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.
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