9输送扩管

9SteppedPipelineSystems扩管系统1INTRODUCTION介绍Wheneitherahighpressureorahighvacuumisusedforpneumaticconveying,itisgenerallyrecommendedthatthepipelineshouldbesteppedtoalargerborepartwayalongthelengthofthelineatleastonce.Thisisthecasewhetherthematerialisbeingconveyedindiluteordensephase,andwhetherthepipelineislongorshort.Steppingofthepipelineisparticularlyrecommendedifthematerialbeinghandlediseitherabrasiveorfriable.Problemsofbotherosivewearandparticledegradationincreasemarkedlywithincreaseinvelocityandsosteppingthepipelinecanhaveaverysignificanteffectonlimitingconveyingairvelocityvalues,andhenceinminimizingthemagnitudeoferosionanddegradation.当使用高压或高真空用于气力输送，一般建议，管道应沿管长至少扩管一次。不管物料是稀相还是密相输送，也不管管道是长还是短。特别是处理的物料是研磨或易碎物料是，更应该扩管。速度增加了，冲蚀磨损和颗粒退化的问题的会明显增加，而扩管对限制输送空气流速特别有用，因此能尽量地减少磨损和退化的程度。Formanymaterialsitispossiblethatthelowervelocityprofileachievedinasteppedpipelinewillalsobringbenefitsintermsofimprovedconveyingperformance.Aparticularproblem,however,isinthelocationofsuchsteps,foriftheyareincorrectlylocated,pipelineblockagecouldresult.对于许多物料，扩管所取得的较低的流速分布也将带来输送性能改善的好处。然而，扩管的位置是个问题，因为如果他们位置不恰当，可能造成管道堵塞。Thecapabilityofpurgingmaterialfromasteppedborepipelineisanotherissuethatmighthavetobetakenintoaccount.Asituationinacontinuouspipelineinwhichthepipelinemayrequiretobereducedindiameter,ratherthanincreasedwhichisgenerallythenorm,iswherethepipelineincorporatesalongsectionofverticallydownwardflow.从扩管管道中清洁物料的能力是另一个的问题，可能需要加以考虑。而对长距离的垂直向下流动管道，可能需要缩小管道口径，而不是通常的扩管。2CONVEYINGAIRVELOCITY输送空气速度Forthepneumaticconveyingofbulkparticulatematerials,oneofthecriticalparametersistheminimumconveyingairvelocitynecessarytoconveyamaterial.Fordilutephaseconveyingthisistypicallyabout3000ft/mm,butitdoesdependverymuchuponthesize,shapeanddensityoftheparticlesofthebulkmaterial.Fordensephaseconveyingitcanbeaslowas600ft/min,butthisdependsuponthesolidsloadingratioatwhichthematerialisconveyedandthenatureoftheconveyedmaterial.Ifthevelocitydropsbelowtheminimumvaluethepipelineislikelytoblock.Itisimportant,therefore,thatthevolumetricflowrateofair,specifiedforanyconveyingsystem,issufficienttomaintaintherequiredminimumvalueofvelocitythroughoutthelengthoftheconveyingsystem.气力输送散料，其中一个重要参数是输送一种物料的最低输送空气流速。对稀相输送，典型的速度是约3000ft/mm，但它主要取决于散料粒子的大小、形状和密度。对密相输送它可低至600ft/mm，但是这取决于物料输送的混合比及物料的特性。如果速度低于最小值，管道可能堵塞。因此，空气的体积流量很重要，对任何输送系统，在输送线整个长度上都要足以维持所需的最低速度值。2.1CompressibilityofAir空气的压缩性ThefollowingequationswerepresentedinChapter5andarepresentedbelowforfurtherdevelopment.ThefirstoftheseisfromEquation5.2andrelatesvolumetricflowratewithconveyingairvelocity:下列等式在第5章中被提出，在这里做进一步推导。首先是由等式5.2而来的与输送气速有关的体积流量：ThesecondistheIdealGasLawfromEquation5.4:第二个等式是由等式5.4推导出的理想气体定律：ThethirdcomesfromEquations5.5and5.6andisthedirectderivativefromtheIdealGasLawthatequatesanytwopointsanywherealongthelengthofapipeline,andwillalsoequatetofreeairconditions:第三个等式来自等式5.5和5.6，是直接由理想气体定律推导而来，为管道管长的任何两点间的等式，同时也将等同于自由空气条件：UsingthisgroupofequationstheproblemofcompressibilitywithairinsingleborepipelineswasdemonstratedwithFigure5.6andthisispresentedhereinFigure9.1forreference.Afreeairflowrateof1500ftVminwasselectedandtheinfluenceofpipelineboreandpressureareclearlyillustrated.Thelinesofconstantpipelineborerepresentthevelocityprofilethroughapipelineinsingleborepipelines.Itwillbeseenthattheslopeofthelinesofconstantpipelineborechangeconstantlywithpressure,andastheairpressurereducestheslopeincreasesconsiderably.Theproblemofairexpansion,therefore,isverymarkedinlowpressuresystemsandparticularlysoinnegativepressuresystems[1]。通过这组等式，已经在图5.6中说明了单口径管道中，由于空气的压缩性所产生的问题，这里在图9.1中再次提出，以供参考。选定的自由空气流量为1500ft/min，而且清楚地说明了管道口径和压力的影响。恒定管道内径线代表了通过单口径管道的流速分布。可以看出很顶管道口径线的斜率随压力而不断地变化，而且当空气压力减低时，斜率大大增加。因此，空气膨胀的问题，在低压系统，尤其是负压系统中表现的非常明显[1]。Figure9.1Theinfluenceofairpressureandpipelineboreonconveyingairvelocityforafreeairflowrateof1500ft3/min.自由空气流量为1500ft3/min时，空气压力和管道口径对输送空气速度的影响。3STEPPEDPIPELINESYSTEMS扩管系统Figure9.1showsquiteclearlythenatureoftheproblemofsingleborepipelineconveying,withrespecttoairexpansionandhenceconveyingairvelocities,particularlywherehighpressuresorvacuumsareemployed.Forbothlongdistance,anddensephaseconveying,itisgenerallynecessarytohaveafairlyhighairpressureatthestartoftheconveyingline.Asthepressureoftheconveyingairdecreasesalongthelengthoftheline,itsdensitydecreases,withacorrespondingincreaseinvelocity,asillustratedabove.图9.1很清楚地显示了单一口径管道输送时由于空气膨胀而引起的输送空气速度变化问题的性质，特别是在采用高压力或真空输送时。对于长距离密相输送，一般需要在输送线始端有一个相当高的空气压力。如上所述，沿管道的长度随着输送空气压力的降低，其密度减小，同时速度相应地增加。Asimplemeansoflimitingtheveryhighvelocitiesthatcanoccurtowardstheendofapipelineistostepthepipelinetoalargerboreonceortwicealongitslength.Bythismeansitwillalsobepossibletokeeptheconveyingairvelocitywithinreasonablelimits[2].为了限制在管道末端可能出现非常高的速度，一个简单的手段是沿管道长度扩管一次或两次。以这种方式也将可能使输送空气流速保持在合理的限度内[2]。Theultimatesolution,ofcourse,istouseataperedpipeline,forinthistheconveyingairvelocitycouldremainconstantalongtheentirelengthofthepipeline.This,however,isneitherpracticalnorpossible,butitdoesprovidethebasisforamodelofwhatisrequired.Asteppedpipeline,therefore,shouldbedesignedtoac