Definition and Details of Flanges - Pressure Rating

Pressure Classes of Flanges
Forged steel flanges, according to ASME B16.5, are made in seven primary Pressure Classes:

150-300-400-600-900-1500-2500

Pressure-Temperature Ratings - Example -
Pressure-temperature ratings are maximum allowable working gage pressures in bar units at the temperatures in degrees celsius. For intermediate temperatures, linear interpolation is permitted. Interpolation between class designations is not permitted.
Pressure-temperature ratings apply to flanged joints that conform to the limitations on bolting and on gaskets, which are made up in accordance with good practice for alignment and assembly. Use of these ratings for flanged joints not conforming to these limitations is the responsibility of the user.
The temperature shown for a corresponding pressure rating is the temperature of the pressurecontaining shell of the component. In general, this temperature is the same as that of the contained fluid. Use of a pressure rating corresponding to a temperature other than that of the
contained fluid is the responsibility of the user, subject to the requirements of applicable codes and regulations. For any temperature below -29°C, the rating shall be no greater than the rating shown for -29°C.
As an example, below you will find two tables with material groups acc. to ASTM, and two other tables with flange pressure-temperature ratings for those ASTM materials acc. to ASME B16.5.
The form lb is actually an abbreviation of the Latin word libra, which could mean a pound, itself a shortened form of the full expression, libra pondo, "pound weight". The second word of this phrase, by the way, is the origin of the English pound