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Join Date: Oct 2009
Posts: 1

Question regarding to the stress classification in ASME VIII div 2

10/25/2009 6:10 AM

Gooday all, i recently started doing FEA analysis on pressure vessel, and i have a question in regards to the stress classification in ASME VIII div 2.
Let me demonstrate by using a simple example of a vessel with a nozzle coming out of the shell, with certain nozzle loads acting on the nozzle, operating under a certain pressure.

According to ASME VIII div 2 part 5 (app 4 in the old version), under design loads (sustained case - weight plus pressure), the loads can be classified into 3 stresses: Pm (general membrane), PL (local membrane) and Pb (bending). From my understanding, in this case, the stresses induce by the pressure is PL and the nozzle loads causes Pb, so for the load case of PL+Pb, the acceptable value is < 1.5S. However, i was told that the nozzle loads are actually already accounted for in PL and Pm, which in this example, will be PL. I am pretty confused, because if that is the case, then what exactly is Pb, and when will we ever see the load case of PL+Pb?

The second part of my question is that, if we move onto the operating loads (sus + expansions), there is now an extra term Q (secondary stress) and the primary + secondary accpetance critieria becomes, Pm+PL+Pb+Q < Sps. As i said earlier that i believe the stress due to internal pressure should be classify as PL, but from the definition of Q, Q also seems to apply to the stress induced by pressure in this case due to constraint at nozzle and shell joint, so should this stress be classified as PL or Q? And what is the differences between PL and Q? Is bending stress a component of secondary stress as well (due to bending stress at joint)? And am i correct to say that there are no Pm and F(peak stresses) in this example?

Thank you all for your help, any answer will be appreciated!

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Participant

Join Date: Sep 2009
Location: ISGEC YAMUNANAGAR HARYANA INDIA
Posts: 4
#1

Re: Question regarding to the stress classification in ASME VIII div 2

10/30/2009 11:32 PM

Dear friend

Membrane stresses - when you average the stress distribution across thickness along a line of reference, what comes out is a membrane stress.

Bending stress- the stress which is linearised across the thickness (along the line of reference) is bending stress

Either membrane or bending can arise due to loads calling for force equilibrium such as pressure, weight or seismic comes under primary.

Either membrane or bending caused due to thermal expansion load or load due to compatibility condition at a junction are all secondary (please note that this is Q and is not categorised as membrane or bending - In secondary membrane and bending is put together ; At the same time the membane at junctions under specific situations are classified as local primary membrane (PL);

Better to go through calssification guidelines given in code (table plus corresponding description) carefully and then discuss further CASE BY CASE

I know that this will not make you all clear but it will be a good start. However I remain to help you clarify further if needed

With best wishes

R.SRINIVASAN

VICE PRESIDENT, PVD DESIGN, ISGEC , YAMUNANAGAR, HARYANA, INDIA

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R.SRINIVASAN VICE PRESIDENT PVD DESIGN ISGEC YAMUNANAGAR HARYANA INDIA
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