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CrossingtheretirementfinishlinewithhightemperaturesteamturbinerotorsByStephenR.Reid,PE,andRachelSweigart,TGAdvisersInc.|Posted:Wednesday,May27,20155:23pm

Background

Newenvironmentalregulationsandtheinstallationofrenewableenergysourceshaveresultedinmanyretirementsofolderlegacysteamturbinegeneratingunits.Mostoftheseolderunitshavespentthemajorityoftheirlifeinoperationbaseloaded.Asaresult,theyhaveloggedsignificantoperatinghourswithfewon/offcycles.Longterm,temperaturedependentdamagemechanismsoftheseturbinerotorsmustbeconsideredtoensuresuccessfuloperationtothedesiredretirementdates.Thisarticlehighlightstwocasestudiesandsomeoperationalstrategiestoextendlifetothedesiredretirementdate.

Considerhightemperature,lifelimitingmechanisms

HPandIProtorswithmaterialpropertiessimilartoASTMA470typicallyoperatewithsteaminlettemperaturesof1,000F.Withtime,creepcrackscaninitiateinhighlystressedareas.Inaddition,thematerialcansuffersignificantembrittlementwithtime,reducingthematerialductilitythatisanimportantpropertyforremaininglifeanalysis.Thesemechanismsareatplayintherotorboreandbladeattachmentsections.

Creep

Creepisatime,temperatureandstressdependentmechanismthatmustbeconsideredforunitswithasignificantnumberofservicehours.Forcreepcracking,rotoroperatingtemperaturesingeneralmustbeabove800Fandasignificantstresslevelmustbepresentatalltimestobeconcerning.Creeplifetargetsof30yearsarefrequentlyquotedbyOEMsforhightemperaturerotordesigns.DesignlifeistypicallydeterminedfromaLarsonMillercurve,whichrelatestime,temperatureandstressforcrackinitiation(seeFigure1).TheLMPapproachdoesnotaccountforcrackpropagationand/ortheinteractionofcreepandfatiguefromonandoffcycling.Theindustryhasutilizedafracturemechanicsapproachtoassesslifeoncecrackshaveinitiated,and/ortoassesspreexistingcracksinhightemperaturezonesofsteamtemperaturerotors.

Figure1.LarsonMillerParameterCurve

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AsnotedinFigure1,therecanbeasignificantscatterofcreepmaterialdata.TheOEMswilladdadditionalsafetymarginsanddefineaDesignCurve,whichisalwayslowerthanthecurveproducedfromtheminimumtestpoints.Asaresult,unitsusuallyfarexceedtheir30yearcreepdesigntarget.Rotorlifeisoftenlimitedbypreexistingflawsthataredevelopedfromtheforgingprocess.Thisisespeciallytrueofrotorsproducedpriortoforgings,whichweremanufacturedusingvacuumdegasingtechnology(i.e.airmeltprocess).

Embrittlement

Dr.SwamiSwaminathanofTurboMetInternational,whileundercontractbyElectricPowerResearchInstitute(EPRIproject24815)performedtestingonaretiredrotortosupportneededmaterialsdataforlifeassessmentsofserviceexposedrotors.Testingprovidedmanyconcerningchangesinmaterialpropertieswithtimeandtemperature.Amainareaofconcernwasthelargedegreeofrotorembrittlementduringtherelativelyshortoperatingperiodoflessthan200Khours.ThedegreeofembrittlementwasquantifiedbyashiftFractureAppearanceTransitionTemperature(FATT).TheFATTdefinesthetemperatureatwhichtherotorsteelexhibits50percentbrittleandductilebehavior.Theresultingcriticalcracksizesthatdefinetheburstconditionforarotoraredirectlyrelatedtothematerialsductility.Thisparameterisimportantsinceithelpstodefinetherequiredrotorsoakperiodtoensurethematerialisductilewhenitisexposedtohighstressesduringcoldstarts.

Swaminathancompletedtestingindifferentsteadystateoperatingtemperaturezonesattheboreandperipherallocations.TheworkprovedlongtermexposuretotemperaturecouldincreasetheFATTbyapproximately200F.Anotherinterestingfactwasthatembrittlementdidnotoccurinthehighestoperatingtemperaturezone.Embrittlementwasmorepronouncedinthezonewheretemperaturewasinthe800Frange.

Asaresultoftheseefforts,preexistingflawsinrotorshaverequiredamoreconservativematerialpropertyboundaryconditiontoassessrisksofbrittlefracture.Inmanycases,embrittlementhaslimitedthenumberofstopsandstartsand/orhasresultedinprematureretirementofagingrotors.

