VIII Div 2 - Part 8 Hydraulic Test Pressure Limits 2

[Ishmael]

I have a question about the upper limit for a hydraulic test pressure according to Sect VIII Div 2, 8.2.1.
According to paragraph e), which refers to 4.1.6.2.a, the test pressure shall not exceed that value which results in the following equivalent stress limits:

Pm<=0.95 SY
and
Pm+Pb<=1.43 SY or (2.43 SY-1.5Pm) if Pm>0.67 SY

where Pm is calculated primary membrane stress, Pb primary membrane stress, and SY is min yielding stress.

I can understand why the limitation applies only to primary components, because the secondary values are a concern only in cyclic application which is not the case of a hydrotest.
What actually puzzles me is why there's no limitation on LOCAL membrane stresses PL.
Shall I discard those stresses in the max pressure test assessment, or shall I assume Pm=PL in the above equations?
Being usually Pm much less than PL the max allowable test pressure will be much larger if I discard local stresses.

Thanks,
Ishmael

 

[Ishmael]

Sorry, Pb is the primary bending stress, of course.

 

[TGS4]

There is no defined limit on PL for the hydrostatic test condition case.

If you want to be completely satisfied that your local membrane stresses are not being "excessive", you could perform the limit load assessment for the Hydrostatic Test Condition, as defined in Table 5.4.

 

[prex]

Local primary stresses are in fact secondary stresses that are classified as primary, and limited as such, to avoid excessive distortion in the transfer of load to the adjoining parts of a vessel.
Typical examples of local primary stresses are the discontinuity membrane component at a shell to head junction and the membrane component around a nozzle, both due to internal pressure. Due to their distributed pattern (in both cases circumferentially uniform around centerline of shell and nozzle), they are not going to create excessive distortion with one or two load applications.
The same reasoning would not apply to an external moment acting on nozzle, but of course there are no external moments in hydrotest.
To be noted also that the local yielding that occurs during the hydrotest is beneficial to the future resistance of vessel to ratcheting and fatigue: so the choice of the hydrotest pressure is a balance between two opposing criteria, 'stress it as much as possible, but of course not too much'.

prex

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[Ishmael]

Thank you so much TGS4 and prex, now it makes much more science. I remember I read in the ASME (probably old DIV2 Appendix 4) that local membrane are actually secondary stresses but treated as primary, but without your help I probably wouldn't have sorted it out.
As per the limit Load, is probably a good idea. A customer is asking for a test pressure that is about 6 time the default asme pressure, so at least I'd want lead them to accept the max ASME value.