diagnostic - troubleshooting hrsg

7/26/2019 Diagnostic - Troubleshooting HRSG

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Diagnostic/Troubleshooting Monitoring to

Identify Damaging Cycle Chemistry orThermal Transients in Heat Recovery SteamGenerator Pressure Parts

Technical ReportL

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LWARNING:

Please read the License Agreement

on the back cover before removingthe Wrapping Material.

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accordance with Section 734.3(b)(3) and published in accordance with Section

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EPRI Project ManagerR. B. Dooley

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Diagnostic/TroubleshootingMonitoring to Identify Damaging

Cycle Chemistry or ThermalTransients in Heat Recovery SteamGenerator Pressure Parts

1008088

Final Report, March 2005


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'SCIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER(INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVEHAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOURSELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD,PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRI

Aptech Engineering Services, Inc.

J. Michael Pearson & Associates Co. Ltd.

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 WillowWay, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169,(925) 609-1310 (fax).

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc.

Copyright 2005 Electric Power Research Institute, Inc. All rights reserved.


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CITATIONS

This report was prepared by

EPRI3412 Hillview AvenuePalo Alto, CA 94304

Principal InvestigatorR. B. Dooley

Aptech Engineering Services, Inc.1253 Reamwood AvenueSunnyvale, CA 94089

Principal InvestigatorS. Paterson

J. Michael Pearson & Associates Co. Ltd.9 Abbit Crescent, RR1Georgetown, Ontario

Canada L7G4S4

Principal InvestigatorM. Pearson

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Diagnostic/Troubleshooting Monitoring to Identify Damaging Cycle Chemistry or ThermalTransients in Heat Recovery Steam Generator Pressure Parts, EPRI, Palo Alto, CA: 2005.

1008088.


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PRODUCT DESCRIPTION

The worldwide fleet of combined cycle units with heat recovery steam generators (HRSG) hasexhibited a disappointing track record with respect to reliability and availability in terms ofHRSG Tube Failures (HTF). This report will assist operators in identifying the harmful chemicaland thermal transient excursions that lead to failure.

Results & FindingsThe report provides a series of road maps to identify, measure, evaluate, correct, and control

those cycle chemistry and thermal transients that result in poor HRSG reliability within thedesign life of a generator. Particular emphasis has been given to eliminating failures in the firstfew years of operation. The appendices of the report include background information on tubetemperature measurement, cycle chemistry monitoring, and estimates of cost.

Challenges & ObjectivesThe most frequently occurring HTF damage mechanisms include thermal and corrosion fatigue,thermal quench cracking, flow-accelerated corrosion, and under deposit corrosion. These areeither influenced by transiently high, thermally induced cyclic stresses or inadequate feedwaterand evaporator chemistries. On the chemistry side, it is clear that the chemistries adopted duringthe design phase of a plant set the stage for later failures. The objective of this work was to

develop a comprehensive approach that will identify and eliminate non-optimum cyclechemistries and avoid potentially damaging thermal transients in the various HRSG circuits asearly in life as possible.

Applications, Values & UseOrganizations that apply the monitoring and diagnostic approaches delineated in this report canoperate HRSGs with added confidence that chemical and thermal effects have been identifiedand will not lead to HTF damage and failure. Adoption of the necessary practices will put anorganizations HRSG on the road to world-class performance.

EPRI Perspective

To address the suite of issues related to HTF, EPRI has developed a series of documents: HRSGCycle Chemistry Guidelines (EPRI report TR-110051), HRSG Tube Failure Manual (EPRIreport 1004503), and Delivering High Reliability HRSGs (EPRI report 1004240). However,these documents by themselves would not lead to a reduction in repeat HTF because many of theinfluencing features leading to failure were an integral part of the original specification anddesign aspects or resulted from inadequate commissioning. Organizations needed an approach torecognize these deficiencies in the early life of an HRSG. The current document provides this


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methodology. The next steps are to conduct a number of case studies to illustrate the efficacy ofthe approach.

ApproachThe EPRI team first developed an interim White Paper on the topic. This document was used to

solicit host HRSGs. The team worked with one host site, reviewed its chemistry and possiblethermal transients, and made suggestions for installation of thermocouples and for cyclechemistry monitoring. Based on this exercise, the team upgraded the white paper to the currentreport.

KeywordsHeat recovery steam generator (HRSG)Combined cycle unitsTube failuresThermal transientsCycle chemistryMonitoring and diagnostics


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EPRI Licensed Material

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ABSTRACT

To identify the root causes of the leading HRSG Tube Failure (HTF) mechanisms, and toidentify damage accumulating from non-optimum cycle chemistry and severe thermal transients,it is necessary to conduct diagnostic monitoring. This report, which is within a series of EPRIreports, provides details and case studies of how to conduct monitoring of HRSG tubing andheader/tubing attachments. A road map approach is provided with numerous examples.


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ACKNOWLEDGMENTS

The authors of this report

R. B. Dooley, EPRIS. Paterson, Aptech Engineering Services, Inc.M. Pearson, J. Michael Pearson & Associates Co. Ltd.

acknowledge the contributions from the following individuals:

Kevin Shields, EPRIKurt Koenig, Plant Engineer, Jasper Plant, SCE&GSteve Palmer, Plant Manager, Jasper Plant, SCE&GJohn Pearrow, Manager System Chemistry, Jasper Plant, SCE&GGalen Bullock, Maintenance Superintendent, Jasper Plants, SCE&G


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CONTENTS

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

1.1 Background ....................................................................................................... 1-1

2STEPS FOR THE IDENTIFICATION, CORRECTION AND CONTROL OFCYCLE CHEMISTRY OR THERMAL TRANSIENT INFLUENCED HRSGPRESSURE PART DAMAGE......................................................................................2-1

2.1 Introduction to Road Map Steps........................................................................ 2-1

2.2 Step 1: Review of Unit Design, Operational and Maintenance Information....... 2-2

2.2.1 Cycle Chemistry Review............................................................................ 2-2

2.2.2 Review of Pre-operational and Layup Practices ......................................2-11

2.2.3 Review of Thermal-Mechanical Parameters ............................................ 2-12

2.3 Step 2: Identifying the Potential Cycle Chemistry or Thermal-MechanicallyInfluenced Damage Mechanisms........................................................................... 2-15

2.4 Step 3: Specify the Type and Locations for the Diagnostic Instrumentation ... 2-16

2.5 Step 4: Install High Priority Diagnostic Instrumentation ..................................2-29

2.6 Step 5: Operate Unit over a Wide Range of Operating Conditions .................2-30

2.7 Step 6: Review and Evaluate the Results of Diagnostic InstrumentationMeasurements ....................................................................................................... 2-31

2.8 Step 7: Evaluate, Engineer and Implement Operational, Maintenance andDesign Enhancements to Ameliorate or Eliminate Damage Influencing CycleChemistry or Thermal-Mechanical Events .............................................................2-33

2.9 Step 8: Verify the Success of the Changes through Additional Monitoringand Evaluation ....................................................................................................... 2-34

2.10 Step 9: Ongoing Monitoring, Evaluation, and Improvements ........................ 2-34

3REFERENCES..........................................................................................................3-1

ABACKGROUND INFORMATION REVIEW ............................................................. A-1

General Plant/Unit Information.................................................................................A-1


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Cycle Chemistry Information ....................................................................................A-2

Thermal Cycling Information ....................................................................................A-2

BIDENTIFYING POTENTIAL DAMAGE MECHANISMS AND DIAGNOSTICMONITORING NEEDS................................................................................................ B-1

Personnel Involved ..................................................................................................B-2

Objectives ................................................................................................................B-2

CMONITORING GAS, FLUID, AND TUBE METAL TEMPERATURES.................... C-1

Thermocouple Installation Process Specification.................................................... C-2

Process Specification.............................................................................................. C-2

Additional Important Installation Requirements....................................................... C-3

Data Recording ....................................................................................................... C-3

Photos of Installation Steps..................................................................................... C-4

Capacitance Discharge Weld Qualification ........................................................... C-11

DCOST FOR A 96 THERMOCOUPLE MONITORING SYSTEM .............................. D-1


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LIST OF FIGURES

Figure 1-1 Cycle chemistry evaluation, and improvement road map: feedwater (all-ferrous cycles)

