压力密封及静压轴承

-1Notes12.AnnularpressuresealsandhydrostaticbearingsFromSanAndrés,L.,2006,IntroductiontoPumpRotordynamics,Lecture3.vonKarmanInstitute-RTOLectureSeries,RTO-MP-AVT-143,DESIGNANDANALYSISOFHIGHSPEEDPUMPS,(REPRODUCEDwithpermission)ABSTRACTThelectureintroducesannularsealsandhydrostaticbearingsinliquidpumps.Theanalysisdetailsthephysicalprincipleforgenerationofadirectstiffnessinthesemechanicalcomponents.Annularsealsasneckringsealsandinterstagesealsrestrictleakagebutalsogenerateforcecoefficients,stiffness-damping-inertia,greatlyaffectingtherotordynamicsofliquidturbopumps,inparticularthosehandlinglargedensityfluids.Highlightsonthebulk-flowanalysisofannularsealsaregivenwithdetailsontheperformanceoftwowaterseals–longandshort,featuringtheadvantagesofananti-swirlbraketoenhancethesealrotordynamicstability.Hydrostaticbearingsrelyonexternalfluidpressurizationtogenerateloadsupportandlargecenteringstiffnesses,evenintheabsenceofjournalrotation.Theloadcapacityanddirectstiffnessesofhydrostaticbearingsdonotdependonfluidviscosity,thusmakingthemidealrotorsupportelementsinprocessfluidpumps.Currentapplicationsintendtoreplaceoillubricatedbearingwithhybridbearingstoimproveefficiencywithshortenrotorspansandlessmechanicalcomplexity.CurrentcryogenicliquidturbopumpsimplementhydrostaticbearingsenablinganallfluidfilmbearingtechnologywithverylownumberofpartsandnoDNlimitoperation.Detailsonthebulk-flowanalysisofturbulentflowhydrostaticbearingsaregivenalongwiththediscussionofperformancecharacteristics,staticanddynamic,forhydrostaticbearingssupportingawaterpump.Angledliquidinjectionproducesahydrostaticbearingwithunsurpasseddynamicforceandstabilitycharacteristics.1.0ANNULARPRESSURESEALSINPUMPAPPLICATIONSSealrotordynamiccharacteristichaveaprimaryinfluenceonthestabilityresponseofhigh-performanceturbomachinery[1].Non-contactingfluidseals,asshowninFigure1,areleakagecontroldevicesminimizingsecondaryflowsinturbomachinesandtypicallyuseprocessliquidsoflightviscosityorprocessgassesastheworkingfluid.Annularseals,althoughgeometricallysimilartoplainjournalbearings,showadifferentflowstructuredominatedbyflowturbulenceandfluidinertiaeffects.Operatingcharacteristicsuniquetosealsarethelargeaxialpressuregradientsandlargeclearancetoradiusratios,whiletheaxialdevelopmentofthecircumferentialvelocityisofimportanceinthegenerationofcross-coupled(hydrodynamic)forces.Texturedstatorsurfaces(macroroughness)toreducetheimpactofundesirablecross-coupleddynamicforcesandimprovesealstabilityarebynowcommonpracticeindampersealtechnology[2].Furthermore,annularsealsasLomakinbearingshavepotentialapplicationassupportelements(dampingbearings)inhighspeedcryogenicturbopumpsaswellinprocessgasapplications(compressors)[3].Notes12.ANNULARSEALSANDHYDROSTATICBEARINGS.©Dr.L.SanAndres2006-2InterstageSealImpellerEyeSealBalancePistonSealFigure1:SealsinaMultistageCentrifugalPumporCompressorTheimportanceofsealflowphenomenonanditsinfluenceonthedynamicresponseofactualturbomachineryhavepromptedalargenumberoftheoreticalandexperimentalinvestigations.Seals,duetotheirrelativepositionwithintherotor-bearingsystem,canmodifysensiblythesystemdynamicbehaviorsincetheseelementstypicallyseelargeamplituderotormotions.Thisassertionisofparticularimportanceonback-to-backcompressorarrangements(seeFigure2).Furthermore,theforcecoefficients–stiffness,dampingandinertia-ofannularsealsinlargedensityliquidpumpscanbeaslargeasthosearisingintheoil-lubricatedbearings;thusthesealelementseffectivelybecomeloadpathsandmodifythepumprotordynamicbehaviour.“Wet”and“dry”criticalspeeds,i.e.thoseaccountingforseals’forcesandnot,canbemarkedlydifferentasnotedin[1,4].Figure2:Straight-ThroughandBack-to-backCompressorConfigurationsand1stModeShapesBlack[5]firstexplainedtheinfluenceofsealforcesontherotordynamicbehaviourofpumps.Since1980,Childsandco-workersatTAMUhaveconductedacomprehensiveprogramfortheanalysisandtestingofthedynamicforceresponseofliquidandgasannularseals.Experimentalprogramswithdampersealsfeaturingvariousstatorsurfacemachinedtextures(macroroughness),seeFigure3,haveconfirmedthebenefitofhighernetdampingforcesandlessleakagethaninsmoothsurfaceseals.Reference[2]detailsmajordevelopmentsingassealapplications,forexample.Notes12.ANNULARSEALSANDHYDROSTATICBEARINGS.©Dr.L.SanAndres2006-3Figure3:HoneycombsealforturbopumpThislecturepresents:a)Thephysicalmechanismbywhichadirectstiffnessarisesinannularpressuresealsevenwithoutjournal(shaft)rotation.Themodelanalyzestheflowbalanceandpressuredropsattheentranceofachannelandontheensuingthinfilmland.Amaximum(optimum)stiffnessisthenpredictedforacertainflowresistancebalancebetweentheentranceandlandpressuredrops.b)Briefdescriptionofthebulk-flowequationsforpredictionoftheflowandforcecoefficientsinannularpressureseals.c)Discussionofpredictionsfortwowaterseals,longandshort,forapplicationasneckringandinterstageseals.Theinfluencesofseallengthandinletswirlontherotordynamicforcecoefficientsarethoroughlydiscussed.RefertoChilds[1]andSanAndrés[6]foracriticalreviewofthearchivalliteraturerelatedtothechronologicaldevelopmentsi