Flanges, made in accordance with American standards such as ASME B16.5, ASME B16.47, ASME B16.36, ASME B16.48, AWWA C-207, are generally designated in “classes” by “ratings” according to their design pressure. The flange pressure rating is expressed in “psi” or, as a common practice, in “pounds”. When we speak for example of “a 300 pound flange”, it is however preferable to simply refer to a “class 300 flange”. The “class 300 flange” does not mean that the flange rated pressure is 300 psi. For class 300 and heavier, the rated working pressure PT is a function of the pressure rating Pr and the allowable stress ST: PT = Pr × ST / 8750. PT = rated working pressure for the specified material at temperature T, psi; Pr = pressure rating class index; ST = flange material allowable stress at temperature T, psi.
As a rule of thumb, at ambient temperature, the maximum operating pressure of a carbon steel flange of class X is approximately 2.4X. This rule particularly well for class 300 and heavier. For example, a class 300 ASTM A105 flange will have a design pressure of 2.4 × 300 = 720 psi at ambient temperature.