Spiral Wound Gasket: How It Works
A spiral wound gasket (SWG) is a semi-metallic sealing element made by winding a preformed metal strip and a soft filler material into a dense, alternating spiral. The metal strip provides structural resilience and recovery, while the filler creates the sealing surface. SWGs are the most widely used gasket type for raised face (RF) flanges in oil and gas, refining, and petrochemical applications.
| Component | Material Options | Function |
|---|---|---|
| Metal winding strip | SS 304, 316, 321, Monel 400, Inconel 600, Hastelloy C-276 | Provides spring-back and structural strength |
| Filler material | Flexible graphite, PTFE, ceramic | Seals micro-gaps between flange faces |
| Outer ring (centering ring) | Carbon steel (painted yellow, green, etc.) | Centers gasket on flange; acts as compression stop |
| Inner ring | Same alloy as winding or higher | Prevents winding buckling into the bore; required for certain services |
| Standard | ASME B16.20 | Dimensions, tolerances, and marking |
| Flange face | Raised face (RF) per ASME B16.5/B16.47 | Standard seating surface |
How a Spiral Wound Gasket Is Made
The manufacturing process begins with a V-shaped or flat metal strip (typically 0.15-0.20 mm thick) and a filler strip of similar width. Both are fed simultaneously onto a rotating mandrel. The winding machine interlocks the metal and filler in concentric spirals, creating a gasket with alternating layers of resilient metal and compressible sealing material.
After winding, the gasket density is controlled by the number of turns and the winding tension. The outer centering ring is spot-welded or tack-welded to the outermost metal winding. If specified, an inner ring is inserted inside the innermost winding to reinforce the bore side.
Metal and Filler Selection
The winding metal must be compatible with the process fluid. The filler must withstand the operating temperature without degradation.
| Service | Winding Metal | Filler | Max Temp |
|---|---|---|---|
| General hydrocarbon | SS 316/316L | Flexible graphite | 450 degC (842 degF) |
| High-temperature steam | SS 321 | Flexible graphite | 500 degC (932 degF) |
| Oxidizing acids | Monel 400 | PTFE | 260 degC (500 degF) |
| Highly corrosive (HCl, H2SO4) | Hastelloy C-276 | Flexible graphite | 450 degC (842 degF) |
| Food-grade / pharma | SS 316L | PTFE | 260 degC (500 degF) |
Flexible graphite is the default filler for most oil and gas services because it handles thermal cycling, resists blowout, and tolerates flange surface imperfections. PTFE filler is specified when graphite contamination is a concern (food, pharma, oxidizing chemicals).
Outer Ring Color Coding
ASME B16.20 mandates color-coded outer rings to identify the winding metal at a glance.
| Outer Ring Color | Winding Material |
|---|---|
| Yellow | SS 304 |
| Green | SS 316 |
| Turquoise | SS 321 |
| Gray | Monel 400 |
| Red | Inconel 600 |
Pressure and Temperature Range
SWGs cover ASME pressure classes 150 through 2500 and temperatures from cryogenic (-200 degC) up to approximately 1000 degC, depending on metal and filler selection. They are specified in virtually all piping classes that use RF flanges at moderate to high pressure.
Spiral wound gaskets pair with stud bolts and heavy hex nuts torqued per ASME B16.5 bolt charts. Proper bolt load is critical to compress the filler without over-stressing the windings.
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