流體機(jī)械的畢業(yè)設(shè)計英漢翻譯化工泵

wordword16/16word文獻(xiàn)名稱〔中文〕化工工業(yè)離心泵摘要：離心泵是通過葉輪的旋轉(zhuǎn)把液體的內(nèi)能轉(zhuǎn)換成動能的一種旋轉(zhuǎn)裝置。液體由吸入口進(jìn)入蝸殼，通過高速旋轉(zhuǎn)的葉輪，液體呈放射狀加速從泵中向外輸出，這時葉輪附近留出一個真空，不斷吸引更多的流體進(jìn)入泵的葉輪附近，這樣由葉輪的旋轉(zhuǎn)來完成液體的進(jìn)出。這篇文章主要講述了關(guān)于離心泵的開展史，離心泵工作原理的分析，汽蝕的根本原理和預(yù)防汽蝕的措施等的一系列問題。從而幫助我們加深對離心泵的理解。關(guān)鍵詞：離心泵；工作原理；汽蝕；汽蝕原理；預(yù)防措施1．介紹泵的提出，最先是用于轉(zhuǎn)移或壓縮液體和氣體的設(shè)備。在所有泵中，我們一步步采取措施來防止氣蝕，氣蝕將減少流量并且破壞泵的結(jié)構(gòu)。用來處理氣體和蒸汽的泵稱為氣體壓縮機(jī)，研究流體的運動的科學(xué)稱為流體力學(xué)。水泵是用管子連接的機(jī)械把水從一個地方傳到另一個地方。水泵的操作壓力從一磅到一萬磅每平方英尺。日常生活中，泵是很多見的，有用于在魚池和噴泉使水循環(huán)和向水中充氣的電泵，還有用于從住宅處把水引走的污水泵。離心泵的早期形式螺桿泵，是通過一個管子連接一根螺桿組成的，它是利用螺桿的旋轉(zhuǎn)把水提升上去。螺旋泵經(jīng)常用在污水處理廠中，因為它們可以運輸大量的水，而不會因為碎片而堵塞。在遠(yuǎn)古的中東，因為對農(nóng)場進(jìn)展灌溉的需求，所以有一種強(qiáng)大的動力去推進(jìn)水泵的進(jìn)程。在這些區(qū)域里，早期的泵是為了將水一桶一桶的從水源或河渠中提升到容器中。古希臘的發(fā)明家和數(shù)學(xué)家阿基米德被認(rèn)為是公元前3世紀(jì)首先提出螺旋泵的發(fā)明家。之后，古希臘發(fā)明家發(fā)明了第一個提水泵。在十七世紀(jì)末和十八世紀(jì)初，英國的工程師ThomasSavory，法國的物理學(xué)家DenisPa]pin，和英國的鐵匠和發(fā)明家TomasNewen,它們發(fā)明了用蒸汽驅(qū)動活塞的水泵。蒸汽驅(qū)動的水泵首先廣泛的被應(yīng)用是在從煤礦往外輸水過程中。現(xiàn)在離心泵使用的例子，是來自于哥倫比亞河上使用的大古利水壩。這個泵有灌溉超過一百萬英畝的土地能力。離心泵被認(rèn)為是旋轉(zhuǎn)泵，它有一個旋轉(zhuǎn)地葉輪，葉輪上有葉片，葉片是侵入液體中的。液體也是由葉輪軸向進(jìn)入泵，并且旋轉(zhuǎn)的葉輪將液體甩向葉片根部。同時葉輪也給液體一個較高的速度，這個速度通過泵的一個固定部件轉(zhuǎn)化成壓力。我們一般稱為擴(kuò)壓器。在高壓泵里，很多葉輪可以被系列選用，并且在一個葉輪后有一個擴(kuò)壓器，也可能含有導(dǎo)輪，可以逐漸的降低液體的速度。對于低壓泵來說，擴(kuò)壓泵一般就是一個螺旋形的通道，成為蝸殼，作用原理是攔截面逐漸增加可以有效降低流體的速度。在泵工作前，葉輪必須被灌注，也就是在泵啟動時，葉輪必須被液體包圍。也可以通過在吸入線上放另一個截止閥來實現(xiàn)，截止閥在泵停止工作時是液體保存在泵內(nèi)。如果截止閥泄露了，泵可以通過閥的入口，從外面的水源比如說蓄水池來取水灌注。一般離心泵在排水線的地方也有一個閥控制流體和壓力。對于小流量和高壓力來說，葉輪作用很大局部是放射狀的。對于高速流體和低壓排水壓力，泵中流體的方向可以近似于與軸的軸向平行，這時泵有一個軸流，這時葉輪就近似于螺旋推進(jìn)器。從一種流動的狀態(tài)轉(zhuǎn)換到另一種流動的狀態(tài)是漸進(jìn)的，對于中間狀態(tài)的設(shè)備可稱為混流泵。2．離心泵離心泵是化工和石油工業(yè)中應(yīng)用最廣泛的一種泵。它能輸送性能非常廣泛的液體和固體含量高的懸浮液，像泥泥漿，可以用多種抗腐蝕材料建造。泵的整個外殼可用像聚丙烯這樣的塑料來建造，或者用腐蝕襯里加工。由于它的高速運轉(zhuǎn)，可將其直接耦合到電動機(jī)上，由電動機(jī)的規(guī)格大小決定流量上下。在這樣的泵中，液體被吸入到旋轉(zhuǎn)葉輪的中心，通過離心作用向外流動。由于高速旋轉(zhuǎn)，液體在吸入口和因動能轉(zhuǎn)化為壓能的出口側(cè)獲得較高的動能和壓力差。葉輪由一系列弧形葉片組成，因此能使液體的流動盡可能平穩(wěn)。葉輪中葉片越多，如此液體的流動方向越好控制，那么液體循環(huán)流動時因波動引起的損失就越少。在開式葉輪中，葉片被固定在中心輪轂上，而在閉式中葉片如此是用兩塊鋼板支撐以減少漏液。由此可以看出，在很大程度上，葉片末端的角度決定了泵的工作特性。流體通常在軸向上通過葉片的上升進(jìn)入泵殼。在這種簡單類型的離心泵中，液體由切向方向隨著橫截面逐步流到蝸殼中。圖(a)所示為旋渦型泵，圖(b)中，在渦輪泵中的液體隨移動的葉輪在一系列固定葉片中形成擴(kuò)散環(huán)。這種旋渦能逐漸改變流體的流動方向，并有效地將動能轉(zhuǎn)化成壓能。固定葉片前緣處的流體應(yīng)該沒有受到?jīng)_擊。沿著葉輪葉片，液體的流動具有一定速度，同時，葉片末端相對于泵體有移動。液體的運動方向相對于泵殼和固定葉片所需的角度一樣，是兩個速度的合成方向。是液體相對于葉片的速度，是葉片上某點的切向速度；將這兩個速度合成即可得到液體的速度。因此，很明顯，在擴(kuò)散環(huán)中所需要的葉輪角由葉輪的產(chǎn)量、旋轉(zhuǎn)速度和葉片的角度決定。所以，泵在很嚴(yán)格的條件下才能有最大的運行效能?！?〕離心泵的有效壓頭當(dāng)流體所剩余的動能全部轉(zhuǎn)化為壓能時，壓力最大。如下文所述，有效壓頭和半徑的平方以與速度成正比，壓力更高時，必須使用多級泵?？紤]到液體在離泵中心到的距離內(nèi)旋轉(zhuǎn)，如圖〔d〕這一局部流體的質(zhì)量為，其中是流體的密度，是這局部流體的寬度。如果流體在與切向方向成角上以速度流動，如此這局部質(zhì)量流體的角動量為。