Gasket Selection —
Types, Materials and
Correct Application

Gasket selection is one of the most consequential minor decisions in pipework design and one of the most frequently made on the basis of habit rather than engineering judgement. The consequence of a wrong selection is not always immediate — a gasket that is marginally under-specified for the service may seal satisfactorily through the initial hydrostatic test and through the first months of operation, only to fail after the first heat cycle, after the seating stress relaxes, or when the process fluid changes composition. By that point the cause is rarely obvious unless the selection process is traced back.

This article covers the principal gasket types in process piping and pressure vessel service, the parameters that determine their suitability, and the practical decision logic for matching a gasket to a flange, a fluid, and an operating condition. It is a companion to the Bolted Flange Joint Integrity article which covers the bolt load calculation; this article focuses on the gasket material selection that precedes it.

What a Gasket Must Do

A gasket seals by conforming to the surface irregularities on the two flange faces it is compressed between, creating a continuous contact path that the pressurised fluid cannot cross. To do this it must:

• Deform sufficiently under bolt load to conform to the flange face finish and close off any potential leak path
• Maintain adequate contact stress when internal pressure is applied and tends to separate the flanges
• Resist creep and relaxation under sustained load, particularly at elevated temperature, so that the initial seating stress is not progressively relieved
• Be chemically compatible with the process fluid — it must not be attacked, softened, or embrittled by the fluid it is sealing
• Survive the operating temperature without degradation of its sealing or mechanical properties

No single gasket material satisfies all of these requirements across the full range of process conditions. The selection decision is always a compromise between seating requirement, chemical resistance, thermal capability, and cost.

Gasket Parameters — m and y Values

The ASME code characterises the seating behaviour of gaskets using two parameters that appear in the bolt load calculation (ASME VIII Appendix 2). A full treatment of these is covered in the Bolted Flange Joint Integrity article; a brief summary for context:

• m (gasket factor) — the ratio of residual gasket contact stress to internal pressure required to maintain the seal under operating conditions. Higher m indicates a harder-to-maintain gasket that demands more bolt load to keep it seated against pressure.
• y (minimum design seating stress) — the minimum initial compressive stress (MPa) required to seat the gasket, deforming it into the flange face and establishing the initial seal before any pressure is applied.

Softer gasket materials have low y values (easy to seat, low bolt load required) but often high m values (hard to maintain under pressure). Harder metallic gaskets have high y values (difficult to seat, high bolt load) but low m values (once seated, maintain contact stress well). The selection optimum is a gasket that seats with the available bolt load and maintains the seal at the maximum operating pressure at the operating temperature — a balance that depends on the specific flange and bolt specification.

Compressed Non-Asbestos Fibre (CNAF)

What it is

Sheet gasket material composed of fibres (glass, aramid, carbon, or mineral fibre) bound in a rubber or elastomeric matrix, compressed and vulcanised into sheet form and cut to size. The direct replacement for compressed asbestos fibre (CAF) sheet following the asbestos ban; modern CNAF grades in quality brands approximate the performance of CAF for most moderate-service applications.

Where it works

CNAF is the general-purpose workhorse for raised face and full-face flanges in low-to-moderate pressure service. Suitable for water, steam (limited), oils, fuels, and a wide range of process chemicals depending on the specific matrix rubber. Inexpensive, easy to cut to any size on site, and familiar to all maintenance teams. For Class 150 and 300 flanges in benign or moderate service, CNAF is typically adequate and economical.

Limitations

• Temperature limit is grade-dependent — standard grades 250–300°C, specialist high-temperature grades to 400°C. Above 300°C, creep relaxation becomes significant and re-torquing after first heat-up is usually necessary.
• High-pressure steam service above approximately 40 bar — CNAF is not recommended. Spiral wound gaskets are the standard alternative.
• The m and y values for CNAF vary significantly between manufacturers and grades — always use manufacturer test data rather than generic ASME table values for engineered joints.
• Susceptible to blowout if severely over-compressed — no inner ring to prevent extrusion.
• Reuse is not acceptable — even if visually undamaged, a used CNAF gasket has permanently deformed and will not provide the same seating performance on reassembly.

Spiral Wound Gaskets (SWG)

What it is

A V-profile metal strip wound in alternating layers with a soft filler material, producing a semi-metallic gasket with spring-like elastic recovery. The metal strip (typically 316L stainless, or Inconel for high-temperature or corrosive service) provides structural backbone and recovery; the filler (graphite or PTFE) provides conformance and sealing. Standard spiral wound gaskets for raised face flanges include a solid outer centering ring (which locates in the raised face and prevents over-compression of the winding) and for ASME B16.20 compliant gaskets, a solid inner ring (which prevents inward buckling of the winding under high bolt load and prevents crevice corrosion on the inner bore).

