Gaskets

What is a Gasket

A gasket is a deformable element placed between two stationary mating surfaces and compressed by an external clamp load, usually bolts, so that it conforms to the surface profile of both faces and blocks the leak path. Because the mating surfaces do not move relative to one another, a gasket is classed as a static seal, which distinguishes it from dynamic seals such as O-rings on reciprocating rods, lip oil seals on rotating shafts, or mechanical seals on pump shafts. The gasket itself does almost nothing: the seal exists only while the bolts hold enough load to keep the gasket flowed into the surface micro-roughness and resisting the internal pressure trying to blow it out.

That single fact, that a gasket is a load-managed component, is the most important idea in gasket engineering. A flange joint is a spring system. The bolts are stiff springs in tension, the gasket is a soft spring in compression, and the two are balanced at assembly. Internal pressure, thermal expansion, gasket creep, and bolt relaxation all shift that balance over time. A leak is almost always a story about lost load, not about a defective gasket material. This is why the same gasket can seal perfectly on one joint and weep on the next: the difference is in the assembly and the load retained, not in the part.

Gaskets span an enormous range of duty. At the low end, a die-cut rubber ring seals a domestic water meter at a few bar and ambient temperature. At the high end, a metal ring-type joint seals an API 6A wellhead at up to 20,000 psi (138 MPa), and a flexible graphite gasket seals a steam line near 450 degrees C. Between those extremes sit the bulk of process industry joints: refinery piping, chemical reactors, heat exchangers, compressor casings, and pump and valve bonnets. The two most common engineered gaskets in that middle band are the spiral wound gasket and the compressed fiber sheet gasket.

Historically the gasket evolved alongside the bolted flange. Early joints used cut leather, cork, paper, and rubber. Compressed asbestos fiber dominated industrial sealing for most of the twentieth century because of its heat resistance, then was phased out for health reasons, replaced by compressed non-asbestos fiber (CNAF), PTFE, and flexible graphite. The spiral wound gasket was developed by Flexitallic in the 1910s to give a resilient, pressure-tolerant seal that recovers some of its compression as bolts relax, and it remains the workhorse of refinery and petrochemical flanges today. The kammprofile (grooved metal) gasket followed as a lower-bolt-load, reusable alternative.

Four engineering questions decide whether a gasket holds: Can the material survive the temperature and the chemistry over the service life? Will it seal at the bolt load this flange class can actually deliver? Will it resist blowout at the design pressure? And will the joint retain enough load after creep, embedment, and thermal cycling? Every chapter that follows maps one or more of these questions onto a specific gasket family, material, or spec parameter, so that a flange joint is engineered rather than guessed.

Gasket Types and Classification

Flange gaskets divide into three broad construction families: soft (non-metallic) cut sheet gaskets, semi-metallic gaskets that combine metal and a soft filler, and solid metallic gaskets. Each family seals at a different bolt-load level, tolerates a different temperature and pressure envelope, and matches a specific flange face. Choosing the wrong family for the flange facing, for example trying to fit a soft sheet gasket where a ring-type joint groove is machined, is a fundamental error that no amount of bolt torque can fix. The table below compares the five mainstream gasket types.

Soft cut sheet gaskets are stamped or water-jet cut from a flat sheet of compressed non-asbestos fiber, PTFE, flexible graphite, or rubber, and dropped onto a raised-face or flat-face flange. They are the cheapest and simplest gaskets, sealing well at low bolt loads, and dominate low-pressure water, air, and mild chemical duty. Their limits are temperature, creep, and blowout resistance: a soft sheet gasket will extrude and blow out long before a metallic gasket. Non-metallic flat gaskets are dimensioned by ASME B16.21 (and EN 1514-1 in Europe), and come in ring (inside-bolt-circle) and full-face patterns.

