A spiral-wound gasket is the cheapest component in a flanged joint and the most frequent cause of that joint leaking. It is specified in a single line on a requisition, arrives in a box that looks interchangeable with any other, and fails for reasons that trace back to that one line. This guide covers what actually has to be on it.
The short answer: a spiral-wound gasket is defined by five things — the winding metal, the filler, the ring configuration (inner, outer, both), the flange size and pressure class it must fit, and the standard it is built to. Specify all five, or the supplier will choose for you.
What the parts do
A spiral-wound gasket is not one material. It is an assembly, and each part has a separate job:
- Winding metal — a V-shaped or W-shaped preformed strip, wound in a spiral. This carries the load and provides the springback that keeps the joint sealed through thermal cycling. Commonly 304 or 316 stainless steel; higher alloys (Alloy 825, Inconel, Hastelloy, titanium) where the process fluid attacks stainless.
- Filler — the soft material wound alternately with the metal. This is what actually seals. Flexible graphite is the general-purpose choice for hydrocarbons and steam; PTFE for aggressive chemicals; mica or ceramic where temperatures exceed graphite's oxidation limit.
- Outer centring ring — a solid carbon-steel ring that locates the gasket concentrically in the bolt circle and acts as a compression stop, preventing over-compression of the windings.
- Inner ring — a solid metal ring on the bore side. It stops the windings buckling inward under high load and shields the filler from erosion.
The failure most people have seen — a gasket that blew out sideways into the pipe bore — is usually a missing inner ring, not a bad gasket.
The five things a line item must state
A complete gasket line reads something like:
Spiral-wound gasket, 4 inch, Class 300, 316L winding / flexible graphite filler, with CS outer ring and 316L inner ring, to ASME B16.20, for raised-face flange to ASME B16.5.
Broken down:
| Element | Why it matters | Typical values |
|---|---|---|
| Size | Must match the flange NPS, not the pipe OD | NPS ½" – 24" and above |
| Pressure class | Sets the gasket's thickness, ring dimensions and whether an inner ring is mandatory | 150, 300, 600, 900, 1500, 2500 |
| Winding metal | Corrosion resistance and springback | 304, 316L, 321, Alloy 825, Inconel 625 |
| Filler | The actual seal; sets the temperature ceiling | Flexible graphite, PTFE, mica |
| Ring configuration | Prevents buckling and over-compression | Outer only, inner + outer |
Two further items belong on the line whenever they apply: the facing type (raised face, flat face, or a ring-type joint groove, which takes a different gasket entirely), and any material certification you need — typically an EN 10204 3.1 certificate for the metallic components.
Choosing the filler
The filler sets the operating envelope more than the metal does.
- Flexible graphite — the default for hydrocarbon service, steam and most utilities. Excellent sealing, very good thermal stability, and it conforms to imperfect flange faces. Its limit is oxidation: in an oxidising atmosphere it begins to degrade well below the temperature it would tolerate in an inert one. Confirm the ceiling against the manufacturer's data for your service, not the headline figure.
- PTFE — for strong acids, caustics and other chemistry that attacks graphite. Chemically near-inert across the pH range, but it has a much lower temperature ceiling and is prone to cold flow (creep) under sustained load, so bolt retorquing matters more.
- Mica / ceramic — specified where the joint sees fire exposure or temperatures beyond graphite's oxidising limit. More expensive and less forgiving of flange-face imperfection.
Colour coding, and why you should still read the box
ASME B16.20 defines a colour code so a gasket can be identified after it leaves its packaging: the outer ring edge colour identifies the winding metal, and a stripe on the outer ring identifies the filler.
It is a genuinely useful system and a genuinely common source of error, because the codes are similar and stores personnel memorise them wrong. Two rules keep you out of trouble:
- Treat the colour as a cross-check, never as the specification. The marking stamped on the outer ring is the specification.
- On receipt, verify the stamped size, class, winding and filler against the purchase order line before the gasket goes into stores. A mis-picked gasket found on the rack costs nothing; the same gasket found during a shutdown costs a shift.
The mistakes that actually cause leaks
Reusing a gasket. A spiral-wound gasket seals by permanent compression set. Once released, it is scrap. Budget them as consumables — one per joint break, plus spares.
Torquing wrong. Bolt load, not bolt torque, seals a joint; torque is only a proxy for it, and a poor one when the nut factor is uncertain. Follow a documented cross-pattern sequence in stages. For critical joints, controlled bolting removes the guesswork — see our guide to hydraulic torque wrench selection for flange bolting.
Mismatched facing. A raised-face gasket in a ring-type joint groove will not seal, and vice versa. Confirm the flange facing from the drawing, not from the flange in front of you.
Ignoring flange condition. A spiral-wound gasket tolerates more surface imperfection than a sheet gasket, but not a scored, pitted or non-parallel face. Inspect the face before you blame the gasket.
Over-compression. The outer ring exists to stop it. A gasket without one, in a joint where the bolts can be pulled well past the design load, is a leak waiting for the next thermal cycle.
Sourcing gaskets in Nigeria
Gaskets are low-value, high-consequence, and consumed continuously — which makes them exactly the wrong thing to run out of and exactly the wrong thing to air-freight one at a time. Two practical points for Nigerian buyers:
- Buy to a schedule, not to an emergency. Gaskets are light, cheap to consolidate and easy to store. A standing order sized to your turnaround plan costs a fraction of the same items expedited.
- Specify the certificate up front. If your QA needs EN 10204 3.1 traceability on the metallic parts, it must be on the enquiry — retrofitting a certificate to material already manufactured is usually impossible. Our guide to importing certified industrial equipment into Nigeria covers where certification enters the process.
FirstSupply supplies spiral-wound, ring-type-joint and sheet gaskets against specification, including the material certification where your requisition calls for it. Browse pipe, fittings & flanges, or send us the flange schedule and we will price it line by line — including the joints where a different gasket type is the better answer.