Where it works

The standard gasket for Class 300 and above in process piping and pressure vessel service, and widely used at Class 150 where the service is demanding. Superior to CNAF in high-pressure, high-temperature, and cyclic service due to significantly lower creep relaxation (particularly with graphite filler) and better recovery after thermal cycling. The go-to upgrade from CNAF for steam service above 40 bar, for hydrogen service, and for any application where joint integrity is critical.

Filler selection

• Graphite filler — lower creep relaxation, better performance in elevated temperature and cyclic service, resistant to fire (non-combustible). Preferred for steam, hydrocarbons, and general high-temperature process service. Graphite is not suitable for strongly oxidising services (fuming nitric acid, concentrated sulphuric acid) — the graphite is attacked.
• PTFE filler — excellent chemical resistance, suitable for aggressive chemical service where graphite is not compatible, and for applications where metal ion contamination from graphite is a concern (pharmaceutical, food grade). Higher creep relaxation than graphite at elevated temperature. Temperature limit approximately 260°C for standard PTFE grades.

Critical installation requirement

The centering ring must be present — it locates the gasket concentrically on the raised face and prevents the outer windings from unwinding during bolt-up. A spiral wound gasket installed without a centering ring on a raised face flange will migrate off-centre and may not seal. For ASME Class 300 and above, the inner ring is also mandatory — it prevents the inner windings from buckling inward under high bolt load and prevents the inner bore of the gasket from acting as a crevice.

Kammprofile (Grooved Metal) Gaskets

What it is

A solid metal core with concentric serrations machined into both faces, overlaid with a thin soft facing layer (typically graphite or PTFE). The serrations bite into the flange face under bolt load, providing positive mechanical keying and a highly reliable metal-to-metal seal backed by the soft facing. Unlike spiral wound gaskets, the kammprofile is a rigid metallic gasket that does not deform significantly — the sealing action is by the serrations embedding into the soft facing and the flange face rather than by bulk compression of the gasket material.

Where it works

Kammprofiles are used where spiral wound gaskets cannot provide adequate sealing performance: very high pressure and temperature combinations, heat exchanger tube-to-shell joints and girth flanges, large bore joints where maintaining bolt load across the full gasket face is challenging, and services requiring fugitive emissions compliance (VOC regulations). They are significantly more expensive than spiral wound gaskets and require higher bolt loads to seat (higher y values), but in return offer lower creep relaxation, better tolerance of flange face imperfections (the serrations can seat across minor corrosion pitting), and longer service life in cyclic conditions.

Ring Type Joint (RTJ) Gaskets

What it is

A solid metal ring — oval or octagonal cross-section — that sits in precision-machined grooves in the flange face. Under bolt load, the ring is compressed into the groove, plastically deforming slightly to produce a metal-to-metal seal on the groove contact faces. The oval ring makes contact at two lines on the groove face; the octagonal ring makes contact across two flat bearing surfaces and achieves higher seating efficiency for the same bolt load.

Where it works

RTJ is the standard for Class 600 and above in oil and gas, petrochemical, and high-integrity pressure service. The metal-to-metal seal provides the highest leak tightness of any standard gasket type and is the default specification for hydrogen service, high-pressure steam, sour gas (H₂S containing) where zero leakage is required, and wellhead and Christmas tree equipment. RTJ gaskets require matching RTJ-faced flanges — the precision groove must be in the flange face, and RTJ gaskets cannot be used on raised face flanges. The ring is always softer than the flange material — carbon steel rings in alloy steel flanges, soft iron rings in stainless flanges — so that the ring deforms into the groove rather than the groove deforming under the ring.

Reuse

RTJ rings are not reusable. Once the ring has been seated and the joint broken, the ring has plastically deformed at the contact lines. Reusing it will not produce the same contact geometry and the joint integrity cannot be relied upon. RTJ rings are consumable items; a stock of spare rings for each size and class in service should be maintained.