1...................................................................................................................1-5

Figure 1-2 Cycle chemistry evaluation, and improvement road map: evaporator water1...........1-6

Figure 1-3 Thermal transient evaluation and optimization road map .........................................1-7

Figure 2-1 Example generic cycle chemistry diagram for a triple pressure HRSG withreheat .................................................................................................................................2-4

Figure 2-2 Cycle chemistry diagram showing the chemical feed and chemistry

monitoring locations in a triple pressure HRSG with reheat that is controlled withammonia additions in the feedwater and trisodium phosphate additions to the IPand HP drums. ...................................................................................................................2-5

Figure 2-3 Total (particulate + soluble) iron levels measured in each of three HRSGsafter a few months of plant operation. Note that the iron concentrations are close tothe desired value of less than 5 ppb for all the sections of the cycle except theintermediate pressure drums. ..........................................................................................2-11

Figure 2-4 Characterization of an aggressive basic thermal shutdown and startup cycle. ......2-15

Figure 2-5 Example of a preliminary 21 day in-situ test to confirm that reducing theinjection of hydrazine does reduce the total level of iron in the feedwater.Subsequently on this unit, the reducing agent was eliminated.........................................2-17

Figure 2-6 Example arrangement of a set of thirteen diagnostic/ troubleshootingthermocouples that identified and quantified the magnitude of cold startup relatedrow-to-row and element-to- element tube temperature differences in an HRSGreheater bundle. Prior to introducing steam flow (at approximately 38 minutes aftercombustion turbine startup) the leading row tubes (Row A) were approximately55

oC (100

oF) hotter than the trailing, Row B tubes. After introducing steam flow, two

of the leading row tubes near the right hand side of the bundle were rapidlyquenched to near the estimated saturation temperature. This forward flow ofsaturated liquid (most likely undrained condensate) resulted in a tube to tubetemperature difference near 140

oC (250

oF). In more recent tests on another design

of HRSG tube-to-tube temperature differences in excess of 167oC (300

oF) were

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

Figure 2-7 Example of the location of twenty eight diagnostic/ troubleshootingthermocouples installed in a vertical tube high pressure economizer tube bundle. .........2-26

Figure 2-8 Seven high pressure economizer tube Row A (just beneath the outletheader, see Figure 2-7) thermocouples measurements during a cold start. At thislocation the measurements suggest that the HP economizer approach temperatureis at least 10

oF (6

oC) and the tube to tube temperature differences are less than

25oF (14

oC). No steaming or severe tube to tube temperature differences would be

indicated from these thermocouples. ...............................................................................2-27


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Figure 2-9 Seven high pressure economizer tube Row B (just after the final upperreturn bend, see Figure 2-7) thermocouples measurements during a cold start. Atthis location the measurements indicate that the water in four of the seven tubes ismore than 25

oF (14

oC) above the estimated saturation temperature (i.e., air-locking

or steaming is occurring in the tubes near the edges of the bundle). This resulted ina tube to tube temperature difference near 80

oF (44

oC)...................................................2-27

Figure 2-10 Seven high pressure economizer tube Row C (just after the first upperreturn bend, see Figure 2-7) thermocouples measurements during a cold start. Atthis location the measurements indicate that the water in one of the seven tubes ismore than 10

oF (6

oC) above the estimated saturation temperature (i.e., air-locking

or steaming is occurring in at C23). Since the water in the other tubes is more than100

oF (55

oC) below the saturation temperature this results in a tube to tube

temperature difference that is greater than 100oF (55

oC).................................................2-28

Figure 2-11 Seven high pressure economizer tube Row D (in the vertical downflowtube just beneath the inlet header, see Figure 2-7) thermocouples measurementsduring a cold start. At this location the measurements indicate that the water in thethree cooler tubes, which are probably representative of the majority of tubes in the

row, remains close to the HP economizer feedwater inlet temperature, whereas thefluid in the other tubes with TCs (that are close to the blind ends of the inlet headerand further from the inlet pipes on the header) operate up to 67

oC (120

oF) hotter

than the coldest tubes due to either flow stagnation or reverse, recirculating flowupwards in the tubes with the lowest pressure drop between the upper inlet andlower return header. Lower flow and high temperatures in the tubes furthest fromthe inlet pipes were caused by higher hydraulic resistance and buoyancy forces inthe water in these tubes. Gas laning and elevated heat absorption in the tubecircuits near the sides of the bundle may have been partially responsible for theincreased buoyancy forces, reduced flow and elevated water temperatureobserved. .........................................................................................................................2-29

Figure 2-12 Time line plot of the bulk temperatures measured up and downstream of a

single parallel pass, two row, vertical tube, finishing (high temperature) reheaterduring a warm start made with a low hot reheat steam temperature setpoint

4. Two

significant temperature drops were observed. The first occurred soon after steamflow was established through the reheater bundles and was attributed to forwardflow of undrained condensate. The second event was more severe and was causedby overspraying of the interstage attemperator too close to saturation temperature.Operating practices that contributed to the overspraying were substantial loweringof the RH steam outlet temperature setpoint and excessively aggressive ramping ofthe CT load and exhaust gas temperature to the maximum gas temperature.Simultaneous raising of HP pressure exacerbated the severity of the event...................2-32

Figure 2-13 Spatial temperature plot of 74 reheater tube thermocouples. This time slicewas associated with the attemperator overspray event shown in Figure 2-12. Sometubes were more than 83oC (150oF) hotter than the adjacent tubes. The tubes nearthe inlet nozzle centered above tube element #9 were severely cooled in both tuberows. Near the other inlet nozzles the dogleg tube row (Row #2) was being cooledwell below the straight, leading row of tubing...................................................................2-33

Figure B-1 Location of thirty two thermcouples to be installed in direct contact with thetube OD surface between the finned tubing and the headers........................................... B-8

Figure B-2 Location of fourteen thermocouples to be installed in direct contact with thetube OD surface between the finned tubing and the headers........................................... B-9


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Figure B-3 Location of eighteen thermocouples to be installed in direct contact with thetube OD surface between the finned tubing and the headers......................................... B-10

Figure B-4 Location of three thermocouples to be installed in direct contact with the ODsurface of the downcomer on the east tube bundle ........................................................ B-11

Figure B-5 Location of fifteen thermocouples to be installed in Bundle A in direct contact

with the tube OD surface between the finned tubing at three locations on 5 tubeelements.......................................................................................................................... B-12

Figure C-1 Removal of a window of lagging and insulation on an HP downcomer.................. C-4

Figure C-2 Removal of surface oxide with gentle grinding ......................................................C-5

Figure C-3 Preheating of cleaned surface and verification of the surface temperaturewith a Tempil stick............................................................................................................. C-6

Figure C-4 Attachment of the work lead (attached with a magnet) and the electrode lead(thermocouple wire held on the surface with a plier which is electrically connectedto the capacitance discharge power supply. .....................................................................C-7

Figure C-5 Completed thermocouple junction welds ............................................................... C-8

Figure C-6 Supporting and guiding the thermocouple wire from the thermocouplejunction to the data acquisition system. It is important that the wire is guided in amanner that provides for thermal expansion and protection from personnel. ................... C-9

Figure C-7 Insulation covering the thermocouple junctions on a HPSH tube. Theinsulation used was spare manway door insulation that was held in place with twowraps of stainless steel wire. .......................................................................................... C-10

Figure C-8 Mock-up T91 tube that was used to qualify the thermocouple junctionwelding procedure. Note the two unsuccessful welds above the completed welds.Unsuccessful weld junctions should be removed by light grinding.................................. C-11

Figure C-9 Polished metallurgical cross-section of one of the mock-up thermocouplejunctions. Although this junction has some minor porosity, undercut and incomplete

fusion it will provide accurate temperature readings and will probably haveadequate reliability. ......................................................................................................... C-11

Figure C-10 Cross-section through another thermocouple junction. This junction hassevere porosity and small cracks (see Figure C-11). This thermocouple junction willprovide accurate temperature readings but is not adequate if longevity andresistance to in-service cracking are important............................................................... C-12

