1、附录英语原文HydraulicConductorsandFittingsEricSandgren*/T/M/CamerontoaccountforuncertaintyaMechanicalEngineering/VirginiaCommonwealthUniversity/601WestMainStreet/P/O/Box843015/Richmond/VA23284-3015/USAReceived19October2001；accepted5June20021/1INTRODUCTIONInahydraulicsystem/thefluidflowsthroughadistributio
2、nsystemconsistingofconductorsandfittings/whichcarrythefluidfromthereservoirthroughoperatingcomponentsandbacktothereservoir/Sincepoweristransmittedthroughoutthesystembymeansoftheseconductinglines(conductorsandfittingsusedtoconnectsystemcomponents)/itfollowsthattheymustbeproperlydesignedinorderforthet
3、otalsystemtofunctionproperly/Hydraulicsystemsuseprimarilyfourtypesofconductors/1/Steelpipes2/Steeltubing3/Plastictubing4/FlexiblehosesThechoiceofwhichtypeofconductortousedependsprimarilyonthesys-temsoperatingpressuresandflowrates/Inaddition/theselectiondependsonen-vironmentalconditionssuchasthetypeo
4、ffluid/operatingtemperatures/vibration/andwhetherornotthereisrelativemotionbetweenconnectedcomponents/Conductinglinesareavailableforhandlingworkpressuresupto10/000Paorgreater/Ingeneral/steeltubingprovidesgreaterplumbingflexibilityandneaterappearanceandrequiresfewerfittingsthanpiping/However/pipingis
5、lessexpensivethansteeltubing/Plastictubingisfindingincreasedindustrialusagebecauseitisnotcostlyandcircuitscanbeveryeasilyhookedupduetoitsflexibility/Flexiblehosesareusedprimarilytoconnectcomponentsthatexperiencerelativemotion/Theyaremadefromalargenumberofelastomeric(rubberlike)compoundsandarecapable
6、ofhandlingpressuresexceeding10/000Pa/Stainlesssteelconductorsandfittingsareusedifextremelycorrosiveenvi-ronmentsareexpected/However/theyareveryexpensiveandshouldbeusedonlyifnecessary/Copperconductorsshouldnotbeusedinhydraulicsystemsbecausethecopperpromotestheoxidationofpetroleumoils/Zinc/magnesium/a
7、ndcadmiumconductorsshouldnotbeusedeitherbecausetheyarerapidlycorrodedbywater-glycolfluids/Galvanizedconductorsshouldalsobeavoidedbecausethegalvanizedsurfacehasatendencytoflakeoffintothehydraulicfluid/Whenusingsteelpipeorsteeltubing/hydraulicfittingsshouldbemadeofsteelexceptforinlet/return/anddrainli
8、nes/wheremalleableironmaybeused/Conductorsandfittingsmustbedesignedwithhumansafetyinmind/Theymustbestrongenoughnotonlytowithstandthesteady-statesystempressuresbutalsotheinstantaneouspressurespikesresultingfromhydraulicshock/Whenevercontrolvalvesareclosedsuddenly/thisstopsthefluid/whichpossesseslarge
9、amountsofkineticenergy/Thisproducesshockwaveswhosepressurelevelscanbetwoorfourtimesthesteady-statesystemdesignvalues/Pressurespikescanalsobecausedbysuddenstoppingorstartingofheavyloads/Thesehigh-pressurepulsesaretakenintoaccountbytheapplicationofanappropriatefactorofsafety/1/2CONDUCTORSIZINGFORFLOW-
10、RATEREQUIREMENTSAconductormusthavealargeenoughcross-sectionalareatohandletheflow-raterequirementswithoutproducingexcessivefluidvelocity/Wheneverwespeakoffluidvelocityinaconductorsuchasapipe/wearereferringtotheaveragevelocity/Theconceptofaveragevelocityisimportantsinceweknowthatthevelocityprofileisno
