Hi esteemed experts,
according to B31.3, if sustained stress doesn't exceed sustained allowable, you can use the difference to increase expansion stress allowable.

If you need to increase expansion stress allowable in a particular point of the piping system to comply the code using "liberal stress allowable", have you to reduce occasional allowable (1.33*sustained allowable) in that point?

It seems that the code doesn't state anything about, so you can use liberal and occasional freely.

Any comment is appreciate.
Thanks.
Best regards

Alessio

In opinion you do not have to reduce the occasional allowable value.

Note that B31.3 now requires you to evaluate the sustained stresses in all positions of the piping system. This is what the Alternate-Sustained load cases are for. Similar to having multiple occasional load cases, you can also have multiple sustained load cases for your system.

Hi there,

Here is just an opinion:

At THIS time, B31 Code Occasional Stress qualification are not correlated with B31 Displacement Stress Range (e.g. thermal expansion/contraction stress) check. Occasional and Sustained stresses are not self-limited stresses and are related to plastic collapse failure, while displacement stresses are considered self-limited stresses and are related to fatigue failure mechanism.

A conservative approach that would provide a safe envelope for B31 Code Occasional Stress qualification, would be to define a MAX SUS Load Case as MAXimum between actual SUS and Alt-SUS cases, and to perform then the ABS (absolute) summation between this MAX SUS case and the individual OCCasional (OCC) load cases.

A more expedite approach would be to perform the ABS summation between MAX SUS and MAX OCC (latest case as MAX between all individual Occasional load cases - might be efficient if you have many Occasional load case - WIND, SEISMIC, Slug Flow, etc.). But such approach might be overly conservative indeed.

Personally, this is the approach I use to employ. If it fails, then we need to go to deeper/more accurate investigations....

Regards,

For me the question makes sense. Is like two brothers buying two different things with the same money borrowed from their cousin.