流體通過泵所產(chǎn)生的扭轉(zhuǎn)力等于角動量對時間的改變量液體的體積流速為：因此，液體在泵中受到總的扭轉(zhuǎn)力由在小標(biāo)1和2之間積分而得，下標(biāo)1引用的是泵入口處的條件，小標(biāo)2是出口時的條件。于是有：〔2〕離心泵的優(yōu)缺點主要優(yōu)點有：1.制造簡單，可用多種材料加工。2.無閥門。3.高速運轉(zhuǎn)〔高達(dá)100赫茲〕，因此可直接耦合到電動機(jī)上。一般地，速度越大，泵和電動機(jī)的效率越小。4.能平穩(wěn)傳送。5.維修費用比其他類型的泵少。6.輸送堵塞時，只要不是長時間運作，泵就不會被損壞。7.與其它泵相比，體積較小，因此可利用電動機(jī)做成密封裝置沉浸在吸收罐中。8.能容易輸送含有高比例懸浮固體的液體。主要缺點有：1.單級泵不能提高壓力。而多級泵能提高壓頭，但價格昂貴而且由于它們的復(fù)雜性不能用抗腐蝕的材料加工建造。通常用較高的速度來減少所需要的級數(shù)。2.只有在有限條件下才能以最高效能運作，尤其是渦輪泵。3.它不能自動注水。4.在輸送和吸收管道中，如果沒有止回閥，液體就會在泵停止工作的瞬間倒流到吸入槽內(nèi)。5.不能有效處理粘性液體。3．離心泵中的汽蝕〔1〕“汽蝕〞一詞來源于拉丁語，這意味著一個中空的空間或空腔。韋氏詞典定義的字是在一個非常低的壓力區(qū)域流動的液體腔內(nèi)迅速形成和崩潰的“腔〞。在離心泵中的任何地方像蒸氣泡沫，氣體泡沫，氣體破洞，氣泡等各種條件長期作用都會造成汽蝕。這是一個各種結(jié)果同時作用的事情，不能簡單地看待。汽蝕的形成討論如下。在離心泵的蝸殼中，汽蝕意味著一個氣泡內(nèi)的液體，他們的形成，成長和隨后通過泵的液體流動崩潰所經(jīng)歷的動態(tài)過程。一般來說，液體內(nèi)氣泡的形成有兩種類型：蒸汽氣泡或氣態(tài)空泡。1.由于一個進(jìn)程正在進(jìn)展的液體汽化而引起的泡沫的形成。蒸汽氣泡的形成和崩潰引起的汽蝕條件通常被稱為霧狀氣蝕。2.泡沫形成的過程中，由于正在往泵中輸送的液體中溶解入氣體〔一般空氣的存在，但可能是系統(tǒng)中的任何氣體〕，由這些氣體的形成和崩潰引起的汽蝕條件通常被稱為氣態(tài)空泡。度CollapseofaVaporBubbleCollapseofaVaporBubble出版日期〔期刊號〕：ChemicalEngineering,1995出版單位：Butterworth-HeinemannLtd參考文獻(xiàn)[M].：宇航，1995[M].：機(jī)械工業(yè)，2002[M].：機(jī)械工業(yè)，2003文獻(xiàn)名稱〔英文〕CENTRIFUGALPUMPSINTHECHEMICALINDUSTRYAbstract:Acentrifugalpumpconvertstheinputpowertokineticenergyintheliquidbyacceleratingtheliquidbyarevolvingdevice-animpeller.Themostmontypeisthevolutepump.Fluidentersthepumpthroughtheeyeoftheimpellerwhichrotatesathighspeed.Thefluidisacceleratedradiallyoutwardfromthepumpchasing.Avacuumiscreatedattheimpellerseyethatcontinuouslydrawsmorefluidintothepump.Thisarticlestressesonaseriesofcentrifugalpumps.Fromabriefintroductiontotheprinciple.Keywords:centrifugalpump;Workingprinciple;Cavitation;MechanismofCavitation;SolutionandRemedies1．IntroductionPump,deviceusedtoraise,transfer,orpressliquidsandgases.Fourgeneralclassesofpumpsforliquidsaredescribedbelow.Inallofthem,stepsaretakentopreventcavitation(theformationofavacuum),whichwouldreducetheflowanddamagethestructureofthepump.Pumpsusedforgasesandvaporsareusuallyknownaspressors.Thestudyoffluidsinmotioniscalledfluiddynamics.Waterpump,devicesformovingwaterfromonelocationtoanother,usingtubesorothermachinery.Waterpumpsoperateunderpressuresrangingfromafractionofapoundtomorethan10,000poundspersquareinch.Everydayexamplesofwaterpumpsrangefromsmallelectricpumpsthatcirculateandaeratewaterinaquariumsandfountainstosumppumpsthatremovewaterfrombeneaththefoundationsofhomes.Onetypeofmodernpumpsusedtomovewateristhecentrifugalpump.Earlyversionofthecentrifugalpump,thescrewpump,consistsofacorkscrew-shapedmechanisminapipethat,whenrotated,pullswaterupward.Screwpumpsareoftenusedinwaste-watertreatmentplantsbecausetheycanmovelargeamountsofwaterwithoutbeingcloggedwithdebris.IntheancientMiddleEasttheneedforirrigationoffarmlandwasastronginducementtodevelopawaterpump.Earlypumpsinthisregionweresimpledevicesforliftingbucketsofwaterfromasourcetoacontaineroratrench.GreekmathematicianandinventorArchimedesisthoughttohavedevisedthefirstscrewpumpinthethirdcenturyBC.LaterGreekinventorCtesibiusdevelopthefirstliftpump.Duringthelate17thandearly18thCenturiesAD,BritishengineerThomasSavery,FrenchphysicistDenisPapin,AndBritishblacksmithandinventorThomasNewencontributedtothedevelopmentofawaterpumpthatusedsteamtopowerthepump’piston.Thesteam-poweredwaterpump’sfirstwideusewasinpumpingwateroutofmines.Modern-dayexamplesofcentrifugalpumpsarethoseusedattheGrandCouleeDamontheColumbiaRiver.Thispumpsystemhasthepotentialtoirrigateoveronemillionacresofland.Alsoknownasrotarypumps,centrifugalpumpshavearotatingimpeller,alsoknownasablade,thatisimmersedintheliquid.Liquidentersthepumpneartheaxisoftheimpeller,andtherotatingimpellersweepstheliquidouttowardtheendsoftheimpellerbladesathighpressure.Theimpelleralsogivestheliquidarelativelyhighvelocitythatcanbeconvertedintopressureinastationarypartofthepump,knownasthediffuser.Inhigh-pressurepumps,anumberofimpellermaybeusedinseries,andthediffusersfollowingeachimpellermaycontainguidevanestograduallyreducetheliquidvelocity.Forlower-pressurepumps,thediffuserisgenerallyaspiralpassage,knownasavolute,withitscross-sectionalareaincreasinggraduallytoreducethevelocityefficiently.Theimpellermustbeprimedbeforeitcanbeginoperation,thatistheimpellermustbesurroundedbyliquidwhenthepumpisstarted.Thiscanbedonebyplacingacheckvalveinthesuctionline,whichholdstheliquidinthepumpwhentheimpellerisnotrotating.Ifthisvalveleaks,thepumpmayneedtobeprimedbytheintroductionofliquidfromanoutsidesourcesuchasthedischargereservoir.Acentrifugalpumpgenerallyhasavalveinthedischargelinetocontroltheflowandpressure.Forlowflowsandhighpressures,theactionoftheimpellerislargelyradial.Forhigherflowsandlowerdischargepressure,thedirectionoftheflowwithinthepumpismorenearlyparalleltotheaxisoftheshaft,andthepumpissaidtohaveanaxialflow.Theimpellerinthiscaseactsasapropeller.2．TheCentrifugalPumpThecentrifugalpumpisbyfarthemostwidelyusedtypeinthechemicalandpetroleumindustries.Itwillpumpliquidswithverywiderangingpropertiesandsuspensionswithahighsolidscontentincluding,forexample,cementslurries,andmaybeconstructedfromaverywiderangofcorrosionresistantmaterials.Thewholepumpcasingmaybeconstructedfromplasticsuchaspolypropyleneoritmaybefittedwithacorrosion-resistantlining.Becauseitoperatesathighspeed,itmaybedirectlycoupledtoanelectricmotoranditwillgiveahighflowrateforitssize.Inthistypeofpump,thefluidisfedtothecentreofarotatingimpellerandisthrownoutwardbycentrifugalaction.Asaresultofthehighspeedofrotationtheliquidacquiresahighkineticenergyandthepressuredifferencebetweenthesuctionanddeliverysidesarisesfromtheconversionofkineticenergyintopressureenergy.Theimpellerconsistsofaseriesofcurvedvanessoshapedthattheflowwithinthepumpisassmoothaspossible.Thegreaterthenumberofvanesontheimpeller,thegreateristhecontroloverthedirectionoftheliquidandhencethesmallerarethelossesduetoturbulenceandcirculationbetweenthevanes.Intheopenimpeller,thevanesarefixedtoacentralhub,whereasintheclosedtypethevanesareheldbetweentwosupportingplatesandleakageacrosstheimpellerisreduced.Aswillbeseenlater,theangleofthetipsofthebladesverylargelydeterminestheoperatingcharacteristicsofthepump.Theliquidentersthecasingofthepump,normallyinanaxialdirection,andispickedupbythevanesoftheimpeller,Inthesimpletypeofcentrifugalpump,theliquiddischargesintoavolute,achamberofgraduallyincreasingcrosssectionwithatangentialoutlet.AvolutetypeofpumpisshowninFig.(a).IntheturbinepumpFig.(b)theliquidflowsfromthemovingvanesoftheimpellerthroughaseriesoffixedvanesformingadiffusionring．Thisgivesamoregradualchangeindirectiontothefluidandmoreefficientconversionofkineticenergyintopressureenergythanisobtainedwiththevolutetype．Theangleoftheleadingedgeofthefixedvanesshouldbesuchthatthefluidisreceivedwithoutshock．Theliquidsflowsalongthesurfaceoftheimpellervanewithacertainvelocitywhilstthetipofthevaneismovingrelativetothecasingofthepump．Thedirectionofmotionoftheliquidrelativetothepumpcasingandtherequiredangleofthefixedvanes—isfoundbypoundingthesetwovelocities．isthevelocityoftheliquidrelativetothevaneandisthetangentialvelocityofthetipofthevane;poundingthesetwovelocitiesgivestheresultantvelocityoftheliquid.Itisapparent,therefore,thattherequiredvaneangleinthediffuserisdependentonthethroughput,thespeedofrotation，andtheangleoftheimpellerblades.Thepumpwillthereforeoperateatmaximumefficiencyonlyoveranarrowrangeofconditions．(1)VirtualheadofacentrifugalpumpThemaximumpressureisdevelopedwhenthewholeoftheexcesskineticenergyofthefluidisconvertedintopressureenergy.Asindicatedbelow,theheadisproportionaltothesquareoftheradiusandtothespeed,andisoftheorderof60mforasingle—stagecentrifugalpump;forhigherpressures,multistagepumpsmustbeused.Considertheliquidwhichisrotatingatadistanceofbetweenandfromthecentreofthepump.Themassofthiselementoffluiddmisgivenby，whereisthedensityofthefluidandisthewidthoftheelementoffluid。Ifthefluidistravelingwithavelocityandatanangletothetangentialdirection.Theangularmomentumofthismassoffluidis.Thetorqueactingonthefluiddτisequaltotherateofchangeofangularmomentumwithtime，asitgoesthroughthepumpThevolumetricrateofflowofliquidthroughthepump：Thetotaltorqueactingontheliquidinthepumpisthereforeobtainedintegratingbetweenthelimitsdenotedbysuffix1andsuffix2，wheresuffix1referstotheconditionsattheinlettothepumpandsuffix2referstotheconditionatthedischarge．