外文翻译浅谈油管液压管路系统设计.docx
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外文翻译浅谈油管液压管路系统设计.docx
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外文翻译浅谈油管液压管路系统设计
TalkingAboutTheDesignofHydraulicConductors
EricSandgren
ThispaperisaccountforuncertaintyMechanicalEngineering,UniversityofCalifornia,SanFrancisco,avialon503WestMainStreet,P.O.Box962531
1.1INTRODUCTION
Inahydraulicsystem,thefluidflowsthroughadistributionsystemconsistingofconductorsandfittings,whichcarrythefluidfromthereservoirthroughoperatingcomponentsandbacktothereservoir.Sincepoweristransmittedthroughoutthesystembymeansoftheseconductinglines(conductorsandfittingsusedtoconnectsystemcomponents),itfollowsthattheymustbeproperlydesignedinorderforthetotalsystemtofunctionproperly.
Thechoiceofwhichtypeofconductortousedependsprimarilyonthesystem’soperatingpressuresandflowrates.Inaddition,theselectiondependsonenvironmentalconditionssuchasthetypeoffluid,operatingtemperatures,vibration,andwhetherornotthereisrelativemotionbetweenconnectedcomponents.
Conductinglinesareavailableforhandlingworkpressuresupto10,000Paorgreater.Ingeneral,steeltubingprovidesgreaterplumbingflexibilityandneaterappearanceandrequiresfewerfittingsthanpiping.However,pipingislessexpensivethansteeltubing.Plastictubingisfindingincreasedindustrialusagebecauseitisnotcostlyandcircuitscanbeveryeasilyhookedupduetoitsflexibility.Flexiblehosesareusedprimarilytoconnectcomponentsthatexperiencerelativemotion.Theyaremadefromalargenumberofelastomeric(rubberlike)compoundsandarecapableofhandlingpressuresexceeding10,000Pa.
Stainlesssteelconductorsandfittingsareusedifextremelycorrosiveenvironmentsareexpected.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-RATEREQUIREMENTS
Aconductormusthavealargeenoughcross-sectionalareatohandletheflow-raterequirementswithoutproducingexcessivefluidvelocity.Wheneverwespeakoffluidvelocityinaconductorsuchasapipe,wearereferringtotheaveragevelocity.Theconceptofaveragevelocityisimportantsinceweknowthatthevelocityprofileisnotconstant.AsshowninChapter5thevelocityiszeroatthepipewallandreachesamaximumvalueatthecenterlineofthepipe.Theaveragevelocityisdefinedasthevolumeflowratedividedbythepipecross-sectionalarea:
Inotherwords,theaveragevelocityisthatvelocitywhichwhenmultipliedbythepipeareaequalsthevolumeflowrate.Itisalsounderstoodthatthetermdiameterbyitselfalwaysmeansinsidediameterandthatthepipeareaisthatareathatcorrespondstothepipeinsidediameter.Themaximumrecommendedvelocityforpumpsuctionlinesis4ft/s(1.2m/s)inordertopreventexcessivelylowsuctionpressuresandresultingpumpcavitation.Themaximumrecommendedvelocityforpressurelinesis20ft/s(6.1m/s)inordertopreventturbulentflowandthecorrespondingexcessiveheadlossesandelevatedfluidtemperatures.Notethatthesemaximumrecommendedvaluesareaveragevelocities.
EXAMPLE1-1
Apipehandlesaflowrateof30gprn.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed20ft/s.
SolutionRewriteEq.(3-26),solvingforD:
EXAMPLE1-2
Apipehandlesaflowrateof0.002
.Findtheminimuminsidediameterthatwillprovideanaveragefluidvelocitynottoexceed6.1m/s.
SolutionPerEq.3-35)wesolvefortheminimumrequiredpipeflowarea:
Theminimuminsidediametercannowbefound,because
SolvingforDwehave
1.3PRESSURERATINGOFCONDUCTORS
Aconductormustbestrongenoughtopreventburstingduetoexcessivetensilestress(calledhoopstress)inthewalloftheconductorunderoperatingfluidpressure.Themagnitudeofthistensilestress,whichmustbesustainedbytheconductormaterial.weseethefluidpressure(P)actingnormaltotheinsidesurfaceofacircularpipehavingalength(L).ThepipehasoutsidediameterD0,insidediameterDi,andwallthicknesst.Becausethefluidpressureactsnormaltothepipe’sinsidesurface,apressureforceiscreatedthatattemptstoseparateonehalfofthepipefromtheotherhalf.