Spiral wound gaskets (SWG) are the semi-metallic workhorse. A preformed V-shaped metal strip, usually 304 or 316 stainless, is wound in a spiral alternating with a soft filler strip of flexible graphite or PTFE. The V-section acts as a spring, giving the gasket resilience that recovers some compression as bolts relax, which is exactly what a long-term flange joint needs. A solid outer centering ring (usually carbon steel) centers the gasket and acts as a compression stop, and a solid inner ring adds radial strength and blowout resistance. ASME B16.20 mandates an inner ring for PTFE-filled gaskets, and for graphite-filled gaskets in the larger and higher-class sizes (for example Class 900 NPS 24 and larger, Class 1500 NPS 12 and larger, Class 2500 NPS 4 and larger).

Kammprofile gaskets (also camprofile or grooved metal gaskets) use a solid metal core machined with concentric serrations on both faces, covered by a thin soft facing of graphite, PTFE, or mica. The serrations concentrate the bolt load onto narrow ridges, so the facing flows into the flange micro-roughness even at relatively low bolt load, and the solid core prevents over-compression and resists blowout. Kammprofile gaskets are reusable (re-faced), tolerant of flange flexing, and increasingly specified for critical heat exchanger and reactor joints. Kammprofile is a tradename associated with Flexitallic.

Metal jacketed gaskets wrap a soft compressible core in a thin metal envelope, traditionally used on heat exchanger pass-partition plates and irregular shapes. Ring-type joint (RTJ) gaskets are solid metal rings seated in machined grooves on RTJ flange faces, and seal by line contact and pressure energization, covered in detail in Chapter 4. They are the standard for high-pressure, high-temperature service from ASME Class 900 upward and for API 6A wellhead equipment to 20,000 psi.

Sealing Materials and Temperature

The filler or sheet material is what actually flows into the flange surface and blocks the leak path, so its temperature limit, chemical compatibility, and creep behavior set the envelope for the whole gasket. The five dominant non-metallic sealing materials are compressed non-asbestos fiber, PTFE, flexible graphite, mica, and elastomer rubbers. The table below gives approximate continuous-service temperature limits and the duty each material owns. Treat these as starting points: the real limit is the combination of temperature, pressure, and chemical attack, and the manufacturer chemical-compatibility chart always governs.

Compressed non-asbestos fiber (CNAF) is the modern replacement for compressed asbestos, made from aramid or other fibers bound with an elastomer (NBR, EPDM, or SBR). It is the default low-cost sheet gasket for water, steam, oil, and mild chemicals, with a continuous limit around 200 degrees C, though short peaks run higher. Its weaknesses are creep relaxation under load and limited chemical range compared with PTFE, which is why critical joints have largely moved to PTFE and graphite.

PTFE (polytetrafluoroethylene) is almost chemically inert, sealing strong acids, caustics, and solvents that destroy other materials, with a service limit of 260 degrees C. Its weakness is cold flow: under sustained load PTFE creeps and relaxes, bleeding off bolt preload and causing leaks. Modern engineered PTFE sheet (filled or expanded grades such as restructured PTFE) dramatically reduces creep and is the standard for chemical-process flanges. Garlock GYLON is a widely cited filled-PTFE reference grade.

Flexible graphite is exfoliated graphite calendered into a foil or sheet, giving excellent high-temperature sealing, fire safety, and steam resistance. It serves continuously to about 450 degrees C in air and far higher in non-oxidizing atmospheres, but oxidizes (burns away) above roughly 450 to 500 degrees C in air, which sets its practical ceiling. Graphite is the dominant filler for spiral wound and kammprofile gaskets in steam and hydrocarbon service. Mica and ceramic materials extend the range to between 500 and over 1000 degrees C for furnace, exhaust, and fire-test duty, at the cost of brittleness.

For metallic windings and ring materials, the common grades are 304/304L and 316/316L stainless steel, with upgrades to 321, 347, alloy 400 (Monel), nickel alloys, and titanium for aggressive media. The same corrosion logic applies as for any wetted part: 316L handles general process media but not chlorides, while nickel alloys and titanium are needed for hot chlorides, wet chlorine, and seawater. Always match the winding and ring metallurgy to the process fluid, not just the filler.

Standards and the m and y Factors

Gasket selection lives inside a framework of standards that govern dimensions, materials, and the calculation of bolt load. The two ASME dimensional standards are paired: ASME B16.20 covers metallic gaskets (spiral wound, metal jacketed, grooved metal kammprofile, and ring-type joint), and ASME B16.21 covers non-metallic flat gaskets (cut sheet). Both are dimensioned to mate with ASME B16.5 and B16.47 flanges. In Europe, EN 1514 series gives flange gasket dimensions and EN 13555 specifies the test method that produces measured gasket parameters for leak-tightness based design.

