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Guide

Flange Gasket Selection — Practical Guide

Of all the components in a piping system, the gasket is the cheapest — and the one most likely to leak. A wrong gasket choice can void the pressure rating of a whole flange assembly, shut down a plant for an unscheduled replacement, or in the worst case release hydrocarbons to atmosphere. This guide walks through the main gasket families, the selection criteria that actually matter, and specific recommendations by service.

What a Gasket Has to Do

A gasket seals the imperfect mating surfaces of two flange faces. Under bolt load it must deform enough to fill surface irregularities but recover enough to maintain the seal as the bolts relax (creep, thermal cycling, vibration). Three properties govern that balance:

  • Seating stress (y) — the minimum compressive stress needed to make the gasket seal initially. A soft rubber gasket seats at < 5 MPa; a solid metal ring seats at > 100 MPa.
  • Gasket factor (m) — a multiplier on the internal pressure, giving the minimum residual stress that must remain on the gasket in service to keep it sealed.
  • PT rating — the maximum pressure/temperature combination the material can tolerate without creeping, extruding, or decomposing.

ASME Section VIII Appendix 2 uses m and y to size bolting. Manufacturers publish these values per product — do not rely on generic textbook figures for a specific gasket.

The Three Gasket Families

Soft (Non-Metallic) Gaskets

Flat sheet gaskets cut from a continuous material. Cheap, easy to fabricate on site, suitable for low to moderate pressure and temperature.

  • Compressed non-asbestos fibre (CNAF) — the modern replacement for compressed asbestos. Good general-purpose sealing up to ~40 bar and ~200 °C. Variants exist for oil, steam, and mild chemical service.
  • Elastomer (EPDM, NBR, FKM/Viton) — for water, refrigerant, and low-temperature chemical service. EPDM for water and steam (short term); NBR for hydrocarbons and oil; FKM for aggressive chemicals and high temperature (up to ~200 °C).
  • PTFE sheet — exceptional chemical resistance; handles nearly any fluid short of molten alkali metals and fluorine gas. Pure PTFE creeps badly under load — most installations use a modified or expanded PTFE (ePTFE) that resists cold flow. Good from cryogenic to ~260 °C.
  • Flexible graphite — dense graphite foil, often reinforced with a tanged stainless insert. Excellent for steam and high-temperature service up to ~450 °C in oxidising atmospheres, higher in inert. Not suitable for strong oxidisers.

Semi-Metallic Gaskets

Combine a metallic structure for strength with a soft filler for sealing.

  • Spiral wound gasket (SWG) — alternating plies of metal strip (typically 304, 316, or Inconel) and a soft filler (graphite or PTFE) wound into a ring. Supplied with an inner ring to prevent inward buckling and an outer centring ring that seats on the bolt circle. The industry workhorse for ASME class 150 through 2500. Graphite fill for steam and hydrocarbons; PTFE fill for aggressive chemicals.
  • Kammprofile (grooved metal core, soft-faced) — a machined metal core with concentric grooves, faced on both sides with thin layers of graphite or PTFE. Handles higher loads than a spiral wound and tolerates flange rotation better. Increasingly preferred for heat exchangers and large-diameter flanges.
  • Metal-jacketed — a soft filler wrapped in a metal jacket. Mostly found on legacy heat exchanger designs; kammprofile is the modern alternative.

Metallic Gaskets

Solid metal rings, used where temperature or pressure exceeds what semi-metallic gaskets can handle.

  • Ring Type Joint (RTJ) — oval or octagonal solid metal rings seated in machined grooves (ASME B16.20). Mandatory on class 900 and above in most piping codes and on API 6A wellhead equipment. Soft iron, low-carbon steel, 316, Monel, Inconel — the ring must be softer than the flange groove so it deforms on tightening.
  • Lens rings, double-cone — specialised solid-metal gaskets for very high pressure service (> 400 bar). Rare outside hydrogen and ammonia synthesis.

Match the Gasket to the Flange Facing

The flange facing dictates which gasket families are even usable.

  • Raised face (RF) — the default for ASME class 150–2500. Accepts soft sheet, spiral wound, and kammprofile.
  • Flat face (FF) — used where a raised face would overstress a brittle mating flange (cast iron pump casings, glass-lined vessels). Use a full-face soft gasket, never a spiral wound or kammprofile.
  • Ring Type Joint (RTJ) — only accepts the matching RTJ ring; the groove geometry is machined to suit.
  • Tongue-and-groove, male-female — retains the gasket laterally. Works well with PTFE and graphite, avoiding the blowout risk that full-face sheet gaskets have at high pressure.