Figureshowsthispressureforce
pushingdownwardonthebottomhalfofthepipe.Topreventthebottomhalfofthepipefromseparatingfromtheupperhalf,theupperhalfpullsupwardwithatotaltensileforceF.One-halfofthisforce(orF/2)actsonthecross-sectionalarea(tL)ofeachwall,asshown.
Sincethepressureforceandthetotaltensileforcemustbeequalinmagnitude,wehave
whereAistheprojectedareaofthelowerhalf-pipecurved-wallsurfaceontoahorizontalplane.Thus,AequalstheareaofarectangleofwidthDiandlengthL,asshowninFigure4-1(b).Hence,
Thetensilestressinthepipematerialequalsthetensileforcedividedbythewallcross-sectionalareawithstandingthetensileforce.Thisstressiscalledatensilestressbecausetheforce(F)isatensileforce(pullsontheareaoverwhichitacts).
Substitutingvariableswehave
where
=Greeksymbol(sigma)=tensilestress.
AscanbeseenfromEq.thetensilestressincreasesasthefluidpressureincreasesandalsoasthepipeinsidediameterincreases.Inaddition,asexpected,thetensilestressincreasesasthewallthicknessdecreases,andthelengthofthepipedoesnothaveanyeffectonthetensilestress.
BurstPressureandWorkingPressure
Theburstpressure(BP)isthefluidpressurethatwillcausethepipetoburst.Thishappenswhenthetensilestress(
)equalsthetensilestrength(S)ofthepipematerial.Thetensilestrengthofamaterialequalsthetensilestressatwhichthematerialruptures.NoticethatanaxialscribelineisshownonthepipeouterwallsurfaceinFig.4-1(a).Thisscribelineshowswherethepipewouldstarttocrackandthusruptureifthetensilestressreachedthetensilestrengthofthepipematerial.Thisrupturewilloccurwhenthefluidpressure(P)reachesBRThus,fromEq.(4-2)theburstpressureis
Theworkingpressure(WP)isthemaximumsafeoperatingfluidpressureandisdefinedastheburstpressuredividedbyanappropriatefactorofsafety(FS).
Afactorofsafetyensurestheintegrityoftheconductorbydeterminingthemaximumsafelevelofworkingpressure.Industrystandardsrecommendthefollowingfactorsofsafetybasedoncorrespondingoperatingpressures:
FS=8forpressuresfrom0to1000Pa
FS=6forpressuresfrom1000to2500Pa
FS=4forpressuresabove2500Pa
Forsystemswhereseverepressureshocksareexpected,afactorofsafetyof10isrecommended.
ConductorSizingBasedonFlowRateandPressureConsiderations
Thepropersizeconductorforagivenapplicationisdeterminedasfollows:
1.Calculatetheminimumacceptableinsidediameter(Di)basedonflow-raterequirements.
2.Selectastandard-sizeconductorwithaninsidediameterequaltoorgreaterthanthevaluecalculatedbasedonflow-raterequirements.
3.Determinethewallthickness(t)oftheselectedstandard-sizeconductorusingthefollowingequation:
4.Basedontheconductormaterialandsystemoperatingpressure(P),determinethetensilestrength(S)andfactorofsafety(FS).
5.Calculatetheburstpressure(BP)andworkingpressure(WP)usingEqs.(4-3)and(4-4).
6.Ifthecalculatedworkingpressureisgreaterthantheoperatingfluidpressure,theselectedconductorisacceptable.Ifnot,adifferentstandard-sizeconductorwithagreaterwallthicknessmustbeselectedandevaluated.Anacceptableconductorisonethatmeetstheflow-raterequirementandhasaworkingpressureequaltoorgreaterthanthesystemoperatingfluidpressure.
ThenomenclatureandunitsfortheparametersofEqs.
BP=burstpressure(Pa,MPa)
Di=conductorinsidediameter(in.,m)
D0=conductoroutsidediameter(in.,m)
FS=factorofsafety(dimensionless)
P=systemoperatingfluidpressure(Pa,MPa)
S=tensilestrengthofconductormaterial(Pa,MPa)
t=conductorwallthickness(in.,m)
WP=workingpressure(Pa,MPa)
=tensilestress(Pa,MPa)
EXAMPLE1-3
Asteeltubinghasa1.250-in,outsidediameteranda1.060-in,insidediameter.ItismadeofSAE1010deadsoftcold-drawnsteelhavingatensilestrengthof55.000Pa.Whatwouldhethesafeworkingpressureforthistubeassumingafactorofsafetyof8?
SolutionFirst,calculatethewallthicknessofthetubing:
Next,findtheburstpressureforthetubing:
Finally,calculatetheworkingpressureatwhichthetubecansafelyoperate:
UseofThick-WalledConductors
Equationsandapplyonlyfor
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