PTFE and ePTFE Gaskets

Full-face PTFE

PTFE sheet gaskets are used primarily on flat-face flanges (cast iron valves, pump bodies, glass-lined equipment) and in services where metallic or graphite contact with the process fluid is not acceptable. PTFE is chemically resistant to almost everything except fluorine gas, molten alkali metals, and certain highly reactive fluorinated compounds. Temperature limit approximately 200°C for standard grades. Susceptible to cold flow under bolt load — PTFE creeps continuously under sustained compressive stress, causing progressive loss of bolt load. Joint re-torquing is usually required.

Expanded PTFE (ePTFE) tape and sheet

Expanded PTFE in tape or sheet form is used for custom-size gaskets, for wrapping in gasket grooves, and for very low-pressure applications where the soft conformance of ePTFE allows sealing on irregular or damaged faces. Chemical resistance matches standard PTFE. Lower compressive strength than filled PTFE sheet — not suitable for applications requiring high bolt load. Used extensively in pharmaceutical, semiconductor, food processing, and laboratory pipework where chemical purity is paramount.

Metallic Flat Gaskets

Solid metal flat gaskets — soft iron, copper, aluminium, stainless — are used in specialised applications where other gasket types cannot meet the operating requirements. Common applications: cylinder head gaskets in reciprocating compressors, heat exchanger floating head covers, and high-pressure gas connections where very high bolt loads are available and the flange faces can be finished to the close tolerances required to seat a metallic flat gasket reliably. Generally not suitable for standard process piping flanges where the available bolt load and flange face finish are not sufficient to seat a solid metal gasket without leakage.

Selection Decision Matrix

Flange Face Finish and Gasket Compatibility

The flange face finish is as important as the gasket selection. A gasket that is specified correctly for the service will still leak if the flange face is too smooth (the gasket cannot grip it) or too rough (a soft gasket will extrude into the surface irregularities rather than sealing across them).

• Spiral wound gaskets — require a serrated concentric (phonographic) finish, typically Ra 3.2–6.3 μm. Smooth finishes below Ra 1.6 μm do not provide enough texture for the winding to grip.
• CNAF and soft sheet gaskets — suitable for a range of finishes from stock finish to Ra 6.3 μm. Radial scratches are more damaging than concentric marks — they create a potential leak path under the gasket that cannot be sealed by compression.
• RTJ ring gaskets — require precision-machined RTJ grooves to ASME B16.20 dimensional tolerances. Surface finish in the groove is critical — typically Ra 0.8 μm or better. Damaged or corroded grooves must be remachined or the flange replaced.
• Kammprofile gaskets — more tolerant of face condition than spiral wound or RTJ. The serrations can bridge minor pitting or irregularity. Stock finish or smooth machined finish is acceptable.

Chemical Compatibility — A Checklist

Beyond temperature and pressure, the gasket material must be chemically compatible with the process fluid. A selection that fails this check will degrade in service regardless of how well it was mechanically specified. Key incompatibilities to check before finalising selection:

• Graphite — incompatible with strongly oxidising agents (fuming nitric acid, concentrated sulphuric acid, chlorine above ~100°C, liquid oxygen). Compatible with almost all other process fluids including hydrocarbons, steam, alkaline solutions, and dilute acids.
• PTFE — incompatible with fluorine gas, molten alkali metals (sodium, potassium), certain fluorinated solvents at elevated temperature. Compatible with virtually all other chemicals including strong acids, alkalis, and solvents.
• NBR rubber matrix (in CNAF) — incompatible with ketones (acetone, MEK), esters, certain chlorinated solvents, and some oxygenated hydrocarbons. Compatible with mineral oils, water, fuels.
• EPDM rubber matrix (in CNAF) — incompatible with mineral oils, hydrocarbons, and petroleum products. Compatible with water, steam (to limit), ozone, ketones. The correct matrix for hot water and steam CNAF applications.
• Soft iron RTJ ring — avoid in chloride-containing aqueous service (corrosion of the ring in the groove). 316 stainless or Alloy 625 rings for chloride service.

Summary

Gasket selection is a three-filter process: first, can the gasket seat with the available bolt load for the flange class and size? Second, will it maintain the seal at the operating pressure and temperature through the service life, including thermal cycling? Third, is it chemically compatible with the process fluid at the operating temperature? A gasket that passes all three filters for its specific application is the correct selection. A gasket specified by habit or precedent without checking all three filters is the reason joints leak.

The most common single improvement to gasket selection practice: switch from CNAF to spiral wound graphite fill for any steam service above 40 bar, for hydrogen service, and for any application where joint integrity is critical and re-torquing between shutdowns is not operationally acceptable. The cost differential is modest; the reliability improvement is substantial.