Industrial gaskets split into three structural families (non-metallic, metallic, and composite), with each type rated against a specific pressure class, temperature window, and chemical envelope per ASME B16.5 flange geometry [S3].
Material choice within each family is the second decision layer: compressed non-asbestos fibre (CNAF), PTFE, flexible graphite, rubber elastomers, and solid metals (soft iron, low-carbon steel, stainless steel, Monel, Inconel) cover the bulk of process-piping service [S3][S4].
Non-metallic gaskets: soft seal for low-pressure flange classes
Non-metallic (soft) gaskets compress under low bolt load and are normally specified for ASME 150 and 300 class flanges, with graphite sheet extending usable temperature up to about 500°C [S3]. Common constructions include CNAF, PTFE, graphite sheet, and rubber or elastomer compounds; full-face geometry suits flat-face (FF) flanges, while inside-bolt-circle geometry suits raised-face (RF) flanges [S3]. Rubber and elastomer gaskets are excluded from hydrocarbon service and are normally limited to utility lines (water, air, mild chemicals) [S3]. A complete gasket taxonomy is anchored by these soft-material designs as the entry-level option.
Metallic gaskets: ring-joint (RTJ) for high-pressure, high-temperature service
Metallic ring gaskets are machined from soft iron, low-carbon steel, stainless steel, Monel, or Inconel, and seat into a groove cut into ASME 600 and higher flange faces, with octagonal and oval cross-sections as the two API 6A / ASME B16.20 stock shapes [S3][S5]. Operating envelope targets above 900 class flanges and high-temperature hydrocarbon service, with high-tension bolting required to achieve the seating stress [S3]. RTJ is the most costly and mechanically robust of the three families.
Composite (semi-metallic) gaskets: spiral wound and kammprofile
Composite gaskets combine a metal carrier with a non-metallic sealing layer; spiral wound (metal strip wound with graphite or PTFE filler), kammprofile (serrated metal core faced with graphite or PTFE), and metal-jacketed constructions cover the broad mid-to-high pressure range in oil, gas, petrochemical, and power generation service [S2][S5][S6]. Spiral wound units dominate heat-exchanger, boiler, and refinery header joints where cyclic temperature and pressure fluctuate, with flexible graphite as the standard filler for oxidising service [S2]. Kammprofile (also called camprofile) is selected for its compression recovery and is common on heat exchangers, pressure vessels, and flanged joints subject to thermal cycling [S5].
Material families and their operating envelopes
PTFE offers broad chemical resistance (pH 0-14) but is generally limited to about 260°C continuous service, making it standard in food, pharmaceutical, and strong-acid/alkali lines [S2][S4]. Flexible graphite handles oxidising media up to roughly 450-500°C in steam and hydrocarbon service and is the filler of choice inside spiral wound and kammprofile facings [S2][S3]. CNAF (compressed non-asbestos fibre) bridges the temperature gap between rubber and graphite, typically rated to about 200-400°C depending on binder, and is the workhorse for Class 150/300 chemical and water service [S3][S4]. Rubber families split by chemistry: natural rubber for general purpose, nitrile (NBR) for oils and fuels, EPDM for steam, water, and weathering, silicone for wide temperature range and FDA food contact, and neoprene for outdoor and chemical resistance [S4][S5]. Cork-rubber composites are retained for vibration-damped automotive and fuel-pump sealing where micro-movement would defeat solid rubber [S5].
Selection criteria: pressure class, temperature, media, and flange finish
Four inputs drive a defensible gasket call-out. (1) Pressure class: below ASME 300, a soft sheet (CNAF, PTFE, rubber) is almost always adequate; 300-600 is a transition zone where spiral wound begins to displace CNAF on thermal cycling; 600 and above, or any ASME 900+ hydrocarbon service, calls for spiral wound, kammprofile, or RTJ [S3]. (2) Temperature: rubber families cap near 120-150°C, PTFE near 260°C, CNAF 200-400°C, flexible graphite near 450-500°C, and solid metal ring joints extend to the ASME B16.5 material limit [S2][S3]. (3) Media: PTFE for aggressive acids and caustics; nitrile for hydrocarbons and oils; EPDM for steam, water, and glycol; graphite for steam, hydrocarbons, and most chemicals except strong oxidisers at high temperature [S2][S4]. (4) Flange face finish: RF flanges accept ring-type gaskets (inside bolt circle), FF flanges require full-face geometry with bolt holes punched through, and RTJ flanges require precision-machined grooves and metal hardness below the groove [S3]. Operating-condition mismatches are the dominant failure mode, not bolt failure, so each axis must be checked before sign-off. For plants that also track emissions and area classification, a sensor-side cross-check matters too; see this portable gas detector sizing and selection walkthrough for a parallel methodology.
Standards, limitations, and sourcing signals
Specifying engineers should anchor each call-out to ASME B16.5 (pipe flanges), ASME B16.20 (metallic gaskets for ring-joint flanges, including RTJ style), ASME B16.21 (non-metallic flat gaskets), and API 6A / 6D for wellhead and pipeline valves, with material compliance to NACE MR0175 for sour (H₂S) service as a separate line item [S3]. Sheet material standards (ASTM F104, F152) govern CNAF and rubber-sheet call-outs, and FDA-grade elastomers (silicone, EPDM, PTFE) are required for food and pharmaceutical lines [S2][S4]. Limitations to watch: RTJ grooves are machined and not field-modifiable, spiral wound gaskets with graphite filler are not suited to strong oxidising service above ~450°C, and PTFE gaskets cold-flow under sustained load and may need a compressive ring or confined geometry in high-load joints. For lamps and light fittings enclosures that need gasket recovery after repeated opening, EPDM or silicone sponge is the usual answer because of its low compression set, not because of temperature rating alone. Sourcing signal worth tracking: vendor stocking of ASME B16.20 RTJ rings in Inconel 625/825 for sour service, and pre-formed spiral wound kits with inner and outer rings for class 150-2500 heat-exchanger change-outs. Both are still routinely available on 4-6 week lead times from major industrial distributors as of 2026.
Specifying engineers should treat gasket selection as a four-axis check (pressure class, temperature, media, flange finish) and pin the call-out to ASME B16.5, B16.20, and B16.21 before vendor selection. Trackable next node: confirm that any soft-gasket replacement on existing flanges matches both the original ASME class and the original facing finish, since retrofitting a CNAF where RTJ was originally machined requires re-machining the groove.
For component-level specifications, see construction machinery and equipment.