流体机械的毕业设计英汉翻译化工泵

wordword16/16word文献名称〔中文〕化工工业离心泵摘要：离心泵是通过叶轮的旋转把液体的内能转换成动能的一种旋转装置。液体由吸入口进入蜗壳，通过高速旋转的叶轮，液体呈放射状加速从泵中向外输出，这时叶轮附近留出一个真空，不断吸引更多的流体进入泵的叶轮附近，这样由叶轮的旋转来完成液体的进出。这篇文章主要讲述了关于离心泵的开展史，离心泵工作原理的分析，汽蚀的根本原理和预防汽蚀的措施等的一系列问题。从而帮助我们加深对离心泵的理解。关键词：离心泵；工作原理；汽蚀；汽蚀原理；预防措施1．介绍泵的提出，最先是用于转移或压缩液体和气体的设备。在所有泵中，我们一步步采取措施来防止气蚀，气蚀将减少流量并且破坏泵的结构。用来处理气体和蒸汽的泵称为气体压缩机，研究流体的运动的科学称为流体力学。水泵是用管子连接的机械把水从一个地方传到另一个地方。水泵的操作压力从一磅到一万磅每平方英尺。日常生活中，泵是很多见的，有用于在鱼池和喷泉使水循环和向水中充气的电泵，还有用于从住宅处把水引走的污水泵。离心泵的早期形式螺杆泵，是通过一个管子连接一根螺杆组成的，它是利用螺杆的旋转把水提升上去。螺旋泵经常用在污水处理厂中，因为它们可以运输大量的水，而不会因为碎片而堵塞。在远古的中东，因为对农场进展灌溉的需求，所以有一种强大的动力去推进水泵的进程。在这些区域里，早期的泵是为了将水一桶一桶的从水源或河渠中提升到容器中。古希腊的发明家和数学家阿基米德被认为是公元前3世纪首先提出螺旋泵的发明家。之后，古希腊发明家发明了第一个提水泵。在十七世纪末和十八世纪初，英国的工程师ThomasSavory，法国的物理学家DenisPa]pin，和英国的铁匠和发明家TomasNewen,它们发明了用蒸汽驱动活塞的水泵。蒸汽驱动的水泵首先广泛的被应用是在从煤矿往外输水过程中。现在离心泵使用的例子，是来自于哥伦比亚河上使用的大古利水坝。这个泵有灌溉超过一百万英亩的土地能力。离心泵被认为是旋转泵，它有一个旋转地叶轮，叶轮上有叶片，叶片是侵入液体中的。液体也是由叶轮轴向进入泵，并且旋转的叶轮将液体甩向叶片根部。同时叶轮也给液体一个较高的速度，这个速度通过泵的一个固定部件转化成压力。我们一般称为扩压器。在高压泵里，很多叶轮可以被系列选用，并且在一个叶轮后有一个扩压器，也可能含有导轮，可以逐渐的降低液体的速度。对于低压泵来说，扩压泵一般就是一个螺旋形的通道，成为蜗壳，作用原理是拦截面逐渐增加可以有效降低流体的速度。在泵工作前，叶轮必须被灌注，也就是在泵启动时，叶轮必须被液体包围。也可以通过在吸入线上放另一个截止阀来实现，截止阀在泵停止工作时是液体保存在泵内。如果截止阀泄露了，泵可以通过阀的入口，从外面的水源比如说蓄水池来取水灌注。一般离心泵在排水线的地方也有一个阀控制流体和压力。对于小流量和高压力来说，叶轮作用很大局部是放射状的。对于高速流体和低压排水压力，泵中流体的方向可以近似于与轴的轴向平行，这时泵有一个轴流，这时叶轮就近似于螺旋推进器。从一种流动的状态转换到另一种流动的状态是渐进的，对于中间状态的设备可称为混流泵。2．离心泵离心泵是化工和石油工业中应用最广泛的一种泵。它能输送性能非常广泛的液体和固体含量高的悬浮液，像泥泥浆，可以用多种抗腐蚀材料建造。泵的整个外壳可用像聚丙烯这样的塑料来建造，或者用腐蚀衬里加工。由于它的高速运转，可将其直接耦合到电动机上，由电动机的规格大小决定流量上下。在这样的泵中，液体被吸入到旋转叶轮的中心，通过离心作用向外流动。由于高速旋转，液体在吸入口和因动能转化为压能的出口侧获得较高的动能和压力差。叶轮由一系列弧形叶片组成，因此能使液体的流动尽可能平稳。叶轮中叶片越多，如此液体的流动方向越好控制，那么液体循环流动时因波动引起的损失就越少。