1643,
This is a Local Building Code Issue. Check with your local jurisdiction. The local building Codes usually refer to the State Code and in some States they refer to one of the "Uniform" Codes. If this "precedence of Codes leads you to the ASME B&PV Code, it will likely have to have a "U" stamp. Sorry I could not be more specific.
Also, you might want to ask this question on the Pressure Vessel forum at this site.
Regards, John.