Do you know what the markings on the spiral wound gasket represent

Leaks at flanged joints start with a gasket that was read wrong. A metallic spiral wound gasket carries its whole identity in the marking stamped on the outer ring, and that line of codes — size, pressure, three materials and a governing standard — is what engineers, buyers and fitters decode before a joint is ever closed. This guide breaks down every element of the marking so the spiral wound gaskets on the shelf match the joint on the drawing.

Seven Data Points on Every Gasket

The marking is not decoration; it is the traceability record and the acceptance basis in one line. Seven elements repeat on almost every product.

• Manufacturer mark — the source of traceability when a joint has to be audited or a part replaced.
• Nominal size (DN) — a standardized series number such as DN150 or DN300, related to the bore of the pipe or flange, not a measured inner diameter.
• Nominal pressure (PN or Class) — the rating band, e.g. PN16, PN40, Class 150, Class 600, covering the load capacity the gasket is designed for at its design pressures and temperatures.
• Inner ring material — the code left of the slash, e.g. 304.
• Filler material — the middle code, e.g. FG for flexible graphite.
• Outer ring material — the right code, e.g. CS for carbon steel; a full string like 304/FG/CS reads inner ring 304, flexible graphite filler, carbon steel outer ring.
• Governing standard — the single authoritative basis for design, manufacture and acceptance, e.g. ASME B16.20.

Size, Pressure and Material Codes

Size decides fit, pressure decides survival, materials decide what the gasket can withstand. A DN150 gasket never goes on a DN125 or DN200 flange: the nominal size must match the mating pipe flanges exactly so the gasket centres inside the bolt circle. The pressure rating sets the winding geometry — more layers and a denser V-strip on higher ratings — and it must be equal to or above the flange design rating; down-rating a gasket in a higher-pressure system is a safety fault, not a saving.

• Ring codes — 304, 316/316L, 321 for stainless service, carbon steel where corrosion is mild, and Monel, Hastelloy or titanium for concentrated acids and strong alkalis.
• Filler codes — FG flexible graphite covers most steam, hot oil and hydrocarbon duty; PTFE covers clean pharmaceutical and food service with strong acids and alkalis; ceramic fibre handles hot flue gas; non-asbestos fibre suits moderate water, oil and solvent service.
• Filler identification by color — gray flexible graphite, white PTFE, light green ceramic, pink mica graphite.

Inner ring grades at a glance:

Temperature bands are the usual published service ranges for the ring grades; confirm the exact limit against the gasket data sheet for the order.

Standards Behind the Marking

The standard number in the marking is the law for manufacturing and acceptance. Common systems:

• GB/T 4622.2-2022 and the GB/T 9129-2022 series — the basic product and pipe flange gasket standards in the Chinese national system.
• HG/T 20610-2022 for PN-series flanges and HG/T 20631-2022 for Class-series flanges — the pair most often specified on chemical project drawings.
• ASME B16.20 — the international reference, used together with ASME B16.5 flanges.
• ISO 21298 — common on international project work.

The rule is simple: the gasket standard must correspond exactly to the standard of the connecting flanges. Read the flange marking or the drawing note first; that is the definitive selector.

Reading a Complete Marking

Take a gasket stamped Qishine 4in 300# 304/FG/CS ASME B16.20.

• Maker — the manufacturer named on the ring answers for the quality.
• Size and pressure — for a flange rated 4 inches nominal diameter at 300# nominal pressure.
• Materials — inner ring 304 stainless, flexible graphite filler, carbon steel outer ring.
• Standard — ASME B16.20, so it installs on a flange marked 4in 300# with a matching face type such as raised face under ASME B16.5.

Verify the full string once more before every installation. Where service conditions are unusual — vacuum, cryogenic duty, thermal cycling — confirm the selection with the manufacturer's technical desk before the joint is closed.

Frequently Asked Questions

What does 304/FG/CS mean on a gasket marking?

It is the material stack: inner ring 304 stainless steel, filler flexible graphite, outer ring carbon steel, in that order.

Can a DN150 gasket be used on a DN200 flange?

No. The nominal size must match the mating flange exactly; a mismatch leaves the gasket off-centre in the bolt circle and the joint leaks.

How do I choose between flexible graphite and PTFE filler?

Flexible graphite covers most steam, hot oil and hydrocarbon service; PTFE is chosen for clean pharmaceutical and food duty with strong acids and alkalis, up to about 260 °C.

Which standard should the gasket follow?

Whichever standard governs the flanges: ASME B16.20 with ASME B16.5 flanges, or the GB/T and HG/T series on Chinese designs. Match the flange marking first.

Why does pressure rating affect gasket thickness?

Higher ratings need more winding layers and a denser V-strip to hold the load, so a Class 600 gasket is visibly thicker and heavier than a Class 150 of the same size.