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142 COMBINED CYCLE JOURNAL, Second Quarter 2008

HRSG USERS GROUP

T

he HRSG UsersGroup conductedits 16 th Annual

Conference & Expo-sition in Austin, Tex, April7-9, hosting a record 357attendees (including 170users) and 69 exhibitors.A pre-conference seminaron chemistry-influencedHRSG tube failures and theopening reception/dinnerdominated the first day.Four formal presentationsanchored the open-forumdiscussions on second andthird days which focused onheat-transfer equipment,water treatment, piping sys-tems, controls, ductwork,dampers, stacks, supple-mentary firing, valves andpiping, environmental sys-tems and balance of plant.

The day beforeThe pre-conference seminar, Chem-istry-Influenced HRSG Tube Fail-ures, was praised by attendees.Dr Barry Dooley, one of the electricpower industrys foremost authori-ties on water treatment, developedthe three-part program which includ-

ed participation by Laney Bisbey andScott Wambeke.

Dooley, who joined StructuralIntegrity Associates as a senior asso-ciate about two years ago, spent morethan two decades at the ElectricPower Research Institute where hislast position was technical executivefor materials and chemistry. Bisbey,an associate of Dooleys at StructuralIntegrity, is an expert on technolo-gies for nondestructive examinationof materials. Wambekes expertiseis HRSG inspection and conditionassessment. He is employed by HRST

Inc, the leading independent boiler

consulting firm in the gas-turbine-based sector of the electric powerindustry.

Dooley began with the workshopagenda:n The FAC mechanism: appearance,

locations, mechanism, and influ-ences.

n Typical and focused HRSG inspec-

tions for flow-accelerated corro-sion.

n Current and future aspects ofNDE for FAC.FAC in low-pressure (LP) evapora-

tors is the HRSG tube-failure mecha-nism (HTF) experienced most often.The others, in order of frequency ofoccurrence, are:n Thermal fatigue in the economiz-

er, superheater, and reheater, andcreep fatigue in the superheaterand reheater.

n Corrosion fatigue in LP evapora-tors and economizers.

n

Under-deposit corrosion in HP

evaporators. Mechanismsinclude hydrogen damage,acid phosphate corrosion,

and caustic gouging.n Pitting.

Dooley estimates thatabout 70% of the HTF inci-dents are influenced by cyclechemistry.

He told the group thatFAC is not an HRSG phe-nomenon, although someO&M personnel who earntheir paychecks at GT-basedcogen facilities and com-bined-cycle plants mightbelieve otherwise. It is rela-tively common in nuclearand fossil-fired steam/elec-tric stations and in indus-trial plants, occurring incondensate and feedwatersystems, piping around theboiler-feed pump (includingdesuperheating lines), pip-ing to the economizer inlet

header, deaerators, feedwater-heatershells and drains, steam-turbineexhaust systems, and air-cooled con-densers.

Dooley has been passionatelywriting and lecturing about FAC forthe better part of his career. Despitehis efforts, and those of a few oth-

ers, it remains at the top of the HTFincident list. Perhaps there are justtoo many new people entering theelectric-power business today with-out a good foundation in chemistryfundamentals. FAC seems to be anout of sight, out of mind kind ofthing. Most often, operations peopledont know they have a problem untila pipe or boiler tube leaks.

FAC continues to occur in moreand more systems despite R&D sincethe 1960s, Dooley continued. Carbon-steel tubes, pipes, valves, headers,and vessels containing flowing sin-

gle-phase water and wet steam (two-

FAC, field hardness testingof P91/T91, piping issuesdominate program

April 6-8Hyatt Regency Jacksonville Riverwalk

Jacksonville, Fla

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COMBINED CYCLE JOURNAL, Second Quarter 2008 143

phase steam and water) are suscep-tible. Mechanism is well understood:Wall thinning caused by dissolutionof magnetite.

