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TableIBearingDescription
No. Type Diameter (Inches) Figure
Bearingno.1
DTP:DoubleTiltPadBearing
(PivottedShoeJournalBearing,ButtonType,Pin
Located,4Pads,1PadDown)
fromKingsburyInc.
12.018
Bearingno.2 16.024
Thiscombinedcyclepowerplant isconsistingof2GasTurbineand1SteamTurbineunits.TheSteamTurbine
unitwasfirstsynchronized in1995andhadbeenexecutedwith5minorinspection(MI)and1majoroverhaul(MO)
outages. TheSteam Turbine has been observed the symthom by condition monitoring which can be analyzed the
trend simplyand speedily.The significantconditionmonitoringdatachosenareOverallShaftVibration,Admission
SteamTurbineInletPressure,LPSteamTurbineInletTemperature,LPSteamExtractionTemperature,CondenserHighVacuumRangePressure,andLubeOilInletTemperature.TheOverallShaftVibrationisrecordedatbearinglocations
andthesetpointacceptedbyGEcriteriaofalarmandtripis150 mppand225mpprespectively.
OnMarch15th
,2008,thevibrationtrendpresentedasthefollowingfigurewasrecordedthetripeventwithout
alarmsignalobservedbyoperator.ThiswasshowntheMVARs(GENVARS)decreasedfrom+10to 10when itwas
broughtaGasTurbineunitdown.ThenSteamTurbinewas trippedby thevibration1X=230 mpp (2X=160 mpp)
within20minutesafter.Moreover,thevibrationatBearingno.1(VIB1X,1YBRG#1)wasnotconsistencyobviously.
TheSubsynchronousvibrationcouldnotbecapturedbyOverallVibrationmonitoring,so itwasneededtomeasure
thespectrumvibration.
Figure2VibrationtrendrecordofTripeventonMarch15th
,2008
Pivoted
shoe
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METHODSAfter the tripeventwith230mpp, thevibrationanalyzing teamhad torecord theVibrationSpectrumofminutes
before trip again. During testing, it was found that the Steam Turbine suddenly had vibrated up to the overall
magnitudeof1X=134 mpp ,1Y=139mppat60MW (52% load)whenMVARchangedandabnormaldifferential
expansionappearedasshowninthefigure3.Asshowninfigure4,theSubsynchronousof812Hzcapturedhadvalue
upto10 mppandthenatthemomentoftripevent itsvalue increased immediatelyupto75mpp.Practicallythe
normalSubsynchronoushadvaluednotover45 mppwhenaSteamTurbineisrunning.Therefore,thisproblemwasimpliedthatthepossiblecausescouldbefluidinducedinstability,theresultofBearingwear,Bearingoverclearance,abnormalofBearingAlignment.
Figure3VibrationtrendrecordofTripeventonMarch17
th
,2008
Figure4SubsynchronousvibrationrecordoftestingonMarch17th
,2008
Thehistoryrecordtoldusthatthevibrationhighproblemwasfoundsince2002.ItishighlyvibratedatBearingno.1
during60MW(52%Load)whileanotherGasTurbinestartedupandSteamwasadmitted;however, Itcouldnotbe
foundany severedamageonanybearings.Therefore,wekeptoncorrecting theproblemby lowspeedbalancing,
field balancing, and bearing replacement depended on which obvious evidences found during such outages.
Furthermore, the root cause analysis diagram in the below figure was the main tool to check the possible causes
duringproblemsolving.
Duringtheevent
Beforetheevent
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Figure4RootcauseanalysisdiagramofVibrationhighproblem
Intheinitialcountermeasure,therewere2options,6dayworkingonCompromisedBalancingand8dayforBearing
Inspection, in order to correct the problem. The 6day working including Generator preparation on Compromised
Balancingwasselected.TheSteamTurbineandVibrationteamworkedtogethertoexecutethethreerunbalancing
andanalyze thevibrationvectors, but the vibration stillhighat1x=200 mpp, instabilityofwhichhad resulted by
Subsynchronousasusual.
ThenwehadthemeetingagainandcomeupwithBearingInspectionfor8days.AftertheBearingno.1disassembly,
the wear of pivoted shoe was shown in the figure 5, so the Bearing had to be replaced. As the following steps,
Realignment (TurbineGenerator Coupling) had to be finished, as well as rubbing reducing by increasing oil seal
clearance.
Figure5ThepivotedshoeofBearingno.1Condition
WhentheunitwasstartedupagainonApril12nd
,2008,itwasfoundthesamesignalthatwehadhavenbefore.
