycle Chemistry Diagram of Once-Through Units on Oxygenated Treatment(core parameters only) .................................................................................................... 7-3

Figure 7-2 Road map for the startup of once-through boilers operated with OT.................... 7-5

Figure 7-3 Shutdown and Operation Guidance for OT Chemistry for Short-TermShutdowns ...................................................................................................................... 7-8

Figure 7-4 Shutdown and Operation Guidance for OT Chemistry for Long-TermShutdowns. Note *: Dependent on wet or dry storage and utilization of nitrogenblanketing (See Section 4) .............................................................................................. 7-9

Figure 7-5 Cycle Chemistry Diagram of Drum Units on Oxygenated Treatment (OT).......... 7-13

Figure 7-6 Road map for the startup of drum boilers operated with OT............................... 7-16

Figure 7-7 Operation and Shutdown Guidance for OT Chemistry for Short-termShutdowns (Drum Boiler Unit) ....................................................................................... 7-20

Figure 7-8 Operation and Shutdown for OT Chemistry for Longterm Shutdowns (DrumBoiler Unit) .................................................................................................................... 7-21

Figure 8-1 Cycle Chemistry Diagram for Drum Type Coal-Fired Boiler on Sodium

Hydroxide Treatment (Plants with Reheat)...................................................................... 8-3

Figure 8-2 CT - Startup of Drum Boilers with All-Ferrous Feedwater Heaters........................ 8-6

Figure 8-3 CT - Shutdown of Drum Boilers with All-Ferrous and Mixed MetallurgyFeedwater Heaters.......................................................................................................... 8-7

Figure 8-4 CT - Startup of Drum Boilers with Mixed Fe-Cu Metallurgy FeedwaterHeaters ......................................................................................................................... 8-11






xxi

LIST OF TABLES

Table 1-1 “Core” Monitoring Parameters (Minimum level of instruments for allplants/units)..................................................................................................................... 1-8

Table 2-1 Transient Effects and Their Amelioration.............................................................. 2-7

Table 2-2 Boiler Tube Damage Mechanisms Influenced by Cycle Chemistry (Adaptedfrom reference 27) (Discussion of each mechanism can be found in Reference 25) .... 2-18

Table 2-3 Turbine Deposits & Damage Mechanisms Influenced by Cycle Chemistry

(Adapted from reference 27) (Discussion of each mechanism can be found inReference 65) ............................................................................................................... 2-19

Table 2-4 Condensate/Feedwater Cycle Damage Mechanisms Influenced by StartupCycle Chemistry (Adapted from reference 27) .............................................................. 2-20

Table 2-5 Generation of Feedwater Corrosion Products by Corrosion and Flow-Accelerated Corrosion, and the Major Unit Transport and Deposition ProblemAreas for All-Ferrous Systems....................................................................................... 2-23

Table 2-6 Generation of Feedwater Corrosion Products by Corrosion and Flow-Accelerated Corrosion, and the Major Unit Transport and Deposition ProblemAreas for Mixed Metallurgy Systems ............................................................................. 2-25

Table 2-7 List of Concerns for Cycling Units (CH indicates the items affected by water

and steam chemistry) .................................................................................................... 2-28

Table 4-1 Shutdown and Layup Alternatives Showing Advantages and Disadvantagesfor Each Alternative......................................................................................................... 4-4






1-1

1

INTRODUCTION

These guidelines cover water and steam chemistry control during transient operationincluding cycling and peaking, cold and warm startups, shutdown, and layup. Theydo not cover mechanical and thermal restraints imposed by equipment manufacturersand cycle design. However, these restraints, which often have the highest priority, areconsidered in the chemical guidelines and limits.

1.1 OVERVIEW OF THE EPRI FOSSIL PLANT CYCLE CHEMISTRYPROGRAM

The Electric Power Research Institute (EPRI) Fossil Plant Cycle Chemistry Program hasthe following goals:

To eliminate boiler tube failures related to cycle chemistry

To eliminate turbine chemical problems (low-pressure blade and disk cracks, andserious deposits throughout the turbine)

To develop optimized feedwater treatment:

— elimination of serious flow-accelerated corrosion (FAC)

— low iron and copper transport (<2 ppb at the economizer inlet)

To eliminate the need for boiler chemical cleaning

To provide simple, reliable cycle chemistry instrumentation and control:

— “core “ levels of instrumentation for all plants

— expert advisor

— direct on-line, in-situ instruments

To shorten the startup period by:




Introduction

1-2

— optimization of shutdown, lay-up and startup chemistry

— the elimination of unnecessary chemical holds in the startup sequence

To develop operational guidelines with action levels for all units, and

To provide the optimum managerial approach in support of cycle chemistry

The program was initiated with the development of the “Interim Consensus Guidelines(ICG)” in 1986

(1).

The ICG was followed by a detailed monitoring program at four US fossil plants(2, 3).Information was also collected at many international plants

(4). The information that was

developed from these efforts led to the identification of four areas where theunderstanding was deficient in the derivation of the ICG chemistry limits:

Volatility of salts and how impurities partition between boiler water and steam

Phosphate chemistry/hideout/corrosion

Deposition around the cycle

Steam chemistry and corrosion in the phase transition zone (PTZ)

EPRI projects have been initiated in response to these areas of deficiency. Brief information on these studies is included below together with information from otherstudies with particular emphasis for developing these current guidelines on

cycling/shutdown/startup and layup.

Volatility of Salts in Steam Cycles

Historically the “ray diagram” has provided a rough estimate for determiningvaporous carryover from the boiler water. But it was confirmed from plantmonitoring

(2, 3) that chloride and sulfate concentrations can be as much as two orders of

magnitude higher in the steam than shown in the ray diagram.

To develop a more thorough understanding of the volatility of salts in steam cycles, the

EPRI research in this area began by investigating the partitioning of ammoniumchloride (NH4Cl) in laboratory-scale experiments from 120°C (248°F) to 350°C (662°F)

(5).

This research revealed that while the dominant chloride species for NH4Cl solutions in

both high and low temperature liquid were NH4

+and Cl-, the species transported to the

equilibrated vapor were predominantly HCl and NH3. An approximately similarpicture has emerged from the continued research on the partitioning from sulfatesolutions in the presence of sodium and ammonium cations, although the hydrolysis




Introduction

1-3

reactions of sulfate ion complicate the speciation. The significant species in solution atlow temperature (condensate, blowdown) under AVT conditions are ammonium ions,ammonia, and hydroxide ions, with impurities of “sulfur” being present in the form of sulfate ions. At boiler operating conditions, equilibrium thermodynamics dictate thatammonia predominates over ammonium ion, whereas bisulfate and sulfate ions are at

much lower, but similar, concentrations. In the high temperature steam phase atequilibrium with this solution, again ammonia predominates over HCl. At lower, butcomparable, concentration levels are ammonium chloride, sodium hydroxide, sulfuricacid, sodium bisulfate and ammonium bisulfate, depending on the relative levels of these impurities in the boiler water. These preliminary calculations predict furtherrearrangement of the relative concentrations of the predominant molecules as the steamcools, with a much larger range in values. Clearly the situation is more complex withthe addition of more potentially-volatile species, particularly those which undergoadditional reactions in the liquid phase, and this complex chemistry goes far beyondthat which can be predicted from the ray diagram. The partitioning constants for

typical fossil plant salts, acids and bases are shown in Figure 1-1, where thepartitioning constant, KD, can be defined for a simple 1:1 electrolyte as the ratio of the

concentration of the neutral molecule in the vapor phase to the activities of thecomponent ions in the liquid phase.






Introduction

1-5

very difficult. Also, an increasing number of utilities experienced serious internalcorrosion (attributed to acid phosphate corrosion) of the boiler waterwalls andsubsequent boiler tube failures when using this phosphate chemistry

(6, 7). The sodium

iron phosphate compound, maricite, has been found to be a magnetite-phosphatereaction product associated with cases of serious corrosion, and a distinguishing

difference from caustic gouging.

An EPRI project(8, 9)

was initiated to answer questions related to boiler tube corrosionand phosphate “hide-out” that have occurred in some boilers operating under CPT andto assist in modifying the ICG. This work generally extended the results of the 1964-68ASME Test Program

(10). The results are in general agreement with the literature

published on this subject. Specifically, no evidence of major corrosion attack was foundusing phosphate based boiler water treatment under conditions of:

Saturation pressure of 2800 psig (19.3 Mpa) and heat flux up to 200,000 BTU/hr ft2

(630kW/m2)

Departure from nucleate boiling (DNB) 1–2 hours in duration

Phosphate concentration to 10 ppm

Sodium to phosphate molar ratios ranging from 1.8–4.0

Magnetite deposition of 4 mg/cm2 (~4 grams/ft

2)

Low chloride and silica contamination

The results provide support for treatment methods which permit low levels (generally<1 ppm) of free caustic, such as equilibrium phosphate treatment under the tubecleanliness conditions tested.

Work conducted by the Canadian Electrical Association(11)

identified the sodium-ironphosphate reactions that take place up to 360°C. The major iron reaction products thatcause hideout (or more specifically in these experiments, “uptake by magnetite” at

Na/PO4 molar ratios near 2.5) were identified from batch experiments as NaFe++PO

4

(maricite) and Na4Fe+++(OH)(PO4)2 ·1/3NaOH. At higher Na/PO4 ratios Na3-2xFex

++

PO4 (a solid solution with Na3PO4) replaces maricite as the stable reaction product. At

360°C (680°F) “uptake by magnetite” behavior is similar except that there appears to beno significant amount of iron (+2) reaction products with Na/PO4 ratios of 2.5 orgreater. If the Na/PO4 ratio is large (>3.5), no “uptake by magnetite” takes place.

Nickel (NiO) reportedly behaves similarly. The Na/PO4 ratio in boiler water requiredto avoid the formation of more acidic phosphate mixtures (maricite + iron III phases)increases from about 2.3 at 315°C (599°F) to about 2.7 at 360°C (680°F). The injection of




Introduction

1-6

solutions with Na/PO4 ratios above 3.0 causes little or no iron-containing phosphatedeposit to form at 360°C (680°F).

EPRI published a revised Guideline(12)

for phosphate treatment for drum units whichtook into account the results of all these studies and relevant utility experiences. This

has been accomplished by providing two phosphate treatments (see Figure 5-2): thefirst, called phosphate treatment (PT), involves a broadening of the control range abovethe sodium-to-phosphate 2.8 molar ratio curve and allows operation with up to 1 ppmof free sodium hydroxide; the second, equilibrium phosphate treatment (EPT), operatesat or below phosphate levels which would lead to hideout. In high performance unitswith low tolerance for phosphate, operation with up to 1 ppm of free hydroxide isallowed. The major philosophy change incorporated has been to try to minimize oreliminate phosphate hideout and the continual correction of the boiler chemistry byaddition of the acid phosphate chemicals (di, and mono-sodium phosphate). PT isessentially an extension of EPT at higher phosphate levels. From a control viewpoint,the major difference is in the level of allowed contaminants, which must be consistentwith the buffering capacity of the treatment in use. Since the guideline was introducedin 1994, the incidence of corrosion has decreased markedly and utilities are able tocontrol the phosphate chemistry with minimum or reduced levels of hideout.Operation with these new phosphate treatments allows cycling of the unit withinchemical control boundaries.

Deposition and Chemical Cleaning

Deposition has a very important influence on waterside failure mechanisms andcomponent performance. The deposition of feedwater corrosion products, and

particularly their minimization, on the waterwalls of the boiler is key to a successful boiler treatment program. EPRI has recently initiated a strategic project to developquantitative understanding of deposition processes throughout the steam and watercycle. In the interim there are a number of published documents, which relate to theoperation of an optimum cycle chemistry program

(12–15), to the minimization of

deposition(16, 17) and to the determination of the need to chemically clean a boiler (18).

Steam, Chemistry and Corrosion in the Phase Transition Zone (PTZ)

Recently EPRI published a State-of-Knowledge document in this area(16)

which included

information on steam chemistry, moisture nucleation, early condensate and depositionwithin the phase transition area of the steam turbine. This work led to the formation of an international collaboration consisting of 23 organizations that are performingdetailed monitoring of these areas in operating turbines, and of extensive modelturbine studies of the PTZ. It is anticipated that the work will lead to a completeunderstanding of the environment in the PTZ, which will ultimately provide better




Introduction

1-7

steam chemistry limits. It has already led to a better understanding of the importanceof providing a suitable shutdown environment to the low pressure turbine.

1.2 EPRI FOSSIL PLANT GUIDELINES AND MANAGEMENT APPROACHES

FOR CYCLE CHEMISTRY

Over the period 1993–1996, EPRI has incorporated the information from all the on-going cycle chemistry projects into individual guidelines which are revisions of theInterim Consensus Guidelines:

Phosphate treatment guidelines to cover phosphate treatment (PT) and equilibriumphosphate treatment (EPT) for drum units

(12)

Oxygenated treatment for once-through and drum units(14)

All-volatile treatment for once-through and drum units(15)

EPRI has also prepared a document(19)

which summarizes the worldwide experiencewith caustic treatment for drum boilers. This treatment is currently utilizedsuccessfully in over 50,000 MW of drum boilers at applied concentrations up to 2 ppmNaOH.

The “Selection and Optimization of Boiler Water and Feedwater” was published in1997

(21). This document is the “glue” which brings the four guidelines together; it helps

a utility to select the optimum treatment for specific units and provides a “road-map”methodology to optimize the feedwater for all-ferrous and mixed metallurgy feedwater

systems.

In parallel to the guidelines development, EPRI has developed a very successful CycleChemistry Improvement Program

(13, 20) and has demonstrated it with nine utilities. This

includes the minimum level of instrumentation that all fossil plants are considered toneed (core parameters) and which was developed as a result of the monitoringprogram

(2) and international data

(4). Table 1-1 shows these parameters/instrumentation

together with the diagnostic parameters, which should be used (a) in cases of contaminant ingress or when target levels are exceeded (troubleshooting parameters),and (b) during commissioning of cycle chemistry.

Also included in the CCIP is the optimum management approach for a utility’s cyclechemistry program, and the methodology to record the costs/benefits of an improvingcycle chemistry.




Introduction

1-8

Table 1-1“Core” Monitoring Parameters (Minimum level of instruments for all plants/units)

Parameters Measurement Locations Usage. On-Line/ Grab

FrequencyMeasurement

Cation Conductivity

CP Discharge O C

Cation Conductivity

Polisher Outlet and Economizer Inlet

O C

Cation

Conductivity2

Blowdown or Downcomer O C

Cation Conductivity

Hot Reheat Steam or Main Steam

O C

Dissolved Oxygen

CP Discharge O C

Economizer Inlet O C

pH (Drum Boilers) Blowdown or Downcomer O CSodium

CP Discharge O C

Sodium

Polisher Outlet or Economizer Inlet

O C

Sodium

Hot Reheat Steam or Main Steam

O C

Additional Monitoring or Diagnostic Parameters

Parameters Measurement Locations Usage. On-Line/

Grab

Frequency

MeasurementpH

Economizer Inlet

C

SpecificConductivity

Economizer Inlet

Treated Makeup

CC

Silica

Treated Makeup

C

Phosphate1

Blowdown or Downcomer

or G C or S

Chloride

Blowdown or Downcomer

or G C or D

Iron

Economizer Inlet G W

Copper

Economizer Inlet G W

Total OrganicCarbon

Condensate Pump Discharge G W

Air In-leakage Air Removal System or G C or DORP

Economizer inlet and feedwater O or G W

1 Drum Boilers on Phosphate Treatments2 Drum Boilers on AVT and OT O - On-LineG - Grab

C - Continuous or Semi-ContinuousS - Grab, Once/ShiftD - Grab, Once/DayW - Grab, Once/Week




Introduction

1-9

1.3 NEED AND DEVELOPMENT FOR

CYCLING/SHUTDOWN/STARTUP/LAYUP GUIDELINES

As can be seen from the previous discussion, there is now comprehensive coverage of

guideline limits for all operating chemistries and unit configurations. There is amethodology to select and optimize the treatment for each specific unit. All thenecessary management approaches to run a successful cycle chemistry program are alsoavailable.

Despite this, only rudimentary information on the effects of cycling, shutdown, startupand layup on cycle chemistry and unit availability/reliability has been available. Nocomprehensive guidelines exist. This has been well identified by the industry, but untilnow the necessary information and underpinnings for such a guideline for chemistsand operators has not been available.

It is clear that improper shutdown and layup can lead to serious plant damage such aspitting, oxidation and corrosion, which during operation can be the initiators of theserious major failure mechanisms affecting plant availability. Two such importantmechanisms are: a) pitting in unprotected reheaters which can lead to multiple reheaterleaks, and b) pits on low pressure turbine blade/disk surfaces in the PTZ, which can beprecursors to stress corrosion cracking and corrosion fatigue.

A recent survey(22)

, conducted of over 60 utilities of their chemistry practices, providessome important clues as to the extent of protection (or lack of it) applied to units duringshutdown. Some of the results include:

13% nitrogen blanket heaters.

35% nitrogen blanket boiler.

Very low % protect turbine during SD.

49% don’t know the Fe and Cu levels on SU.

46% don’t use deaerated water on SU.

40% put makeup directly into cycle.

66% put makeup into vented storage tank.

Hydrazine is most common scavenger (66%).

28% don’t use scavenger during layup (this is normal and OK for OT units).




Introduction

1-10

Short term shutdown is biggest concern.

Damage due to cycling: BTF, condenser leaks, and LP blade problems

58% think guideline is high priority.

38% think it is urgently needed.

It is also well understood that the cycle chemistry can often be outside of guidelinelimits during the startup period, and that the level of corrosion products, flowing fromthe feedwater system and within the boiler, are highest during this period. Poorshutdown and layup only exacerbate these problems. The choice of chemistry for aunit is important, not only when the unit is operating, but also during the transientconditions of shutdown and startup. For instance: the choice of OT over AVT for drumand once-through units reduced markedly the level of corrosion products duringstartup; the choice of EPT over CPT should remove the possibility of hideout andhideout return during startup and shutdown respectively.

There has been much discussion about whether phosphate or caustic should be addedto a drum boiler during the startup period, either as an automatic addition or inresponse to a contaminant.

These new guidelines assembled in this document address all these concerns.

1.4 OBJECTIVES OF THESE GUIDELINES

The overall objective of these guidelines is to minimize the impact of cycling and

peaking operation, and to achieve availability and efficiency similar to the base loadedunit. Specific objectives are to:

Maximize unit life and minimize forced outages

Maximize power production and thermodynamic efficiency

Minimize scale formation and corrosion, and transport

Minimize impurity ingress, generation, and transport

Maximum unit life and low forced outage rate are achieved by a combination of controlof operating stresses, and chemistry of water and steam. High operating stresses aremostly caused by temperature transients in heavy section components such as boilerdrums, superheater and reheater headers, and turbine rotors and casings. Highstresses are also produced in boiler waterwall tubes by heating, cooling, and circulationproblems. The above conditions can lead to low cycle fatigue, low cycle corrosionfatigue, and stress corrosion cracking.




Introduction

1-11

In marginally designed highly stressed components, such as for example LP turbine blade attachments, the stress cycling produced by frequent shutdowns can also reducecorrosion fatigue life.

Corrosion caused by ingress of cooling water impurities and oxygen and carbon

dioxide which enter through air inleakage, reduces the useful life of all waterside andsteamside component surfaces.

High power production and efficiency is achieved by minimization of impurity ingresswhich leads to generation and transport of iron and copper oxides. These oxides formscale in boiler and feedwater heater tubes, reducing heat transfer. In the turbine, metaloxides and salts form deposits which reduce the flow capacity and, through surfaceroughness, reduce thermodynamic efficiency.

1.5 SCOPE OF THESE GUIDELINES

These guidelines cover the following steam cycles, water treatments, types of operation,and pressure control:

Drum boiler units above 600 psi (4Mpa) drum pressure and once-throughsubcritical and supercritical boiler units

Units with and without condensate polishers

All-ferrous and mixed metallurgy feedwater systems

Superheaters, reheaters and turbines

Water treatments: Phosphate treatment and equilibrium phosphate treatment, all-volatile treatment, oxygenated treatment and caustic treatment

Base load, peaking, and cycling operation

Constant and sliding pressure

1.6 REFERENCES

1. Interim Consensus Guidelines on Fossil Plant Cycle Chemistry. Electric Power ResearchInstitute, Palo Alto, Calif. CS-4629. June 1986.

2. Monitoring Cycle Water Chemistry in Fossil Plants: Volume 1, Monitoring Results.Electric Power Research Institute, Palo Alto, Calif. EPRI GS-7556, Vol. October 1991.




Introduction

1-12

3. Monitoring Cycle Water Chemistry in Fossil Plants: Volume 3, Project Conclusions andRecommendations. Electric Power Research Institute, Palo Alto, Calif. EPRI GS-7556,Vol. 3, October 1991.

4. Monitoring Cycle Water Chemistry in Fossil Plants: Volume 2, International Water

Treatment Practices in Fossil Fuel Units. Electric Power Research Institute, Palo Alto,Calif. EPRI GS-7556, Vol. 2. December 1992.

5. Behavior of Ammonium Salts in Steam Cycles. Electric Power Research Institute, PaloAlto, Calif. EPRI TR-102377. Final Report. December 1993.

6. R.B. Dooley and S. Paterson, “Phosphate Treatment: Boiler Tube Failures Lead toOptimum Treatment”. Proceedings: 55th International Water Conference. EngineersSociety of Western Pennsylvania. 1994.

7. R.B. Dooley and W.P. McNaughton, Boiler Tube Failures: Theory and Practice. EPRI

Book TR-105261. 1996.

8. S.L. Goodstine and R.B. Dooley, “Behavior of Sodium Phosphates Under BoilerConditions”. Proceedings: 54th International Water Conference. Engineers Society of Western Pennsylvania. 1993.

9. Behavior of Sodium Phosphate Under Boiler Conditions. Electric Power ResearchInstitute, Palo Alto, Calif. TR-102431. June 1994.

10. P. Goldstein and C.L. Burton, “A Research Study on Internal Corrosion of HighPressure Boilers Final Report”. Transactions of the ASME, Journal of Engineering for

Power. April 1969.

11. P.R. Tremaine, L.G.S. Gray, B. Wiwchar, P. Taylor and J. Stodola, “PhosphateInteractions with Metal Oxides Under High Performance Boiler HideoutConditions”. Proceedings: 54th International Water Conference. Engineers Society of Western Pennsylvania. 1993.

12. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Treatment for Drum Units.Electric Power Research Institute, Palo Alto, Calif. EPRI TR-103665. Final Report.December 1994.

13. Cycle Chemistry Corrosion and Deposition: Correction, Prevention and Control. ElectricPower Research Institute, Palo Alto, Calif. TR-103038. Final Report. December1993.

14. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. Electric PowerResearch Institute, Palo Alto, Calif. EPRI TR-102285. December 1994.




Introduction

1-13

15. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. Electric PowerResearch Institute, Palo Alto, Calif. EPRI TR-105041. April 1996.

16. Turbine Steam, Chemistry, and Corrosion. Electric Power Research Institute, Palo Alto,Calif. EPRI TR-103738. August 1994.

17. R.B. Dooley, J. Mathews, R. Pate and J. Taylor, “Optimum Chemistry for ‘All-Ferrous’ Feedwater Systems: Why Use an Oxygen Scavenger?”. Proceedings: 55thInternational Water Conference. Engineers Society of Western Pennsylvania. 1994.

18. Guidelines for Chemical Cleaning of Fossil-Fueled Steam Generating Equipment. ElectricPower Research Institute, Palo Alto, Calif. TR-102401. Final Report. June 1993.

19. Sodium Hydroxide for Conditioning the Boiler Water of Drum-Type Boilers. ElectricPower Research Institute, Palo Alto, Calif. TR-104007. January 1995.

20. Cycle Chemistry Improvement Program. Electric Power Research Institute, Palo Alto,Calif. TR-106371. April 1997.

21. Selection and Optimization of Boiler Water and Feedwater Treatments for Fossil Plants.Electric Power Research Institute, Palo Alto, Calif. TR-105040. March 1997.

22. Proceedings: Fifth International Conference on Fossil Plant Cycle Chemistry. Edited byR. B. Dooley and J. Mathews. Electric Power Research Institute, Palo Alto, Calif.TR-108459. November 1997.






2-1

2

METALLURGICAL, DESIGN, AND OPERATING

CONSIDERATIONS

2.1 INTRODUCTION

In this Section, general and specific considerations relating water and steam chemistryand steam cycle design and operation to corrosion and deposit formation are discussed.Water chemistry, metallurgical, design, and operating aspects are combined becausethey all strongly interact, particularly in cycling units.

Operator and chemist actions influence the generation, cycle transport, and removal of corrosion products and ingress of impurities. Designers, operators, and chemistsinfluence the impurity concentration on heat transfer and turbine surfaces. Control of stresses and the number of stress cycles which interact with the chemistry in producingequipment damage (corrosion fatigue, stress corrosion, etc.) is by operation and design.

In Section 2.1 - Introduction, the impurity generation, transport, and corrosion effects,

and steam cycle materials are described. The basics of material corrosion and cyclecomponent mechanisms are given in Section 2.2, differences between the all-ferrous vs.copper-containing preboiler systems are outlined in Section 2.3, priorities for transientoperation in Section 2.4, and the effects of steam cycle component design and operationare discussed in Section 2.5. A brief assessment of the alternative water treatmentchemicals is presented in Section 2.6.

The steam cycle startup and cycling sequence may cover all or some of the followingmain steps:

shutdown

short or longterm layup

system draining and filling

water cleanup before firing




Metallurgical, Design, and Operating Considerations

2-2

boiler firing and warmup

steam turbine bypass operation

turbine roll and warmup

turbine speed ramp with holds

synchronization

load ramping

All the above steps involve water chemistry considerations and control limits andequipment considerations relating to thermal stresses, resonant speeds for rotatingmachinery, deaeration, water hammer and water induction, rubbing and cavitation of pumps, and other concerns. Schematically, the philosophy of the three legs of a

milking stool supporting reliable operation applies (see Figure 2-1) even more totransient operation than to base load.

Cycling operation

C o n

t r o l

o f

s t r e

s s

F a i l u r e r e s i s t a n t m a t e r i a l s

C o n t r o l o f w a t e r a n d

s t e a m c

h e m i s t r y

Figure 2-1 Three supports for reliable cycling operation

While these guidelines concentrate on water and steam chemistry, the main emphasisfor operators of the startup and cycling operation is on the control of thermal andvibratory stresses which can interact with the environment and result in corrosioncracking (corrosion fatigue and stress corrosion).





2-3

Impurity Generation, Transport, and Corrosion Effects

To minimize impurity ingress, generation, and transport, the sources of impurities,effects of layup, and the startup, shutdown, and operational chemical transportcharacteristics of each cycle should be periodically determined during commissioning

and thereafter about every five years(1-6)

. Important characteristics which need to berecognized include:

oxidation of cycle materials and deposited copper during layup

precipitation of suspended solids in feedwater and boiler water due to changingpH, O2, redox potential, and temperature during layup and startup

introduction of aerated (O2 + CO2) makeup from storage tanks during system filland from the deaerator storage tank during startup

deaeration in the condenser and deaerator

boiler carryover and drum level control

transport of exfoliated oxides into the turbine and around the cycle

transport of metal oxides from dead legs and mud drums

sloughage of impurities from condensate polisher resins

Typical water chemistry and corrosion effects of layup, startup, and cycling are

illustrated for a drum boiler cycle in Figure 2-2 and Table 2-1. Figure 2-3 is a summaryof the sources of contaminants, most of them active during startup and cycling. Inparticular: air inleakage, corrosion and exfoliation products, condenser leaks, aeratedmakeup water from storage tanks, condensate polishers, and, sometimes, thecombustion products entering leaking reheater tubes (and via this route to the IP andLP turbines) during initial firing when the reheater is under vacuum. Figure 2-4 showsthe steam cycle components and thermodynamic conditions where various impuritiescan cause corrosion. These regions change with load and during shutdown andstartup.





2-4

Condensatepolisher

Deaerator-good deaerationat all loads

Fe foulingand rapidexhaustionby CO2

HP heaters

HP turbine – System

filled with

aeratedwater

– pH andredoxchangeslead todissolution,precipitation,anddepositionof oxides

LP heaters

NH3 + O2 + CO2- Poor deaeration- Corrosion of

Cu alloys

Attemperation

High Fe, Cu intoboiler duringstartup

– Cu alloy and C-Steel oxidation during layup – Oxide transport and deposition during startup

Impurity ingressCorrosion

Deposition

Turbine: – Pitting during layup – Washing of

deposits

high carry-over due todrum level control andsuspended solids

Boiler

Makeup

Condenser

IPturbine

LPturbine

Figure 2-2 Typical water chemistry and corrosion effects of layup, startup andcycling for a drum boiler cycle





2-5

RegenerationDesignResin testing

monitoring/ operation

Resin testingMonitoringNitrogen sparging of storage tanks

Selection of chemicalsControl of cleaning

Material selectionAvoid copper

Water chemistryDesignExfoliationMaintenance

Design

Reheatermaintenance

QC: MaintenancePurchasing

Design, MaterialsPreventive maintenanceMonitoring

Selection of treatmentQC-Purchasing

Waterand

steam

*

*

* *

* **

*

*

C

o n d e n s e r l e a k s

P a i n t s , s o l v e n t s ,p r e s e r v a t i v e s , e t c .

C o m b

u s t i o n

p r o d u

c t s

A i r i n

l e a k

a g e C

o r r o

s i o n p r o d u c t s

C h e m i c a l c l e a n i n g

Ma k e u p wa t e r

C o n

d e n s

a t e p o l i s h e r s

W a t e r t r e a t m

e n t c h e m i c a l s

Figure 2-3 Sources of contaminants enhanced by cycling operation and examplesof engineering solutions.





2-6

Caustic stresscorrosion cracking

Boiling and highheat flux zones

IP turbine

LP turbine S

u p e r h e a t e

r

R e h

e a t e

r

Salt zone

3 0 % NaCl s o l u t i o n s

S a t u ra

tion li n e C o r r o s i o n - e r o

s i o n l o w p H , h i g h v e l o c i t y )

(Cau s e s : C O 2 , a c i d s ,

W a t e r d r o p l e t e r o s i o n

C o p p

e r c o r r o

s i o n

( c a u

s e s N

H 4 + O 2 )

G e n

e r a l

c o r

r o s i o n

o f c a r b o

n s t e e l

70%

50%

20%

C o n d e n

s e r

Enthalpy

Entropy

Superheat

T

P

20% Moisture

4%

6%

8%

12%

10%

Pitting, stress corrosioncracking, corrosion fatigue(Causes: Cl, SO4, CO3 O2, CuO, Acetate,...)

N a O H s o l u t i o n s

B o i l e

r

H P

t u r b i n

e

E x t r a

c t i o

n s t o

f e e d w

a t e r

h e a t e

r s

Note: This diagram illustrates regions where impurities will concentrate and promote corrosion. Points in the diagramshould relate to actual conditions at component surfaces, not to the theoretical average flow path conditions. Heattransfer, surface cleanliness, crevices, and surface-flow stagnation conditions determine the actual surfacetemperatures and pressures.

Figure 2-4 Mollier diagram for a fossil cycle



EPRI Licensed MaterialMetallurgical, Design, and Operating Considerations

2-7

TransientEffectsandTheirAmelioration

Transient Effects How to Reduce the Impact

Unprotected cycle components corrodeduring layup. Layup generated oxides aretransported during the startup sequencearound the cycle (see Figure 2-2).

Proper layup, startup cleanup,condensate polishing or filtration,drain and fill, washing condenserwith turbine hood spray.

If not removed, these oxides deposit in thefeedwater heaters and the boiler and somecan be carried with attemperating spraysinto the superheater and reheater and theturbine.

Boiler blowdown, feedwater cleanup(may need to retrofit cleanup loops,turbine bypass).

pH, oxygen, and temperature changes causedissolution and precipitation of metal oxideson boiler tube and other surfaces (see Figures2-6 to 2-9).

Maintain proper boiler water andfeedwater pH, fill with deaeratedwater.

The presence of salt deposits in the turbineand humid air cause pitting corrosion.

Turbine layup with dehumidified airor nitrogen, turbine washing.

An increase of air inleakage during low load

operation introduces oxygen and carbondioxide, which can lead to general andpitting corrosion; carbonic acid can influenceflow accelerated corrosion.

Fix air inleakage.

The efficiency of deaeration in the condenseris poor during low load operation.

Improve condenser deaeration bysteam sparging, additional airejectors or vacuum pumps.

Carbonate generated from the CO2 whichentered the cycle during startup and low load

operation, and the aerated makeup from thedeaerator storage tank and boiler fill act as aneluent, replacing the already exchangedanions on the condensate polisher resin.

Fix air inleakage, fill with deaeratedwater (pegging steam to deaerator,

nitrogen blanketing and sparging of condensate storage tank)





2-8

High transient stresses in heavy sectioncomponents resulting from temperaturetransients, stresses in boiler water wall tubesresulting from irregular circulation, andvibration resonance and increased alternatingstresses in the rotating equipment interactwith marginal water and steam chemistryproducing accelerated stress corrosion andcorrosion fatigue crack propagation.

Follow vendor instructions forstartup and cycling, retrofit cyclingdesigns and controls, maintain goodwater and steam chemistry.

During shutdown, exfoliation of superheater,reheater, and steam pipe oxides occurs,leading to solid particle erosion in the HP

and IP turbine during the subsequent startup,and transport and deposition of magnetitearound the cycle.

Minimize the rate of metaltemperature changes, removeexfoliated oxides through main

steam drains, use condensatepolishers and/or filters.

Superheater and reheater corrosion due towetting of previously deposited salts.

Optimum is to use dehumidified airfor prolonged off-load periods.However, wet protected storage canalso be used (see Figure 4-8)

Hideout and hideout return of many

chemical impurities and additives occursduring transients.

Optimize phosphate control, keep

boiler, superheater, and reheaterclean, use condensate polishers.

Steam Cycle Materials and Their Properties

In fossil utility cycles, the steam cycle component materials which need to be protected by the chemical treatment and during layup, include carbon steels, austenitic andferritic stainless steels, and copper alloys. These common materials can be subject to

corrosion damage during operation and layup. They can be attacked by theconcentrated impurities formed in steam cycles, by hot water and steam, and by acombination of humid air and corrosive deposits during layup.

Typical materials for the key cycle components are:

Boiler: carbon and low alloy steels





2-9

Superheater and reheater: carbon, low alloy and austenitic stainless steels

Steam piping: low alloy or carbon steel

Turbine: low alloy steels for rotors and HP and IP cylinders, ferritic stainless

steels and titanium alloys for blades, austenitic stainless steels for stationary blades, super alloys for high temperature bolting

Feedwater heaters and condensers: carbon steel for shells and feedwater heatertubing, austenitic and ferritic stainless steels, titanium, and copper alloys forfeedwater heater and condenser tubing

Feedwater piping: carbon steel, low alloy and stainless steels for sectionssusceptible to FAC

Material Properties

For the evaluation of the effects of cycling on steam cycle components, mechanical,fracture, fatigue, creep, and corrosion properties need to be known

(6-26). These

properties must cover behavior of smooth and notched surfaces, materials with andwithout defects, and crack propagation behavior under fatigue and corrosion crackingconditions.

New materials have been developed for replacement of the components, the life of which has been exhausted and for the retrofits for cycling service. These have betterfracture, fatigue, and creep properties.

2.2 STEAM CYCLE COMPONENT CORROSION AND DEPOSITS

During cycling and other transient operation, the corrosion situation in steam cycles isaggravated by increased steady and vibratory stresses (corrosion cracking) and often bymarginal water and steam chemistry, i.e., ingress of air, system filling with aeratedwater, transport of corrosion products, washing of deposited impurities, chemicalhideout, fast exhaustion of condensate polishers, condenser leaks (particularly duringstartups), etc.

Basics of Material Corrosion

The cycle materials can be subject to many corrosion mechanisms including:

general corrosion, and high temperature steam oxidation

galvanic corrosion





2-10

stress corrosion

low cycle and high cycle corrosion fatigue

crevice corrosion

pitting

flow-accelerated corrosion (erosion-corrosion)

exfoliation

fretting

Where in the steam cycle these mechanisms are active depends on the thermodynamicand flow conditions, water and steam chemistry (see Figure 2-4), materials, and

stresses. Corrosion data can be found in hundreds of technical papers, EPRI reports,and book compilations(15, 23-26)

.

General Corrosion - is the most important for: a) carbon steels which are used for boilertubes, feedwater piping, pressure vessel shells, turbine casings, condenser shell, andfeedwater heater tubing, and b) for copper alloys, which are used for condenser,feedwater heater, and auxiliary heat exchanger tubing. General corrosion is affected bythe chemistry at the metal surface; most significantly by oxidizing-reducing potential(redox), pH at temperature, oxygen, and concentrated salts, acids, and hydroxides. It isalso exponentially dependent on temperature. Typical relationships for corrosion of carbon steel are given in Figures 2-5 and 2-6. Selected corrosion data for copper alloys

are shown in Figures 2-7 to 2-9. For an evaluation of the effects of the corrosiveenvironment, the conditions at the surface, including impurity concentration, corrosionpotential, and temperature, must be considered (Figure 2-4).





2-11

0.5

0

-0.5

-1.0

Potential (Driving Force for Corrosion) (volts)

42 6 8 10

Acidcorrosion

Immunity

12 14

Passivation

Normal boileroperating range

Alkalinecorrosion

Stable Species at 300°C

Metallic iron

Hematite (Fe2O3)Magnetite (Fe3O4)

Acid Neutral Alkaline

pH

Figure 2-5 Potential - pH diagram for carbon steel in 300 °C water(19)





2-12

Corrosion

101

100

10-1

10-2

10-3

10-4

Neutral

Solubility, mmol/kg: Corrosion Rate, mm/year

Conc. HCl, mol/kg Conc. NaOH, mol/kg

pH 300°C

Linear

2 4 6 8 10 12

100 10-2 10-4 10-4 10-2 100

Linear

Fe(OH)2

Fe(OH)2-4

Fe2+

CubicParabolic

Fe(OH)+

Fe(OH)-3

Magnetitesolubility

Figure 2-6 Corrosion of mild steel and solubility of magnetite at 300°C, showingcorrosion rate laws

(15,20.21)





2-13

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

pH

2 4 6 8 10 16

CuO

CuO

Cu

Cu(NH3)4++

Cu(NH3)2+

Cu++

Cu2O

Cu2O

0 1412

Potential, V(H)

Figure 2-7 Potential - pH diagram for copper in ammonia solutions at 25°C

(22)





2-14

8.5 9.0 9.5

pH at 25°C

8

6

0

4

2

Copper Release (ppb )

14

12

10

20

18

16

26

24

22

30

28

CDA-706. (Cu/Ni: 90/10),

274°F, hydrazine added

CDA-706, 375°F,

hydrazine added

SoMs 71 brass, 86°F,

alkalized with NH3

CDA-443 (Admiralty brass),

193°F, hydrazine added

Aluminum brass,

temperature unknown

Figure 2-8 Average copper release as a function of pH

Adapted from Corrosion-Product Transport in PWR Secondary Systems, EPRI NP-2149,December 1981, and Effects of Hydrazine and pH on the Corrosion of Copper-Alloy Materialsin AVT Environments with Oxygen, EPRI NP-2654, December 1982.





