A gasket is chosen in one line on a requisition and blamed for the leak three months later. In most cases the gasket was made correctly — it was simply the wrong family for the joint, and the decision that put it there was made without working through a sequence anyone had written down.
The short answer: the flange facing decides the family, the pressure class decides whether a soft gasket is viable at all, and the temperature and media decide the filler or facing material. Work in that order and the choice usually makes itself.
Step one: the facing decides the family
This is the step that eliminates the most options, and the one most often skipped.
| Flange facing | What it takes | What will not work |
|---|---|---|
| Raised face (RF) | Spiral wound, kammprofile, or sheet depending on class | An RTJ ring — there is no groove to seat it |
| Flat face (FF) | Full-face sheet gasket | A raised-face gasket, which can crack a cast-iron flange on bolt-up |
| Ring-type joint (RTJ) | An RTJ ring to ASME B16.20 | Anything flat — a spiral wound in an RTJ groove will not seal |
Read the facing from the flange drawing or the piping class, not from the flange in front of you. A flange that has been in service can be hard to read, and a flange that has been replaced may not match the drawing at all — which is itself worth knowing before a shutdown.
Step two: pressure class decides whether soft is viable
Soft sheet gaskets seal by conforming. That works while the bolt load needed stays reasonable, and stops working as class rises:
- Class 150, utilities, water, low-pressure air — compressed sheet or PTFE sheet is appropriate and economical.
- Class 300 and above, hydrocarbon service — semi-metallic or metallic. The bolt load required to seat a soft gasket at these classes becomes impractical, and the material creeps or extrudes under sustained load.
- Class 900 and above — metallic, and usually RTJ by the time you reach wellhead pressures.
Step three: temperature and media decide the material
Within a family, the soft element sets the operating envelope more than the metal does. Flexible graphite is the general-purpose choice for hydrocarbons and steam but oxidises in air well below the temperature it tolerates in an inert atmosphere. PTFE handles aggressive chemistry across the pH range but has a much lower ceiling and creeps. Mica and ceramic go where graphite cannot, at a cost.
The metal follows the process: 304 or 316L for ordinary service, and higher alloys — 321, Alloy 825, Inconel 625 — where the fluid attacks stainless.
The five families, and where each belongs
Spiral wound
The default for raised-face pipework from Class 150 to Class 2500. A spiral of preformed metal strip wound alternately with a soft filler, usually with an outer centring ring and, above Class 300 on graphite fillers, an inner ring.
Widely stocked, well understood and inexpensive. It is the right answer often enough that it gets specified reflexively, which is where the failures come from — a spiral wound in an RTJ groove, or without the inner ring the class demanded.
We have written a full specifier's guide: spiral-wound gaskets and flange sealing covers winding and filler selection, ring configuration, the B16.20 colour code and the mistakes that cause leaks.
Kammprofile
A solid serrated metal core faced with a thin soft layer — graphite or PTFE. The serrations concentrate load onto narrow rings of contact, so it seals at lower bolt load than a spiral wound.
Choose it for heat exchangers, joints opened on a routine, flanges where available bolt load is limited, and anywhere a spiral wound has buckled before. The core is robust enough that a kammprofile can often be refaced rather than scrapped. It costs more per unit and is less commonly held in stock in Nigeria, so it is worth ordering to a plan.
Ring type joint (RTJ)
A solid metal ring seating in a machined groove on both flange faces. Used where pressure, temperature or criticality rule out a flat gasket — pipeline valves, wellheads, high-pressure separators.
Three styles, all in ASME B16.20:
- Style R — octagonal or oval section, for ASME B16.5 and API 6B flanges. Octagonal seals better and is preferred where the groove allows it.
- Style RX — pressure-energised: the geometry lets process pressure act on the ring and tighten the seal as pressure rises. Fits the same API 6B grooves as style R.
- Style BX — for API 6BX flanges at the highest wellhead working pressures, typically 5,000 psi and above. Not interchangeable with R or RX.
The ring number is the part number: R 20, RX 24, BX 155. It fixes size, groove and pressure class in one token, which is why an RTJ line item should quote it rather than describing the ring in words.
Two practical points. The ring must be softer than the flange groove — soft iron or low-carbon steel against carbon-steel flanges, 316 or Inconel where the process demands — because the ring is the sacrificial part and a groove is far more expensive to repair than a ring is to replace. And groove condition is the real failure mode: a scored, corroded or out-of-round groove will leak with a perfect ring in it. Inspect the groove before blaming the gasket.
Sheet and cut gaskets
Compressed fibre, PTFE, graphite laminate or rubber, cut to shape. Correct and economical for utilities, water, low-pressure air and Class 150 duties, and available as full-face for flat-face flanges.
They are not a cost-saving substitute on hydrocarbon service at pressure. Specify the sheet material and thickness, not just "gasket sheet" — and for anything cut locally, confirm the material actually is what the offcut was sold as.
Insulating kits
A non-conductive gasket with sleeves and washers that electrically isolate the two halves of a joint. Used to section a cathodic protection system, or to break a galvanic couple between dissimilar metals.
These are engineered assemblies. Specify the kit, the flange rating and the service — do not assemble one from loose parts, and do not leave the bolt sleeves off because they looked optional in the box.
What a complete gasket line item says
Whatever the family, an unambiguous line states size, pressure class, the facing it must suit, the materials, the standard, and the certification required. For a spiral wound:
Spiral-wound gasket, 4 inch, Class 300, 316L winding / flexible graphite filler, CS outer ring and 316L inner ring, to ASME B16.20, for raised-face flange to ASME B16.5. EN 10204 3.1 on metallic components.
For an RTJ, the ring number carries most of that on its own:
Ring type joint gasket, style RX, ring number RX 24, soft iron, to ASME B16.20, for API 6B flange. EN 10204 3.1.
A line that omits the class, the facing or the ring number is a line the supplier will complete on your behalf, using an assumption you never see.
Sourcing gaskets in Nigeria
Gaskets are low-value, high-consequence and consumed continuously — the worst combination to run out of, and the worst to air-freight one at a time.
- Buy to a schedule. Gaskets are light, easy to consolidate and easy to store. A standing order sized to your turnaround plan costs a fraction of the same items expedited during a shutdown.
- Put the certificate on the enquiry. If QA needs EN 10204 3.1 traceability on the metallic components, it has to be stated up front. Retrofitting a certificate to material already made is usually impossible. Our guide to importing certified industrial equipment into Nigeria covers where certification enters the process.
- Order the joint, not the gasket. Bolt load seals a joint; the gasket only responds to it. See hydraulic torque wrench selection for getting that load right, and pipe, fittings and flanges in Nigeria for the flange side of the same joint.
FirstSupply supplies spiral-wound, kammprofile, ring-type-joint, sheet and insulating gaskets against specification, with material certification where your requisition calls for it. Browse pipe, fittings & flanges, or send the flange schedule and we will price it line by line — including the joints where a different family is the better answer.