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2 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

T

he HRSG Users Group, wellinto its second decade of ser-vice to the industry, contin-

ues to deliver fresh, vitalinformation on the design, opera-tion, and maintenance of HRSGsand their associated steam systems.That certainly was evident to thenearly 350 attendees who participat-ed in the organizations 15th Annu-al Conference & Exhibition, heldin Kansas City (Mo), March 26-28.

The HRSG Users Group hostsone of the largest and most dynamicmeetings for owners and operatorsof gas-turbine-based combined-cycleand cogeneration plants. Several fac-tors account for this success, begin-ning with the meetings format: Aseries of Open Discussions dominatethe agenda, enabling users to fullyexplore the specific issues theyrecurrently facing at their plants. Theissues may address any aspect ofthe HRSG, as well as the entire,integrated steam cycle the HRSGservesfrom the makeup-water pre-treatment system to the last-stagesteam-turbine blading.

A second factor is the type andnumber of people actively contributingto the meeting. In contrast to manyuser forums, all HRSG Users Groupsessions are open to all participants

including manufacturers, EPC con-tractors, water-treatment suppliers,engineering consultants, insurancecarriers, and so on. This infuses theroom with the wide range of techni-cal knowledge and robust diversity ofopinion needed to tackle the most vex-ing of technical problems.

Increasingly, the participants arediversified in geography, as well,reports Executive Director RobSwanekamp. Registered membersof the HRSG Users Group are nowlocated in over 50 countries, furtherexpanding the talent and resources

the organization brings to bear.

Perhaps the most important fac-tor in the meetings successandthe most difficult to emulateis thelengthy experience and considerablemoderating skills of Chairman BobAnderson. A 30-yr industry veteran,Anderson has chaired this confer-ence for most of its existence, eachyear further honing his ability tokeep the discussions on-point, toput esoteric details into perspective,and to extract key information fromsometimes-bashful attendees. Ander-son says with pride, Dont attendthis meeting if you want to sit quietlyin the back row and avoid having tolearn anything. We work hard tomake this a participatory event, forusers and suppliers alike.

Heat transferAl th ough the Op en Di sc us si ons

are wide-ranging in content, theyare tackled in an orderly manner.Swanekamp presorts the scores ofquestions submitted by users intonine technical categories, which getaddressed sequentially as the meet-ing progresses. Damage to super-heaters, reheaters, evaporators, andeconomizers tend to dominate HRSGuser concerns, so the rightful leadoffcategory for the Open Discussions isheat-transfer equipment.

This year, a plant manager fromGeorgia displayed photos of fin-tubecorrosion occurring in the latter stag-

es of his HRSGs, and opened up an

hour-long discussion when he askedabout the potential causes and curesof this so-called cold-end corrosion.

Consensus was that it occurs acrossall OEM lines, that it can affect base-load as well as cycling plants, andthat several different mechanismscan cause it.

Sulfuric acid is one potential cul-prit, explained an attendee. Plantsfiring liquid fueleven oncearemore likely to experience this prob-lem, but gas-only facilities can be vul-nerable, too, because normal sulfurlevels in some pipeline natural gasruns as high as 15 ppmand high-er during upsets. A combined-cycleplant in western Washington, search-ing for the cause of its cold-end cor-rosion, found that the sulfur contentin its suppliers pipeline averagedbetween 20 and 30 ppm, and peakedat nearly 60 ppm last year.

Another potent ial culprit is animproper cold-layup procedure,said a former OEM engineer turnedHRSG consultant. Making an anal-ogy to the condensation he gets on hiscold garage floor when warm, moistair hits it in the springtime, the con-sultant explained that moisture inthe ambient air will condense on thetubes of an HRSG in cold layup.

HRSGs tend to sit idle more fre-

quently during the fall and springtraditionally the wet seasonsthanthey do in winter and summer,increasing the potential for this phe-nomenon. Rainwater and humidityentering through an open stack orleaking roof-casing pipe penetrationsexacerbate the problem.