Figure C-11 Close-up of the thermocouple junction shown in Figure C-10. Note thesevere porosity, and cracks that extend toward the tube surface. The tube surfacebeneath the weld will have a shallow, but very hard heat affected zone. Thecombination of porosity, microcracks, and the hard base metal heat affected zonecould lead to cracking. .................................................................................................... C-12


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LIST OF TABLES

Table 2-1 Example of Measured or Recommended Cycle Chemistry Parameters for aTriple Pressure HRSG with Reheat Treated with Ammonia in the Feedwater andTrisodium Phosphate in the IP and HP Drums. Selected measured valuesmeasured at full load are shown in the right-hand column. (The values within thistable are not all in line with the EPRI Guideline values and should not be applied tothe readers unit. Please see the EPRI Guidelines

1)...........................................................2-6

Table 2-2 HRSG Cycle Chemistry Monitoring Parameters ......................................................2-18

Table 2-3 HRSG Thermal-Hydraulic Monitoring Parameters...................................................2-21

Table A-1 DCS Attribute Grouping Useful for Identifying and Characterizing Thermal-Mechanical Cycles ............................................................................................................ A-3

Table B-1 Location and Number of Diagnostic Thermocouples .............................................. B-7


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1INTRODUCTION

1.1 Background

There have been numerous heat recovery steam generator (HRSG) pressure part failures thathave resulted from less than optimal cycle chemistry and damaging thermal transients. It isestimated that poor cycle chemistry and damaging thermal transients are responsible for morethan 80% of the pressure part failures that have been experienced in HRSGs.

To address this concern EPRI developed the Interim HRSG Cycle Chemistry Guidelines

1

. Theseguidelines provided road map approaches to the monitoring, optimization, and control of cyclechemistry in single-, double- and triple-pressure combined cycle HRSGs. Next EPRI preparedthe HRSG Tube Failure (HTF) Manual

2. This manual identified all the current HTF and ways of

preventing repeat failures. EPRI then recognized that there was a need for a more proactiveapproach to prevention of HRSG pressure part damage. A report entitled Delivering HighReliability Heat Recovery Steam Generators

3was prepared. This document presented needed

actions during the design, commissioning, and operation phases, etc. to prevent HRSG pressurepart damage and failures.

This latter effort, and recognition that pressure part failures caused by transiently high, thermallyinduced cyclic stresses are becoming increasingly frequent, led to the need to compile

information on the world wide design codes dealing with fatigue and their deficiencies, and thusthe need to provide better guidance for the assessment of creep fatigue in components thatnormally operate at higher temperatures, thermal fatigue in lower temperature components orcorrosion fatigue of HRSG pressure parts. It also led on the chemistry side to the clearunderstanding that many of the chemistries adopted during the design phase are designed to fail.Historically, poor commissioning practices have generally failed to identify or evaluate thedamaging consequences of either the thermal transients or the non-optimum chemistry. Thisreport provides a road map approach for identifying non-optimum chemistry regimes anddamaging thermal transients in the various circuits.

The most frequently occurring HRSG tube failure (HTF) damage mechanisms include:

1. Corrosion fatigue

2. Flow accelerated corrosion, single and two-phase

3. Underdeposit corrosion (hydrogen damage, acid phosphate corrosion, caustic gouging)

4. Pitting corrosion

5. Thermal quench-induced fracture


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Introduction

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6. Thermal fatigue

7. Creep fatigue

Damage mechanisms 2, 3 and 4 are predominately cycle chemistry influenced and mechanisms5, 6 and 7 are thermal mechanical mechanisms. Damage mechanism 1 requires the simultaneous

occurrence of a corrosive environment and high thermally induced stresses.

Some common root cause factors for the cycle chemistry influenced failures include:

a) Initiating startup operation with a corrosive environment in the water, increasing the risk ofcorrosion fatigue damage.

Deficient chemistry control.

Improper chemical cleaning.

Deficient startup operating practices.

Deficient design that incorporates features and details that develop high localized thermalstresses in pressure parts.

b) Operation with a corrosive environment in flowing feedwater and/or steam-water mixtures,increasing the risk of flow-accelerated corrosion.

Reducing feedwater chemistry (single-phase FAC) (i.e., oxidizing-reducing potential,ORP, is in the negative range).

Entrained water droplets of low pH in steam-water mixture (two-phase FAC).

c) Operation with a corrosive environment beneath waterside deposits, increasing the risk of

underdeposit corrosion mechanisms including hydrogen damage (HD), acid phosphatecorrosion (APC) and caustic gouging (CG).

Excessive waterside deposits (all mechanisms).

Waterside flow disruptions (all mechanisms).

Improper gas side conditions (all mechanisms).

Improper selection/control of evaporator chemical treatments (all mechanisms).

Water treatment plant upsets (CG and HD).

Condenser leaks (HD).

Improper chemical cleaning (HD).

Nonoptimal phosphate treatment such as congruent phosphate treatment (APC).

d) Creation of a corrosive environment during idle periods.

Improper wet layup with stagnant oxygenated water.

Non use of nitrogen blanketing during shutdown.


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Introduction

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Improper chemical cleaning.

Carryover of sodium sulfate and deposition in reheaters (rare in HRSGs because of thelow number of units with condensate polishers).

Some common root cause factors for the thermally induced failures include:

a) In horizontal gas path HRSGs, transient tube-to-tube temperature variations in vertical tubesconnected to common upper and lower headers. These temperature differences are intendedto be very small. Typically well under 50

oC (90

oF). Tube-to-tube temperature differences

above 100oC (180

oF) have been measured in numerous HRSGs. There are a variety of root

causes of these unanticipated and damaging temperature differences including:

Startup conditions with gas temperatures and flows that are excessive.

Forward flow of undrained condensate in HPSH or RHs during startups due to poordesign and/or arrangement of lower connecting pipes and the drains removal and disposalsystems or to incorrect operation.

Introduction of saturated or subcooled water into the HPSH or RH by over sprayingduring startups and shutdowns or poor design and/or arrangement of the interstageattemperators.

Delays in establishing natural circulation in successive rows of evaporator tubes duringcold startups.

Circulation of stratified, subcooled liquid in the HP evaporators during startups followingovernight or weekend shutdowns.

Introduction of cold feedwater into a hot stagnant LP economizer or preheater duringstarts and drum top ups.

Introduction of hot recirculation water into the cool inlet of the LP preheater followingtrips or shutdowns.

Steaming or air lock in some economizer tubes due to lack of or inadequate venting.

Periodic reverse or stagnant flow of feedwater in some economizer tubes during startupsor continuous flow recirculation on load.

Leakage of cooler feedwater into hotter sections of economizers through passingmaintenance drains.

b) Tube-to-header or through wall header temperature gradients must also be controlled to avoidlocalized yielding and cyclic damage. Localized permanent cyclic damage may occur whenthese gradients exceed a critical value that is dependent on the local geometry. Thesedamaging transients are caused by:

Excessively fast steam temperature and/or steam pressure increases during startups.

Headers are too thick and/or of large diameter.

Introduction of saturated or subcooled water such as undrained condensate orattemperator spray water into the HPSH or RH.


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Introduction

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Introduction of cold feedwater into the economizer/evaporator headers/drum.

Periodic reverse or stagnant flow of feedwater in the economizer, during startups.

EPRI developed a benchmarking process where an organizations HRSG Dependability Programranks on a world class basis

6. It is clear that although thermal- and cycle chemistry-induced

HRSG failures predominate, there has currently been very little effort by HRSG operators toidentify the precursors of failure on their units. EPRI has also developed a set of attributes thatare needed to put an organizations HRSG on the road to worldclass

7. The need to identify the

thermal transients and deficiencies in cycle chemistry early in the life is one of the key features.

The objective of diagnostic/ troubleshooting monitoring is to identify and eliminate non-optimum cycle chemistries and avoidable, potentially damaging thermal transients in the variousHRSG circuits as early in the life of unit as possible. This can be accomplished by applying aroad map approach (Figures 1-1 through 1-3) to the identification, measurement, evaluation,correction, and control of those affects that may result in poor HRSG reliability within itsintended design life, with particular emphasis on eliminating failures in the first few years ofoperation.

