Gaskets for flanged connections are the largest single use of PTFE sheet. The common failure is not chemical. It is bolt-load loss.
Types
Skived or moulded virgin PTFE sheet. Chemically universal and clean. Prone to creep, so it needs a controlled low-stress or confined design.
Filled PTFE sheet. Glass, silica, barium sulphate or other mineral fillers cut cold flow substantially. Filler choice sets chemical limits (silica and glass are attacked by hydrofluoric acid and strong alkalis; barium sulphate is better).
Expanded PTFE (ePTFE). Stretched PTFE with a soft microporous structure. Conforms to rough or uneven flanges at low bolt load. Often supplied as tape (joint sealant) or as sheet.
Envelope gasket. A compressible core (non-asbestos fibre, rubber, metal) wrapped in a PTFE skin. The skin resists the fluid, the core supplies resilience.
Modified PTFE gasket. Denser, less permeable and creeps less than standard.
Selection guide
| Duty | Suggested type |
|---|---|
| Glass-lined or plastic flanges (low bolt load) | ePTFE |
| Strong acids, bases, solvents | Virgin or modified |
| Steam, thermal cycling | Filled or envelope |
| High purity or pharma | Virgin or modified with certificate |
| Irregular flanges | ePTFE or envelope |
| Hydrofluoric acid | Barium sulphate-filled or virgin; avoid glass or silica |
Installation points
- Clean, flat, undamaged flange faces.
- Tighten in a star pattern in stages. Do not over-torque; excessive bolt load simply squeezes PTFE out.
- Retighten after the first temperature cycle as the supplier advises.
- Use a stop (a groove, a spacer or a confining ring) when you must not exceed a certain compression.
- Use dimensioned data: gasket factors (m and y values) from the gasket supplier, in the right flange standard.
Common failures
Extrusion (blow-out) at the inside edge, creep-relaxation leaks after heating, chemical attack of the filler, and leakage from rough or damaged flange faces. Most are solved by grade choice and installation, not by buying a thicker gasket.