2-15

50.0

20.0

10.0

5.0

2.0

1.0

0.5

0.2

pH98 10

70Cu/30Ni

0

Corrosion Product Release Rate, g/m2 • yr

90Cu/10Ni

Figure 2-9 Effect of pH on steady state release rates for 90Cu/10Ni and 70Cu/30Niexposed to ammonia solutions containing 8-21 µg/kg oxygen, flowing at ~1ft/s(0.3m/s) and at a temperature of 35° - 38°C

(22)

Corrosion Fatigue - depends on stress or strain amplitude, mean (or average) stress,

and the environment, including temperature

(15,18,24)

. Pitting can significantly reduce thetime for crack initiation and failure. An example of the effect of environment oncorrosion fatigue is shown in Figure 2-10 which is a corrosion fatigue diagram for LPturbine disk and rotor steel

(18).

The 45° line represents the mean stress line or no alternating stress. The farther the datafalls away from the mean stress line, the higher the alternating stress that can bewithstood for a given mean stress and stay below the fatigue strength of the material.Agressive environments (increasing NaCl) represented by the lines below the 45° line,and high mean stresses lead to marked acceleration of corrosion fatigue cracks.

Corrosion fatigue of carbon steel tubing used for boiler waterwalls has been extensivelyinvestigated

(25) and strong effects of off-limit boiler water chemistry (pH, oxygen) on

crack initiation and propagation were found.





2-16

600

500

400

300

200

100

0

100

-200

-300

Mean Stress, σm, (MPa)

0 100 200 300 400 600

A

i r

Alternating Stress, σm6σa, (MPa)

500

S e r v i c e

c o n d i t i o

n s

Figure 2-10 Corrosion fatigue diagram for NiCrMoVLP turbine disk and rotor steel

18

Stress Corrosion Cracking (SCC) - is a corrosion damage mechanism resulting from acombination of high tensile stress and environment. The tensile stress can be anoperating stress or a residual stress, such as residual welding stress. SCC is alsostrongly temperature dependent. Sodium hydroxide is a common steam cycle impuritywhich has the strongest effect on stress corrosion, however, other impurities such asacids and salts can also induce SCC. For higher strength materials, such as the LPturbine disk low alloy steels, even pure water and wet steam can cause cracking. Anexample of stress corrosion behavior is shown in Figure 2-11, which shows the

dependence of the threshold stress SCC, threshold stress intensity KISCC, and stress





2-17

corrosion crack propagation rate (da/dt) on yield strength for an LP turbine diskmaterial

(18).

600

500

400

300

200

Yield Strength (0.2%), MPa

400 500 600 700 800 1000

d a / d t , m m / s

900

σ S C C ,

( M P a )

K I S C C M P a • m 1 / 2

10-5

10-6

10-7

10-8

10-9

σSCC

da/dtKISCC

1100 1200

100

80

60

40

20

Figure 2-11 Stress corrosion of NiCrMoV disk steel vs. yield strength for "good"water and steam

(18)

Cycle Component Damage Mechanisms

Startup and cycling operation often lead to acceleration of corrosion and other damagemechanisms of steam cycle components. This is because there are increased thermaland vibratory stresses at the same time as when water and steam chemistry aremarginal, and because the corrosion products generated during layup are transportedand deposited around the cycle.

Major damage mechanisms influenced by cycle chemistry and the effects of startup andcycling are listed in Tables 2-2 to 2-4, expanded from reference 27.





2-18

Table 2-2Boiler Tube Damage Mechanisms Influenced by Cycle Chemistry (Adapted fromreference 27) (Discussion of each mechanism can be found in Reference 25)

Mechanism Chemistry Influence Effects of Cycling

Corrosion fatigue Poor water chemistry, shutdown orlayup practices, and improper chemicalcleaning (with HCl) worsen thecontribution of the damage-causingenvironment

Marginal chemistry,circulationproblems in natl.circ. boilers, thermalstresses

Hydrogen damage Excessive deposits from feedwatercorrosion products combined withcontamination by acids or salts

Marginal chemistry,deposition of oxides

Caustic gouging Excessive deposits from feedwatercorrosion products combined withcaustic contamination

Hideout, dirty tubes

Fireside corrosion Mechanism accelerated by increasedmetal temperatures resulting fromexcessive tube deposits

Tube deposits

Short- and long-term overheating

Increased tube metal temperaturesresulting from excessive deposits; orificeplugging by feedwater corrosionproducts prevents cooling

Dirty tubes,circulation

Flow-acceleratedcorrosion of economizer

Reducing conditions and low pHfeedwater

CO2+ low pH

Pitting ineconomizer

Stagnant, oxygenated water duringshutdown

Layup

Pitting in reheater Concentration of salts or H2SO4 at lower bends during layup; carryover or fluegas

More frequent salts+ condensed steam+ air





2-19

Table 2-3Turbine Deposits & Damage Mechanisms Influenced by Cycle Chemistry (Adapted fromreference 27) (Discussion of each mechanism can be found in Reference 65)

Mechanism Chemistry Influence Effects of Cycling

Stresscorrosioncracking of LPdisks, rotors,HP bolts, ...

Excessive corrodents present in steam,and drying of liquid films and moisturecombined with synergistic effects of tensile stress and materials, depositionof corrodents

Pitting duringunprotected layupaccelerates SCC,marginal startupchemistry

High cyclecorrosionfatigue of LP blades and

disks

Excessive corrodents present in steamcombined with cyclic stresses

Pitting duringunprotected layupaccelerates CF, rotatingmachinery through

critical speed - vibration

Low cyclecorrosionfatigue

Deposited corrodents, particularlyNaOH

Pitting duringunprotected layup, stressand strain cycling due tostartups and thermalstresses

Pitting Salts or acidic corrodents in steam,unprotected layup - create sites of SCCor corrosion fatigue

Deposited salts duringunprotected layup

HP bladedeposits

Copper in steam deposits on HP bladesand nozzles decreasing efficiency andMW output

Copper alloys anddeposited copper oxidizeduring unprotectedlayup, transport duringstartup

Silica depositson LP blades

Precipitation of silica in steam -excessive deposits lead to partialpluggage of seals, mis-operation of valves, deformation of blades anddiaphragms and efficiency losses

Marginal startupchemistry

Solid particleerosion - HP,IP

SH and RH tube exfoliation is notinfluenced by chemistry, but causes Feloading during startups

Exfoliation duringshutdown, transportduring startup





2-20

Table 2-4Condensate/Feedwater Cycle Damage Mechanisms Influenced by Startup CycleChemistry (Adapted from reference 27)

Location Mechanism Chemistry Influence

Condensateand feedwatersystems

Carbon steel andcopper alloycorrosion andcorrosion producttransport

Low pH, acid constituents, excess carbondioxide and oxygen present in condensate;alternating oxidizing and reducingconditions, excess hydrazine solubilizingmagnetite

Heater drainand feedwater

piping

Flow-acceleratedcorrosion (FAC) of

carbon steel

Attack by reducing feedwater conditionsand high velocities, excessive hydrazine

with "zero" oxygen, low pH

Copper alloycondenser andheater tubes

Ammoniaattack/condensatecorrosion

Simultaneous excess ammonia, oxygen, andCO2 in steam synergistically oxidizing andsolubilizing copper

Feedwaterheater tubes

Stress corrosioncracking Cu/Ni,Monel, StainlessSteel

Excessive corrodents in steam synergisticwith tensile stress, corrodent concentrationin crevices, dry - wet transition

Condenserand heatertubes

Admiralty Brassstress corrosioncracking

Excessive ammonia/chloride present insteam synergistic with residual stress attubesheets and in u-bends

Feedwaterheater tubes

Copper/nickelexfoliation

Excessive oxygen on shutdown combinedwith thermal cycling and thermal stresses





2-21

2.3 PREBOILER SYSTEMS - ALL FERROUS VS. MIXED METALLURGY

The concerns with the pre-boiler systems for cycling duty include:

reliability of components (see Table 2-4)

generation and transport of corrosion products(3-6,15,22)

deaeration (see Section 2.5)

The generation and transport of corrosion products (iron, copper, nickel, and zincoxides) can cause boiler waterwall tube scale accumulation which can lead to tubefailures and turbine deposits which can lead to MW and efficiency loss. Coppertransport and accumulation in the superheater can act as a continuing source of copperfor the turbine.

The high pressure drum boiler utility units with all-ferrous metallurgy (no copperalloys in feedwater heaters) usually have better efficiency and reliability than thesimilar units with mixed metallurgy. It has been concluded that copper alloys shouldnot be used for the heat exchanger tubing applications in steam cycles

(22). The main

water treatment dilemma is that there are different pH requirements for the control of general corrosion and flow-accelerated corrosion for carbon steel and copper alloys.Figure 2-12 shows that the feedwater pH for the best protection of carbon steel againstgeneral corrosion under deoxygenated reducing conditions should be above 9.4, but forthe protection of copper/zinc alloys, below 9.1.

Other than using an oxidizing cycle where the oxidizing-reducing potential, ORP >0mV the optimum protection of carbon steel against flow-accelerated corrosion (FAC) iswith pH>9.6 and oxygen concentration about 10 ppb. While to minimize coppercorrosion, excess hydrazine is required (ORP < 0mV), which may further accelerateFAC of carbon steel.

Auxiliary Heat Exchangers - such as the hydrogen cooler and gland steam condensershould be considered when deciding on feedwater control because they may containcopper alloys even while the feedwater system does not.

Layup - practices are very important for protection and corrosion product generationfor both ferrous and copper alloy materials (see Section 4).





2-22

12

10

8

6

4

2

C o p p e r a s p p b C u

I r o n a s p p b F e

Copper

Iron

pH (25°C)

8.5 8.7 8.9 9.1 9.3 9.79.5 9.9

12

10

8

6

4

2

Figure 2-12 The effect of pH on iron and copper concentration at the economizerinlet. Source: D. Frey, Mechanics of Corrosion Product Formation andTransport

(14)

All-Ferrous Feedwater Systems (3, 4)

The corrosion behavior of all-ferrous feedwater systems depends on the materials usedfor the condenser and feedwater heater tubing, feedwater chemistry, and local flowconditions. Where only carbon steels and ferritic stainless steels are used, slightlyelevated oxygen concentration levels can reduce iron oxide generation. However,where austenitic stainless steels are used, interaction of oxygen and chloride ions must be considered in controlling feedwater and steam chemistry, particularly duringtransients, because these materials are susceptible to pitting and stress corrosioncracking.

In units with condensate polishing, austenitic stainless steels can tolerate higher oxygenlevels because the chloride in feedwater can be better controlled than in the unitswithout condensate polishers.

In an attempt to reduce feedwater corrosion products, the older operating guidelines55

(including EPRI's Interim Consensus Guidelines(1)

) indicated that the feedwater oxygen





2-23

levels should be less than 5 ppb. Reduction of air inleakage did not always markedlychange the key indicators of preboiler system corrosion, such as the time betweenchemical cleans; this indicates that this philosophy did not produce a serious reductionin feedwater corrosion products. In the same time period, utilities have been adding,increasing, and changing oxygen scavengers. The result has generally been a reduction

in economizer inlet oxygen levels and a concomitant oxygen scavenger increase. Thisresults in severe reducing conditions (oxidizing-reducing potentials of less than -350mV) in the feedwater leading to an increase in feedwater corrosion products in all-ferrous systems: the opposite result to the initial consideration.

Table 2-5 provides an indication of the typical areas suffering from corrosion and flow-accelerated corrosion, and generation of feedwater corrosion products, and the majorcycle problem areas.

Table 2-5Generation of Feedwater Corrosion Products by Corrosion and Flow-AcceleratedCorrosion, and the Major Unit Transport and Deposition Problem Areas for All-Ferrous Systems

Generation Transport and Deposition

Low/high pressure heater tubes and shells and drainsDeaeratorEconomizer InletFeedwater Piping

Boiler deposits and increased boiler pressure dropRipple magnetite formation on the waterwalls of once-through unitsAt least five boiler tube failure mechanisms

affected by depositsFrequent need for chemical cleaning of boilerBoiler feedpump foulingOrifice fouling/plugging can lead to boiler tube failures by overheat (creep)

For all-ferrous systems with excellent feedwater chemistry, it is clear from the pastexperience that the optimum feedwater chemistry involves a transition to moreoxidizing conditions and a recognition that oxygen scavengers may not be needed

(66),

and that higher dissolved oxygen levels can eliminate flow-accelerated corrosion. The

initial steps involve gradually eliminating the oxygen scavenger under controlled testconditions

(3), with the ultimate benefit accruing from the use of oxygenated treatment

(30).

This treatment has been applied to hundreds of once-through boiler units andnumerous drum boiler units. This direction is reflected in Sections 3 and 4 of the newEPRI AVT Guidelines

(31) where it is suggested that oxygen levels should not be allowed

to drop below 1 ppb in units with all-ferrous feedwater systems.





2-24

The selection and optimization of feedwater treatment for all-ferrous systems isdiscussed in detail in Section 4 of the Selection and Optimization Document

(3).

Mixed Metallurgy Feedwater Systems (Copper Containing) (3, 4, 22, 28)

Mixed metallurgy feedwater systems are more common in drum boiler units where, inhigh pressure units, the carryover of copper oxides can cause a loss in generatingcapacity and efficiency due to turbine deposits. In once-through boiler units, copperalloys may be used for condenser tubing only and the copper induced problems areless frequent. Once-through systems also have condensate polishing which aids incopper corrosion product removal.

As shown in Figure 2-12, it is difficult and sometimes impossible to control corrosion of both carbon steel and copper alloys in the preboiler cycle. All copper alloys used in LPand HP feedwater heaters corrode, and there is also the effect of temperature and flow

velocity.

Besides the corrosion of copper alloy feedwater heater and condenser tubing, the mainproblem is deposition of copper and its oxides on the inlet stages of high pressureturbines. This results in rapid loss of the MW generating capacity and turbineefficiency, requiring chemical or mechanical cleaning of the turbine. Copper alloycorrosion is aggravated by higher air inleakage during cycling operation. Both, oxygenand carbon dioxide from air accelerate the corrosion. Significant ingress of air occurswhen the feedwater and boiler are filled with aerated water prior to a cold startup. Thepractice of boiling out the ammonia and hydrazine dosed boiler layup water duringstartup can produce a large quantity of copper corrosion products by the corrosionattack of ammonia on condenser and feedwater heater tubes.

Additional problems caused by copper transport around the water and steam cycleinclude: deposition on waterwall tubes and more complicated chemical cleaning andaggravation of corrosion of cycle components by the deposited copper, nickel, and zincoxides (on boiler tubes, turbine blades, and austenitic stainless steel tubing). Table 2-6.provides an indication of typical areas suffering from corrosion and flow-acceleratedcorrosion, and generation of feedwater corrosion products.





2-25

Table 2-6Generation of Feedwater Corrosion Products by Corrosion and Flow-AcceleratedCorrosion, and the Major Unit Transport and Deposition Problem Areas for MixedMetallurgy Systems

Generation Transport and DepositionLow/high pressure heaters,Condenser

HP turbine deposits leading to MW lossBoiler deposits and increased boiler pressure dropAt least five boiler tube failure mechanismsFrequent need for chemical cleaningOrifice foulingSuperheater deposits

Copper Transport

Transport of copper into the turbine occurs both during startups and normal operation.Feedwater concentration of copper and its oxides (mostly as colloids) during coldstartup can be as high as 10 ppm. Depending upon the copper alloys used in the cycle,there is also an elevated concentration of nickel and zinc.

These feedwater corrosion products deposit in the boiler and, after concentration in the boiler water, are carried over into steam as mechanical and vaporous carryover.Volatility of copper oxides at high boiler pressures (>2400 psi (17Mpa)) is very high,and up to 30% of boiler water copper can be carried over into the main steam. Becauseof this carryover, hundreds of pounds of copper can be deposited in superheaters and

slough-off and exfoliate. The third mode of copper transport into the turbine is byattemperating sprays.

Minimization of the negative effects of copper can be achieved by replacement of copper alloy tubing, prevention of corrosion during inactive periods by proper layup of heaters and boilers, filling with deaerated water, stringent control of air inleakage, useof condensate polishers, and control of oxygen in makeup water, particularly duringstartups. The key feature here is to keep the environment reducing (ORP < 0mV)during all periods of operation and shutdown

(22).

Layup - After years of corrosion of copper alloys, the corrosion products are

transported and deposited throughout the steam cycle, including heaters, boiler,superheater, and the turbine. The deposited copper is often in the form of metalliccopper and cuprous oxide (Cu2O), because during operation there is a reducingenvironment. During an unprotected layup, these deposited species may oxidize tocupric oxide (CuO) which can then be dissolved and transported downstream.





2-26

The selection and optimization of feedwater treatment for mixed metallurgy systems isdiscussed in detail in Section 4 of the Selection and Optimization Document

(3).

For mixed metallurgy systems, it is clear that reducing conditions (excess oxygenscavenger) are required for the non-ferrous materials

(22). The EPRI AVT Guidelines

(31)

suggest that oxygen levels should be kept below 5 ppb at the economizer inlet.However, the most appropriate oxygen scavenger concentration and residual oxygenlevel can only be determined by carrying out a series of tests. (See road map in Section4, Ref. 31 for mixed metallurgy feedwater systems.)

Because the copper alloy corrosion is caused by ammonia, oxygen, and ammoniumcarbonate and bicarbonate, other amines have been tried and also other oxygenscavengers besides hydrazine (because hydrazine decomposes into ammonia).However, these efforts were not successful in high pressure units, because the alternate(organic) chemicals decompose, forming organic acids and CO

2 which are also

corrosive.

2.4 PRIORITIES FOR TRANSIENT OPERATION

The operation and controls related to safety and mechanical damage of equipment havepriority over the water and steam chemistry control steps. However, it should berealized that some damage mechanisms, such as corrosion fatigue and stress corrosioncracking, are the result of an interaction of mechanical stresses with the environment.Also, there are high impact catastrophic failures such as LP turbine disc burst due tostress corrosion cracking, deaerator failures, and piping failures due to flow-acceleratedcorrosion which often occur during transients when a component, weakened by theslow corrosion damage (cracking or wall thinning), is overloaded beyond its fracturetoughness.

In failure prevention, the knowledge of the maximum loading conditions which oftenoccur during transients is as important as the knowledge of the corrosion damage. Thisis being achieved by inspections and by diagnostic monitoring of defects, pressures,stresses, and temperatures during all types of operation and by life predictinginstrumentation such as turbine rotor stress

(32), and boiler drum stress monitors

(33).

For the above reasons, the priorities are:

1. All safety-related operation, layup, and testing procedures such as safety valves,critical speeds, combustion, and water hammer.

2. Thermal and low cycle fatigue damage prevention such as control of drum, heater,and turbine rotor and piping stresses, and boiler circulation.





2-27

3. Other operation-related problem prevention including control of boiler carryover,condenser vacuum, deaerator water hammer and water piston, deaerator storagetank water flashing and steam space collapse, pump cavitation (particularly for boiler feed pumps), feedwater heater non-condensable gas removal, water levelcontrols, drains, and superheater and reheater overheating.

4. Water and steam chemistry control including:

Control of ingress, generation, transport, and deposition of impurities

Control of all types of corrosion and erosion

Control of deposits in boiler, superheater, reheater, and turbine which can leadto corrosion, overheating, and loss of MW capacity and efficiency

Control of hideout of phosphate and other chemicals

Table 2-7 lists the items of concern that were identified in EPRI report EL-975(7)

. Thereport summarizes a survey of the cycling capabilities of the fossil-fired generatingunits in the US and Canada.

Additional concerns include increased risk of condenser leaks, increased ingress of aerated makeup, and high air inleakage during startups.

For the cycling units, in addition to the recommended physical changes to theconventional plant configuration, one fundamental solution to the problems of cyclingis to give closer attention to optimization of startup, shutdown and load changesequences. Such an optimization process can be performed most readily through theuse of effective analytical tools and control/monitoring instruments designedspecifically for this purpose.

The longterm reliability and availability of cycling units rely on the operator's attentionto, and recognition of, the impact of life expenditure due to thermal cycles and othertransient conditions on plant components. Since the effect on life expended of eachcycle is small, the overall effect may not be readily apparent to plant operators duringthe event and may not be recognized and properly considered in making the day-to-day operating decisions. Yet, the longterm cumulative effect of such cycles can result in

extended and expensive forced outages requiring major repairs or componentreplacement.





2-28

Table 2-7List of Concerns for Cycling Units(CH indicates the items affected by water and steam chemistry)

increased boiler component cyclic stress with loss of life CH

increased turbine rotor cyclic stress with loss of rotor life CH

increased thermal stress on turbine rotor, steam chests, CHvalves, and inner casing leading to corrosion fatigue

increased solid particle erosion of HP and IP turbine bladingfrom superheater and reheater tube exfoliation

turbine vibration during startup/shutdown CH

possibility of furnace implosion/explosion duringstartup/shutdown

steam requirements for maintaining condenser vacuum

the need for improved boiler control to minimize unit restart CHand reloading times

acid dew point condensation - corrosion and plugging of air preheaters and back end ductwork

flame scanner monitoring problems requiring much attention

burner turndown ratio/flame stability at low loads - feederturndown and mill response times during load ramps too slow

increased frequency of chemical cleaning and more CHmonitoring and control of steam and water chemistry needed

poor back end equipment performance at low load conditions

increased possibility of mill fires and explosions duringfrequent starting and stopping of a unit; particularly true for themore volatile western coal

distortion and internal rubbing of the boiler feed pump, if pump is not fully warmed up at startup

By conducting appropriate training programs on operational considerations for cyclicduty, plant operators become more aware of conditions in which thermal stresses onplant components might be particularly excessive, and be better able to take corrective

actions to avert them. In addition, the use of effective analytical tools (such ascomputer modeling to simulate plant operation under different conditions) anddiagnostic monitoring

(34-38) and control of instruments/equipment (such as boiler stress

analyzer and turbine rotor stress indicator) can provide valuable benefits and guidanceto plant operators for reliable and efficient operation under cyclic duty.

The monitoring should include water chemistry, deposits, exfoliation, and corrosion.





2-29

2.5 EFFECTS OF STEAM CYCLE DESIGN AND OPERATION

The cycle design goals for base load, cycling and peaking operation are indicated bythe concerns listed in Table 2-7 and briefly described in this Section. Retrofits have been developed aimed at improving water chemistry control, temperature matching,

and thermal stresses during cycling operation.

Operator actions significantly influence the cycle chemistry and cycle componentcorrosion during cycling operation. The chemistry is influenced by the actions aimed atthe prevention of ingress and removal (blowdown, use of condensate polishers, fill anddrain, etc.) of impurities. Operator actions which control cycle component stresses(temperature matching, ramping rates) also control the stress induced corrosionmechanisms.

A 1977 EPRI workshop on cycling(8)

gathered together utilities with experience inhandling cycling problems when using formerly base-loaded fossil plants. Among therecommendations resulting from that workshop were the following items, most of which relate to capital expenditure items for improving cycling performance

(17). The

items marked with CH indicate effects on water chemistry or corrosion.

Incorporate steam bypass systems CH

Incorporate full arc admission

Add additional controls and monitoring equipment CH

Incorporate variable pressure operation CH

Use two half-sized boilers

Install turning gears on ID fans

Incorporate a condensate polishing system CH

Incorporate better turbine seals

Use integral separators on once-through units CH

Install smaller coal mills for low load operation

Install feedwater cleanup loop CH





2-30

Drum Boiler vs. Once-through Boiler Units (7-12)

Generally, both types of units can be converted or originally designed for cyclingoperation. The most important part of such conversion is matching the steam andturbine metal temperatures. This is usually easier for drum boilers. In once-through

supercritical boilers, the transition through the critical region and from the evaporationmode to the once-through mode, and carryover of chemical impurities from the flashtank are of concern.

Sliding Pressure Operation (17,39-42)

Changing the turbine controls from partial arc admission to full arc admission andsliding pressure operation can reduce thermal stresses and improve efficiency andwater and steam chemistry control. The advantages of sliding pressure operation are:

1. Steam temperature distributions within the turbine are more uniform. Minimumvariation of first stage shell temperature.

2. Improved overall power plant efficiency - feed pump and other auxiliaries.

3. Reduced pressure results in lower heat transfer coefficient and correspondinglylower thermal stresses.

4. Reduced pressure - eases components' duty cycle.

5. Improved and extended control of primary and reheat steam temperature - due to

an increase in the latent heat of vaporization as pressure is decreased.

6. Improved water and steam chemistry control by reducing boiler carryover,improving boiler circulation in the natural circulation drum boilers, and possiblyreducing scale growth in waterwall tubes at lower pressures.

Boiler Concerns(15-17,25,39,40,43-46)

Boiler consideration for cycling operation cover the areas of stress generation,circulation in waterwall tubes, boiler water chemistry, and steam chemistry. The

problems are prevented by a combination of operator and chemist actions and boilerdesign.

The major factors affecting boiler design considerations for cyclic duty are:

number of cycles

heating and cooling rates





2-31

component thickness, diameter and material

operating temperature level

waterwall tube circulation in drum boilers

Heavy thick-walled steam generator components (steam drums, superheater headertees, valves, etc.) should be watched closely for possible failure due to excessivethermal stress (with possible corrosion effects of water and steam chemistry). Startuprates of boilers containing such components have been limited by simple but effectiverules governing the rate of temperature change in these components. In the past, theselimitations did not substantially restrict unit availability because such units wereoperated in the base-load mode. With the conversion of such units to cycling duty,these simple but conservative startup limitations may no longer be adequate to protectagainst excessive loss-of-life.

Cyclic stresses resulting from such temperature changes must, therefore, bereconsidered in the boiler as well as the turbine. They result from either

temperature differences through the thickness of a containment; or

temperature differences between components attached to each other.

The steam drum and superheater outlet headers are the two thickest parts of a boilerand must be considered when thermally cycling the boiler.

There are two general areas in a boiler where parts with different temperaturecharacteristics are attached to each other. The first area covers superheater and reheatertube legs, which penetrate the enclosure and connect to an outlet header. These legsmust have sufficient flexibility to permit one end to move with the header at final steamtemperature, and the other end to move with the enclosure wall at saturationtemperature. The flexibility can be designed into a new boiler, but it may be limitingon an older one, particularly a wide one.

The second area is the attachment of nonpressure parts, such as windboxes andvestibules, to tube walls that are at saturation temperature. These parts respond to airor flue-gas temperature rather than saturation temperature, and change temperature

more slowly.

EPRI developed a boiler thermal stress and condition analyzer to evaluate the conditionof boiler components during episodes of high temperature and pressures or highthermal stresses

(33). This analyzer operates on-line to accumulate a history of damaging

incidents.





2-32

In one EPRI study of a cycling boiler (CS-2438)(44)

, it was found that on/off daily loadcycling of the boiler requires design changes before this type of operation is performedon a regular basis. Even with design changes, the thermal shock of the boiler thatoccurs with operation of once-through boilers when "cold" water enters a "hot" boilermay eventually lead to failure of boiler components. Recommended design changes

included:

Change boiler furnace to spiral design.

Change bypass system, use full pressure separator design, or full pressure separatorrecirculation pump design.

Make provision for quick water cleanup and provide for feedwater heating beforefiring boiler.

Add flame monitoring system.

Provide new burners and burner control system.

Upgrade unit control system.

Add new and larger computer to better monitor temperatures in boiler and turbine.

Boiler Carryover - in drum boilers is sensitive to the drum level which could beelevated along the whole drum length or locally during shutdowns, startups, and rapidload or boiler pressure changes. Operating events such as the use of circulating pumps,coal mills and burners, and soot blowing can significantly influence the drum level. It

is imperative to experimentally determine the carryover for all modes of operation andafter equipment changes are made, particularly after installation of new burners whichmay change the boiler heat flux patterns.

The water chemistry parameters which can increase boiler carryover during cyclingoperation and startups include: high dissolved and suspended solids and highhydroxide alkalinity, particularly when combined with organic matter.

Exfoliation - of oxides (mostly magnetite) from superheater, reheater, and steam pipingcan be accelerated by more frequent shutdowns. The exfoliation occurs during

shutdown because the ID of the tubes and piping cools faster than the metal, and because of the difference in the coefficient of thermal expansion between the oxides andthe steel

(47). The exfoliated oxides collect in the lower bends of the SH and RH pendant

platens and, during the following startup, are carried into the turbine causing solidparticle erosion

(48). These oxides also increase the iron and copper concentration in

condensate and feedwater and cause overloading of condensate polishers. The oxidesextracted through turbine extractions go through feedwater heaters and heater drains





2-33

into the suction of the boiler feed pump and then back into the boiler, and thus bypassthe condensate polishers.

Mud Drums - or lower headers often collect large quantities of oxides (up to hundredsof pounds) in the form of sludge. The quantity of sludge can be reduced markedly by

blowing down the lower boiler drains just prior to shutdown at 50 to 70 psi (0.3-0.5MPa), by drain and fill, by manual cleaning during shutdown, and by blowing thelower drains during the early startup. During startups, circulation from the muddrums through the economizer into the boiler transports large quantities of iron andcopper

(28).

Turbine(18,42,49,50)

Cycling and peaking operation can affect the turbine by generation of corrosivedeposits which reduce generating capacity and efficiency, by producing high thermal

and vibratory stresses, by introduction of exfoliated oxides from the superheater andreheater (leading to solid particle erosion), and by introducing humid air which can,together with corrosive deposits, lead to pitting during layup. A positive effect of cycling can be washing of the accumulated deposits during startup.

Some of the possible turbine modifications that have been considered for cycling dutyinclude:

layup dehumidification or nitrogen blanketing to reduce corrosion

change from constant to sliding pressure(40-42)

change from partial arc to full arc admission(49)

turbine by-pass(49-50)

design changes to decrease thermal strains (decreasing notch effects, better heatingand cooling)

(50)

materials with higher ductility

changing water glands to steam glands

bearing and turning gear modifications

instrumentation and control





2-34

The implementation of any or some of these modifications or processes is clearly afunction of the benefit-cost ratio, which includes the cost of generation for thatparticular plant.

Life Expenditure(50)

- Cycling duty, which can range from daily load changes to daily

startup and shutdown, imposes a much more severe duty on a turbine-generator than base load operation does. Inherent with cycling are large and frequent changes intemperature (see Figure 2-13) which accelerate the expenditure of component life.Once the component life is expended, cracks will be initiated, and (depending onmaterial properties, operating stress levels and stress concentration, the severity of thetransient condition and corrosiveness of steam and deposits) the cracks may propagaterapidly. If the resulting crack is not detected early, propagation can progress to a pointwhere a permanent repair cannot be made and the component must be replaced.

Figure 2-13 Calculated and measured HP rotor temperatures - startup

The cycling duty will also affect the unit's alignment, clearances, etc. There are,however, methods to reduce the magnitude of these temperature changes in the turbineand also to make modifications which will better enable the unit to accommodate thedetrimental effects of cycling duty.





2-35

Turbine related limitations during rapid load changes, shutdowns, and startupsinclude:

thermal stress

differential expansion

rotor and blade vibration

stress and strain cycling of the highly stressed areas due to startups (LP bladeattachments, shrunk on disks), i.e., low cycle fatigue

The critical turbine limitation during cyclic duty are the transient thermal stresses inthe large high temperature components, particularly the rotor, HP and IP innercylinders, during heating and cooling. Cyclic thermal stresses are especiallyaccentuated during periods of rapidly increasing load, as in the case of two-shift

cycling where the unit has to be brought quickly on-line. These thermal stresses havethe potential for causing high rotor bore stresses or local surface yielding which can, if severe enough, result in premature initiation of surface cracks.

Thermal stresses in the turbine arise, in part, because of an inherent difference in therate of temperature change, with respect to time, between the boiler and the turbine.For example, after shutdown of a unit, the boiler cools at a faster rate than the turbine.As a result of this, the subsequent restart of the unit is characterized initially by a steamtemperature which is below the turbine metal temperature. At a later time in thestartup sequence, steam temperature has a tendency to become excessively high as

compared to turbine metal temperature. This mismatching of steam temperature andturbine metal temperature is the driving potential for thermal stresses in the turbine.

The magnitude of thermal stress depends on the total required temperature change andon the temperature ramp rate. Thermal stress is, therefore, the most important factor inestablishing the rate at which turbine operating loads can be varied. Turbine fatigueindex showing the number of startup cycles for initiation of fatigue cracks for differentrates of first stage temperature change is shown in Figure 2-14. On-line thermal stressanalyzers have been developed for turbines which monitor the behavior of the turbineduring startup or load change. Any adverse condition which results in loss-of-life of the rotor forgings is tracked, and episodes are accumulated to give an estimate of cyclic

life expenditure and the remaining rotor life. An important benefit of continuous stressevaluation is that it offers guidance to plant operators during transient cyclingoperations. By monitoring calculated stresses, operators can make more efficient use of the equipment and thus better satisfy the plant cycling requirements. For the particulartype of transient operation, it is necessary to select a cyclic life expenditure target value.This selected value then establishes the allowable thermal surface stress limit whichgoverns the turbine loading rate.





2-36

As already discussed, sliding pressure operation improves many operating problems,including thermal stresses in the heavy sections of turbines. The change of turbinevalve operation from partial arc admission to full arc has similar beneficial effects

(49,50).

These two modes of operation are illustrated in Figure 2-15. Rotor thermal stress as afunction of time is for the constant pressure, partial arc and sliding pressure, full arc

operation shown in Figure 2-16(49)

. The full arc operation can extend the rotor life from1,000 to over 100,000 cycles.

In combination with corrosive impurities, the stresses resulting from the abovesituations can lead to stress corrosion and corrosion fatigue cracking. In the hightemperature turbine sections, sodium hydroxide is the only active corrosive chemical.Salts and acids either evaporate or are dry (non-corrosive). Many corrosive substancesare active in the LP turbine.

(65)

In units which synchronize at very low loads, the following problems can beencountered:

shift of the corrosive salt zone to the highly stressed L-0 blades

reversed circulation of steam at the LP exhaust (windage) leading to high vibratorystresses and water droplet erosion of the L-0 and L-1 trailing edges

600

500

400

300

200

100

0

Time to Change Load/Throttle Conditions (minutes)

8040 1200

First Stage Temperature Change (°F)

6020 100

1000 cycles2000 cycles

3000 cycles5000 cycles

10,000 cycles20,000 cycles

316

260

204

149

93

38

0

°C

Figure 2-14 Turbine fatigue index vs. temperature change and time(51)





2-37

Open

Closed

Stop valve

HP turbine

Stop valve

From boiler

Control valves

Partial Arc Admission

Stop valve

HP turbine

Stop valve

with internalby-pass

From boiler

Controlvalves(all open)

Full Arc Admission

Open

Closed

Figure 2-15 Turbine valves for partial arc and full arc admission

30

20

10

0

10

20

30

Time, minutes

10 20 30 40 50

Partial arc

Full arc

0 7060

C o m p r e s s i v e

S t r e

s s ,

K S I

T e n s i l e

S t r e s s ,

K S I

Cyclic life 10,000

>100,000

Figure 2-16 Rotor thermal stress as a function of time with sliding pressure.Initial throttle to metal temperature difference = +50°F, throttle temperature ramp =200°F/h, loading rate = 2% per minute





2-38

Turbine Bypass Systems(43,49.50)

Large external turbine bypass systems (see Figure 2-17) enhance the startup flexibilityand load changing capability of the unit by better control of thermal stresses and the

impurities carried into the turbine with the steam. With turbine bypass systems, unitscan be made equally suitable for peaking and base-load duties. Full capacity turbine bypass systems permit rapid reloading of the unit even after full load rejection orcontinuous operation at auxiliary house load. European utilities have been using largeexternal turbine bypass systems for over 30 years and in recent years European designshave been trending toward full 100% external turbine bypass systems.

Large external 100% bypass systems allow the full boiler flow at any load to betransferred from the turbine to the bypass system and vice versa without any majorpressure changes. In addition, with a 100% bypass, large volumetric flows at lowpressures can be established during startup, thus guaranteeing a high steam velocity inthe superheat and reheat boiler sections.

Turbine bypass systems offer improvement that can be summarized as follows:

1. Starting and Loading Characteristics - Steam flow in the reheater is established at anearly time in the startup. Therefore, control of the firing rate is limited by theallowable rate of drum heating, not by concern for protecting the reheater. In thisway the overall startup time can be reduced.

2. Independent Boiler/Turbine Operation - Steam is not admitted from the boiler to

the turbine during startup while the steam temperature is excessively lower than theturbine metal temperature. This can minimize the temperature mismatch whichplays an important factor in cyclic life expenditure of major turbine components.

3. Decreased Solid Particle Erosion - During startup, exfoliated oxides (magnetite)carried over from the superheater, reheater, and steam pipes present an erosiveproblem to the turbine and valves.

4. Prevention of turbine contamination by corrosive impurities.

5. Reduction of accumulation of HP turbine deposits of copper and phosphate which

can cause reduction of the MW generating capacity and efficiency.





2-39

Steamfrom boiler

superheater

Generator

CondenserControlvalve

H.P.bypassvalve

L.Pbypassvalve Condenser

Ventilatorvalve

Reverseflow valve

Interceptvalve

Reheater

L.P. bypasswater control

valve

H.P. bypasswater coltrol

valve

H.P.turbine

R.H.turbine

Figure 2-17 Turbine bypass system

Feedwater System Cleanup Loops(14,45,46,52,53,54)

Cycling units should have an auxiliary sub-loop between the condenser and the outletof the heaters to facilitate the cleanup of the preboiler cycle (Figure 2-18). The purposeof the cleanup loop is to remove the metal oxides, which enter the feedwater from thesurfaces of feedwater heaters, and the oxides which deposit on feedwater surfaces due

to the changes of pH and temperature which occur during system layup, fill, andstartup. The principal items for the successful operation of the cleanup loop are: a) alow-pressure cycle line, and b) a condensate polisher or filter to process at least 25% of the rated flow. Following an outage, the condensate must be properly treated andrecirculated through the preboiler cycle to permit deaeration and removal of suspendedsolids. Recirculation, when performed at sufficiently high velocity, removescontaminants from preboiler surfaces.