Section 2 of this report provides a nine step approach which can be followed to accomplish theobjectives stated above. Three appendixes provide: (i) a list of background information thatshould be reviewed (Appendix A), (ii) examples illustrating the logical selection of locations fortube temperature measuring thermocouples (Appendix B), (iii) a protocol for the installation oftube temperature measuring thermocouples (Appendix C), and estimates of the costs to installapproximately 100 thermocouples in a tube temperature monitoring system (Appendix D).


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Figure 1-1Cycle chemistry evaluation, and improvement road map: feedwater (all-ferrous cycles)

1


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Figure 1-2Cycle chemistry evaluation, and improvement road map: evaporator water

1


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Figure 1-3

Thermal transient evaluation and optimization road map


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2-1

2STEPS FOR THE IDENTIFICATION, CORRECTION AND

CONTROL OF CYCLE CHEMISTRY OR THERMALTRANSIENT INFLUENCED HRSG PRESSURE PART

DAMAGE

2.1 Introduction to Road Map Steps

Time, cost, and difficult access to certain tube bundle locations places limits on the location,type, and extent of diagnostic instrumentation that can be used to identify potential cyclechemistry and thermal transient influenced HRSG pressure part damage. With the plethora ofHRSG designs it is also not possible to have a single approach that can be used for all units. Onthe other hand, it is possible to provide a set of steps that can be followed to prescribe diagnosticinstrumentation requirements and use these measurements to make appropriate correctiveactions. The primary steps in this process include (Figure 1-3):

1. Review of unit design, operational and maintenance information.

2. Identify the potential cycle chemistry or thermal-mechanically influenced damagemechanisms.

3. Specify the type and locations for the diagnostic instrumentation.

4. Install high priority diagnostic instrumentation.

5. Operate unit over a wide range of operating conditions.

6. Review and evaluate the results of diagnostic instrumentation measurements.

7. Evaluate, engineer and implement operational, maintenance and design enhancements toameliorate or eliminate damage influencing cycle chemistry or thermal-mechanical events.

8. Verify the success of the changes through additional monitoring and evaluation.

9. Ongoing monitoring, evaluation, and improvements.

This report is focused on the first four of these steps but includes some guidance on each step. Acompanion report Evaluation of Creep-, Corrosion- and Thermal-Fatigue of HRSG PressureParts

5addresses Step 6. Other projects are underway that will provide additional guidance and

case studies providing examples for every step in this process.

Each of the nine road map steps are described in more detail in the following sections.


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Steps for the Identification, Correction and Control of Cycle Chemistry or Thermal Transient Influenced HRSGPressure Part Damage

2-2

2.2 Step 1: Review of Unit Design, Operational and MaintenanceInformation

Working with the plant staff, the information listed in Appendix A should be collected andreviewed. The objective of this review is to gain an understanding of the current design, and

operation and to identify major operational or design issues. It is important to emphasize duringthis initial review that many of the more subtle, off-design problems that have caused HRSGpressure part failures are sometimes impossible to identify with existing plant instrumentation.

2.2.1 Cycle Chemistry Review

The cycle chemistry review is usually relatively straightforward compared with the review of thethermal hydraulic features. The primary effort required for the cycle chemistry review is thepreparation and review of a heat balance/cycle chemistry diagram for the unit. A custom cyclechemistry diagram for each unit should be developed if these are not already available. Thegeneric cycle chemistry diagrams in EPRIs Interim Cycle Chemistry Guideline

1for HRSGs

(e.g., Figure 2-1) can be used as a guide but these should be customized to reflect the uniquecircuitry, feed points and monitoring instrumentation of each unit (e.g., Figure 2-2).

The basis of these diagrams is the unit heat balance diagram. Superimposed on this diagram arethe continuous on-line instruments employed at each monitoring point around the unit. Thentypical values for each monitored parameter will provide good indications of how the unit isrunning. Chemical injections into the feedwater and drum provide an instant indication of howclose the unit is to optimum treatments. Finally the grab sample analyses are added; mostimportant here are the total iron levels in the feedwater and each drum. After the cycle chemistrydiagram is prepared, a review to see if core cycle chemistry monitoring instrumentation ispresent should also be made.

For example, from a comparison of the cycle chemistry diagram shown in Figure 2-2 with EPRIGuidelines

1it clear that the following additional continuous monitoring instrumentation is

needed on this unit:

Demineralized makeup water effluent silica measured once per shift.

Cation conductivity of the feedwater downstream of the ammonia feed.

Cation conductivity of the LP drum saturated steam.

Cation conductivity and sodium of the LP superheat outlet steam.

Cation conductivity of the IP drum saturated steam.

Cation conductivity and sodium of the hot reheat steam.

Next the results of recent full load grab sample and on-line monitoring results should bereviewed (e.g., Table 2-1 and Figure 2-3) to see what attributes are measured, what control oraction limits have been set and which parameters have been outside the optimum ranges. Usingthis approach it is always possible to quickly identify non-optimal conditions that should becorrected prior to any further operational transient studies. For example, although comprehensive


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2-3

chemistry results were not available, the available measurements listed in Table 2-1 and Figure2-3 identified the following issues:

The makeup and preheater inlet water have excessive conductivity and dissolved oxygencontents.

The IP and HP evaporator water is showing phosphate and conductivity instabilitiessuggesting moderate phosphate hideout and return may be occurring.

The IP evaporator water has high iron levels.

Based on these findings a more thorough investigation to determine the source of the highconductivity and oxygen readings in the makeup and turbine condenser condensate water wouldbe performed, starting with independent measurements and instrument calibration or morefrequent and comprehensive grab sample analyses. Measurements of air inleakage would also bemade.

Ongoing IP drum water iron monitoring including some detailed monitoring performed during

thermal transients and following IP drum blowdown would be performed to attempt tounderstand the reason for the elevated levels of iron in the IP evaporator.

The elevated conductivity and phosphate instabilities in the IP and HP evaporator water wouldbe investigated by checking the calibration of the instruments, and monitoring sodium,phosphate, pH and cation conductivity and iron more frequently and over a range of operatingconditions. If further evidence of phosphate hideout was found then an internal videoprobeexamination of the leading row of HP and IP evaporator tubing would be performed. If watersidedeposits were observed then instrumentation to measure the gas temperature profile, circulationratios, and steam quality at various fired and unfired operating conditions in the HP and IPevaporators would be performed. This detailed monitoring might include adding gas and midwallchordal thermocouples into sections of the leading row tubing experiencing deposition. Todetermine if the circulation and steam quality are acceptable may require installing flow meterson each of the downcomers and a few selected riser tubes and pressure gages and thermocoupleson the top and bottom of the downcomers and deposition prone riser tubes.


32/98


Steps for the Identification, Correction and Control of Cycle Chemistry or Thermal Transient Influenced HRSG Pressure Pa

2-4

Figure 2-1Example generic cycle chemistry diagram for a triple pressure HRSG with reheat


33/98



2-5

Figure 2-2

Cycle chemistry diagram showing the chemical feed and chemistry monitoring locationsin a triple-pressure HRSG with reheat that is controlled with ammonia additions in thefeedwater and trisodium phosphate additions to the IP and HP drums.


