Hi,

Pressure stiffening on bend is allowed to use In ASME B31.1.
As you know, pressure stiffening effect makes SIF decreased, piping flexibility as well.

Normally I implement stress analysis taking only P1 as a design pressure and consequently elbow stiffening pressure is applied Pmax, design pressure, in default option.

but in terms of SIF, applying Pmax brings less conservative results. Therefore, i think it would be reasonable to apply operating pressure only at elbow pressure stiffening. Moreover it would be good for piping flexibility considering nozzle load.

Ultimately which is more reasonable? taking design pressure or operating pressure in elbow pressure stiffening.

Please let me know your opinion.

thanks

In my opinion, the answer depends on the reason for which you choose a design pressure substantially greater than operating pressure. Often this is what is called "good practice" but exceeds Codes requirements.
Proceeding in this way, you/ your Company people/ are conservative adding a substantial margin to get a design pressure and later realize the result may be not conservative versus flexibility/ stress calculation. "More reasonable" is to revise this procedure in case you got real problems following it.

There's only one setting that is unilaterally conservative in this business - reducing allowable stress.

In all other cases, the status of any other setting being conservative will be wholly contingent on which set of blinders you put on.

I'll preface my further statements that I'm largely unfamiliar with B31.1's requirements - so I'm assuming it's non-specific on the subject - and I'm also assuming your client and company policies are also non-specific.

But if (since) it's unclear which is more conservative, it's unclear to me why you wouldn't consider the entire pressure range in addition to the temperature range, aside from the multiplicative nature of combining load cases.