Based on experience with cleanup systems, a velocity of 0.6 m/s (2 fps) (approximately25% of the maximum continuous rating, MCR), is recommended for the current designsof feedwater heaters. Recommended procedures incorporating the bypass system,

condensate polishing, and deaeration for cyclic units have been discussed(54). Cleanuptime after long shutdowns (greater than 4 days) requires about 8 hours to reduceimpurities in the feedwater to levels suitable for use in boilers. Units started up afteroutages of less than 4 days generally require 3-4 hours to clean up the feedwater. Nopreboiler cleanup is required after a hot restart if the condensate quality meets specifiedlimits.





2-41

Condensate Filtering and Polishing(15,45,54,67)

Removal of corrosion products (iron and copper oxides) and other impurities fromfeedwater during the cleanup operation of cyclic units is one of the best water

chemistry control measures. It can be achieved with deep-bed demineralizers,powdered-resin demineralizers, magnetic or other filters. With proper flow anddeaeration, it is possible to remove 85-95% of the suspended contaminants with amixed-bed condensate demineralizer. Optimum filtration efficiency during startup isachieved when the flow rates are greater than 0.02 m/s (25 gpm/ft

2) of resin area for

deep-bed demineralizers. Contaminant breakthrough can occur rapidly duringcleanup if suspended solids levels are high. Protection against this condition ispossible by limiting the demineralizer runs (to 24 hours or less as required) or to adifferential pressure of 345 kPa (50 psig).

Powdered-resin demineralizer filtration efficiency equals that of a deep-bed system.Design flow rates for these units are approximately 0.3 cm/s (4 gpm/ft

2) of resin area.

While these units may have less total ion exchange capacity than deep mixed-bed units,they are relatively free of incidents of "crud throw" or the release of filtered metal oxideparticulates reported with deep mixed-bed units. This is an important consideration instartup/cleanup for a cyclic unit.

Condensate polishers also remove undesirable cations and anions such as sodium,chloride, sulfate, organic acid anions, and carbonate. These impurities are at higherconcentrations during startups, and their concentration is also influenced by loadchanges during cycling. Carbonate formed from CO2 entering with air inleakage can

act as an eluent and replace the already exchanged anions from the polisher resin.

Air Inleakage and Deaeration(15,45,53-61)

During layup, startup, and cycling, large quantities of air can be introduced by:

system filling with aerated water from storage tanks,

air saturation of water in condenser and deaerator storage tank, and

increased air inleakage because a larger portion of the cycle is under vacuum.

While both tray and spray deaerators perform well at all loads (see Figure 2-19),condensers do not deaerate well until 30-50% of the thermal load is reached (see Figure2-20)

(62). Condenser deaeration can be improved by increasing the deaerating capacity

and by retrofitting steam blanketing and other means(9,57-61)

such as ensuring that themakeup water is introduced above the tubesheet.





2-42

Air infiltration in cycling units can result from design and operation as follows(55)

:

(a) Some system designs include heater drip pumps which return drips to thecondensate or feedwater. At low loads, oxygen contents as high as 500 ppb in thedrips have been noted.

(b) Reduction of a system load to below 60% of capacity generally results in a reductionof one or two of the low pressure heaters from a positive pressure to belowatmospheric pressure. Leakage of air through the various seals results in oxygencontamination of the steam condensing on the tube bundles.

(c) Addition of undeaerated makeup water directly to the condensate or feedwatercauses extra oxygen introduction to the cycle. In base-load systems, contaminationfrom makeup sources such as storage tanks is less important since little water istransferred to and from surge tanks. In peaking units, it can be significant sincethere is considerably more shrinkage and expansion occurring in condensate surge

tanks due to frequent load changes. When undeaerated makeup or surge water isintroduced, portions of the preboiler system will become contaminated with airunless it is introduced into the deaerating section of the condenser or to a deaerator.If the makeup and surge water storage reservoirs are not resistant to oxygen attacknor protectively lined, corrosion products may also be introduced.

(d) Reduced air removal efficiency of deaerating equipment can be caused by cyclingoperation. Systems that are shut down and started frequently, such as in two-shifting and peaking operation, are subject to significant corrosion problems if aproper design to exclude air infiltration is not used by the plant designer. In this

mode, it is possible to add more oxygen into the cycle than in several months of normal base load operation. Cyclic units that may be idle overnight or on weekendsshould include the following operating and design features:

— Maintain condenser vacuum and turbine seals during brief shutdowns. Thiswill protect both low- and high-pressure condensate systems from oxygen.Turbine water glands are not effective seals at low load. Use steam seals.

— Provide auxiliary steam to pressurize the deaerator and keep deaeratorpressurized during short outages. If auxiliary steam is not available, peggingsteam from an adjacent boiler or the drum should be used. For longer outages,

nitrogen blanketing may be more convenient.

— Blanket the boiler with nitrogen or steam under pressure. The nitrogen orsteam-injection systems should be automatic with multiple points of addition on boiler, deaerator, superheaters, and feedtrain vents. While a unit that is base-loaded may be able to function with manual systems for introducing nitrogen





2-43

and layup chemicals, a unit with frequent shutdowns requires a rapidlyresponding automatic system to ensure the introduction of steam or nitrogen.

The effect of turbine steam seals and nitrogen blanketing on iron concentration duringa cold startup is illustrated in Figure 2-21.

120

110

100

90

80

70

60

50

40

30

20

10

0

Dissolved Oxygen in Effluent (ml/l)

0.004 0.0060.001 0.0050.0030 0.0080.0070.002

% Loading

N o r m

a l

e x p e c

t e d d

i s s o

l v e

d o

x y g e n

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

d o x y g e n

Tray Spray

Figure 2-19 Expected dissolved oxygen at the deaerator outlet vs. load for trayand spray deaerators





2-44

40

30

20

10

0

Dissolved Oxygen (ppb)

With jet deaerating device

Source: F.J. Pocock, Prepared Discussion to J. Brown and R.E. Massey, "Condensate, Feedwater, SteamSampling and Analysis in Ontario Hydro Thermal Generating Stations. "Proceedings of the 41stInternational Water Conference, October, 1980

With bubbling device

Without auxilliary deaerating device

Heat Load (%)

0 20 6040 10080

Figure 2-20 Condenser deaerating capacity with and without retrofitted devices(62)





2-40

Condenser Cleanup - There is often an accumulation of corrosion products on the bottom of the hotwell and even on condenser tubes. The hotwell corrosion productsshould be cleaned during shutdown, and the hotwell water can be cleaned prior to astartup by bypass filtration or circulation through condensate polishers. The tube bundle and the condenser structure and walls can be cleaned by washing with the

turbine hood sprays prior to a startup.

Use of these procedures will ensure feedwater that is low in iron, copper and silicacontamination.

Source: B.T. Hagewood, H.A. Klein, and D.E. Voyles, "The Control of InternalCorrosion in High-Pressure Peaking Unit," Proceedings of the American PowerConference, Vol. 30, Chicago, Ill., 1984.

Hotwell

Polisher

Condenser

Dearator

Recirculation valve

IPturbine

LPturbine

Drip pump

HP heaters

HPturbine

Boiler

LP heaters

Cleanup loops

Figure 2-18 Condensate/feedwater cleanup loops(46)





2-45

Source: F.J. Pocock, Prepared Discussion to J. Brown and R.E. Massey, "Condensate, Feedwater, SteamSampling and Analysis in Ontario Hydro Thermal Generating Stations. "Proceedings of the 41stInternational Water Conference, October, 1980

25,000

20,000

15,000

10,000

5,000

0

Iron (ppb Fe)

Condensatebooster pump

discharge

Hotwell Economizerinlet

Cycloneoutlet

Primaryfurnaceoutlet

Without turbinesteam seal and

nitrogen blanket

With turbinesteam seal

and nitrogenblanket

Secondarysuperheater

outlet

Figure 2-21 Cycle iron concentration during a cold startup for two layuppractices

(62)

With the use of pegging steam, the oxygen content of the feedwater can be reduced to below 10 ppb during startup. Pressurization of the system in a banked condition willprevent oxygen infiltration in the deaerator storage water. Pegging steam must beprovided to maintain a pressure of 69-103 kPa (10-15 psig) until the turbine extractionsteam is available to the deaerators

(59,60). If the supply of steam is inadequate to heat all

the incoming feedwater to a temperature above 100°C (212°F), air will be aspirated intothe deaerator and contaminate the incoming feedwater with oxygen. It is importantthat either steam or nitrogen pressure be maintained on the deaerator during outages.If the water in the deaerator storage tank becomes aerated, the elimination of oxygencannot be accomplished during startup unless there is a facility for recirculation.

In systems not having a deaerating heater, it is impractical to pressurize or excludeoxygen from the feedwater cycle when the unit is out of service. In such systems,deaeration is achieved solely in the condenser. In order for the condenser to deaerate, avacuum must be maintained in it. This is seldom practical during an extended outage.A more reasonable approach is to recycle and deaerate the condensate in the systemprior to startup. This requires a recycle line to be connected from the discharge of thefeedwater heaters back to the deaerating section of the condenser. This will prevent the





2-46

air-rich feedwater in the preboiler system from being introduced into the boiler. Theturbine must be sealed and condenser vacuum established prior to recycle. To establishvacuum and ensure a more rapid exclusion of noncondensible gases, the condensateshould be heated to about 79°C (175°F) at the outlet of the heaters, during recycle. Thiscan be accomplished by injecting steam into the steam side of one of the feedwater

heaters.

Makeup Addition - Addition of undeaerated makeup water to the condensatesignificantly upsets the oxygen control (see Figure 2-22) and increases corrosionpotential in the preboiler cycle. Makeup water should be atomized and treated bysteam in the upper part of the condenser tube bundle to provide effective deaeration.

120

100

80

60

40

20

0

Condensate Oxygen Concentration (ppb)

0.7%

0.5%

0.3%

1% makeup rate(% of main steam flow)

Makeup Water Temperature (°F)

Saturated Oxygen Cencentration (mg/l)

Source: Y.H. Lee and D.M. Sopocy, "Cost Benefit Analysis of Backfiting Makeup Degasifiers," Materials Performance, Vol. 24, No. 3, May 1985

6 7 8 109 11 12

113 100 90 7080 5060 45

58 38 32 2127 1016 7°C

Figure 2-22 Effect of air-saturated makeup water on condensate oxygen level(59)





2-47

At startup, or at low loads, the heater drips may contain as much as 500 ppb of oxygenand, therefore, they should be introduced into the condenser through atomizingdevices. Drains at temperatures below saturated steam temperature should beintroduced above the tube bundle; drains with temperatures above saturated steamtemperature should be introduced between the tube bundle and the hot well.

Condenser Deaeration

Efficient air removal is essential for achieving good condenser deaeration at all loads.Most condensers provide adequate deaeration at high loads; however, at low loads oron startup of peaking or two-shifting units, the deaeration is marginal (see Figure 2-20).It can be improved by retrofitting steam sparging and other means.

The prime factors affecting condenser deaeration at low loads are the temperature riseof the circulating water, air inleakage, and deaerating capacity of the vacuum pumps or

air ejectors. At low loads, the temperature rise of the cooling water across thecondenser tubes is less than at full load. This results in a reduced condenser pressureand corresponding saturated steam temperature causing a reduction in mass-steamcondensation area and an increase in the air cooler area. Under these conditions, anincreased oxygen concentration is caused by subcooling. Since air ejector capacity isconstant, and normally designed for full-load operation, more wet steam and less gas isejected. The remainder of the gas accumulates within the condenser shell, increasingthe partial pressure of the gas throughout the bundle.

Tests have been run which indicate that air inleakage at 25% load can be double that atfull load. At low loads, or startup, it is necessary to remove the air to ensure acondenser vacuum. Air ejector overloads lead to increased condenser pressure anddissolved oxygen content, which in turn promotes corrosion in the cycle. Thiscondition and that due to insufficient sweeping of air deep in the tube bundle, can beobviated by using an additional vacuum pump or ejector. With proper design, 12.7mm (0.5 in.) or as low as 6.35 mm (0.25 in.) Hg, absolute suction pressure can beattained.

2.6 ALTERNATIVE WATER TREATMENT CHEMICALS(3,63,64)

During the past 15 years, many new amine, oxygen scavengers, and polymericdispersants have been introduced. These chemicals are mostly used in industrial steamcycles and their use in high pressure utility units is controversial. All types of utilityunits can be operated with the water treatments and chemicals recommended in EPRIguidelines. The main concern with the use of the alternative organic water treatmentchemicals is their thermal decomposition (breakdown) and production of organic acidsand carbon dioxide.





2-48

Before applying these new chemicals, their properties should be carefully determinedand experience with each should be verified. Then, within a few weeks of the firstapplication of the new chemical, the cycle chemistry should be analyzed in much moredetail than during the normal operation and any breakdown products should beidentified

(3).

The selected treatment should protect all cycle components and prevent generation of corrosion products, and general and localized corrosion. These chemicals and their breakdown products should be compatible with all cycle component materials and thelayup practices, and with all environmental and health regulations. Decomposition of organic compounds at the elevated and high temperatures in feedwater and boilers canlead to increased feedwater and steam cation conductivity and reduced pH. Most of the applications are for low- and medium-pressure industrial units, but some of thesechemicals are also being used in utility cycles. The use of some of these products canlead to corrosion, buildup of deposits, and other problems. It should be kept in mindthat the overall philosophy of the EPRI fossil plant cycle chemistry program is to keepthe cycle as pure as possible with as few chemical additions as possible.

The alternative water treatment chemicals (all organic) fall into the followingcategories:

neutralizing and filming amines for feedwater, steam, and condensate

reducing agents (oxygen scavengers)

dispersants and chelating agents for prevention of boiler scale and removal of

hardness and corrosion products (these should only be considered and used in lowpressure (<1000 psi, 7MPa) units)

There are hundreds of formulations of the above chemicals and their mixtures.

To evaluate the effects of any water treatment chemical, data pertinent to its chemicaltransport, decomposition temperature and products, cycle material corrosion, depositand scale buildup, and analytical interferences should be known. These needed datainclude:

hydrothermal stability in the cycle

kinetics of reactions

decomposition products and their effects

analytical interferences





2-49

how to monitor/analyze

toxicity of the product, decomposition products, deposits, etc.

measured effects on pH, specific conductivity, cation conductivity, iron, and copper

concentrations in the feedwater, boiler water and steam

stability in chemical addition tanks and storage containers

solubility and volatility of the chemical and its decomposition products

behavior of dried-out solutions (deposits in reheaters, superheaters, turbines, valve“gluing”)

behavior under short- and long-term layup conditions and during startup(decomposition - acid formation, scale formation, disposal, etc.)

The utility users of water treatment chemicals need to know the pressure andtemperature range of their application and the nature and behavior of thedecomposition products. These are not usually supplied by the chemicalmanufacturers, and a utility will need to perform comprehensive monitoring beforeconsidering their application

(3).

2.7 REFERENCES AND BIBLIOGRAPHY FOR SECTION 2

1. Interim Consensus Guidelines on Fossil Plant Cycle Chemistry. EPRI CS-4629,

RP2712-1, June 1986.

2. Guidelines on Cycle Chemistry for Fluidized-Bed Combustion Plants. EPRI TR-102976, September 1993.

3. Selection and Optimization of Boiler Water and Feedwater Treatments for FossilPlants. EPRI TR-105040, March 1997.

4. Cycle Chemistry Corrosion and Deposition: Correction, Prevention, and Control.EPRI TR-103038, December 1993.

5. “Transport of Chemicals in the Steam Cycle.” In Ref. 15.

6. O. Jonas. “Transport of Chemicals in the Steam Cycle”. Paper No. 245,Corrosion/85, NACE, March 25, 1985, Boston.

7. Survey of Cyclic Load Capabilities of Fossil-Steam Generating Units, EPRI EL-975,Final Report, February 1979.





2-50

8. Cycling Ability to Large Generating Units Workshop, EPRI WS-77-50, November1977.

9. “Proceedings: 1983 Fossil Plant Cycling Workshop.” EPRI Report CS-3979, April1985.

10. “Proceedings: 1985 Fossil Plant Cycling Workshop.” EPRI Report CS-4723,September, 1986.

11. “Proceedings: 1987 Conference of Fossil Plant Cycling,” EPRI Report CS-6048,December, 1988.

12. “Fossil Plant Cycling.” EPRI Conference, Washington, D.C., December 4-7, 1990.

13. Cycling of High-Pressure Steam Power-Generating Units with Drum Boilers. EPRICS-2340, Final Report, April 1982.

14. Improvement of Chemistry Control During Startup of Fossil Units. ASMEWorkshop, St. Louis, MO, April 9-10, 1997.

15. The ASME Handbook on Water Technology for Thermal Power Systems. PaulCohen, Editor-in-Chief. EPRI Research Project RP-1958-1, ASME, 1989.

16. Combustion - Fossil Power Systems, Combustion Engineering, Inc. 1981.

17. A. F. Armor and F. K. L. Wong. “Fossil Plant Cycling Program.” In Ref. 9.

18. O. Jonas. “Understanding Steam Turbine Corrosion”, Paper No. 55, Corrosion/84,NACE April 2-6, 1984, New Orleans.Also: “Steam Turbine Corrosion,” Materials Performance, 24, 2, February 1985, pp.9-18.

19. R. Garnsey, Combustion, Vol. 52, No. 2, p. 39, 1980.

20. G. M. W. Mann. “The Oxidation of Iron Base Alloys Containing Less Than 12% Crin High Temperature Aqueous Solutions.” in High Temperature, High PressureElectrochemistry in Aqueous Solutions, NACE, Houston, Texas, 1976. pp. 34-47.

21. G. M. W. Mann. “History and Causes of On-Load Waterside Corrosion.” Br.Corrosion J. 12 (no. 1, January 1977): pp. 6-14.

22. State-of-Knowledge of Copper in Fossil Plant Cycles. EPRI TR-108460, September1997.

23. Metals Handbook, Volume 19 - Fatigue and Fracture, ASM International 1986.





2-51

24. Metals Handbook, Volume 13 - Corrosion, ASM International 1987.

25. B. Dooley and W. McNaughton. Boiler Tube Failures: Theory and Practice. EPRIBook, TR-105261, 1996.

26. D. D. Macdonald and G. A. Cragnolino. “Corrosion of Steam Cycle Materials.” InRef. 15.

27. J. Mathews. “The Importance of Startup Chemistry to the Long-term Reliability of Power Generating Equipment.” In Ref. 14.

28. O. Jonas, et al. “Copper Deposition and MW Loss Problem Solutions.” Paperpresented at the International Water Conference, Pittsburgh, October 1996.

29. Corrosion Product Transport in a Cycling Fossil Plant. EPRI Report CS-5033,February, 1987.

30. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI TR-102285, December 1994.

31. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. EPRI TR-105041, April 1996.

32. Monitoring & Diagnostic Center: An Overview of Operating Activities. EPRI GS-7407, July 1991, pp. 4-22 to 4-38.

33. Boiler Stress and Condition Analyzer. EPRI, RN6315B(1), August 1986.

34. G. Touchton, et al. “Predictive Maintenance for the '90s: EPRI Keynote.” EPRIFourth Incipient Failure Detection Conference, Philadelphia, PA, October, 1990.

35. A. F. Armor. “On-Line Diagnostics for Fossil Power Plants: The Promise and theReality.” EPRI Workshop on Incipient Failure Detection for Fossil Plants, Hartford,CT, August 1982.

36. O. Jonas. “Incipient Failure Detection and Predictive Maintenance.” Power, January 1992, p. 61.

37. O. Jonas. “On-Line Diagnosis of Turbine Deposits and First Condensate.”Pittsburgh, PA, Oct. 31-Nov. 1-2, 1994.

38. O. Jonas. “Monitoring of Superheater and Reheater Exfoliation and Steam Blow.”56th International Water Conference, Pittsburgh, October 1995.





2-52

39. B. E. Laney, et al. “Supercritical Unit Boiler Circuitry and Control SystemModifications for Improved Unit Turndown Capability.” In Ref. 9.

40. W. P. Gorzegno. “Retrofitting High Efficiency Steam Generators for CyclingService.” In Ref. 9.

41. I. Martinez, et al. “Supercritical Steam Generator Designs for Sliding PressureOperation.” American Power Conference, Chicago, 1981.

42. H. Termuehlen. “Variable-Pressure Operation and External Turbine BypassSystems to Improve Power Plant Cycling Performance.” ASME paper 79-JPGC-Pwr-9, Joint Power Generation Conference, Charlotte, NC, Oct. 1979.

43. G. P. Schatzmann. “Economic Peak-load Coverage by Retrofitting Existing PowerPlants.” In Ref. 9.

44. Study of Universal Pressure Boiler for Cycling Operations. EPRI CS-2348, June1982.

45. F. Gabrielli, et al. “Water Chemistry Aspects of Cyclic Operation for Older HighPressure Drum-Type Boilers.” In Ref. 9.

46. B. T. Hagewood, et al. “The Control of Internal Corrosion in High Pressure PeakingUnit.” Proceedings of the American Power Conference, Vol. 30, Chicago, IL, 1984.

47. The Spalling of Steam-Grown Oxide from Superheater and Reheater Tube Steels.EPRI FP-686, February 1979.

48. Solid Particle Erosion Technology Assessment. EPRI TR-103552, December 1993.

49. D. D. Rosard and W. G. Steltz. “Assessment of Fossil Steam Bypass Systems.” InRef. 9.

50. C. R. Ernest and W. G. Gorman. “Upgrading Steam Turbine-Generators for CyclingOperation.” In Ref. 9.

51. J. Bellows. “Startup Procedures and Limits: A Manufacturer's Perspective.” In Ref.14.

52. R. L. Coit. “Balance of Plant Options for Cyclic Duty Operation.” In Ref. 9.

53. W. A. Micek and K. L. Atwood. “Design Factors in Water Chemistry Control forBoilers in Cyclic Service.” Proceedings of the American Power Conference 41, 1979,pp. 905-911.





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54. F. Gabrielli, and W. R. Sylvester. “Water Treatment Practices for Cyclic Operationof Utility Boilers.” International Water Conference, Pittsburgh, PA, October 31-November 2, 1978.

55. H. Grabowski. “Management of Cycle Chemistry.” In Ref. 15.

56. O. Jonas. “Deaerators, An Overview of Design, Operation, Experience, and R & D.”Proceedings of the Amer. Power Conference, Vol. 49, p. 979, 111. Institute of Technology, 1987.

57. W. Pearl, et al. “Deoxygenation in Cycling Fossil Plants.” 1990 Conference onFossil Plant Cycling, Washington, DC, December, 1990.

58. O. Jonas. “Controlling Oxygen in Steam Generating Systems.” Power, May 1990.

59. Y. H. Lee and D. M. Sopocy. “Cost Benefit Analysis of Backfitting Makeup

Degasifiers.” Materials Performance, Vol. 24, No. 3, May 1985.

60. I. Oliker. “Deaeration.” In Ref. 15.

61. R. Coit. “Condensers.” In Ref. 15.

62. F. J. Pocock, prepared discussion of the paper: J. Brown and R. E. Massey.“Condensate, Feedwater, Steam Sampling and Analysis in Ontario Hydro ThermalGenerating Stations.” Proceedings of 41st International Water Conference, October,1980.

63. “Use of Organic Water Treatment Chemicals.” VGB Conference, OrganischeKonditionierungs-und Sauerstoffbindemittel, Lahnstein, Germany, March 1994.

64. O. Jonas. “Beware of Organic Impurities in Steam Power Systems.” Power, 126, 9,pp. 103-107, September 1982.

65. T. McCloskey, B. Dooley and W. McNaughton. “Steam Path Failures: Theory andPractice.” Two Volume EPRI Book TR-108943.

66. R.B. Dooley, J. Mathews, R. Pate and J. Taylor, “Optimum Chemistry for ‘All-Ferrous’ Feedwater Systems: Why Use an Oxygen Scavenger?”. Proceedings: 55th

International Water Conference. Engineers Society of Western Pennsylvania. 1994.

67. Condensate Polishing Guidelines. TR-104422. Palo Alto, Calif.: Electric PowerResearch Institute, September 1996.





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3-1

3

GENERAL ASPECTS COMMON TO MOST UNITS

3.1 DEFINITIONS

Cycling

Cycling is a load following operation. The unit load fluctuates with system demand,with the unit synchronized at very low loads during low-demand periods. A typicalload variation for cycling units might range from 30% to 100% of design capacity.

Peaking is a form of cycling in which the unit is operated only during peak powerdemand periods. At off-peak hours the unit is on hot or cold standby, depending uponthe estimated time between restarts. Two-shift operation is typical of peaking units,which generally furnish power for the morning and evening high demand hours.

Duration of Shutdown

The duration and description of shutdown periods have different definitions within the

various utility systems. For the purposes of this Guideline document, shutdownperiods are defined as follows:

Short-Term Shutdown (Wet)

Overnight to through-a-weekend. This might be typical of cycling-type operation. Thechemistry conditions for boiler water and pre-boiler systems are usually kept in thenormal operating range. The boiler should be full and under pressure.

Intermediate Shutdown (Wet and Dry)

This condition applies for periods extending more than a weekend and up to one week.It could typify a shutdown for equipment repair of modest complexity.

Under wet conditions the chemistry is maintained in the normal operating range andthe boiler is allowed to cool. Positive nitrogen pressure is applied and maintained toprevent air-ingress as pressure decays below positive pressure.




General Aspects Common to Most Units

3-2

Under dry conditions (needed for such operations as boiler tube repair) the boiler andassociated systems are drained hot and purged with nitrogen to remove all traces of moisture. Air-ingress is controlled by maintaining a positive nitrogen blanket on the boiler, superheater, and associated steam spaces until moisture is removed and themetal cools.

For safety reasons nitrogen must be purged from all areas being serviced beforepersonnel ingress for repairs.

Longterm Shutdown (Wet and Dry)

This condition applies when the unit is out-of-service for more than one week. Thiscould include major equipment repair, planned outage, or a unit mothballing scenario.

Under wet conditions, hydrazine concentrations are elevated, pH is maintained above 9(25°C, 77°F) and a positive nitrogen pressure is maintained to exclude air from un-

flooded spaces.

Under dry conditions, the hot boiler is drained and purged of all moisture withnitrogen. A positive pressure of nitrogen is maintained in the boiler and associatedsteam spaces until the boiler cools. Nitrogen blankets may be maintained for extendedperiods. Alternatively the unit may be stored indefinitely under properly controlleddry dehumidified conditions.

Peaking

If the unit is utilized for peaking service, the applicable hot-standby or short-termlayup condition with properly controlled chemistry should be utilized.

Again, the length of shutdown plays a major role in the type of layup procedureselected (See Layup Section 4). The rapidity with which units can be returned toservice may place constraints on how the unit is shut down or the procedures used forlayup.

Forced Shutdown

There are several types of situations which would demand an emergency shutdown.Each situation may affect the type of chemistry treatment which can be provided andthe method of layup

(1a):





3-3

System Failure but no Equipment Failure

A system failure may be caused by a fault on the system over which the utility has littlecontrol, such as a system blackout or loss of a critical transmission line. This wouldresult in rapid shutdown without the possibility of close chemical control. Most layups

under these conditions would be short-term.

Major Equipment Failure

A major equipment failure might include boiler tube failures, turbine vibration, boilerfeed pump malfunction, etc. This type of failure would normally result in rapidshutdown without the possibility of close chemical control. Layup would probably belongterm.

3.2 USE OF POLISHERS AND CONDENSATE FILTRATION

Condensate Polishing and/or Filtration (1b)

Condensate polishing and/or filtration is a definite asset for all operating units, andparticularly those in cycling operation. These options materially reduce startup timesand prevent high concentrations of corrosion products from entering the boiler. Acomplete discussion of the benefits of condensate polishing can be found in the EPRICondensate Polishing Guidelines

(2).

Clean-up loops (Figure 2-18) that include either or both condensate polishing and

filtration provide for rapid cleanup of the pre-boiler system and will reduce startuptime by removing corrosion products and other contaminants such as silica.

Makeup Water Treatment

The makeup water quality is especially important with frequent startups/shutdowns, because of the additional water usage required during these operations. The makeupwater limits provided in the phosphate

(3), AVT

(4) and OT

(5) guidelines are comparable to

the requirements for the condensate cycle.

Properly designed and operated makeup systems are generally reliable in providing aproduct meeting the requirements of the plant with respect to both quantity and purity.Unfortunately, the makeup system is sometimes taken for granted. During cycling andpeaking operations, it is vitally important that routine surveillance of the makeup plantis maintained to meet standard makeup water quality requirements

(6).





3-4

3.3 MONITORING IMPORTANCE AND REQUIREMENTS

Sampling and Monitoring

Monitoring of core parameters (see Table 1-1) is essential for transient operation.During shutdown, additional monitoring of iron and copper, and monitoring of thewater treatment additives used during layup is necessary. Water chemistry and othermonitoring during layup is described in Section 4-6.

During startup, additional monitoring may include more frequent sampling for ironand copper, analysis for organics, and analysis of the makeup and the condensatestorage tank water. Membrane Filter Charts and membrane filtering of feedwater have been found to be a rapid and useful method of evaluating corrosion product transportduring start-up and re-starts

(7). This is a very simple and available method which can

provide a direct indication of whether the shutdown and layup procedures have been

successful.

Sampling Problems

During low load operation, pressure and flow characteristics of the sampled streamschange and often there is insufficient sample flow to analyzers and grab samplingports. In improperly designed sampling systems, the changing sample flows can resultin sampling errors up to several hundred percent. The sampling system characteristicsneed to be tested and the sampling system improved if necessary, particularly forcycling and peaking units.

During startups, re-starts, rapid load changes, significant amounts of corrosionproducts can be transported from the pre-boiler system to the boiler

(9-11). After extended

outages and where systems have been opened for inspection and repair, the quantitiesmay be large (even in the ppm range). Figure 3-1 shows an example of iron and copperlevels measured at the economizer inlet of a drum unit startup

(9,10). Thus, it is very

important to improve the sampling systems for cycling and peaking service in orderthat a proper assessment of corrosion product transport to the boiler can be madeduring this type of operation

(8).





3-5

700

600

500

400

300

200

100

0

C o

p p e r C o n c e n t r a t i o n ( p p b )

I r o n C

o n c e n t r a t i o n ( p p b )

Copper

Iron

Time (hours)

80

70

60

50

40

30

20

10

00 1 2 3 4 5

Figure 3-1 Metals Concentrations in Feedwater During Startup Operations(9,10)

3.4 MAJOR CHEMICAL TRANSIENT

A major chemical transient might include a major condenser leak.

A small condenser leak of low solids water (cooling pond, river, etc.) would generallypermit continued operation while isolating the location of the condenser leak and formaking suitable repairs, such as plugging the offending tube(s).

The incidences of condenser leaks tend to increase during cycling operation.

A brackish or sea water leak presents a more difficult problem. The presence of chlorides in the sea water, particularly magnesium chloride, will produce an acidiccondition in the boiler (hydrochloric acid) and cause severe tube damage via hydrogendamage if allowed to continue for even short periods of time.





3-6

With the availability of a condensate polisher on the unit, an orderly shutdown may bepermitted during sea water leakage, especially if a deep bed polisher is provided, andif the sea water leak is small. Utilities with deep bed polishers should retain one ormore beds in the hydrogen form to provide additional capacity to handle the condenserleak.

Powdered resin condensate polishers contain less capability for removal of dissolvedimpurities than deep bed polishers

(2). If the leak is small, the use of powdered resin

systems may permit an orderly unit shutdown.

Chemistry monitoring is especially important when a sea water condenser leak issuspected. Boiler water pH is critical and condensate cation conductivity and sodiumwill assist in estimating the extent of the leak

(8).

In any event, the unit should be shut down to repair the condenser leak. Depending onan evaluation of cycle and boiler chemistry, the shutdown will be immediate ororderly, as outlined above. If the unit has a divided waterbox then the load could bereduced to half depending on the seriousness of the leak.

Chemical Transients and Equipment Failures

Chemical contamination may occur from several sources:

— Makeup Demineralizer

Both caustic and acid contamination of the boiler have been reported as a result of

demineralizer regenerant solutions inadvertently entering the system throughequipment malfunctions or operator error.

— Deep Bed Polisher

Contamination similar to that from the makeup demineralizer (above) can occurand for the same reasons.

— Chemical Cleaning

Acidic contamination has been reported after chemical cleanings because of improper rinsing. Superheater contamination has been reported caused byinadequate superheater isolation procedures.

Intrusion of chemical contamination from the above (and possibly other) sourcesrequires immediate unit shutdown, draining and flushing the unit. Inspection of critical areas of the system (boiler, superheater, turbine, etc.) should be performed toassess the effects of chemical intrusion on the system. Chemical cleaning of the boiler,superheater and turbine may be required depending upon the results of the inspection.





3-7

Equipment repairs may also be required. In such case, a longterm layup will benecessary. (See Layup Section 4.)

The use of a condensate/feedwater cleanup loop, such as shown in Figure 2-18, ishighly effective in removing contaminants from the cycle, permitting more rapid starts

and less contamination entering the boiler.

3.5 MINIMIZATION OF AIR IN-LEAKAGE

Minimization of air inleakage (oxygen and carbon dioxide) is essential to preventincreased corrosion during startup. Oxygen and carbon dioxide can be controlled byone or more of the following measures:

Makeup water deaeration

Protecting condensate storage tanks from air

— Floating covers

— Diaphragms

— Nitrogen Purge

Maintaining condenser vacuum during shutdowns

Hotwell sparging

Use of a heat cycle deaerator

Proper maintenance procedures, particularly for equipment operating undervacuum conditions

Optimization of air removal equipment

Steam or nitrogen blanketing steam-side surfaces during downtime periods

To minimize corrosion, the dissolved oxygen content at the economizer inlet and boiler

water during startup and before firing should be less than 100 ppb, the iron in thefeedwater should be less than 100 ppb and the copper in the feedwater should be lessthan 10 ppb.





3-8

3.6 CORRECTIVE ACTIONS

Typical corrective actions to respond to out-of-specification steam and/or waterchemistry conditions for drum units on PT, EPT, AVT, OT and CT are presented in the“Corrective Actions” sections of the respective EPRI guidelines

(3-5,12). Individual

corrective action tabulations are given for the following:

Makeup treatment system effluent

Condensate storage tank effluent

Condenser leak detection trays

Air removal system exhaust

Condensate pump discharge

Deaerator inlet

Deaerator outlet

Economizer inlet

Boiler water

Reheat steam

For the specific suggestions during shutdown and startup, the reader is referred toSections 5–8 in this Guideline.

3.7 HOW TO USE THE PRESENT EPRI GUIDELINES FOR CYCLING

OPERATION

All the current EPRI Guidelines(3-5,12)

for drum units present a series of curves for boilerwater concentrations of sodium, chloride, sulfate and silica vs. boiler drum pressurewhich are considered satisfactory for “normal operation,” consistent with longtermsystem reliability. The AVT Guidelines

(4) also provide similar information for cation

conductivity. The Guidelines for once-through units also present “normal” operatinglimits on cycle diagrams. Four additional action levels are also recognized:

Action Level 1

There is potential for the accumulation of contaminants and corrosion. Returnvalues to normal levels within 1 week.





3-9

Action Level 2

The accumulation of impurities and corrosion will occur. Return values tonormal levels within 24 hours.

Action Level 3

Experience indicates that rapid corrosion could occur, which can be avoided byshutdown of the unit within 4 hours.

Immediate Shutdown (for drum units)

This action level is related to low pH, without regard to boiler pressure. Levelsof pH below 8.0 in the boiler water require immediate shutdown to preventrapid boiler tube damage.

Drum Units

For further guidance, maximum annual exposure to contaminant conditions are givenfor both base-load and cycling units. The cumulative hours per year is a useful guidefor evaluating the operation of cycling units relative to water chemistry.

The following tabulation is an excerpt from the phosphate guidelines(3)

, and pertains to both PT, EPT, AVT and oxygenated treated units with and without reheat:

Maximum Annual Exposure to Contaminant Conditions

Cumulative Hours per Year

Targets Base Load Cycling

Normal – –

Action Level 1 336 672

Action Level 2 48 96

Action Level 3 8 16

Immediate Shutdown 1 2

During cycling operation, a substantial time is spent at low load operation. A review of the previously referenced curves of boiler water chemical concentration vs. pressureindicates that boiler water chemical concentrations can be higher at these lower boiler





3-10

pressures. This relationship provides a further guide to water chemistry during startupof drum units under PT, EPT, AVT, OT and CT. Chemical concentration curves forvariable pressures are included.

Once-Through Units

The maximum annual allowable exposure to contaminant conditions for once-through boilers on both all-volatile treatment and oxygenated treatment is the same as that fordrum boilers.

Cycle diagrams for once-through boilers detailing chemistry limits are included in theguidelines

(4,5). If the guidelines cannot be met, then a thorough review of chemistry

operations, sampling and monitoring capabilities must be made.

3.8 OPERATING PROCEDURES

Cycling and peaking operations require careful control to prevent the formation of corrosion products in the feedwater system. This can be accomplished with very closecontrol of out-of-service conditions especially the elimination of air ingress along withproper chemistry control assessed by good sampling and monitoring protocols.

Particular attention of operators needs to be given to mixed-metallurgy feedwatersystems. Here it is extremely important that a reducing environment is present duringall periods of operation and shutdown. The reader is referenced to the latestinformation on copper based alloys in the feedwater system

(13).

Retrofit of a by-pass cleanup system with a condensate polisher is considered wellworth the expense as it shortens the startup (with fuel savings) as mentioned before bymaking it possible to meet proper water chemistry conditions at acceptably low levelsof corrosion product transport. The means of justifying retrofitting condensatepolishers into fossil plants has recently been consolidated

(2,14).

3.9 ENVIRONMENTAL CONSIDERATIONS

The effects of increased oxide generation removed by draining and increased

blowdown, disposal of layup chemicals, and increase in removal of volatile chemicalsthrough deaerator lines and condenser deaeration require special attention for theirhandling and disposal. They must be addressed in relation to local and nationalenvironmental regulations. Reduced control of corrosion product transport willincrease chemical cleaning frequencies with associated downtime and waste disposalcosts.





3-11

3.10 REFERENCES

1. ASME Handbook On Water Technology for Thermal Power Systems. ASME, UnitedEngineering Center, East 47

th Street, New York, NY, 10017.

(a) Chapter 22 Postoperational Treatment, Lay-up, and Flushing

(b) Chapter 13 In-Cycle Processing Principals and Equipment

2. Condensate Polishing Guidelines. TR-104422. Palo Alto, Calif.: Electric PowerResearch Institute, September 1996.

3. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Treatment for Drum Units. TR-103665. Palo Alto, Calif.: Electric Power Research Institute, December 1994.