Thus，(2)TheadvantagesanddisadvantagesofthecentrifugalpumpThemainadvantagesare：1.Itissimpleinconstructionandcan，therefore，bemadeinawiderangeofmaterials.2.Thereisapleteabsenceofvalves．3.Itoperatesathighspeed(upto100Hz)and，therefore，canbecoupleddirectlytoanelectricmotor.Ingeneral，thehigherthespeedthesmallerthepumpandmotorforagivenduty．4.Itgivesasteadydelivery．5.Maintenancecostsarelowerthanforanyothertypeofpump．6.Nodamageisdonetothepumpifthedeliverylinebeesblocked，provideditisnotruninthisconditionforaprolongedperiod．7.Itismuchsmallerthanotherpumpsofequalcapacity．Itcan，therefore，bemadeintoasealedunitwiththedrivingmotorandimmersedinthesuctiontank．8.Liquidscontaininghighproportionsofsuspendedsolidsarereadilyhandled．Themaindisadvantagesare：1.Thesingle—stagepumpwillnotdevelopahighpressure．Multistagepumpswilldevelopgreaterheadsbattheyareverymuchmoreexpensiveandcannotreadilybemadeincorrosion—resistantmaterialbecauseoftheirgreaterplexity．Itisgenerallybettertouseveryhighspeedsinordertoreducethenumberofstagesrequired．2.Itoperatesatahighefficiencyoveronlyalimitedrangeofconditions;thisappliesespeciallytoturbinepumps．3.Itisnotusuallyself-priming.4.Ifanon-returnvalveisnotincorporatedinthedeliveryorsuctionline,theliquidwillrunbackintothesuctiontankassoonasthepumpstops．5.Veryviscousliquidscannothehandledefficiently．3.Cavitationincentrifugalpump(1)Theterm‘cavitation’esfromtheLatinwordcavus,whichmeansahollowspaceoracavity.Webster’sDictionarydefinestheword‘cavitation’astherapidformationandcollapseofcavitiesinaflowingliquidinregionsofverylowpressure.Inanydiscussiononcentrifugalpumpsvarioustermslikevaporpockets,gaspockets,holes,bubbles,etc.areusedinplaceofthetermcavities.Theseareoneandthesamethingandneednotbeconfused.Thetermbubbleshallbeusedhereafterinthediscussion.Inthecontextofcentrifugalpumps,thetermcavitationimpliesadynamicprocessofformationofbubblesinsidetheliquid,theirgrowthandsubsequentcollapseastheliquidflowsthroughthepump.1.Vaporbubblesareformedduetothevaporisationofaprocessliquidthatisbeingpumped.ThecavitationconditioninducedbyformationandcollapseofvaporbubblesismonlyreferredtoasVaporousCavitation.2.Gasbubblesareformedduetothepresenceofdissolvedgasesintheliquidthatisbeingpumped(generallyairbutmaybeanygasinthesystem).ThecavitationconditioninducedbytheformationandcollapseofgasbubblesismonlyreferredtoasGaseousCavitation.Totalpressure：Thesumofstaticpressureanddynamicpressureisdefinedasthetotalpressure.Itisameasureoftotalenergyofthemovingfluidstream.i.e.bothpotentialandkineticenergy.Vaporpressureisthepressurerequiredtokeepaliquidinaliquidstate.Ifthepressureappliedtothesurfaceoftheliquidisnotenoughtokeepthemoleculesprettyclosetogether,themoleculeswillbefreetoseparateandroamaroundasagasorvapor.Thevaporpressureisdependentuponthetemperatureoftheliquid.Higherthetemperature,higherwillbethevaporpressure.