Pipe flanges that are made to standards called out by ASME B16.5 or ASME B16.47 are typically made from forged materials and have machined surfaces. B16.5 refers to nominal pipe sizes (NPS) from ½” to 24″. B16.47 covers NPSs from 26″ to 60″. Each specification further delineates flanges into pressure classes: 150, 300, 400, 600, 900, 1500 and 2500 psi for B16.5; B16.47 delineates its flanges into pressure classes 75, 150, 300, 400, 600, 900, 1500, 2500.
Materials for flanges are usually under ASME designation: SA-105 (Specification for Carbon Steel Forgings for Piping Applications, Pressure Vessel Components), or SA-182 (Specification for Forged, Pipe Flanges, and Parts for High-Temperature Service). In addition, there are many “industry standard” flanges that in some circumstance may be used on ASME work.