The heart of flange design is the calculation of required bolt load, and that calculation uses two gasket constants defined in the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Mandatory Appendix 2, Table 2-5.1. The y factor is the minimum gasket seating stress in psi needed to flow the gasket into the flange face at assembly, with no internal pressure present. The m factor is a dimensionless maintenance multiplier applied to the internal pressure to compute the load needed to keep the joint tight once it is pressurized. The Code is explicit that these are suggested values, not mandatory, and that the manufacturer may supply better data. The table below lists representative m and y values.

Reading the table top to bottom shows the logic of gasket selection. A soft elastomer seats at essentially zero stress (y = 0) but holds almost no pressure (m = 0.5), so it suits low-pressure duty. A spiral wound gasket needs 10,000 psi of seating stress but holds pressure well (m = 3.0). A stainless ring joint needs a very high 26,000 psi to seat but is nearly immune to blowout (m = 6.5). Higher seating stress means you need either more or larger bolts, or a smaller gasket contact area, to deliver that load, which is why high-pressure flanges have heavy bolting. The m and y framework, dating from research in the 1940s, is now supplemented in critical service by the EN 13555 parameters and PVRC tightness-based method, which characterize actual leak rate rather than a simple seated or not-seated judgment.

For high-pressure metallic sealing, ring-type joint (RTJ) gaskets follow ASME B16.20 and API 6A. They come in three styles: style R with an octagonal (more common) or oval cross section that seats in a standard groove; style RX, a pressure-energized variant where internal pressure increases the seating force; and style BX, a high-pressure design for API 6A 6BX service to 20,000 psi (138 MPa) used with pressure-passage holes. A critical assembly rule is hardness: the ring must be softer than the flange groove, typically 15 to 20 Brinell lower, so the ring deforms into the groove without galling or scoring the flange. Soft iron rings are about 90 HB; stainless and alloy rings are matched to harder flange materials.

Beyond dimensions and load, ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) governs how the joint is actually built: target bolt stress, legacy cross-pattern tightening passes, flange face inspection, thread lubrication with a known nut factor, controlled torque or tensioning, and re-torque after thermal cycling. PCC-1 exists because field experience shows that the majority of flange leaks trace to assembly, not to the gasket, a point reinforced in Chapter 6.

Key Specification Parameters

A gasket data sheet or RFQ line item is defined by a compact set of parameters. Eight of them drive selection: gasket type and style, dimensions and flange match, materials (winding, filler, rings, or sheet), pressure and temperature rating, the m and y design factors, thickness, surface finish requirement of the mating flange, and certification. Each is explained below so a buyer can read a spec sheet without guessing.

Type and style fix the construction and the flange face it mates with: spiral wound style CG (with outer ring) or CGI (with inner and outer ring), kammprofile, RTJ style R/RX/BX, metal jacketed, or soft cut sheet (ring or full-face). The style must match the flange facing, raised face, flat face, tongue-and-groove, male-female, or RTJ groove. This is the single most common selection error, because a gasket that fits the bolt circle can still be wrong for the face.

Dimensions and flange match are stated as nominal pipe size and pressure class against a flange standard, for example NPS 4 Class 300 per ASME B16.5, which fully fixes inside and outside diameters under B16.20 or B16.21. For EN flanges the call-out is DN and PN per EN 1514. A spiral wound gasket for an ASME B16.5 Class 150 to 2500 flange and B16.20 typically has a 4.5 mm (0.175 in) nominal winding thickness with 3.2 mm (0.125 in) solid rings, unless otherwise specified.

Materials are listed per element: winding metal (304, 316L, 321, Monel, nickel alloy, titanium), filler (flexible graphite, PTFE, mica, ceramic), centering ring (carbon steel, stainless), and inner ring. For soft gaskets the single sheet material and its binder are stated. Under ASME B16.20 these are also encoded in the color marking: a solid color band on the centering ring for the winding metal (yellow = 304, green = 316) and a colored stripe for the filler (gray = flexible graphite, white = PTFE). Always cross-check the color, the laser marking, and the material certificate.

