管道应力师应知问题集锦.docx
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管道应力师应知问题集锦
AUDITFORCHINESESTRESSENGINEER
A)UNDERSTANDINGCAESARII
1.WhatkindofcalculationandevaluationisnotcoveredbyC2inpipingsystem?
蠕变分析,管道或设备的局部应力分析,管道失稳等
HowtocheckifpipingsystemexposetobucklingproblembyusingC2?
负压、偶然荷载、集中荷载、埋地管的土壤约束等荷载都会引起管道的失稳;
但CAESARII并不能评定管道的失稳(需要借助压力容器设计软件进行管道的失稳设计);
CAESARIIaddressesnegativepressuresasfollows:
theabsolutevalueofthelongitudinalpressurestress(PD/4t)termisaddedtotheappropriatecodeequations;pressurethrustforcesappliedtoexpansionjointendswillbecompressive;andbucklingisnotaddressedinCAESARII
临界失稳压力:
E—弹性模量(MPa);
I—截面惯性矩(mm4/mm);
v—泊松比,0.35—0.45;
Dm—管道平均直径;
C—椭圆修正系数,
,r1—椭圆管道的弯曲半径,r0—圆形管道半径;
允许的管壁冲击荷载:
2.Global/localloadsinC2output,Thereasonwhyweneedthelocalelementsloads?
为了获得作用在设备管口上准确的外载,以进行局部应力分析;另外通过局部坐标推力的分析,可以仔细察看管道单元内部的相互作用关系,协助发现问题,并尽快找到答案。
3.Oninputsheet,whatisdifferentforForce,Uniformload,Rigidelementwithweight.-thetemperaturebaseofElasticmodulusinput?
集中力(力或弯矩作用于管道上一点,一般模拟外载力的作用效果,对于一些额外重量,如支架根部,最好不用力来输入,因为如果计算地震,这些质量都不存在。
)、均布载荷(荷载均布于管道所有点上,我们一般用它来模拟雪载)、带重量的刚性件(横向刚度无穷大横向没有弯曲,刚性件只传导位移和力,我们一般用它来模拟阀门,管道设备等,但要注意,刚性件有直径和壁厚,他们对他的横向变形会有影响。
)。
软件缺省的弹性模量是冷态的。
但用户可以选择。
4.Whatisnon-linearsystem?
Areathereanyspecialcautionsininput/outputfornon-linearcase?
非线性系统就是:
依据胡克定律F=K*X,我们认为k是变化的。
管道在使用支架、吊杆,支架加间隙,摩擦力的情况下,系统变成非线性的。
管系从冷态—>热态或热态—>偶然工况过程中由于非线性约束的作用导致管系刚度发生改变;
如果你输入过多的非线性支架,可能导致软件计算不收敛。
陷入死循环。
我们一般先判断,根据直观感觉决定一些点,加入非线性约束。
其他处根据计算结果,在调整支架。
5.Howtocheckthelift-offsupportpoint?
WhatshouldEngineercheckforlift-offproblem?
Whatishotsustainedstress?
检查约束报告,垂直方向受力为0的点即支架托空点;
支架托空点应考虑采用弹性支吊;
Hotsustainstress—计算热态持续应力。
依据B31.3AppendixP,用户可以校核热态持续应力。
主要是考虑支架脱空情况下持续应力。
GenerallyHotsustainedstressiscarriedoutbyremovingthesupportatthepointofliftoff,&checkforsustainedstresses.Myquestionis:
-Isthereanyguidelinesoranyproperbasisavailableforneglectingsmallliftoffforhotsustainedstresscheck.IassumethatthemeaningofsmallliftoffmeansLiftoffbysmallpositivedisplacement..
Supposeu'rsystemhasthreecases
1operating
2design
3Upset
Andinthesystemu'rtwosupportaregoingtoliftoff(maybebysmalldisplacement)andduetotheremovalofthesetwosupportu'rsystemgetfailedinsustainedcase...Whatucandohereisjustcheckwhetherthessetwosupportsareliftinginthesamecaseordifferent..iftheyliftindifferentcasecheckforremovalofonesupportatonetime...inmyviewushouldnotignoresmalldeflectionbecausetheywillactasconstantactingload....whichwillcausefailure
6.WhatisLiberalstressallowable?
