外文翻译液压机械装置振动问题处理.docx
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外文翻译液压机械装置振动问题处理.docx
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外文翻译液压机械装置振动问题处理
原文
SolvingVibrationProblemsInHydraulicMachinery
Abstract
Inthecurrentpaper,variouscasesofvibrationproblemsdetectedinhydraulicmachineryarepresented.Thesecaseswerefoundduringseveralyearsofvibrationmonitoring.theproblemshavebeenclassifieddependingontheirorigin.Forallofthem,asystematicapproachisgivenindicatingthesymptoms,theexciatationprovokingthem,thepossibilitiesofamplificationduetoresonanceandtheremediesthathavebeenapplied.
Introduction
Vibrationproblemsarecommoninhydraulicmachinery.Solvingthemhelpstoincreasethemachinelifeandtoreducemaintenancecosts.
Manycaseshavebeenfoundandsolvedduringlastyearsofmonitoring(Egusquiza1998andEgusquizaet.2000).Someofthemhavebeenduetodesignormountingproblemsandothersduetodamageinmachineelements.Thecasespresentedherecorrespondtolargemachinewithverticalshaftandrigidcoupling.Theproblemsfoundhavebeenclassifiedinthefollowingtypes:
Type1:
Excessiveexcitationsofhydraulicorigin
Type2:
Hrustbearingproblems
Type3:
Unbalanceandmisalignment
Type4:
Electromagneticproblem
Amethodologytosolvethesetypesofvibrationproblemsisproposedinthepaper.ThestepstofollowareindicatedinFig,1.Oncetheabnormalhighvibrationamplitudeshavebeendetectedthroughscheduledmonitoringormachinemalfunctioning.Vibrationanalysishastobeproformdetoidentifytheoriginoftheexciationprovokingthem.Varioustechniquesareavailabledependingonthetypeofexcitationunderconsideration,whetheritishydraulic,mechanicor
Eletromagnetion.
Theresultinghighvibrationlevelsmayalsobeduetosometypeofresonaceinthehydraulicsystemorinthemechanicalcompoents.Therefore,thispossibilitymustbechecedbeforeacorrectdiagnosticcanbemade.
Finally,remediestotheprobiemareproposedandtheirsuccessisconfirmedcomparingthevibrationsafterthemodificationwiththepreviousones.
Type1:
ExcessiveHydraulicExcitation.
Typicalvibrationsofhydraulicoriginaregeneratedbyrotor-statorinteraction(RSI)andbycacitation.
RSIisduetotheinterferencebetweentherunnerbladesandguidevanes.Itcangeneratepressurepulsationsofhighamplitudeinhydraulicmachines.Asaresult,cracksinrunnersandexcessivevibrationlevelsinmachineandpipingcanbeproduced.RSIidentificationisnormallyeasywithspectralanalysisofvibration.Theproblemistoknowifthevibrationishighduetotheexcitationitself,toahighhydraulicsystemishighduetotheexcitationresonseortoarunner/motorresonance.Thevibrationwillbeatafrequencyfbgivenby:
Fb=n*f*zb=
Withmaximumamplitudeatthelowestdiametricalmodeexcitedkaccordingtothefollowingequation(tanaka1990):
n*zv+-k=m*zb
Anothertypicalphenomenonthatcangeneratevibrationsiscavitation.Cavitationcantakeseveralformsandcanresultinvibrations,lossesofperformanceanderosion.Themostcommontypeofcavitationispartloadsuige.Itsnegativeeffectsareusuallymitigatedbymeansofairentrapmentindrafttube.Ontheotherhand,inletcacitationprovokingerosionofrunnerbladesisalsoaconcernduetoitsdestructiveeffects.
Heretherearesomeexamplesforbothtypesofhydraulicexcitations.
Highvibrationsinthecasingofamultistagepump
Amultistagepumphadhighvibrationlevelsinthecasingduringoperation.Thegeneratedvibrationsproducedtheburstofpipesandotherelements.Initially,itwasthoughtthatthevibrationswereduetowearordamage(malfunctioning).Asaresult,thepumpwascompletelydismantledandrepaird.Surprisingly,thevibrationdidnotdisappear.
Ageneralschemeofthepumpisgiveninfig,2.Duringpumpingthemachinedeliversaflowrateof2.8m3/stoaheadof935m.OthermachinecharacteristicsarelistedinTable1.
Tounderstandthecauseofthehighvibertionlevelssomeexperimentalmeasurementsandatheoreticalanalysiswerecarriedout.
Fromthespectralanalysisitwasfoundthatthevibrationamplitudeoccurredat150Hzwhichisfb(seeFig,5).therefore,thevibrationhasafluiddynamicoriginanditshighamplitudemightbeduetoseveralreasonssuchashighRSIexcitationduetodesign,mounting,damage,orresonance.Inthiscase,theRSIanalysisgivesapressurepulsationaroundtheimpellerwithadiametricalmodek=-2rotatingintheoppositedirectionoftheimpellerstfb.
Firstofall,thehydraulicsystemresponseofthereturnchannelwascalculatedusingatransfermatrixmethod.Nofrequenciesaround150Hzwerefound.Furthermore,theanalysisofthemeasurements(phaseandamplitudevariation)atdifferentroatingspeedsneithershowedresonancearoundtheexcitationfrequency.