(3)CavitationDamage：Cavitationcandestroypumpsandvalves,andcavitationcausesalossofefficiencyinpumpsimmediately,andalsoacontinuouslyincreasinglossofefficiencyastheequipmentdegradesduetoerosionofthepumpponentsbycavitation.ThereforeItisimportanttounderstandthephenomenasufficientlytopredictandthereforereducecavitationanddamagefromcavitation,andalsotodiagnoseandfindpracticalsolutionstocavitationproblems。1.CavitationEnhancedChemicalErosionPumpsoperatingundercavitationconditionsbeemorevulnerabletocorrosionandchemicalattack.Metalsmonlydevelopanoxidelayerorpassivatedlayerwhichprotectsthemetalfromfurthercorrosion.Cavitationcanremovethisoxideorpassivelayeronacontinuousbasisandexposeunprotectedmetaltofurtheroxidation.Thetwoprocesses(cavitation&oxidation)thenworktogethertorapidlyremovemetalfromthepumpcasingandimpeller.Stainlesssteelsarenotinvulnerabletothisprocess.2.MaterialsSelectionThereisnometal,plastic,oranyothermaterialknowntoman,thatcanwithstandthehighlevelsofenergyreleasedbycavitationintheformsofheatandpressure.Inpracticehowever,materialscanbeselectedthatresultinlongerlifeandcustomervalueintheirabilitytowithstandcavitationenergies,sothatattentiontopumpconstructionmaterialsisvaluableandproductive.Wherecavitationisnotaproblemornotpredictedtobeaproblem,monmaterialssuchascastironandbronzearesuitableforpumpconstruction.Therearemillionsofcastironandbronzepumpsthatworkfinefor20yearsormorewithoutanyproblemeventhoughmanyofthosepumpsexperiencesomecavitation.(4)MechanismofCavitation:ThephenomenonofcavitationisastepwisecrocessasshowninFigure(below).StepOne,Formationofbubblesinsidetheliquidbeingpumped.Thebubblesforminsidetheliquidwhenitvaporisesi.e.phasechangefromliquidtovapor.Buthowdoesvaporizationoftheliquidoccurduringapumpingoperation?Vaporizationofanyliquidinsideaclosedcontainercanoccurifeitherpressureontheliquidsurfacedecreasessuchthatitbeesequaltoorlessthantheliquidvaporpressureattheoperatingtemperature,orthetemperatureoftheliquidrises,raisingthevaporpressuresuchthatitbeesequaltoorgreaterthantheoperatingpressureattheliquidsurface.Forexample,ifwateratroomtemperature(about)iskeptinaclosedcontainerandthesystempressureisreducedtoitsvaporpressure(about0.52psia),thewaterquicklychangestoavapor.Also,iftheoperatingpressureistoremainconstantatabout0.52psiaandthetemperatureisallowedtoriseabove,thenthewaterquicklychangestoavapor.CollapseofaVaporBubbleJustlikeinaclosedcontainer,vaporizationoftheliquidcanoccurincentrifugalpumpswhenthelocalstaticpressurereducesbelowthatofthevaporpressureoftheliquidatthepumpingtemperature.CollapseofaVaporBubbleStep