Thcrm3J end reactions can be pre~ictedfor jacketed piping carrying a processfluid in an inner pipe heated by st8:1m

b:.:f',jeen the pipe and jackGt

JACKETED -PIPING requires special stress anaiysis: Com-monly used to convey very viscous pr~cess fluids in anmner pIpe, neated by steam between the outer jacket ans!inner pipe, thermal end reactions can be predicted. Vac-cuum jacketing is also used as an insulator for cryogenicfluicls and can be analyzed using the same calculationmethod for heated jacketed piping.By example, stress problems occuring with this type

of piping system will be descrihed. The example assumesa process fluid requiring const.1nt, uniform heat not avail-able through steam or electrical tracing systems. It furtherass,:mcs-the need for a stainless steel Lnner pipe or coreand an outer carbon steel pipe or jacket. Since both pipesare rigidly attached to flanges in spool pi.eces and eachpire has a difTerent. thermal e~pansion _coefficient, ~xial

- str~se~ will be _an_a_ly_·z_e_d_._

Example. An isometric dra\',ing of the sample problemis shown on Fig. 1. The construction is a three inchschedule '10 stainless steel core pipe with a four inchschedu1e ,10 carbon steel jacket. The jacket covers the corefor its entire length and flanges arc used for system dis-rnantlillg. The heating medium is 180 psi-saturated ste<\m;proper collection and removal of condens:lte has beenassumed. I t has also been assumed that the core pipeand j:lck~t pipe are at thenn:l! equilibrium.

Core/jacket stn:ss anaIJ~si". This ex:tIllple has a pro-cess core pres,ure of.3'O'·[?sig";jwith jacket pressure at.J8CJpsig, the.: core is su~ed to an' extern:ll pressurc of150 psig. The core pipe must be investigated for col-lapse or local buckling frolll the extemal pressure load,COll\cntional techniques as outlined in the ANSI andAS:\fl~ Coue.:s can be used.

Since the core is at the saine temper:lture as t~1<'jacket,in this eX:lI11ple, thr only di!lcn.:utial exp:lnsion would beexpcricllccd fn.>lll the uifl""ll:IJ"': in tIll: coefficieJlt of cx-pallSi",n hct\\cI'lI t!w st;liJlIt-" eof/.' alld carboJl steeljad:t.:t. The cIJJllpn:ssi\'c :1Il<1 teJlsile stl<>:,('~Oli b;; teadily

!~ t4~"

I /;: YJ

/'/'/'//' n

/' /'/' /'

I /'

~I ,(0> I I)

1 I I 'I I

~ v5pacers shown as a restraint

i~I II II II }- 3-in sch. 1055 core pipe

I II 1,-4·in, sch,40 C stl jacket

I I~~~'" I I I-( I I

I I

6.\frJ?

± 24,600 psi

._ 6J --:-?}')~0.2:37:

PRst

180(2.25) _ 2( -810) (I 76) (22-'0.237 0.237' . :J)

2(-230) _ 0 _

± 0.237 (1./6t (2.2.)

= 1710 + 27,100 -t- 13,550 = +2,360 psi outside-15,310 psi inside

~3 (Radial) = 0<I, will not be combined with other loadings for thepurpose of this analysis, but should always be evaluated.The longitudinal discontinuity stress then becomes: <I,

- <I3 24,600 - 0 = 24,600 psiCombined stresses at the juncture are then evaluated

which include: (a) discontinuity stresses, (b) axial col-umn IOZ!f1jpg froJn dissimilar core and jacket, (c) pres-sure stresses, (d) deadweight bending and (e) stressesim?oscd from thermal external bending loads. Thesestress levels should then be weighed against a limit thatwiJl ellSure cyclic elastic behavior at the joint. For thisanalysis we ha\'e taken the approach of ANSI B31.3for total stress evaluation, Sa = 1.25 (Se +Sh) == Sr, whereS e and Share 20,000 psi each:

24,600 psi (Longitudinal vDirection)

6,860 psi (Jacket) V(b) Axial column loading, <Ie

(c) Longitudinal PressureStress = Y:z hoop stress= (PR./2t)

(d) Deadweight BendingJ155 psi (Jacket)2,.QOo.psi, (Assumed)

34,315 psi

The difference of combined stresses from (a) to (d),compared to allowables can then be used as a minimumfor: (e) thermal external bending loads.