Dooley outlined the parametersinfluencing FAC, specifically:n Water chemistry: oxidizing/

reducing potential, pH, andtemperature.

n Hydrodynamics: flow/turbu-lence and mass transport,component geometry andupstream influences, andsteam quality.

n Pipe/vessel material com-position: chromium, copper,molybdenum.He then dissected each of

these parameters for attend-

ees, showing exactly how theyimpacted FAC and what strat-egies were most practical/costeffective for mitigating theireffects. Dooleys remarks aresummarized below:n Single-phase FAC can be

controlled by feedwaterand evaporator chemistry.Requirements: Positive ORP(oxidizing/reducing potential)and no reducing agent.

n Some two-phase FAC can beaddressed by materials solu-tions (use of steel containing

1.25% chromium); some by

focusing on LP evaporator chemis-try (increasing ammonia or addingtrisodium phosphate or sodiumhydroxide if allowed).

n Monitor iron levels and maintainthem at less than 2 ppb in feedwa-

ter and less than 5ppb in drums.

W r a p p i n g

up, Dooley stressed that you haveto be proactive to avoid FAC dam-age. Address the root cause if it hasoccurred. Maintain tight control ofchemistry, perform regular inspec-tions, and repair/replace damagedcomponents as soon as practicable.

Wa mbeke began with a dia-gram of the most common FAC risk

locations on a simplified boilersketch (figure). He said that anexperienced inspector relieson his or her eyes, ultrasonictesting (UT), and sludge evalu-ation for an accurate assess-ment of conditions.

For example, shiny black,flat black, and polished areas

often are associated with

FAC. Some reddish ormagenta color is a goodsign because it indicatessome oxygen, which isgood for FAC protection.Orange-peel texture isindicative of single-phaseFAC, tiger stripes two-phase. Variables impact-ing sludge evaluationinclude location, quan-tity, magnetic properties,color, and particle size.Note that the absenceof sludge doesnt mean

theres no problem.

Druminternals Steam drum

Downcomer

Economizer

Feeder manifold

Feeder

Tubes

Upperheaders

Risers

Lowerheaders

Approach FAC inspection by first ranking areas of impor-tance based on risk. Next UT suspect areas, starting with

the highest risk first. Borescope for telltale patterns

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144 COMBINED CYCLE JOURNAL, Second Quarter 2008

Wambeke addressed two com-mon misconceptions about FAC oftenheard in plants:n FAC doesnt occur in straight

tubes and piping. Typically thatstrue but it does happen and moreoften than you might want tobelieve.

n Solving the problem in one areadoes not automatically move FACdamage elsewhere. FAC does nothave to happen; it can be stopped,or at least greatly minimized.Bisbee set the stage for an in-

depth review of NDE technologiesfor FAC assessment by stating thecase for regular inspections. Avoid-ing unscheduled outages caused bycomponent failures was at the top ofthe list. Other benefits include com-

ponent life extension and avoidanceof unnecessary repair/replacementwork because of early identificationand correction of issues.

Important to accurate componentcondition assessment is a plan, likethe one Bisbee proposed to seminarattendees:n Review environment, operating

conditions, materials, service his-tory, and industry experience ofplant components.

n Identify critical components, pos-sible damage mechanisms, andsusceptible locations.

n

Select the appropriate inspection

techniques for the detection andcharacterization of subject dam-age.

n Perform a baseline inspection toquantitatively establish compo-nent condition.

n Assess component serviceabilitybased on inspection results.

n Establish an inspection intervalfor re-examination to monitor pro-gression of damage or degradation;or, determine repair/replacementrequirements based on componentserviceability predictions.There are several available meth-

ods for inspecting/examining anygiven component. Each has a definedrange of capabilities, limitations, costof implementation, support require-ments, and reliability. Common

sense tells you the selection processmust match the most appropriateinspection method and scope to thespecific component and inspectionobjective in the most time-efficientand cost-effective way.

