Overview
Gaskets create a seal between two flange faces by deforming under bolt load to fill microscopic imperfections on the flange surface. The correct gasket must be compatible with the fluid, pressure, and temperature; resilient enough to maintain seal during thermal cycling; and strong enough to resist blowout under operating pressure. Selection involves matching gasket type, material, and dimensions to the service conditions and flange standard.
Gasket Types
Non-Metallic (Soft) Gaskets
Compressed Fiber (CAF / CNAF)
The historical standard (formerly asbestos, now aramid/glass/mineral fiber in nitrile or EPDM binder).
- Temperature range: -40 to +400°C (depending on fiber/binder)
- Pressure: up to Class 300 (occasionally Class 600)
- Fluid compatibility: wide range with appropriate binder (nitrile for oil, EPDM for water/steam, neoprene for mild chemicals)
- Good for general water, oil, low-pressure steam
- Caveat: aging; sensitive to bolt over-load; requires flat flange faces
PTFE (Teflon)
Universal chemical resistance, but cold flows (creeps) under load.
- Temperature: -200 to +260°C
- Pressure: Class 150-300 (filled PTFE up to Class 600)
- Excellent for strong acids, chlorine, pharmaceutical, food
- Types: virgin PTFE (creeps badly); filled PTFE (glass, carbon, silica-filled to reduce creep); expanded PTFE (ePTFE, softer, lower seating stress)
- Caveat: requires lower bolt stress; re-torque after initial relaxation
Flexible Graphite
Excellent high-temperature performance.
- Temperature: -200 to +550°C (oxidizing); up to 3000°C inert
- Pressure: Class 150-2500 (with metal reinforcement)
- Fire-safe, zero creep under load, resistant to most chemicals (NOT for strong oxidizers: nitric acid, hot concentrated sulfuric)
- Available as plain sheet, laminated with tanged stainless steel core, or as filler in spiral-wound gaskets
- Preferred for high-temperature steam and refinery service
Rubber / Elastomer
Very low seating stress; used for low-pressure flat-face and raised-face flanges.
- Nitrile (NBR): oil/grease, -40 to 120°C
- EPDM: water/steam/chemical, -50 to 150°C
- Viton (FKM): chemicals/high temp, -20 to 200°C
- Pressure: Class 150 max
Semi-Metallic (Composite)
Spiral-Wound (Most Common for Process Industry)
Alternating V-shaped metal windings (provides resilience/springback) and soft filler (seals against flange faces).
- Metal windings: 304 SS, 316 SS, 321 SS, Inconel, Hastelloy, Monel
- Filler: flexible graphite (most common), PTFE, mica (for very high T)
- Inner ring: prevents inward buckling, required for Class 900+, vacuum service, high velocity
- Outer (guide) ring: centers gasket in flange, prevents over-compression
- Temperature: -200 to +870°C (metal dependent)
- Pressure: Class 150 through Class 2500
- Excellent resilience for thermal cycling
- Standard for raised-face (RF) flanges in chemical, oil & gas, power
Kammprofile (Grooved Metal with Soft Layer)
Solid metal core with concentric grooves, faced with soft material (graphite or PTFE).
- Excellent for heat exchangers, large vessel flanges
- Good recovery; easy to remove and re-face (replace the soft facing)
- Temperature to 1000°C; pressure to Class 2500
- Lower seating stress than solid metal gaskets
Metallic Gaskets
Ring Type Joint (RTJ)
Solid metal rings (oval or octagonal cross-section) for RTJ-flanged joints (B16.5 RJ facing). The ring deforms plastically under bolt load to seal the groove.
- Materials: soft iron, low-carbon steel, 304/316 SS, Alloy 825, Inconel, Hastelloy
- Temperature to 1000°C; pressure to Class 2500+
- Extremely reliable for high pressure/temperature (wellheads, subsea, refinery high-pressure)
- Single use: deforms plastically; never reuse
- Groove must be clean and undamaged
Metal Jacketed
Soft core (graphite, ceramic) inside a metal jacket. Mostly for heat exchanger gaskets and valve bonnets.