在开式叶轮中，叶片被固定在中心轮毂上，而在闭式中叶片如此是用两块钢板支撑以减少漏液。由此可以看出，在很大程度上，叶片末端的角度决定了泵的工作特性。流体通常在轴向上通过叶片的上升进入泵壳。在这种简单类型的离心泵中，液体由切向方向随着横截面逐步流到蜗壳中。图(a)所示为旋涡型泵，图(b)中，在涡轮泵中的液体随移动的叶轮在一系列固定叶片中形成扩散环。这种旋涡能逐渐改变流体的流动方向，并有效地将动能转化成压能。固定叶片前缘处的流体应该没有受到冲击。沿着叶轮叶片，液体的流动具有一定速度，同时，叶片末端相对于泵体有移动。液体的运动方向相对于泵壳和固定叶片所需的角度一样，是两个速度的合成方向。是液体相对于叶片的速度，是叶片上某点的切向速度；将这两个速度合成即可得到液体的速度。因此，很明显，在扩散环中所需要的叶轮角由叶轮的产量、旋转速度和叶片的角度决定。所以，泵在很严格的条件下才能有最大的运行效能。〔1〕离心泵的有效压头当流体所剩余的动能全部转化为压能时，压力最大。如下文所述，有效压头和半径的平方以与速度成正比，压力更高时，必须使用多级泵。考虑到液体在离泵中心到的距离内旋转，如图〔d〕这一局部流体的质量为，其中是流体的密度，是这局部流体的宽度。如果流体在与切向方向成角上以速度流动，如此这局部质量流体的角动量为。流体通过泵所产生的扭转力等于角动量对时间的改变量液体的体积流速为：因此，液体在泵中受到总的扭转力由在小标1和2之间积分而得，下标1引用的是泵入口处的条件，小标2是出口时的条件。于是有：〔2〕离心泵的优缺点主要优点有：1.制造简单，可用多种材料加工。2.无阀门。3.高速运转〔高达100赫兹〕，因此可直接耦合到电动机上。一般地，速度越大，泵和电动机的效率越小。4.能平稳传送。5.维修费用比其他类型的泵少。6.输送堵塞时，只要不是长时间运作，泵就不会被损坏。7.与其它泵相比，体积较小，因此可利用电动机做成密封装置沉浸在吸收罐中。8.能容易输送含有高比例悬浮固体的液体。主要缺点有：1.单级泵不能提高压力。而多级泵能提高压头，但价格昂贵而且由于它们的复杂性不能用抗腐蚀的材料加工建造。通常用较高的速度来减少所需要的级数。2.只有在有限条件下才能以最高效能运作，尤其是涡轮泵。3.它不能自动注水。4.在输送和吸收管道中，如果没有止回阀，液体就会在泵停止工作的瞬间倒流到吸入槽内。5.不能有效处理粘性液体。3．离心泵中的汽蚀〔1〕“汽蚀〞一词来源于拉丁语，这意味着一个中空的空间或空腔。韦氏词典定义的字是在一个非常低的压力区域流动的液体腔内迅速形成和崩溃的“腔〞。在离心泵中的任何地方像蒸气泡沫，气体泡沫，气体破洞，气泡等各种条件长期作用都会造成汽蚀。这是一个各种结果同时作用的事情，不能简单地看待。汽蚀的形成讨论如下。在离心泵的蜗壳中，汽蚀意味着一个气泡内的液体，他们的形成，成长和随后通过泵的液体流动崩溃所经历的动态过程。一般来说，液体内气泡的形成有两种类型：蒸汽气泡或气态空泡。1.由于一个进程正在进展的液体汽化而引起的泡沫的形成。蒸汽气泡的形成和崩溃引起的汽蚀条件通常被称为雾状气蚀。2.泡沫形成的过程中，由于正在往泵中输送的液体中溶解入气体〔一般空气的存在，但可能是系统中的任何气体〕，由这些气体的形成和崩溃引起的汽蚀条件通常被称为气态空泡。度CollapseofaVaporBubbleCollapseofaVaporBubble出版日期〔期刊号〕：ChemicalEngineering,1995出版单位：Butterworth-HeinemannLtd参考文献[M].：宇航，1995[M].：机械工业，2002[M].：机械工业，2003文献名称〔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