krishnan,
You should go back and read the Clients "BED" (Basis for Engineering & Design) Document and also take note of the type of Project.

Normally:
If the Project is a Power Plant then ASME B31.1 would apply to all Boiler and Steam piping as well as all other piping.

If the Project is a Refinery or Chemical Plant then ASME B31.1 would apply to the boiler piping up to the boiler isolation valves and ASME B31.3 would apply to all steam piping beyond that point as well as all other piping.