why in design of cylindrical shell as per ASME 2004 SECTION VIII-2 , they are subtracting 0.5 times P from the denominator.? look at this..

thickness t = PR/(SE-0.5P)

Why they dont use t = PR/SE ?

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Paresh Gujarati
M.TECH (Industrial Process Equipment Design)

Because it is written in the Code.

Thanks for the reply.
I have read from the book that it was taken due to taking consideration of nonlinearity of stresses in the shell. but i wanted the how they considered? how is the stress pattern? can u explain.......

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Paresh Gujarati
M.TECH (Industrial Process Equipment Design)

some has quote it's written.
however, please find below point why it's written.
1. 0.5P is consideration of radial stress. as we know radial stress equal to internal pressure at inside the surface & zero at outside the surface. therefore, average equal 0.5P. i would provide the details derivation of code equation, pl. mail me on mentioned address : vishaltailor03@gmail.com

Pl find an details reply.
hoop stress (Sh) = PR/t
Radial Stress @ inside, Sr1 = -p, & @ outside Sr2 = 0. therefore, Average Radial stress = -p/2

Now, as per tresca theory difference of maximum principle stress = Yield stress (here we will consider Code allowable SE)

Therefore, Hoop - Avg. Radial = SE
Therefore, PR/t - (-0.5P) = SE
Therefore, PR/t + 0.5P=SE
Therefore, PR/t = SE-0.5P
Therefore, PR/(SE-0.5P) = t ----> it's code formula. Hope this is clear now.

Hi everyone I have just joined the forum a few days back.
Thanks for providing the derivation. Can we use the same logic in deriving the shell thickness formula for Sec VIII Div 1 substituting the value of radial stress as 0.6P. If yes why does Div 1 use the value of 0.6P and not 0.5P for radial stress. I would be very grateful if someone explains this and clears the confusion.

Regards

Yaseer

Yaseer
it's div-2 forum. Hence, i may add this question in Div-1 separately, if required. However, pl. note below brief explanation for 0.6.

div-1 formula is based of lame's equation for thick & thin shell theory. code has simplified lame's equation of thin shell. Thin shell Equation has been simplified in such way that it can be used for thick shell with certain restriction. Hence, code has limited applicability up to t/r ratio not more than 0.5, against this simplification 0.6 has been added, as it has been observed that Difference in value of stress to pressure ratio obtained by thick and thin shell theory =0.6.
Please, refer STRUCTURAL ANALYSIS & DESIGN OF PROCESS EQUIPMENT BY Jawad & farr, where details description is given.