ASME Gasket Design Factors
ASME BPVC Section VIII Div.1 Appendix 2 defines two design parameters for each gasket type:
| Gasket | y (seating stress, psi) | m (operating factor) |
|---|---|---|
| Compressed fiber | 3,700-6,500 | 2.00-2.75 |
| Rubber (elastomer) <75 Shore A | 0-1,000 | 0.50-1.00 |
| PTFE (virgin) | 2,500-5,000 | 2.00-3.00 |
| Flexible graphite sheet | 2,500-4,500 | 1.50-2.50 |
| Spiral-wound w/ graphite filler | 3,000-10,000 | 2.50-3.50 |
| RTJ (soft iron) | 20,000+ | 5.50 |
| Solid metal flat | 20,000-40,000 | 3.50-6.50 |
yis the minimum stress to "seat" the gasket (deform it into flange imperfections) at assemblymis the factor for minimum gasket stress during operation (must hold against hydrostatic end force + m × pressure)- These govern required bolt load per ASME flange calculation
Selection by Service
Water / Low-Pressure Steam
- Class 150-300
- Compressed fiber with EPDM/nitrile binder OR rubber
- Cost-effective; forgiving installation
High-Temperature Steam (>200°C)
- Class 300-2500
- Spiral-wound 316 SS / graphite filler with inner/outer rings (ASME B16.20)
- Hot re-torque after 24 hours
Hydrocarbon / Oil & Gas
- Class 150-2500
- Spiral-wound (316 SS/graphite) for RF flanges
- RTJ for Class 600+ on wellheads and manifold
- Fire-safe certification (API 6FB) required for flammable service
Chemical / Corrosive
- Match gasket material to fluid chemistry (use manufacturer compatibility chart)
- PTFE or ePTFE for acids, chlorine, pharmaceuticals
- Alloy windings (Hastelloy C-276, Alloy 20) for severe service
- Check gasket binder compatibility (not just facing)
High Pressure (>Class 900)
- RTJ or Kammprofile
- Verify flange groove condition
- Hydraulic bolt tensioning often required for accurate preload
Vacuum Service
- Spiral-wound with inner ring mandatory
- PTFE envelope or Kammprofile for low leak rate
- Avoid rubber (permeates at high vacuum)
Food / Pharmaceutical / Sanitary
- PTFE, EPDM (FDA-compliant)
- Tri-clamp fittings with molded gaskets, not bolted flanges
Compatibility Matrix Highlights
| Fluid | Recommended Gasket | Avoid |
|---|---|---|
| Strong acids (HCl, H₂SO₄) | PTFE, Viton | Fiber, graphite |
| Caustics (NaOH) | EPDM, PTFE (at high T) | Buna-N |
| Hydrocarbons | Nitrile, Viton, spiral-wound/graphite | Natural rubber, EPDM |
| Steam (>200°C) | Graphite spiral-wound | Rubber, PTFE (limited T) |
| Chlorine, oxygen | Alloy C-276 spiral, Monel RTJ, copper | Graphite, any organic |
| Refrigerants | Neoprene, nitrile | Butyl |
Installation Best Practice Reminders
- New gasket every time — compressed gaskets do not re-seal reliably
- No gasket cement/sealant on spiral-wound or RTJ
- Check flange face finish: RA 125-250 µin (3.2-6.3 µm) for soft gaskets; RA 63-125 µin for spiral-wound; RTJ groove per API
- Store gaskets flat, dry, away from heat/oil/UV
- Verify gasket dimensions against flange size and class (use B16.21/B16.20 tables)
Summary
Select gaskets by matching material chemistry, temperature, and pressure to the service conditions. Spiral-wound (316 SS with graphite filler, inner and outer rings) is the default choice for most process piping (Class 150-2500 raised-face flanges). RTJ gaskets are standard for high-pressure and subsea service. PTFE is the universal chemical-resistant option but requires lower bolt stress due to creep. Compressed fiber is economical for water/low-pressure steam but is aging-sensitive. Always follow ASME PCC-1 installation and use a new gasket on every joint break — 70% of flange leaks trace to poor selection or installation, not design.