In this example Sa = 1.25 (20,000 + 20.,000;---·== 50,OOOpsi'"

Total (a) through (d)=--3"'.f;31S psi (Sl)Allowable t~errnal bending stress (e) = 15,685 psi

Composite core/jacket analysis. Prediction of terminalend reactions of the composite corel jacket sectioll can bedetermined by simulating a jacket sheJl thickness that isrepresentative of the combined moments of inertia of thecore and the jacket pipe. In our example, the followingapplies: ~3-il1. Scltt 10 Q.2.re,I .:=; 1.82 in4 E =- 28.3 (lOG)4~i-;-;Scl,lCl Jacket, 1== 7.32 in4 E =- 27.9( IV';)Sidce, for the analysis, we will use the carbon steel

jacket with a moclulus of elasticity of 27.9 (lOG), anadjustmclIt in the core moment of inertia would be:I = 1.82 (28.3 127.9) = 1.85 in·Total equi valenl I == 1.85 in' + 7.32 in4 = 9.17 in4

Using an OJ) of 4.5 inch, the equivalellt shell thickne:;sm; •..· be c:dclJlatcd :

I

9.1730.2

.~ft~r t~.;__L'qLi~\'3!f~:1t<~en tLiL~<;. n3s h~~':II c·'"mined: cor:-'LI"i(Jll;-,I flexibility all:llysis tedl! .•iq,..·~, Glil be:applied to calculate stres,cs and end re;lc,ifJ!ls.

Step by ste;J approach. :\ reasonable ap?roJ.ch to the:

design and ana1ysis of jacketed systems is:1. Establioh jJ,H,1:nctc'rs f(JI fLl'!:;" and spacer!gu:dcs loca-tions. This is a significant consideration when there is dii-ferential expansion between the core and jacket.2. Perform a flexibility analysis of the core pipe when re-quired by the constr-aints of No. I above.3. Evaluate all combined stresses, i.e. discontir;uit;·, ther-mal, etc., on an indi\'idual basis.4. Perform a flexibility analysis, predicting stresses andend reactions, of the combined corel jacket section whereinherent flexibility appears to be minimal or strain-sensi-tive equipment is used.5. 'Pay special attention to the local effects of vents, pres-sure taps, drains, etc. for their contrihution to the problemof differential core I jacket expansions.

In this example, a computer analysis was perfol1lled forthe piping configuratioll shown on Fig. 1. Predicted jacketthennal bending stresse, were within the stn'ss limits estah-lished by the analysis of the combined loading,;. \,\'ere thisnot the case, additional flexibility would be required bygeometry changes or a bellows type expansion joint wouldbe provided on the jacket pipe. The bellows would, forpractical purposes, eliminate the axial column loading anddiscontinuity stresses.

NOi\IENCLATUREAm, Cross-sectional metal area of the carbon steel jacket.A m. Cross-sectional metal area of the stainless steel core.e Constant = 2 for a column with one end fixed and the

other free.E Modulus of elasticity, psiI Momcnt of inertia, in.'

K Ratio of ring radius to shell radiusL Unsupported column length = 240 m. for a 20 ft. pipe

sectionM. Momcnt, in.-lbs,

P Pressure within the jacket = 180 psir. Radius of gyration = 1.2 for SS core pipe, in.r. Equivalent shcll radius, in.R Radius of carbon steel jacket, m.

R. Ring radius at 300· F, in..R. Shell radius, in.S. Allowable strcss, psiS. Stress at ambient temperature. = 20,000 psi for example

from ANSI B31.3S. Stress at operating temperature = 20,000 psi for examplc

from ANSI B31.3S, Longitudinal strcss, psi

t Thickncss of the carbon sted pipe, in.t. Equivalent shell thickncss representing the comhined mo-

menl; uf inertia of the composite corc and jackct!1T. Tempcrature difTcrcnce bctwccn avcrage ring tempcrature

and ambient, 0 FV. Shear, Ibs.

w Width of two pipc flanges, in.W Max. or critical load on a structural column, Ibs.fJ Total linear thcrmal exp:wsion betwcen 700 F and tcm-

perature of st,-alll at 180 psi, in.! I00 ft.a Mcan coefficient of linear thermal cxransion hetwecn 70" F

and stealll at 180 l'si.a, a of carbon steel pipc cOllsiJered as a shclla. a of sl"inle>s stecl f1allge cUllsiJercJ as a ring-(1 Stress, psit Str;lin, in.!in.

~" DifTcrcn,'(" in strain bctWCCll .the core and j:\cket =, [/1 (SScore) - fJ (C st!. jacket)] /1200 whcrc 1200 = lenilth of100 ft. pipe in inchcs

A 1.285/V1<.. t

Sub.<cripl'5 PcnaillS to st'-linll S5 steel curcC Pnl"ills tu c"rholl stet'! jacket

L1TrR.\TURE CITED1C..-atud.d and ScbnciJcl-, "Sltc:'~l:3 in a Pln'.Utl.: Vt::s~d with Circuru(r:rcn·tial Rill~ Stifft"nl'rs." a