But even units that are staying hotduring daily cycling can experiencecold-end corrosion, Chairman Ander-son pointed out, so users need to lookat additional factors. For example, hesaid that HRSGs equipped with selec-tive catalytic reduction (SCR) systems

will often get ammonia salt deposits

Discussion forums, formal

presentations, exhibition provideguidance needed to improve plantperformance, availability

AndersonSwanekamp

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6 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

num steel (9Cr-1Mo) for critical sec-tions of the HRSG and steam plant.Referred to as Grade 91or morecommonly P91 for piping and T91for tubingthis high-temperature,high-strength alloy can dramaticallyreduce thermal stresses in superheat-ers, reheaters, and main-steam pip-ing, and thereby improve service life.

However, its superior mechanicalproperties require that a very specificmicrostructure be obtained duringoriginal steel production, and main-tained throughout its service life. Anyaction that alters the microstructureof the alloysuch as the hot bending,forging, or welding that regularlyoccurs during component fabrica-tion, plant construction, and outagerepairscan seriously degrade thealloys properties (Fig 2).

As a result, quality-control stepstypically include precise temperaturerecords of all heat-treatment process-es like PWHT. One convenient wayto verify the microstructure of Grade

91 is with hardness testing, hencethe users concern about his differenthardness readings.

Presenting at the 2007 conferencewas one of the leading authoritieson this particular topica metal-lurgist who chairs the ASME TaskGroup currently revising how theASME Code addresses Grade 91 andother creep-strength-enhanced fer-ritic steels. This was no coincidence,since the HRSG Users Group meet-ing tends to draw leading authoritiesfrom throughout the industry.

The metallurgist was one of sever-

al participants who responded to the

users question, explaining first of allthat its not uncommon after PWHTon Grade 91 for the weld metal to beslightly harder than the base metal.For example, if the base-metal hard-ness is around 200 or 210, the weld-metal hardness generally will comein around 230, he said. If this is the

magnitude of difference the user wasseeing, then he shouldnt be too con-cerned with the readings, the metal-lurgist advised.

However, he was troubled by thestatement that the PWHT graphslooked nearly perfect. Generally,when the PWHT chart looks thatgood, the metallurgist warned, thenyou need to start asking questions.Because there are some people outthere who are much more skilled atmaking good charts than they areat making good heat treatments.Though his comment generated some

laughs, users understood his seriouspoint about possibly fraudulent docu-mentation.

Much discussion ensued about thetypes of failures users are experienc-ing in P91 and T91 components, andthe quality-control methods neededto properly handle the alloy. Weveseen a number of T91 failures in ourlaboratory, reported one participant,and overheating is one of the causes.He explained that duct burners cancause a transient spike in temper-ature that exceeds the materialscritical temperature. This causesthe microstructure to change, whichweakens the material and sets it upfor probable failure.

Another problem reported is thespecification of Grade 91 in low-tem-perature areas of the HRSG. Appar-ently, some owners believe the oldcliche that if some is good, more isbetter. However, components madeof T91 need to be exposed to tempera-tures approaching 1000F, in order forthe residual stresses from fabricationto relax. If used, for example, in theevaporator and economizer sections,T91 tubing wont be exposed to thesetemperatures and always will be

subjected to higher-than-expectedresidual stresses.

Another failure mechanism dis-cussed in detail was dissimilar-metalweldsthe joining of Grade 91 toeither Grade 11, Grade 22, or auste-nitic stainless steels. One US plantowner was forced to repair the weldsbetween the main-steam pipingmade of P91 and the steam-turbinestop valve made of 1.25Cr low-alloysteelnot on one, but on three ofits 500-MW F-class combined-cycleplants. The oldest plant, which hadless than 5000 service hours, found

a through-wall crack 135 deg around

the weld, and 24.5 in. long, while thesteam line was in service.

