Section VIII Div 2 question 2

[heaterguy]

From Figure 4-130.1 "Stress Categories and Limits of Stress Intensity", what stress classification(s) should be used for a blind flange that is bolted to a pressure vessel:

Choices are:
Pm - Primary General Membrane
Pl - Primary Local Membrane
Pb - Primary Bending
Q - Secondary Membrane plus Bending
F - Peak

 

[TGS4]

Your answer is in Table 4-120.1.

 

[heaterguy]

Unfortunely for me, a bolted blind flange is not listed in Table 4-120.1. That would be too easy.

 

[TGS4]

OK - time for some hand-holding... Try the flat head (unless your blind flange isn't flat...). The answer to your question is that it depends on the type of stress and the location of the stress. But you probably figured that out already by seeing the structure of Table 4-120.1.

 

[heaterguy]

Yes, hand holding is required.

A flat head, center region is classified as Pm and Pb, but I have been told that there is no membrane stress on a bolted blind flange. May you please explain?

 

[prex]

Yes Pm=0 in a bolted blind flange under pressure load only, and if you calculate the stresses with the thin plates formulae (I'm not suggesting to use thick plate formulations, but you could).
Not to complicate things, but, if you want to be 100% correct, you should calculate Pb with Poisson's ratio [ν]=0. The increment in stresses due to [ν]>0 is due to self-constraint in the structure, so it is really a secondary stress (although you won't find this in Table 4-120.1).

prex

http://www.xcalcs.com

Online tools for structural design

 

[heaterguy]

How can 3Sm exist, anyway? Isn't this past the point of no return?

 

[jte]

Read the definition of a Secondary Stress.

jt

 

[heaterguy]

OK, I think I get it, Sm has a 3.5 safety factor, right?

 

[prex]

Can't see your point on a safety factor of 3.5 .
3S_m is in fact 2Y (not really that, but often close to): the limit on P+Q is in fact that it must not cause ratcheting (accumulation of plastic deformation due to load cycling), but plastic deformation for it is indeed allowed.
The safety factor is somewhat given by strain hardening, but in fact it is not necessary: secondary stresses will never cause a sudden collapse before excessive deformation is eventually detected.

prex

http://www.xcalcs.com

Online tools for structural design

 

[TGS4]

heaterguy,

I would highly recommend that you get your hands on a document published by ASME titled "Criteria of the ASME Boiler and Pressure Vessel Code for Design By Analysis in Sections III and VIII, Division 2" - not sure of the publication date. It answers all of the questions that you have here, and many more. If you can't find it, e-mail me at the address on my profile, and I'll see what I can do.