Flange Rating

Rating, as applied to flanges, may best be defined as the maximum pressure allowed by the pressure piping code for the specific temperature at which the flange will be operating. Flanges and nozzles are sized according to pressure ratings established by the American National Standards Institute (ANSI). These pressure ratings, sometimes called pound ratings, are divided into seven categories for forged steel flanges. They are 150#, 300#, 400#, 600#, 900#, 1500#, and 2500#. Cast iron flanges have pound ratings of 25#, 125#, 250#, and 800#. Pound ratings, when combined with the temperature of the commodity within the pipe, are used to select the appropriate size, rating, and type of flange. This pressure/ temperature relationship will allow any given flange to be used in a number of different applications. For example, a 150# forged steel flange is rated to perform at 150# PSIG at 500°F. If the temperature were decreased to 100°F, this same flange could be used for 275# PSIG. However, if the temperature were increased to 750°F, the flange could only be used for 100# PSIG. As you can see, the pressure/temperature relationship is important. When temperature decreases the allowable pressure increases, and vice versa. Pound ratings are also used to establish the outside diameter and thickness of a flange. Typically as pound ratings increase, so will the flange’s diameter and thickness.