A spiral wound gasket is built from a metal winding strip and a soft filler wound together in alternating layers, usually with an outer centring ring and often an inner compression ring.
Under ASME B16.20 the outer ring carries a colour code so the gasket can be identified after it leaves the box. The base colour of the outer ring identifies the winding metal — for example yellow for 304 stainless steel and green for 316L stainless steel. The stripe on the edge identifies the filler material, such as grey for flexible graphite and white for PTFE.
The inner ring matters more than people expect. It prevents inward buckling of the winding, protects the filler from the process fluid and reduces turbulence at the bore. For Class 900 and above, and for all PTFE-filled gaskets, an inner ring is generally required.
Always match the gasket class and size to the flange it seats in. A Class 300 gasket in a Class 150 flange will not seal correctly even though it may appear to fit.
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