Power boiler external piping is typically governed by B31.1

However, you are still going to have to determine the scope of supply and engineering responsibility. There should be some kind of interface point delineated.

You will need to account for the movements, restraints, and any allowable loads at the interface given by the vendor as part of the analysis of the general steam piping that is beyond the block valve - assuming your firm is the one doing the overall piping design of the plant.