ASME
ASME B16.34
Valves — Flanged, Threaded and Welding End
Pressure-temperature ratings, wall thickness and testing for valves. It is what a valve body rating is quoted against, and it defines the standard, special and limited class distinctions.
This is an explanation, not the standard. AlpineTutor is not affiliated with ASME and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.