However,theSteamTurbinewasabletosupplyhigherload,longerasitcouldrun.Inthemeanwhile,of90MW(78%
Load)loadandtheVibrationatBearingNo.2shown130 mpp,theSteamTurbineteamtriedtoincreaseviscosityof
inletlubeoilinorderthattheBearingmechanismwouldbeincreaseditsstiffness.Thetemperaturewasadjusted
from49oCto46
oC,buttheVibrationwasstillhigh.Thenthetemperaturewasbroughtbacktotheprevious.It
eventuallytrippedatVibration2X=300 mpp(1X=220 mpp)whileMVARwasraised.
Followingourexperiencestoresolvethisdifficulty,itcouldbesummarizedthatSubsynchronouscouldbeselfexcited
from every single source of rotating behaviour such as Load exciting, Load increasing, MVAR increasing, Steam
Admission,Startupmodeswitching,abnormaldifferentialexpansion,etc.TheSteampathwas likelytobeseriously
clogged,soithadnosimpleremedyforseveraldays.Ifwecontinuedthecorrection,wewouldneedmoreinputdata
SteamTurbineHighVibration
AABBNNOORRMMAALL
BBEEAARRIINNGG LLOOAADD
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from theOEM (OriginalEquipmentManufacturer)Engineering,accuratehistoryrecord,besidesour technical team
experiences.
WithGErecommendations, Bearingboreswereadjustedandtherotorpositionwasmovedtoproperposition,butthe
result was that the Steam Turbine was heavily vibrated at 5001,000 rpm. Consequently, the discussing meeting
betweenthemanufacturer,GEanduser,EGAThadtheadditionalissueaboutgenerator2poleshortturnproblem.GE
technicaladvisor(TA)recommendedthatthefirstprioritywasimprovingtheSteamTurbineconditiontobereadyto
testrunforthegeneratorsolvingissue.
Inthesummaryresultofthemeeting,itwasshownthatwecouldnotreducethevibrationwithoutSteamTurbineinternalpartcorrectionanymore.Hencethat45dayMajorOverhaulwasperformedtorehabilitatetheentireoftheSteamTurbinerotor. Infactofany
unforeseen evidences enhancing Subsynchronous vibration, all parts of rotor obstructed the Steam path, working
fluid,hadtobecloselytakencarebyGETAandEGATteam.
Afterupperhalfcasing removal, the formeractivitiesperformedweredividing flange level inspection, frontcasing
weightunbalancingtest(horndroptest),andBearinglevelcomparedwithdesignlevel.Theinspectionwasfoundthat
theLeftsideofdividingflangelevelwaslowerthantherightsideof3mm.,69%offrontcasingweightunbalancing,andBearingno.1 level is lowerthandesignof2.7mils.Notonlywastheevidencefound,thewater inductiontrace
alsoappeared.On June3rd
,2008, the front casingweightunbalancingwasadjusted tobewithin10%as shown in
figure6,causingTurbinecasingLifting.Sothefollowingstepwas InternalAlignment,alldiaphragmstagecentering,
andthelatteractivitiesarerubbingdecreasing,realignment,expandingtheanchorboltholes,Bearinginspectionand
setting. In spite of foundation deformation found, the corrective action could not be done because of time
constraints.Ithascontinuouslymonitoredtoprotecttheproblemintime.
Figure6HydraulicliftingtheFrontcasingintheprocessofHorndroptest
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Figure7WaterInductionDamageandRubbingRepairing
Figure8ExpandingtheanchorboltholesofBaseplate
Figure9BearingInspectionandSetting
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RESULTSExecuting most of remedial action steps, we started up the unit again on July 20
th, 2008. Notwithstanding
Subsynchronousappearingofthe100MW(87%load),theTurbinecouldbearonhigherSubsynchronousmagnitude
of80mppcomparedwith10mppoftheprevioustripevent.Infact,theSteamTurbineconditionwasbetter,butit
furtherneeded fine tunning in thestartupprocessbyCompromisedbalancing tohandleTurbinecarefully.Because
theBearingno.1 isverysensitive, itcanbestimulatednatural frequencyeasily.Thereforesuitableoilviscositywas
adjustedbyinletLubricationoiladjustingtohelpsteadilyrotatablebearings.TheresultwasthattheSubsynchronousmagnitudereducedgradually.Itwasfirstlydecreasedfrom49
oCto46.5
oC,thentheSubsynchronouswasreduced as
shown infigure10.Nextthetemperaturewasreducedto45oC,andtheSubsynchronouswasreduceddramatically.