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2-6

Table 2-1Example of Measured or Recommended Cycle Chemistry Parameters for a Triple PressureHRSG with Reheat Treated with Ammonia in the Feedwater and Trisodium Phosphate inthe IP and HP Drums. Selected measured values measured at full load are shown in theright-hand column. (The values within this table are not all in line with the EPRI Guidelinevalues and should not be applied to the readers unit. Please see the EPRI Guidelines

1)

Location/Parameter

Sample Normal ActionLevel 1:Returntonormalwithin 1wk, Nomorethan 2wk/yr

ActionLevel 2:Returntonormalvalueswithin 24hr, Nomorethan 48hrs/yr

ActionLevel 3:Shutdown unitwithin 4hrs toavoiddamage,No morethan 8hrs/yr

Immediateshutdown

Measuredfull loadvalues

Condenser Air RemovalExhaust

Air inleakageSCFM/100MW

Daily 1 >1

Condenser Leak Detection Trays or HotwellZones

Cationconductivity,S/cm

Continuous

Demineralized Makeup Water

* Silica, ppb Once per shift 10 15 20 25* Specific

conductivity,S/cm

Continuous 0.1 >0.2 0.50 to1.02

pH 7.1 to 8.1

Hardness 0 >0

Total organiccarbon, ppb

Weekly orTroubleshooting

300

Condensate Pump Discharge

* Cationconductivity,S/cm

Continuous 0.20 0.35 0.65 > 0.65 0.40 to0.84

* Dissolvedoxygen, ppb(if notmeasured ateconomizerinlet)

Continuous 20 40 > 40 69 to 78

Sodium, ppb Continuous 5 10 20 > 20


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2-7

Location/Parameter

Sample Normal ActionLevel 1:Returntonormalwithin 1wk, No

morethan 2wk/yr

ActionLevel 2:Returntonormalvalueswithin 24

hr, Nomorethan 48hrs/yr

ActionLevel 3:Shutdown unitwithin 4hrs toavoid

damage,No morethan 8hrs/yr

Immediateshutdown



Weekly orTroubleshooting

200 > 200

Iron, ppb Weekly orTroubleshooting

5 >5 3 to 5

Preheaterinlet water

* Cation

conductivity,S/cm

Continuous 0.20 0.35 0.65 > 0.65

* Dissolvedoxygen, ppb(if notmeasured atCPD)

Once per shift 20 40 > 40

Ammonia Consistent with pH, no limit

pH Continuous 9.2 to9.6

< 9.2

Iron, ppm Weekly 5 >5

Sodium, ppb 4 5 10 > 20

ORP, mV Troubleshooting To + 50

Specific conductivity, S/cm 4.6 to 6.0


Troubleshooting 200 > 200

Hardness 0

LP drum water effluent (IP and HP economizer and attemperator water influent)


Continuous 0.20 0.35 0.65 > 0.65 0.27 to0.62

* Dissolved

oxygen, ppb(if notmeasured atCPD)

Once per shift 1 to 10 15 20 > 20 96 to 103

pH Continuous 9.8 to10.2

< 9.8 or > 10.2 < 8.5 or>12

9.8 to 9.9

Silica, ppb 10 15 20 > 20

Iron, ppb Weekly 5 >5 4 to 7


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2-8

Location/Parameter


morethan 2wk/yr





Immediateshutdown


Copper, ppb 5

Ammonia,ppm

Consistent with pH, no limit 2.8 to 4.3

Chlorides,ppb

2

ORP, mV Troubleshooting To + 50

IP drum water (blowdown or

downcomer)


Continuous

* Phosphate,ppm

Once per shift 3 < 1 or > 4 0.1 to 4.1

* pH Continuous 9.2 to 9.6 < 9.2 < 8.5 or>12

9.2 to10.0

Sodium, ppm Continuous

Silica, ppb Once per day 0.6 >0.6

Iron, ppm Weekly 5 >5 7 to 23

Phenolphthalen alkalinity, ppm 6Methyl purple alkalinity, ppm 15

Hydroxide,ppm

Troubleshooting

HP drum water (blowdown ordowncomer)


Continuous 1.3 to17.6

* Phosphate,ppm

Once per shift 3 < 1 or > 4 0.1 to 3.8

* pH Continuous 9.2 to 9.6 < 8.5 or>12

9.4

Sodium, ppm Continuous

Silica, ppb Once per day >0.6

Iron, ppm Weekly 5 >5 3 to 7

Phenolphthalen alkalinity, ppm 6

Methyl purple alkalinity, ppm 15

Hydroxide, Troubleshooting


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2-9

Location/Parameter


morethan 2wk/yr





Immediateshutdown


ppm

LP drumsaturatedsteam


Continuous 0.30 0.55 1.0 > 1.0

Sodium, ppm Continuous 4 10 20 > 20Silica, ppb Once per day 10 20 40 > 40

pH 9.2 to 9.6 10.2

IP drumsaturatedsteam


Continuous 0.30 0.55 1.0 > 1.0

Sodium, ppm Continuous 4 10 20 > 20

Silica, ppb Once per day

10

20

40 > 40pH 9.2 to 9.6 9.8 to

10.0

HP drumsaturatedsteam


Continuous 0.30 0.55 1.0 > 1.0 0.29 to0.35

Sodium, ppm Continuous 4 10 20 > 20

Silica, ppb Once per day 10 20 40 > 40 3.3 to

10.7pH 9.2 to 9.6 9.7 to

10.0

LP superheater outlet steam


Continuous 0.30 0.55 1.0 > 1.0


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2-10

Location/Parameter


morethan 2wk/yr





Immediateshutdown


* Sodium, ppm Continuous 4 10 20 > 20

Silica, ppb Once per day 10 20 40 > 40

pH 9.2 to 9.6

IP superheater outlet steam


Continuous 0.30 0.55 1.0 > 1.0



pH 9.2 to 9.6

HP superheater outlet steam


Continuous 0.30 0.55 1.0 > 1.0



pH 9.2 to 9.6

Reheatoutlet steam


Continuous 0.30 0.55 1.0 > 1.0 0.11 to0.15


Silica, ppb Once per day 10 20 40 > 40 0.12

pH 9.2 to 9.6

Notes: * = EPRI "core" parameters

HP/IP/LP drum pressure: 2074 psig/528 psig/ 83 psig


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2-11

Average of 10 dail y iron readings in Units #1, #2, and #3

0

5

10

15

20

25

30

Con

dens

ate

LP#1

LP#2

LP#3

IP#1

IP#2

IP#3

HPEc

on#1

HPEc

on#2

HPEc

on#3

HP#1

HP#2

HP#3

Totalironc

oncentration,ppb

Figure 2-3Total (particulate + soluble) iron levels measured in each of three HRSGs after a fewmonths of plant operation. Note that the iron concentrations are close to the desired valueof less than 5 ppb for all the sections of the cycle except the intermediate pressure drums.

2.2.2 Review of Pre-operational and Layup Practices

The layup practices that have been and will be used should be thoroughly reviewed to identifyopportunities for improvement since poor layup practices can cause severe, rapid and numerousHRSG pressure part failures.

As part of this review it may also be worth finding out when and how the unit was stored prior tocommercial operation, especially during periods of suspended construction. It is useful to learnhow the final pressure test hydrotest water was treated and how long it remained in the pressureparts. The pre-operational chemical cleaning reports should be reviewed to learn if the unitentered commercial operation with clean, well passivated internal surfaces.

One of the wet layup issues that often needs to be thoroughly reviewed and evaluated, especiallyfor units that experience numerous stop/start cycles, involves the feeding of a reducing agentsuch as hydrazine into the water during shutdown periods. The objective of this practice is toprovide a means of controlling the level of dissolved oxygen in the water. Unfortunately thispractice can often make the situation worse because it changes the electrochemistry of the watersufficiently to alter the stability of the passive oxide layer that has formed on the water touched


40/98



2-12

surfaces. Under alternating on-line oxidizing and off line reducing conditions the protective ironoxides will become non-adherent and during restarts very high concentrations of oxide willrelease from the water touched surfaces, flow forward and deposit in the high heat flux sectionsof the IP and HP evaporators.

One of the goals of the cycle chemistry program, especially for units that are required to stop andstart frequently should be to condition the water in a way that makes the pH fluctuate as little aspossible and maintains the oxidation-reduction potential (ORP) either positive (oxidizing) ornegative (reducing). If the on-line chemistry requires a reducing (negative) ORP then the off-linewet layup should also be designed to produce water that is reducing. On the other hand, if themore desirable practice (for most modern, all-ferrous pressure part HRSGs) of treating thefeedwater with chemicals that produce a positive ORP is used then the wet layup practicesshould be designed to produce water that also maintains a positive ORP. The practices toaccomplish these objectives are documented in the EPRI Interim Cycle Chemistry Guidelines

1.

EPRI is currently working on a HRSG layup guideline which will provide an update on the bestoptions for wet and dry layups.

For units that dont use a feedwater reducing agent except for wet layups the feedwater and LP,IP and HP drum water iron concentrations should be measured during some cold starts that werepreceded by wet layup and starts following extended weekend shutdowns. Grab samples takenevery 15 minutes during the startup will provide a baseline for comparison tests performed usingwet layups that only use a properly applied nitrogen blanket and cycle chemistry that is the sameas the operating chemistry.

