What Is a Soft Gasket?
Soft gaskets seal by compressing between flange faces under bolt load. Unlike spiral wound gaskets or RTJ rings, they contain no metal winding or machined metallic profile. Their low seating stress makes them suitable for cast iron flanges (ASME B16.1), glass-lined equipment, and any application where excessive bolt load could damage the flange.
Soft Gasket Materials
| Material | Max Temp | Max Pressure | Chemical Resistance | Common Applications |
|---|---|---|---|---|
| PTFE (virgin) | 260 degC (500 degF) | ~40 bar (600 psi) | Excellent (nearly universal) | Chemical, pharma, food, oxidizing acids |
| Expanded PTFE (ePTFE) | 315 degC (600 degF) | ~80 bar (1,160 psi) | Excellent | Chemical plants, aggressive media |
| Flexible graphite | 450 degC (842 degF) | ~100 bar (1,450 psi) | Good (not for strong oxidizers) | Steam, hydrocarbons, high-temp service |
| Compressed aramid fiber (CAF) | 200 degC (400 degF) | ~60 bar (870 psi) | Moderate | Water, steam, oils, general utilities |
| Rubber (EPDM, NBR, Viton) | 100-200 degC | ~10 bar (150 psi) | Varies by elastomer | Water, low-pressure utilities, HVAC |
| Ceramic fiber | 1,000 degC (1,832 degF) | Low | Good | Furnace doors, exhaust systems |
Flexible graphite and PTFE dominate industrial piping. CAF (formerly asbestos-based, now asbestos-free) is still used in utility and low-criticality services.
Gasket Forms
Soft gaskets come in two basic forms:
- Full-face gasket: Covers the entire flange face including the bolt holes. Required for flat face (FF) flanges to distribute bolt load evenly across the flange.
- Ring gasket (ID to bolt circle): Sits inside the bolt circle on raised face (RF) flanges. Uses less material and is easier to install.
| Gasket Form | Flange Face | Bolt Holes in Gasket? |
|---|---|---|
| Full-face | Flat face (FF) | Yes |
| Ring | Raised face (RF) | No |
Thickness Selection
Standard thicknesses for soft gaskets per ASME B16.21 are 1/16” (1.5 mm), 1/8” (3.0 mm), and 1/4” (6.0 mm). Thinner gaskets (1/16”) are preferred because they minimize creep relaxation and blowout risk. Thicker gaskets are used only when flange faces are rough or warped and need additional compressibility to fill surface irregularities.
Advantages and Limitations
| Advantage | Limitation |
|---|---|
| Low bolt load required | Not suitable for high-pressure classes (600+) |
| Will not damage soft flange faces | Prone to creep and relaxation over time |
| Low cost | Limited temperature range vs. metallic gaskets |
| Easy to cut and install | Cannot handle severe thermal cycling |
| Wide chemical compatibility (PTFE) | May blow out under pressure surges |
Soft gaskets require proper bolt torque to compress without over-stressing. Unlike metallic gaskets, soft gaskets continue to creep after initial bolt-up. Hot re-torque after the first thermal cycle is recommended for services above 150 degC.
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