附錄英語原文HydraulicConductorsandFittingsEricSandgren*,T.M.CamerontoaccountforuncertaintyaMechanicalEngineering,VirginiaCommonwealthUniversity,601WestMainStreet,P.O.Box843015,Richmond,VA23284-3015,USAReceived19October2001；accepted5June20021.1INTRODUCTIONInahydraulicsystem,thefluidflowsthroughadistributionsystemconsistingofconductorsandfittings,whichcarrythefluidfromthereservoirthroughoperatingcomponentsandbacktothereservoir.Sincepoweristransmittedthroughoutthesystembymeansoftheseconductinglines(conductorsandfittingsusedtoconnectsystemcomponents),itfollowsthattheymustbeproperlydesignedinorderforthetotalsystemtofunctionproperly.Hydraulicsystemsuseprimarilyfourtypesofconductors:1.Steelpipes2.Steeltubing3.Plastictubing4.FlexiblehosesThechoiceofwhichtypeofconductortousedependsprimarilyonthesys-temsoperatingpressuresandflowrates.Inaddition,theselectiondependsonen-vironmentalconditionssuchasthetypeoffluid,operatingtemperatures,vibration,andwhetherornotthereisrelativemotionbetweenconnectedcomponents.Conductinglinesareavailableforhandlingworkpressuresupto10,000Paorgreater.Ingeneral,steeltubingprovidesgreaterplumbingflexibilityandneaterappearanceandrequiresfewerfittingsthanpiping.However,pipingislessexpensivethansteeltubing.Plastictubingisfindingincreasedindustrialusagebecauseitisnotcostlyandcircuitscanbeveryeasilyhookedupduetoitsflexibility.Flexiblehosesareusedprimarilytoconnectcomponentsthatexperiencerelativemotion.Theyaremadefromalargenumberofelastomeric(rubberlike)compoundsandarecapableofhandlingpressuresexceeding10,000Pa.Stainlesssteelconductorsandfittingsareusedifextremelycorrosiveenvi-ronmentsareexpected.However,theyareveryexpensiveandshouldbeusedonlyifnecessary.Copperconductorsshouldnotbeusedinhydraulicsystemsbecausethecopperpromotestheoxidationofpetroleumoils.Zinc,magnesium,andcadmiumconductorsshouldnotbeusedeitherbecausetheyarerapidlycorrodedbywater-glycolfluids.Galvanizedconductorsshouldalsobeavoidedbecausethegalvanizedsurfacehasatendencytoflakeoffintothehydraulicfluid.Whenusingsteelpipeorsteeltubing,hydraulicfittingsshouldbemadeofsteelexceptforinlet,return,anddrainlines,wheremalleableironmaybeused.Conductorsandfittingsmustbedesignedwithhumansafetyinmind.Theymustbestrongenoughnotonlytowithstandthesteady-statesystempressuresbutalsotheinstantaneouspressurespikesresultingfromhydraulicshock.Whenevercontrolvalvesareclosedsuddenly,thisstopsthefluid,whichpossesseslargeamountsofkineticenergy.Thisproducesshockwaveswhosepressurelevelscanbetwoorfourtimesthesteady-statesystemdesignvalues.Pressurespikescanalsobecausedbysuddenstoppingorstartingofheavyloads.Thesehigh-pressurepulsesaretakenintoaccountbytheapplicationofanappropriatefactorofsafety.1.2CONDUCTORSIZINGFORFLOW-RATEREQUIREMENTSAconductormusthavealargeenoughcross-sectionalareatohandletheflow-raterequirementswithoutproducingexcessivefluidvelocity.Wheneverwespeakoffluidvelocityinaconductorsuchasapipe,wearereferringtotheaveragevelocity.Theconceptofaveragevelocityisimportantsinceweknowthatthevelocityprofileisnotconstant.AsshowninChapter5thevelocityiszeroatthepipewallandreachesamaximumvalueatthecenterlineofthepipe.Theaveragevelocityisdefinedasthevolumeflowratedividedbythepipecross-sectionalarea:Inotherwords,theaveragevelocityisthatvelocitywhichwhenmultipliedbythepipeareaequalsthevolumeflowrate.Itisalsounderstoodthatthetermdiameterbyitselfalwaysmeansinsidediameterandthatthepipeareaisthatareathatcorrespondstothepipeinsidediameter.Themaximumrecommendedveloc-ityforpumpsuctionlinesis4ft/s(1.2m/s)inordertopreventexcessivelylowsuctionpressuresandresultingpumpcavitation.Themaximumrecommendedvelocityforpressurelinesis20ft/s(6.1m/s)inordertopreventturbulentflowandthecorrespondingexcessiveheadlossesandelevatedfluidtemperatures.Notethatthesemaximumrecommendedvaluesareaveragevelocities.EXAMPLE1-1Apipehandlesaflowrateof30gprn.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed20ft/s.SolutionRewriteEq.(3-26),solvingforD:EXAMPLE1-2Apipehandlesaflowrateof0.002.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed6.1m/s.SolutionPerEq.3-35)wesolvefortheminimumrequiredpipeflowarea:Theminimuminsidediametercannowbefound,becauseSolvingforDwehave1.3PRESSURERATINGOFCONDUCTORSTensileStressAconductormustbestrongenoughtopreventburstingduetoexcessivetensilestress(calledhoopstress)inthewalloftheconductorunderoperatingfluidpressure.Themagnitudeofthistensilestress,whichmustbesustainedbytheconductormaterial,canbedeterminedbyreferringtoFigure4-1.InFig.4-1(a),weseethefluidpressure(P)actingnormaltotheinsidesurfaceofacircularpipehavingalength(L).ThepipehasoutsidediameterD0,insidediameterDi,andwallthicknesst.Becausethefluidpressureactsnormaltothepipesinsidesurface,apressureforceiscreatedthatattemptstoseparateonehalfofthepipefromtheotherhalf.Figure4-1(b)showsthispressureforcepushingdownwardonthebottomhalfofthepipe.Topreventthebottomhalfofthepipefromseparatingfromtheupperhalf,theupperhalfpullsupwardwithatotaltensileforceF.One-halfofthisforce(orF/2)actsonthecross-sectionalarea(tL)ofeachwall,asshown.Sincethepressureforceandthetotaltensileforcemustbeequalinmagnitude,wehavewhereAistheprojectedareaofthelowerhalf-pipecurved-wallsurfaceontoahorizontalplane.Thus,AequalstheareaofarectangleofwidthDiandlengthL,asshowninFigure4-1(b).Hence,Thetensilestressinthepipematerialequalsthetensileforcedividedbythewallcross-sectionalareawithstandingthetensileforce.Thisstressiscalledatensilestressbecausetheforce(F)isatensileforce(pullsontheareaoverwhichitacts).Substitutingvariableswehavewhere=Greeksymbol(sigma)=tensilestress.AscanbeseenfromEq.(4-2),thetensilestressincreasesasthefluidpres-sure