Valve design for shell integrity
Valve design for shell integrity
(OP)
The specifications (ANSI B16.5, MSS SP72) I've seen pertaining to housing/flange shell integrity require a 50% safety factor over the cold temperature rated working pressure. Is this to mean that, in a pressure surge of more than 50% over max. working pressure, one can expect catastrophic failure ? Anyone familiar with additional shell safety factor requirements ?





RE: Valve design for shell integrity
Regarding your reference to ASME B16.5 and MSS-SP-72 and your comment of 50% safety factor, I'm not sure I understand where you got this number. Neither of theses standards address or discuss margins of safety.
Perhaps you are refering to the requirement of valves being shell pressure tested at 1.5 times the rated cold working pressure. Please note that these are required production tests to verify leak tightness and are not for the purpose of determinng a margin of safety.
RE: Valve design for shell integrity
As peaseman said B16-34 is conservative in its approach when calculating wall sections and as you alluded to will be tested at 1.5 times the cold rating. If you calculated a shell thickness based on say B31.3 you would find a significant difference in sections.
Look at Annex F1.3 and the 1.5 factor shown in the calculation for Wall Thickness.
RE: Valve design for shell integrity
ANSI requires that a shell-integrity test be done at 1.5X the rated cold working pressure. Note that API is somewhat different--they require 2X the cold working pressure for 5K and less, and 1.5X at 10K and up.
Your design needs to consider both the rated working pressure AND the shell test requirement. The relationships are as follows:
At working pressure: Sm = (2/3) Sy
At test pressure: St = 0.83 Sy
Your selected material must have sufficient strength to satisfy these stress limits.