Notablecasestudies

IProtorembrittlement

AsaresultofarecentfailureofanIPstage2dovetail,TGAdvisershadanopportunitytoevaluatetheembrittlementconditiononarotorthatloggedmorethan425Kofoperatinghours.TheresultingshiftinFATTwassignificantlygreaterthanexpected.TheFATTforthestageofconcernhadincreasedtomorethan600Fwhichwasapproximately300Fhigherthananonembrittledrotorsteelofthesamechemistry.

Withretirementplannedinthenearfuture,theutilitydecidedtoremovethestageandevaluate

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methodstoensurereliableoperationuntiltheunitsretirementdate.Duringtheremainingoperatingwindow,theunitwasexpectedtohavejustafewcoldstarts,whicharethemostconcerningforanembrittledrotor.AdecisionwasmadebytheevaluatingteamtoassessmodificationsoftheexistingrotorcoldstartproceduretoaccountfortheFATTshift.Multiplestartupsimulationswererunvaryingspeed,holdtimesandrampratestoensurehighstressesoccurredonlyaftertherotorachievedaductilestate.Theevaluationalsoincludedanassessmentofdifferentialexpansion,whichwassignificantlychangedwithlongersoakperiods.

Theresultsoftheanalysisprovedtheembrittledrotorscouldachieveductilepropertiesbyextendingsoakperiodsatlowerspeedswithoutdetrimentallyaffectingdifferentialexpansion.Theplotbelowshowsapotentialimprovementof400percentincriticalcracksizeforamodifiedcoldstart.Adaptingtherevisedstartupprocedureshasallowedthisutilitytostayontrackwithitsretirementplan.Followupanalysisoftheactualrevisedstartuppracticehasshownaclosematchtotherecommendedcurves.

Controlstagefailure

AnothernotablefailurewasexperiencedonaninletstageofanHPturbinethathadaccumulatedmorethan300Khoursofoperation(seeFigure2).

Thefailureprovidedanopportunityfortheutilitytotestlargersectionsofthecontrolstagedisctodetermineifaweldrepairwasfeasible.Thetestprogramincluded:RotorBoresonicTests(MT,UT,visual)MaterialTests(actualrotormaterial)TensileTests(detectcreepsoftening)ImpactTests(estimatefracturetoughness)AcceleratedCreepRuptureTest

Testingprovedthefailuretobeattributedtohightemperaturecreepnotchsensitivityinthestressconcentratedareaoftherotordovetails.Followinganassessmentoftherotormaterialpropertiesatvariousradiallocations,acutlinewasestablishedandtherotorwasbuiltup,stressrelieved,remachinedandnewbladesinstalled.Therepairwassuccessfulandtheunitachieveditsdesiredretirementdateofapproximately10yearsaftertherepair.

Acknowledgements

TGAdviserswouldliketothanktheConsumersEnergyteamofMatthewT.Helms,VladimirTrbulinandMarkE.WittbrodtfortheirtechnicalsupportanddirectionwiththeHPandIProtorassessments.TGAalsowouldliketothankDaveSheasley,RogerKarnandMarkMillerofNRGforthetechnicalsupportwiththeHProtorcontrolstageassessmentefforts.

StephenR.Reid,P.E.,ispresidentofTGAdvisersInc.andhasmorethan30yearsofturbineand

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rotatingmachineryexperience.Reidandhisteamprovideturbinetroubleshooting,healthassessmentsandexpertwitnessservicestomajorenergycompaniesintheU.S.andhaveprovidedconditionassessmentevaluationsonmorethan100turbinegeneratorsintheU.S.ReidalsoisashortcourseinstructorforEPRI,ASME,ElectricPowerandPOWERGEN,hasnumerouspatentdisclosuresandawards,andpublishedmorethan20technicalpapersandarticles.Reidwastherecipientofthe1993ASMEGeorgeWestinghouseSilverMedalAwardforhiscontributionstothepowerindustryandispastchairmanoftheASMEPowerGenerationOperationsCommittee.HeisaregisteredprofessionalengineerinthestateofDelaware.Youmaycontacthimbyemailing

.editorial@woodwardbizmedia.com

RachelSweigartjoinedTGAdvisersin2014asaconsultingengineer.Shehasprovidedlifeassessment,analyticalmodelingandtroubleshootingservicesformainturbinegeneratorslocatedthroughoutthecountry.SweigartisamechanicalengineeringgraduatefromLafayetteCollege.Youmaycontactherbyemailing .editorial@woodwardbizmedia.com