Spiral Wound Gaskets and Flange Gasket Selection: A Practical Guide
The spiral wound gasket is the workhorse flange seal for industrial piping and valves. Choosing the right winding metal, filler, and ring configuration - and applying the correct bolt load - is what separates a leak-free flange from a fugitive-emission problem. This guide covers construction, materials, colour coding, and selection.
In This Article
- 1.Construction
- 2.Winding Metals and Filler Materials
- 3.Selecting by Service
- 4.Installation and Bolt Load
- 5.Standards and Documentation
Most valve and pipe flanges in refineries, chemical plants, and power stations are sealed with spiral wound gaskets (SWG). They combine a preformed metal winding with a soft filler to give a resilient, high-recovery seal that tolerates pressure cycling, thermal movement, and modest flange imperfections far better than a flat sheet gasket. But a spiral wound gasket is only as good as its material selection and the bolt load applied to it - the wrong filler or an under-torqued flange is a common root cause of persistent leaks and fugitive emissions.
Construction
A spiral wound gasket is built from these elements:
- Winding - a V- or W-profile metal strip spiral-wound with a soft filler strip between the metal turns; this is the sealing element.
- Outer (centring / compression) ring - a solid metal ring that centres the gasket in the flange and acts as a compression limiter to prevent over-compression.
- Inner ring - a solid metal ring on the bore side that prevents inward buckling of the winding, protects against erosion, and fills the annular gap; recommended for many services and mandatory for certain ratings and fillers.
- Filler - the soft sealing material wound with the metal (graphite, PTFE, mica, or ceramic) selected for the temperature and chemical service.
Winding Metals and Filler Materials
The winding metal is chosen for corrosion resistance in the service, and the filler for temperature and chemical compatibility. ASME B16.20 assigns a standard colour code (outer ring colour identifies the winding metal; a stripe identifies the filler) so gaskets can be verified visually. Common filler selections:
| Filler | Typical Temperature Range | Best For | Avoid |
|---|---|---|---|
| Flexible graphite | -200 C to about 450 C (higher in non-oxidising) | Steam, hydrocarbons, most general service | Strong oxidisers; high-temp oxidising above graphite limit |
| PTFE | -200 C to about 260 C | Aggressive acids, chlorine, caustic, high-purity | High temperature above PTFE limit |
| Mica / vermiculite | up to about 1000 C | Fire-safe and high-temperature oxidising service | Where maximum chemical resistance needed |
| Ceramic | up to about 1000 C+ | Very high temperature, exhaust, furnace flanges | General moderate-temperature duty (over-specified) |
Selecting by Service
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Match the winding metal to the process fluid the same way you would a valve trim - Type 304/316 stainless for general and mildly corrosive service, 316L for chlorides at moderate levels, and higher alloys such as Alloy 20, Hastelloy, Monel, or titanium for aggressive acids, seawater, or specific chemicals. The centring ring is usually carbon steel (zinc plated) or stainless. For chlorine, strong acids, and high-purity service use a PTFE filler; for steam and most hydrocarbon service use flexible graphite; for fire-safe and very high-temperature flanges use mica or ceramic filler. Always specify an inner ring for PTFE-filled gaskets, for Class 900 and above, and for large diameters, to prevent winding buckling.
Installation and Bolt Load
A spiral wound gasket seals by being compressed to a controlled thickness. Under-load and it leaks; over-load and it can crush the winding. Correct installation is procedural:
- 1Inspect flange faces - the gasket seats on a raised-face or flat-face finish within the correct roughness range; scratches across the face cause leaks.
- 2Use new gaskets every time - spiral wound gaskets take a permanent set and must not be reused.
- 3Lubricate bolt threads and nut faces with the specified anti-seize to achieve the target preload from the applied torque.
- 4Tighten in a cross (star) pattern in several passes, building up to the final torque so the flange pulls down evenly.
- 5Compress the winding until the flange faces contact the outer ring - the ring acts as the compression stop that sets final thickness.
- 6Re-check torque after thermal cycling where the procedure calls for hot re-torque on high-temperature service.
Standards and Documentation
Spiral wound gasket dimensions and colour coding follow ASME B16.20; flange dimensions and ratings follow ASME B16.5 (up to NPS 24) and ASME B16.47 (larger). Gasket seating stress and the bolt load required to seat and keep the joint tight are handled through the flange design factors (the m and y factors) in the applicable code. When ordering, state the flange size and rating, facing, winding metal, filler, and inner-ring requirement. Vajra Industrial Solutions supplies flanged valves complete with correctly specified spiral wound gaskets and fasteners matched to the service and flange rating, with material certification and colour-code-verified gaskets to ASME B16.20.
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