ASME
ASME B16.5
Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24
The dimensional and pressure-temperature standard for flanges up to NPS 24. Its rating tables are why a Class 150 flange does not hold 150 psi at every temperature — the allowable pressure falls as the metal gets hotter, and the material group decides how fast.
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.