As per UW40 (a)(4), when PWHT by internal firing method the internal
pressure should be kept as low as practicable, but shall
not exceed 50% of the maximum allowable working pressure
at the highest metal temperature expected during the
postweld heat treatment period.

How to calculate MAWP at PWHT temp (~ 600°C) since allowable stress for SA 516 Gr. 70 is available upto 524°C only.

Wow, tricky problem.
I suggest to keep the pressure atmospheric during heating. That should avoid problems. Why would you want to pressurize during PWHT?

DK

Gitsunil,
It's easy to guess what would be the answer for a request for interpretation "does it mean the Code does not allow pressurised PWHT at non-allowed service temperature?"- it would surely be "not allowed". Manufacturing is not service, but I don't think that the intention of the code is allowning exceeding the allowable pressures&temperatures just because the vessel is not sent to the user and it's not a service condition yet. By the way, you are required not to exceed those during hydrotesting, so why should you be exempt from this principle during PWHT?

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Konrad Anikiel