Gasket Material Selection: Matching Material to Service

Gasket material selection starts with the service conditions, not with a material shortlist. Temperature, the media being sealed, pressure, flange size and the bolt load your flange can actually generate decide the answer between them. Pick the material first and you end up defending a datasheet number instead of a joint.

The failure mode is consistent. A headline figure gets read as a rating, and it describes conditions the joint will never operate under.

Work the service conditions first

The sealing industry has a mnemonic for this. Garlock's own training material calls STAMPS the six pieces of information needed to recommend any sealing product: Size, Temperature, Application, Media, Pressure and Shaft speed.

Its origin is worth stating accurately. Writing in the Fluid Sealing Association's Sealing Sense column in October 2016, Lee Gillette attributes the acronym to the association's Compression Packing Technical Manual. That is a dynamic seal, which is why the list ends with shaft speed and asks about stuffing box dimensions.

A flanged gasket joint has no shaft. Drop the final S and work STAMP.

For a static flanged joint, that leaves five questions to answer before you open a catalogue:

• Size. Flange standard, class, nominal bore, face type and gasket dimensions.
• Temperature. Continuous operating temperature, plus any spike during start-up, steam-out or upset.
• Application. The equipment and the duty, including thermal cycling and vibration.
• Media. What is being sealed, at what concentration, and what else it contacts.
• Pressure. Operating pressure, test pressure, and whether vacuum is ever pulled.

Most of the selection errors worth writing about happen because someone answered Temperature and stopped.

What the datasheet numbers actually mean

Three different temperatures get printed for the same material, and they are not interchangeable.

Continuous is what the material will live at. Peak or intermittent is a spike it will survive. And for some materials the number changes entirely with the surrounding atmosphere.

Klinger's comparison data states the rule directly: maximum temperature and maximum pressure should not be used at the same time, and all values depend on the application. Garlock encodes the same truth as a P times T ceiling, a product of pressure and temperature that caps the two together even when each sits inside its own limit.

Take Garlock Style 5500, a compressed non-asbestos fibre sheet. The published figures are 290 degrees C continuous, 425 degrees C peak, and 1,200 psig maximum, with a P times T ceiling of 400,000 psig degrees F. You cannot have the top temperature and the top pressure at once. The ceiling says so.

That is one material with a 135 degree C gap between two of its own numbers.

The shortlist, with sourced numbers

Figures below come from the manufacturers' published technical data. Date-stamp them and check the current edition before you specify.

Note the last three rows. Moving from nitrile to fluoroelastomer buys 83 degrees C and not one extra psi. You are paying for temperature, not pressure.

For spiral wound gaskets the filler sets the temperature ceiling, not the winding. Teadit's filler data runs PTFE at about 260 degrees C, flexible graphite at 450, a high-temperature grade to 815, mica to 1,000 and ceramic fibre to 1,090.

We have deliberately left the pressure column empty for spiral wound. A spiral wound gasket's pressure capability comes from the flange class it is dimensioned to, not from the gasket.

PTFE gives you the chemistry and takes back the stability

Expanded PTFE holds pH 0 to 14, excepting molten alkali metals and elemental fluorine at high temperature and pressure. Nothing else on the shortlist comes close.

The cost is creep. Garlock's technical manual illustrates it with a test at 2,000 psi and 260 degrees C for one hour, captioned to show the uneven cold flow of conventional PTFE.

Creep is a bolt-load-loss mechanism, which is what makes it serious. The gasket thins, bolt stretch relaxes, and the joint starts leaking without anyone touching it.

The fixes are structural rather than chemical. Compare the two PTFE rows above: the metal-inserted grade gains no temperature at all, but doubles the pressure limit and the P times T ceiling. You are buying mechanical stability.

Read a single temperature figure for graphite and you will get it wrong too.

Graphite's real limit is the atmosphere, not the material

Garlock rates its flexible graphite at 454 degrees C in atmosphere and 650 degrees C in steam. Same material, a 196 degree spread, decided entirely by whether oxygen reaches the exposed edge.

Klinger states the same physics more starkly, quoting 460 degrees C in oxidising conditions against a figure in the thousands where no oxygen is present. Teadit arrives independently at 450 degrees C for graphite filler, which corroborates the air-service number across two manufacturers.

This is why high-temperature spiral wound fillers exist at all, and why the Fluid Sealing Association maintains a dedicated oxidation test for flexible graphite materials.

A spec that quotes graphite's high number without naming the atmosphere is quoting a number your joint will never see.

Why the highest-spec material is often the wrong choice

Two reasons, and neither is cost.

The flange may not be able to seat it. For saturated steam service on compressed fibre, Garlock calls for a minimum assembly stress of 4,800 psi and prefers 6,000 to 10,000 psi, with a retorque before pressurising. Its stated maximums bracket the window from above at 15,000 psi for compressed fibre and filled PTFE, and 10,000 psi for flexible graphite. A flange that cannot generate the seating stress cannot flow the gasket into the face irregularities, and an under-seated premium gasket leaks worse than a correctly seated ordinary one.

Some flanges forbid the torque outright. Garlock notes that for glass-lined, PTFE-lined, fibre-reinforced plastic and PVC flanges, the flange manufacturer's maximum torques are usually lower, and often much lower, than it would otherwise recommend. On those joints you need the most conformable material available at low stress, which points toward expanded PTFE rather than toward anything high-spec.

There is a thickness trade-off underneath this as well. Thinner gaskets cost less, emit less and resist blowout better, but they seal fewer flange irregularities and demand flatter flanges. The P times T ceiling drops with thickness too, from 350,000 to 250,000 on filled PTFE and from 400,000 to 275,000 on compressed fibre when you move to an eighth of an inch.

On reuse, Garlock's position is short. It does not recommend the practice.

What the standards do and do not tell you

This trips up more specifications than any material property.

ASME B16.20, in its 2023 edition, covers materials, dimensions, tolerances and markings for ring joint, spiral wound and metal jacketed gaskets against flanges dimensioned to B16.5, B16.47 and API 6A. It is a dimensional and marking standard. It does not rate a gasket for pressure or temperature. Compliance with it says your gasket fits, not that it seals your service.

ASME B16.21, 2021 edition, does the same job for nonmetallic flat gaskets.

ASTM F104 is a classification system for nonmetallic gasket materials, assigning a coded callout so buyer and seller mean the same thing. The standard warns about its own limits: because not all properties contributing to gasket performance are included, its use as a basis for selecting materials is limited. Rubber compounds sit outside it under ASTM D2000.

For the flange calculation itself, Mandatory Appendix 2 of Section VIII Division 1 of the Boiler and Pressure Vessel Code introduces the two gasket inputs. The gasket factor m is the ratio of residual gasket stress to internal pressure under operating conditions. The minimum design seating stress y is the stress needed to flow the gasket into the flange face. Both are tabulated in the code, and self-energising gaskets may take zero for each.

In Europe, the EN 1514 series is the dimensional counterpart, covering nonmetallic flat gaskets up to PN 63 and spiral wound gaskets from PN 10 to 160.

For another example of how we write, see our other writing samples.