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外文翻译--化工工业离心泵.doc

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外文翻译--化工工业离心泵.doc

1CENTRIFUGALPUMPSINTHECHEMICALINDUSTRYAbstractAcentrifugalpumpconvertstheinputpowertokineticenergyintheliquidbyacceleratingtheliquidbyarevolvingdeviceanimpeller.Themostcommontypeisthevolutepump.Fluidentersthepumpthroughtheeyeoftheimpellerwhichrotatesathighspeed.Thefluidisacceleratedradiallyoutwardfromthepumpchasing.Avacuumiscreatedattheimpellerseyethatcontinuouslydrawsmorefluidintothepump.Thisarticlestressesonaseriesofcentrifugalpumps,Fromabriefintroductiontotheprinciples.Keywordscentrifugalpump,Introduction,Workingprinciple,Cavitation,MechanismofCavitation,SolutionandRemedies1.IntroductionPump,deviceusedtoraise,transfer,orcompressliquidsandgases.Fourgeneralclassesofpumpsforliquidsaredescribedbelow.Inallofthem,stepsaretakentopreventcavitationtheformationofavacuum,whichwouldreducetheflowanddamagethestructureofthepump.Pumpsusedforgasesandvaporsareusuallyknownascompressors.Thestudyoffluidsinmotioniscalledfluiddynamics.Waterpump,deviceformovingwaterfromonelocationtoanother,usingtubesorothermachinery.Waterpumpsoperateunderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.Everydayexamplesofwaterpumpsrangefromsmallelectricpumpsthatcirculateandaeratewaterinaquariumsandfountainstosumppumpsthatremovewaterfrombeneaththefoundationsofhomes.Onetypeofmodernpumpsusedtomovewateristhecentrifugalpump.Earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrewshapedmechanisminapipethat,whenrotated,pullswaterupward.Screwpumpsareoftenusedinwastewatertreatmentplantsbecausetheycanmovelargeamountsofwaterwithoutbecomingcloggedwithdebris.IntheancientMiddleEasttheneedforirrigationoffarmlandwasastronginducementtodevelopawaterpump.Earlypumpsinthisregionweresimpledevicesforliftingbucketsofwaterfromasourcetoacontaineroratrench.GreekmathematicianandinventorArchimedesisthoughttohavedevisedthefirstscrewpumpinthethirdcenturyBC.LaterGreekinventorCtesibiusdevelopthefirstliftpump.Duringthelate17thandearly18thCenturiesAD,BritishengineerThomasSavery,FrenchphysicistDenisPapin,AndBritishblacksmithandinventorThomasNewcomencontributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepumppiston.Thesteampoweredwaterpumpsfirstwideusewasinpumpingwateroutofmines.ModerndayexamplesofcentrifugalpumpsarethoseusedattheGrandCouleeDamontheColumbiaRiver.Thispumpsystemhasthepotentialtoirrigateoveronemillionacresofland.Alsoknownasrotarypumps,centrifugalpumpshavearotatingimpeller,alsoknownasablade,thatisimmersedintheliquid.Liquidentersthepumpneartheaxisoftheimpeller,andtherotatingimpellersweepstheliquidouttowardtheendsoftheimpellerbladesathighpressure.Theimpelleralsogivestheliquidarelativelyhighvelocitythatcanbeconvertedintopressureinastationarypartofthepump,knownasthediffuser.Inhighpressurepumps,anumberofimpellermaybeusedinseries,andthediffusersfollowingeachimpellermaycontainguidevanestograduallyreducetheliquidvelocity.Forlowerpressurepumps,thediffuserisgenerallyaspiralpassage,knownasavolute,withitscrosssectionalareaincreasinggraduallytoreducethevelocityefficiently.Theimpellermustbeprimedbeforeitcanbeginoperation,thatis,theimpellermustbesurroundedbyliquidwhenthepumpisstarted.Thiscanbedonebyplacingacheckvalveinthesuctionline,whichholdstheliquidinthepumpwhentheimpellerisnotrotating.Ifthisvalveleaks,thepumpmayneedtobeprimedbytheintroductionofliquidfromanoutsidesourcesuchasthedischargereservoir.Acentrifugalpumpgenerallyhasavalveinthedischargelinetocontroltheflowand2pressure.Forlowflowsandhighpressures,theactionoftheimpellerislargelyradial.Forhigherflowsandlowerdischargepressure,thedirectionoftheflowwithinthepumpismorenearlyparalleltotheaxisoftheshaft,andthepumpissaidtohaveanaxialflow.Theimpellerinthiscaseactsasapropeller.Thetransitionfromonesetoffloeconditionstotheotherisgradual,andforintermediatecondition,thedeviceiscalledamixedflowpump.2.TheCentrifugalPumpThecentrifugalpumpisbyfarthemostwidelyusedtypeinthechemicalandpetroleumindustries.Itwillpumpliquidswithverywiderangingpropertiesandsuspensionswithahighsolidscontentincluding,forexample,cementslurries,andmaybeconstructedfromaverywiderangofcorrosionresistantmaterials.Thewholepumpcasingmaybeconstructedfromplasticsuchaspolypropyleneoritmaybefittedwithacorrosionresistantlining.Becauseitoperatesathighspeed,itmaybedirectlycoupledtoanelectricmotoranditwillgiveahighflowrateforitssize.Inthistypeofpump,thefluidisfedtothecentreofarotatingimpellerandisthrownoutwardbycentrifugalaction.Asaresultofthehighspeedofrotationtheliquidacquiresahighkineticenergyandthepressuredifferencebetweenthesuctionanddeliverysidesarisesfromtheconversionofkineticenergyintopressureenergy.Theimpellerconsistsofaseriesofcurvedvanessoshapedthattheflowwithinthepumpisassmoothaspossible.Thegreaterthenumberofvanesontheimpeller,thegreateristhecontroloverthedirectionoftheliquidandhencethesmallerarethelossesduetoturbulenceandcirculationbetweenthevanes.Intheopenimpeller,thevanesarefixedtoacentralhub,whereasintheclosedtypethevanesareheldbetweentwosupportingplatesandleakageacrosstheimpellerisreduced.Aswillbeseenlater,theangleofthetipsofthebladesverylargelydeterminestheoperatingcharacteristicsofthepump.Theliquidentersthecasingofthepump,normallyinanaxialdirection,andispickedupbythevanesoftheimpeller.Inthesimpletypeofcentrifugalpump,theliquiddischargesintoavolute,achamberofgraduallyincreasingcrosssectionwithatangentialoutlet.AvolutetypeofpumpisshowninFig.a.IntheturbinepumpFig.btheliquidflowsfromthemovingvanesoftheimpellerthroughaseriesoffixedvanesformingadiffusionring.Thisgivesamoregradualchangeindirectiontothefluidandmoreefficientconversionofkineticenergyintopressureenergythanisobtainedwiththevolutetype.Theangleoftheleadingedgeofthefixedvanesshouldbesuchthatthefluidisreceivedwithoutshock.Theliquidsflowsalongthesurfaceoftheimpellervanewithacertainvelocitywhilstthetipofthevaneismovingrelativetothecasingofthepump.Thedirectionofmotionoftheliquidrelativetothepumpcasingandtherequiredangleofthefixedvanesisfoundbycompoundingthesetwovelocities.InFig.c,3c.vuisthevelocityoftheliquidrelativetothevaneandtuisthetangentialvelocityofthetipofthevanecompoundingthesetwovelocitiesgivestheresultantvelocity2uoftheliquid.Itisapparent,therefore,thattherequiredvaneangleinthediffuserisdependentonthethroughput,thespeedofrotation,andtheangleoftheimpellerblades.Thepumpwillthereforeoperateatmaximumefficiencyonlyoveranarrowrangeofconditions.VirtualheadofacentrifugalpumpThemaximumpressureisdevelopedwhenthewholeoftheexcesskineticenergyofthefluidisconvertedintopressureenergy.Asindicatedbelow.theheadisproportionaltothesquareoftheradiusandtothespeed,andisoftheorderof60mforasinglestagecentrifugalpumpforhigherpressures,multistagepumpsmustbeused.ConsidertheliquidwhichisrotatingatadistanceofbetweenrandrdrfromthecentreofthepumpFig.d.dThemassofthiselementoffluiddmisgivenby2πrdrdρ,whereρisthedensityofthefluidand6isthewidthoftheelementoffluid。