4. Cycle Chemistry Guidelines for Fossil Plants, All-Volatile Treatment. EPRI TR-105041,Final Report, April 1996.

5. Cycle Chemistry Guidelines for Fossil Plants, Oxygenated Treatment. EPRI TR-102285,Final Report, December 1994.

6. Guidelines for Make-Up Water Treatment. EPRI GS-6699, March 1990.

7. Membrane Filter Comparison Charts, available from Babcock & Wilcox - ServiceTechnology, P.O. Box 351, 20 S. Van Buren Avenue, Barberton, Ohio 44203-0351.

8. Guideline Manual on Instrumentation and Control. CS-5164. Palo Alto, Calif.: ElectricPower Research Institute, April 1987.

9. J. Brown and P. McSweeney. “Feedwater Line Corrosion”. Proceedings of the American Power Conference, Volume 39. 1977.

10. J. Brown and P. McSweeney. “Feedwater Line Corrosion”, Combustion, Volume 49,No. 2, August 1977.

11. Mathews, J. “The Importance of Start-up Chemistry to the Long-Term Reliability of Power Generating Equipment.” ASME Workshop on Improvement of ChemistryControl During Start-up of Fossil Units, St. Louis, Mo. April 9-10, 1997.

12. Sodium Hydroxide for Conditioning the Boiler Water of Drum-Type Boilers. EPRI, TR-105041, April 1996.

13. State-of-Knowledge of Copper in Fossil Plant Cycles. EPRI TR-108460, September 1997.

14. Cycle Chemistry Improvement for Fossil Power Plants. RP 2712-11, Palo Alto, Calif.:Electric Power Research Institute.





3-12




4-1

4

SHUTDOWN AND LAYUP CONSIDERATIONS

COMMON TO MOST UNITS

4.1 INTRODUCTION

Severe corrosion damage to all power plant cycle components has been experienced because of insufficiently protected metal surfaces during inactive periods. Examples of such damage include the following:

turbine blade and disk pitting;

boiler drum and tube, feedwater heater, and condenser pitting and oxidation; and

stress corrosion of condenser and feedwater heater tubing in stagnant oxygenatedwater.

Corrosion damage can occur on the water and steam-side surfaces as well as on other

surfaces, including pipe surfaces under insulation.

In addition to irreversible corrosion damage, the generation of excessive amounts of metal oxides and the contamination of layup water, if used, with oxygen and carbondioxide adversely influence water and steam chemistry during subsequent startup andoperation. One commonly experienced effect of corrosion damage during layup is aprolonged startup period. Other considerations of layup include its cost (chemicals,equipment, manpower) and the proper disposal of layup water when practicing wetlayup and using chemicals such as ammonia and hydrazine.

The shutdown and layup periods should be viewed as a continuum of the good

practices used during operation. The primary purpose of the cycle chemistry is toprovide protective oxide surfaces on all components throughout the steam and watercircuits; the primary purpose of the shutdown and layup periods should be to maintainthose protective surfaces. A couple of examples will illustrate the guiding principlesfor shutdown and layup:




Shutdown and Layup Considerations Common to Most Units

4-2

Use of OT with all-ferrous feedwater systems is to provide surfaces completelyprotected by FeOOH. Thus during shutdown and layup those surfaces should not be exposed to reducing chemistries by application of hydrazine or alternatives;

In contrast with mixed metallurgy feedwater systems, it is necessary to maintain

reducing conditions during operation so that cuprous oxide is the protective oxideof choice. Thus during shutdown and layup these surfaces should not be exposedto oxidizing chemistries.

Preventative measures for protecting steam cycle equipment during shutdown forinactive periods, including short-term and longterm layup, are presented in thissection. More specific layup considerations are found in subsequent Sections(phosphate treated units—Section 5; AVT units—Section 6; OT Units—Section 7; caustictreated units—Section 8)

4.2 LAYUP PRACTICES

The procedures for layup of idle equipment fall into two general categories: the wetand the dry procedures. In general, with the exception for units on oxygenatedtreatment (OT), wet layup requires filling of most of the system with an alkalinereducing solution (ammonia and hydrazine) and preventing air ingress bypressurization with an inert gas (nitrogen). Dry layup requires drainage while hot, andremoval of all water followed by pressurization with a moisture-free inert gas or by useof dehumidified air to maintain a low moisture environment. In selecting the properlayup procedure for a specific boiler or steam generator and its related equipment, onemust consider the following

(1):

the compatibility between the chemistry required for layup and that used duringoperation;

maintenance of the protective oxides formed during operation;

the possibility the boiler or steam generator may be required for operation on shortnotice;

facilities available for proper disposal of layup solutions;

the possibility of freezing;

a realistic assessment of the practicality of maintaining all the required conditions of a given procedure, i.e., complete dryness in dry layup or completely filled reducingconditions in wet layup (except for OT units);

local atmospheric conditions, e.g., salt air environment; and





4-3

the availability of sufficient high quality condensate, deaerated demineralizedwater, nitrogen or dehumidified air during a unit outage.

A number of guidelines and other information have been published relative toshutdown and layup and are available for review

(2-7).

A comparison of the advantages and disadvantages for various shutdown and layupalternatives is given in Table 4-1

(18).

Short-term vs. Longterm Layup

Current layup practices vary widely, from the protection of all cycle componentsduring longterm layup, to providing no protection to any component. There shouldnot, however, be any difference in the degree of corrosion protection provided during ashort-term or longterm layup. The most significant differences between the two are in

the cost of layup chemicals, layup preparation, and maintenance.

For short-term layup periods, the following shutdown procedures have proveneffective:

Maintain condenser vacuum and turbine seals to protect the condensate systemfrom air ingress.

Provide auxiliary steam to blanket the deaerator. If auxiliary steam is unavailable,pegging steam from an adjacent unit or from the drum should be provided. Forlonger outages, nitrogen blanketing may be more convenient.

Nitrogen or steam blanket the boiler. The nitrogen or steam inerting systemsshould be automatic with multiple injection points on the boiler, deaerator,superheater and feedtrain vents. A unit with frequent shutdowns requires a fastresponse system, thus indicating an automatic system.

For short outages, a turbine steam bypass system will permit the boiler to operate at alow firing rate while taking the turbine off-line. This procedure may result in a netenergy savings while facilitating rapid return to service once load demand increases.Several references

(8,9) can be consulted relative to these turbine bypass systems.





4-4

Table 4-1Shutdown and Layup Alternatives Showing Advantages and Disadvantages forEach Alternative

Advantages Disadvantages

Wet storage withammonia/hydrazinesolution*

1. No concern aboutrelative humidity

2. Easily maintained3. Easily tested4. With proper

installation, leaks caneasily be detected

5. Superheaters andreheaters may bestored safely

6. If facilities are

installed, solutionmay be reused

1. Possible pollutionwhen draining

2. Need to recirculateregularly

3. Hydrazine possiblecarcinogen

4. High waterconsumption prior tostartup; solution must be drained andpossibly rinsed

5. Regular monitoring6. Excessive ammonia

must not be added if copper or copperalloys are present inthe system

7. Tight isolations areprerequisite

8. Not recommended if freezing may occur

9. Draining if work is to

be carried out10. Pure water (demin)

must be used

Nitrogen

__________________*Requires nitrogen blanket

1. System need not becompletely dry

2. Completelyindependent of climatic conditions

3. May be used as acapping of normaloperating fluid duringoutages

1. Very dangerous;asphyxiation of workers if notproperly vented before access

2. Preferably to becarried out whilesystem is beingdrained





4-5

Dry air 1. Readily available basicconstituent

2. Maintenance on plant

performed withoutproblems3. Easy monitoring4. No risk to personnel5. Whole plant may be

stored dry if drainableor dryable

6. Independent of ambient temperature if air dry enough

7. Residual heat in boilersteelwork utilized for

drying

1. Drying equipment and blowers required

2. Climatic conditions

may cause rapiddeterioration in storageconditions

3. Hermetical sealingmay be required toprevent 2, above

4. System must becompletely dry

5. Sediment may causecorrosion if hygroscopic

6. SO2 and dust must be

excluded from the airused

7. If work to be carriedout on part of driedsystem, that part of system must beisolated and redriedafterwards

8. Even draining hot andunder pressure doesnot ensure complete

water removal





4-6

Some of the major advantages cited for turbine bypass systems are(10)

:

Flexibility of operation

Ability to hold the generator output during startup without undesirable turbine

cooling

Ability to recover following a load rejection before restarting or reloading theturbine

Ability to match turbine metal temperatures on hot restart

Some of the disadvantages cited for turbine bypass systems are:

Increased plant cost

Complexity of control

Additional valve maintenance

Possibility of turbine or condenser damage from malfunction or failure of bypasscomponents

Increase in plant heat rate because of greater condenser heat loss during periods of bypass system operation

Successful extended boiler layups have been accomplished using one of the followingoptions:

a wet layup with a pH of up to 10.0 achieved with ammonia, up to 200 ppm of hydrazine, and condensate-quality water plus a pressurized nitrogen blanket.(Note: The use of hydrazine is not recommended for units on oxygenated treatment.Refer to Section 7 for recommendations for OT units.) Lower level reducing agenttreatments (for example 5-10 ppm hydrazine) have been found to be successful andallow a quick return to service (See example in Section 4.7 and Step 6 in Section 4.8).High concentrations of ammonia should be prevented from coming into contactwith copper alloys.

a wet layup with treated good quality boiler water of the same chemicalcomposition as that used during operation; or

a dry layup in which a hot boiler is drained and purged with nitrogen ordehumidified air.





4-7

Good experience has been reported(12)

for the second variant of wet layup of drum boilers, utilizing a nitrogen blanket while maintaining the boiler water at the samecomposition as during operation, without the need for the addition of a reducing agent.With this procedure the individual boiler design must be carefully considered whendetermining the number and location of nitrogen feed points: no boiler part at any time

should be exposed to vacuum. The multiple nitrogen feed points for this particularapplication are illustrated in Figure 4-1; one feed point is not considered sufficient.

Waterwalls

Economizer

Superheater

N2

Economizer

Superheater

N2

Superheater

N2

Waterwalls

Figure 4-1 Nitrogen blanketing of a drum boiler showing the nitrogenconnections

(12).





4-8

4.3 WET LAYUP(12-15)

Wet layup is a popular method of “protecting” a unit when it might have to bereturned to service on relatively short notice. It generally involves filling the unit with

demineralized water containing an excess of a reducing agent (oxygen scavenger).Depending on design, the oxygen scavenger may be eliminated assuming a viablenitrogen blanketing system is available. Circulation may be maintained, a head tankmay be used, or positive nitrogen pressure may be maintained throughout theshutdown with water at normal operating levels. Wet layup can generally be used forrelatively short periods of up to 6 months although longer idle times may beexperienced.

Extensive use of nitrogen blanketing is recommended in conjunction with wet storage,not only for the boiler, but also with other heat cycle components. Excellent layupprotection has been reported using a bulk nitrogen system comprised of liquid nitrogenstorage and provided with evaporators to convert the liquid nitrogen to gas.

A nitrogen cap:

Allows boiler and feedwater equipment to remain full

Requires no excessive addition of chemicals

Permits nitrogen to rush in when steam collapses, preventing oxygen from enteringthe system.

The following procedures are utilized with a bulk nitrogen system:

Main Condenser and Turbine

Nitrogen addition starts while the turbine is still spinning down

Nitrogen is added quickly at first, then slowly as the vacuum approaches zero (Thecondenser is the largest user of nitrogen)

Deaerator and Storage Tank

Nitrogen is added when the deaerator is still hot

Nitrogen is purged for about 20 minutes followed by the maintenance of a smallnitrogen positive flow





4-9

Feedwater Heaters

Nitrogen is supplied through a shell-side vent line

Steam Drum

Nitrogen enters the drum through vent lines

Nitrogen feed is started while the drum is still hot.

During wet layup, the oxygen scavenger concentration and the ORP must bemonitored. Also, the boiler and economizer waters should be circulated routinely toprevent stagnant conditions developing.

Corrosion in the form of pitting frequently occurs under wet layup conditions due topoor circulation of the treated water or failure to maintain a positive nitrogen pressure.

More serious is the fact that cracking has been found associated with welds in someunits. The cracking is the result of a corrosion fatigue mechanism similar to that foundin deaerators. The corrosion occurs during the layup period, with cracking followingsoon after startup. Cracking has occurred around nozzles, particularly in the steamdrum but has been found, to a lesser extent, on the head to shell welds as well.

The use of a nitrogen cap, as outlined above, improves startup chemistry, reduceslayup corrosion, reduces boiler tube deposits and lengthens the time between chemicalcleanings.

Because nitrogen gas does not support human life, safety issues are very important.Therefore, before any equipment that has been laid-up with nitrogen can be entered bypersonnel, all nitrogen supply lines must be disconnected, the equipment purged withair, and oxygen levels verified as safe by proper oxygen test procedures.

4.4 DRY LAYUP USING DEHUMIDIFIED AIR

The use of clean, dehumidified air to purge the boiler and auxiliary equipment duringlayup periods is routinely practiced internationally, and is gaining in popularity in theUS for both long- and short-term layup periods.

The justification for the use of dehumidified air to protect ferrous surfaces is depictedgraphically in Figure 4-2, which is a plot of corrosion rate vs. humidity of air. Thisgraph illustrates that corrosion can be mitigated by maintaining air in contact withcorrosion prone surfaces at a relative humidity of 60% or less.





4-10

Humidity of the air (percent)

80400

C o r r o s i o n R a t e

6020 100

Figure 4-2 Corrosion Rate of Steel Relative to Humidity of Air

A desiccant dehumidifier commonly used for layup of boilers and auxiliaries, is shownin Figure 4-3

(17). The dehumidifier consists of a wheel of ceramic material that has been

corrugated, so air can pass lengthwise down the fluted corrugations. A desiccant isimpregnated into the structure. Moisture is attracted from the air onto the desiccant asthe air passes through the wheel.

Other dehumidifier components include two fans, one each to pull the process andreactivation air streams through the wheel, a drive motor to turn the wheel, and a

heater to warm the reactivation air so it can dry the desiccant. Finally, an electricalcontrol panel coordinates the operation of the fans, drive motor, and heater.

The wheel rotates slowly between two air streams (about one revolution every 10minutes). The first air stream, called the process air, is dried by the desiccant. Thesecond air stream, which is heated and runs through the wheel in the other direction, iscalled the reactivation air. Reactivation air transfers heat to the wheel, heating the





4-11

desiccant to remove and carry away its moisture so the desiccant can be reused tocollect more moisture from the process air.

The power system components must be made as air tight as possible. The dry aircirculation systems are then sized to provide

ten air changes per hour for water/steam-side components,

one air change per hour for flue gas-side components, and

five to ten air changes per hour for gas turbine components and generatingequipment.

If the installation does not allow the systems to be air-tight, larger values are used. If the systems are exceptionally tight, smaller values can be used, or only a portion of thecirculating air can be processed through the dehumidifier.

Dry air outlet

Reactivation airinlet filter

Reactivationair heater

Dry air fan

Desiccant wheel

Wet airoutlet

Reactivationfan

Reactivationsector

Humid airinlet filter

Figure 4-3 Rotary Desiccant Dehumidifier(17)

The system utilized to supply dry air to the various components of the feedwater,steam and boiler circuits may be customized to adapt to various heat cycleconfigurations. One example is the flow diagram depicted in Figure 4-4

(18).





4-12

Boilers

Feed heating plant

LPheaters

Feedpumps

HPheaters

Dehumidifier installed insystem to dry out andcirculate dehumidifiedair to control relative

humidity <30%

Condenser

LPturbine

HPturbine

Figure 4-4 Block Diagram of Dehumidifier Steam/Feed Cycle(18)





4-13

Another variation is shown in Figure 4-5(19)

. The flow path for Figure 4-5 can bedescribed as follows:

Dry air is discharged from the dehumidifier (DH) into the hotwell, and then flowsthrough the low pressure turbine and continues through all turbine sections to the boiler, backward with respect to steam flow. Dry air flows through the feedwater sideof the heaters and is discharged out of the system, back to the DH. Condensate pumpsreceive dry air from the hotwell and discharge it back to the DH from the dischargecheck valves. Extractions are left open so dry air can reach the feedwater heaters, fromwhich air is returned to the DH. Drip pumps and crossover heaters are protected in thesame manner. Dry air is extracted from each waterwall header and returned to the DH.

Air moisture levels should be checked as air enters and as it exits the reheat section.Two humidistats are installed in the return plenum of the steam side DH. They should be set to turn the DH reactivation heaters and blower off when returning air humidity

decreases to 15% and turned on when it increases to 25%. (The percentage of time theheaters stay off is a function of ambient humidity; the approximate on time is about40%.)

Condpumps

XO HP IP

BFP

LP#3 LP#2 LP#1

Gland

cond

BFP

LIP

Boiler

Hotwell

HP turbine IP turbine LP turbine

Steamside D.H.

unit

S.H. drain

Feedwaterheaters

Process air discharge

Process air return

Figure 4-5 Steamside Dehumidification Flow(19)





4-14

One report(26)

notes that it is difficult to dry a system with “hanging” superheaters(vertical tubes with bends) by the use of dehumidified air circulation. The samedifficulty is noted for non-drainable headers or connecting lines. The following dryingprocedure was recommended for these instances:

Dry the systems by utilizing the standard vacuum equipment supplied with thegenerating unit. The use of additional heating (operating the steam-heated air heater)facilitates drying during the vacuum process. Vacuum drying is reported to becomplete within 10-36 hours, depending upon the unit. It is important that the vacuumdoesn’t suck in any fireside environment (flyash and SO2)through small leaks, whichcould lead to corrosive acids and salts.

Another customization(20)

involves blowing dry air through the turbine and boiler in theopposite direction of normal steam and water flow. The air is dried by using acommercial rotary-type air dryer (see Figure 4-3) capable of delivering a maximumflow of 2250 scfm at a pressure of 13 inches (33 cm) of water.

The relative humidity is below 60% in less than 20 hours and less than 30% in 36 hours.To ensure effective dehumidification, the boiler is flash drained at 250 psig (1.7 MPa)drum pressure. Draining at 250 psig (1.7 MPa) pressure prevents condensation in thesecondary superheater and reheater U-bends in the hanging pendant sections.

Dehumidified air is discharged into the LP turbine as soon as the boiler steam drumreaches atmospheric pressure. All turbine valves necessary to allow air flow throughthe turbine steam cycle are opened. Low pressure turbine extraction piping andheaters are dehumidified through the normal extraction piping, in the normal direction

of steam flow. Low point drains on the shell sides of the heaters are opened to facilitateair flow.

The humidity is monitored at several locations to determine the status of thedehumidification process. Relative humidity data indicates an adequate passivation inmost areas of the boiler-turbine cycle after 48 hours.

Additional applications of the use of dehumidified air for layup can be found in theliterature

(21-24).

4.5 FEEDWATER HEATERS, CONDENSER, REHEATER AND TURBINE

Particular care must be exercised during shutdown and layup not only for the boiler, but also for the remaining components of the heat cycle. Some considerations for theremainder of these components follow.





4-15

These components are generally considered as a group, since they cannot be isolatedwithout special facilities being incorporated. These components are generally storeddry. The reheater may be stored wet (for very longterm storage) when isolated fromthe turbine (see later discussion of Figure 4-8), however a better practice is to store thereheater dry, as wet conditions dissolve any salt deposits, leading to off-load corrosion

and pitting.

During major outages some utilities conduct a reheat soak with demineralized water todissolve any deposited salts. The process can be repeated until acceptable contaminantlimits are reached.

Turbine

An example of dry layup of a 515 MW turbine is shown in Figure 4-6(23)

. It is necessaryto preclude any steam ingress into the laid up turbine by installing additional vents

and drains (with a 8 in. (200 mm) siphon). The turbine has to be equipped withadditional connection points for dry air or venting. In this case

(23), two air changes per

hour were sufficient for the steam turbine and condenser. In another example of drylayup of a 107 MW turbine, Figure 4-7 shows the values of temperature and airhumidity when using two air dehumidifiers (one with 1.1 kW and one with 5.4 kW).

Turbine dry layup using dehumidified air can also be combined with dry layup of theunit steamside circuits. Figures 4-4 and 4-5 show examples.

H.P. I.P. L.P. 1 L.P. 2 L.P. 3

Condenser

ClosedFreshair

Freshair

Airdrier Air

drier

Non-returnflap removed

Heading line

Manhole





4-16

Figure 4-6 Turbine dry layup using dehumidified air(23)

35.5/18.2°Cφ = 16%

I.P. LP

37.0/18.3φ = 14%

30.7/16.0°Cφ = 20%

Air drier1.1 kW

29.7/18.5°Cφ = 35%

48.8/20.8°Cφ = 5%

Air drier5.4 kW

23.8/16.5°Cφ = 47%

56.5/22.1°Cφ = 2%

18.2/10.2°Cφ = 33%

18.3/10.2°Cφ = 33%

21.2/10.8°Cφ = 25%

H.P.

Figure 4-7 Dry layup of 107 MW turbine showing measured values of temperature(°F/°C) and air humidity

Feedwater Side of Condensers and Feedwater Heaters

The metallurgy of these components must be carefully considered when establishingthe feedwater chemistry for intermediate or long term storage. A recent EPRIpublication

(15) provides guidance for proper chemistry for mixed metallurgy systems.

For systems containing copper alloys, it is most important to maintain a reducingenvironment (ORP < 0 mV) at all times to prevent excessive corrosion of the copperalloys. This is generally accomplished by the use of hydrazine during layup.Ammonia additions must be reduced to provide a pH of 9.0-9.2, and oxygen ingressmust be avoided. Hydrazine concentrations should be maintained at around 40-50ppm for this application

(16).

Shell Side Feedwater Heaters

The shell sides of feedwater heaters should be protected by a nitrogen blanket or asteam blanket (short term layup) when the unit is out of service.

Superheater

If the superheater is stored wet, then it should be back filled with treated water of acomposition identical to that used for layup of the boiler. A nitrogen cap should beused to prevent air ingress.





4-17

Deaerator and Storage Tank

The deaerator and deaerator storage tank should be protected by a steady, smallnitrogen purge.

4.6 LAYUP MONITORING

All layup conditions, dry or wet, should be either continuously or periodicallymonitored to ensure that the layup water or air quality is maintained. Existing samplepoints may be used to draw water samples for chemical analysis. Should the layup andwater chemistry deteriorate to corrosive conditions in a particular component, thatcomponent should be drained and refilled with properly conditioned water, oradditional chemicals added, assuming proper mixing can be provided.

4.7 ENVIRONMENTAL CONSIDERATIONS

The disposal of layup solutions containing high concentrations of alkaline chemicalssuch as ammonia, and/or high concentrations of reducing agents, such as hydrazine orhydrazine substitutes, poses problems from an environmental standpoint. Solutions tothese problems may require modifications to existing waste treatment facilities. Thecooperation of regulatory authorities should be a part of the investigative processnecessary to resolve these environmental issues.

A low level chemical layup procedure(25)

has been used to protect the environment bynot requiring draining of drum boilers prior to startup. This has been possible through

the use of adequate layup monitoring and an efficient nitrogen blanketing system. Theprocedure is as follows:

With the boiler off-line, inject an oxygen scavenger at 5-10 ppm hydrazine equivalentinto the boiler when the boiler pressure decays to 200 psi (1.4 MPa) (typically 3 days).The chemical injection is made using the normal chemical feed system. Natural boilercirculation at 200 psi (1.4 MPa) is sufficient to mix adequately the chemicals with the boiler water.

When the boiler decays to 5 psi (0.03 MPa) pressure (typically 7 days) a nitrogen cap isapplied to the boiler.

Upon return to service, the boiler is fired, without draining the layup solution, and thedrum vents are opened until 25 psi (0.2 MPa) is reached to remove excess ammoniafrom the system.

Since it typically takes 3 days before the pressure decays to 200 psi (1.4 MPa), weekendoutages normally do not require a chemical injection treatment.





4-18

4.8 ROAD MAP FOR SHUTDOWN AND LAYUP

Figure 4-8 provides a generic road map for implementing shutdown and layupprocedures common to most units. Because of variations in design, some generatingunits may require deviations to Figure 4-8, or may require customization to adapt to a

particular utility’s needs. Also please refer to Sections 5, 6, 7 and 8 for proceduresspecific to phosphate, AVT, OT and CT respectively.

Fill feedwater systemwith 200 Hydrazine*10 ppm Ammonia**

Fill boiler with 10 ppmAmmonia; and up to200 ppm Hydrazine*

Maintain feed waterwithout change

Add Nitrogen capwhen boiler pressure

decays to 5 psi(0.03 MPa)

Add 5-10 ppmHydrazine when boiler

pressure decaysto 200 psi (1.4 MPa)

Back fill superheater200 ppm Hydrazine*

10 ppm ammonia.Nitrogen cap

Wet layup

Traditional Low O2 scavenger

Add Nitrogen to deaeratorand storage tank while stillhot.Maintain slow N2 flow

Add Nitrogen to condenserwhile turbine spins down.Maintain slow N2 flow.

Maintain Nitrogen capon shell side of

feedwater heaters

Very long termstorage

Isolate reheater

Backfill reheater andsuperheater with 200ppm Hydrazine*10 ppm AmmoniaNitrogen cap

For maintenance:purge with air all N2

from equipment to bemaintained.Test to ensure safeenvironment.

Short term shutdown

Maintain condenservacuum and turbine

seals

Inert the deaeratorand heater shells

Inert the boiler withautomatic system

Maintain chemicallimits per guidelines .See sections 5, 6, 7,and 8

Evacuate reheaterwith condenser

vacuum

Break reheatervacuum with

Nitrogen purge

Drain condenserunder Nitrogen

Intermediate andlong term

Dry layup

Dry air

Drain system toremove all water

Follow Figures 4-4 and4-5 or customize

Maintain smallNitrogen flow through

condenser, turbineand deaerator

Pressurize with N2 allwetted parts

Drain system to

remove all water

Yes

Step 8

Step 9

Step 7

Step 6

Step 2

YesNo

Yes

Yes

NoNo

Yes

Yes No

Yes

Step 3 Step 4

Step 1

Notes: *No Hydrazine for OT units**Limit pH to 9.0 to 9.2 if units

have copper alloys in cycle;maintain Hydrazine at40-50 ppm

Yes

Step 5

No

Establish boilerNitrogen cap of 5 psi

(0.03 MPa)

Figure 4-8 Road Map to Develop Shutdown and Layup Guidelines Common toMost Units

Notes: * No hydrazine for oxygenated units**Limit pH to 9.0 to 9.2 if units have copper alloys in cycle;

maintain hydrazine at 40-50 ppm





4-19

Figure 4-8 is divided into 9 considerations, options or steps, which are furtherdescribed as follows:

Step 1—Short-Term Layup

Short-term layup presumes that the unit will be required to operate within a relativelyshort timeframe. In consideration of this, no major changes are required from normaloperating conditions, with the exception being that the unit must be protected from airingress. The condenser vacuum and turbine seals are maintained; the deaerator, heatershells and boiler are inerted with nitrogen or steam; and, the feedwater chemistry ismaintained according to the requirements of the treatment philosophy employed (seephosphate treatment—Section 5; AVT—Section 6; oxygenated treatment—Section 7;and caustic treatment—Section 8).

Step 2—Intermediate and Longterm Layup Common to Dry and Wet Layup

Intermediate and longterm layup require additional steps to be taken to preventcorrosion during intermediate periods of layup, such as for maintenance andindeterminate cycling or peaking requirements; and during longterm layup forindefinite periods of time. Certain procedures are common regardless of whether theunits are to be laid up dry or wet. There is a danger of off-load corrosion (pitting) if there are salts present. Consideration needs to be given to whether the reheatersupports can take the weight if the reheater is to be filled with water.

During shutdown, the turbine, condenser (steam side) and reheater are generallyconsidered together because, unless special facilities are incorporated, there is no

practical way to isolate them. With special facilities incorporated, the reheater can beisolated from the turbine and may be stored wet (see Step 8). The turbine, however,can only be laid up dry as indicated previously in this section. The condenser may beflooded and laid up wet, but several factors limit the feasibility of this procedure

(10):

The condenser must be supported from the bottom to handle the extra weight of aflooded condenser.

The expansion joints between the turbine and condenser are not designed tosupport the weight of a flooded condenser.

A flooded condenser tends to pull the turbine bearings out of alignment.

Water in the condenser may cause moisture vapor to enter the turbine causingcorrosion.

For these reasons, the steam side of the condenser is normally laid up dry.





4-20

For Step 2, during shutdown, the reheater is evacuated by utilizing the vacuum in thecondenser. The vacuum is then broken using nitrogen pressure. The condenser isdrained under nitrogen.

If the turbine steam is supplied from a header system, all valves must be tight to

prevent moisture entrance into the turbine.

Step 3—Dry Air Layup

The dry air layup procedure requires that all components of the system be drained.There are several methods of maintaining dry air flow through the equipment, andsome of these are illustrated in Figures 4-4 and 4-7. The advantages and disadvantagesof dry air layup and the equipment required have been previously described in thisSection (Table 4-1).

Step 4—Dry Layup with Nitrogen

Nitrogen can be used for blanketing equipment, which is drained but not completelydry, or for blanketing equipment either filled with water or not, to prevent air ingress.

Step 4 is similar to Step 3, except nitrogen is used for a positive pressure on allcomponents rather than a dry air purge. A small continuous purge of nitrogen isrequired, however, to protect the turbine, deaerator and deaerator storage tank.

Step 5—Wet Layup: Traditional Method (Boiler and Feedwater Heaters)

The traditional method of wet layup involves filling the boiler, feedwater cycle andsuperheater with demineralized water containing a volatile alkaline, reducing solution.Up to 10 ppm of ammonia and up to 200 ppm of hydrazine have been used for thispurpose. However, for units on oxygenated treatment, hydrazine should beeliminated.

For those units having copper alloy condenser tubes and/or feedwater heaters,feedwater pH should be limited to 9.0 to 9.2 by reducing ammonia. Hydrazine is heldat 40-50 ppm. For these units, it is most important to maintain reducing conditions(ORP < 0 mV) to prevent increased attack of the copper alloys associated with a changeof the surface oxide layers from cuprous to cupric oxide.

(15).

A nitrogen cap of 5 psi (0.03 MPa) is maintained on the boiler.

Step 6—Wet Layup: Low Oxygen Scavenger Method (Boiler and Feedwater Heaters)

Many alternatives to the traditional wet layup method have been used successfully bymarkedly reducing the level of hydrazine. These procedures may not require boiler





4-21

draining prior to startup, thereby protecting the environment and not requiring specialdisposal techniques or permits. In one procedure, 5 to 10 ppm of hydrazine equivalentis injected into the boiler when the boiler pressure decays to 200 psi (1.4 MPa) (typically3 days). The chemical injection is made using the normal chemical feed system.Natural boiler circulation at 200 psi (1.4 MPa) is sufficient to mix adequately the

chemicals in the boiler water. No other changes need be made to the boiler or to thefeedwater chemistry. When the boiler pressure decays to 5 psi (0.03 MPa) (typically 7days) a nitrogen cap is applied to the boiler.

Upon return to service, the boiler may be fired without draining the layup solution.Also, since it typically takes 3 days before the boiler pressure decays to 200 psi (1.4MPa), weekend outages normally do not require a chemical injection treatment.

Step 7—Wet Layup (Balance of Cycle)

As the turbine spins down, nitrogen is added to the condenser, which also purges thereheater system. A continuous purge of nitrogen is necessary to account for leakagethrough the turbine steam seals.

Nitrogen is added to the deaerator and storage tank while this system is still hot. Asmall flow of nitrogen is provided to purge this system.

A nitrogen cap is maintained on the shell side of the feedwater heaters. Thesuperheater is back filled with a solution of up to 200 ppm of hydrazine and 10 ppmammonia and a nitrogen cap maintained. (Units on oxygenated treatment willeliminate the use of hydrazine for this application.)

Step 8—Very Long Storage

For very long storage periods, the reheater is isolated from the turbine, and back filledwith a solution of up to 200 ppm hydrazine and 10 ppm ammonia, and capped withnitrogen. The superheater is treated similarly.

Step 9—Maintenance

Only equipment requiring maintenance should be drained (wet storage only), andnitrogen (if used) purged with air to provide an environment suitable for entry of

personnel. Since nitrogen does not support human life, it is extremely important thatnitrogen is completely displaced by air. The atmosphere within the equipment to bemaintained should be tested with suitable test equipment to ensure the equipment issafe for personnel entry.





4-22

4.9 REFERENCES

1. The ASME Handbook on Water Technology for Thermal Power Systems. AmericanSociety of Mechanical Engineers, New York, N.Y. 1989.

2. VGB Guidelines. “Preservation of Power Plant Systems.” VGB-R116H. VGBTechnische Vereiniging der Grosskraftwerksbetreiber, 1981.

3. EPRI Guidelines. Long-Term Layup of Fossil Plants. CS-5112. Palo Alto, Calif.:Electric Power Research Institute, April 1987.

4. CEGB: “Long Term Storage of Power Plants.” General Considerations andPreservation Techniques. April 1978.

5. “Guidelines for the Long-Term Storage of Power Plants. ESKOM. Ref. NWG7021.

August 1991.

6. Monitoring Cycle Water Chemistry in Fossil Plants, GS-7556, Volume 2. Palo Alto,Calif.: Electric Power Research Institute, October 1991.

7. Cycle Chemistry Improvement for Fossil Power Plants. Electric Power ResearchInstitute, Palo Alto, Calif.: TR-104422, September 1996.

8. D. D. Rosard and T. McCloskey. “Bypass Systems Increase Cycling Capability of Drum Boilers”, Power, July, 1984.

9. J. Reasons. “Steam Bypass Systems for Drum Boilers. How Much Capacity Do YouNeed”, Power, July, 1984.

10. D. B. DeWitt-Dick. “Protection of Utility Steam Generating Systems During IdlePeriods”, A.S.M.E. Workshop “Improvement of Chemistry Control During Startupof Fossil Units”, April 9

th and 10

th, 1997, St. Louis, Missouri.

11. “Combustion Fossil Power”, J. Singer, Editor. Published by CombustionEngineering, Inc./ASEA Brown Boveri, 1991.

12. A. Bursik and R. Richter. “Hints for the Steam Generator Layup Practice” (inGerman). VGB Kraftwerkstechnik Vol. 60, No. 9, pp. 714-718.

13. R. J. Twigg. “Mothballing—The Impossible Solution?” Fossil Plant Layup andReactivation Conference. EPRI TR-101250, October 1992.





4-23

14. W. Turowski and D. Daniels. “Routine Use of Nitrogen Caps”, A.S.M.E. Workshop“Improvement of Chemistry Control During Startup of Fossil Units”, April 9

th and

10th, 1997, St. Louis, Missouri.

15. State-of-Knowledge of Copper in Fossil Plant Cycles. TR-108460, Palo Alto, CA: Electric

Power Research Institute, September 1997.

16. S. R. Pate and R. C. Turner. “Minimizing Corrosion Product Transport at GeorgiaPower.” ASME Workshop, St. Louis, MO, Apr. 9-10, 1997.

17. D. Kosar. “Power Plant Preservation Using Desiccant Dehumidifiers.” Fossil PlantLayup and Reactivation Conference. EPRI TR-101250, October 1992.

18. J. Jenkins and T. Moss. “The Storex Project.” Fossil Plant Layup and ReactivationConference. EPRI TR-101250, October 1992.

19. D. B. Griffin and H. D. Thomas. “Fossil Plant Layup and UnanticipatedReactivation.” Fossil Plant Layup and Reactivation Conference. EPRI TR-101250,October 1992.

20. M. E. Walker. “Passivation of Unit 3 State Line Station Through Dehumidification.”Fossil Plant Layup and Reactivation Conference. EPRI TR-101250, October 1992.

21. T. Gostenkors. “Layup of Units in Power Station Gersteinwerk” (in German). Der Maschinenschaden, Vol. 49, No. 6, p. 236ff, 1976.

22. W. Kahlert, “Fast Startup Procedure for Standby Units in the VEW Power Station

Gersteinwerk” (in German). VGB Kraftwerkstechnik, Vol. 52, No. 5, p. 425ff, 1972.

23. H. Steger. “Standby Corrosion Prevention in Power Plants” (in German). Der Maschinenschaden, Vol. 49, No. 1, pp. 23-27, 1976.

24. T. H. Pike. “Corrosion Prevention of Turbines During Extended Outages (CaseHistories.” Proceedings of the 48th International Water Conference, Pittsburgh, PA, No.2-4, 1987.

25. W. H. Stroman and N. L. Rentle. “Declining Pressure Method for Boiler Storageand Boiler Cleanliness Assessment by Ultrasonic Technique at San Diego Gas andElectric’s South Bay Unit 4”, Fossil Operations and Maintenance InformationServices, Clearwater Beach, Florida, June 15-18, 1992.

26. VGB Guidelines, “Layup of Power Plants”, VGB-R116H, VGB KraftwerkstechnikGmbH, Essen, 1983.






5-1

5

PHOSPHATE TREATED DRUM UNITS

5.1 INTRODUCTION

The utilization of phosphate salts for internal boiler water treatment is more than 70years old. During this time there have been several philosophies developed relative tothe proper use and proper concentration of phosphates in the boiler drum. Also, therehave been problems reported with these various treatments, notable of which are

phosphate hideout, caustic gouging, hydrogen damage, and “acidic phosphate” underdeposit corrosive attack. These problems have been related to the older treatmentmethods of coordinated phosphate treatment and congruent phosphate treatment, asdepicted in Figure 5-1.

To mitigate these problems, two new phosphate treatment approaches have beenidentified

(1) as phosphate treatment (PT) and equilibrium phosphate treatment (EPT), as

depicted in Figure 5-2. For PT, the treatment philosophy involves broadening of thecontrol range above the sodium-to-phosphate 2.8 molar ratio curve, and allowsoperation with up to 1 ppm of free sodium hydroxide. For EPT, the treatmentphilosophy involves operations at or below phosphate levels which would lead tohideout. A comparison of PT, EPT and the more familiar congruent phosphatetreatment (CPT) is shown in Figure 5-2.

The phosphate guidelines(1)

require a lower feedwater pH for mixed metallurgysystems than for all-ferrous systems. Also, while there are reports of successfuloperation of all-ferrous systems with reduced or even no hydrazine in the feedwater

(2),

it is most important to provide reducing conditions (ORP < 0 mV) at all times in mixedmetallurgy systems

(3) including the shutdown periods. Reducing conditions in mixed

metallurgy systems will prevent excessive corrosion of copper alloys. The mostcommon method of ensuring reducing conditions is through the use of hydrazine in the

feedwater cycle.