11、tconstant/AsshowninChapter5thevelocityiszeroatthepipewallandreachesamaximumvalueatthecenterlineofthepipe/Theaveragevelocityisdefinedasthevolumeflowratedividedbythepipecross-sectionalarea/Inotherwords/theaveragevelocityisthatvelocitywhichwhenmultipliedbythepipeareaequalsthevolumeflowrate/Itisalsounde
12、rstoodthatthetermdiameterbyitselfalwaysmeansinsidediameterandthatthepipeareaisthatareathatcorrespondstothepipeinsidediameter/Themaximumrecommendedveloc-ityforpumpsuctionlinesis4ft/s(1/2m/s)inordertopreventexcessivelylowsuctionpressuresandresultingpumpcavitation/Themaximumrecommendedvelocityforpressu
13、relinesis20ft/s(6/1m/s)inordertopreventturbulentflowandthecorrespondingexcessiveheadlossesandelevatedfluidtemperatures/Notethatthesemaximumrecommendedvaluesareaveragevelocities/EXAMPLE1-1Apipehandlesaflowrateof30gprn/Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed20ft/s/
14、SolutionRewriteEq/(3-26)/solvingforD/EXAMPLE1-2Apipehandlesaflowrateof0/002/Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed6/1m/s/SolutionPerEq/3-35)wesolvefortheminimumrequiredpipeflowarea/Theminimuminsidediametercannowbefound/becauseSolvingforDwehave1/3PRESSURERATINGOF
15、CONDUCTORSTensileStressAconductormustbestrongenoughtopreventburstingduetoexcessivetensilestress(calledhoopstress)inthewalloftheconductorunderoperatingfluidpressure/Themagnitudeofthistensilestress/whichmustbesustainedbytheconductormaterial/canbedeterminedbyreferringtoFigure4-1/InFig/4-1(a)/weseethefl
16、uidpressure(P)actingnormaltotheinsidesurfaceofacircularpipehavingalength(L)/ThepipehasoutsidediameterD0/insidediameterDi/andwallthicknesst/Becausethefluidpressureactsnormaltothepipesinsidesurface/apressureforceiscreatedthatattemptstoseparateonehalfofthepipefromtheotherhalf/Figure4-1(b)showsthispress
17、ureforcepushingdownwardonthebottomhalfofthepipe/Topreventthebottomhalfofthepipefromseparatingfromtheupperhalf/theupperhalfpullsupwardwithatotaltensileforceF/One-halfofthisforce(orF/2)actsonthecross-sectionalarea(tL)ofeachwall/asshown/Sincethepressureforceandthetotaltensileforcemustbeequalinmagnitude
18、/wehavewhereAistheprojectedareaofthelowerhalf-pipecurved-wallsurfaceontoahorizontalplane/Thus/AequalstheareaofarectangleofwidthDiandlengthL/asshowninFigure4-1(b)/Hence/Thetensilestressinthepipematerialequalsthetensileforcedividedbythewallcross-sectionalareawithstandingthetensileforce/Thisstressiscal
19、ledatensilestressbecausetheforce(F)isatensileforce(pullsontheareaoverwhichitacts)/Substitutingvariableswehavewhere=Greeksymbol(sigma)=tensilestress/AscanbeseenfromEq/(4-2)/thetensilestressincreasesasthefluidpres-sureincreasesandalsoasthepipeinsidediameterincreases/Inaddition/asex-pected/thetensilest
20、ressincreasesasthewallthicknessdecreases/andthelengthofthepipedoesnothaveanyeffectonthetensilestress/BurstPressureandWorkingPressureTheburstpressure(BP)isthefluidpressurethatwillcausethepipetoburst/Thishappenswhenthetensilestress()equalsthetensilestrength(S)ofthepipematerial/Thetensilestrengthofamaterialequalsthetensilestressatwhichthematerialruptures/NoticethatanaxialscribelineisshownonthepipeouterwallsurfaceinFig/4-1(a)/Thisscribelineshowswherethepipewouldstarttocrackandthusruptureifthetensilestressreachedthetensilestrengthofthepipe
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