We have the need for reducing a refrigerating compressor MDMT for better productivity.

We are applying stress ratio for piping circumferential stress and longitudinal stress and valve/flange comparison of pressure with pressure alloable for flange rating after deducting piping corrosion allowance and mnfg tolerance for sustained condition and considering nominal thickness for expansion load calculation.

But, what about the static and rotating equipment MDMT reduction - how will this be checked - will allowable get derated by the stress ratio ? From the intent of B31.3, is not this approach right ?

reg,
sam

Check the datasheet. If the equipment already comes pre-rated for the temperature you're trying accomplish, then there is no derate required.

If operating outside the equipment's capabilities (which would be surprising), then ideally the manufacturer would re-qualify the equipment, including nozzle loads.

B31.3 doesn't cover nozzle capabilities. The equipment should tell you which code / standard it was designed to, and thus what you should do with nozzle load capacities if you are attempting to re-rate the equipment yourself.