Pressure and Temperature Ratings
The classes 150#, 300#, ... etc. are just considered as a categorization or family name for each group of flanges.
The calling up is first created by ANSI under ANSI B16.5 and ANSI B16.47, and starting from 1998 the publications issued under ASME B16.5 (for flanges NPS 1/2" up to NPS 24") and ASME B16.47 (for flanges NPS 26" and larger) for determining what we called Pressure-Temperature Ratings.
Note. 150# ≡ Class 150 ≡ Rating 150, all three names are correct, but don't mean that the flange will withstand a pressure of 150 lb/in2 (pound per inch square).
The tabulated pressure in which the flange will resist is located from ASME B16.5 and ASME B16.47 which depends on type of flange Material and Design Temperature.
To calculate the max. design pressure of any flange with a such rating, 1st you have to select the flange material and 2nd you have to select the design temperature.
