I have a short period high temperature(less than 50 hour/year) case due to decoking (690C) in one piping system.Is it possible to use other allowable stress(higher) than that described in B31.3(Table A) for the material???
In affirmative case, which???

Hello Mr. Christofi,

The short answer is maybe.

You will have to closely read ANSI/ASME B31.3 paragraph 302.2.4 and see if the condition that you are describing can qualify as an "Occasional Variation". You should especially examine B31.3, Appendix V. In any event, you should involve the owner in the decision.

Regards, John.