薄壁容器的应力分析.ppt

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1、第四章第四章 压力容器设计压力容器设计CHAPTER 4Design of Pressure VesselMechanical Design of Process EquipmentMechanical Design of Process EquipmentOurtopiccoversthoseaspectsofthemechanicaldesignofchemicalplantthatareofparticularinteresttochemicalengineers.Themaintopicconsideredisthedesignofpressurevessels.Thedesignof

2、storagetanks，centrifugesandheat-exchangertubesheetsarealsodiscussedbriefly.Thechemicalengineerwillnotusuallybecalledontoundertakethedetailedmechanicaldesignofapressurevessel.Vesseldesignisaspeciafisedsubject，andwillbecarriedoutbymechanicalengineerswhoareconversantwiththecurrentdesigncodesandpractice

3、s，andmethodsofstressanalysis.However，thechemicalengineerwillberesponsiblefordevelopingandspecifyingthebasicdesigninformationforaparticularvessel，andneedstohaveageneralappreciationofpressurevesseldesigntoworkeffectivelywiththespecialistdesigner.ThebasicdataneededThebasicdataneededbythespecialistdesig

4、nerwillbe：1.Vessel function.2.Process materials and services.3.Operating and design temperature and pressure.4.Materials of construction.5.Vessel dimensions and orientation.6.Type of vessel heads to be used.7.Openings and connections required.8.Specification of heating and cooling jackets or coils.9

5、.Type of agitator.10.Specification of internal fittings.Themisnostrictdefinitionofwhatconstitutesapressurevessel，butitisgenerallyacceptedthatanyclosedvesselover150mmdiametersubjecttoapressuredifferenceofmorethan1barshouldbedesignedasapressurevessel.It is not possible to give a completely comprehensi

6、veaccount of vessel design in one chapter.The designmethods and data given should be sufficient for thepreliminarydesignofconventionalvessels.Sufficientforthechemicalengineertocheckthefeasibilityofaproposedequipment design；to estimate the vessel cost for aneconomicanalysis；andtodeterminethevesselsge

7、neralproportions and weight for plant layout purposes.For amoredetailedaccountofpressurevesseldesignthereadershould refer to other useful books on the mechanicaldesignofprocessequipment.Anelementaryunderstandingoftheprinciplesofthe“StrengthofMaterials”（MechanicsofSolids）willbeneeded to follow this c

8、hapter.Readers who are notfamiliar with the subject should consult one of themany textbooks available；such as those by FaupelandFisher（1981）.ThebookbyFaupelandFisherisparticularlyrecommendedasageneralintroductiontomechanicaldesignforchemicalengineers.Classification of pressure vesselsForthepurposeso

9、fdesignandanalysis，pressure vessels are sub-divided into twoclassesdependingontheratioofthewallthickness to vessel diameter：thin-walledvessels，withathicknessratiooflessthan1：10；andthick-walledabovethisratio.Theprincipalstressesactingatapointinthewallofavessel，toapressureload，areshowninFigure3.1.Ifth

10、ewallisthin，theradialstress3willbesmallandcanbeneglectedincomparisonwiththeotherstresses，andthelongitudinalandcircumferentialstresses1and2canbetakenasconstantoverthewallthickness，Inathickwall，themagnitudeoftheradialstresswillbesignificant，andthecircumferentialstresswillvaryacrossthewall.Themajorityo

11、fthevesselsusedinthechemicalandalliedindustriesareclassified as thin-walled vessels.Thick-walled vessels areusedforhighpressures.Membrane stresses in shells of revolutionAshellofrevolutionistheformsweptoutbyalineorcurve rotated about an axis.（A solid of revolution isformedbyrotatinganareaaboutanaxis

12、.）Mostprocessvesselsaremadeupfromshellsofrevolution：cylindricalandconicalsections；andhemispherical，ellipsoidalandtorisphericalheads.The walls of thin vessels can be considered tobe“membranes”；supporting loads without significantbending or shear stresses；similar to the walls of aballoon.The analysis

13、of the membrane stresses induced inshellsofrevolutionbyinternalpressuregivesabasisfordeterminingtheminimumwallthicknessrequiredfor vessel shells.The actual thickness required willalso depend on the stresses arising from the otherloadstowhichthevesselissubjected.Consider the shell of revolution of ge

14、neral shapeshowninFigure，underaloadingthatisrotationallysymmetric；thatis，theloadperunitarea（pressure）on the shell is constant round thecircumference，butnotnecessarilythesamefromtoptobottom.第一、第二曲率半径第一、第二曲率半径LetP=pressure，t=thicknessofshell，=themeridional（longitudinal）stress，thestressactingalongameri