Modalanalysisfromtheimpactsdonewithaninsteumentedhammerwascarriedouttocheckthepossibilityofmechuanicalresonanceinthecasingorimpeller.
Forthecasing,noresonancewasfoundataroundthefrequencyof150HzasitcanbeseeninFig,3.AtheoreticalanalysiswithFEMgavesimilarresults.
ThefrequencyresponsefunctionsobtainedfromimpactsintheimpellerareindicatedinFig,4.Theyshowthepresenceofamodeat472Hz.Thismodewassusceptibleofexcitationbecauseitcorrespondedtodiametricalmode2.Estimatingareductionfactorof0.45to0.5inordertoconsidertheaddedmassandthecasingboundary,theactualnaturalfrequencywouldliebetween212and236Hz.So,therewasalowprobabilityofresonance.Infact,nohighvibrationlevelswerepresentatthebearings.
Finally,analyzingthephasingofthepressurepulsationandvibrationinthecasing,thediagnosiswasthatthehighvibrationswerecausedbytheinteractionofthepressurepulsationinsidethemachine.
Inthiscasethesolutionwastochangetherelativepositionbetweenimpellers.Afterthat,vibrationwasreducedconsiderablyasitisshowninFig,5whereRSIvibrationsbeforeandafterrepairarecompare.
Vibrationonrunnerofapump-turbine.
Inthiscase,themachinewasasinglestagereversiblepump-turbine.Itsmaincharacteristicsarelistedintable2.thispumpsufferedformcracksintheimpellerblades.
FortheZbandZvcombinations,thediametricalmodek=-2occursat2*fb(=140Hz)andshouldbethemostimportantexcitation.ThisisexactlywhatcanbeseeninthevibrationspectrumshowninFig,6.
Afteranalyzingthesystemresponse,checkingthenaturalfrequenciesofrunnerandrotor,itwasfoundthatarotornaturalfrequencywaswasclosetotheexcitationfrequency.Asthesystemcouldnotbechangedphysically,thesolutionoftheproblemwasdevisedasmodifyingtherunnerlacationwherecracksappearedtoreducestressesonblades.
Partialloadsurgeandinletcavitationerosiononbladesofafrancisturbine.
Thisuniewasaverticalshaftfrancisturbineoperatinguptoamaximumoutputpowerof65MW.Thetotalnominalflowrateandthenetheadwere57.5m3/sand122.5m.therestofcharacteristicsarelistedinTable3.
Inthiscasetwoproblemshadbeendetected.Thefirstonewasexcessivevibrationlevelsinthedrafttube.Thesecondonewasadvancederosiononthesuctionsideoftheblades.
InFig,7overallvibrationlevelsmeasuredfortheentirerangeofpoweroutputsindicatethatamplitudesaremoreimportantduringoperationatpartialloads.IntheFig,8,spectralanalysisofshaftdisplacementusingproximityprobescanbeobservedat20and55MW.Whenoperatingat20MW,afrequencypeakat0.27*ffpredominatesbutdisappearsat55MW.Thisanalysisindicatedthepresenceofahubropeinthedrafttubeatpartialloads.
Spectralanalysisisenoughtodetectpartialloadsurgebutitisnotusefulforothertypesofcavitation.Thisisthecaseoferosivecavitationisespeciallydestructive.
Anothertechniquetobeusedistodemodulatehighfrequencyvibrations.InFig,9,thespectraoftheenvelopeinthefrequencybandform30kto40kHzareplotted.Again,at20MWthepresenceofthepartloadsurgeiswelldetectedinthetopoftheFig.
So,thepossibleremediessuchasoptimizationofairinjectionfinsarecurrentlybeinganalyzed.Meanwhile,ithasbeenrecommendedtoavoidoperationatloadsbelow35MW.
Forthedetectionoferosivecavitationhighfrequencyvibrationsalsohadtobemeasured(Escaleretal.2002).Amplitudedemodulationofhighfrequencybands,showninthebottomofFig,9,indicatedthepresenceofapulsatingcavityintherunneratfvat55MW.Themaximumamplitudeofthispeakwasfoundat60MW,thusindicatingthemaximumcavitationaggressiveness.Therefore,thesolutionconsistedinlimitingthetimeofoperationaround60MWwheneverpossible.Arefurbishingoftherunner(newhydrodynamicdesign)isexpectedtobethesolutioninthiscase.
Type2:
thrustbearingproblems
Anothertypeofproblemsometimesfoundinverticalshaftmachinesisrubbinginthrustbearingwhichcanprovokeitsrapiddestruction.Inguidebearings,radialloadingisusuallylowandtheyarenotsoaffected.
Machinesarepronetohavefrictionduringstart-upandcoast-downifonlyhydrodynamiclubricationexists.Duringoperating,vibrationcanbegeneratedespeciallywhenloadonthepadsisnotevenlydistributed.Apossibilityfordetectionistoinstallaverticalproximityprobeoranabsolutevibrationsensorlocatednexttothebearingpadsinaxialdirection.Theuseofjoint-timefrequencyanalysisisveryadequatetoidenfutysuchfrictionsthatcanoccurinveryshortperiodsoftime.
Atsteadyoperation,spectralanalysishelpstoidentifypotentialproblemsbylookingatthepadpassingfrequency.Forinstance,inFig,10,thisfrequencydisappearsfromthevibrationsignatureafterrepairingthebearing.Althoughdetectioniseasy,toquantifythelevelofthedamageisdiff
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