chương 4- giãn nở nhiệt đường ống.pdf

8/11/2019 Chng 4- gin n nhit ng ng.pdf

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Chng 4Gin n nhit ng ng

- Schnh lchnhitcah thngngnghocnhitcathitbtrn ngngkhi vnhnh vi nhit mi trng s gy ra nhngchuyn v (dch chuyn) trong ng ng.Chuyn v ny gi l chuyn v nhit (thermal

displacement)- Chuynvnhitsinh ra cc lcko, nn, m men

un, mmen xon tc dng ln ng ng, gi,gi treo

- Do vycnphitnh ton xc nhcc chuynvnhit kim nghim bn ng ng v v trni, tnh ton thitkh thnggi treo, ging thi a ra cch b tr ng hp l nhmgimbttc ngcachuynvnhit.


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4.1 Chuyn v nhit v mmen lin kt

4.1.1 Chuyn v nhit

- Chuynvnhitcangngtheo phngdctrcctnh ton theo cng thc

Trong : (in): chuynvnhit (in/in.0F): hschuynvnhit L0(in): chiudi ban ucang T (0F): chnh nhit


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- ng sut sinh ra trong ng do chuyn v nhit

T thy rng, nu s chnh nhit T ln cth sinh ra ng sut vt qu ng sut cho phpv dn n ng b ph hy.


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4.1.2 Mmen un chuyn v nhit

M men ni lc ti v tr lin kt ngm (v tr nivi thit b) trn ng ng khi c chuyn vnhit c tnh nh sau

Trong : M (lb.in): m men ni lc ti v tr lin kt ngm trn

ng ng E (psi): mu un n hi ca vt liu I (in4) : m men qun tnh ca mt ct ngang ca ng L (in) : chiu di on ng hp th chuyn v nhit

(in) : chuyn v nhit


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MA=6.E.I.y /L2

AB

MC=6.E.I.x /L2BC


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MA=6.E.I.1 /a2

MC=6.E.I.2 /b2


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4.2 iukindocangng

- ngngnigia2 thitblm vicnhit

cao schu cc ngsutdo chuynvnhit,do khi thit k ng ng ngoi vic tnh tonngngtheo p suttrong cn phich ccninggia2 thitbnhmgimngsutnhitnmcnhnht.

- Bin php n gin nht khi ni ng gia haithitbl ingtheo hnh chL, Z, U


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- iukin theo kinh nghimmbodokhi chyng

D./(L-U)2 30SA/EcTrong :

D (in): ngknh ngoi cang L (in): tngchiudi cangng U (in): khongcch giahai thitb (in) : chuyn v nhit ca ng ng ni

hai thitb SA (psi): ng sut cho php khi gin n

nhitcavtliu Ec(psi): muunnhicavtliunhit

vnhnh


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V d1: Hy kimtra iukindocasningsau v tnh m men untiA v B

ng NPS10, A-106B, nhit mi trng 700F,nhit lm vic 8000F, L1=12(ft), L2=13(ft),muun n hi E=29,7.106(psi), h s gin nnhit =7,75.10-6(ft/ft.0F), Sc =20000 (psi); Sh=10800 (psi), N=7000.


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- Tnh do khi chy ng theo hnh ch L v Z lnhnhau, nhngchyngtheo hnh Z hay c

sdngnhiuhn


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- Chiudi nghnh chU mbotnh dokhi cchuynvnhit

L=(W+2W)=(3..Ec.D/S)0,5Trong

L(in): chiudi onnghnh chU W(in) :chiu di onngchy song song vi

ngnihai thitb S (psi): ngsutcho php tinhitlm vic 2W(in): chiu di on ng vung gc vi

ngnihai thitb

D(in): ngknh ngoi cang (in): chuynv


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V d 2.

Cho mt ng ng NPS 10, L=475 ft, vn ticht lngc nhit2340F, nhitmi trng

700

F, ngsutcho php tinhitlm vicS=15000 (psi), muunnhiE=27,9.106(psi), hs gin n nhit =6,5.10-6 (in/in.0F). Xc nhkch thccaonngchU mbotnhdocho ngng


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- Trong trnghpcch btr ingkhng pngcyu cuv tnh dohocc shnchv

khng gian ing th ngita thngdng thitb b chuyn v nhit (Expansion Joint) gimbtngsutnhitsinh ra trong ng.


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4.3 Xc nh chuyn v nhit trong h thngngng

- Mcchcavicxc nhchuynvnhittronghthngngngnhmtnh ton, lachncc loigi treo v gingsao cho mboiukinbn.

- Quy tcxc nhchuynvnhit: chuynvnhittun theo quy lutbcnhtivichiudi ng.


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Cho h thngngngnhhnh v,hy tm cc chuynvtheo phng thng ng ti

cc v tr Hi (i=1-9) bit ccchuynvticc imA, B, C(nhhnh v)v chuynvtiH-4=0,=9,653.10-5 (in/ft.0F)T=980 0F


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Xc nh chuyn v H-1, H-2


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Xc nh chuyn v ti H-3


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Xc nh chuyn v ti J, H-5, H-6, H-7


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Xc nh chuyn v ti G, H


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Xc nh chuyn v ti H-7, H-8


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Bng tng hp chuyn v theo phng thng ng

chương 4- giãn nở nhiệt đường ống.pdf

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