15、dian，=thecircumferentialortangentialstress，thestressactingalongparallelcircles（oftencalledthehoopstress），R1=themeridionalradiusofcurvature，R2=circumferentialradiusofcurvature.或当角度（弧度）很小时，sinxx（拉力的一个分量）两个分量，平衡力大小相互抵消微元近似一个平面，这是微积分的思想两个分量，平衡力大小相互抵消微元近似一个平面，这是微积分的思想Equations13.5and13.6arecompletelygene

16、ralforanyshellofrevolution.13.513.6Cylinder（圆筒形壳体圆筒形壳体）Acylinderissweptoutbytherotationofalineparalleltotheaxisofrevolution,so:whereDisthecylinderdiameter.Substitutioninequations13.5and13.6gives:带有椭圆孔的带有椭圆孔的 薄板上的应力集中薄板上的应力集中Sphere（球形壳体球形壳体）hence:Cone（锥形壳体锥形壳体）Aconeissweptoutbyastraightlineinclinedan

17、angletotheaxis:Substitutioninequations13.5and13.6gives:ThemaximumvalueswilloccuratEllipsoid（椭圆形壳体，封头椭圆形壳体，封头）Foraellipsewithmajoraxis2aandminoraxis2b，itcanbeshownthat（seeanystandardgeometrytext）：Fromequations13.5and13.6Atthecrown（top）Attheequator（bottom），soItshouldbenotedthatif1，willbenegative（compr

18、essive）andtheshellcouldfailbybuckling.Thisconsiderationplacesalimitedonthepracticalproportionsofellipsoidalhead.Torispherical heads（准球形封头，准球形封头，碟形封头，折边封头碟形封头，折边封头）Atorisphericalshape，whichisoftenusedastheendclosureofcylindricalvessels，isformedfrompartofatorusandpartofasphere.Theshapeisclosetothatofa

19、nellipsebutiseasierandcheapertofabricate.istheknuckleradius（theradiusofthetorus）andthecrownradius（theradiusofthetorus）.Forthesphericalportion：Forthetorus：dependsonthelocation，andisfunctionofand；itcanbecalculatedaswell.Theratiooftheknuckleradiustocrownradiusshouldbemadenotlessthan6/100toavoidbuckling

20、.Thestresswillbehigherinthetorusthanthesphericalsection.andthecrownradiusshouldnotbegreaterthanthediameterofthecylindricalsection.Secondary stresses（二次应力二次应力）1In the stress analysis of pressure vessels andpressurevesselcomponentsstressesareclassifiedasprimary or secondary.Primary stresses can bedefi

21、nedasthosestressesthatarenecessarytosatisfytheconditionsofstaticequilibrium.Themembranestresses induced by the applied pressure and thebendingstressesduetowindloadsareexamplesofprimary stresses.Primary stresses are not self-limiting；if they exceed the yield point of thematerial，gross distortion，and

22、in the extremesituation，failureofthevesselwilloccur.Secondary stresses（二次应力二次应力）2Secondarystressesarethosestressesthatarisefromtheconstraintofadjacentpartsofthevessel.Secondarystressesareself-limiting；localyieldingorslightdistortionwillsatisfytheconditionscausingthestress，andfailurewouldnotbeexpecte

23、dtooccurinoneapplicationoftheloading.The“thermalstress”setupbythedifferentialexpansionofpartsofthevessel，duetodifferenttemperaturesortheuseofdifferentmaterials，isanexampleofasecondarystress.Secondary stresses（二次应力二次应力）3Other sources of secondary stresses are theconstraintsarisingatflanges，supports，a

24、ndthechangeofsectionduetoreinforcementatanozzleoropening.Thoughsecondarystressesdonotaffectthe“burstingstrength”ofthevessel，theyareanimportantconsiderationwhenthevesselissubjecttorepeatedpressureloading.Iflocalyieldinghasoccurred，residual stress will remain when thepressureloadisremoved，andrepeatedp

25、ressurecyclingcanleadtofatiguefailure.一次应力与二次应力一次应力又叫基本应力，它是由外载荷引起的。这类应力的出现是为了抵抗外载对容器的破坏作用，它们随外载的增大而增大，当外载增大到使这些应力达到材料的屈服极限时，容器筒壁就屈服。当外载继续增加时，这类应力也会因材料的强化而继续上升，直至达到材料的强度极限，容器发生破裂为止。二次应力是由于容器部件的自身约束或相邻部件的相互约束而产生的正应力或剪应力。这类应力不是为平衡外载而产生，而是由于受到自身或外部的约束而引起。如果限制增大到使应力达到了材料的屈服极限，使器壁金属在这些受限制的区域发生了塑性变形，那么这里的限制将被突破，相互的约束得到缓解，应力便会自动地限制在一定范围之内。所以二次应力有时被称为“自限性”应力。取一段单位长度圆环取一段单位长度圆环