Warned by this problem, theowner conducted liquid-penetranttesting on a second plant and founda crack that was 20 deg around thejoint but fortunately had not yet pen-etrated the wall. The youngest of the

three plants could not detect a crackwith liquid-penetrant testing, butreplaced the weld anyway becauseits joint design was the same as theother two plants.

Attendees were advised that whenjoining Grade 91 to other metals,they need to (a) pay strict attentionto welding and PWHT procedures; (b)use cold-spring in the piping to makeup for the different relaxation ratesof the different metals, and (c) care-fully design the transition geometry.

Whenever there are dissimilar-metal joints, the ASME Task Group

chairman explained, a weak zone isinevitably present, as a result of thedecarburization that occurs at theinterface between different chromiumlevels. Therefore, the transition geom-etry must place that weak zone in thelowest-stress region of the joint.

Experience with P91/T91 use inHRSGs and high-temperature HPsteam systems was addressed atthe CTOTF Spring Turbine Foruma few weeks after the HRSG UsersGroup meeting. Presentations bya well-known industry consultantand two users are summarized inthe CTOTF report elsewhere in thisissue (see sections on the Generic andGE Roundtables).

Controls

For several years, HRSG usershave reported difficulty in control-ling superheater and reheater out-let temperature while the GT is atlow loads. This is especially trouble-some in HRSGs operating behind GEEnergys 7FA gas turbines. In Kan-sas City, the topic received in-depthattention.

Once again, some brief background

is in order: When a 7FA is changingload in the range between approxi-mately 40 and 100 MW, its exhausttemperature increases rapidly fromabout 1050F to the so called the iso-thermal limit, just above 1200F.This exhaust-temperature spike isnecessary to maintain flame stabilityin the combustor.

Unfortunately, exhaust tempera-ture peaks at a time when steamflows in the HRSG are low. Thismakes it very difficult for the super-heater and reheater attemperators tomaintain steam outlet temperatures

within design limits, while at the

2. The microstructure of P91 com-ponents can be damaged by improperhot bending, forging, or welding,

thus degrading the alloys mechani-cal properties. Catastrophic materialfailuresuch as this pipe rupturecan result

GE

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8 COMBINED CYCLE JOURNAL, Second Quarter 2007

HRSG USERS GROUP

same time avoiding desuperheateroverspray. As a result, a mixture ofsaturated steam and water dropletscan enter the downstream super-heater or reheater coils, potentiallycausing severe fatigue damage.

Aware of the problem, GE devel-oped a control-system modificationcalled OpFlex, which some usersare beginning to install. An earlyadopter asked other users at the con-ference about their experience withthe modification, which touched off along discussion on both OpFlex soft-ware and controls in general.

As further evidence of the qualityof attendees at the meeting, the prod-uct manager for OpFlex was in theaudience. He explained how userscan get the most out of the offering,including the use of a turndownoption that further assists in low-load operation.

Ductwork, dampers,

stacksA question in the next categoryStructuresunderscored the groupsincreasing diversity in geography,and the value in assembling an inter-national audience. An OEM engi-neer from the Netherlands inquiredabout acid-dewpoint corrosionnotof the tubing, as discussed earlierin the meetingbut of the HRSGcasing. Most HRSGs, particularlythe horizontal-gas-path type mostpopular in North America, have acold casing design. This engineerwho has more experience with the

vertical-gas-path type popular in

Europewondered why chemicaltheory predicts sulfuric-acid dew-point corrosion will occur in cold-casing HRSGs, but in real life he hadnot seen such corrosion.

An HRSG specialist from the USpromptly raised his hand. Yes, thereis experience with this type of corro-sion damage, he reported. Ive seenit personally, and I know of at leastone other site that experienced it.Several other reports added to hisassertion that, yes, sulfuric-acid dew-point corrosion of a cold casing actu-ally has occurred.

An HRSG OEM engineer raisedthe if some is good, more is notnecessarily better argument as itrelates to this issue. Apparently,installing too much thermal insula-tion in a cold-casing system can causethe outer casing to operate below theacid dewpoint, resulting in severecasing corrosion.

