B31.3 defines design pressure and design temperature to establish wall thickness. That's it.
Stress and load calculations in B31.3 do not mention "design" values.
So use actual, expected temperatures and pressures if you wish. BUT I am concerned that someone, in the future, will read those design values, assuming that the system can be run at those conditions without further evaluation, and that may prove to be trouble.
Note that CAESAR II does not explicitly request design pressure and design temperature.
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Dave Diehl