Finally,theunitcouldbebroughtbacktodispatchingcontrolcentertoThailandnationalelectricgridonAugust1st
,
2008.
Figure10TheSubsynchronous reducingfrominletLubeoiltemperatureAdjustingDuring2010minorinspectionoutage,wehadtoanalysebythepreviousrootcauseanalysisdiagramalongwiththe
correctingactivitiesinoutage.Thelastpossiblecausewhichisabnormalbearingloadwasfoundbyanalysingtheshaft
positionplotofallbearingpositions.ItwascorrectedbyliftingtheBearingno.3supporttodistributetheloadfrom
Bearingno.2tootherbearingsandthentheSubsynchronouswasdisappearedcompletely.
AccordingtotheexperiencesinSteamTurbineproblemanalysisandcorrection,mostlyinternalturbinepartcouldbe
abletorepairinourEGATworkshop,soourteammustconsiderthemosteconomicaloptionwithtimeandmethod
tradeoffdependingonourworkshopresourcesand teamcapabilities. Even though thechecklistof therootcause
analysisfishbonediagramforthiscomplicatedproblemcouldbeapplied, inrealitywecounteredthebarriersduring
countermeasureinsite.Themostimportantfactormustbeconsideredisselectingtherightmethodintherighttime.
Forthisreason,theremedialactionstepofSubsynchronousvibrationtripcouldbesummarizedasfollowings.For Bently Nevada vibration monitoring system, this steam turbine is observed the rotating condition by Overall
vibrationwhichhadthe150 mppand225 mppasalarmandTripsetpointsrespectively.
IftheTripeventishappenedimmediatelyafteranymanuallyorautomaticallyprocessesoccur,measurethespectrum
vibration and observe whether Subsynchronous is shown. If it is appeared, then indicate the magnitude of
Subsynchronousvibration.TheprocessescanbeloadreducingbyshuttingdownoneofGasTurbine,loadincreasingbystartinguponeoftheGas
TurbineandSteamadmission,MVARsraising,loadexciting,startupmodeswitching,etc.
Incase,themagnitudeofSubsynchronousisnotover10 mpptobeartripevent.TheSteamTurbineconditionismost
severe.TheinternalpartsofSteamTurbinehavetobeinspectedandcorrected.Checkthehorndroptestandcorrect
ifneeded,centering,rubbingPartsreducing,Bearingsandpivotedshoesinspection,couplingalignmentetc.
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In case, the magnitude of Subsynchronous is about 80 mpp to bear trip event. It is meant that Steam Turbine
conditionisabnormal.Thereare3optionswithoutupperhalfcasingremovaltochoose.
1. Adjustinginletlubricationoilfor12dayduringrunningCaution: Bearing metal temperature at other location might be high because of viscosity change, so it is
neededcloselymonitoringduringadjustment.
2. Compromisedbalancingfor6day:This isalsodependingon theposition to insertrequiredweights. If it isinsertedatTurbineGeneratorcoupling,thebalancingisonlyneeded2daycorrection.
Caution:ThismethodwillsupporttheTurbinerunningatfull load,but itmightsuddenlyvibratedhighlyat
75%load.Hencetheoperatorhastocarefullycarrytheturbinepassingthroughthisconditionsmoothly.
3. Bearinginspectionfor8dayCaution:Thedisadvantageofthismethodisthatrealignmentduringhighturbinemetaltemperatureisquite
difficult.
Normally, if it does not have trip event, but the Subsynchronous signal sometimes appears during operation. The
Steam Turbine Condition is quite normal. Keep monitoring the data: Bearing Metal Temperature, Lube Oil Inlet
Temperature,Noiseat laststagebladeposition,BackPressure,StatorCoolingTemperature,MWandMVARChange
comparedwithSubsynchronoustrend.RecordSubsynchronous andcomparewiththepreviousandpreparetoinspect
andcorrectpossiblerubbingpartsand BearingsinspectionduringscheduledOverhaul.
CONCLUSIONSIn Conclusion, the Subsynchronous vibration problem analysis needed to consider the entire area of rotating
behaviour.TheEGATteamalsohadtomakedecisionundertimelimitandlesseffortrestriction.Moreover,themost
importantbeforefollowingtheActionStep,theSteamTurbineconditionandaccuratemaintenancerecordhadtobe
considered.TheobviousexampleisthatthefirstactionofCompromisedBalancingandLubeoiltemperatureadjusting
wasnotsucceededbecauseoftheseriousdamagefromwaterinduction.Therefore,itprimarilyneededtocorrectall
possiblerubbinginternalparts.
