EVALUATION OF FLANGED CONNECTIONS DUE TO PIPING L.OAO

7/27/2019 EVALUATION OF FLANGED CONNECTIONS DUE TO PIPING L.OAO

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EVALUATION OF FLANGED CONNECTIONS DUE TO PIPING L.OAOD. L. Mdnhtn , Dlvi1lon M1negerPiping ~ ~ n l c . l O!v!don

ABSTRACT

L.C. P111gPrincipal Salt EnglnMrTheM. W. KUaw ComPIOI'

l-iOU$ton, Tnaa

The pipe loading applied to flanged connactiona has been a p i ~ i n g engineer'a conetant concern, TRia p r e b l e ~ haa been treated by varioua authors,The lateat tenAency in evaluation ia leaning t ~ a r d tha eo called equivalentpressure approach. The paper deacribea briefly the two popular a p p r o ~ c h e a ,namely rating table ~ e t h o d and direct atreat calculation mtthod, With extentivaatreaa calculations performed on the atandard flange&, i t ia ahowa that thestandard flangea not only can taka the rating praaaure allowed, but also havereterve attength available to raailt aignificant pipe lo&dl. Caution i l al1ogiven to non-tandard flange designed by ASHE BOPV Code rulaa, Without properconsideration of pipe load, a euatom deaigned non-at1ndard flansa i l prona.tolaalt.NOMENCLATURE

D o ~ t a i d e d i a ~ e t e r of pipet Thickneaa of pipeP Internal preaaurePe Equivalent preuaure due to pipe loadM Bending m o ~ ~ ~ a n t from pipeF Axial force from p:l.peG Diameter at location of gasket load reactionb - Effective zakat vidthSll - Longitudinal acraaa in hubSll. R.adial atreu in flange

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S'f Tangential stress in t langeSc Basic nll01o1able s t ress a t ambient temjlerature (cold)Sb - Basic allowable stress a t ojllirating temperature (hot)INTRODUCTION

The pipe load applied to a flanged connection has been a constant concernto piping engineers. An excessive pipe load can c: \ ~ e leaking or even fa i lure .l'his problem hall been investigated by Mllrkl and George, Blick, and others (1,2).From the extensive tes ts made on 4 inch claas 300 ANSI flanges, Markl found thateven under unullually severe bending r>tresees, .flange oosemblies did not fa i l inth e f lange proper, or by fracture of th e bol ts , or by leakage across th e jo intface. Structural fai lure occurred almost invariably in th e pipe adjacent totile flange, and in rare instunccs, across an unusually 1Neak attachment weld.He stat.;,d ~ ' - ~ leakage well in advance of fai lure was observed only in the caseof thre flanges. Although th e tee t resul ts were def in i te , they are no meanstell:l tha t th e pipe load on a flanged connection should not be a concern.B c c a ~ he wide rw1ges of factors such as aize, class , type, and materia linvoh ,, a lea/;( or fai lure can s t i l l occur a t fla:oge connections if the pipelr'ld i s exces!live. Proper precautions ahould be taken to ensure the integri tyo. ,e connections.The current tendency o evaluating the pipe load i s leaning toward theequivalent pressure approach. This conservative approach is relat ively simple.l l i s especia l ly s u i t ; ~ b l e for high volume dai ly production analyses, However,40 considerably dif feren t methods have e ~ r g e d from th e application of the


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(1)1\"Gb

After simplification, Equation (1) becomes(2)

Above equivalence i s b.lieved to be conservative bee11use tlle Wl:dmum gasket stress produced by the pipe load exists only at the BXtreme edge of thegasket, ~ h e r e a s th e stress generated by the pressure is uniform, Equation (2)can be considered as being in the same spirit as the area replacement approachused in the deslgn of the re-tnforcement Rt branch connections, It i s simple andre l iable .By converting the pipe load totile s tandanl problem of f lange undersolved by ASME B&PV code procedure.becomes

Ptotal - P + Pe

equivalent pressure, the problem becOmesin ternal pres11ure which can be readi lyThe total equivalent design pressure

(3 )Equation (3 ) las been used by th e petrochemical industry (4) fo r more than30 years, and wan formally adopted by the nuclear pipe Code (S) for evaluatingclass Z piping flange connections, In fact Equation (Z) has gained such apopularity that it is altlo used in converting torsional moment to equivalentpressure {5), although the nature of the stress generated from torsion is different from the one created by pressure.

A P P L I C A T I O ~ OF EQUIVALENT P R E S S L ~ EWith th e total equivalent pressure determined, the rest of the evaluationis strai-ght forward. One can perform a stress calculation based on this pre.ssure or simply select a [lange that has th e rating suitable fo r this pressure,Although both methods are acceptable the results c.an be quite different. Thisdifference can be i l lustrated by the following example: Assuming there is amoderate pipe load of 8400 lbf-f t (11390 K-M) bending moment acting on a 8,625inch (219mm) outside diameter, 0.322 inch (8.l8rm:n) thick pipe opernting a t300F ( 1 4 ~ C ) t ~ ~ p e r a t u r e and 200 psi (1379 KPa) internal pressure. The bendingmoment will produce a bending stress of 6000 psi (41364 KPa) in the pipe. Ifa flanged connection is needed at this point, a design engineer will take theoperating pressure and temperature and look up th e rating table from standards

such as A::trcss calculation willshow th


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(a) Longitudinal s t ress in hub, SH(b ) Radial atJ:eas in flallge, SR(c) Tangential s t ress in flange, ST