This review should also include a detailed evaluation of the nitrogen capping feedpoints, andpractices. A common error is to provide the nitrogen too late (after some air entered the pressureparts). It is important that the nitrogen cap be added to the main condenser and turbine, deaeratorand storage tank and steam drums while there is still residual heat and pressure present. This

positive pressure of nitrogen must also be maintained throughout the layup period. For wet layupthat last longer than a few days the water in evaporator and economizer circuits may need to becirculated to avoid prolonged stagnant conditions.

With regard to diagnostic monitoring issues and wet layups some consideration should be givento instrumentation requirements and sampling intervals to ensure that the pH and oxygen levelsin the economizer, evaporator, condenser and feedwater water are maintained within acceptablelevels. If dry layups are being used then instrumentation requirements for monitoring thehumidity throughout the HRSG, turbine, condenser and feedwater system should be addressed.

2.2.3 Review of Thermal-Mechanical Parameters

The thermal mechanical design and operating practices review can begin in a fashion that issimilar to the cycle chemistry review but usually requires considerably more information andprevious experience analyzing the results from diagnostic monitoring of other HRSGs becausemany of the key damage influencing operating targets and control limits are often not known andthe damaging thermal events can be very localized and of short duration.


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2-13

The review of steady state operating parameters at full load and at part loads at the lower end ofthe normal operating range might occasionally provide some indication of problems in HPsuperheater or reheater. However, even when bulk values of economizer approach temperature orevaporator pinch points are within their expected ranges the unit may still be experiencingsevere, damaging thermal transients because the tube temperature anomalies usually do not causediscernible deviations in the bulk outlet fluid temperature measurements.

The next step is to review the transient conditions for a broad sampling of shutdowns andstartups. Unless the unit already has definitions for different transients the following definitionscould be used:

The usual shutdown procedure used at the plant

Shutdown with controlled combustion turbine firing

Forced cooling or rapid maintenance shutdown

Trip (Combustion turbine (CT), Steam turbine (ST), or HRSG induced)

Rapid hot restarts - with the unit offline for less than ~ 5 hours

Hot starts made after longer shutdowns where the HP drum pressure prior to the startupremains above approximately 35 barg (500 psig). These are associated with shutdowns thatare around 5 to 12 hrs, depending on the method used for the shutdown and the leak tightnessof the HP section of the HRSG.

Warm starts made after the HP drum pressure prior to the startup is between approximately 7barg (100 psig) to 35 barg (500 psig) , provided the unit was boxed up after the shutdownwith high HP drum pressure and is leak tight. These are associated with weekend shutdowns.Some units will not be able to maintain HP drum pressure above 7 barg (100 psig) for morethan approximately 12 hrs to 24 hrs offline.

Extended warm starts made after the HP drum pressure has approached ambient conditions(0.5 barg to 7 barg, 7 psig to 100 psig). These are typical of long weekend shutdowns 24 to60 hours although some units may experience HP drum pressure decay to ambient pressure in30 hours or less offline. Although rare, some units are able to maintain pressure aboveambient for time periods approaching 72 hrs or more.

Cool starts made from 0 barg (0 psig) pressure, but where the HP drum water temperatureremains above about 75

oC (167

oF).

Cold starts made after the HP drum water are close to ambient conditions.

Unless a stop/start algorithm has been programmed into the plant historian it will probably onlybe practical to review a small number of recent shutdown and startup events.

If the plant historian does not have a software algorithm to identify the date, time, type andrelative severity of the stop/start transient then plots of the following parameters can be used toquickly identify the beginning and ending date/time of specific shutdowns and startups:

Combustion turbine speed, exhaust temperature, and load

HP drum pressure


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2-14

HP steam outlet temperature and pressure

Steam turbine load

The basic thermal cycle comprises a CT/HRSG shutdown, followed by a period of naturalcooling while offload, followed by the CT/HRSG startup and reloading to high load, followed by

the high temperature dwell part of the cycle during operation on load. The beginning of thisthermal cycle is identified by the beginning of combustion turbine load decrease from the highestoperating load. The beginning of the start is identified by the purge conditions (the combustionturbine speed increases with no load or increase in exhaust gas temperature). The end of thestartup is when the combustion turbine reaches stable load with the HRSG producing steam atmaximum temperature and pressure.

The category of start for the HRSG is determined by the HP drum pressure just prior to thestartup. Figure 2-4 illustrates the constituent parts of one basic thermal cycle. The proceduresused in Figure 2-4 for both the shutdown and startups parts of the cycle are aggressive andconflict with those recommended in Section 4.3 of Reference 3 and are likely to lead to

premature damage or failure in critical parts of the HRSG if cycled.

Once the time periods of the shutdown and startup phases of a range of thermal cycles have beenidentified then more detailed data plots and evaluations for each of these time periods should beperformed. Appendix A list various DCS data attributes that have been found to provide usefulinsights into the potential for specific types of thermal-mechanical damage. The reader isencouraged to start by identifying a pressure part and design feature of concern then identify theavailable DCS instrumentation attributes that provide insight about the thermal-mechanical loadsassociated with the pressure part feature. Appendix A provides examples of timeline plotattributes that have been useful for specific components/features.


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2-15

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0120

240

360

480

600

720

840

960

1080

1200

1320

1440

Time, minutes

Pressure

(ps

ig),Tempera

ture

(oF)

0

20

40

60

80

100

120

140

160

180

200

Loa

d(MW),Spee

d(%)

HP drum pressure CT exhaust temp CT load CT speed

~16 hrs off lin e

~ 1-1/2 hr warm start

Min. HP drum pressure 350 psig

Purge

Shutdown

(1) No cont rolled coo ling (CT speed = 0)

(2) Pressure decay rate (~-1 psi/min)

Figure 2-4Characterization of an aggressive basic thermal shutdown and startup cycle.

The procedures used for this shutdown are not recommended for units intended for

cycling service. Note that the shutdown was performed with rapid deloading andshutdown of the combustion turbine (before the superheater headers had been gentlycooled close to the saturation temperature). Next is seen a natural cooling and pressuredecay of the HP drum. This is followed by a short purge signifying the commencement ofthe startup (identified by increased combustion turbine speed with a slight decrease incombustion turbine exhaust temperature and decay in drum pressure) and anapproximately 1-1/2 hr restart. The HP drum pressure decay to approximately 350 psig(saturation temperature 224

oC (435

oF)) is within the range characterized as a warm start.

2.3 Step 2: Identifying the Potential Cycle Chemistry or Thermal-Mechanically Influenced Damage Mechanisms

After completion of the initial review, good engineering judgment is also needed to assess thepotential for damaging thermal-mechanical transients that may not be picked up with existinginstrumentation. Appendix B provides an example of an engineering judgment based preliminaryassessment of a triple pressure, horizontal gas path HRSG with reheat. These judgment basedassessments rely heavily on knowledge and the past experience gleaned from combined cycleand conventional fossil fuel fired power plant industry experience. Much of this industryexperience information has been widely communicated in public forums and literature. Even so,it is extremely useful to include an industry expert or two in these reviews.


44/98



2-16

The EPRI HRSG Tube Failure Manual2and the EPRI report on Delivering High Reliability Heat

Recovery Steam Generators3provide numerous examples of cycle chemistry and thermal-

mechanical influenced HRSG pressure part failures and the underlying factors that influencethese failures. These references should be thoroughly reviewed to identify potential operational,maintenance or design features that may be relevant to the unit/pressure part/feature beingassessed.

It has also been found useful with regard to thermal-mechanical influenced damage to review theloading modes that have been associated with failures. Some of the key loading modes that haveled to pressure part failures have been summarized in the Evaluation and Control of Creep-,Corrosion- and Thermal Fatigue of HRSG Pressure Parts

5report. These should be reviewed for

each section/pressure part/feature of the HRSG to identify where damage may occur and whatdiagnostic/troubleshooting instrumentation is needed to quantify the severity of the loading ofconcern.

Appendix B provides an example of the process used to identify potential issues. During thisevaluation every section of the HRSG from the first gas touched tube bundles (e.g., the final

reheater) to the last gas touched tube bundles (e.g., the preheater) should reviewed and assessed.For each section of the HRSG the following questions should be addressed:

What thermal-mechanical load influenced pressure part failures could occur in this section ofthe HRSG or have occurred in units with similar operating, maintenance, or designattributes?