ACKNOWLEDGMENTAsintheprevioussection,IhavereliedoncommentsandinformationfromanumberofengineerswithwhomIhave
workedandcooperatedonanumberofprojectsoverthepastyears.Theircommentsandobservationshavebeenof
considerable assistance in identifying areas where more detailed information is required and in bringing to my
attention subjects that I may not havebeen, at that time, addressing in sufficientdetail,oreven atall. For these
reasons,theirassistanceandadvicehasbeeninvaluable.
As in this paper, my special thanks go to my colleagues Supaniti Songsawas, Manassavee Thuphom, and Cheewin
Youngngern for allowing me to interrupt his work. Their review and input from their experience have added
considerably toanyvalueotherplantengineersmayfindinthispaper.Theirconceptandthetimetakentoreadand
commentonthetextandthewayithasbeenarrangedhavebeeninvaluable.
Iwouldalso like toacknowledge the input from the followings.Eachhasmade informationavailable tomeorhas
been prepared to discuss certain aspects of units and our joint experiences. Santhiti Chanthauthai, head of
EngineeringSectioninSteamTurbineDepartment,SupawatKladthong,KobchaiWasuthalainan, ApichatPaugsopatai,
Sompong Pantmuk of Steam Turbine Department, Sahathep Joipradit of other departments in Mechanical
Maintenance Division, Pramote Kajornpol of South Bangkok Power Plant, EGAT, Athikom Jumrutsri, GE Technical
Advisor.
Tomakethetextclearer,Ihaveusedseveraloffiguresandtables.Someofthesewerepresentedatthepowerplant
site,conferences,andseminarsIattended.
Specialthanksgotomyofficestaff inSteamTurbineDepartment,fortheirassistance,especiallyWasuthepSinmayo
andPhipobChomboonchoy.Theirhelpinparticularwasinvaluable.
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REFERENCES[1] General Electric Company, Electrical Generating Authority of Thailand: South Bangkok Combined CyclePlant, South Bangkok, Thailand, File No. 62.1001 Steam Turbine Generator 103, Maintenance and SystemDescription,TurbineNo.270T238,GeneratorNo.290T238,Schenectady,NewYork:GE,1992[2] Grissom,B(2002),FundamentalsofRotatingMachineryDiagnostics,Minden,Nevada[3] H.P.Bloch,andM.P.Singh,SteamTubineSecondEdition:Design,applicationsand rerating, theUnitedStatesofAmerica,2009.[4] ElectricityGeneratingAuthorityofThailand,Rootcauseanalysisreport:SteamTurbineVibrationHighTrip,SBC10,Thailand,2008[5] Electricity Generating Authority of Thailand, Learning organization knowledge management report:RubbingCorrectionofSubsynchronousVibrationSteamTurbine,SBC10,Thailand,2008[6] GeneralElectric,SteamTurbineVibrationReport:ForcedOutageVibration,Thailand,2008
Miss Parichart Suttiprasit obtained a Bachelors degree of Mechanical Engineering fromChulalongkornUniversity,Bangkok,Thailand in1999.SincejoiningElectricityGeneratingAuthorityof Thailand (EGAT) as a mechanical engineer in steam turbine department, she had carried outseveral works regarding problem solving on steam turbine operation and maintenance includingauxiliaries inEGATpowerplants.Currently,sheworksasaplanningengineer inbusinessplanningdepartmentofgenerationdeputy.Mr.ChaiyantTibdeeobtainedaBachelorsdegreeofMechanicalEngineeringfromSongklanakarin University, Hatyai, Thailand. In 1999, he initially worked as mechanical engineer in boilerdepartment;hehadcarriedoutseveralworksregardingproblemsolvingonboileroperationandmaintenance.Afterjoining the testingengineeringdepartment,heworksasamechanical testingengineerandhehasbeenexecutedmanyvibrationanalysistestingprojectsinEGATpowerplants.Mr.NaritWongsaobtainedahighvocationalcertificateinmechanicalareaandhasworkedassteamturbinesitemanagerinsteamturbinedepartment.SincejoiningEGATin1980,hehasexecutedalotof steam turbine outages in EGAT power plants as technical skill, foreman, and site manager.Regardingtohisworkexperiences,heexecutedtheissuesonsteamturbineandauxiliariessuchasproblemsolving,functiontesting,sitemanaging,etc.andhealsosuppliedthedetailsofthistopicinhisresponsibleareas.
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