一次应力的余量可以添加到二次应力的许用应力上来,我们将这个余量称为Liberalstressallowable。
Perhapsitwouldbeofsomebenefittoreviewthemeaningoftheterm?
liberalallowablestressrange?
.Firstofall,thisisnotatermthatisusedbytheASMEB31PressurePipingCodes.CaesarII(C2)usesthetermasasortof?
short-hand?
forreferringtotheincreaseinthermal(displacement)allowablestressrangethatisallowedinB31.1(paragraph102.3.2(D))andB31.3(paragraph302.3.5(d).Intheseparagraphs,theB31Codesallowthepositivedifferencebetweenthecalculatedcombinedlongitudinalstresses(duetoweight(bending)andlongitudinalpressure)andthemaximumallowablestress,Sh,tobeaddedtotheterm?
0.25Sh?
(thesecondterm)intheequationforcalculatingthemaximumallowablestressrange,SA.TheB31Codessaythatit?
MAY?
beaddedintothe?
0.25Sh?
term,therebymakingitoptional.Iftheanalystelectstoignoretheadditionalallowablestressrangethedecision(allotherthingsbeingequal)wouldbejudgedconservative.Iftheanalystelecttoemploythisadditionalallowablestressrange,theresultingallowablestressrangeissometimestermed?
liberal?
.Itis,ofcourse,allsemantics.TheCodeallowstheadditionalallowablestressrangeforaverygoodreason.
TheconceptunderlyingthisruleisverywelldescribedinthefamousbookbyS.W.Spielvogle(PipingStressCalculationsSimplified,FifthEdition,1955).SpielvogleexplainsthattheB31rulesintendfortheanalysttobeabletousetheentirerangeofstressfromthematerialyieldpointattheoperating(hot)temperaturetothematerialyieldpointattheambient(cold)temperature(lessafactorofsafety).SinceSh(neglectingthepossibilityofcreep)issetat2/3Syforboththehotandcoldconditions,wecancalculatethehotyieldstressasSh*1.5andwecancalculatethecoldyieldstressasSc*1.5.Takentogetherthetotalallowablestressrangeforthecombinedloadingsofweight(bending),longitudinalpressure(tension)andthermalexpansion(displacement)wouldbe(1.5*Sc)+(1.5*Sh),or1.5(Sc+Sh).Thisrangeofallowablestresshasbeenreducedslightlytoallowforthevagariesofmaterialandforother?
realworld?
inaccuracies.TheCodephilosophywouldthenpermitthetotalallowablestressrange(afterthefactorofsafetyisapplied)forallthecombinedloadingdescribedabovetobe1.25(Sc+Sh)(if,inthisdiscussion,weneglectthestressrangereductionfactorforsimplicity).ButtheCodeuses1.0Shforthesustainedloadingsofweightandlongitudinalpressureandthisleaves1.25*Sc+0.25*Shfortheallowablethermalexpansion(displacement)stressrange(betyouwonderedwherethatcamefrom).BecausetheCodeintendsfortheentirestrengthofthematerial(fromhotyieldtocoldyield)tobeused(exceptforthe?
adjustment?
madeforvagaries),itfollowsthattheruleintheparagraphscitedaboveallowstheanalysttoputtheunused(differencebetweencalculatedsustainedlongitudinalstressesandtheallowable1.0*Sh)portiontouseinincreasingtheallowablethermalexpansion(displacement)stressrange.Youwillrecognizethatthe?
excess?
sustainedcaseallowablestresswillvaryacrossthesystembeinganalyzedandthatthevariationwilldirectlyreflecthowwellsupportedthesystemis(bendingstresseswillhavethegreatereffect).Thisvariationin?
excess?
sustainedcaseallowablestressfromnodetonodeinthemodelwill(whenthe?
liberal?
optionisused)resultintheallowablestressrange,Sa,beingdifferentateverynodewhentheCodecompliancereportisviewed.