What thermal-mechanical loads control the potential damage?

What troubleshooting/diagnostic instrumentation is needed to quantify the potentialdamaging thermal-mechanical loads and provide the information needed to identify the rootcauses of the loads, and verify that corrective actions to eliminate or ameliorate the damagehave been successful?

2.4 Step 3: Specify the Type and Locations for the DiagnosticInstrumentation

Working with the plant staff, specify the location and type of instrumentation that will need to bemonitored for the cycle chemistry and that will need to be installed for the thermal transientdiagnostic/troubleshooting/improvement tests. In the cycle chemistry area this will also includegrab sampling for iron in the feedwater and evaporator circuits. The specific procedures andcontrols to be used prior to and during the operational transients that will be evaluated will alsobe defined and communicated.

For example, it may be recommended that some preliminary tests, evaluations and improvementof the full load chemistry be performed and implemented prior to performing transient thermaland cycle chemistry tests. These might involve testing oxidation-reduction potential (ORP), feedand drum water iron level (with and without the feedwater reducing agent) then adjustingfeedwater ammonia level and the drum water solid alkali additives to optimal levels (Figure 2-5).


45/98



2-17

0

2

4

6

8

10

12

14

16

18

20

6/10/2003

6/11/2003

6/12/2003

6/13/2003

6/14/2003

6/15/2003

6/16/2003

6/17/2003

6/18/2003

6/19/2003

6/20/2003

6/21/2003

6/22/2003

6/23/2003

6/24/2003

6/25/2003

6/26/2003

6/27/2003

6/28/2003

6/29/2003

6/30/2003

7/1/2003

ppb

Hydrazine

Dissolved Oxygen

Iron Level

Figure 2-5Example of a preliminary 21 day in-situ test to confirm that reducing the injection ofhydrazine does reduce the total level of iron in the feedwater. Subsequently on this unit,the reducing agent was eliminated.

Most of the combined cycle units will have an adequate array of core cycle chemistry monitoringinstrumentation. A listing of cycle chemistry monitoring parameters and monitoring points isprovided in Table 2-2. The core parameters are considered the minimum level of surveillancethat is needed for all HRSG units. In general, use of on-line analyzers for continuous analysis ofchemistry is preferred. However, some provision is made for use of shared instrumentation(dissolved oxygen) and laboratory analysis of grab samples (silica and phosphate). The

monitoring approaches suggested recognize limitations on manpower at many HRSG plants andthe fact that some analyzers require significant maintenance attention to perform reliably.

The listing of Suggested Additional Monitoring or Diagnostic Parameters indicated inTable 2-2 represents those chemistry surveillance measures most likely to be included incustomized chemistry programs for specific plants and units. It is anticipated that this existinginstrumentation will need to be supplemented by some grab sampling (particularly of iron).


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2-18

Table 2-2HRSG Cycle Chemistry Monitoring Parameters

Core Monitoring Parameters (Minimum Surveillance for Most Units)

Parameter/Monitoring Approach Monitoring PointsCation ConductivityContinuous by On-Line Analyzer

Condensate Pump Discharge Condensate Polisher Effluent

Feedwater (or Economizer Inlet) Blowdown or Downcomer Saturated Steam, and Main or Hot Reheat Steam

Specific ConductivityContinuous by On-Line Analyzer

Treated Makeup Condensate Polisher Effluent (OT) Blowdown or Downcomer (PT, EPT and CT)

pHContinuous On-Line Analyzer

Blowdown or Downcomer (Drum Units)

Dissolved OxygenContinuous by On-Line Analyzer(May Be Shared)

Condensate Pump Discharge Feedwater (or Economizer Inlet)

SodiumContinuous by On-Line Analyzer

Main or Hot Reheat Steam

Phosphate

Grab Sample Analysis Each Shift(Or Continuous by On-Line Analyzer)

Blowdown or Downcomer (PT and EPT)

SilicaGrab Sample Analysis Each Shift(Or Continuous by On-Line Analyzer)

Treated Makeup

Suggested Additional Monitoring; Troubleshooting or Diagnostic Parameters

Parameter/Monitoring Approach Monitoring PointsSpecific ConductivityContinuous by On-Line Analyzer

Feedwater (Economizer Inlet)

pHContinuous by On-Line Analyzer


SodiumContinuous by On-Line Analyzer(May Be Shared)

Condensate Pump Discharge Condensate Polisher Outlet Blowdown or Downcomer (PT, EPT and CT)

Saturated SteamSilicaGrab Sample Analysis Daily

Blowdown or Downcomer Saturated Steam

ChlorideGrab Sample Analysis Daily Blowdown or Downcomer (EPT, CT, AVT and OT)

Sodium HydroxideGrab Sample Analysis Daily(Or Derived from Conductivity Data)

Blowdown or Downcomer (CT)

HydrazineGrab Sample Analysis Each Shift(Or Continuous by On-Line Analyzer)


IronGrab Sample Analysis Weekly


ORPContinuous by On-Line Analyzer (when used)


Total Organic CarbonGrab Sample Analysis Weekly Treated Makeup Condensate Pump Discharge

Air InleakageCheck Air Removal Rate Daily

Condenser Air Removal System

Still other chemistry parameters that may be monitored under special circumstances include thefollowing:

Ammonia in Feedwaterto improve control of feedwater treatment or to optimize thetreatment approach.


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2-19

Copper in Feedwaterto evaluate feedwater treatment in HRSG units with copper alloys(HRSG/combined cycles usually have copper-free feedwater systems, but may have copperalloys in the condenser.).

Sulfate and Chloride in Treated Makeup, Condensate, Condensate Polisher Effluent andSteamto evaluate cycle contamination and transport involving these species.

Sodium in Treated Makeupto evaluate demineralizer performance.

Total Organic Carbon (TOC) in Feedwater, Boilerwater and Steamto assess the effect oforganic based proprietary treatments and makeup contamination on cycle chemistry.

Analyzers suitable for low level analysis of anions and organics are not expected to be includedin an HRSG plant laboratory. Such testing would need to be contracted to an outside laboratorywith an ion chromatograph (for anions) and a TOC analyzer (for organics).

Water chemistry commissioning should be a part of the total commissioning effort during whicha new unit or a retrofitted unit is transferred from the supplier to the operator. The main

objectives of the commissioning are: To prevent equipment damage due to malfunction of water chemistry related equipment.

To determine the chemical transport characteristics of the cycle and final selection of watertreatment, water and steam chemistry limits.

To assess major sources of impurities and corrosion.

To quantify total carryover.

This is the primary focus of the monitoring campaigns to be performed within this project toselect the optimum evaporator water and feedwater treatments. There have been too many units

where commissioning has not been performed at all or has been performed insufficiently,resulting in major equipment damage within weeks or months of the initial operation. Typicalproblems that can be avoided by proper commissioning include:

Destruction of the magnetite on HRSG boiler tube surfaces and flow-accelerated corrosion(FAC).

Severe hideout of water treatment chemicals, such as sodium phosphate, resulting in depositsand corrosion.

Dryout or onset of departure from nucleate boiling in evaporator tubing which may lead tooverheating or underdeposit corrosion such as caustic gouging, hydrogen damage or acidphosphate corrosion.

High carryover leading to superheater, reheater, and turbine deposits and superheater andreheater overheating failures.

Dirt, debris or corrosion products left in the boiler or elsewhere in the system resulting inbuildup of deposits or foreign object damage.

The water chemistry-related systems and functions that should be tested during commissioninginclude:


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2-20

Choice of boiler water and feedwater treatments.

Chemical oxygen scavenging.

Feedwater corrosion product generation and transport (determination of the points where thecorrosion occurs).

Evaporator carryover.

Chemical addition equipment.

Sampling and analytical equipment.

Deaeration (condenser, deaerator).

Purity of returned condensate.

Blowdown and cycles of concentration.

Makeup system and associated regeneration equipment.

Pretreatment system. Condensate polishers (if used) and associated regeneration equipment.

Condensate and feedwater storage systems.

Cleanliness and passivation effectiveness after preoperational chemical cleaning of cyclecomponents.