A multi-disciplinary approachprobably will be necessary to accom-modate the variety of components,operating environments, and damagemechanisms encountered. This mayinvolve use of two, three, or possiblyall of the following:n Nondestructive testing (NDT),

to determine the current state of

damagespecifically, to detect

and characterize flaws, defects,and degradation.

n Metallurgical examination, toevaluate material condition andidentify damage mechanisms.

n Engineering analysis, to predictfuture damage accumulation andserviceable life.

n Online monitoring, to improveprediction capabilities and iden-tify operating modes conducive todamage.FAC is characterized by a loss in

wall or component thickness. NDEmethods that apply for detecting metalloss, and how much, include these:n Ultrasonic testing (UT), including

simple, B-scan, C-scan, phasedarray, and guided wave.

n Electromagnetic acoustic trans-

ducers (EMAT).n Pulsed eddy-current testing.n Digital radiography.

Bisbee reviewed how UT worksand how its used to measure wallthickness. Most plant personnel arefamiliar with this. UT typically isaccurate to within 1 mil and is appli-cable for a large thickness range.But its generally impractical if largeareas must be inspected and it hasa low probability of detecting smallsurface-thickness variations, such asthose caused by pitting.

UT technology keeps improv-

ing. Scanners combined with digital

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COMBINED CYCLE JOURNAL, Second Quarter 2008 145

data acquisition systems now rapidlymeasure and store large volumes ofinspection data. New software per-mits analysis and presentation ofthickness data in convenient formats.Plus, material wastage rates can bedetermined by crunching data fromsuccessive inspections.

Here are some notes on types ofUT that you may not be familiarwith:n B-scan is conventional UT com-

bined with an encoded scannerhaving single-axis scanning capa-bility. Scanner can be manual orautomated and plots of wall thick-ness and location are provided.Cross-sectional view of the testarea is in two dimensions.

n C-scan goes a step further. It has

multi-axis (x-y) scanning capabil-ity and creates a 3-D cross-section-al view of the test location.

n Phased-array UT, which hasgained favor in gas-turbine inspec-tion, finds defects that convention-al UT would miss and providestop-quality characterization ofthose defects.Advantages of B- and C-scan and

phased-array for wall-thickness test-ing are their ability to scan continu-ously and quickly, provide an actualthickness measurement, producean image of a pipe-surface contour,

and serve as a good follow-up tool for

screening techniques. Drawbacks:Difficult to scan over corroded sur-faces, require access to the pipe sur-face, and not practical for screeninglarge areas.

Guided-wave, or G-scan, can dothings the other technologies cant,but the systems are expensive. Forexample, it can screen insulated pipefor wall-loss degradation as well aspipe thats buried or running throughpenetrations. G-scan is capable ofexamining up to 300 ft of pipe fromone location, making it especiallycost effective in difficult-to-accesslocations.

It works off a ring of transducersthat are wrapped around the pipe.No couplant is required and gener-ally no surface preparation. The ring

for pipes 2 to 8 in. diam is solid andcan be mounted in less than a min-ute provided theres a 3-in. clearancearound the pipe. For pipes up to 36in., the ring is inflatable and requiresa clearance of 2 inches. Guided wavesare sent both up and down the pipeand the resultant reflections areanalyzed; the entire pipe cross sec-tion is inspected. Results are semi-quantitative. G-scan is considered anexcellent screening tool.

Keep in mind that the moresophisticated the inspection tools, thehigher their cost, the more training

operators need, and the more support

functions they require. To illustrate:The sophisticated analysis and pat-tern recognition necessary to inter-pret G-scan results require years ofexperience.

EMAT, an alternative to UT,works by producing and detectingacoustic waves in metal using electro-magnetic fields. It covers less distancethan G-scan and generally requiresaccess to the pipe surface. However,it is insensitive to surface conditionsand can operate at high temperaturesand inspect at high speed.

Pulsed eddy-current technol-ogy relies on a pulsed magnetic fieldto produce eddy currents affected bymaterial thickness. It is suitable forgeneral wall-loss determinations, notlocalized damage, because the thick-

ness determined is an average overthe probes footprint. PETC requiresno coupling or surface preparation, canwork through insulation, and is capa-ble of high-temperature online testing.