So,onemightask,whywouldananalystopttonotusethe?
liberal?
allowablestressrangeforcomparisontocalculatedexpansion(displacement)stressrange?
Thisisanengineeringjudgement.Forexample,ifthesustainedstresseswerecalculatedas80or90percentofShandthesystemwereoperatedinthematerial?
screeprange,thedesignermightwanttotaketheconservativedecisiontonotusethe?
liberal?
allowablestressrangewhenevaluatingthermal(displacement)stressranges.Anotherexamplemightbeofferedasacasewhenthesystemisinseverecyclicservice(seeB31.3paragraph300.2forthedefinition)andthedesignerislookingforalongerfatiguelife.Goingthe?
conservative?
routemightalsoappealtothedesigner(orowner)ifthesystemwouldbeoperatingwithinthepressure/temperaturevariationsdescribedinparagraph302.2.4inB31.3orparagraph102.2.4inB31.1.Ifwehavesomedegreeofuncertainty,weemployanadditionalmeasureofconservatism.Asthesayinggoes,?
whenindoubt,builditstout?
.
Thankyouforbringingupthetopicfordiscussion.Goodluckwithyourprojects.
Ofcourse,alltheaboveisjustmyopinionanddoesnotreflecttheopinionofASMEoranyCodeCommittee.John.
AconservativeformulationoftheallowableexpansionstressrangeformanycodesinCAESARIIiscalculatedfrom:
f(1.25Sc+.25Sh)Whentheuserrequeststhatthe"LiberalAllowable"beused,thedifferencebetweenShandSl,providedSh>Sl,willbeaddedtotheterminsidetheparenthesis,i.e.SA(Liberal)=f[1.25Sc+.25Sh+(Sh-Sl)]Theliberalexpressionwillonlybeemployedwhenthereisatleastonesustainedstresscaseintheloadset.Ifthereismorethanonesustainedstresscaseinasingleproblem,thenthelargestofSl,consideringallofthesustainedcases,foranysingleelementendwillbechosentosubtractfromSh.Becausethesustainedstressvariesfromonepipetoanother,theallowableexpansionstresswillalsovary.
7HowtoincorporatetheinstallationtemperatureinC2?
可在CII环境变量中更改环境温度;环境温度在管道应力分析中是个很重要的设计条件,我们一般会去年平均的温度来进行考虑。
真正的环境温度是指管道安装最后焊缝施工时的环境温度,这样就可以决定安装的初态。
8.Whatisthermalbowingeffect?
管道暴露在阳光下,管道上部的温度和下部的温度会不同,管道会在这个温度作用下出现热拱效果。
9.WhatkindofpipingsystemshouldtheBourdoneffectbeconsideredto?
包尔登效应,盘管在压力作用下有伸直的效果。
一般是指弯头在压力作用下,有要伸直的效果。
Thereisapressure-sensingdevicecalledabourdontubewhichisacoiledflexibletubewithfluidpressureontheinside.Aspressureincreases,thecoiltendstowanttostraightenoutagainstspringpressure(sincethereisabiggersurfaceareaaroundtheoutsidecurveofthecoilthantheinsidecurveofthecoil...withsamepressureappliedonboth).
Iwouldassumethatapipingelbowwantstostraightenoutforthesamereason.Thehigherthepressure,thehighertheforce...alsoprobablydependsonthegeometryoftheelbow.
thebourdoneffectisthedifferencebetweenthepressureinsidethetubeorpipeinrelationtothepressureoutsidethetubeorpipe.Iftheinsidepresurerisgreaterthantheoutsidepresurethetubeorpipewillexpand.Andifthetubeorpipeisbentinacircularform,theexpansionisnoticableandcanbeconvertedtoacalibratedreadingasinapressuregauge.
Electricpeteiscorrect.Internalpressurecanaffectthepipebendorelbowin2ways.TheBourdoneffectdescribesthetendencyforthepipebendto"open"underinternalpressureand"close"underexternalpressure.ThereasonisasdescribedbyElectricpete.Theotheref