The importance of comprehensive monitoring/commissioning for all types of HRSGs units andtreatments cannot be overemphasized.

Most combined cycle units will also have an adequate array of core thermal hydraulic monitoring

instrumentation (Table 2-3). It is very likely that additional diagnostic/ troubleshootingthermocouples will need to be installed. The locations recommended for attachment ofthermocouples to tubes and headers will be based on experience obtained from previous projectsperformed on HRSGs with somewhat similar design features in conjunction with the followinggeneral guidelines applicable to horizontal gas path (HGP) HRSGs.


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2-21

Table 2-3HRSG Thermal-Hydraulic Monitoring Parameters

Core Monitoring Parameters (Minimum Surveillance for Most Units)

Parameter/Monitoring Approach Monitoring Points

Combustion turbine speed and power output &estimatedexhaust gas flow

Combustion turbine and generator

Combustion turbine exhaust temperature (e.g., grid of16 thermocouples)

In duct upstream of the HRSG

Duct burner gas flow Gas inlet pipe

Duct gas temperatures Upsteam of the HPSH

Upstream of duct burner

Downstream of duct burner

SCR Catalyst Inlet

Upstream of IPSH, HPEcon, LPEvap

Downstream of LP econ or Feedwater heaterWithin exhaust stack

Fluid flow rates HPSH, IPSH/RH, LPSH outlet steam

HP, IP, LP Economizer Inlet

Boiler Feedwater to HP, IP, LP Attemperators

Fluid pressure HPSH, IPSH/RH, LPSH outlet

HP, IP, LP drum

Deaerator

HP, IP, LP Economizer Inlet

Fluid temperatures HPSH, IPSH, LPSH, RH outlet header steam

HP, RH desuperheater inlets and outletIntermediate HPSH headers

HP, IP, LP Steam drum

HP, IP, LP Economizer outlet

Fluid levels LP, IP, HP drum levels

Valve positions Boiler feedwater to HP, IP, LP economizerposition

HP, RH desuperheater valve position

HPSH, IPSH/RH, LPSH bypass valve position

HPSH, IPSH, LPSH vent valve position

Pegging Steam Valve Position


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Suggested Additional Monitoring; Troubleshooting or Diagnostic Parameters

Parameter/Monitoring Approach Monitoring Points

Gas temperatures and velocity/ thermocouples As needed, thermocouples typically attachedto vibration restraint bars

Metal temperatures/surface or buried thermocouples Selected tubes, header surfaces,

downcomers

Metal temperature gradients/ surface and buriedthermocouples

Selected header outside and near-insidesurfaces (requires a drilled hole), header stubtube and header outside surface

Pressure drop/ added pressure taps Locations such as the HP evaporatordowncomers and selected riser tubes

Heat absorption rate/ flux domes, chordalthermocouples

Typically in sections downstream of ductburners, HP evaporator, SHs, RHs

Fluid velocity/ annubar flow meters In HP evaporator downcomers and selectedriser tubes

Dew point meters Attached to the unfinned tubing at the

feedwater inlet end of the preheater or lowpressure economizer

HPSHs and RHs of all designs of HRSG are vulnerable to damaging tube-to-tube temperaturedifferences caused by undrained condensate blown forward when significant steam flow isinitiated during each CT/HRSG startup due to one or more of the following design deficiencies:

No means provided to drain condensate from all blind ends of headers, pipes and manifolds.

Drain connections on headers, pipes and manifolds are too small to quickly remove allcondensate during starts from low pressure in HPSH or RH.

Drain pipes and isolating valves are too small.

Drains from different sections of HPSH (or RH) are interconnected with drains from othersections of HPSH (or RH) that normally operate at different pressures, (instead of eachsection that operates at a different pressure being individually piped all the way to themanifold on the blowdown vessel).

Condensate pools along the bottom of long headers, interconnecting pipes and manifoldsbecause they were installed with inadequate fall in the direction of normal steam flow in coldand/or hot operating conditions to ensure all condensate does drain to drain connections.

Inadequate height difference between bottom drain points on HRSG and drain manifold onthe blowdown vessel to accommodate condensate collection pots, positive fall in drain pipesall the way to the tank, sufficient static head on RH drain lines to provide adequate drain flow

rate from RH when at or close to atmospheric pressure.

Drain flow rates from HPSH drains are not automatically regulated as a function of HPpressure to prevent excessive HPSH drains flow from overpressurizing the blowdown vessel.

HPSH and RH drains taken to the same blowdown vessel and causing reverse flow of amixture of steam and slugs of water from blowdown vessel into RH when simultaneouslydraining HPSH and RH.


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Condensation occurs in HPSH tubes whenever the gas temperature at inlet to the HPSH tubes isbelow the saturation temperature in the HPSH tubes. This is likely to occur during decelerationof the CT after firing ceases at every shutdown and in substantial quantities during every prestartpurge before CT firing commences. The second common source of significant quantities ofcondensate in HPSH sections is in-leakage of feedwater at interstage desuperheaters due toleakage past the spraywater control valve when notionally closed if the block valve is open andthe feedpump running, and even past the block valve when notionally closed due to erosion atthe valve seat. When HP superheater pressure is low the high pressure drop across the spraywatervalves when the boiler feedpump is running will pass significant quantities of feedwater throughsmall leakage paths.

Condensation in RH tubes is most prevalent when cooling during offload periods and duringprestart purges of a cold or cool HRSG with the HP drum at or close to atmospheric pressure.However, experience from testing several HRSGs installed with TCs on HPSH and RH tubes isthat RH tubes are particularly susceptible to tube-to-tube temperature differences caused byblowing forward of undrained condensate, not just during cold and cool startups, but also duringhot and warm startups. Furthermore, RH tubes usually experience much larger, very damaging

tube-to-tube temperature differences than HPSH tubes because steam flow and forward pressuredrop cannot be established in the RH until a later stage in the startup than in the HPSH by whenthe gas temperature and that of the uncooled tubes is higher. One common sources of undrainedcondensate in RHs during startups is in-leakage of attemperator feedwater past passing valveswhen the boiler feed pump is running. Another source of potentially substantial quantities ofundrained condensate in the lower manifold, interconnecting pipes and headers of RHs is reverseflow through the RH drain pipes of substantial quantities of saturated steam, in some cases ofsubcooled water, from the common blowdown vessel used for HPSH and RH drains, whichbecomes pressurized to above the RH pressure when simultaneously attempting, as necessary, todrain the HPSH and RH sections prior to establishing forward steam flow through the HPSH andthen the RH. A further source of damage in reheaters is condensate migration forward from cold

reheat pipes when steam flow from the HP steam turbine is established. Substantial condensateforms during warming from long, large cold reheat pipes between the steam turbine and theHRSG, and from leakage past eroded seat of the HP bypass attemperator spraywater blockvalves, which deficient cold reheat pipe drainage arrangements often cannot quickly remove. Anexample of the results from an instrumented RH is shown in Figure 2-6.


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Figure 2-6Example arrangement of a set of thirteen diagnostic/ troubleshooting thermocouples thatidentified and quantified the magnitude of cold startup related row-to-row and element-to-element tube temperature differences in an HRSG reheater bundle. Prior to introducingsteam flow (at approximately 38 minutes after combustion turbine startup) the leading row

tubes (Row A) were approximately 55

o

C (100

o

F) hotter than the trailing, Row B tubes. Afterintroducing steam flow, two of the leading row tubes near the right hand side of the bundlewere rapidly quenched to near the estimated saturation temperature. This forward flow ofsaturated liquid (most likely undrained condensate) resulted in a tube to tube temperaturedifference near 140

oC (250

oF). In more recent tests on another design of HRSG tube-to-tube

temperature differences in excess of 167oC (300

oF) were recorded.

Significant tube-to-tube temperature differences have been measured in both HPSH and RHsteam heating sections downstream of the desuperheaters during loading ramps at startups andduring deloading ramps at normal shutdowns. The propensity for overspraying is greatest onHRSGs supplied with exhaust gas from the GE 7/9 FA combustion turbine because of its veryhigh exhaust gas temperature at relatively low CT generator outputs when steam flow rates are

low. When the HP pressure is simultaneously being raised

diagnostic - troubleshooting hrsg

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