The differences between all-ferrous and mixed metallurgy feedwater systems aretreated comprehensively in terms of startup, shutdown and layup in the drum unit(Sections 6.3 and 6.4) for AVT. The reader is referenced to these sections for furtherinformation which are also directly applicable to phosphate treated units




Phosphate Treated Drum Units

5-2

5.2 CURRENT NORMAL OPERATING GUIDELINES

Cycle chemistry guidelines for fossil plants operating with phosphate treatment haverecently been published by EPRI

(1). This publication provides chemistry guidelines for

the two phosphate treatments previously mentioned, PT and EPT. A road map has

been included in the Phosphate Guideline to allow utilities to develop the optimumtreatment for their units.

The new phosphate Guidelines present a series of curves for boiler waterconcentrations of sodium, chloride, sulfate and silica vs. boiler drum pressure whichare considered satisfactory for “normal operation,” consistent with longterm systemreliability. A series of three additional action levels, and an immediate shutdown levelare also presented, and are described in Section 3.

Cycle diagrams present chemistry target values both for comprehensive monitoringand for “core” parameters. “Core” parameters (Table 1-1) are defined as those samplepoints used for routine chemistry monitoring and control, as differentiated from thosesample points which may be added for troubleshooting and/or plant commissioning.For example, the core parameter cycle chemistry diagram for EPT for reheat units andthe associated pressure related curves for sodium, chloride, sulfate and silica are shownin Figures 5-3 to 5-7. The diagrams for PT (both reheat and non-reheat) and theremaining diagrams for EPT (non-reheat) are available in the phosphate guidelines

(1).





5-3

a) Operating range of boiler water on coordinated phosphate treatmentppm PO4

E q u i v a l e n t N a O

H

C o n c e n t r a t i o n ( p

p m )

0 1 2 3 4 5 6 7 8 9 10

Na/PO4

3.0 (TSP)

4.0

3.0

2.0

1.0

0.4

0.3

0.2

p H

a t 2 5 ° C

10.0

9.0

9.5

8.5

b) Operating range of boiler water on congruentphosphate treatment

E q u i v a

l e n t N a O H

C o n c e n t

r a t i o n ( p p m )

4.0

3.0

2.0

1.0

0.4

0.3

0.2

10.0

9.0

9.5

8.5

p H

a t 2 5 ° C

0 1 2 3 4 5 6 7 8 9 10

ppm PO4

Na/PO4

2.6

Na/PO4

3.0 (TSP)

Figure 5-1 Older Forms of Phosphate Treatment





5-4

ppm PO4

1 3 4 5 6 7 8 92

p H

a t 2 5 ° C

E q u i v a l e n t N a O H

C o n c e n t r a t i o n ( p p m )

100

TSP + 1 ppm NaOH

Na: PO4 = 3.0

Na: PO4 = 2.8

Na: PO4 = 2.6

PT

CPTEPT

3.0

0.4

4.0

1.0

0.3

0.2

8.5

9.5

9.0

10.0

Figure 5-2 Schematic of Operating Ranges of Boiler Water on EquilibriumPhosphate Treatment (EPT), Congruent Phosphate Treatment (CPT) andPhosphate Treatment (PT)

(1). The CPT is shown to its maximum Na:PO

4 molar ratio

of 2.8; the normal operating range is below the Na:PO4 molar ratio of 2.6.



EPRI Licensed Material

5-5

Figure 5-3 Cycle Chemistry Diagram for a Drum Unit on Equilibrium Phosphate Treatment (Plants With Reheat)—Core Parameters Marked.







5-7

600 700 900 1100 1300 1500 1700 1900 2100 2300 2500 2700 2850

Drum pressure (psia)

S o d i u m ( p

p m N

a )

2400

Note: Use of phosphate treatment above 2400 psia shouldbe carefully evaluated during commissioning.

Normal

0.3

0.4

0.5

0.6

0.70.80.91.0

0.2

0.1

2

3

4

5

6

7

89

10

15

20

Action level 1

Action level 3

Action level 2

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 5-4 Equilibrium Phosphate Treatment: Boiler Water Sodium vs. OperatingPressure (Plants With Reheat)




5-8

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

Pressure (psia)

3.0

2.0

1.5

1300 1700700 15001100600 2500 28502100 270023001900900

C h l o r i d e ( p p m C

l )

Actionlevel 2

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

Normal

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 5-5 Equilibrium Phosphate Treatment: Boiler Water Chloride vs. OperatingPressure (Plants With Reheat)





5-9

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

Drum Pressure (psia)

3.0

2.0

1.5

1300 1700700 15001100600 2500 28502100 270023001900900

S u l f a t e ( p p m S

O 4

)

Actionlevel 2

Normal

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 5-6 Equilibrium Phosphate Treatment: Boiler Water Sulfate vs. OperatingPressure (Plants With Reheat)




5-10


1300 1700700 15001100600 2500 28502100 270023001900900

S i l i c a ( p p m S

i O 2 )

Actionlevel 2

NormalActionlevel 1

Action level 3

0.8

10

87

2

6

5

4

3

9

0.10

0.080.070.06

0.05

0.04

0.03

0.09

20

15

1.0

0.7

0.2

0.6

0.5

0.4

0.3

0.9

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 5-7 Equilibrium Phosphate Treatment: Boiler Water Silica vs. OperatingPressure (Plants With Reheat)





5-11

5.3 STARTUP PROCEDURES

Section 3 explains how the chemistry curves and action levels can be utilized duringunit startup. Basically, during startups, the initial lower boiler pressure permits boilerwater chemical concentrations to be higher than those at normal unit operating

pressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units as compared to baseloaded units.

A road map for startup of PT or EPT units is given in Figure 5-8.

No

Yes

Polishers?

Fire boiler

Monitor

Fire boiler.Open vents to 25

psi (0.17 MPa)(Section 4)

Reduce O2 to100 ppb

Fe to 100 ppb

Cu to 10 ppb

Proceed with

cleanup loop(Section 3)

Is system filled with chemicals per

PO4 guidelines

Is system filled with

low O2 scavenger(Section 4)

Maintaintemperature ramp

per boiler and

turbine MFG’s,requirements

Keep Na, SiO2,Cl and SO4 withinphosphate guide-

lines by controllingpressure andblowdown

Drain system

under nitrogenand fill per PO4

guidelines

Proceedwith startup

Step 6

Step 5

Step 4Step 4Step 3

No

Yes

Step 2

Step 3

Step 4

Step 3

No

Yes

Issystem

full?Step 1

Low oxygenscavengerSection 4

Fill systemper phosphate

guidelines

Achieve fullpressure and full

load

Step 4

No

Step 2

Step5

Step5

Yes

Figure 5-8 Road Map for Startup of PT or EPT Units




5-12

Road Map

The road map for startup of units operating with PT or EPT, Figure 5-8, can be dividedinto 6 steps:

Step 1—System Stored Dry

If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for PT or EPT units

(1).

Step 2—System Stored Wet: Low Oxygen Scavenger (Refer to Section 4)

If the system has been stored using the low oxygen scavenger procedure (Step 6 inSection 4.8), the boiler can be fired immediately. The boiler vents are kept open to 25psi (0.2 MPa) to remove any excess ammonia.

Step 3—System Stored Wet: Excess Ammonia and Hydrazine (Refer to Section 4)

If the system has been stored wet with a surplus of ammonia and hydrazine, it must bedrained under nitrogen and refilled with water meeting the PT or EPT Guidelines (1).(See Figure 5-3 for units operating with EPT). The startup of the unit can then proceed.

Step 4—Cleanup and Firing Boiler

The following limits for the boiler feedwater are required prior to firing the boiler:

Oxygen

100 ppb

Iron

100 ppb

Copper

10 ppb

During startup, the levels of corrosion products (iron and copper) can be very highinitially and silica may also be a problem.

Achieving the above limits is greatly facilitated through the use of condensate polishingand/or condensate filtration (See Section 3). When these limits are attained, the boiler

can be fired.

Attainment of the prefiring limits will be more difficult in units not equipped withcondensate polisher and/or filtration equipment. Best control will require closecompliance with the guidelines for unit shutdown and layup.





5-13

Step 5—Monitoring

After firing the boiler, monitoring must be fully implemented, both for chemistryparameters, and to ensure that temperature ramps are maintained according to theturbine and boiler manufacturer’s specifications.

During startup, the concentration vs. pressure curves can be utilized to control sodium,silica, chlorides and sulfates (for example see Figures 5-4 to 5-7 for EPT, and thephosphate guidelines for PT

(1)). Boiler pressure should remain at reduced levels such

that these limits are maintained before pressure can be increased to the next stage.Maximum use of blowdown and condensate polishing (if available) will minimizestartup times.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or, most likely, condenser leakage. The source must be found andthe problem corrected at once. Excursions affecting (lowering) boiler water pH must becorrected immediately by feeding trisodium phosphate or 1-2 ppm of caustic.

Effects of cycle contamination are magnified at startup due to relatively low flow ratesfor condensate, feedwater and steam. Cation conductivity may increase as a result of air ingress due to either aeration of water during the shutdown period or air in-leakageduring startup. The change to boiler water chemistry will be minimal compared tocontamination involving the makeup system, chemical cleaning activities, condensatepolishers or condenser leaks.

Step 6—Full Load

Full load can be achieved when chemical limits are within PT or EPT Guidelines(1)

.

5.4 CYCLING AND PEAKING UNITS

Cycling and peaking units have been previously defined (Section 3). These units aresubject to frequent startups and shutdowns, with generally short-term layupprocedures practiced, when required (see Section 4).

5.5 SHUTDOWN PROCEDURES

A road map for shutdown of PT or EPT units is presented in Figure 5-9. Shutdownshould be closely related to layup (Section 4) which in turn depends on the anticipatedoutage length. Based upon this road map, shutdown should proceed in the followingmanner:




5-14

Yes

Chemical transient

No

No

Yes

Step 1 YesStep 1

Proceed withshutdown for

short term layup(Section 4)

Estimate outagelength - proceed with

short to long termlayup after adjusting

system chemistry(See Section 4)

Isolate leak andrepair - usually while

system continuesto operate

Orderly shut down

with polisher -immediate without

Adjust boilerpH to >8.0

Orderly shutdownwith polisher

Immediateshutdown without

polisher

Normal cycling orpeaking loadreduction orshutdown

Planned outage

Condenser leak,freshwater

Sea water?

Chemicalintrusion

Yes

Unplanned outagesystem or

equipment failure

No

No

Yes

No

Yes

No

Yes

Step 2

Step 2

Step 3

Step 3

Step 4

Step 5 Step 5

Step 5

Step 5

Step 2

Step 3

Figure 5-9 Road Map for Shutdown of PT or EPT Units(This should be used in conjunction with Figure 4-8.)





5-15

Road Map

The road map for shutdown for PT and EPT units, Figure 5-9, should be used inconjunction with Figure 4-8, “Road Map to Develop Shutdown and Layup GuidelinesCommon to Most Units.” Figure 5-9 can be divided into 5 steps:

Step 1—Normal Cycling or Peaking: Load Reduction or Shutdown

Normal cycling or peaking load reduction or shutdown presumes a short term layup asdescribed in Section 4.

An orderly reduction of load can be performed for routine cycling operations. Also, anorderly reduction of load or unit shutdown can be performed when contamination isminimal, as determined by chemical monitoring during such transients.

During orderly load reductions, the condensate cycle and boiler chemical limits should

be adjusted to conform with the phosphate chemistry guidelines(1) for PT or EPT, asapplicable. (Layup procedures are covered in Section 4 of this document.)

Particular care should be exercised to prevent oxygen ingress during this period, and blowdown should be maintained at an appropriate level to remove contaminants fromthe system.

During shutdown, any indication of phosphate hideout return (increased levels of phosphate, change of pH, etc.) indicates that the boiler has not been operating at theequilibrium level during normal operation. This should be corrected during normal

operation by following the procedure given in the phosphate guidelines to determinethe optimum (equilibrium) level of phosphate(1)

.

Step 2—Outages

Planned or unplanned outages may be short or long term, depending upon systemdemand or the extent of work required to return the unit to operation. The length of time required for maintenance must be estimated and, depending upon this estimate,short term or long term layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options delineated in Section 4,Figure 4-8.

An unplanned outage due to an equipment or system failure can be treated as for aplanned outage.




5-16

Step 3—Chemical Transients: Condenser Leak (Fresh Water)

For condenser leaks with fresh, relatively low solids cooling water, the leak cangenerally be isolated and repaired while the unit is still operational under reduced load(divided water box). Otherwise the unit should be shut down, and leaks isolated andrepaired.

Step 4—Chemical Transients: Condenser Leaks (Sea Water)

Serious damage can occur to units within a short period of time with intrusion of seawater. Without condensate polishing, the boiler must be shut down immediately uponidentifying a significant condenser leak. The addition of extra trisodium phosphate or1-2 ppm of caustic may also be required as the boiler water pH drops.

With condensate polishing, the unit can generally be shut down in an orderly fashion,

especially if the polishers are of the deep bed type. With deep bed polishers, it isprudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.

Powdered resin condensate polishers have less capability for removal of dissolvedimpurities than deep bed polishers. If the leak is small, the use of powdered resinsystems may permit an orderly unit shutdown.

Chemistry monitoring is especially important when a sea water condenser leak issuspected. Boiler water pH is critical and condensate cation conductivity and sodiumwill assist in estimating the extent of the leak.

In any event, the unit should be shut down to repair the condenser leak. Depending onan evaluation of cycle and boiler chemistry, the shutdown will be immediate ororderly, as outlined above.

Substantial intrusion of sea water into the boiler will require that the unit be drained,flushed and refilled with condensate quality water plus chemicals consistent withoperating requirements of PT or EPT, as applicable.

Step 5—Chemical Transients: Chemical Intrusion

Some of the many causes of chemical intrusion (in addition to condenser leakage)include:

Makeup water system malfunction

Polisher leakage





5-17

— Poor regeneration

— Acid or caustic contamination

Chemical cleaning residue; acid, caustic, other

Maintenance chemicals and preservatives

Silica from flyash or other contaminants from maintenance activities

Minor chemical intrusions can be controlled by increasing boiler blowdown, andemployment of idle condensate polishers, if available.

More serious chemical intrusions may require adjustment of boiler pH throughaddition of more trisodium phosphate or 1-2 ppm of caustic. If these treatments areunsuccessful, the unit must be shutdown (orderly with polisher, immediate without

polisher) if the pH falls below 8.0 (Figure 5-3). The unit then requires carefulinspection to determine possible damage, and the necessity for repair and possiblechemical cleaning prior to restart.

Intrusion of chemical contamination requires immediate unit shutdown, draining andflushing the unit. Inspection of critical areas of the system (boiler, superheater, turbine,etc.) should be performed to assess the effects of chemical intrusion on the system.Chemical cleaning of the boiler, superheater and turbine may be required dependingupon the results of the inspection. Equipment repairs may also be required. In suchcase, a longterm layup will be required. (See Section 4.)

During emergency shutdowns, such as for major sea water leaks and extensivechemical contamination, immediate unit shutdown is required. Therefore, little can bedone to adjust cycle chemistry during shutdown since rapid action is required. Duringsuch occasions, the unit should be drained, flushed and inspected to assess damages, asoutlined previously. Restarting the unit will be contingent upon this inspection, as willlayup provisions.

5.6 MIXED METALLURGY SYSTEMS

As mentioned in Section 4, the metallurgy of condensers and feedwater heaters must becarefully considered when establishing chemistry parameters for startup, shutdown,and layup. A recent EPRI report

(3) on the State-of-Knowledge of Copper in Fossil Plant

Cycles describes the procedures required to prevent serious problems related to copperalloy corrosion, such as copper volatilization and deposition. For systems containingcopper alloys, it is most important to maintain a reducing atmosphere (ORP < 0mV) atall times. During operation this is generally accomplished by the use of suitableconcentrations of hydrazine with the addition of ammonia to regulate pH in the range




5-18

of 8.8-9.1. The reducing environment must also be maintained during the shutdownand layup periods. Oxygen ingress must be avoided, as this will increase the corrosionrate of copper alloys by changing the predominant surface oxide to cupric oxide.

The cycling of units, accompanied by periods of layup, materially increases the risk of

copper alloy attack, because of the opportunities for oxygen ingress with associatedincreases of ORP into the oxidizing range. Therefore, chemistry control under theseconditions, requires much more attention than for base loaded units.

The reader is referred to Sections 6.3 and 6.4 for more comprehensive coverage of mixed versus all-ferrous feedwater systems for drum units operating with AVTfeedwater.

5.7 CORRECTIVE ACTIONS

Typical corrective actions to respond to out-of-specification steam and/or waterchemistry conditions for PT and EPT units are presented in Section 6 of the phosphateguidelines

(1) (“Corrective Actions”). Individual corrective action tabulations are given

for the following:

Makeup treatment system effluent

Condensate storage tank effluent

Condenser leak detection trays

Air removal system exhaust

Condensate pump discharge

Deaerator inlet

Deaerator outlet

Economizer inlet

Boiler water

Reheat steam

5.8 LAYUP

Layup procedures are presented in Section 4, and a layup road map, which can beapplied to phosphate treated (PT) and equilibrium phosphate treated (EPT) units, is





5-19

depicted in Figure 4-8. Layup procedures fall into two categories—wet or dry. Withineach method, customization is possible (and encouraged) to suit the needs of the utilityor individual unit.

As described in Section 4, nitrogen blanketing for the boiler and feedwater system

components has been used to great advantage to prevent air ingress and subsequentcorrosion.

Particular attention should be given to maintaining proper layup conditions for theturbine (Section 4).

Also, the use of minimal levels of hydrazine or hydrazine substitute, introduced intothe boiler upon layup has permitted unit startups without the necessity to drain andrefill. This procedure reduces startup time and is environmentally sound. Thisprocedure is referenced in Step 6 in Section 4.8.

5.9 REFERENCES

1. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Treatment for Drum Units. TR-103665. Palo Alto, Calif.: Electric Power Research Institute, December 1994.

2. B. Dooley, J. Matthews, R. Pate and J. Taylor. “Optimum Chemistry for ‘All-Ferrous’ Feedwater Systems: Why Use an Oxygen Scavenger?” IWC Paper I94-53,International Water Conference, Pittsburgh, PA, Oct. 31–Nov. 2, 1994.

3. State-of-Knowledge of Copper in Fossil Plant Cycles. TR-108460. Palo Alto, CA: Electric

Power Research Institute, September 1997.






6-1

6

ALL-VOLATILE TREATMENT

6.1 INTRODUCTION

All-volatile treatment must be used for once-through units, to avoid deposition of non-volatile compounds in the boiler water/steam circuit. With the “traditional” form of all-volatile treatment (AVT), ammonia and hydrazine is added to the feedwater toprovide chemical conditions that protect the feedwater heaters, boiler, superheaters,reheaters and turbine against corrosion, without further chemical additions.

An alternative chemical treatment for once-through units is oxygenated treatment (OT -see Section 7), where instead of hydrazine, oxygen is added in addition to theammonia. An intermediate form of conditioning is also being used, where onlyammonia, without hydrazine or oxygen, is added to the feedwater. With all threealternatives, the degree of protection against corrosion provided by the chemicaladditives is strictly limited. Therefore, it is essential to maintain high purity feedwaterand to prevent the ingress of impurities, both while on-load and during off-loadconditions.

All-volatile treatment can also be used for units with drum-type boilers, provided highpurity feedwater is available and the buildup of impurities in the boiler water is strictlycontrolled. The absence of a solid alkalizing chemical in the boiler water gives lessprotection against corrosion, but, because there should be lower concentrations of impurities in the boiler water, it also reduces the risk of carrying over boiler water saltsand solid alkalizing chemicals into the steam. If the high purity feed and boiler waterconditions cannot be maintained during startup, operation and shutdown, it may benecessary to resort to another form of chemical conditioning, such as phosphate orcaustic treatment. The Selection and Optimization document provides advice andguidance on the optimum boiler water and feedwater choices for drum units

(1).

Guidance on the use of AVT was given in the EPRI Interim Consensus Guidelines(2)

in1986. Cycle diagrams were provided for once-through and drum-type boilers with allferrous and mixed Fe-Cu metallurgy feedwater heating systems. The normal limits andthree action levels were given for sodium, chloride, sulfate, silica and cationconductivities for feedwater and steam for once-through boilers. A range of pressureswas considered for drum boilers, with and without reheat, with additional pressurecurves for the normal limits and three action levels for sodium, chloride, sulfate and




All-Volatile Treatment

6-2

silica in the boiler water. In addition, some general guidance was also included onstartup, cycling and peaking operation.

The AVT guidelines for once-through and drum boilers were revised in 1996(3)

and“core” parameters for a minimum level of instrumentation were given (Table 1-1), in

addition to the comprehensive list of parameters. Guidance was included for all-ferrousand mixed Fe-Cu metallurgy for drum-type boilers, but was restricted to all-ferrousfeedwater heating systems for once-through boilers. Revised guidelines and actionlevels were given and guidance was included on optimizing AVT.

Further minor revisions to the “core” parameters were incorporated in the EPRI reporton Selection and Optimization of Boiler Water and Feedwater Treatment of FossilPlants

(1).

As an aid to planning and obtaining the optimum operating conditions for cycling,startup, shutdown and layup, it is important to know as accurately as possible, what

notice will be given of shutdown, the period of outage and for startup.

6.2 ONCE-THROUGH UNITS

Current Guidelines

In considering cycling, startup, shutdown and layup of units operating on AVT, it isassumed, in accordance with the latest EPRI Cycle Chemistry Guidelines

(3), that the

units with once-through boilers under consideration have condensate polishing plantsand all-ferrous feedwater heating systems. The cycle diagram for all parameters

(including the “core parameters”) is given in Figure 6-1 for once-through unitsoperating with AVT.

Startup

During off-load conditions, deposited impurities may have been released into thecircuit or have entered from outside, e.g. condenser leaks, CO2 with air in-leakage.Therefore, it is important to consider the effects on the feedwater and steam cationconductivity, silica, sodium, chloride and sulfate concentrations during startup. If thereis a leak in the reheaters, CO 2 and, more importantly, flyash and SO2, may be drawn in,resulting in acidic condensed steam in the reheaters. There is some evidence that silica

is washed off turbines during shutdown and startup. Oxygen and hydrazineconcentrations in the feedwater require special consideration. Restrictions will need to be applied to these parameters, but some relaxation from the normal EPRI guidelinevalues should be possible until steady state operating conditions are achieved. It should be possible to eliminate hydrazine entirely, for once-through units with all-ferrousfeedwater heating systems

(1) (see also Section 7 - Oxygenated Treatment).



EPRI Li censed Mat eri al

Specific conductivitya

Cation conductivity, µS/cma

HP

turbine

LP

turbine

Deaerator

Attemperat

Boiler

LP Condensate

polisher

Makeup

treatment

system

Condensate

storage tank

Condenser

IP

turbine

HP heaters

N (Normal)

Maximum Annual Exposure to

Contaminant Conditions

Targets


Base Load Cycling

1 (Action Level 1)

2 (Action Level 2)

3 (Action Level 3)

—

336 ( 2 weeks)

48 (2 days)

8

1Immediate Shutdown

—

672 (4 weeks)

96 (4 days)

16

2

Hydrazine, ppb

Deaerator Outlet

Parameter

TargetSample N

C < 20 ppb

Oxygen, ppb

Deaerator Inlet

Parameter

TargetSample N

T < 10

Hydrazine, ppb

Low-Pressure Steam (Optional)

ParameterTarget Injection

Dosage

< 20

Air inleakage,

scfm/100 MWe

Air Removal System Exhaust

Parameter

TargetSample N

D ≤ 1

Cation conductivitya

or sodium

Condenser Leak Detection Trays orHotwell Zones (If applicable)

Parameter

Target

Sampl

C

Con

Pa

Sod

Oxy

Tot

•

Cat

µS/

Cation conductivity,

µS/cma

Sodium, ppb

Condensate Polisher Effluent

ParameterTarget

Sample N

Silica, ppb C ≤ 10

C ≤ 0.15

1

> 10

≤ 0.2

2

—

3

—

C ≤ 3 ≤ 6 ≤ 12 > 12CR •

CR • ≤ 0.3 > 0.65

CR •

CR •

Economizer Inlet and Attemperation Water

Parameter Target Sample N

All ferrous metallurgy C 9.2-9.6

1

< 9.2

> 9.6

2

—

3

—

Ammonia D

C

pHa

Consistent with pH

C ≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.65

Iron, ppb

Copper, ppb

Oxygen, ppb

W ≤ 5 > 5 — —

W ≤ 2 > 2 — —

C 1-10 ≤ 15 ≤ 20 > 20

CR •

CR •

Reheat Steam

ParameterTarget

Sample N

Sodium, ppb C ≤ 3

To ta l o rg an ic c arb on , p pb W ≤ 100


T —


µS/cma

C

Silica, ppb T ≤ 10

Chloride, ppb T ≤ 3

Sulfate, ppb T ≤ 3

1

≤ 6

> 100

—

≤ 20

≤ 6

≤ 6

2

≤ 12

—

—

≤ 40

≤ 12

≤ 12

3

> 12

—

—

> 40

> 12

> 12

CR •

CR • ≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.65

N = Normal

1 = Action Level 1

2 = Action Level 2

3 = Action Level 3

Target Values

Legend

C = continuous

S = grab, once per shift

D = grab, once per day

W = grab, once per week

T = troubleshooting and commissioning

Sample Frequency

Footnotes

a = Conductivity and pH measured at 25° C

e = Target values may be adjusted to reflect capabilities of installed equipment

Figure 6-1 Cycle Chemistry Diagram for a Once-Through Unit on All-Volatile Treatment




6-4

Depending on the plant, and the shutdown and layup conditions adopted, several stepswill be required for startup (in addition to filling the boiler following a long shutdownand safely venting the nitrogen, if used). The steps used during startup will be plantspecific and, depending on the practicality, the following stages should be monitored,at least during a trial period:

StageduringStartup CircuitMonitoringa)Circulationviadeaerator pH, ironandcationconductivity b)Circulationviaeconomizer pH, Fe, cationconductivity, oxygenandhydrazinec)Circulationviatheboiler pH, Fe, cationconductivity, silica, sodiumandchlorided)Allowfiring pH, ironandcationconductivity

e)Allowsteamtoturbine pH, ironandcationconductivityf)Turbinetoonethirdload pH, ironandcationconductivityg)Tofullload pH, ironandcationconductivityOnfull load, thenormal EPRIGuidelinesvaluesgiveninFigure6-1shouldbeachieved. Theremayberoomforrelaxation, particularly, asregardssilicainsteam,duringstartup, theearlierstagesofoperationandduringlowloadoperation, providedtheparametersarewithintheEPRIactionlevels1-3. (Notethattheguidelinesarethesameforall operatingpressuresforonce-throughboilers.)Thecoreparameters,sodium, cationconductivityandoxygengivenintheEPRIGuidelinesshouldbemonitoredatthecondensatepumpdischarge, condensatepolisheroutlet, economizerinletandsteam, andalsopH, silicaandiron, ifpossible, atleastduringatrial period.Thenormal operatinglevels, asgivenintheEPRIguidelines, shouldbeachievedassoonaspossible. ThesearegiveninFigure6-1.Aroadmap(decisiontree)showingaschemeforstartupofaonce-throughunitoperatingwithAVTisgiveninFigure6-2. Thetimetakentoachievetherequiredtargetvalueswill beplantdependent. Thisshouldbedeterminedforeachunit, usedasguidanceforsubsequentstartups, andamendedinthelightofoperatingexperience.Theprocedurecanusuallybecurtailedforwarmandhotstarts.Theroadmapforstartupofonce-throughunitsoperatingonAVT, Figure6-2, canbedividedinto7steps.




6-5

No

Yes

Step 1

Fill system perAVT guidelines

Monitor chemicalparameters

Achieve full pressureand load

Safely vent N2 (if used)Refill per AVT guidelines

Maintain Na, SiO2, pH, Cl,SO4 and cation conductivity

within the AVT guidelines

Is system full?

Is system filled withlow O2 scavenger?

(Section 4)

Is system filled withwater per AVT

guidelines?

Proceed withstartup

Maintain temperature rampwithin boiler and turbine

manufacturers requirements

Proceed progressively withthe polishers, deaerator,

economiser in cleanup loop

Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Reduce O2 to 100 ppband Fe to 30 ppb

Fire to boiler

Figure 6-2 Startup of Once-through Units with All-Ferrous Feedwater Heaters

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that for once-through boilers, very high purity water is required for the boiler,as well as for feedwater. Control of boiler water purity cannot be achieved by blowdown, as is the case with drum boilers, and the absence of the “reservoir” of the boiler and a steam separation stage in once-through boilers, means that the acceptablelevel of impurities is much less than for drum boilers.




6-6

Step 1 - System Stored Dry

If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for once-through units operating on AVT (3).

Step 2 - Systems Stored Wet: Low Oxygen Scavenger

If the system has been stored using the low oxygen scavenger procedure (Section 4), the boiler can be fired immediately.

Step 3 - System Stored Wet: Excess Ammonia and Hydrazine

If the system has been stored wet with a surplus of ammonia and hydrazine (Section 4),it must be drained under nitrogen and refilled with water meeting the EPRI Guidelinesfor once-through units operating on AVT

(3).

Step 4 - System Stored using Nitrogen

If the system has been filled with nitrogen or filled with water, using nitrogen capping,the nitrogen must be safely vented and the boiler refilled, if necessary, with watermeeting the EPRI Guidelines for once-through units operating on AVT

(3). Startup of the

unit can then proceed.

Step 5 - Cleanup and Firing Boiler


Oxygen < 100 ppb

Iron < 30 ppbDuring startup, the levels of corrosion products (iron) can be very high initially andsilica may also be a problem. Achieving these limits is greatly facilitated by usingcondensate polishing (See Section 3). When these limits are attained, the boiler can befired.

Step 6 - Monitoring

After firing the boiler, monitoring must be fully implemented, both for chemistryparameters, and to ensure that temperature ramps are maintained according to the boiler and turbine manufacturer’s specifications.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problemcorrected at once.




6-7

Step 7 - Full Load

Full load can be achieved when chemical limits are within the EPRI Guidelines foronce-through units operating on AVT(3).

Shutdown

Information is given in Section 4.8 on Shutdown and below for various plannedshutdown conditions.

Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized. Maintain condensate circulation and deaerator pressure. Raise the pHwhen coming off-load.

Intermediate shutdown (weekend/week) - Leave plant pressurized, raise pH and thehydrazine concentration (if used), when coming off-load.

Long shutdown (longer than 1 week) - Drain under nitrogen or from high temperature(e.g. 130°C) and maintain the boiler, superheaters and reheaters dry. Continue asdescribed under “Layup”.

Unplanned shutdowns also occur and, by their very nature, are unpredictable.Therefore, it is difficult to give specific advice, except that, by using circulation,endeavor to achieve the most appropriate conditions given above.

During shutdown, if there is a leak in the reheaters, CO2 and, more importantly, flyash

and SO2, may be drawn in, resulting in acidic condensed steam in the reheaters. There

is also evidence that silica is washed off turbines during shutdown.

A road map showing a scheme for shutdown of a once-through boiler operating withAVT is given in Figure 6-3. The time taken to achieve stable shutdown conditions will be plant dependent. This should be determined for each unit, as guidance forsubsequent shutdowns and amended in the light of operating experience.

The road map for shutdown of once-through units operating on AVT, Figure 6-3, can be divided into 5 steps.




6-8

Estimate outage length.Use layup appropriatefor duration of outage

No

Step 1

Use short termlayup procedure

(Section 4)

Isolate and repairleak, usually allow

continued operation

Step 2

Step 3

Step 3

Step 2

Yes

Consider chemicaltransients due to:-

Orderly shutdown,if polishers utilized,

otherwiseimmediate shutdown

Normal cycling andpeaking operation

Unplanned outage

Yes

Yes

Yes

Yes

Condenser leakfresh water cooling

Condenser leaksea water cooling

Planned outage

Step 4

Step 2

No

No

Step 3

Step 4

Step 5

Step 5

Orderly shutdown, ifpolishers utilized,


YesChemical intrusion

No

Figure 6-3AVT - Shutdown of Once-Through Units with All-Ferrous Feedwater Heaters

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that for once-through boilers, very high purity water is required for the boiler,as well as the feedwater, and control of the boiler water purity cannot be achieved by blowdown.

Step 1 - Normal Cycling or Peaking: Load Reduction or Shutdown

Normal cycling or peaking load reduction or shutdown presumes a short-term layup as

described in Section 4.


Step 2 - Outages




6-9

Planned or unplanned outages may be short or long term, depending on systemdemand or the extent of the work required to return the unit to operation. The length of time required for maintenance must be estimated and, depending on this estimate,short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options delineated in Section 4.8

(Figure 4-8).


Step 3 - Chemical Transients: Condenser Leaks (Fresh Water)

For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the condensate polishing plant maintainsthe feedwater quality.

Step 4 - Chemical Transients: Condenser Leaks (Sea Water)

Serious damage can occur to units within a short period of time with intrusion of seawater. With condensate polishing, the unit can generally be shut down in an orderlyfashion, especially if the polishers are of the deep bed type. With deep bed polishers, itis prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.


Chemical monitoring is especially important when a sea water leak is suspected.Sodium and cation conductivity will assist in estimating the extent of the leak.

In any event, the unit should be shut down to repair the condenser leak. Depending onan evaluation of the cycle chemistry, shutdown will be immediate or orderly, asoutlined above.

Substantial intrusion of sea water into the boiler will require that the unit is drainedand refilled with water meeting the EPRI Guidelines for once-through units operatingon AVT

(3).




6-10

Step 5 - Chemical Transients: Chemical Intrusion



Polisher leakage





Silica from flyash or other contaminants from maintenance activitiesMinor chemical intrusions can be controlled by employment of idle condensatepolishers, if available.

Intrusion of chemical contamination requires immediate unit shutdown, draining andflushing the unit. Inspection of critical areas of the system (boiler, superheater, turbine,etc) should be performed to assess the effects of chemical intrusion on the system.Chemical cleaning of the boiler, superheater and turbine may be required, dependingon the results of the inspection. Equipment repairs may be required. In such case, along term layup will be required. (See Section 4.)

During emergency shutdowns, such as for major sea water leaks and extensivechemical contamination, immediate unit shutdown is required. Therefore, little can bedone to adjust cycle chemistry during shutdown, since rapid action is required. Duringsuch occasions, the unit should be drained, flushed and inspected to assess damage, asoutlined previously. Restarting the unit will be contingent upon this inspection, as willlayup provisions.

Cycling and Peaking

Special features may be required for once-through units to cycle, such as a turbine bypass (Section 2).

As an aid to planning and obtaining optimum conditions, it is important to know asaccurately as possible, the frequency and duration of cycling and peaking, the noticegiven and the variation of load patterns.

Essentially, cycling and peaking plant should operate according to the outline givenabove for startup, load changes and shutdown.




6-11

Layup

Layup procedures are presented in Section 4 and a road map suitable for once-throughunits operating on AVT is depicted in Figure 4-8.

Store under nitrogen or, if shutdown from a high temperature, maintain the boiler,superheaters and reheaters dry. Possibly dehumidify the feedheaters or allow them toremain wet. Avoid air ingress into the feedheaters. Monitor oxygen and pH, andnitrogen and moisture, as appropriate.

The layup conditions are going to be dependent on the method of layup chosen. Thisshould be determined for each unit and each method of layup used. The informationobtained should then be used as guidance for subsequent repeat operations andamended in the light of operating experience.

6.3 DRUM BOILERS WITH ALL-FERROUS FEEDWATER HEATING

SYSTEMS

In general, the features for the feedwater and steam for units with once-through boilers,also apply to drum boilers. However, these need to be supplemented by additionalconsiderations for the boiler water. Some units with drum boilers may be equippedwith a condensate polishing plant, but many units are not and, therefore, will be moresusceptible to the effects of condenser leaks.

For completeness and ease of reference for AVT, phosphate treatments (Section 5), andcaustic treatment (Section 8), drum boilers with all-ferrous and mixed Fe-Cumetallurgy feedwater heating systems are considered separately in the next twosections.

Current Guidelines

The current guidelines for drum boilers with all-ferrous feedwater heating systems aregiven in Figure 6-4 for units with reheat, including the “core” parameters. The normallimits and three action levels for sodium, chloride, sulfate, silica and cationconductivity in boiler water are given in Figures 6-5 to 6-9 for a range of operatingpressures. The values given for plants without reheat are twice those for plants withreheat, except for pH, oxygen, TOC, iron and copper. They are not reproduced here

and reference should be made to the original documents(1,3)

.






6-13

Action level 3

Normal

10

8

7

2

6

5

4

3

9

S o d i u m ( p

p m N

a )

1500 19001100 17001300

Pressure (psia)

900 27002300 285025002100

Action level 1

Action level 2

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

0.1

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.6

Drum Pressure (MPa)

Figure 6-5 All Volatile Treatment: Drum Boiler Water Sodium vs. OperatingPressure (Plants With Reheat)




6-14

Pressure (psia)

1300 1700700 15001100600 2500 28502100 270023001900900


l )

Actionlevel 2

Normal

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

3.0

2.0

1.5

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-6 All-Volatile Treatment: Drum Boiler Water Chloride vs. OperatingPressure (Plants With Reheat)




6-15

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9


3.0

2.0

1.5

1300 1700700 15001100600 2500 28502100 270023001900900


O 4

)

Actionlevel 2

Normal

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-7 All-Volatile Treatment: Drum Boiler Water Sulfate vs. OperatingPressure (Plants With Reheat)




6-16

10

87

2

6

5

4

3

9


0.10

0.080.070.06

0.05

0.04

0.03

0.09

20

15

1300 1700700 15001100600 2500 28502100 270023001900900


i O 2 )

Actionlevel 2

Normal Actionlevel 1

Action level 3

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-8 All-Volatile Treatment: Drum Boiler Water Silica vs. Operating Pressure(Plants With Reheat)




6-17

100

80

70

20

60

50

40

30

90


1300 1700700 15001100600 2500 28502100 270023001900900

C a t i o n C o n d u c t i v i t y - µ S / c m

Normal

Actionlevel 1

Actionlevel 2

Action level 3

10

87

2

6

5

4

3

9

1

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-9 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs.Operating Pressure (Plants With Reheat)




6-18

Startup

During off-load conditions, impurities may have been released into the circuit or haveentered from outside, e.g. condenser leaks, CO2 with air ingress. Therefore, duringstartup, it is important to consider the effects of these on the feedwater, boiler waterand steam cation conductivity, silica, sodium, chloride and sulfate concentrationsduring startup. If there is a leak in the reheaters, CO2 and, more importantly, flyash andSO

2, may be drawn in, resulting in acidic condensed steam in the reheaters. There is

some evidence that silica is washed off turbines during shutdown and startup. Oxygenand hydrazine concentrations in the feedwater also require special consideration.Restrictions will need to be applied to these parameters, but some relaxation from thenormal EPRI guideline values may be possible until steady state conditions areachieved.

