Last updated: April 2026
A gasket is a deformable mechanical seal that fills the microscopic space between two mating surfaces — typically a pair of flanges — preventing fluid, gas, or pressure from escaping the joint. By compressing under bolt load, the gasket flows into the surface imperfections of the flange faces and creates a continuous, leak-tight barrier for the service life of the assembly.
Gaskets are used everywhere industrial fluids are contained under pressure: oil & gas refineries, water-treatment plants, chemical reactors, power stations, food processing, HVAC systems, and pressure vessels of every description. The correct gasket for a duty is determined by the media being sealed, the pressure and temperature, the flange type and finish, and the service life required.
What Does a Gasket Actually Do?
Two machined flange faces, no matter how well finished, are never perfectly flat at the microscopic level. Put them together and tiny channels remain between the peaks and valleys of the surface roughness. A gasket, made of a softer material, is compressed into those channels by the bolt load — filling them and forming a continuous seal. Three things have to happen for the joint to stay leak-tight:
- Seating. Enough compressive stress on the gasket to initially force it into the flange surface (the y factor in ASME VIII design).
- Operating. Enough residual stress to keep the seal during service, as internal pressure tries to blow the joint apart (the m factor).
- Retention. The bolt load must not drop below the operating stress, despite creep relaxation, thermal cycling, or vibration loosening.
What Are the Main Types of Gaskets?
Gaskets are grouped into three broad families, each suited to different flange types, pressures, and media:
Soft-Cut (Non-Metallic) Gaskets
Cut from sheet material: compressed non-asbestos fibre (CNAF), PTFE, rubber (EPDM, neoprene, nitrile, Viton), cork, or graphite. Used on low- to medium-pressure flat-face flanges, water lines, HVAC, and food or potable-water service. Inexpensive and easily cut to shape.
Semi-Metallic Gaskets
Combine a metallic structural element with a soft filler. Spiral wound gaskets (stainless winding + graphite or PTFE filler) are the default for ASME B16.5 flanges from Class 150 to 2500. Kammprofile gaskets (metal core with soft facing) offer higher recovery for vibration or thermal-cycling duties.
Metallic Gaskets
Solid metal rings machined to a specific cross-section. Ring-joint (RTJ) gaskets sit in a machined groove on API 6A or high-pressure flanges and seal by metal-to-metal contact. Used for Class 600 and above, wellheads, and high-temperature hydrocarbon service.
How Is a Gasket Selected?
Gasket selection is a process of matching six constraints against the catalogue:
| Constraint | Drives your choice of |
|---|---|
| Media (chemical) | Filler or sheet material (PTFE, graphite, rubber grade) |
| Temperature | Filler upper limit (PTFE 260°C, graphite 450°C, mica 800°C) |
| Pressure (flange class) | Gasket type — soft-cut → semi-metallic → metallic as pressure rises |
| Flange type and finish | Gasket profile (flat ring, spiral wound, kammprofile, RTJ) |
| Service requirement | Fire-safe, food-contact (FDA), potable water (WRAS, AS/NZS 4020) |
| Bolt load available | Gasket seating stress (soft gaskets need less; harder gaskets need more) |
What Is a Gasket Made Of?
- Elastomers — EPDM, nitrile (NBR), neoprene, Viton (FKM), silicone. Low-to-mid temperature, water and general chemical service.
- PTFE — near-universal chemical resistance, -200 to +260°C, FDA-compliant.
- Flexible graphite — high temperature, excellent recovery, the workhorse filler for refinery and steam service.
- Compressed non-asbestos fibre (CNAF) — aramid/glass fibre in a rubber binder, a general-purpose replacement for legacy asbestos sheet.
- Stainless steel — winding metal for spiral wound and kammprofile gaskets; solid rings for RTJs.
How Long Does a Gasket Last?
A correctly selected and installed gasket on a static flange can last the design life of the equipment — often 10 to 20 years. Three factors shorten that life: under-compression (inadequate seating stress), thermal cycling (loss of bolt preload through creep relaxation), and media attack (chemical incompatibility between gasket and process). Periodic re-tightening of critical flanges and adherence to ASME PCC-1 bolt-up procedures extend gasket service life substantially.
Frequently Asked Questions
A gasket is a soft, deformable ring or sheet clamped between two mating surfaces (such as flanges) to seal the joint. When bolts are tightened, the gasket compresses into the microscopic roughness of the flange faces and stops fluid or gas from leaking out.
Soft-cut (non-metallic) gaskets made from sheet material such as rubber, PTFE, graphite or compressed non-asbestos fibre; semi-metallic gaskets such as spiral wound and kammprofile, which combine a metal structural element with a soft filler; and metallic gaskets such as ring-joint (RTJ) gaskets, which are solid metal rings used on high-pressure flanges.
Match the gasket to six constraints: the media being sealed, the operating temperature, the flange pressure class, the flange type and surface finish, any service requirements (FDA, potable water, fire-safe) and the bolt load available. For most ASME B16.5 refinery and chemical flanges from Class 150 to 2500, a spiral wound gasket with graphite or PTFE filler is the default.
A gasket is a static seal — it sits between two non-moving surfaces. A seal (such as an O-ring on a rotating shaft or reciprocating piston) works between surfaces that move relative to each other. Both fill the gap and prevent leakage, but gaskets are designed for assembled joints, not dynamic service.
The three most common causes of gasket failure are under-compression (bolts not tightened to the correct preload), chemical attack (the gasket material is incompatible with the media) and loss of bolt load over time through thermal cycling, creep relaxation, or vibration loosening. Following ASME PCC-1 bolt-up procedures and selecting a gasket rated for the full duty envelope eliminates most premature failures.
Related Articles
- Types of gaskets — a practical overview
- Gasket materials — how to pick the right one
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Sources
- ASME Section VIII Division 1 — Appendix 2 (gasket m & y factors)
- ASME PCC-1 — Guidelines for Pressure Boundary Bolted Flange Joint Assembly
- ASME B16.20 / API 601 — metallic gasket standards