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Stress classification in ASME VIII div 2

  1. Oct 22, 2009 #1
    Hi guys, 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!
     
  2. jcsd
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