What the Color Bands on a Spiral Wound Gasket Are Telling You
A spiral wound gasket carries its entire specification on its outer edge in two painted bands, and most crews installing them could not read that code without a chart. The bands identify the winding metal and the filler material, which together determine what temperature, pressure, and chemistry the gasket can survive. Getting the combination wrong produces a joint that seals on hydrotest and fails months into service. ASME B16.20 governs how these gaskets are built and marked, and understanding that structure turns gasket selection from a stocking question into a specification decision.
How Is a Spiral Wound Gasket Constructed?
A spiral wound gasket is built from alternating layers of a preformed metal strip and a softer filler, wound together into a spiral and welded at the start and finish points. The metal strip provides resilience and resistance to blowout, while the filler provides the conformability that lets the gasket seal against imperfect flange faces. Compression flattens the windings and forces the filler into surface irregularities, and the metal strip springs back as the joint cycles through temperature and pressure changes. That combination of sealing and recovery is what separates spiral wound construction from a flat sheet gasket.
Most spiral wound gaskets in flanged piping include an outer centering ring and, depending on the service, an inner ring. The outer ring positions the gasket concentrically in the bolt circle and acts as a compression stop that prevents over compression of the windings. The inner ring sits at the bore side and controls a set of failure modes that arise when the windings are exposed directly to the process stream. The construction standard for raised face flanges is published by ASME as B16.20, which fixes the dimensions, materials, and marking system.
What Is the Outer Ring Doing Mechanically
The outer centering ring performs three functions that are easy to underestimate. It centers the gasket so the sealing element sits evenly under the flange face rather than offset toward one side of the bolt circle. It limits how far the bolting can compress the windings, which protects the gasket from being crushed past its recovery point during assembly. It also carries the identification marking that lets an inspector confirm the correct gasket went into the joint without disassembling anything.
Why Does the Filler Material Drive the Selection?
Filler material determines the chemical and thermal envelope of the gasket far more than the winding metal does in most services. Flexible graphite is the most common filler in refinery and petrochemical work because it handles high temperature and a broad range of hydrocarbons without degrading. PTFE fillers handle aggressive acids and caustics that would attack graphite, and they suit services where the sealing element cannot contribute any contamination. Mica and ceramic fillers appear where temperatures exceed what graphite can sustain in an oxidizing atmosphere.
The filler also sets the practical limit on how the gasket behaves during thermal cycling. Graphite oxidizes above a defined temperature in air, which means a graphite filled gasket serving a hot line can degrade from the outside inward at the exposed edge while the interior remains sound. PTFE cold flows under sustained load, which affects long term bolt load retention in joints that are not retorqued. Matching the filler to both the process chemistry and the cycling pattern avoids selecting for one and failing on the other.
What Do the Color Codes Identify?
ASME B16.20 assigns a color to the outer ring that identifies the winding metal, and a stripe on that ring identifies the filler. Reading both together gives the full material specification without reference to paperwork. The table below summarizes the most common combinations encountered in industrial piping, and the governing standard should be consulted for the complete list and for any manufacturer specific variations.
| Marking | Identifies | Common designation |
|---|---|---|
| Yellow ring | Winding metal | Type 304 stainless steel |
| Green ring | Winding metal | Type 316 stainless steel |
| Gray ring | Winding metal | Type 321 stainless steel |
| Silver stripe | Filler | Flexible graphite |
| White stripe | Filler | PTFE |
| Pink stripe | Filler | Ceramic |
| Light green stripe | Filler | Mica graphite |
When Should an Inner Ring Be Specified?
An inner ring is not a universal requirement, and specifying one where it is not needed adds cost without benefit. The cases where it earns its place are specific and predictable. Inner rings prevent inward buckling of the windings in higher pressure classes, they shield the windings from erosion by the process stream, and they eliminate the annular gap between the flange bore and the gasket where product can accumulate.
- Class 900 and higher pressure ratings, where inward buckling risk increases
- PTFE filled gaskets, where the filler alone offers limited radial support
- Services where the process stream would erode exposed winding edges
- Applications requiring a smooth bore transition with no cavity for product buildup
- High temperature service where graphite oxidation at the inner edge is a concern
How Does the Inner Ring Interact With Flange Bore
An inner ring sized incorrectly protrudes into the flow path or leaves a step at the transition, and either condition creates turbulence and a deposition site. The ring inside diameter should correspond to the flange bore rather than to a nominal pipe size, which matters most on heavy wall pipe where the bore is meaningfully smaller than the nominal. Confirming the schedule of the mating pipe before ordering avoids a gasket that fits the bolt circle correctly and interferes with the bore.
How Does Flange Class Affect Gasket Thickness and Compression?
Flange pressure class determines the available bolt load, and bolt load determines whether a given gasket reaches its required seating stress. A spiral wound gasket needs a minimum compression to seal and has a maximum beyond which the windings are damaged, and both limits sit within a narrower band than a sheet gasket allows. Higher pressure classes deliver more bolt load through larger and more numerous studs, which is why the same nominal gasket size behaves differently across classes.
Thickness selection follows from the flange face condition and the class rather than from preference. Standard thickness suits machined raised faces in good condition, while thicker construction may be indicated where face damage or distortion is present, at the cost of reduced blowout resistance. The relationship between flange dimensions, bolting, and gasket seating is covered in more depth in our overview of industrial flange specifications for pipeline and process piping.
Where Spiral Wound Construction Reaches Its Limit
Spiral wound gaskets cover a wide band of industrial service, and they are not the answer at both extremes. Below their seating stress requirement, on light flanges or thin wall connections, a softer sheet gasket seals with less bolt load. Above their pressure and temperature envelope, metal to metal ring joint construction takes over, and the flange face itself changes to a machined ring groove. Recognizing where a joint sits relative to those boundaries prevents specifying a spiral wound gasket into a service it cannot hold.
High temperature steam and superheat service sits close to the upper boundary and deserves particular attention on filler selection, a pattern examined in our discussion of high temperature gasket materials for stainless steel steam flanges. Selection in those services depends as much on the thermal cycling pattern as on the peak temperature.
Building a Gasket Specification That Survives the Warehouse
A complete spiral wound specification names the winding metal, the filler, the outer ring material, whether an inner ring is required, the nominal size, and the flange class. Anything left unstated gets filled from stock, and a substitution of filler or winding metal changes the service envelope without changing the appearance of the part. Recording the full specification on the isometric and on the gasket schedule keeps the correct part reaching the correct joint through a turnaround.
Flange face condition, bolting grade, and gasket construction are one system rather than three separate purchases, and specifying them together produces joints that hold through thermal cycles. Teams assembling a gasket schedule across a unit can work through the material and ring requirements with our team alongside the matching flange and industrial gasket requirements for the project.