Section 3 explains how the chemistry curves and action levels can be utilized during

unit startup. Basically, during startups, the initial lower boiler pressure permits boilerwater chemical concentrations to be higher than those at normal unit operatingpressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.

If the boiler has been stored under nitrogen or filled with ammonia and hydrazinesolution, it is necessary to ensure that this is safely vented or disposed.

It is important to prevent high concentrations of oxygen and chloride in the boilerwater, as these can act synergistically and may initiate corrosion

(4). For this reason, the

oxygen concentration of the boiler water should not be allowed to exceed 0.2 ppmduring boiler operation. It is also important to ensure that the pH, cation conductivityand chloride concentration are within the limits given in the EPRI

(1,3) guidelines for

boiler water. It might be possible to eliminate hydrazine in plants with all-ferrousfeedwater heating systems(1) (see Section 7 - Oxygenated Treatment).





6-19

Stage during Startup Circuit Monitoring

a) Addition of hydrazine (if used) and

ammonia to the feedwater

pH, cation conductivity and oxygen

b) Additional boiler blowdown pH, cation conductivity, chloride andsuspended corrosion products

c) Care to avoid additional carryover intosteam

Cation conductivity, silica and sodium

At full load, the normal EPRI guideline values given in Figure 6-4 should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figures 6-5 to 6-9. The core

parameters, sodium, cation conductivity and oxygen, given in the EPRI guidelines(1,3)

should be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH, andsteam. The normal operating levels, as given in the EPRI guidelines

(1,3), should be

achieved as soon as possible. These are given in Figure 6-4.

A road map showing a scheme for startup of a drum boiler with an all-ferrousfeedwater heating system operating with AVT is given in Figure 6-10. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups and amended in the light of operating experience. The procedure can normally be curtailed for warm and hot starts.

If the high purity feed and boiler water conditions cannot be maintained duringstartup, it may be necessary to resort to another form of chemical conditioning, such asphosphate or caustic treatment. The Selection and Optimization document providesadvice and guidance on the optimum boiler water and feedwater choices for drumunits

(1).




6-20

No

Yes

Step 1


Fire to boiler



Safely vent N2 (if used)Drain storage solutionRefill per AVT guidelines



within AVT guidelines bycontrolling pressure and blow-down. Avoid excess carry-over

of impurities into steam

Is system full?


(Section 4)


guidelines?

Proceed withstartup



Proceed progressively withpolishers (if fitted), deaerator,economiser in clean-up loop

Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Figure 6-10 AVT - Startup of Drum Boilers with All-Ferrous Feedwater Heaters




6-21

The road map for startup of a drum-type unit operating on AVT with all-ferrousfeedwater heaters, Figure 6-10, can be divided into 7 steps.

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that higher purity water is required for units operating with AVT. Unlike once-through units operating on AVT, described in Section 6.2, control of boiler water puritycan be achieved by blowdown with drum units.


If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for drum units operating on AVT

(3).


If the system has been stored using the low oxygen scavenger procedure (Section 4.8),

the boiler can be fired immediately.Step 3 - System Stored Wet: Excess Ammonia and Hydrazine

If the system has been stored wet with a surplus of ammonia and hydrazine(Section 4.8), it must be drained under nitrogen and refilled with water meeting theEPRI Guidelines for drum units operating on AVT

(3).


If the system has been filled with nitrogen or filled with water, using a nitrogen cap,the nitrogen must be safely vented and the boiler refilled, if necessary, with water

meeting the EPRI Guidelines for drum units operating on AVT(3)

. Startup of the unit canthen proceed.



Oxygen < 100 ppb

Iron < 100 ppb

During startup, the levels of corrosion products (iron) can be very high initially and

silica may also be a problem. Achieving these limits is greatly facilitated by usingcondensate polishing (if fitted) (See Section 3). When these limits are attained, the boilercan be fired.





6-22

Step 6 - Monitoring


During startup, the concentration vs. pressure curves can be utilized to control sodium,silica, chloride and sulfate (for examples see Figures 6-5 to 6-9). Boiler pressure shouldremain at reduced levels such that these limits are maintained before pressure can beincreased to the next stage. Maximum use of blowdown and condensate polishing (if available) will minimize startup times.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problem

corrected at once. Excursions affecting (lowering) boiler water pH must be correctedimmediately by feeding trisodium phosphate or 1-2 ppm of sodium hydroxide. Avoidexcess carryover of boiler water impurities into the steam.


Step 7 - Full LoadFull load can be achieved when chemical limits are within the EPRI Guidelines fordrum boilers operating on AVT

(3).

Shutdown

Information is given in Section 4 on shutdown (see Figure 4-8) and below for variousplanned shutdown conditions:

Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized.

Intermediate shutdown (weekend-week) - Leave plant pressurized or store the boilerunder nitrogen.

Long shutdown (longer than 1 week) - Drain the boiler under nitrogen or blowdownfrom high temperature (e.g. 130°C, 266°F) and maintain the boiler, superheaters andreheaters dry. Continue as described under “Layup”.




6-23



and SO2, may be drawn in, resulting in acidic condensed steam in the reheaters. Thereis also evidence that silica is washed off turbines during shutdown.

A road map showing a scheme for shutdown of a drum boiler with an all-ferrousfeedwater heating system operating with AVT is given in Figure 6-11. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience.


No

Step 1

Use short term

layup procedure(Section 4)


continued operation

Step 2

Step 3

Step 3

Step 2

Yes





Unplanned outage

Yes

Yes

Yes

Yes



Planned outage

Step 4

Step 2

No

No

Step 3

Step 4

Step 5Step 5

Adjust pH >8.0, orderly

shutdown, if polishersutilized, otherwise

immediate shutdown

Yes Chemical intrusionseverely affecting pH

No

Figure 6-11 AVT - Shutdown of Units with Drum Boilers with All-Ferrous andMixed Metallurgy Feedwater Heaters




6-24

The road map for shutdown of a drum-type unit operating on AVT with all-ferrousfeedwater heaters, Figure 6-11, can be divided into 5 steps.

Again, the steps are essentially similar to those described in Section 5 for phosphatetreatment. Unlike once-through boilers operating on AVT, described in Section 6.2,

control of the boiler water purity can be achieved by blowdown with drum boilers.


Normal cycling or peaking load reduction or shutdown presumes a short-term layup asdescribed in Section 4.



be adjusted to conform with the AVT guidelines for drum units(3).

Particular care should be exercised to prevent oxygen ingress during this period and blowdown should be maintained at an appropriate level to remove contaminants fromthe system.

Step 2 - Outages

Planned or unplanned outages may be short or longterm, depending on systemdemand or the extent of the work required to return the unit to operation. The length of time required for maintenance must be estimated and, depending on this estimate,

short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options delineated in Section 4.8(Figure 4-8).


Step 3 - Chemical Transients: Condenser Leak (Fresh Water)

For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the unit is still operational under reducedload (divided water box) or if the condensate polishing plant (if fitted) maintains thefeedwater quality.


Serious damage can occur to units within a short period of time with intrusion of seawater. Without condensate polishing, the boiler must be shutdown immediately upon




6-25

identifying a significant condenser leak. The addition of trisodium phosphate or 1-2ppm of sodium hydroxide may also be required as the boiler water pH drops.

With condensate polishing (if fitted), the unit can generally be shut down in an orderlyfashion, especially, if the polishers are of the deep bed type. With deep bed polishers, it

is prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.


Chemical monitoring is especially important when a sea water leak is suspected. Boilerwater pH is critical, and sodium and cation conductivity will assist in estimating theextent of the leak.

In any event, the unit should be shutdown to repair the condenser leak. Depending on

an evaluation of the cycle chemistry, shut down will be immediate or orderly, asoutlined above.

Substantial intrusion of sea water into the boiler will require that the unit is drainedand refilled with water meeting the EPRI Guidelines for drum boilers operating onAVT

(3).




Polisher leakage






Minor chemical intrusions can be controlled by employment of idle condensatepolishers, if available.

More serious chemical intrusions may affect (lower) the boiler water pH, necessitatingadjustment of boiler water pH through the addition of trisodium phosphate or 1-2 ppm




6-26

sodium hydroxide. If these treatments are unsuccessful, the unit must be shut down(orderly with polishers, immediate without polishers) if the pH falls below 8 (Figure6-4). The unit then requires careful inspection to determine possible damage, and thenecessity for repair and possible chemical cleaning prior to restart.


During emergency shutdowns, such as for major sea water leaks and extensivechemical contamination, immediate unit shutdown is required. Therefore, little can bedone to adjust cycle chemistry during shutdown, since rapid action is required. Duringsuch occasions, the unit should be drained, flushed and inspected to assess damage, as

outlined previously. Restarting the unit will be contingent upon this inspection, as willlayup provisions.

Cycling and Peaking


Essentially, cycling and peaking plants should operate according to the outline givenabove for startup, load changes and shutdown.

Layup

Layup procedures are presented in Section 4, and a road map suitable for units withdrum boilers operating on AVT is depicted in Figure 4-8.

Store the boiler under nitrogen or, if blowdown from a high temperature, maintain the boiler, superheaters and reheaters dry. Possibly dehumidify the feedwater heaters orallow them to remain wet. Avoid air ingress into the feedwater heaters. Monitoroxygen and pH, and nitrogen and moisture, as appropriate. It is also possible to storethe boiler filled with a solution containing ammonia and hydrazine.

The layup conditions are going to be dependent on the method of layup chosen. Thisshould be determined for each unit and each method of layup used. The informationobtained should be used as guidance for subsequent repeat operations and amended inthe light of operating experience.




6-27

6.4 DRUM UNITS WITH MIXED METALLURGY FEEDWATER HEATING

SYSTEMS

EPRI(5)

has recently produced a report on the State-of-Knowledge of Copper in FossilPlant Cycles as the first stage of the “Program Copper” project. This was in response to

a demand from members, particularly in the US, for improved performance of powerplants containing copper alloys. Traditionally, these have been widely used incondensers, but, in some plants, brass and/or cupro-nickel have also been used for heatexchange surfaces in low and high pressure feedwater heaters. Copper released fromfeedwater heaters deposits in the boiler, increasing the locations where impurities canconcentrate on boiler waterwalls. Thick deposits can lead to overheating and thepresence of copper in deposits complicates chemical cleaning.

Copper in high pressure boilers can be carried over into the steam and deposited inhigh pressure turbines, where even as little as 1 kg can reduce the output capacity of the turbine by 1 MW. This is particularly true for plants operating at more than 2400 psi(16.6 MPa) and is exacerbated further by increasing pressure. The presence of oxygenand the absence of reducing conditions in the feedwater during all periods of operationand shutdown is the main cause of copper transport round the circuit.

It is worth reiterating that startups are generally considered to be the periods of maximum copper transport activity in the cycle. This relates directly to the feedwatersystem not being protected during shutdown periods, i.e. that a reducing environment(ORP < 0mV) is not maintained. Air in-leakage into the LP feedwater heating circuitsincreases the growth of non-protective copper oxides and copper transport. The EPRI“Guiding Principles” for successful operation of units with copper alloys

(5) are:

Keep feedwater copper levels at guideline values (< 2 ppb at the economizer inlet)during normal operation.

Establish conditions which favor cuprous oxide (Cu2O) rather than cupric oxide(CuO) under all operating conditions.

Maintain reducing chemistry (oxidizing-reducing potential, ORP < 0mV) at alltimes, including shutdown and startup.

Control feedwater pH in the range 8.8-9.1.

Implement shutdown procedures and layup programs which effectively minimizecopper transport activity upon return to service.

Consider volatility effects in controlling drum pressure; if possible, maximizeoperating pressure in the range of 2400-2500 psi (16.5-17.2 MPa) and avoid over-pressure operation above this range.




6-28

Current Guidelines

The current guidelines for drum boilers with mixed Fe-Cu metallurgy feedwatersystems are given in Figure 6-4 for reheat plants, including the “core” parameters. Thenormal limits and three action levels for sodium, chloride, sulfate, silica and cation

conductivity in boiler water are given in Figures 6-5 to 6-9 for a range of operatingpressures. Except for pH, oxygen, TOC, iron and copper, the limits for non-reheat plantare generally higher by a factor of two. They are not reproduced here and referenceshould be made to the original documents

(1,3).

Startup

During off-load conditions, impurities may have been released into the circuit or haveentered from outside, e.g. condenser leaks, CO2, with air ingress. Therefore, duringstartup it is important to consider the effects of these on the feedwater, boiler water andsteam cation conductivity, silica, sodium, chloride and, sulfate concentrations. If there

is a leak in the reheaters, CO 2 and, more importantly, flyash and SO2, may be drawn in,resulting in acidic condensed steam in the reheaters. There is some evidence that silicais washed off turbines during shutdown and startup. Oxygen, hydrazine and copperconcentrations in the feedwater and copper concentrations of the steam will alsorequire special consideration, since the presence of oxidizing conditions increase therelease and transport of copper around the circuit.


It is important to prevent high concentrations of oxygen and chloride in the boilerwater, as these can act synergistically and may initiate corrosion. For this reason, theoxygen concentration of the boiler water should not be allowed to exceed 0.2 ppmduring boiler operation. The presence of oxygen also assists the transport of copper intothe steam, increasing the risk of deposition in the superheater and high pressureturbine. It is also important to ensure that the pH, cation conductivity and chlorideconcentration are within the limits given in the EPRI

(1,3) guidelines for boiler water.

Hydrazine (or volatile reducing agents) should not be eliminated from plants withmixed Fe-Cu feedwater heating systems.

Depending on the plant, and the shutdown and layup conditions adopted, several steps

will be required for startup (in addition to filling the boiler following a long shutdownand safely venting the nitrogen, if used). The steps used during startup will be plantspecific and, depending on the practicality, the following stages should be monitored,at least during a trial period:


a) Addition of hydrazine and ammonia to pH, cation conductivity, oxygen and




6-29

the feedwater copper


c) Care to avoid additional carryover into

steam

Cation conductivity, silica, sodium and

copper

At full load, the normal EPRI guidelines values given in Figure 6-4 should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figures 6-5 to 6-9. The coreparameters, sodium, cation conductivity and oxygen given in the EPRI

(1,3) guidelines

should be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH, andsteam, including copper. The normal operating levels, as given in the EPRI

(1,3)

guidelines, should be achieved as soon as possible. These are given in Figure 6-4.

A road map showing a scheme for startup of a drum boiler with mixed Fe-Cufeedwater heating system operating with AVT is given in Figure 6-12. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups, and amended in the light of operating experience. The procedure can normally be curtailed for warm and hot starts.

The road map, Figure 6-12, can be divided into 7 steps.

Section 3 explains how the chemistry curves and action levels can be utilized duringunit startup. Basically, during startups, the initial lower boiler pressure permits boilerwater chemical concentrations to be higher than those at normal unit operatingpressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.

If the high purity feed and boiler water conditions can not be maintained duringstartup, it may be necessary to resort to another form of chemical conditioning, such asphosphate or caustic treatment. The Selection and Optimization document providesadvice and guidance on the optimum boiler water and feedwater choices for drumunits

(1).

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that higher purity water is required for units operating with AVT. Unlike once-through units operating on AVT, described in Section 6.2, control of boiler water puritycan be achieved by blowdown with drum units. However, compared with units withall-ferrous feedwater heating systems described in Section 6.3, additionalconsiderations are required for units with mixed Fe-Cu feedwater heaters. To reducethe risk of copper corrosion and transport, the ingress of oxygen must be minimized




6-30

and chemically reducing conditions must be maintained during all periods of operation, shutdown and layup.

No

Yes

Step 1


Fire to boiler




Add N2H4, if necessary.Reduce O2 to 100 ppb, Fe

to 100 ppb, Cu to 10 ppb

Maintain Cu, Na, SiO2, pH, Cl,SO4 and cation conductivity

within AVT guidelines bycontrolling pressure and blow-down. Avoid excess carryover


Is system full?


(Section 4)


guidelines?

Proceed withstartup



Proceed progressively withpolishers (if fitted), deaerator,economizer in cleanup loop

Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Figure 6-12 AVT - Startup of Drum Boilers with Mixed Metallurgy FeedwaterHeaters

For ease of reference, full details of the steps for the startup of drum units with mixedFe-Cu feedwater heating systems, operating on AVT, are given below:




6-31


If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for drum units operating on AVT (3).


If the system has been stored using the low oxygen scavenger procedure (Section 4.8),the boiler can be fired immediately.


If the system has been stored wet with a surplus of ammonia and hydrazine (Section 4),it must be drained under nitrogen and refilled with water meeting the EPRI Guidelinesfor drum units operating on AVT

(3).


If the system has been filled with nitrogen or filled with water, using nitrogen capping,the nitrogen must be safely vented and the boiler refilled, if necessary, with watermeeting the EPRI Guidelines for drum units operating on AVT

(3). Startup of the unit can

then proceed.



Oxygen < 100 ppb

Iron < 100 ppbCopper < 10 ppb

During startup, the levels of corrosion products (iron and copper) can be very highinitially and silica may also be a problem. Achieving these limits is greatly facilitated by using condensate polishing (See Section 3). When these limits are attained, the boilercan be fired.


Step 6 - Monitoring





6-32

During startup, the concentration vs. pressure curves can be utilised to control sodium,silica, chloride and sulfate (for examples see Figures 6-5 to 6-9). Boiler pressure shouldremain at reduced levels such that these limits are maintained before pressure can beincreased to the next stage. Maximum use of blowdown and condensate polishing (if available) will minimize startup times.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problemcorrected at once. Excursions affecting (lowering) boiler water pH must be correctedimmediately by feeding trisodium phosphate or 1-2 ppm of sodium hydroxide.

Effects of cycle contamination are magnified at startup due to relatively low flow ratesfor condensate, feedwater and steam. Cation conductivity may increase as a result of air ingress due to either aeration of water during the shutdown period or air in-leakage

during startup. The change to boiler water chemistry will be minimal compared tocontamination involving the makeup system, chemical cleaning activities, condensatepolishers or condenser leaks.

Avoid excessive carryover of boiler water impurities into the steam. This includescopper, whose transport can be minimized by avoiding ingress of oxygen andmaintaining chemically reducing conditions.

Step 7 - Full Load

Full load can be achieved when chemical limits are within the EPRI Guidelines for

drum boilers operating on AVT

(3)

.

Shutdown


Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized, avoid air ingress to the feedheaters.

Intermediate shutdown (weekend-week) - Leave plant pressurized or store the boiler

under nitrogen. Avoid air ingress to the feedwater heaters.

Long shutdown (longer than 1 week) - Drain the boiler under nitrogen or blowdownfrom high temperature (e.g. 130°C) and maintain the boiler, superheaters and reheatersdry. Continue as described under “Layup”.




6-33


During shutdown. if there is a leak in the reheaters, CO2 and, more importantly, flyash


A road map showing a scheme for shutdown of a drum boiler with a mixed Fe-Cu-feedwater heating system operating with AVT is given in Figure 6-11. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience.

The road map for shutdown of a unit with a drum boiler operating on AVT with mixedmetallurgy feedwater heaters, Figure 6-11, can be divided into 5 steps.

Again, the steps are essentially similar to those described in Section 5 for phosphatetreatment, except that higher purity water is required for units operating with AVT.Unlike once-through boilers operating on AVT, described in Section 6.2, control of the boiler water purity can be achieved by blowdown with drum boilers. However,compared with units with all-ferrous feedwater heating systems described in Section6.3, additional considerations are required for units with mixed Fe-Cu feedwaterheaters. To reduce the risk of copper corrosion and transport, the ingress of oxygenmust be minimized and chemically reducing conditions must be maintained.




During orderly load reductions, the condensate cycle and boiler chemical limits should be adjusted to conform with the AVT guidelines for drum units

(3).

Particular care should be exercised to prevent oxygen ingress during this period tominimize pickup of copper. Blowdown should be maintained at an appropriate level toremove contaminants from the system.




6-34

Step 2 - Outages


short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options deliniated in Section 4.8(Figure 4-8).



For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the unit is still operational under reduced

load (divided water box) or if the condensate polishing plant (if fitted) maintains thefeedwater quality.


Serious damage can occur to units within a short period of time with intrusion of seawater. Without condensate polishing, the boiler must be shutdown immediately uponidentifying a significant condenser leak. The addition of trisodium phosphate or 1-2ppm of sodium hydroxide may also be required as the boiler water pH drops.

With condensate polishing (if fitted), the unit can generally be shut down in an orderlyfashion, especially, if the polishers are of the deep bed type. With deep bed polishers, itis prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.





(3).




6-35




Polisher leakage

Poor regeneration

Acid or caustic contamination





More serious chemical intrusions may affect (lower) the boiler water pH, necessitatingadjustment of boiler water pH through the addition of trisodium phosphate or 1-2 ppmsodium hydroxide. If these treatments are unsuccessful, the unit must be shut down(orderly with polishers, immediate without polishers). The unit then requires carefulinspection to determine possible damage, and the necessity for repair and possiblechemical cleaning prior to restart.







6-36

Cycling and Peaking



Layup

Layup procedures are presented in Section 4, and a road map suitable for drum boilersoperating on AVT is depicted in Figure 4-8.

Store the boiler under nitrogen or, if blowdown from a high temperature, maintain the boiler, superheaters and reheaters dry. Possibly dehumidify the feedwater heaters or

allow them to remain wet. Avoid air ingress to the feedwater heaters, as this leads tothe increased corrosion of copper alloys, particularly in the presence of ammonia.Monitor oxygen and pH, and nitrogen and moisture, as appropriate. It is also possibleto store the boiler filled with a solution containing ammonia and hydrazine. Avoidcontact of solutions with high ammonia concentrations with the copper alloys.


6.5 REFERENCES

1. Selection and Optimization of Boiler and Feedwater Treatment for Fossil Plants. ElectricPower Research Institute, Palo Alto, Calif. EPRI TR-105040. March 1997

2. Interim Consensus Guidelines on Fossil Plant Cycle Chemistry. Electric Power ResearchInstitute, Palo Alto, Calif. EPRI CS-4629. June 1986

3. Cycle Chemistry Guidelines for Fossil Plants: All Volatile Treatment. Electric PowerResearch Institute, Palo Alto, Calif. EPRI TR-105041. April 1996

4. G. M. W. Mann and R. Garnsey, “Waterside Corrosion Associated with Two-ShiftBoiler Operation on All-Volatile Treatment Chemistry.” Corrosion 79 Conference.Materials Performance, October 1980, pp 32-38

5. State-of-Knowledge of Copper in Fossil Plant Cycles. Electric Power Research Institute,Palo Alto, Calif. EPRI TR-108460, September 1997




6-1

6

ALL-VOLATILE TREATMENT

6.1 INTRODUCTION

All-volatile treatment must be used for once-through units, to avoid deposition of non-volatile compounds in the boiler water/steam circuit. With the “traditional” form of all-volatile treatment (AVT), ammonia and hydrazine is added to the feedwater toprovide chemical conditions that protect the feedwater heaters, boiler, superheaters,reheaters and turbine against corrosion, without further chemical additions.

An alternative chemical treatment for once-through units is oxygenated treatment (OT -see Section 7), where instead of hydrazine, oxygen is added in addition to theammonia. An intermediate form of conditioning is also being used, where onlyammonia, without hydrazine or oxygen, is added to the feedwater. With all threealternatives, the degree of protection against corrosion provided by the chemicaladditives is strictly limited. Therefore, it is essential to maintain high purity feedwaterand to prevent the ingress of impurities, both while on-load and during off-loadconditions.

All-volatile treatment can also be used for units with drum-type boilers, provided highpurity feedwater is available and the buildup of impurities in the boiler water is strictlycontrolled. The absence of a solid alkalizing chemical in the boiler water gives lessprotection against corrosion, but, because there should be lower concentrations of impurities in the boiler water, it also reduces the risk of carrying over boiler water saltsand solid alkalizing chemicals into the steam. If the high purity feed and boiler waterconditions cannot be maintained during startup, operation and shutdown, it may benecessary to resort to another form of chemical conditioning, such as phosphate orcaustic treatment. The Selection and Optimization document provides advice andguidance on the optimum boiler water and feedwater choices for drum units

(1).

Guidance on the use of AVT was given in the EPRI Interim Consensus Guidelines(2)

in1986. Cycle diagrams were provided for once-through and drum-type boilers with allferrous and mixed Fe-Cu metallurgy feedwater heating systems. The normal limits andthree action levels were given for sodium, chloride, sulfate, silica and cationconductivities for feedwater and steam for once-through boilers. A range of pressureswas considered for drum boilers, with and without reheat, with additional pressurecurves for the normal limits and three action levels for sodium, chloride, sulfate and





6-2

silica in the boiler water. In addition, some general guidance was also included onstartup, cycling and peaking operation.

The AVT guidelines for once-through and drum boilers were revised in 1996(3)

and“core” parameters for a minimum level of instrumentation were given (Table 1-1), in

addition to the comprehensive list of parameters. Guidance was included for all-ferrousand mixed Fe-Cu metallurgy for drum-type boilers, but was restricted to all-ferrousfeedwater heating systems for once-through boilers. Revised guidelines and actionlevels were given and guidance was included on optimizing AVT.

Further minor revisions to the “core” parameters were incorporated in the EPRI reporton Selection and Optimization of Boiler Water and Feedwater Treatment of FossilPlants

(1).

As an aid to planning and obtaining the optimum operating conditions for cycling,startup, shutdown and layup, it is important to know as accurately as possible, what

notice will be given of shutdown, the period of outage and for startup.

6.2 ONCE-THROUGH UNITS

Current Guidelines

In considering cycling, startup, shutdown and layup of units operating on AVT, it isassumed, in accordance with the latest EPRI Cycle Chemistry Guidelines

(3), that the

units with once-through boilers under consideration have condensate polishing plantsand all-ferrous feedwater heating systems. The cycle diagram for all parameters

(including the “core parameters”) is given in Figure 6-1 for once-through unitsoperating with AVT.

Startup

During off-load conditions, deposited impurities may have been released into thecircuit or have entered from outside, e.g. condenser leaks, CO2 with air in-leakage.Therefore, it is important to consider the effects on the feedwater and steam cationconductivity, silica, sodium, chloride and sulfate concentrations during startup. If thereis a leak in the reheaters, CO 2 and, more importantly, flyash and SO2, may be drawn in,resulting in acidic condensed steam in the reheaters. There is some evidence that silica

is washed off turbines during shutdown and startup. Oxygen and hydrazineconcentrations in the feedwater require special consideration. Restrictions will need to be applied to these parameters, but some relaxation from the normal EPRI guidelinevalues should be possible until steady state operating conditions are achieved. It should be possible to eliminate hydrazine entirely, for once-through units with all-ferrousfeedwater heating systems

(1) (see also Section 7 - Oxygenated Treatment).






HP

turbine

LP

turbine

Deaerator

Attemperat

Boiler

LP Condensate

polisher

Makeup

treatment

system

Condensate

storage tank

Condenser

IP

turbine

HP heaters

N (Normal)



Targets


Base Load Cycling

1 (Action Level 1)

2 (Action Level 2)

3 (Action Level 3)

—

336 ( 2 weeks)

48 (2 days)

8

1Immediate Shutdown

—

672 (4 weeks)

96 (4 days)

16

2

Hydrazine, ppb

Deaerator Outlet

Parameter

TargetSample N

C < 20 ppb

Oxygen, ppb

Deaerator Inlet

Parameter

TargetSample N

T < 10

Hydrazine, ppb

Low-Pressure Steam (Optional)

ParameterTarget Injection

Dosage

< 20

Air inleakage,

scfm/100 MWe

Air Removal System Exhaust

Parameter

TargetSample N

D ≤ 1

Cation conductivitya

or sodium

Condenser Leak Detection Trays orHotwell Zones (If applicable)

Parameter

Target

Sampl

C

Con

Pa

Sod

Oxy

Tot

•

Cat

µS/


µS/cma

Sodium, ppb


ParameterTarget

Sample N

Silica, ppb C ≤ 10

C ≤ 0.15

1

> 10

≤ 0.2

2

—

3

—

C ≤ 3 ≤ 6 ≤ 12 > 12CR •

CR • ≤ 0.3 > 0.65

CR •

CR •




1

< 9.2

> 9.6

2

—

3

—

Ammonia D

C

pHa

Consistent with pH

C ≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.65

Iron, ppb

Copper, ppb

Oxygen, ppb

W ≤ 5 > 5 — —

W ≤ 2 > 2 — —

C 1-10 ≤ 15 ≤ 20 > 20

CR •

CR •

Reheat Steam

ParameterTarget

Sample N

Sodium, ppb C ≤ 3

To ta l o rg an ic c arb on , p pb W ≤ 100


T —


µS/cma

C

Silica, ppb T ≤ 10

Chloride, ppb T ≤ 3

Sulfate, ppb T ≤ 3

1

≤ 6

> 100

—

≤ 20

≤ 6

≤ 6

2

≤ 12

—

—

≤ 40

≤ 12

≤ 12

3

> 12

—

—

> 40

> 12

> 12

CR •

CR • ≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.65

N = Normal

1 = Action Level 1

2 = Action Level 2

3 = Action Level 3

Target Values

Legend

C = continuous

S = grab, once per shift


W = grab, once per week

T = troubleshooting and commissioning

Sample Frequency

Footnotes


e = Target values may be adjusted to reflect capabilities of installed equipment

Figure 6-1 Cycle Chemistry Diagram for a Once-Through Unit on All-Volatile Treatment




6-4


StageduringStartup CircuitMonitoringa)Circulationviadeaerator pH, ironandcationconductivity b)Circulationviaeconomizer pH, Fe, cationconductivity, oxygenandhydrazinec)Circulationviatheboiler pH, Fe, cationconductivity, silica, sodiumandchlorided)Allowfiring pH, ironandcationconductivity

e)Allowsteamtoturbine pH, ironandcationconductivityf)Turbinetoonethirdload pH, ironandcationconductivityg)Tofullload pH, ironandcationconductivityOnfull load, thenormal EPRIGuidelinesvaluesgiveninFigure6-1shouldbeachieved. Theremayberoomforrelaxation, particularly, asregardssilicainsteam,duringstartup, theearlierstagesofoperationandduringlowloadoperation, providedtheparametersarewithintheEPRIactionlevels1-3. (Notethattheguidelinesarethesameforall operatingpressuresforonce-throughboilers.)Thecoreparameters,sodium, cationconductivityandoxygengivenintheEPRIGuidelinesshouldbemonitoredatthecondensatepumpdischarge, condensatepolisheroutlet, economizerinletandsteam, andalsopH, silicaandiron, ifpossible, atleastduringatrial period.Thenormal operatinglevels, asgivenintheEPRIguidelines, shouldbeachievedassoonaspossible. ThesearegiveninFigure6-1.Aroadmap(decisiontree)showingaschemeforstartupofaonce-throughunitoperatingwithAVTisgiveninFigure6-2. Thetimetakentoachievetherequiredtargetvalueswill beplantdependent. Thisshouldbedeterminedforeachunit, usedasguidanceforsubsequentstartups, andamendedinthelightofoperatingexperience.Theprocedurecanusuallybecurtailedforwarmandhotstarts.Theroadmapforstartupofonce-throughunitsoperatingonAVT, Figure6-2, canbedividedinto7steps.




6-5

No

Yes

Step 1




Safely vent N2 (if used)Refill per AVT guidelines


within the AVT guidelines

Is system full?


(Section 4)


guidelines?

Proceed withstartup



Proceed progressively withthe polishers, deaerator,

economiser in cleanup loop

Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7


Fire to boiler

Figure 6-2 Startup of Once-through Units with All-Ferrous Feedwater Heaters

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that for once-through boilers, very high purity water is required for the boiler,as well as for feedwater. Control of boiler water purity cannot be achieved by blowdown, as is the case with drum boilers, and the absence of the “reservoir” of the boiler and a steam separation stage in once-through boilers, means that the acceptablelevel of impurities is much less than for drum boilers.




6-6


If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for once-through units operating on AVT (3).


If the system has been stored using the low oxygen scavenger procedure (Section 4), the boiler can be fired immediately.


If the system has been stored wet with a surplus of ammonia and hydrazine (Section 4),it must be drained under nitrogen and refilled with water meeting the EPRI Guidelinesfor once-through units operating on AVT

(3).


If the system has been filled with nitrogen or filled with water, using nitrogen capping,the nitrogen must be safely vented and the boiler refilled, if necessary, with watermeeting the EPRI Guidelines for once-through units operating on AVT

(3). Startup of the

unit can then proceed.



Oxygen < 100 ppb

Iron < 30 ppbDuring startup, the levels of corrosion products (iron) can be very high initially andsilica may also be a problem. Achieving these limits is greatly facilitated by usingcondensate polishing (See Section 3). When these limits are attained, the boiler can befired.

Step 6 - Monitoring


During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problemcorrected at once.




6-7

Step 7 - Full Load

Full load can be achieved when chemical limits are within the EPRI Guidelines foronce-through units operating on AVT(3).

Shutdown

Information is given in Section 4.8 on Shutdown and below for various plannedshutdown conditions.

Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized. Maintain condensate circulation and deaerator pressure. Raise the pHwhen coming off-load.

Intermediate shutdown (weekend/week) - Leave plant pressurized, raise pH and thehydrazine concentration (if used), when coming off-load.

Long shutdown (longer than 1 week) - Drain under nitrogen or from high temperature(e.g. 130°C) and maintain the boiler, superheaters and reheaters dry. Continue asdescribed under “Layup”.



and SO2, may be drawn in, resulting in acidic condensed steam in the reheaters. There

is also evidence that silica is washed off turbines during shutdown.

A road map showing a scheme for shutdown of a once-through boiler operating withAVT is given in Figure 6-3. The time taken to achieve stable shutdown conditions will be plant dependent. This should be determined for each unit, as guidance forsubsequent shutdowns and amended in the light of operating experience.

The road map for shutdown of once-through units operating on AVT, Figure 6-3, can be divided into 5 steps.




6-8


No

Step 1


(Section 4)


continued operation

Step 2

Step 3

Step 3

Step 2

Yes





Unplanned outage

Yes

Yes

Yes

Yes



Planned outage

Step 4

Step 2

No

No

Step 3

Step 4

Step 5

Step 5

Orderly shutdown, ifpolishers utilized,


YesChemical intrusion

No

Figure 6-3AVT - Shutdown of Once-Through Units with All-Ferrous Feedwater Heaters

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that for once-through boilers, very high purity water is required for the boiler,as well as the feedwater, and control of the boiler water purity cannot be achieved by blowdown.


Normal cycling or peaking load reduction or shutdown presumes a short-term layup as

described in Section 4.


Step 2 - Outages




6-9

Planned or unplanned outages may be short or long term, depending on systemdemand or the extent of the work required to return the unit to operation. The length of time required for maintenance must be estimated and, depending on this estimate,short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options delineated in Section 4.8

(Figure 4-8).


Step 3 - Chemical Transients: Condenser Leaks (Fresh Water)

For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the condensate polishing plant maintainsthe feedwater quality.


Serious damage can occur to units within a short period of time with intrusion of seawater. With condensate polishing, the unit can generally be shut down in an orderlyfashion, especially if the polishers are of the deep bed type. With deep bed polishers, itis prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.


Chemical monitoring is especially important when a sea water leak is suspected.Sodium and cation conductivity will assist in estimating the extent of the leak.


Substantial intrusion of sea water into the boiler will require that the unit is drainedand refilled with water meeting the EPRI Guidelines for once-through units operatingon AVT

(3).




6-10




Polisher leakage





Silica from flyash or other contaminants from maintenance activitiesMinor chemical intrusions can be controlled by employment of idle condensatepolishers, if available.


During emergency shutdowns, such as for major sea water leaks and extensivechemical contamination, immediate unit shutdown is required. Therefore, little can bedone to adjust cycle chemistry during shutdown, since rapid action is required. Duringsuch occasions, the unit should be drained, flushed and inspected to assess damage, asoutlined previously. Restarting the unit will be contingent upon this inspection, as willlayup provisions.

Cycling and Peaking

Special features may be required for once-through units to cycle, such as a turbine bypass (Section 2).






6-11

Layup

Layup procedures are presented in Section 4 and a road map suitable for once-throughunits operating on AVT is depicted in Figure 4-8.

Store under nitrogen or, if shutdown from a high temperature, maintain the boiler,superheaters and reheaters dry. Possibly dehumidify the feedheaters or allow them toremain wet. Avoid air ingress into the feedheaters. Monitor oxygen and pH, andnitrogen and moisture, as appropriate.

The layup conditions are going to be dependent on the method of layup chosen. Thisshould be determined for each unit and each method of layup used. The informationobtained should then be used as guidance for subsequent repeat operations andamended in the light of operating experience.

6.3 DRUM BOILERS WITH ALL-FERROUS FEEDWATER HEATING

SYSTEMS

In general, the features for the feedwater and steam for units with once-through boilers,also apply to drum boilers. However, these need to be supplemented by additionalconsiderations for the boiler water. Some units with drum boilers may be equippedwith a condensate polishing plant, but many units are not and, therefore, will be moresusceptible to the effects of condenser leaks.

For completeness and ease of reference for AVT, phosphate treatments (Section 5), andcaustic treatment (Section 8), drum boilers with all-ferrous and mixed Fe-Cumetallurgy feedwater heating systems are considered separately in the next twosections.

Current Guidelines

The current guidelines for drum boilers with all-ferrous feedwater heating systems aregiven in Figure 6-4 for units with reheat, including the “core” parameters. The normallimits and three action levels for sodium, chloride, sulfate, silica and cationconductivity in boiler water are given in Figures 6-5 to 6-9 for a range of operatingpressures. The values given for plants without reheat are twice those for plants withreheat, except for pH, oxygen, TOC, iron and copper. They are not reproduced here

and reference should be made to the original documents(1,3)

.






