机械类外文翻译声发射检测初生空化及其应用60KW离心泵最佳效率点案例研究文档格式.docx

机械类外文翻译声发射检测初生空化及其应用60KW离心泵最佳效率点案例研究文档格式.docx

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Abstract

PumpsplayasignificantroleinIndustrialplantsandneedcontinuousmonitoringtominimizelossofproduction.Todate,thereislimitedpublishedinformationontheapplicationofAcousticEmission（AE）toincipientpumpcavitation.ThispaperpresentsacasestudywhereAEhasbeenappliedfordetectingincipientcavitationanddeterminingthebestefficiencypoint（BEP）ofa60KWcentrifugalpump.ResultspresentedarebasedonNPSH（NetPositiveSuctionHead）andperformancetests.InconclusiontheAEtechniquewasshowntoofferearlydetectionofincipientcavitation,furthermore,thetechniquehasdemonstratedtheabilitytodeterminetheBEPofapump

Keywords:

AcousticEmission;

bestefficiencypoint;

cavitation;

conditionmonitoring;

Pumpperformance

1.Introduction

TypicallythepumpmanufacturerwillundertakeperformanceandNPSH（NetPositiveSuctionHead）testsonsuppliedpumps,thesignificanceofthelatteristodeterminethe3%dropinheadatwhichseriouscavitationswilloccur.TheNPSHcanbeexpressedasthedifferencebetweenthesuctionheadandtheliquidsvapourhead.TheconceptofNPSHwasdevelopedforthepurposeofcomparinginletconditionofthesystemwiththeinletrequirementofthepump.Cavitationcausesalossofpumpefficiencyanddegradationofthemechanicalintegrityofthepump.Itmustbenotedthatcavitationstartstodevelopbeforethe3%dropinhead.Itisgenerallyacceptedthatthecriticalpressureforinceptionofcavitationisnotconstantandvarieswithoperationfluidphysicalpropertiesandthesurfaceroughnessofthehydraulicequipment.

ApplicationofthehighfrequencyAcousticEmission（AE）techniqueinconditionmonitoringofrotatingmachineryhasbeengrowingoverrecentyears[1-9].TypicalfrequenciesassociatedwithAEactivityrangefrom20KHzto1MHz.Themostcommonlyusedmethodforidentifyingthepresenceofcavitationisbasedonobservationsofthedropinhead.Whilstothertechniquessuchasvibrationanalysisandhydrophoneobservationsforpumpfaultdiagnosisarewellestablished,theapplicationofAEtothisfieldisstillinitsinfancy.Inaddition,therearealimitednumberofpublicationsontheapplicationofAEtopumphealthandcavitationmonitoring.Derakhshanetal[10]investigatedthecavitationbubblecollapseasasourceofacousticemissionandcommentedthatthehighamplitudepressurepulseassociatedwithbubblecollapsegeneratedAE.WiththeAEsensorwasplacedontheactualspecimenexperiencingcavitationDerkhshanobservedincreasingAEr.m.slevelswithincreasedpressureofflowandcavitation.However,withtheAEsensormountedonthetankwallthereversewasobserved,decreasingAEr.m.slevelswithincreasingpressureandcavitation.Thiswasattributedtoavisiblebubblecloudthatincreasedwithpressure.ItwascommentedthatthiscloudattenuatedtheAEsignaturepriortoreachingthetransduceronthewallcasing.Neilletal[11,12]assessedthepossibilityofearlycavitationdetectionwithAEandalsonotedthatthecollapseofcavitationbubbleswasanimpulsiveeventofthetypethatcouldgenerateAE.ItwasobservedthatwhenthepumpwasundercavitationtheAEoperationalbackgroundlevelsdroppedincomparisontonon-cavitatingconditions.InconclusionNeillstatedthatlossinNPSHbeforethe3%drop-offcriterionwasdetectablewithAEandevidenceofincipientcavitationwasdetectableinthehigherfrequencyband（0.5to1MHz）.

ThepapersreviewedabovehaveclearlyassociatedAEwiththecollapseofcavitationbubbles.ThepresenceofcavitationhasbeenshowntoincreaseordecreaseoperationalAEnoiselevels[10,11,12].ThispaperpresentsacasestudytoascertaintheapplicabilityoftheAEtechniquefordetectingincipientcavitation,and,toaccesstheopportunitiesofferedbytheAEtechniquefordeterminingthebestefficiencypoint（BEP）ofapump.

2.Experimentalsetup

AseriesofperformanceandNPSHtestswereundertakenonatwostage‘DavidBrown’60KWcentrifugalpump（ModelDB22）withamaximumcapacityof204m³

/hatanefficiencyof70.6%.ThesetestswereundertakenusingaclosedlooparrangementwithavacuumfacilityinaccordancewithBS9906.ItmustbenotedthatbestendeavourswereundertakentoreducethetimetakentoreachtherequiredflowrateduringperformanceandNPSHtests.

AcousticEmissionsensorswerelocatedatadistanceof0.5mfromsuctionflange;

atthesuctionflange;

onthepumpcasinginthevicinityofimpellersuctioneye;

onthecasinginthevicinityoftheimpellerdischargetip;

0.5mfromdischargeflange,seefigure1.