For flange dimensions and bolt patterns, see our flange dimensions reference and bolt-hole orientation guide.

Recommendations by Service

| Service | First Choice | Notes | | --- | --- | --- | | Cold water, utility | EPDM or CNAF | Full-face on FF flanges; RF flanges tolerate either | | Saturated steam ≤ 10 bar | CNAF (steam-rated) | Replace every major outage — fibre gaskets creep | | Saturated / superheated steam > 10 bar | Spiral wound, graphite fill, 316 winding | Inner ring mandatory above class 300 | | General hydrocarbons (liquid or vapour) | Spiral wound, graphite, 316 | PTFE fill if temperature < 260 °C and chemical is graphite-sensitive | | Hydrogen, light gases, high pressure | Spiral wound with Inconel winding, or RTJ (class 900+) | Check for embrittlement with solid-metal ring alloy | | Aggressive acids, caustics, solvents | ePTFE (expanded PTFE) sheet or PTFE-envelope | PTFE-enveloped gaskets use a graphite or CNAF core inside a PTFE jacket | | Compressed air, instrument air | CNAF or NBR sheet | Oil-free systems need oil-free gasket — verify binder | | Refrigerant (R-134a, ammonia, CO2) | NBR (NH3 use EPDM; never copper-bearing with ammonia) | Check compatibility per OEM — some refrigerants attack elastomers | | Cryogenic (LNG, LO2, LN2) | PTFE or spiral wound with PTFE fill | Avoid compressed fibre — loses flexibility at low temperature | | Caustic soda, strong alkali > 100 °C | PTFE-enveloped or kammprofile with PTFE | Graphite attacks at high pH and temperature | | Wellhead / sour service | RTJ, soft iron or 316 with NACE MR0175 certification | Octagonal profile on newer flanges; oval is legacy |

Common Mistakes

  • Using a spiral wound gasket on a flat-face flange. Without the raised face, the outer centring ring bottoms out before the winding reaches its seating stress. It won't seal.
  • Skipping the inner ring. On graphite-filled spiral wound gaskets above class 300, the winding can buckle inward without an inner ring, releasing graphite into the process and opening a leak path.
  • Reusing gaskets. Once compressed, gaskets do not recover their original thickness. Replace every time the joint is broken, even if the gasket "looks fine".
  • Wrong bolt torque. Both under-torque (insufficient seating stress) and over-torque (gasket crushing, bolt yield) cause leaks. Use the gasket manufacturer's torque values, calibrated wrenches, and a star pattern in at least three passes.
  • Ignoring thermal cycling. Services that cycle from ambient to > 300 °C relax bolt preload after the first cycle. Re-torque after the first heat-up (hot bolting) if the operating conditions allow it.
  • Matching gasket ID to flange ID, not pipe ID. Spiral wound and kammprofile gaskets are sized so the inner ring matches the pipe bore — an undersized ID protrudes into the flow, a bias that will bite you if there is an orifice plate downstream. (See orifice plate installation.)

Standards to Know

  • ASME B16.20 — metallic gaskets for pipe flanges (spiral wound, jacketed, RTJ).
  • ASME B16.21 — non-metallic flat gaskets for pipe flanges.
  • ASME B16.5 / B16.47 — flange dimensions and pressure-temperature ratings.
  • API 6A — wellhead and Christmas tree equipment, including RTJ gaskets.
  • NACE MR0175 / ISO 15156 — materials for sour service.
  • EN 1514 (European equivalent of B16.21) — soft gaskets for EN flanges.

Start with the Flange, Finish with the Gasket

Gasket selection is a consequence of the flange class and service conditions, not a standalone decision. Pick the right pressure class first, confirm the flange facing is compatible with your service, then match the gasket to the fluid and temperature. When in doubt, default to a graphite-filled spiral wound for anything above class 150 steam or hydrocarbon service — it is the most forgiving choice in the widest range of conditions.

For sizing the rest of the piping system, try our free friction loss calculator, pipe sizing calculator, and valve Cv/Kv calculator. To model a full network end to end, SimuPipe simulates pressure, flow, and velocity across every pipe and fitting in the browser.