When you perform a hydrotest, you can't apply too high pressure because the stress developed may be excessive. My question is during hydrotest, how high stress can be allowed. On what rule, should the maximum allowable hydrotest stress for flat heads or tubesheets be determined?
ASME VIII Div.2 gives a limit of 90% of min.yield stress for general primary membrane stresses and of 135% of yield for primary membrane plus bending stresses.
The ASME codes(fired and unfired pressure vessels) will show you the formulae needed to calculate the MAWP's of these item you mentioned. The hydrostatic test will be at 150% MAWP.
