How do ANSI classes relate to PSI?

There are different ANSI/ASME classes that designate the pressure rating of flange connections for pipes with contents under pressure. Flanges specifically use ANSI classes rather than referencing PSI, they are different because they use bolts as fasteners rather than a threaded means of connecting pipes. While these ANSI classes can often be seen on some mechanical parts side by side with PSI ratings, 150 lb/150# (flange rating) is not interchangeable with 150 psi. This is because PSI (pounds per square inch) is a static measurement while the ANSI/ASME rating is variable depending on the material that is being used, its heat treatment, and the pressure class.

The seven classes used for flanges are 150, 300, 400, 600, 900, 1500, and 2500. The main differences between these classes of flanges is the thickness, the number of bolt holes, the raised face thickness, and the outer diameter of the pipe. Depending on what kind of material is under pressure and what the flange material is determines what ANSI class of flange is needed. Specifications for these classes are located in ANSI/ASME B16.5. It goes over the pressure temperature ratings, materials, dimensions, marking, testing, tolerances, and methods for designing openings fitted for pipe flanges.

There is a way to calculate the ANSI class if the PSI is the only number listed and vice versa. It is a more concrete explanation on why the two concepts are intertwined. The type of material must be determined and found in the ANSI B16.5 standard. Then you can convert the classes to mega pascals (MPA) of which the conversion rate is 1 PSI = 0.0069MPA. The PSI rating can be multiplied by 0.0069 and that will give you the MPA related to the ANSI class.