If Power plant has to be build in process Refinery then where specification break between B31.1 & B31.3??

plz clarify .i am confused about this question

Can any one just help to sort it out.

The power boilers must be in accordance with BPV Code Section I and associated sections of BPV.

Boiler external piping is required to conform to B31.1.

IMO, for your case, Non-boiler External Piping can be either B31.1 or B31.3, depending on the costs vs. benefits your Client wants to assign to.

See B31.1/ B31.3 "Scope and Definitions" chapters.