The term “flange rating” (or class) refers to the maximum pressure (in psi or bars) that a flange withstands at increasing temperatures. Flanges with a higher rating (class) are stronger than flanges with lower ratings, as they bear more pressure at increasing temperatures. Flanges made in different materials show different pressure-temperature performance at the same rating. In the article, you find the pressure rating charts for carbon, alloy, and stainless steel flanges and some guidance about selecting the proper rating for your piping application.
Pressure rating is defined as the maximum allowed pressure that a flange can withstand at increasing temperatures. According to the ANSI/ASME B16.5 specification, there are seven flange pressure ratings: 150, 300, 400, 600, 900, 1500, and 2500.