Ifthefluidistravelingwithavelocityuandatanangleθtothetangentialdirection.TheangularmomentumofthismassoffluiddMurcosθThetorqueactingonthefluiddτisequaltotherateofchangeofangularmomentumwithtime,asitgoesthroughthepumpDτdMα/αturcosθ2πrbρdrα/αturcosθ2.1ThevolumetricrateofflowofliquidthroughthepumpQ2πrbα/αt2.24DrQρdurcosθ2.3Thetotaltorqueactingontheliquidinthepumpisthereforeobtainedintegratingdτbetweenthelimitsdenotedbysuffix1andsuffix2,wheresuffix1referstotheconditionsattheinlettothepumpandsuffix2referstotheconditionatthedischarge.Thus,τQρ2u2rcos21u1rcos1TheadvantagesanddisadvantagesofthecentrifugalpumpThemainadvantagesare1Itissimpleinconstructionandcan,therefore,bemadeinawiderangeofmaterials2Thereisacompleteabsenceofvalves.3Itoperatesathighspeedupto100Hzand,therefore,canbecoupleddirectlytoanelectricmotor.Ingeneral,thehigherthespeedthesmallerthepumpandmotorforagivenduty.4Itgivesasteadydelivery.5Maintenancecostsarelowerthanforanyothertypeofpump.6Nodamageisdonetothepumpifthedeliverylinebecomesblocked,provideditisnotruninthisconditionforaprolongedperiod.7Itismuchsmallerthanotherpumpsofequalcapacity.Itcan,therefore,bemadeintoasealedunitwiththedrivingmotorandimmersedinthesuctiontank.8Liquidscontaininghighproportionsofsuspendedsolidsarereadilyhandled.Themaindisadvantagesare1Thesinglestagepumpwillnotdevelopahighpressure.Multistagepumpswilldevelopgreaterheadsbattheyareverymuchmoreexpensiveandcannotreadilybemadeincorrosionresistantmaterialbecauseoftheirgreatercomplexity.Itisgenerallybettertouseveryhighspeedsinordertoreducethenumberofstagesrequired.2Itoperatesatahighefficiencyoveronlyalimitedrangeofconditionsthisappliesespeciallytoturbinepumps.3Itisnotusuallyselfpriming.4Ifanonreturnvalveisnotincorporatedinthedeliveryorsuctionline,theliquidwillrunbackintothesuctiontankassoonasthepumpstops.5Veryviscousliquidscannothehandledefficiently.3.Cavitationincentrifugalpump(1)ThetermcavitationcomesfromtheLatinwordcavus,whichmeansahollowspaceoracavity.WebstersDictionarydefinesthewordcavitationastherapidformationandcollapseofcavitiesinaflowingliquidinregionsofverylowpressure.Inanydiscussiononcentrifugalpumpsvarioustermslikevaporpockets,gaspockets,holes,bubbles,etc.areusedinplaceofthetermcavities.Theseareoneandthesamethingandneednotbeconfused.Thetermbubbleshallbeusedhereafterinthediscussion.Inthecontextofcentrifugalpumps,thetermcavitationimpliesadynamicprocessofformationofbubblesinsidetheliquid,theirgrowthandsubsequentcollapseastheliquidflowsthroughthepump.Generally,thebubblesthatforminsidetheliquidareoftwotypesVaporbubblesorGasbubbles.1.Vaporbubblesareformedduetothevaporisationofaprocessliquidthatisbeingpumped.ThecavitationconditioninducedbyformationandcollapseofvaporbubblesiscommonlyreferredtoasVaporousCavitation.2.Gasbubblesareformedduetothepresenceofdissolvedgasesintheliquidthatisbeingpumped

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