Graphite vs PTFE Filler
Flexible graphite and PTFE (polytetrafluoroethylene) are the two primary filler materials used in spiral wound gaskets (SWGs). The filler is the soft layer wound between the metal strips that forms the actual sealing surface. Selecting the correct filler determines the gasket’s temperature limit, chemical compatibility, and long-term sealing reliability.
When to Use Each Filler
Graphite filler is the default for oil and gas, refining, and power generation applications. PTFE filler is specified when graphite is incompatible with the process fluid or when contamination must be avoided.
- Use graphite filler for: hydrocarbons, steam, caustic, hydrogen, ammonia, and general refinery/petrochemical service
- Use PTFE filler for: oxidizing acids (HNO3), halogens (Cl2, F2), food and pharmaceutical, and any service where graphite contamination is unacceptable
Head-to-Head Comparison
| Property | Flexible Graphite | PTFE |
|---|---|---|
| Max temperature | 450 degC (842 degF) in air; 650 degC (1,200 degF) in steam | 260 degC (500 degF) |
| Min temperature | -200 degC (-328 degF) | -200 degC (-328 degF) |
| Chemical resistance | Good (fails with strong oxidizers | Excellent) nearly universal |
| Sealability (low stress) | Excellent | Good |
| Creep resistance | Good | Moderate (PTFE cold-flows under load) |
| Fire safety | Excellent (non-combustible) | Poor (emits toxic fumes above 300 degC) |
| Blowout resistance | Excellent | Good |
| Fugitive emissions | Very low (tight seal at low stress) | Low |
| Cost | Moderate | Moderate to high |
Temperature Limits by Winding Metal
| Winding Metal | With Graphite Filler | With PTFE Filler |
|---|---|---|
| SS 304 | 450 degC (842 degF) | 260 degC (500 degF) |
| SS 316/316L | 450 degC (842 degF) | 260 degC (500 degF) |
| SS 321 | 500 degC (932 degF) | 260 degC (500 degF) |
| Monel 400 | 450 degC (842 degF) | 260 degC (500 degF) |
| Inconel 600 | 650 degC (1,200 degF) | 260 degC (500 degF) |
| Hastelloy C-276 | 450 degC (842 degF) | 260 degC (500 degF) |
PTFE is always the temperature-limiting component when used as filler. Regardless of how heat-resistant the winding metal is, the maximum service temperature is capped at 260 degC with PTFE filler.
Chemical Compatibility Matrix
| Process Fluid | Graphite | PTFE | Recommended Filler |
|---|---|---|---|
| Crude oil, refined products | Compatible | Compatible | Graphite (higher temp) |
| Saturated/superheated steam | Compatible | Not recommended | Graphite |
| Natural gas | Compatible | Compatible | Graphite |
| Dilute H2SO4 (<70%) | Compatible | Compatible | Either |
| Concentrated HNO3 | Incompatible | Compatible | PTFE |
| Chlorine gas (Cl2) | Incompatible | Compatible | PTFE |
| Caustic (NaOH) | Compatible | Compatible | Graphite (better temp) |
| Food/pharmaceutical | Potential contamination | Compatible | PTFE |
| Oxygen service | Incompatible | Compatible | PTFE |
| Hydrogen | Compatible | Compatible | Graphite |
Outer Ring Color Codes (ASME B16.20)
The outer centering ring color identifies the winding metal, not the filler. However, PTFE-filled gaskets are sometimes marked with a white stripe or tag to differentiate them from graphite-filled equivalents with the same winding metal.
Both graphite and PTFE fillers require proper bolt torque applied using A193 B7 stud bolts per the ASME bolt chart. Graphite filler generally requires 5-10% lower seating stress than PTFE for the same gasket dimensions.
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