Understanding the Two Gasket Families
When you specify a flange gasket, the first decision is whether the application calls for a metallic or a nonmetallic design. The ASME standards that govern these products are clear about their scope: ASME B16.20 covers metal ring-joint, spiral-wound, and metal-jacketed gaskets, while ASME B16.21 covers nonmetallic flat gaskets. Both standards define dimensions, tolerances, and marking requirements, but they do not tell you which gasket is right for your process. That decision rests on a careful evaluation of service conditions, flange design, and assembly practices.
For EPC engineers and procurement managers, the distinction matters because the two families behave very differently under pressure, temperature, and chemical exposure. Metallic gaskets are typically used in high-pressure or high-temperature services where nonmetallic materials would fail. Nonmetallic gaskets, on the other hand, are often chosen for lower-pressure applications where chemical compatibility and cost are primary concerns. However, there is no universal rule—each joint must be evaluated on its own merits.
What ASME B16.20 and B16.21 Actually Cover
ASME B16.20 specifies materials, dimensions, tolerances, and markings for three types of metallic gaskets: ring-joint, spiral-wound, and metal-jacketed. These gaskets are dimensionally suitable for flanges described in
