P91 PWHT

Each code have its own factor of safety in addition to its unique criteria of acceptance, and ASME Section I "Rules for Construction of Power Boiler" is belong to ASME BPVC committee, where ASME B31.1 " Power Piping" is belong to another pressure piping committee B31.

For the same piping committee B31, if you make a comparison between their codes ASME B31.1 "Power Piping" and ASME B31.3 "Process Piping", you will find that the allowable tensile strength of such a pipe material in B31.1 is differs from the allowable tensile strength of the same pipe material at B31.3 at the same specified temperature, and that depending upon the factor of safety used by the code. And if you proceed a piping design calculations, you will find that for the same loading conditions (pressure & temperature) and for the same pipe size and same pipe material, there are a different pipe wall thicknesses (schedules) per code compared with the other code.