A guide to industrial flange pressure classes under ASME B16.5

What is a flange pressure class?

Standard ASME B16.5 classifies industrial flanges into standard pressure classes. Each class defines the maximum allowable working pressure at a given temperature for a given flange material. The important point is that the permissible pressure of any class falls as the line's operating temperature rises — so selecting a class on the ambient-temperature pressure alone is not enough; the actual service temperature must be taken into account.

Standard pressure class table (carbon steel, ambient temperature)

How buyers should select a pressure class

• Line design pressure: The starting point is not the day-to-day operating pressure but the line's maximum design pressure, taken from the project engineering documents.
• Service temperature: At high temperatures — steam lines or hot petrochemical processes, for example — the pressure capacity of the same class falls. The class must be chosen from the standard's pressure–temperature table, not from the ambient-temperature figure alone.
• Matching the existing equipment on the line: The flange must be the same pressure class as the valves, pumps and other fittings already on that line. Mixing classes on a single line is not permitted.
• Type of service: On lines carrying corrosive media or in sour service, the flange material and coating must also be selected to the relevant standard (such as NACE MR0175), separately from the pressure class.

How this relates to hammer unions

On temporary high-pressure well operation lines — cementing, acidising, fracturing — the usual choice instead of a flange is Hammer union which are classified by figure number and follow much the same selection logic. The full figure classification table is on the Flanges and hammer unions is available.