Pressure and temperature rating is the joint envelope the gasket must survive, derived from the flange class and the lowest temperature limit among the gasket elements. The filler usually sets the temperature ceiling (graphite ~450 degrees C, PTFE 260 degrees C, CNAF ~200 degrees C). The m and y factors from Chapter 4 feed the flange bolt-load calculation and indirectly determine the bolting required.

Thickness, surface finish, and certification close out the spec. Thinner gaskets generally seal better (less material to creep and relax) provided the flange faces are flat and parallel, which is why kammprofile and spiral wound gaskets out-perform thick soft sheet on demanding joints. The mating flange typically needs a serrated or concentric-groove finish in the range of 3.2 to 6.3 micrometre Ra (125 to 250 microinch) for soft and spiral wound gaskets; too smooth a finish can actually reduce the grip on a soft gasket. Certification covers material test reports, PMI (positive material identification), ASME B16.20 marking, fire-safe (API 6FB) testing, and fugitive-emission test data per EN 13555 or API 622/624 for critical hydrocarbon and emissions-controlled service.

Selection Decision Factors

To turn the preceding chapters into a specific gasket call-out, work the decision sequence below in order. Most gasket failures come not from a single wrong choice but from skipping a step, especially the flange-face and bolt-load checks. These steps double as a fixed RFQ template.

1. Identify the flange face first: raised face, flat face, tongue-and-groove, male-female, or RTJ groove. This decides the gasket family before anything else. An RTJ groove demands a metal ring; a raised face accepts soft, spiral wound, or kammprofile gaskets.
2. Set pressure class and dimensions: NPS and Class per ASME B16.5 / B16.47, or DN and PN per EN 1092 / EN 1514, which fixes the gasket inside and outside diameters and the standard (B16.20 metallic or B16.21 non-metallic).
3. Match material to fluid and temperature: select filler or sheet (graphite, PTFE, CNAF, mica, elastomer) for the chemistry and the temperature ceiling per Chapter 3, then select winding and ring metallurgy (316L, alloy 400, nickel alloy, titanium) for corrosion.
4. Check available bolt load against the y factor: confirm the flange bolting can deliver the seating stress the gasket needs. Soft gaskets seat at low load; spiral wound needs ~10,000 psi; ring joints need 18,000 to 26,000 psi. A gasket that cannot be seated will always leak.
5. Verify blowout and overpressure margin: use the m factor and the inner-ring requirement to confirm the gasket resists the design pressure plus transients. ASME B16.20 inner rings are mandatory for PTFE-filled and for larger or higher-class graphite spiral wound gaskets.
6. Specify thickness and flange surface finish: prefer the thinnest gasket the flange flatness allows; confirm the mating face finish (typically 3.2 to 6.3 micrometre Ra for soft and spiral wound gaskets) and inspect for scoring, warpage, and parallelism.
7. Set certification and testing: material test reports, PMI, B16.20 marking and color code, fire-safe (API 6FB) where required, and fugitive-emission data (EN 13555, API 622/624) for emissions-controlled service.
8. Define the assembly procedure: specify ASME PCC-1 target bolt stress, cross-pattern tightening, controlled torque or tensioning with a known nut factor, and a re-torque after thermal cycling. The gasket is only as good as the load that is installed and retained.

One last commonly overlooked dimension is assembly discipline and serviceability. Field studies consistently find that the large majority of flange leaks are assembly problems: uneven bolt load, no cross-pattern sequence, no torque control, reused crushed gaskets, and short bolts that lose preload to creep and thermal relaxation. A modest gasket installed per ASME PCC-1 out-seals a premium gasket installed by feel. On the supply side, confirm the maker provides traceable material certificates, B16.20 marking, EN 13555 or PVRC tested data for critical joints, and reliable lead time on replacements. Flexitallic, Garlock, Teadit, Klinger, Lamons, and Valqua are established references; verify certificates and PMI on any lower-cost source before use in hydrocarbon or high-pressure service.