外文翻译液压机械装置振动问题处理.docx

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