After much discussion a consensusseemed to emerge that so long as the

casing is kept relatively dry, thenthe problem will be relatively rare.The real bad actor, one attendeeexplained, is the free moisture, ora stream of moisture from a leak,because that allows the SO2 to gointo the water, and then you get bad,acidic reactions. So if you keep thecasing dry, you should be in prettygood shape.

Valves

Turbine bypass valves are critical tothe operation of an advanced com-

bined-cycle plant. Unfortunately,

improper selection and sizing of thesevalves has been a common problem inthe industry, resulting in unscheduledunit trips and, in some cases, cata-strophic piping damage. Users reportthat turbine bypass valves have beenreplacedan expensive and time-consuming taskin as little as two

years of service. Other common designproblems with turbine bypass valvesinclude inappropriate logic control,unreliable valve mechanics, andaswas discussed in Kansas Cityweldfailure on the downstream (letdown)side of the valves.

A user in Florida who was expe-riencing such weld failures quicklyfound colleagues with the same trou-ble. A lengthy discussion of the prob-lem suggested that the underlyingcauses are not yet fully understood,and that nobody has a panacea. Asa result, the HRSG Users Group

Steering Committee identified Main-taining and repairing severe-servicevalves as a topic to be explored at itsSteam-Plant Workshop scheduled forlater this year (Sidebar).

Rounding out the nine categoriesof Open Discussion were supplemen-tal firing, environmental systems,and balance-of-plant. In addition,there were four formal presentationsat the 2007 conference:n Latest industry experience with

P91/T91, Jeff Henry, associate,Structural Integrity Associates.

n Evaluation of gas-side tube clean-ing options, Maggie Lelak, chemi-cal engineer, Duke Energy.

n Pros and cons of heavy duct-burnerfiring, William Byrd, operationsmanager, Entegra Power GroupLLC, Gila River Station.

n Condenser performance/trouble-shooting, William Lutz, performanceengineer, GE Energy.

Integrated exhibition

A companion exhibition, supportedby 84 suppliers of combined-cycleequipment and services, was inte-grated with the conference sessions.

Exhibitors were not restricted to theexpo hall, but rather encouraged toparticipate in all of the technical ses-sions. This not only keeps suppliersattuned to user concerns, but affordsthem the opportunity to get involvedin technical questions during theconference, then invite interestedrespondents to meet them at theirbooth, over coffee, or during one ofthe meal functions for follow-up dis-cussions.

The 2008 Conference & Exhibitionwill be held April 7-9, in Austin. Visitwww.HRSGusers.org for details as

they become available.ccj

Steam-plant workshop co-locates with PowerGen

In addition to its spring conferenceand trade show, the HRSG UsersGroup conducts a Steam-PlantWorkshop in the fall. The annualmeeting is characterized by a free-ranging discussion open to the

broad array of issues that usersrequest; the workshops provide in-depth seminars focused only on oneor two topics selected by the steer-ing committee.

This years workshop will beco-located with PowerGen Interna-tional in New Orleans, December11-13. This enables registrants alsoto participate in the largest power-generation expo in the world.

Theme for Day One of the 2007workshop is Evolution of steam-plant design. Presentations includethe following:

n Developing specifications foryour next project.

n Scaling up the once-through

steam generator.n Plant designs for a water-con-

strained world.n New ASME, NFPA, and European

design codes.n Using P91/T91 and other

advanced alloys.Theme for Day Two is Mid-life

maintenance for combined-cycle/cogen plants, including:n HRSG retubing.n Chemical cleaning of HRSGs.n Servicing transformers, switch-

gear, and generators.n Maintaining and repairing severe-

service valves.n Cooling-tower overhauls.

Workshop registration includesfull access to the PowerGen Expo;group discounts are available to

users via the Energy Provider Pro-gram. Visit www.HRSGusers.org, orcall 918-831-9160.

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