Digital radiography (RT) is oftwo types:n Digital computed RT, which uses

a photostimulable image plateinstead of RT film. It works thisway: RT exposes the image plate,which is scanned. The latent imageis digitized and recorded; softwareprocessing allows image analysis.

n Digital direct RT instantly dis-

plays the image on a screen, there-

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146 COMBINED CYCLE JOURNAL, Second Quarter 2008

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COMBINED CYCLE JOURNAL, Second Quarter 2008 147

by eliminating subsequent scan-ning. Software processing allowsimage analysis as it does for digi-tal computed RT.Radiation is less than for conven-

tional RT but still requires evacua-tion of test locations. Computed RTprocessing takes almost as long aschemical processing of conventionalradiographs. Imaging is two-dimen-sional; however, by adjusting imagebrightness, a range of thicknessescan be examined in one shot.

FormalpresentationsThe first presentation, on Tuesdaymorning, was Operational Flexibili-ty Enhancements of Combined-Cycle

Power Plants, by Dr Stefan Murzaof Siemens Energy. Murzas mainpoint was that operational flexibil-ity is a key success factor for plantsdesigned for base-load service nowbeing operated in the intermediate/cycling mode because of changingmarket conditions.

He said the as-designed startup-time for a combined cycle installed inthe late 1990s can be reduced by morethan half with enhancements such asa fast-start stress controller for thesteam turbine, efficient piping war-mup system, auxiliary boiler for seal-

ing steam, condensate polisher, etc.

Flexibility improvements to theHRSG are critical, Murza continued,to enable fast starts. For exampleheat losses must be prevented withenhancements such as automateddrains and vents, stack damper,standby heating; stress limits reas-sessed based on future operatingscenarios; and a fatigue monitoringsystem installed to manage creepand low-cycle fatigue over the unitslifetime.

He then discussed how the OEMconducted a site assessment, theresults of which are used to developand implement a custom upgradepackage. After listening to the pre-sentation, it was difficult to imaginethat a US owner would hire a turbineOEM with no boiler manufacturing

experience to perform all the tasksproposed. Siemens certainly wouldbe involved in developing and imple-menting startup optimization pro-grams for the gas and steam turbine/generators, but users questionedafter the session did not associate thename with HRSGs, water treatment,and some other areas mentioned.

Interestingly, a paper on integrat-ed technologies that enhance power-plant operating flexibility, writtenby US-based Siemens personnel forthe 2007 Power-Gen Internationalmeeting in New Orleans, focused on

the companys key expertise areas in

rotating equipment and plant controlsystems.

Field hardness testing. AndyAllen of Progress Energy made theformal presentation Tuesday after-noon on Field Hardness Testing ofP91 Materials, based on work con-ducted by him, Neal Holden, andother colleagues. Allen said his com-pany had been struggling with con-flicting portable hardness test resultson P91 for a few years.

The dilemma, he continued, isthat inspection costs are a very sig-nificant line item in O&M budgets soit was important to identify and usethe portable field method that couldbe verified as accurate in the 190-245HB Brinnel hardness range based oncomparative tests run on laboratory

devices. Another consideration: Sur-face preparation is costly; how muchis really necessary?

The objective was to develop afield testing procedure for determin-ing representative hardness mea-surements on P91 piping. Four sur-face-preparation methods and fiveportable test methods were evalu-ated. The field test method developedhad these key elements:n Light disk grind (5 mils or less),

flapper wheel 120 grit.n If initial result is in the acceptable

range, accept it.n

If high, reject as too hard.

The pneumaticallypowered ESCO MillhogTube Fin Removal Toolremoves fins from the tubeO.D. quickly and easily.The standard tool removes4 in. of fin in less than twominutes and bevels the tubeend all in the same step.Other features of the toolinclude: no reaction torque

to operator, easy to use,helps reduce outage time.

