Semi-metallic gasket built from alternating plies of preformed metal winding and pliant filler, assembled into a centring ring. Made to ASME B16.20 for raised-face and flat-face flanges to ASME B16.5 and ASME B16.47.
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A spiral wound gasket is wound from a preformed metal strip laid in alternating turns with a soft filler. Under bolt load the winding acts as a spring while the filler flows to fill the flange surface irregularities, so the gasket keeps working through thermal cycling and bolt relaxation. That resilience is why it is the standard joint for refinery, petrochemical and steam service where a flat sheet gasket would relax and leak.
ASME B16.20-2007 fixes the construction. The metal strip is between 0,15 mm and 0,23 mm thick, the filler must be flush with, but not below, the winding on both faces, and the assembly is furnished in a centring ring 2,97 mm to 3,33 mm thick that locates the gasket in the flange and acts as a compression stop. Nominal winding thickness is 4,45 mm, and the gasket is designed so that a uniform bolt stress of 207 MPa compresses it to 3,30 mm plus or minus 0,13 mm; for NPS 1/2, 3/4 and 1 in Classes 150, 300 and 600 the design bolt stress is 172 MPa.
Inner rings matter more than they look. The standard requires an inner ring on every PTFE-filled gasket, and on flexible-graphite-filled gaskets unless the purchaser specifies otherwise, because a spiral wound gasket can buckle inward without one. The inner ring material should match the winding.
Identification is by colour and it is defined by the standard, not by the maker. A continuous colour around the outer edge of the centring ring identifies the winding metal - green for 316L, yellow for 304, silver for carbon steel, blue for 347, turquoise for 321 and so on - while intermittent stripes on that edge identify the filler: white for PTFE, grey for flexible graphite, pink for mica-graphite and light green for ceramic. When ordering, state the flange standard, nominal size and pressure class, and the winding, filler and ring materials.
| Standard | ASME B16.20-2007, for use with raised-face and flat-face flanges |
|---|---|
| Size and Class | Identified by flange size (NPS) and pressure class; NPS 1/2 to NPS 24 for ASME B16.5 Classes 150, 300, 400, 600, 900, 1500 and 2500 |
| Construction | Alternate plies of preformed metal winding and pliant filler, spirally wound; filler flush with but not below the winding on both faces |
| Winding Strip Thickness | 0,15 mm to 0,23 mm (0.006 in to 0.009 in) |
| Nominal Thickness | Winding 4,45 mm; centring ring 2,97 mm to 3,33 mm |
| Compressed Thickness | 3,30 mm +/- 0,13 mm at a uniform bolt stress of 207 MPa (30,000 psi); 172 MPa (25,000 psi) for NPS 1/2, 3/4 and 1 in Classes 150, 300 and 600 |
| Inner Ring | Required on all PTFE-filled gaskets; furnished on flexible-graphite-filled gaskets unless the purchaser specifies otherwise; and required whatever the filler for NPS 24 and larger in Class 900, NPS 12 and larger in Class 1500 and NPS 4 and larger in Class 2500. Inner ring material should match the winding. |
| Winding Colour Code | Silver carbon steel; yellow 304; green 316L; maroon 317L; blue 347; turquoise 321; orange Monel 400; red Nickel 200; purple titanium; black 20Cb-3; brown Hastelloy B; beige Hastelloy C; gold Inconel 600/625; white Incoloy 800/825 |
| Filler Stripe Colour Code | White PTFE; grey flexible graphite; pink mica-graphite; light green ceramic. Four stripes 90° apart for NPS 1 1/2 and larger, minimum two stripes 180° below that. |
| Required Marking | Manufacturer, flange size (NPS), pressure class, winding / filler / ring material abbreviations, flange identification and the ASME B16.20 designation on the centring ring; lettering at least 2,5 mm high |