6-13

Action level 3

Normal

10

8

7

2

6

5

4

3

9

S o d i u m ( p

p m N

a )

1500 19001100 17001300

Pressure (psia)

900 27002300 285025002100

Action level 1

Action level 2

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

0.1

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.6

Drum Pressure (MPa)

Figure 6-5 All Volatile Treatment: Drum Boiler Water Sodium vs. OperatingPressure (Plants With Reheat)




6-14

Pressure (psia)

1300 1700700 15001100600 2500 28502100 270023001900900


l )

Actionlevel 2

Normal

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

3.0

2.0

1.5

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-6 All-Volatile Treatment: Drum Boiler Water Chloride vs. OperatingPressure (Plants With Reheat)




6-15

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9


3.0

2.0

1.5

1300 1700700 15001100600 2500 28502100 270023001900900


O 4

)

Actionlevel 2

Normal

Actionlevel 1

Action level 3

0.10

0.080.07

0.02

0.06

0.05

0.04

0.03

0.09

0.01

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-7 All-Volatile Treatment: Drum Boiler Water Sulfate vs. OperatingPressure (Plants With Reheat)




6-16

10

87

2

6

5

4

3

9


0.10

0.080.070.06

0.05

0.04

0.03

0.09

20

15

1300 1700700 15001100600 2500 28502100 270023001900900


i O 2 )

Actionlevel 2

Normal Actionlevel 1

Action level 3

1.0

0.80.7

0.2

0.6

0.5

0.4

0.3

0.9

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-8 All-Volatile Treatment: Drum Boiler Water Silica vs. Operating Pressure(Plants With Reheat)




6-17

100

80

70

20

60

50

40

30

90


1300 1700700 15001100600 2500 28502100 270023001900900

C a t i o n C o n d u c t i v i t y - µ S / c m

Normal

Actionlevel 1

Actionlevel 2

Action level 3

10

87

2

6

5

4

3

9

1

11.7 13.19.06.2 17.2 19.614.57.6 10.3 15.8 18.64.8

Drum Pressure (MPa)

Figure 6-9 All-Volatile Treatment: Drum Boiler Water Cation Conductivity vs.Operating Pressure (Plants With Reheat)




6-18

Startup

During off-load conditions, impurities may have been released into the circuit or haveentered from outside, e.g. condenser leaks, CO2 with air ingress. Therefore, duringstartup, it is important to consider the effects of these on the feedwater, boiler waterand steam cation conductivity, silica, sodium, chloride and sulfate concentrationsduring startup. If there is a leak in the reheaters, CO2 and, more importantly, flyash andSO

2, may be drawn in, resulting in acidic condensed steam in the reheaters. There is

some evidence that silica is washed off turbines during shutdown and startup. Oxygenand hydrazine concentrations in the feedwater also require special consideration.Restrictions will need to be applied to these parameters, but some relaxation from thenormal EPRI guideline values may be possible until steady state conditions areachieved.

Section 3 explains how the chemistry curves and action levels can be utilized during

unit startup. Basically, during startups, the initial lower boiler pressure permits boilerwater chemical concentrations to be higher than those at normal unit operatingpressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.


It is important to prevent high concentrations of oxygen and chloride in the boilerwater, as these can act synergistically and may initiate corrosion

(4). For this reason, the

oxygen concentration of the boiler water should not be allowed to exceed 0.2 ppmduring boiler operation. It is also important to ensure that the pH, cation conductivityand chloride concentration are within the limits given in the EPRI

(1,3) guidelines for

boiler water. It might be possible to eliminate hydrazine in plants with all-ferrousfeedwater heating systems(1) (see Section 7 - Oxygenated Treatment).





6-19


a) Addition of hydrazine (if used) and

ammonia to the feedwater





At full load, the normal EPRI guideline values given in Figure 6-4 should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figures 6-5 to 6-9. The core

parameters, sodium, cation conductivity and oxygen, given in the EPRI guidelines(1,3)

should be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH, andsteam. The normal operating levels, as given in the EPRI guidelines

(1,3), should be

achieved as soon as possible. These are given in Figure 6-4.

A road map showing a scheme for startup of a drum boiler with an all-ferrousfeedwater heating system operating with AVT is given in Figure 6-10. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups and amended in the light of operating experience. The procedure can normally be curtailed for warm and hot starts.

If the high purity feed and boiler water conditions cannot be maintained duringstartup, it may be necessary to resort to another form of chemical conditioning, such asphosphate or caustic treatment. The Selection and Optimization document providesadvice and guidance on the optimum boiler water and feedwater choices for drumunits

(1).




6-20

No

Yes

Step 1


Fire to boiler






within AVT guidelines bycontrolling pressure and blow-down. Avoid excess carry-over


Is system full?


(Section 4)


guidelines?

Proceed withstartup



Proceed progressively withpolishers (if fitted), deaerator,economiser in clean-up loop

Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Figure 6-10 AVT - Startup of Drum Boilers with All-Ferrous Feedwater Heaters




6-21

The road map for startup of a drum-type unit operating on AVT with all-ferrousfeedwater heaters, Figure 6-10, can be divided into 7 steps.

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that higher purity water is required for units operating with AVT. Unlike once-through units operating on AVT, described in Section 6.2, control of boiler water puritycan be achieved by blowdown with drum units.


If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for drum units operating on AVT

(3).


If the system has been stored using the low oxygen scavenger procedure (Section 4.8),

the boiler can be fired immediately.Step 3 - System Stored Wet: Excess Ammonia and Hydrazine

If the system has been stored wet with a surplus of ammonia and hydrazine(Section 4.8), it must be drained under nitrogen and refilled with water meeting theEPRI Guidelines for drum units operating on AVT

(3).


If the system has been filled with nitrogen or filled with water, using a nitrogen cap,the nitrogen must be safely vented and the boiler refilled, if necessary, with water

meeting the EPRI Guidelines for drum units operating on AVT(3)

. Startup of the unit canthen proceed.



Oxygen < 100 ppb

Iron < 100 ppb

During startup, the levels of corrosion products (iron) can be very high initially and

silica may also be a problem. Achieving these limits is greatly facilitated by usingcondensate polishing (if fitted) (See Section 3). When these limits are attained, the boilercan be fired.





6-22

Step 6 - Monitoring


During startup, the concentration vs. pressure curves can be utilized to control sodium,silica, chloride and sulfate (for examples see Figures 6-5 to 6-9). Boiler pressure shouldremain at reduced levels such that these limits are maintained before pressure can beincreased to the next stage. Maximum use of blowdown and condensate polishing (if available) will minimize startup times.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problem

corrected at once. Excursions affecting (lowering) boiler water pH must be correctedimmediately by feeding trisodium phosphate or 1-2 ppm of sodium hydroxide. Avoidexcess carryover of boiler water impurities into the steam.


Step 7 - Full LoadFull load can be achieved when chemical limits are within the EPRI Guidelines fordrum boilers operating on AVT

(3).

Shutdown


Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized.

Intermediate shutdown (weekend-week) - Leave plant pressurized or store the boilerunder nitrogen.

Long shutdown (longer than 1 week) - Drain the boiler under nitrogen or blowdownfrom high temperature (e.g. 130°C, 266°F) and maintain the boiler, superheaters andreheaters dry. Continue as described under “Layup”.




6-23




A road map showing a scheme for shutdown of a drum boiler with an all-ferrousfeedwater heating system operating with AVT is given in Figure 6-11. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience.


No

Step 1

Use short term

layup procedure(Section 4)


continued operation

Step 2

Step 3

Step 3

Step 2

Yes





Unplanned outage

Yes

Yes

Yes

Yes



Planned outage

Step 4

Step 2

No

No

Step 3

Step 4

Step 5Step 5

Adjust pH >8.0, orderly

shutdown, if polishersutilized, otherwise

immediate shutdown


No

Figure 6-11 AVT - Shutdown of Units with Drum Boilers with All-Ferrous andMixed Metallurgy Feedwater Heaters




6-24

The road map for shutdown of a drum-type unit operating on AVT with all-ferrousfeedwater heaters, Figure 6-11, can be divided into 5 steps.

Again, the steps are essentially similar to those described in Section 5 for phosphatetreatment. Unlike once-through boilers operating on AVT, described in Section 6.2,

control of the boiler water purity can be achieved by blowdown with drum boilers.


Normal cycling or peaking load reduction or shutdown presumes a short-term layup asdescribed in Section 4.



be adjusted to conform with the AVT guidelines for drum units(3).

Particular care should be exercised to prevent oxygen ingress during this period and blowdown should be maintained at an appropriate level to remove contaminants fromthe system.

Step 2 - Outages


short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options delineated in Section 4.8(Figure 4-8).



For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the unit is still operational under reducedload (divided water box) or if the condensate polishing plant (if fitted) maintains thefeedwater quality.


Serious damage can occur to units within a short period of time with intrusion of seawater. Without condensate polishing, the boiler must be shutdown immediately upon




6-25

identifying a significant condenser leak. The addition of trisodium phosphate or 1-2ppm of sodium hydroxide may also be required as the boiler water pH drops.

With condensate polishing (if fitted), the unit can generally be shut down in an orderlyfashion, especially, if the polishers are of the deep bed type. With deep bed polishers, it

is prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.



In any event, the unit should be shutdown to repair the condenser leak. Depending on

an evaluation of the cycle chemistry, shut down will be immediate or orderly, asoutlined above.


(3).




Polisher leakage







More serious chemical intrusions may affect (lower) the boiler water pH, necessitatingadjustment of boiler water pH through the addition of trisodium phosphate or 1-2 ppm




6-26

sodium hydroxide. If these treatments are unsuccessful, the unit must be shut down(orderly with polishers, immediate without polishers) if the pH falls below 8 (Figure6-4). The unit then requires careful inspection to determine possible damage, and thenecessity for repair and possible chemical cleaning prior to restart.




Cycling and Peaking


Essentially, cycling and peaking plants should operate according to the outline givenabove for startup, load changes and shutdown.

Layup

Layup procedures are presented in Section 4, and a road map suitable for units withdrum boilers operating on AVT is depicted in Figure 4-8.

Store the boiler under nitrogen or, if blowdown from a high temperature, maintain the boiler, superheaters and reheaters dry. Possibly dehumidify the feedwater heaters orallow them to remain wet. Avoid air ingress into the feedwater heaters. Monitoroxygen and pH, and nitrogen and moisture, as appropriate. It is also possible to storethe boiler filled with a solution containing ammonia and hydrazine.





6-27

6.4 DRUM UNITS WITH MIXED METALLURGY FEEDWATER HEATING

SYSTEMS

EPRI(5)

has recently produced a report on the State-of-Knowledge of Copper in FossilPlant Cycles as the first stage of the “Program Copper” project. This was in response to

a demand from members, particularly in the US, for improved performance of powerplants containing copper alloys. Traditionally, these have been widely used incondensers, but, in some plants, brass and/or cupro-nickel have also been used for heatexchange surfaces in low and high pressure feedwater heaters. Copper released fromfeedwater heaters deposits in the boiler, increasing the locations where impurities canconcentrate on boiler waterwalls. Thick deposits can lead to overheating and thepresence of copper in deposits complicates chemical cleaning.

Copper in high pressure boilers can be carried over into the steam and deposited inhigh pressure turbines, where even as little as 1 kg can reduce the output capacity of the turbine by 1 MW. This is particularly true for plants operating at more than 2400 psi(16.6 MPa) and is exacerbated further by increasing pressure. The presence of oxygenand the absence of reducing conditions in the feedwater during all periods of operationand shutdown is the main cause of copper transport round the circuit.

It is worth reiterating that startups are generally considered to be the periods of maximum copper transport activity in the cycle. This relates directly to the feedwatersystem not being protected during shutdown periods, i.e. that a reducing environment(ORP < 0mV) is not maintained. Air in-leakage into the LP feedwater heating circuitsincreases the growth of non-protective copper oxides and copper transport. The EPRI“Guiding Principles” for successful operation of units with copper alloys

(5) are:










6-28

Current Guidelines

The current guidelines for drum boilers with mixed Fe-Cu metallurgy feedwatersystems are given in Figure 6-4 for reheat plants, including the “core” parameters. Thenormal limits and three action levels for sodium, chloride, sulfate, silica and cation

conductivity in boiler water are given in Figures 6-5 to 6-9 for a range of operatingpressures. Except for pH, oxygen, TOC, iron and copper, the limits for non-reheat plantare generally higher by a factor of two. They are not reproduced here and referenceshould be made to the original documents

(1,3).

Startup

During off-load conditions, impurities may have been released into the circuit or haveentered from outside, e.g. condenser leaks, CO2, with air ingress. Therefore, duringstartup it is important to consider the effects of these on the feedwater, boiler water andsteam cation conductivity, silica, sodium, chloride and, sulfate concentrations. If there

is a leak in the reheaters, CO 2 and, more importantly, flyash and SO2, may be drawn in,resulting in acidic condensed steam in the reheaters. There is some evidence that silicais washed off turbines during shutdown and startup. Oxygen, hydrazine and copperconcentrations in the feedwater and copper concentrations of the steam will alsorequire special consideration, since the presence of oxidizing conditions increase therelease and transport of copper around the circuit.


It is important to prevent high concentrations of oxygen and chloride in the boilerwater, as these can act synergistically and may initiate corrosion. For this reason, theoxygen concentration of the boiler water should not be allowed to exceed 0.2 ppmduring boiler operation. The presence of oxygen also assists the transport of copper intothe steam, increasing the risk of deposition in the superheater and high pressureturbine. It is also important to ensure that the pH, cation conductivity and chlorideconcentration are within the limits given in the EPRI

(1,3) guidelines for boiler water.

Hydrazine (or volatile reducing agents) should not be eliminated from plants withmixed Fe-Cu feedwater heating systems.

Depending on the plant, and the shutdown and layup conditions adopted, several steps

will be required for startup (in addition to filling the boiler following a long shutdownand safely venting the nitrogen, if used). The steps used during startup will be plantspecific and, depending on the practicality, the following stages should be monitored,at least during a trial period:


a) Addition of hydrazine and ammonia to pH, cation conductivity, oxygen and




6-29

the feedwater copper


c) Care to avoid additional carryover into

steam

Cation conductivity, silica, sodium and

copper

At full load, the normal EPRI guidelines values given in Figure 6-4 should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figures 6-5 to 6-9. The coreparameters, sodium, cation conductivity and oxygen given in the EPRI

(1,3) guidelines

should be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH, andsteam, including copper. The normal operating levels, as given in the EPRI

(1,3)

guidelines, should be achieved as soon as possible. These are given in Figure 6-4.

A road map showing a scheme for startup of a drum boiler with mixed Fe-Cufeedwater heating system operating with AVT is given in Figure 6-12. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups, and amended in the light of operating experience. The procedure can normally be curtailed for warm and hot starts.

The road map, Figure 6-12, can be divided into 7 steps.

Section 3 explains how the chemistry curves and action levels can be utilized duringunit startup. Basically, during startups, the initial lower boiler pressure permits boilerwater chemical concentrations to be higher than those at normal unit operatingpressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.

If the high purity feed and boiler water conditions can not be maintained duringstartup, it may be necessary to resort to another form of chemical conditioning, such asphosphate or caustic treatment. The Selection and Optimization document providesadvice and guidance on the optimum boiler water and feedwater choices for drumunits

(1).

The steps are essentially similar to those described in Section 5 for phosphate treatment,except that higher purity water is required for units operating with AVT. Unlike once-through units operating on AVT, described in Section 6.2, control of boiler water puritycan be achieved by blowdown with drum units. However, compared with units withall-ferrous feedwater heating systems described in Section 6.3, additionalconsiderations are required for units with mixed Fe-Cu feedwater heaters. To reducethe risk of copper corrosion and transport, the ingress of oxygen must be minimized




6-30

and chemically reducing conditions must be maintained during all periods of operation, shutdown and layup.

No

Yes

Step 1


Fire to boiler




Add N2H4, if necessary.Reduce O2 to 100 ppb, Fe

to 100 ppb, Cu to 10 ppb


within AVT guidelines bycontrolling pressure and blow-down. Avoid excess carryover


Is system full?


(Section 4)


guidelines?

Proceed withstartup




Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Figure 6-12 AVT - Startup of Drum Boilers with Mixed Metallurgy FeedwaterHeaters

For ease of reference, full details of the steps for the startup of drum units with mixedFe-Cu feedwater heating systems, operating on AVT, are given below:




6-31


If the system has been stored dry (Section 4), it should be filled with water which meetsthe EPRI Guidelines for drum units operating on AVT (3).


If the system has been stored using the low oxygen scavenger procedure (Section 4.8),the boiler can be fired immediately.


If the system has been stored wet with a surplus of ammonia and hydrazine (Section 4),it must be drained under nitrogen and refilled with water meeting the EPRI Guidelinesfor drum units operating on AVT

(3).


If the system has been filled with nitrogen or filled with water, using nitrogen capping,the nitrogen must be safely vented and the boiler refilled, if necessary, with watermeeting the EPRI Guidelines for drum units operating on AVT

(3). Startup of the unit can

then proceed.



Oxygen < 100 ppb

Iron < 100 ppbCopper < 10 ppb

During startup, the levels of corrosion products (iron and copper) can be very highinitially and silica may also be a problem. Achieving these limits is greatly facilitated by using condensate polishing (See Section 3). When these limits are attained, the boilercan be fired.


Step 6 - Monitoring





6-32

During startup, the concentration vs. pressure curves can be utilised to control sodium,silica, chloride and sulfate (for examples see Figures 6-5 to 6-9). Boiler pressure shouldremain at reduced levels such that these limits are maintained before pressure can beincreased to the next stage. Maximum use of blowdown and condensate polishing (if available) will minimize startup times.

During startup, any chemical excursions must be dealt with quickly and effectively.Increases in feedwater sodium and cation conductivity may indicate contaminationfrom the makeup system, contamination from chemical cleaning operations, condensatepolisher malfunction or condenser leakage. The source must be found and the problemcorrected at once. Excursions affecting (lowering) boiler water pH must be correctedimmediately by feeding trisodium phosphate or 1-2 ppm of sodium hydroxide.

Effects of cycle contamination are magnified at startup due to relatively low flow ratesfor condensate, feedwater and steam. Cation conductivity may increase as a result of air ingress due to either aeration of water during the shutdown period or air in-leakage

during startup. The change to boiler water chemistry will be minimal compared tocontamination involving the makeup system, chemical cleaning activities, condensatepolishers or condenser leaks.

Avoid excessive carryover of boiler water impurities into the steam. This includescopper, whose transport can be minimized by avoiding ingress of oxygen andmaintaining chemically reducing conditions.

Step 7 - Full Load

Full load can be achieved when chemical limits are within the EPRI Guidelines for

drum boilers operating on AVT

(3)

.

Shutdown


Short shutdown (overnight/weekend) - No change to chemical conditions, leave plantpressurized, avoid air ingress to the feedheaters.

Intermediate shutdown (weekend-week) - Leave plant pressurized or store the boiler

under nitrogen. Avoid air ingress to the feedwater heaters.

Long shutdown (longer than 1 week) - Drain the boiler under nitrogen or blowdownfrom high temperature (e.g. 130°C) and maintain the boiler, superheaters and reheatersdry. Continue as described under “Layup”.




6-33


During shutdown. if there is a leak in the reheaters, CO2 and, more importantly, flyash


A road map showing a scheme for shutdown of a drum boiler with a mixed Fe-Cu-feedwater heating system operating with AVT is given in Figure 6-11. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience.

The road map for shutdown of a unit with a drum boiler operating on AVT with mixedmetallurgy feedwater heaters, Figure 6-11, can be divided into 5 steps.

Again, the steps are essentially similar to those described in Section 5 for phosphatetreatment, except that higher purity water is required for units operating with AVT.Unlike once-through boilers operating on AVT, described in Section 6.2, control of the boiler water purity can be achieved by blowdown with drum boilers. However,compared with units with all-ferrous feedwater heating systems described in Section6.3, additional considerations are required for units with mixed Fe-Cu feedwaterheaters. To reduce the risk of copper corrosion and transport, the ingress of oxygenmust be minimized and chemically reducing conditions must be maintained.




During orderly load reductions, the condensate cycle and boiler chemical limits should be adjusted to conform with the AVT guidelines for drum units

(3).

Particular care should be exercised to prevent oxygen ingress during this period tominimize pickup of copper. Blowdown should be maintained at an appropriate level toremove contaminants from the system.




6-34

Step 2 - Outages


short term or longterm layup should be initiated (Section 4). Chemistry should beadjusted prior to shutdown, as indicated in the various options deliniated in Section 4.8(Figure 4-8).



For condenser leaks with fresh, relatively low dissolved solids cooling water, the leakcan generally be isolated and repaired, while the unit is still operational under reduced

load (divided water box) or if the condensate polishing plant (if fitted) maintains thefeedwater quality.


Serious damage can occur to units within a short period of time with intrusion of seawater. Without condensate polishing, the boiler must be shutdown immediately uponidentifying a significant condenser leak. The addition of trisodium phosphate or 1-2ppm of sodium hydroxide may also be required as the boiler water pH drops.

With condensate polishing (if fitted), the unit can generally be shut down in an orderlyfashion, especially, if the polishers are of the deep bed type. With deep bed polishers, itis prudent to maintain one or more vessels in the hydrogen form for added protectionagainst condenser leakage.





(3).




6-35




Polisher leakage

Poor regeneration

Acid or caustic contamination





More serious chemical intrusions may affect (lower) the boiler water pH, necessitatingadjustment of boiler water pH through the addition of trisodium phosphate or 1-2 ppmsodium hydroxide. If these treatments are unsuccessful, the unit must be shut down(orderly with polishers, immediate without polishers). The unit then requires carefulinspection to determine possible damage, and the necessity for repair and possiblechemical cleaning prior to restart.







6-36

Cycling and Peaking



Layup

Layup procedures are presented in Section 4, and a road map suitable for drum boilersoperating on AVT is depicted in Figure 4-8.

Store the boiler under nitrogen or, if blowdown from a high temperature, maintain the boiler, superheaters and reheaters dry. Possibly dehumidify the feedwater heaters or

allow them to remain wet. Avoid air ingress to the feedwater heaters, as this leads tothe increased corrosion of copper alloys, particularly in the presence of ammonia.Monitor oxygen and pH, and nitrogen and moisture, as appropriate. It is also possibleto store the boiler filled with a solution containing ammonia and hydrazine. Avoidcontact of solutions with high ammonia concentrations with the copper alloys.


6.5 REFERENCES


2. Interim Consensus Guidelines on Fossil Plant Cycle Chemistry. Electric Power ResearchInstitute, Palo Alto, Calif. EPRI CS-4629. June 1986

3. Cycle Chemistry Guidelines for Fossil Plants: All Volatile Treatment. Electric PowerResearch Institute, Palo Alto, Calif. EPRI TR-105041. April 1996

4. G. M. W. Mann and R. Garnsey, “Waterside Corrosion Associated with Two-ShiftBoiler Operation on All-Volatile Treatment Chemistry.” Corrosion 79 Conference.Materials Performance, October 1980, pp 32-38

5. State-of-Knowledge of Copper in Fossil Plant Cycles. Electric Power Research Institute,Palo Alto, Calif. EPRI TR-108460, September 1997




7-1

7

OXYGENATED TREATMENT

7.1 INTRODUCTION

For the application of oxygenated treatment (OT) in units with once-through and drum boilers, there are four indispensable prerequisites:

All-ferrous feedwater heater metallurgy (copper alloys may be used only in

condenser tubing).

Cation conductivity < 0.15 µS/cm (at 25°C) in condensate, feedwater, and steam.

Hydrazine and other oxygen scavengers are not used.

Condensate polishing

OT reduces or eliminates most of the typical AVT problems such as(1)

:

1. Flow-accelerated corrosion in the feedwater system and in the economizer inlet

tubes and headers.

2. Deposition of feedwater corrosion products on the boiler feed pump.

3. Corrosion product transport into the boiler resulting in orifice fouling, boilerdeposits and pressure drop problems, thermal fatigue boiler tube failures,overheating boiler tube failures, and frequent chemical cleaning.

4. Turbine fouling.

5. Copper alloy condenser tube failures resulting from ammonia grooving (when

operating with OT at reduced ammonia levels).

Whereas the OT Guideline(1)

covers in detail the transition from AVT to OT and normaloperation on OT, the following sections deal with startup and shutdown procedures,cycling and peaking operation, and layup procedures for once-through and drum boiler units operated on OT.




Oxygenated Treatment

7-2

7.2 ALL-FERROUS CYCLES WITH ONCE-THROUGH BOILERS

Current Normal Operating Guidelines

Oxygenated treatment (OT) uses high purity water to minimize corrosion and flow-accelerated corrosion (FAC) in the feedwater train. The normally desired cation

conductivity level in all plant cycle streams is <0.15

S/cm (at 25C); lower values are

preferred and attainable. OT can be applied only in plant cycles with all-ferrousmetallurgy and full-flow condensate polishing downstream of the condenser. With OTfor once-through units, an oxygen level of 30-150 ppb is maintained across the wholeplant cycle. The use of oxygen as a corrosion inhibitor allows satisfactory operationover a large pH range (7-10). Thus, a marked reduction in plant cycle pH comparedwith all-volatile treatment (AVT) is possible. The application of a pH range from 8.0 to8.5 results in a reduction of condensate polisher regeneration frequency and theassociated costs.

During normal operation the vents on the deaerator are closed. It is also veryimportant with OT that the optimum heater vent position is maintained to ensure theheater drains are fully protected from FAC. This usually involves the operatorensuring that an oxidizing environment is present in the drains (ORP > 0mV)

(8).

Figure 7-1 shows the cycle chemistry diagram of a cycle with a once-through boileroperated on OT

(1). Here, the normal target values and the action levels for condensate

pump discharge, combined condensate polisher effluent, economizer inlet, and steampurity as well as for makeup treatment system effluent are provided.

Oxygenated treatment causes very stable conditions regarding the minimum corrosionproduct transport in the plant cycle. A temporary oxygen or ammonia feed loss is notconsidered to be a very serious situation. Efforts should be make to restore the feed of both chemicals as soon as practical. Overfeed of ammonia and oxygen is likewise not aserious event. However, if the condenser tubing is made of admiralty brass, thenammonia overfeed could result in ammonia grooving. Again, efforts should be madeto establish the appropriate dosing as soon as practical. Particularly ammonia overfeeddoes have cost consequences because of additional loading of the condensate polisherswhen operated in the hydrogen-hydroxyl form.




CR

HP

turbine

LP

turbine

Deaerator

Attemperation

Boiler

LP heaters Condensate

polisher

Makeup

treatment

system

Condensate

storage tank

Condenser

IP

turbine

HP heaters

N (Normal)



Targets


Base Load Cycling

1 (Action Level 1)

2 (Action Level 2)

3 (Action Level 3)

—

336 ( 2 weeks)

48 (2 days)

8

1Immediate Shutdown

—

672 (4 weeks)

96 (4 days)

16

2

Con

Pa

Sod

Ox

Ca

µS/


µS/cma

Sodium, ppb


Parameter

Target

Sample N

C < 0.15

1

< 0.2

2 3C ≤ 3 > 3 > 6 > 24

– –


ParameterTarget Sample N


1 2

–

3

– pHa

Steam

ParameterTarget

Sample N

Sodium, ppb C ≤ 3


µS/cma

C

1

≤ 6

2

≤ 12

3

> 12CR

≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.65


C ≤ 0.15 ≤ 0.2 ≤ 0.3 > 0.3

Oxygen, ppb C 30-150 – –

CR

N = Normal

1 = Action Level 1

2 = Action Level 2

3 = Action Level 3

Target Values

Legend

C = continuous

S = Grab. once per shift


W = grab. once per week

Sample Frequency

Footnotes


Oxygen, ppb C 30-150 – – –

–

–

CR

CR

CR

CR

CR

Air scf

Air R

Par

Figure 7-1 Cycle Chemistry Diagram of Once-Through Units on Oxygenated Treatment (core




7-4

Startup Procedures

Startup is accomplished by essentially the same startup procedure as is used for AVT(Section 6.2). Some minor variations in startup procedures exist, with the variationdepending upon the type of unit shutdown and subsequent layup procedure which

preceded the startup. For a short outage, no layup actions other than discontinuingoxygen feed are recommended. For short duration layups, a relatively quick startup isanticipated.

For longterm layups, increased levels of ammonia are suggested, and some additionalrecirculation and venting are required to reduce ammonia levels and to reduce cationconductivity to acceptable levels during startup. Normal station startup should beperformed as with AVT including ammonia addition but absolutely without hydrazineor other oxygen scavenger addition. Startup should progress through cold and hotcleanup, startup, and ramping activities as customary with AVT. Ammonia addition

begins with the first use of condensate polishers or when the condensate pumps arestarted. Deaerator pegging and venting are performed until oxygen addition is started.

Oxygen addition does not begin until cation conductivity reaches 0.15 µS/cm (at 25°C)and is continuing to downtrend. Deaerator vents should then be positioned ascustomary in steady-state operation. The same is true for heater vents.

Once-through boilers utilize a by-pass system to facilitate cleanup of the pre-boilersystem during startup. This is a system of piping, valves and flash tank (verticalseparator) utilized during starting, stopping and low load operations. Firing a once-through boiler is not permitted unless minimum design limits for feedwater flow aresatisfied. Since once-through boilers have the flow going directly from the economizerthrough the boiler to the turbine, the by-pass system provides protection of the turbineduring startup by isolating the turbine from the boiler by valves.

The by-pass system allows for circulation through the boiler, the feedwater heaters andthe waste cleanup (polishing) system to meet water quality requirements prior to firing.Downstream of the flash tank, the by-pass system directs heater condensate and steamto the deaerator and first point feedwater heater for heat recovery. Steam is suppliedfrom the flash tank for turbine cooling, loading and low load operation up to the once-through transfer load point. At the once-through transfer point, steam flow to the

turbine equals minimum boiler feedwater flow.

One of the most important advantages of OT is that startups can be accomplished muchfaster than when the unit is operated with AVT. There should be no holds or otherramping activities if the OT is operated in the optimum fashion according to the EPRIguidelines(1). For startups following longterm layup, the feedwater iron levels should




7-5

Figure 7-2 shows a road map for startup of once-through units operating with OT.

Layupduration

<4 weeks?

Yes

No

No

No

Yes

Dry

No

Wet

Short-term Longterm

Yes

Start oxygen dosingand reduce pH to8-8.5 if necessary

Is feedwatercation conductivity

<0.15 µS/cm?

Fill or fill up withdeoxygenated

water (O2<10 ppb,pH 8-8.5)

Fill system withdeoxygenated

water (O2<10 ppb,pH >9)

Refill system withdeoxygenated


Issystem filledfor startup?

Startupafter short-term

or longtermlayup?

Step 1: Startup preparation

Step 2:Systemcleanup

Step 3: Fire to boiler

Step 4: Transition to OT

Commence startup.Proceed with

available cleanuploops; ventingvery important

Commence startupwithout any

cleanup actions exceptventing


<0.65 µS/cm?

Yes

Dry or wetlayup?

Fire to boiler.Maintain temperatureramp within boiler andturbine manufacturers

requirements

Figure 7-2 Road map for the startup of once-through boilers operated with OT.




7-6

As mentioned above, the application of OT in all-ferrous cycles with once-through boiler cycles markedly simplifies and shortens the startup procedure. For this reason,the road map for startup of once-through units operating with OT (Figure 7-2) consistsof only 4 main steps.

Step 1 - Startup Preparation

Startup after short-term or longterm layup?

The first startup actions depend on the layup duration. In case of short-term orintermediate layup, the procedure is simpler and faster compared to the case of a unitwhich was the subject of a longterm layup.

Startup after short-term layup

Is system filled for startup?

Systems filled for startup can start up very fast without any cleanup actions except forheater and deaerator venting. Otherwise the system has to be filled with deoxygenatedwater (O2 < 10 ppb, pH 8-8.5). The cation conductivity of the water used for filling thesystem should meet the EPRI OT Guideline. The startup after short-term layup is anexception because it is, in contrary to startup after longterm layup, performed with afeedwater pH of 8-8.5.

Startup after longterm layup

Dry or wet layup?

If the system has been stored dry it should be filled with deoxygenated water (O2 < 10ppb, pH > 9) which meets the EPRI AVT guideline. If the system has been stored wet,e.g. nitrogen cap combined with an oxygen free (< 10 ppb oxygen) ammonia treated(pH > 9) water, it should be refilled with deoxygenated water (O

2 < 10 ppb, pH > 9)

which meets EPRI AVT Guideline.

Layup duration < 4 weeks?

According to the general OT operation experience, it is possible to commence startup of

units stored for less than 4 weeks without any special cleanup actions except deaeratorand heater venting. In other cases, continue with available cleanup loops and take careof adequate deaerator and heater venting.




7-7

Step 2 - System Cleanup

The startup of units operated with OT occurs without any oxygen dosing as in unitsoperated with AVT. The only difference to AVT is the feedwater pH of 8-8.5 in the caseof units starting up after short-term layup.

Is feedwater cation conductivity < 0.65 µS/cm and downtrending?

If the feedwater cation conductivity is > 0.65 µS/cm, then the by-pass of the pre-boilersystem (cleanup loop) has to be used to remove cycle contamination. Otherwise thestartup could begin without any cleanup actions.

An important part of this startup step is the deaerator and heater venting.

Step 3 - Fire to Boiler

It is required that the feedwater cation conductivity is < 0.65 µS/cm and downtrendingprior to firing the boiler. Naturally, during startup the temperature ramp has to bemaintained within boiler and turbine manufacturers’ requirement.

The main cycle chemistry surveillance parameter in this and in the following startupstep is the feedwater cation conductivity. The other plan cycle core parameterscomplete the information on the current cycle chemistry. Sampling and analysis forcorrosion products on a regular basis are, as a rule, not necessary. Nevertheless, it may be meaningful to check the feedwater corrosion product level during some selectedstarts to obtain more information on the required cleanup duration.

Step 4 - Transition to OT

Is feedwater cation conductivity < 0.15 µS/cm?

The transition to OT (oxygen dosing and, if necessary, pH reduction to 8-8.5) takesplace only when the feedwater cation conductivity drops below 0.15 µS/cm (at 25°C).Deaerator and heater vents should then be positioned as customary in steady-stateoperation. Monitoring of all plant cycle core parameters is mandatory.

Shutdown Procedures

The procedures generally applicable to shutdown are included in Section 4.

Hydrazine or other oxygen scavengers should not be utilized in a unit operating on OT.The preferred practice is to stop the oxygen feed at least one hour before shutdownand, for longterm shutdowns, in addition, to increase the pH in the cycle. Oxygen and




7-8

ammonia addition must be stopped for all types of shutdown. In order to exclude thepossibility of oxygen in-leakage into the shutdown unit by way of leaking valves, aphysical disconnection of the oxygen supply from the oxygen dosing line is advised.The checklist for securing sample flows and on-line analyzers should be gone throughpoint by point.

Short-Term Shutdown.

Figure 7-3 graphically depicts guidance for short-term shutdown(1)

. The guidanceconsists simply of stopping oxygen feed at least one hour before shutdown of the unit.Deaerator vents should be opened, if they are not normally open, or the deaeratorventing should be increased to aid in cycle deaeration in conjunction with stopping theoxygen feed. At the same time, the low pressure and high pressure heaters should becarefully vented. Shutdown of ammonia feed should occur simultaneously with unitshutdown.

pH

NH3

O2

(injected)

Cationconductivity

Operation 1 hour Shutdown

8-8.5

30-150 ppb

0 ppb

<0.15 µS/cm

Figure 7-3 Shutdown and Operation Guidance for OT Chemistry for Short-TermShutdowns




7-9

Longterm Shutdown.

Figure 7-4 graphically depicts guidance for longterm shutdown. The guidance consistsof stopping oxygen feed at least one hour before shutdown of the unit and of increasingammonia feed rates. Simultaneously, the deaerator vents should be opened, if they are

not normally open, or the deaerator venting should be increased to aid in cycledeaeration in conjunction with stopping the oxygen feed. At the same time, the lowpressure and high pressure heaters should be carefully vented. The object of theincreased ammonia feed rates is to achieve a pH higher than 9.0.

Then, both the oxygen level in the whole cycle and the cycle pH are comparable withthose typical for AVT (feedwater oxygen <10 ppb and pH >9.0). During the longtermshutdown, the cycle can be regarded and treated as a cycle operated on AVT

(2) (see

Section 6.2). The only exception is the already mentioned elimination of hydrazine orother oxygen scavenger dosing even for a wet layup.

pH

NH3

O2 (injected)

Cation conductivity

Operation 1 hour Shutdown*

8-8.5

>9.0

0 ppb

≥0.2 µS/cm

<0.15 µS/cm

30-150 ppb

Figure 7-4 Shutdown and Operation Guidance for OT Chemistry for Long-Term Shutdowns.Note *: Dependent on wet or dry storage and utilization of nitrogen blanketing (See Section 4)




7-10

Emergency Shutdown.

In case of an emergency shutdown for reasons of a technical fault (e.g., defects in theelectrical equipment, a boiler tube failure), the unit is shut down with the optimumoperating chemistry. If it is foreseeable that the unit will be brought back into service

very fast (e.g., overnight or over a weekend), additional chemical measures areunnecessary. In case the shutdown will continue for a longer period of time (e.g.,several days or longer), the replacement of oxygen-containing water with an oxygen-free (<10 ppb oxygen) ammonia treated (pH >9) water makes sense. Customary layupprocedures have to be employed for prolonged shutdowns.

Shutdown as a Result of a Serious Chemistry Excursion.

Condenser cooling water in-leakage, makeup water contamination, condensate storagetank contamination or improper condensate polisher regeneration can lead to a

dangerous increase in cation conductivity or sodium or silica content. With OT, acation conductivity excursion is the most serious of chemical transients and must bedealt with very seriously and promptly. In case of an increasing cation conductivity,the following actions are possible:

Cation Conductivity Action Required

S/cm (at 25

C)

<0.15 Normal operating value, continue normal operation.>0.2 and <0.3 Increase system pH to AVT level (9.2-9.6).>0.3 Stop oxygen feed; operate on AVT without the use of

hydrazine or other oxygen scavengers.

>2.0 for 5 minutes or>5.0 for 2 minutes

Stop firing.

After a shutdown forced by a cation conductivity excursion, a unit drain and cleanupusing a non-contaminated water treated in accordance with the AVT requirements isrecommended

(2). With this, the standard cleanup loops including condensate polisher

(if the polisher is not the source of contamination) can be employed.

Cycling and Peaking Operation (3-7)

During cycling and peaking operation, as well as a consequence of a sliding pressureoperation, cation conductivity excursions, oxygen content variations, and pHexcursions may occur. The actions required for cation conductivity excursions arediscussed in the section “Shutdown Procedures”.

Particularly during peaking operation, longer periods with slightly higher cation

conductivity than the steady-state normal operating value (i.e., >0.15

S/cm) are not an

exceptional case. In many cases, even a cation conductivity of <0.3

S/cm (at 25C)




7-11

cannot be reached because of the relatively short duration of the operating period; thenthe unit can be operated only on AVT. There is a risk that such a unit could sooner orlater lose its passivation. In such cases, a longer steady-state operation with a renewedcycle passivation has been proven as the best remedy for reestablishing low corrosionproduct transport throughout the cycle.

In many units the oxygen and ammonia feed are not automatically controlled. As aresult of load fluctuations both the oxygen content and the pH may vary considerably.It is recommended to set the manually adjusted dosing to a lower value within therecommended range for oxygen content (30 ppb) and pH (8.0) during full loadoperation to preclude unnecessary overdosing in low load periods.