3.Dataacquisitionsystems

TheAEsensorsusedforalloftheexperimentswerebroadbandtypesensorswitharelativeflatresponseintheregionbetween100KHzto1MHz（Model:

WD,‘PhysicalAcousticsCorporation’）.TheoutputsignalfromtheAEsensorswaspre-amplifiedat40dB.ContinuousAEr.m.svalueswerecalculatedinrealtimebytheAnaloguetoDigitalConverter（ADC）.Thesamplingrateforacquisitionwassetat100msforalltestsandthetimeconstantforcalculatingtheAEr.m.swasalsosetat100ms.

4.Experimentalresultsandobservation

4.1Performancetest

Figure2detailstheperformancecharacteristicsofthepump,highlightingtheBEPat94.5m/hr.Theperformancetestwereundertakentwicetoensurerepeatability.ObservationsofAEr.m.sactivityduringtheperformancetestaredisplayedinfigure3.Duringtheperformancetest,AEactivityfromthesensorlocatedinthevicinityofimpelleronpumpcasingwasfoundtohavethelargestmagnitude,providingthebestpositionforcorrelatingAEactivitytopumpperformance.ItwasobservedthattheminimumAEr.m.svaluewasobtainedforaflowrateof94.5m/hr.AtthisflowratetheAEactivitygeneratedfromthefluidflowwithinthepumpandpipeswaslowestincomparisontootherflowrates.EithersideofthisflowrateresultedinincreasingAEr.m.sactivitywithincreasedflowrates.BasedontheseobservationsitwasconcludedthattheBEPmustoccurwheretherewasminimalflowturbulenceinthesystem,andhenceminimumAEactivity.Thepredictedefficiencypointof94.5m/hrwascheckedwiththemanufacturer’sperformancetestandwasfoundtobeaccurate.InterestinglythisisthefirstknowncorrelationbetweenAEactivityandtheBEPandagreeswithobservationsofMcNulty[13],thoughMcNulty’sinvestigationwascentredatfrequenciesintheaudiblerange;

lowerthantheAErange.TheadvantageofferedbytheAEtechniqueistheinherentrejectionoftypicalmechanicalandprocessoperationalbackgroundnoise（lessthan20KHz）

4.2NPSHtest

AtotalofthreeNPSHtestswereundertakenatflowratesof101,141and180m/hr,seefigure4.Aswiththeperformancetests,thebestAEsignatureresponsewaslocatedonthepumpcasinginthevicinityoftheimpellereye

Figures5to7detailtheassociatedAEr.m.slevelsforthethreeflowratesconsidered.Thefollowingobservationswerenoted:

Ataflowrateof101m/hanincreaseof165%inAEr.m.slevelswasobservedfromanNPSHvalueof8.2mto7m.RelativelyconstantlevelsfolloweduntilanNPSHof5.8mwhenarapiddecreaseinAEr.m.slevelswasnoted.WithfurtherreductionsinNPSH,spikesinAEr.m.ssignalvelsassociatedwithcavitationwasalsoobservedbyNeill[11].ObservationsofAElevelsfromthesuctionanddischargepipesmirrorthisobservation.

aflowrateof141m/hanincreaseof43%inAEr.m.swasobservedatNPSHvalueof12.7mto9.3m.ArapiddecreaseinlevelwasnotedatanNPSHof9.3m.WithfurtherreductionsinNPSH,spikesinAEr.m.ssignalwereobservedaswiththetestaflowrateof101m/hr.Again,observationsofAElevelsfromthesuctionanddischargepipesmirroredthisobservation.

Ataflowrateof181m/hanincreaseinAEr.m.sof223%wasobservedatNPSHvalueof11mto7.3m.AgradualdecreaseintheAElevelsfollowedtoanNPSHof1.7m,whereanincreaseinther.m.svaluewasobserved.

5.Discussions

5.1Performancetest

TheobservationsofAEactivityduringtheperformancetestwereveryencouraging.TheabilitytopredictasystemsBEPbyobservingvariationsintheAEr.m.sresponseofferprocessengineersapowerfultoolformonitoringplantperformance.WhilstfurtherresearchisstillrequiredtheopportunitiesofferedbysuchatoolcouldbeappliedtodeterminingsystemBEPirrespectiveofthetypeofmedium（liquid,gases,semi-solids,etc）inthesystem.TheobservationsnotedinthisinvestigationcorrelatewithhydrophonemeasurementsundertakenbyMcNulty[13],wheretheminimumsoundintensitycoincidedwiththepumpBEP.

5.2NPSHtest

ItisessentialtounderstandthecavitationsequenceifitistobecorrelatedtoobservedAEactivity.Oncethesuctionpressurestartstodecrease,vortexesstarttooccurattheimpellerbladetips.Withfurtherreductioninpressurethesevortexestaketheformoftravellingbubblesintheliquid.Thesebubblesareinitiallycreatedinlowerpressureareaonthesuctionsurfaceoftheblades.Eventuallythebubblesmovetohigher-pressureareaswheretheycollapse.Withevenfurtherreductioninthesuctionpressure,thebubblescombineintolargercavities.Thesecavitiesareusuallyformedontheimpellerbladesuctionsurface.

ForallNPSHtestsanincreaseinAEr.m.slevelswasnotedasvaluesofNPSHstartedtodecrease.AmaximumlevelofAEr.m.swasreachedafterwhichfurtherreductionsinNPSHresultedinadecreaseinAEr.m.slevels.Thiswasalsoobservedonthesensorslocatedonthesuctionanddischargeflanges.Itispostulatedth