Sec VIII Div 1 calculation formula 2

[FSB01]

Hello,
I have a question regarding a shell calculation. Why is code using different formulas if it is outside or inside diameter?

If long. stress formula is Sl=P*Rm/2t, why is code using P*(Ri-0.4*t)/2*E*t.
Or if Circ Stress is Sc=P*Rm/t why it is used P*(Ri+0.6t)/E*t

Why is, for the longitudinal stress Rm=Ri-0.4*t and why is Rm=Ri+0.4*t for the circumferential stress?

And shouldn't Ri always be used if it is internal pressure?

I have posted same question on Pressure vessel forum, but I did not get the answer.

 

[TGS4]

Your question was asked and answered there thread794-215298. Is this a different questions? In that post you mentioned that you are interviewing for a job - have you actually worked in the pressure vessel industry before? If not, don't sweat it, if someone asks you those types of questions and disqualifies you for your answer (if you base it on the answers provided in the other topic), you don't want to work there anyway.

 

[FSB01]

Yes, it is the same question. I didn’t see a definite answer.

It is not a job interview and I am working in pressure vessel industry for some time. I did number of calculations and never questioned why formulas are different. I just took them for granted.

 

[TomBarsh]

The "definite" answer is that in some cases it is more convenient to work to the given inside diameter, in other cases it is more convenient to work to the given outside diameter.

Because the surface to which the load (pressure) is applied differs between the two case the formulas themselves must work differently. This will be the case regardless of whether thin shell or thick shell theory is applicable.

However, as shown in your original thread the two formulas agree "in the limit", ie: when the required thickness equals the nominal thickness the ID and OD formulas produce equal numerical result.

 

[FSB01]

Shouldn’t internal pressure always act on the inner wall of a vessel?
My conclusion is that the Code formulas are using Rm:
For circ stress:

Rm=Ri+0.6t
Rm=Ro-0.4t

For longitudinal stress:

Rm=Ri-0.4t
Rm=Ro-1.4t

Why?

 

[TomBarsh]

I recommend that you obtain some textbooks or manuals describing the theoretical mechanics of pressure vessel analysis, and how the Code rules are developed.

Try:

"Theory and Design of Modern Pressure Vessels" by Harvey
"Pressure Vessel Design Handbook" by Bednar
"Guidebook for the Design of ASME Section VIII
Pressure Vessels" by Farr and Jawad (ASME Press)

Tom Barsh
Codeware Technical Support

 

[TGS4]

Well said Tom. The formulae have nothing to do with the surface upon which the pressure is acting, and everything to do with the nature of the pressure-retaining structure. Thin-shell, thick-shell, and various in-between.

I would add Timoshenko "Theory of Plates and Shells".

 

[CodeRef]

Tom is correct, a little more exposure would be good. Try this link at the NRC, it may be a little old (2001) but it is still good reading.

Click on the link below and then go to Web Based Search, and there enter the following ADAMS accession number:
ML040150682
This will open up a link to a PDF file that gives you a Short Course on ASME Boiler and Pressure Vessel design by John Mooney, starting at page 93. Up to page 93 is all about repairs (NBIC) to same (if you want to read it)

Here is the link:

http://www.nrc.gov/reading-rm/adams/web-based.html

 

[FSB01]

Thank you all. I will try to get some of those books (some of them I have) and see if I can find the answer.

 

[jte]

I dunno. Maybe the next time I'm evaluating a vessel for half vac I'll apply 15 psia to the external surface and 7.5 psia to the internal surface...

Does a vessel with an MAWP of 15 psig, 1" ID, 2" OD have no circumferential stress? I have 30 psia acting on a 1*pi perimeter with an opposing stress of 15 psia acting on a 2*pi perimeter. So, deriving the thin (ahem) wall free body diagram, if I take a section cut on a 180° section, I have no resultant force on the cut surface. No force means no stress.

I think I just came up with another interview question!

jt