Layup Practices

Section 4, “Shutdown and Layup Considerations Common to Most Units” describes the

general information on layup practices for all types of chemistry.

An OT specific variant of wet layup for once-through boilers is the short-term layupwith optimum operating chemistry after an emergency shutdown. If it is foreseeablethat the unit will be brought back into service very fast (e.g., overnight or over aweekend), additional chemical measures are unnecessary. In case the shutdown willcontinue for a longer period of time, additional chemical measures are indispensable.

As an alternative to the usual replacement of oxygen containing system contents withoxygen free (< 10 ppb oxygen) ammonia treated (pH > 9) water, keeping the optimumoperating chemistry even during layup makes sense. With this alternative, a frequentchange or a continuous rinsing of the system contents with an oxygen containing (30-150 ppb oxygen) ammonia treated (pH 8.0-8.5) water has proved to be worthwhile.The sense of such measures is to ensure that the optimum redox conditions are kept inthe system even during the idle period. Naturally, the cation conductivity must notexceed 0.3 µS/cm (at 25°C).

For a longterm layup both dry layup and a nitrogen cap combined with an oxygen free(< 10 ppb oxygen) ammonia treated (pH > 9) water within the system are practicable.Keeping a flow of oxygenated water through the boiler is also possible but in mostcases not economical.

Because OT represents the best available treatment for all-ferrous systems with once-through steam generators, a proper layup of a unit operated with OT is very importantparticularly in the case of prolonged idle periods. The cycle parts at most risk are thesteam and feedwater side of the LP and HP heaters (including drains and vents), thesuperheaters and reheaters, and the LP turbine. These cycle components should always be included in the planned layup measures. See Discussion in Section 4.




7-12

7.3 ALL-FERROUS CYCLES WITH DRUM BOILERS

Current Normal Operating Guidelines

The use of OT for drum units is very similar to that of for once-through units describedin the previous sub-section. Oxygenated treatment (OT) uses high purity water tominimize corrosion and flow-accelerated corrosion (FAC) in the feedwater train. Thenormally desired cation conductivity level in feedwater and steam is < 0.15 µS/cm (at25°C) whereas in the boiler water it is < 1.5 µS/cm (at 25°C); lower values are preferredand attainable. OT can be applied only in plant cycles with all-ferrous feedwatermetallurgy and full-flow condensate polishing downstream of the condenser. With OTfor drum units, an oxygen level of 30-50 ppb is maintained in feedwater and steam.The application of a pH range from 9.0-9.5 enables a slight possible reduction of condensate polisher regeneration frequency. Since a contaminant concentration in boiler water (downcomer) is conceivable even with the best feedwater, the oxygen level

at the drum boiler downcomer is limited to < 10 ppb.

During normal operation the vents on the deaerator are closed. It is also veryimportant with OT that the optimum heater vent position is maintained to ensure theheater drains are fully protected from FAC. This usually involves the operatorensuring that an oxidizing environment is present in the drains (ORP > 0mV)

(8).

Figure 7-5(1)

shows the cycle chemistry diagram of a cycle with a drum boiler operatedon OT. Here, the normal target values and the action levels for condensate pumpdischarge, combined condensate polisher effluent, economizer inlet, boiler water(downcomer), and steam as well as for makeup treatment system effluent are provided.



N (Normal)



Targets


Base Load Cycling

1 (Action Level 1)

2 (Action Level 2)

3 (Action Level 3)

—

336 ( 2 weeks)

48 (2 days)

8

1Immediate Shutdown

—

672 (4 weeks)

96 (4 days)

16

2

Con

Pa

Sod

Ox

Ca

µS/

CR

CR

HP

turbine

Boilerwater

HP heaters

Deaerator

Attemperation

IP

turbine

LP

turbine

LP heaters Condensatepolisher

Condenser

Boiler

Blowdown

Condensatestorage tank

Makeuptreatmentsystem

Boiler Water (Downcomer)

ParameterTarget Sample N

C 8.5-9.2

1 2

< 8.5> 9.2

3

– pH

CR Cation conductivity, µS/cma

C ≤ 1.5 < 3.0 < 5.0

Oxygen, ppb C 5 > 10

–

ImmediateShutdown

< 7.5

Economizer Inlet



1 2

< 9.0> 9.6

3

–

pH


C ≤ 0.15 > 0.2 > 0.3

Oxygen, ppb C 30-50 c –

CR


Sodium, ppb

Combined Condensate Polisher Effluent

ParameterTarget

Sample N

C < 0.15

1

> 0.2

2 3

C ≤ 3 > 3

≤ 6

> 6

≤ 12

> 24

– –

– – Oxygen, ppb C 30-50

Steam

ParameterTarget

Sample N

Sodium, ppb C ≤ 3


µS/cma

C

1

> 3

2

> 6

3

> 12CR

≤ 0.15 > 0.2 > 0.3 – CR

> 0.65

Footnotes

a = Conductivity and pH measured at 25° Cc = Dependent upon economizer inlet/downcomer oxygen

CR

Air

scf

Air R

Par

N = Normal

1 = Action Level 1

2 = Action Level 2

3 = Action Level 3

Target Values

Legend

C = continuous

S = Grab. once per shift


W = grab. once per week

Sample Frequency

Figure 7-5 Cycle Chemistry Diagram of Drum Units on Oxygenated Treatment (OT)




7-14

Oxygenated treatment causes very stable conditions regarding the minimum corrosionproduct transport in a plant cycle. A temporary oxygen feed loss is not considered to be a very serious situation. Efforts should be made to restore the feed of oxygen assoon as practical. A temporary ammonia feed loss could result in an undesirable boilerwater pH reduction. Efforts should be made to restore the feed of ammonia as soon aspossible. Overfeed of ammonia is likewise not a serious event. Again, efforts should be made to establish the appropriate dosing as soon as practical. The ammoniaoverfeed does have cost consequences because of additional loading of the condensatepolishers when operated in hydrogen-hydroxyl form. An overfeed of oxygen couldendanger the waterwalls when contamination is present, and for that reason, has to beprecluded by means of appropriate technical measures.

Startup Procedures

Startup is accomplished using essentially the same startup procedure as used for AVT(Section 6.3). Some minor variations in startup procedures exist, with the variationdepending upon the type of unit shutdown and subsequent layup procedure whichpreceded the startup. Since for a short outage, no layup actions other thandiscontinuing oxygen feed are recommended, a relatively quick startup is anticipated.

For longterm layups, in which increased levels of ammonia are used, some additionalrecirculation and venting are required to reduce ammonia levels and to reduce cationconductivity to acceptable levels. Normal station startup should be performed as withAVT including ammonia but absolutely without hydrazine or other oxygen scavengeraddition. Startup should progress through cold and hot cleanup, startup, and ramping

activities as customary with AVT. Ammonia addition begins with the first use of condensate polishers. Deaerator pegging and venting are performed until oxygenaddition is started. The blowdown is used to reduce the boiler water cationconductivity.

Oxygen addition does not begin until feedwater cation conductivity reaches 0.15µS/cm (at 25°C) and is continuing to downtrend and until the boiler water(downcomer) cation conductivity has reached < 1.5 µS/cm (at 25°C). Deaerator ventsshould then be positioned as customary in steady-state operation. The same is true forheater vents.

One of the most important advantages of OT is that startups can be accomplished muchfaster than when the unit is operated with AVT. For startups following short-termlayup, there should be no holds or other ramping activities if the OT is operated in theoptimum fashion according to the EPRI guidelines

(1). For startups following longterm

layup, the feedwater iron levels still should not be a critical parameter. Here the mainmonitoring parameters are economizer inlet cation conductivity, oxygen, and pH (by




7-15

means of specific conductivity) as well as boiler water cation conductivity, oxygen, andpH.

Section 3 explains how the chemistry curves and action levels can be utilized duringunit startup. Basically, during startups, the initial lower boiler pressure permits boiler

water chemical concentrations to be higher than those at normal unit operatingpressures. Also, the cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.

Figure 7-6 shows a road map for startup of drum-type units operating with OT.




7-16

Layupduration

<4 weeks?

Yes

No

Yes

No

Yes

Dry

No

Wet

Short-term Longterm

Yes


<0.15 µS/cm?

Fill or fill up withdeoxygenated


Fill system withdeoxygenated


Refill system withdeoxygenated


Issystem filledfor startup?

Startupafter short-term

or longtermlayup?

Step 1: Startup preparation

Step 2:Systemcleanup

Step 3: Fire to boiler

Step 4:Transitionto OT

Commence startup.Proceed with

available cleanuploops; ventingvery important

Commence startupwithout any

cleanup actions exceptventing


<0.65 µS/cm?

No

Dry or wetlayup?

Increase blowdown

Is boiler watercation conductivity

<1.5 µS/cm?

Is the boilerload higher than

minimum OTload?

Yes

YesNo

Start oxygendosing and reduce

blowdown

Fire to boiler.Maintain temperatureramp within boiler andturbine manufacturers

requirements

No

Figure 7-6 Road map for the startup of drum boilers operated with OT.




7-17

The application of OT in all-ferrous cycles with drum-type boilers markedly simplifiesand shortens the startup procedure over that for AVT. For this reason, the road mapfor startup of drum-type units operating with OT (Figure 7-6) consists of only 4 mainsteps.

Step 1 - Startup Preparation

Startup after short-term or longterm layup?

The first startup actions depend on the layup duration. In case of short-term orintermediate layup, the procedures are simpler and faster than in the case of a unitwhich was the subject of a longterm layup.

Startup after short-term layup

Is system filled for startup?

Systems filled for startup can start up very fast, without any cleanup actions except forheater and deaerator venting. Otherwise the system has to be filled with deoxygenatedwater (O2 < 10 ppb, pH > 9). The cation conductivity of the water used for systemfilling should meet the EPRI AVT Guideline

(2).

Startup after longterm layup

Dry or wet layup?

If the system has been stored dry, it should be filled with deoxygenated water (O2 < 10ppb, pH > 9) which meets the EPRI AVT guideline. If the system has been stored wet,e.g. nitrogen cap combined with an oxygen free (< 10 ppb oxygen) ammonia treated(pH > 9) water, it should be refilled with deoxygenated water (O 2 < 10 ppb, pH > 9)which meets EPRI AVT Guideline.

Layup duration < 4 weeks?

According to the general OT operation experience, it is possible to commence startup of units stored for less than 4 weeks without any special clean-up actions except deaeratorand heater venting. In other cases, continue with available cleanup loops and take care

of adequate deaerator and heater venting.

Step 2 - System Cleanup

The startup of units operated with OT occurs without any oxygen dosing, as with AVT.





7-18

If feedwater cation conductivity is > 0.65 µS/cm, the by-pass of the pre-boiler system(cleanup loops) can be used to remove cycle contamination (if a by-pass is available).Otherwise the startup could begin without any cleanup actions. An important part of this startup step is the deaerator and heater venting.

Step 3 - Fire to Boiler

It is required that the feedwater cation conductivity reaches < 0.65 µS/cm and tends todowntrend prior to firing the boiler. Naturally, during startup the temperature ramphas to be maintained within boiler and turbine manufacturers’ requirement.

The main cycle chemistry surveillance parameter in this and in the following startupstep is the feedwater cation conductivity and pH as well as boiler water cationconductivity and pH. The other plant cycle core parameters complete the informationon the current cycle chemistry. Sampling and analysis for corrosion products on aregular basis are, as a rule, not necessary. Nevertheless, it may be meaningful to checkthe feedwater corrosion product level during some selected starts to get moreinformation on the required cleanup duration.

Step 4 - Transition to OT


A feedwater cation conductivity below 0.15 µS/cm (at 25°C) is the first precondition forthe transition to OT. Deaerator and heater vents should then be positioned ascustomary in steady-state operation.

Is boiler water conductivity < 1.5 µS/cm?

Reaching this value is the second precondition for the transition to OT. In case of need,the blowdown is used to reduce the boiler water cation conductivity.

Is the boiler load higher than minimum OT load?

The transition to OT is possible only if the boiler load reaches the unit-specificminimum OT load. See EPRI Guidelines

(1) for information on establishing the oxygen

recirculation ratio and the minimum load acceptable for oxygen addition. This is the

last precondition before the transition to OT. Otherwise, the unit has to continueoperation with AVT.

Start oxygen dosing.

Only when the three above mentioned preconditions are fulfilled (feedwater cationconductivity < 0.15 µS/cm, boiler water cation conductivity < 1.5 µS/cm, and boiler




7-19

load higher than minimum OT load), is oxygen dosing permitted. If possible, the blowdown can be reduced. Deaerator and heater vents should then be positioned ascustomary in steady-state operation.

Monitoring of all plant cycle core parameters is important and indispensable during

Step 4.

Shutdown Procedures

The procedures generally applicable to shutdown are included in Section 4.

Hydrazine and other oxygen scavengers should not be utilized in a unit operating onOT. The preferred practice is to stop the oxygen feed at least one hour beforeshutdown and, for longterm shutdowns, in addition, to increase the pH in the cycle.The maximum tolerable pH value depends on the condenser metallurgy. If the

condenser tubing is made of admiralty brass, then a higher pH could result in ammoniagrooving. Oxygen and ammonia addition must be stopped for all types of shutdown.In order to exclude the possibility of oxygen in-leakage into the shutdown unit by wayof leaking valves, a physical disconnection of the oxygen supply from the oxygendosing line is advised. The checklist for securing sample flows and on-line analyzersshould be gone through point by point.

Short-Term Shutdown.

Figure 7-7 graphically depicts guidance for short-term shutdown(1)

. The guidance

consists simply of stopping oxygen feed at least one hour before shutdown of the unit.Deaerator vents should be opened, if they are not normally open, or the deaeratorventing should be increased to aid in cycle deaeration in conjunction with stoppingoxygen feed. At the same time, the low pressure and high pressure heaters should becarefully vented. Shutdown of ammonia feed should occur simultaneously with unitshutdown.




7-20

Feedwater pH

Feedwater NH3

Low PowerOperation

High PowerOperation Shutdown

9-9.6

0 ppb

>9.0

0 ppb

1 hour

Feedwater O2

(injected)

Feedwatercation conductivity

Downcomercation conductivity

Downcomer oxygenresidual (downcomer)

30-50 ppb

<0.15 µS/cm

<1.5 µS/cm

≤10.0 ppb

MinimumOT load

Figure 7-7 Operation and Shutdown Guidance for OT Chemistry for Short-termShutdowns (Drum Boiler Unit)

Longterm Shutdown.

Figure 7-8 graphically depicts guidance for longterm shutdown. The guidance consistsof stopping oxygen feed at least one hour before shutdown of the unit and of increasingammonia feed rates. Simultaneously, the deaerator vents should be opened, if they arenot normally open, or the deaerator venting should be increased to aid in cycle

deaeration in conjunction with stopping the oxygen feed. At the same time, the lowpressure and high pressure heaters should be carefully vented. The object of theincreased ammonia feed rate is to achieve a pH higher than 9.0.




7-21

Feedwater pH

Feedwater NH3

Low PowerOperation

High PowerOperation Shutdown

9-9.6

0 ppb

>9.0

0 ppb

1 hour

Feedwater O2

(injected)

Feedwatercation conductivity

Downcomercation conductivity

Downcomer oxygenresidual (downcomer)

30-50 ppb

<0.15 µS/cm

<1.5 µS/cm

≤10.0 ppb

Minimum OT load

Figure 7-8 Operation and Shutdown for OT Chemistry for Longterm Shutdowns(Drum Boiler Unit)

Then, both the oxygen level in the whole cycle and the cycle pH are comparable withthose typical for AVT (feedwater oxygen < 10 ppb and pH > 9.0). During the longtermshutdown, the cycle can be regarded and treated as a cycle operated on AVT

(2). The

only exception is the already mentioned elimination of the hydrazine or other oxygenscavenger dosing even for a wet layup.

Emergency Shutdown.

In case of an emergency shutdown for reasons of a technical fault (e.g., defects in theelectrical equipment, a boiler tube failure), the unit is shutdown with the optimumoperating chemistry. If it is foreseeable that the unit will be brought back into servicevery fast (e.g., overnight or over a weekend), additional chemical measures areunnecessary. In case the shutdown will continue for a longer period of time (e.g.,




7-22

several days or longer), the replacement of oxygen containing water with an oxygenfree (< 10 ppb oxygen) ammonia treated water (pH > 9) makes sense. Customary layupprocedures have to be employed for prolonged shutdowns.

Shutdown as a Result of a Serious Chemistry Excursion.

Condenser cooling water in-leakage, makeup water contamination, condensate storagetank contamination, or improper condensate polisher regeneration can lead to adangerous increase in cation conductivity or sodium or silica content. With OT, acation conductivity excursion is the most serious of chemical transients and must bedealt with very seriously and promptly. In case of an increasing cation conductivity,the following actions are possible:

Feedwater CationConductivity

Action Required

S/cm (at 25

C)<0.15 Normal operating value, continue normal operation.

>0.2 and <0.3 Increase system pH to AVT level (9.2-9.6).

>0.3 Stop oxygen feed; operate on AVT without the use of hydrazine or other oxygen scavengers.

Other chemical transients (e.g. sodium and silica) must be dealt with in the same wayas with AVT.

A boiler water cation conductivity excursion is also a serious chemical transient and

must be dealt with very seriously and promptly. In case of an increasing boiler water(downcomer) cation conductivity, the following actions are possible.

Boiler Water CationConductivity

Action Required

S/cm (at 25

C)

<1.5 Normal operating value (preferably <1.0), continue normaloperation.

1.5 -3.0 Increase boiler blowdown

>3.0 Stop oxygen feed; increase feedwater pH to AVT level (9.2-9.6). Do not use hydrazine or other oxygen scavengers.Follow the AVT Guideline

(2).




7-23

Cycling and Peaking Operation

During cycling and peaking operation, as well as a consequence of a sliding pressureoperation, cation conductivity excursions, oxygen content variations, and pHexcursions may occur. The actions required for cation conductivity excursions are

discussed under “Shutdown Procedures”.

Particularly during peaking operation longer periods with slightly higher feedwatercation conductivity than the steady-state normal operating value (i.e., < 0.15 µS/cm at25°C) are not an exceptional case. In many cases, even a feedwater cation conductivityof < 0.3 µS/cm (at 25°C) cannot be reached because of the relatively short duration of the operating period; then the unit can be operated only on AVT. There is a risk thatsuch a unit could sooner or later lose its passivation. In such cases, a longer steady-state operation with a renewed cycle passivation has been proven as the best remedyfor reestablishing low corrosion product transport throughout the cycle.

During the conversion to OT, the oxygen recirculation ratio and a minimum loadacceptable for oxygen addition should have been selected

(1). If the boiler is operated at

a lower load than the selected minimum OT load, the oxygen feed should be shut off.

Layup Practices

Section 4, “Shutdown and Layup Considerations Common to Most Units” describes thegeneral information on layup practices for all types of chemistry.

An OT specific variant of wet layup for drum boilers is the short-term layup with

optimum operating chemistry after an emergency shutdown. If it is foreseeable that theunit will be brought back into service very fast (e.g., overnight or over a weekend),additional chemical measures are unnecessary. In case the shutdown will continue fora longer period of time, additional measures are indispensable.

For a longterm layup both dry layup and a nitrogen cap combined with an oxygen free(< 10 ppb oxygen) ammonia treated (pH >9) water within the system are practicable.

OT represents one of the best available treatments for all-ferrous systems with drum boilers. Nevertheless, a proper layup of such a unit operated with OT is very

important particularly in case of prolonged idle periods. The cycle parts at most riskare the steam and feedwater side of the LP and HP heaters (including drains andvents), the waterwalls and the drum, the superheaters and reheaters, and the LPturbine. All mentioned cycle components should always be included in the plannedlayup measures. See discussion in Section 4.




7-24

7.4 REFERENCES

1. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI TR-102285,Dec. 1994.

2. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. EPRI TR-105041,April 1996.

3. A. H. Rudd, J. M. Tanzosh, Developments Applicable to Improved Coal-Fired PowerPlants, Nov 19-21, 1986. Electric Power Research Institute, Palo Alto, Calif. USA.

4. Babcock & Wilcox 7A-1K32, Nov. 1995. Operating Instructions for Universal PressureBoilers. Babcock & Wilcox-A McDermott Co., 20 South Van Buren, Barberton, Ohio

USA.

5. Babcock & Wilcox 7A-1, 1-K32 (FPG) Aug 9, 1982. Initial Waterside Clean-upUniversal Pressure Boilers (Boilers with Integral Primary Superheaters) , Babcock &Wilcox-A McDermott Co., 20 South Van Buren, Barberton, Ohio, USA.

6. A. H. Rudd, Variable Pressure Boiler Operation, Canadian Electrical Association, Sept.18-20, 1972, Calgary, Alberta, Canada BR-978 PGTP 72-53. Babcock & Wilcox-AMcDermott Co., 20 South Van Buren Avenue, Barberton, Ohio, USA.

7. Combustion Fossil Power, Published by Combustion Engineering, Windsor Locks,

Connecticut, 1991, 4th Edition Chapter 7, Central Station Steam Generators.

8. R. B. Dooley and J. Matthews. “The Current State of Cycle Chemistry for FossilPlants.” Fifth International Conference on Fossil Plant Cycle Chemistry. ProceedingsEd. By R. B. Dooley and J. Matthews. EPRI TR-108459, November 1997.




8-1

8

CAUSTIC TREATMENT FOR DRUM BOILERS

8.1 INTRODUCTION

With the “traditional” form of all-volatile treatment, ammonia and hydrazine areadded to the feedwater to provide chemical conditions that protect the feedwaterheaters, boiler, superheater, reheater and turbine against corrosion, without furtherchemical additions.

All-volatile treatment can also be used for drum-type boilers(1)

, provided high purityfeedwater is available and the build-up of impurities in the boiler water is strictlycontrolled. The absence of a solid alkalizing chemical in the boiler water gives lessprotection against corrosion, but also reduces the risk of carrying over boiler water saltsand solid alkalizing chemicals into the steam. If the high purity feed and boiler waterconditions can not be maintained during startup, operation and shutdown, it may benecessary to adopt another form of chemical conditioning, such as phosphate (seesection 5) or caustic treatment, as discussed below.

Preliminary guidance on the use of caustic treatment of boiler water was published in

an EPRI(2) report on Sodium Hydroxide for Conditioning the Boiler Water of Drum-Type Boilers in 1995. Cycle diagrams were provided for drum-type boilers with allferrous and mixed Fe-Cu metallurgy feedwater heating systems. Only high pressuredrum boilers with reheat were considered and EPRI interim guidance with the normallimits and three action levels and “core” parameters were proposed for 2500 psi (17MPa) coal fired drum-type boilers. Guidance was included on the application,experience, benefits, limitations and implementation of caustic treatment.

Further considerations of caustic treatment were included in the EPRI report onSelection and Optimization of Boiler Water and Feedwater Treatment of Fossil Plants

(3).

As an aid to planning and obtaining the optimum operating conditions for cycling,startup, shutdown and layup, it is important to know as accurately as possible, whatnotice will be given of shutdown, the period of outage and for startup.

In general, the features for the feedwater and steam for all-volatile treatment (AVT) fordrum boilers, also apply to caustic treatment (CT). However, these need to besupplemented by additional considerations for the boiler water. Some units with drum




Caustic Treatment for Drum Boilers

8-2

boilers may be equipped with a condensate polisher, but many units are not and,therefore, will be more susceptible to the effects of condenser leaks.

Caustic treatment can be used for drum boilers with all-ferrous and mixed metallurgyfeedwater heating systems. As with phosphate treatment, adding sodium hydroxide as

a solid alkalizing agent to the boiler water, increases the tolerance of the boiler toingress of corrosive contaminants, such as chloride. However, if present in too high aconcentration, it can lead to caustic gouging and increased boiler corrosion. In addition,special care has to be taken to prevent carryover of boiler water into the steam, as thepresence of sodium hydroxide in water droplets could lead to stress corrosion crackingof austenitic components, such as superheaters and turbines.

Many of the features discussed in Section 6 on All-Volatile Treatment also apply toCaustic Treatment, with the proviso that particular care has to be taken in controllingthe concentration of sodium hydroxide in the boiler water and carry over into thesteam. In order to avoid too much repetition, the reader is referred to the appropriateparts of the AVT guidance, Section 6.3 for All-Ferrous Feedwater Heating Systems andSection 6.4 for Mixed Metallurgy Feedwater Heating Systems. This Section 8 of theguidelines will concentrate on the additional considerations for Caustic treatment.

8.2 ALL-FERROUS FEEDWATER HEATING SYSTEMS

Current Guidance Document

The current guidance on caustic treatment (CT) for drum boilers with all-ferrousfeedwater heating systems is given in Figure 8-1 for reheat plants, including the “core”parameters. The normal limits and three action levels for sodium, chloride, sulfate,silica and cation conductivity in boiler water are given in the EPRI

(2) report on Sodium

Hydroxide Conditioning the Boiler Water of Drum-Type Boilers. Non-reheat plantswere not considered in the report.

Startup

The basic considerations outlined in Section 6.3 for AVT apply during startup forCaustic Treatment, with the addition that special care has to be taken to prevent anycarryover of sodium hydroxide in the boiler water, as this could lead to stress corrosion

of austenitic components in the steam circuit.

Section 3 explains how the action levels can be utilized during unit startup. Thecumulative operating hours per year for which the various action levels can beexceeded are twice the values for cycling units, as compared to base loaded units. Nochemistry pressure curves were developed for Caustic Treatment, but the target valuesgiven in Figure 8-1 should be achieved.






8-4

As mentioned previously, special care has to be taken to avoid excessively highconcentrations of sodium hydroxide in the boiler water, and carryover of sodiumhydroxide into the steam. This can be prevented by carefully monitoring sodium insteam. The steps used during startup will be plant specific and, depending on thepracticality, the following stages should be monitored, at least during a trial period:


a) Addition of hydrazine (if used)andammonia to the feedwater


b) Additional boiler blowdown pH, NaOH or specific conductivity, cationconductivity, chloride, and corrosionproducts



At full load, the values given in the EPRI guidance document(2)

should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figure 8-1. The core parameters,sodium, cation conductivity and oxygen given in the EPRI guidance document

(2) should

be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH andspecific conductivity or NaOH, and steam. The normal operating levels, as given in theEPRI guidance document(2), should be achieved as soon as possible. These are given inFigure 8-1.

A road map showing a scheme for startup of a drum boiler with an all-ferrousfeedwater heating system operating with CT is given in Figure 8-2. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups, and amended in the light of operating experience. The procedure can normally be curtailed for warm and hot starts.If the high purity feed and boiler water conditions cannot be maintained duringstartup, it may be necessary to adopt another form of chemical conditioning, such as

phosphate. The Selection and Optimization document provides advice and guidance onthe optimum boiler water and feedwater choices for drum units

(3). The road map,

Figure 8-2, can be divided into 7 steps.

The steps described in Figure 8-2 are essentially similar to those described in Section 6.3for All-Volatile Treatment, except that reference should be made to the CT

(2) guidelines,

instead of the AVT guidelines. Note that additional care is required to avoid excessive




8-5

carryover of boiler water impurities and conditioning chemicals, sodium hydroxide,into the steam.

Under Step 6, excursions affecting (lowering) the pH must be corrected immediately byfeeding 1-2 ppm of sodium hydroxide to the boiler water. Match the sodium hydroxide

to the chloride concentration. Do not overfeed sodium hydroxide. Avoid excesscarryover of boiler water impurities into the steam.

Shutdown

The basic considerations outlined in Section 6.3 for AVT apply during shutdown forCaustic Treatment, with the addition that special care has to be taken to prevent anycarryover of sodium hydroxide from the boiler water, as this could lead to stresscorrosion of austenitic components in the steam circuit.

A road map showing a scheme for shutdown of a drum boiler unit with an all-ferrousfeedwater heating system operating with CT is given in Figure 8-3. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience. The road map, Figure 8-3, can bedivided into 5 steps.

The steps described in Figure 8-3 are essentially similar to those described in Section 6.3for All-Volatile Treatment, except that reference should be made to the CT

(2) guidelines,

instead of the AVT guidelines. Note that additional care is required to avoid excessivecarryover of boiler water impurities and conditioning chemicals, sodium hydroxide,

into the steam.

Under Steps 4 and 5, excursions affecting (lowering) the pH must be correctedimmediately by feeding 1-2 ppm of sodium hydroxide to the boiler water. Match thesodium hydroxide to the chloride concentration. Do not overfeed sodium hydroxide.Avoid excess carryover of boiler water impurities into the steam.

Again, the steps are essentially similar to those described in Section 5 for phosphatetreatment, except that additional care is required to avoid carryover of boiler waterimpurities and conditioning chemical, sodium hydroxide, into the steam. Control of the

boiler water purity can be achieved by blowdown with drum boilers.

Cycling and Peaking





8-6

No

Yes

Step 1

Fill system perCT guidelines

Fire to boiler

Monitor chemical parameters,especially sodium in steam


Safely vent N2 (if used)Drain storage solutionRefill per CT guidance



within CT guidelines bycontrolling pressure and blow-down. Avoid excess carryover


Is system full?


(Section 4)

Is system filled withwater per CT

guidance?

Proceed withstartup




Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 7

Yes

Figure 8-2 CT - Startup of Drum Boilers with All-Ferrous Feedwater Heaters




8-7


No

Step 1


(Section 4)


continued operation

Step 2

Step 3

Step 3

Step 2

Yes





Unplanned outage

Yes

Yes

Yes

Yes



Planned outage

Step 4

Step 2

No

No

Step 3

Step 4

Step 5Step 5

Adjust pH >8.0, orderlyshutdown, if polishers

utilized, otherwiseimmediate shutdown


No

Figure 8-3 CT - Shutdown of Drum Boilers with All-Ferrous and Mixed MetallurgyFeedwater Heaters

Layup

The layup procedure is the same as described in Section 6.3 for All-Volatile Treatment.

8.3 MIXED METALLURGY FEEDWATER HEATING SYSTEMS

EPRI(4)

has recently produced a report on the State-of-Knowledge of Copper in Fossil

Plant Cycles as the first stage of the “Program Copper” project. This was in response toa demand from members, particularly in the US, for improved performance of powerplants containing copper alloys. Traditionally, these have been widely used incondensers, but, in some plants, brass and/or cupro-nickel have also been used for heatexchange surfaces in low and high pressure feedwater heaters. Copper released fromfeedwater heaters deposits in the boiler, increasing the locations where impurities can




8-8

concentrate on boiler waterwalls. Thick deposits can lead to overheating and thepresence of copper in deposits complicates chemical cleaning.

Copper in high pressure boilers can be carried over into the steam and deposited onhigh pressure turbines, where even as little as 1 kg can reduce the output capacity of

the turbine by 1 MW. This is particularly true for plants operating at more than 2400 psi(16.6 MPa) and is exacerbated further by increasing pressure. The presence of oxygenand the absence of reducing conditions in the feedwater is the main cause of coppertransport around the circuit.

It is worth reiterating(4)

that startups are generally considered to be the periods of maximum copper transport activity in the cycle. This relates directly to the feedwatersystem not being protected during shutdown periods, i.e. that a reducing environment(ORP < 0mV) is not maintained. Air in-leakage into the LP feedwater circuits increasesthe growth of non-protective copper oxides and copper transport. The EPRI

(4) “Guiding

Principles” for successful operation of units with copper alloys are:







Many of the features discussed in Section 6 on All-Volatile Treatment also apply toCaustic Treatment, with the proviso that particular care has to be taken in controlling

the concentration of sodium hydroxide in the boiler water and carry over into thesteam. This Section 8.3 of the guidlines will concentrate on the additionalconsiderations for Caustic Treatment and mixed Fe-Cu metallurgy feedwater heatingsystems.




8-9

Current Guidelines

The current guidance on caustic treatment for drum boilers with mixed Fe-Cumetallurgy feedwater systems are given in Figure 8-1 for reheat plants, including the“core” parameters. The normal limits and three action levels for sodium, chloride,

sulfate, copper, silica and cation conductivity in boiler water are given in the EPRI(2)

report on Sodium Hydroxide Conditioning the Boiler Water of Drum-Type Boilers.Non-reheat plant were not considered.

Startup

The basic considerations outlined in Section 6.4 for AVT with mixed Fe-Cu metallurgyfeedwater heating systems, also apply during startup for Caustic Treatment, with theaddition that special care has to be taken to prevent any carryover of sodium hydroxidein the boiler water, as this could lead to stress corrosion of austenitic components in the

steam circuit. Care is required to minimise the ingress of oxygen to reduce thecorrosion of copper alloys.

The presence of oxygen also assists the transport of copper into the steam, increasingthe risk of deposition in the superheater and high pressure turbine. It is also importantto ensure that the pH, cation conductivity and chloride concentration are within thelimits given in the EPRI

(2) guidance document for boiler water. Hydrazine (or volatile

reducing agents) should not be eliminated from plants with mixed Fe-Cu feedwatersystems.

As mentioned previously, special care has to be taken to avoid excessively high

concentrations of sodium hydroxide in the boiler water and carryover of sodiumhydroxide into the steam. This can be prevented by carefully monitoring sodium insteam. The steps used during startup will be plant specific and, depending on thepracticality, the following stages should be monitored, at least during a trial period:


a) Addition of hydrazine and ammonia tothe feedwater

pH, cation conductivity, oxygen andcopper

b) Additional boiler blowdown pH, NaOH or specific conductivity, cationconductivity, chloride, and corrosionproducts


Cation conductivity, silica, sodium andcopper




8-10

At full load, the values given in the EPRI(2)

guidance document should be achieved.There may be room for relaxation, particularly, as regards silica in steam, duringstartup and the earlier stages of operation and during low load operation, provided theparameters are within the EPRI action levels 1-3, see Figure 8-1. The core parameters,sodium, cation conductivity and oxygen given in the EPRI guidance document

(2) should

be monitored at the condensate pump discharge, condensate polisher outlet (if installed), economizer inlet, boiler water (preferably at downcomer), including pH andspecific conductivity or NaOH, and steam, including copper. The normal operatinglevels, as given in the EPRI guidance document, should be achieved as soon as possible.These are given in Figure 8-1.

A road map showing a scheme for startup of a drum boiler with mixed Fe-Cufeedwater heating system operating with CT is given in Figure 8-4. The time taken toachieve the required target values will be plant dependent. This should be determinedfor each unit, used as guidance for subsequent startups, and amended in the light of operating experience. Section 3 explains how the action levels can be utilized duringunit startup. The cumulative operating hours per year for which the various actionlevels can be exceeded are twice the values for cycling units, as compared to baseloaded units.

If the high purity feed and boiler water conditions cannot be maintained duringstartup, it may be necessary to adopt to another form of chemical conditioning, such asphosphate. The Selection and Optimization document provides advice and guidance onthe optimum boiler water and feedwater choices for drum units

(3). The procedure can

normally be curtailed for warm and hot starts. The road map, Figure 8-4, can bedivided into 7 steps.

The steps described in Figure 8-4 are essentially similar to those described in Section 6.4for All-Volatile Treatment, except that reference should be made to the CT(2) guidelines,instead of the AVT guidelines. Note that additional care is required to avoid excessivecarryover of boiler water impurities and the conditioning chemicals, sodiumhydroxide, into the steam. Compared to units with all-ferrous feedwater heatingsystems, additional considerations are required for units with mixed Fe-Cu feedwaterheaters to reduce the risk of copper corrosion and transport, due to the ingress of oxygen.

Under Step 6, excursions affecting (lowering) the pH must be corrected immediately byfeeding 1-2 ppm of sodium hydroxide to the boiler water. Match the sodiumhydroxide to the chloride concentration. Do not overfeed sodium hydroxide. Avoidexcess carryover of boiler water impurities into the steam.




8-11

No

Yes

Step 2

Fill system perCT guidelines

Fire to boiler


Safely vent N2 (if used)Drain storage solutionRefill per CT guidelines

Add N2H4, if necessary.Reduce O2 to 100 ppb, Feto 100 ppb, Cu to 10 ppb


within CT guidelines bycontrolling pressure and blow-down. Avoid excess carryover


Is system full?


(Section 4)

Is system filled withwater per CTguidelines?

Proceed withstartup




Step 4

Step 5

Step 5

Step 6

Step 6Step 6

Step 5

Step 3

Step 2

Yes

Yes

No

Step 12

Monitor chemical parameters,especially sodium in steam

Yes

Figure 8-4 CT - Startup of Drum Boilers with Mixed Fe-Cu Metallurgy FeedwaterHeaters

Shutdown

Information is given in Section 4 on the procedures generally applicable to shutdown,and below for various planned shutdown conditions.

The basic considerations outlined in Section 6.4 for AVT with mixed Fe-Cu metallurgyfeedwater systems, also apply during shutdown for Caustic Treatment, with the




8-12

addition that special care has to be taken to prevent any carryover of sodium hydroxidefrom the boiler water, as this could lead to stress corrosion of austenitic components inthe steam circuit. Care is required to minimise the ingress of oxygen to reduce thecorrosion of copper alloys.

A road map showing a scheme for shutdown of a drum boiler an with mixed Fe-Cufeedwater heating system operating with CT is given in Figure 8-3. The time taken toachieve stable shutdown conditions will be plant dependent. This should bedetermined for each unit, used as guidance for subsequent repeat operations andamended in the light of operating experience. The road map, Figure 8-3, can be dividedinto 5 steps.

The steps described in Figure 8-3 are essentially similar to those described in Section 6.for All-Volatile Treatment, except that reference should be made to the CT(2) guidelines,instead of the AVT guidelines. Note that additional care is required to avoid excessivecarryover of boiler water impurities and the conditioning chemicals, sodiumhydroxide, into the steam. Compared to units with all-ferrous feedwater heatingsystems, additional considerations are required for units with mixed Fe-Cu feedwaterheaters to reduce the risk of copper corrosion and transport, due to the ingress of oxygen.

Under Steps 4 and 5, excursions affecting (lowering) the pH must be correctedimmediately by feeding 1-2 ppm of sodium hydroxide to the boiler water. Match thesodium hydroxide to the chloride concentration. Do not overfeed sodium hydroxide.Avoid excess carryover of boiler water impurities into the steam.

Cycling and Peaking


Layup

The layup procedure is the same as described in Section 6.4 for All-Volatile Treatment.

8.4 REFERENCES

1. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. Electric PowerResearch Institute, Palo Alto, Calif. EPRI TR-105041. April 1996

2. Sodium Hydroxide for Conditioning the Boiler Water of Drum-Type Boilers. ElectricPower Research Institute, Palo Alto, Calif. EPRI TR-105041. April 1996




8-13


4. State of Knowledge of Copper in Fossil Plant Cycles. Electric Power Research Institute,Palo Alto, Calif. EPRI TR-108460, September 1997



Documents

EPRI Start Up, Lay Up & Shut Down Guidelines for Chemist