What Is a PTFE Envelope Gasket?
A PTFE envelope gasket consists of a non-metallic filler insert (typically flexible graphite, compressed fiber, or rubber) enclosed in a PTFE (polytetrafluoroethylene) jacket that wraps around the entire gasket. The PTFE envelope provides near-universal chemical resistance while the internal filler supplies the compressibility and resilience needed for effective sealing. PTFE envelope gaskets are the standard choice for flanged connections handling corrosive, toxic, or high-purity fluids in chemical plants, pharmaceutical facilities, and food processing.
When to Use PTFE Envelope Gaskets
PTFE envelope gaskets are specified when the process fluid attacks conventional gasket materials:
- Concentrated acids (HCl, H2SO4, HF, HNO3) that corrode metal gasket components
- Strong alkalis (NaOH, KOH) that degrade graphite
- Chlorinated solvents and fluorinated chemicals
- Pharmaceutical and food-grade applications where contamination must be eliminated
- Glass-lined and plastic-lined equipment where bolt load must be limited
- Flat face (FF) flanges per ASME B16.1 on cast iron equipment
| Feature | Specification |
|---|---|
| Outer material | Virgin PTFE (0.5-1.0 mm thick) |
| Filler options | Flexible graphite, CAF, rubber (EPDM, Viton), glass fiber |
| Standard | ASME B16.21 (non-metallic gaskets) |
| Flange face | Flat face (FF) or raised face (RF) |
| Max temperature | 260 degC (500 degF); limited by PTFE |
| Max pressure | ~25 bar (360 psi) typical |
| Chemical resistance | Virtually universal (pH 0-14) |
Construction Types
| Type | Description | Filler | Typical Use |
|---|---|---|---|
| Type A | PTFE envelope with no filler (hollow) | None | Low-pressure, non-critical |
| Type B | PTFE envelope with flat insert | Compressed fiber, rubber | General chemical service |
| Type C | PTFE envelope with corrugated metal insert | SS 304/316 corrugated sheet | Higher pressure, better recovery |
| Type D | PTFE envelope with solid metal insert | SS 316 flat ring | Maximum rigidity, blowout resistance |
Type B with a flexible graphite insert is the most common configuration in chemical plants. The graphite provides excellent compressibility and recovery while the PTFE envelope prevents graphite from contacting the process fluid.
Pressure-Temperature Limits
| Filler Type | Max Temp | Max Pressure | Notes |
|---|---|---|---|
| Graphite insert | 260 degC (500 degF) | 25 bar (360 psi) | PTFE is the temperature limiter |
| CAF insert | 200 degC (392 degF) | 20 bar (290 psi) | CAF degrades before PTFE |
| Rubber (Viton) insert | 200 degC (392 degF) | 15 bar (218 psi) | Best for low-pressure chemical |
| Corrugated metal insert | 260 degC (500 degF) | 40 bar (580 psi) | Metal core increases pressure rating |
PTFE begins to creep at temperatures above 100 degC (212 degF). At higher operating temperatures, specify spiral wound gaskets with PTFE filler instead of PTFE envelope gaskets for better long-term sealing.
PTFE Envelope vs Full PTFE Sheet
| Feature | PTFE Envelope | Full PTFE Sheet |
|---|---|---|
| Creep resistance | Better (filler limits cold flow) | Poor (PTFE flows under load) |
| Recovery | Good (filler provides spring-back) | Poor |
| Blowout resistance | Good | Low |
| Chemical resistance | Excellent (PTFE on all surfaces) | Excellent |
| Cost | Moderate | Low |
Pair PTFE envelope gaskets with stud bolts and apply controlled bolt torque per ASME B16.21 guidelines. Avoid over-torquing; PTFE will cold-flow and extrude if compressed beyond its design limit.
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