Flange Ratings: Reading a Pressure-Temperature Table Correctly

Class is not pressure

ASME B16.5 class designations refer to a family of pressure-temperature ratings, not a single pressure. A Class 150 carbon steel flange is rated well above 150 psi at ambient temperature and well below it at elevated temperature.

The applicable rating always depends on the material group in Table 1A and the design temperature.

Material group differences

Group 1.1 carbon steel, group 2.2 stainless and duplex or nickel alloy groups each carry different curves. Substituting a material of the same class without checking the group can quietly reduce the allowable pressure.

Above NPS 24, ASME B16.47 Series A and B apply, and the two series are not interchangeable dimensionally.

What to state on the enquiry

Specify class, facing, finish, material grade and applicable dimensional standard together. Any one of them missing usually leads to a re-quote.