These atJ:eases can be easily calculated using the Code Rules (3 ) baaed onthe bending moment acting a t the flauge bol t ciJ:cle circumference .However,the most important point for pJ:actical design i s to establ i sh a proper allowable s t ress . For example, the Class 2 Nuclear Piping Code (5 ) l ~ i t s each ofthe above stresses to 1. 5 times the b.1.aic allowable s t ~ : e a a a t operat ing tempel'ature, I t alao requires tha t longitudinal pressure s t ress be included in thelongitudinal hub stresS. The basic allowable s t ress is the code tabulateds t ress mainly to be used in i n t e ~ : n a l pressure design,Most flange designs, other than for nuclear as above, 18 covered by ANSI/ASHE Sec. VI I I - Div. 1 (3) for vessels, and by ANSI/ASME B)l Piping Codes (7 )for piping. Rules for allowahle J > t r e ~ > s e s are contained in each of the pert inentCodes. The piping Codes refer back to ref , (3 ) for s t ress calculat ion procedure,Allowahle s t resses for flanges (other than cast iron flangea) are generally asshown below.

sH equal to O< leii


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for the AFl STD 605 flaugea as shown in Figure 3, If also decreaaea for thehigher class flanges. Figure 4 shows th e relation for ANSI B16,5 class 300flanges. 1 t is clear that the r e s e ~ : v e strength of a class 300 dip-asses sufHcient strength to resii:.t th e rated pressure losd plua sustantilllreserve strength to res is t th e pipe load. This reserve strength varies fromflange to flange and is not known unti l a stress analysis is performed, I tis also highly dependent on th e thickness of the connecting pipe when a boreof the flange is specified to be th e same as the inside diameter of the pipe.ln designing a special flange, i t is necessary to provide some allowancefo r the pipe load. The allowance can be either based on the ~ c t u a l expectedpipe load or based on the load tha t will produce s bending stress equivalentto one half of the basic allowable stress at the connecting pipe. ln anycase care should be exercised in the f inal piping system design to keep thepipe load from exceeding the allowance provl,Je.d,REFERENCES

1. Markl, A.R:C. and George, H.U., "Fatigue Testa on Flanged Aaaembl-

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iea" Trans. ASME Vol. 72 No. 1, Jan . , 1950, PP, 77-87,2. Blick, R,G., "Bending Moments and Laakaga a t Flanged Joints ,"Petroleum Refiner, Vol, 29, 1950, pp , 1 2 ~ 1 3 3 ,

3. ASME, Boiler and Pressure Vessel Code, Section VIII , Division l ,Appendix 2.4. M. W. Kellogg, "Local Components," Design of Piping Systems, Revised2nd ed. , John Wiley & Sons, Inc . , New York, 1956,5. ASME, Boiler and Pressure Vessel Code, Section III, Division l ,Subsection NC.6. ANSI Bl6. 5, "Steel Pipe F l a n g e < ~ and Flanged Fi t t ings , 1 published byASHE.

1. ANSI B31, "Code for Pressure Piping," published by ASME,8. API STD 605, "Lar.gc-Diameter Carbon S ted FlangeD,'' Americnn Petroleuill Institute.

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30000

~ 20000~ ~,;"" 10000

0

15000

10000~~

'" 5000"

0

Allowable Stress- 1.5 x 2 1 = ~ ~ 31950 p : ~ ~-- - -- - -t--t- -t -+- -r-ANSI Bl6.S CltH>s 150 Welding NeckASTM A-105, 1/16'' Asbe6tos G11sket230 ps i a t 3011'F ( 1 4 ~ C )

Reserve Strength Available fo r Pipe Load .L++- - -- -- -- -I 1--- --1-- j""Flan>';e s t re ss due to pressure2 4 6 8 10 12 14 16 18 20 22 ,,,

Pipe Sizes , (inch)Figure 1. A:-;sr JH6.5 Class-150 Flanges

(1 inch " 25.4mm, 1 psi "' 6.89!! KPa)

.A.'


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30000

20000

10000

0

30000

" 20000"'"

10000

0

1--

26

' - . Flange Stress DLte to Pressure ,

30

;\PI STO 605, Class 150AS'Il1 A-105, 1/16" Comp. Asbestos260 pai a t JOQPF (1490C)Bore i s so specified that pressurehoop stress is about 10000 psi

42 " 54Pipe Sizes, (inchFigure J. API Std-605 Class-150 Flanges{1 inch 25.4mm, 1 ps i 6.894KF_a)

- ..;: ~ K . : : l ~ : i u s s r t : t : : P a ~ T _--(5)_{. r -c lass 2 Nuclear Piping A l l o ~ a b l e,__ r--- --/ ...._Slip-Qn(L-oose Ty?e)

/ II I -:- U:, Flange ress "\. I-' Welling ti!ckDue to Pressllre- - - - - ~ S l Bl6, 5, Class 300, ASTM A-1051/16" compressed asbestos gasket

535 ps i a t 7000F (371)xs ~ e i g h t pipe

2 4 6 8 10 12 14 16 18 20 22 24Pipe Sizes, (inch)

:figure 4. M St Bl6.5 Clasa-300 Flanges(1 inch 25.4mm, l ps i 6.894KPa)

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A ~ S l B16.5 Class 1500, Welding Neck30000 ASTM A-182 Gr F-22, Sch 160 PipeP 1340 psi a t 1 0 0 0 0 F ~ : t 5 3 a C )

r- M . . . ! : ! . : . . . . ~ 1 W L ~ e s t e W ~ a a ~ - - r--20000

~";'" 10000" Class 2 Nuclear Piping Allowable (5 )- .-,. -" v - t ~ a n g e Stresa Due to Pressure0

2 4 6 8 10 12 14 16 18 20 22 24Pipe Shes, (inch)

Figure 5. ANSI Bl6.5 Class-1500 Flanges(1 inch - 25.4mm, 1 psi 6.894 KPa)

EVALUATION OF FLANGED CONNECTIONS DUE TO PIPING L.OAO

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