Esco Tool, 50 Park St., Medfield, MA 02052Tel: 800-343-6926, 508-359-4311Fax: 508-359-4145E-mail: millhog@escotool.comPlease visit our website at www.escotool.com

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148 COMBINED CYCLE JOURNAL, Second Quarter 2008

n If low, disk grind 20 mils using aflapper wheel 120 grit.

n If still low, examine further (rep-lica, core, or boat sample).Allen said that the Telebrineller

hardness tester (Teleweld Inc) withthe HiLight scope manufacturedby NewAge Testing Instruments Incoffered the highest accuracy amongthe instruments tested. It also per-mits a screen inspection with less

surface preparation.The decarburization layer is soft

and produces low readings. Influ-ence of the decarburization layer issignificant with the DynaMIC 20(GE Inspection Technologies Inc)205 tip (UCI method), less so withthe Telebrineller. The MIC20 UCI isoperator-sensitive. Specifically, theprobe must be perpendicular to thepipe and flat for a good reading.

Rebound methods (EQUOtip andMIC 20 Dyna D) exhibit error signifi-cantly to the low side with -in.-wallpipe. They are not recommended for

typical reheater piping.

High-energy piping failures.Wednesday mornings formal pre-sentation, Lessons from YesterdaysHigh-Energy Piping Failures forTodays Combined-Cycle Projects,was made by Dr Martin Prager ofThe Materials Properties Council Inc(MPC) and Dennis Sullivan of Ame-ren Corp.

They reviewed experience withwelded piping operating in the creep

rangein particular, the causes offailures at electric utilities over thelast 25 years. Several of the morethan 50 slides were scary photo-graphs of large steam pipes thathad split open along longitudinalwelds. Primary reasons for the fail-ures boiled down to a lack of sophis-tication in engineering calculationsand welding technology.

Prager and Sullivans purpose wasto alert attendees that the new alloysfor HRSGs pose even greater risks tothe industry, and what they believedhad to be done to protect against in-

service failures.

MPCs position on P91/T91, whichhas become the preferred materi-al high-temperature sections of theHRSG and main-steam piping system,is as follows: The vanadium-modi-fied 9% chromium steel is a complexalloy requiring great care and controlof composition, heat treatment, and

details of processing to achieve its opti-mized, high-strength (although poten-tially unstable) properties.

The tolerance bands on composi-tion and heat treatment are narrowand the alloy can be ruined. It is farless tolerant of errors, or lack of con-trol of processing details, than thealloys traditionally used in utilitypowerplants. In addition, the 91 alloyis part of a class of materials highlysusceptible to Type IV failures in theweld heat-affected zone (HAZ).

This is an insidious failure modeattributed to local differences of

material within the HAZ where theoptimum microstructures for creepresistance cannot be retained afterwelding.

Prager and Sullivan compiledthree slides worth of bullet pointson why problems occur with chrome-moly steelsa valuable checklistfor anyone involved in the specifica-tion, fabrication oversight, or repair/replacement of these materials. Forthis report, the summary of thatmaterial will suffice:n Highly sensitive, poorly under-

stood alloys requiring stringentcontrols are, when entrusted toinexperienced fabricators, a recipefor big headaches.

n The microstructure, difficult to seebecause of its fineness, is destabi-lized by strain, time, and tempera-ture.The bottom line:

n Leave nothing to chance.n Control and qualify all aspects of

materials, manufacturing, pro-curement, and erection.

n Inspections are necessary, butR&D on methods is needed.Piping Failures Downstream

of Attemperators, presented by

Steve Freitas and Daniel Watson ofCCI-Control Components Inc, wasthe final formal presentation on theprogram. A large portion of what wasdiscussed can be found in these fourreferences available at www.com-binedcyclejournal.com/archives.html:Click 1Q/2005, click Attemperators.. . on the cover of that issue; click2Q/2005, click Cycling, P91/T91. . .on the cover and scroll to the sidebaron Attemperator frustrations. . .;click 3Q/2006, click Monitoring andmaintaining. . .; click 3Q/2007, clickKey elements of successful PM pro-

grams. . . .ccj