Ring joint gaskets are covered by ASME B16.20, which defines the types, the ring numbers, the dimensions and the materials. Three types cover almost all work. They are not interchangeable, and the differences are not cosmetic.
Type R, the standard ring
The conventional ring, for ASME B16.5, B16.47 Series A and API 6B flanges. It comes in two cross sections.
Octagonal
Eight flat faces. The angled flats bear directly against the angled groove walls, giving a wide, well defined contact band. This is the better sealer and it is what you should specify on new work.
Oval
A rounded cross section. It exists because older flanges were machined with a rounded groove bottom, and an octagonal ring may not seat properly in one. Contact is tangential rather than flat, so the contact band is narrower and the seal less robust.
An oval ring will sit in a modern flat-bottomed groove, and an octagonal ring may not sit correctly in an old rounded one. If you are working on existing plant and do not know the groove profile, check before ordering.
Type RX, pressure-energized
RX rings fit the same groove as an R ring on API 6B flanges, so they are a drop-in alternative on that flange type. The ring itself is taller, and its outside diameter is offset so that internal pressure acts on it in a helpful direction.
The effect is that rising line pressure pushes the ring harder against the outer groove wall. The seal tightens as pressure increases rather than being worked open by it.
That makes RX the better choice where the line sees pressure cycling, vibration or shock, which is why it is standard on drilling and wellhead assemblies. It also makes the joint more tolerant of slight bolt relaxation.
Type BX, for the highest pressures
BX rings are for API 6BX flanges, covering 5,000 through 20,000 psi. This is a different flange family, not a variant of 6B, with its own dimensions.
The defining feature is that a BX joint brings the flange faces into metal to metal contact. The faces bottoming out sets the compression on the ring precisely and gives the whole assembly its stiffness. That is what makes 20,000 psi service practical.
The vent hole in a BX ring is supposed to be there. It lets pressure equalise across the ring so the seal energises as designed. It is not a manufacturing defect, it must not be plugged, and a ring without one is not a BX ring.
Ring numbers
Every combination of flange size and class takes a specific ring number, R-24, R-45, BX-154 and so on. Neighbouring numbers are close enough in size to be confused by eye and far enough apart not to seal.
The flange, the groove and the ring number all have to agree. Put the ring number on the purchase order and on the line list, and check it at the joint before assembly. An installed wrong-number ring can hold a hydrotest and fail later.
Ring material and hardness
The principle is simple: the ring must be softer than the flange groove. The ring is the consumable and it is supposed to yield. If the ring is harder, it damages the groove instead of sealing against it, and a groove repair is a machining job.
| Ring material | Where it is used |
|---|---|
| Soft iron | Traditional, very soft, general service. Corrodes if left wet |
| Low carbon steel | Common general purpose choice |
| 304 and 316 stainless | Corrosive service and where the flange is stainless |
| Monel | Seawater and specific chemical service |
| Inconel | High temperature and aggressive service |
Specify the material and the maximum hardness. Hardness is the requirement people leave off, and it is the one that protects the flange. A stainless ring supplied in the wrong condition can be harder than the groove it is going into.
Coatings matter too. Rings are often supplied with a light coating or plating to help seating and to prevent corrosion in storage. Do not clean it off, and do not add grease that was not specified.
Handling and assembly
- Never reuse a ring. The seal is formed by permanent deformation. A used ring will not seal again reliably.
- Inspect before fitting. Any nick, flat or handling damage on the seating faces means a new ring. They are cheap.
- Keep them in their packaging until the moment of assembly, and do not stack them loose in a bucket.
- Clean the groove. Old sealant, rust scale and paint in the groove will stop the ring seating.
- Tighten evenly in a crossing pattern. A ring pulled in cocked does not seal, and it can damage the groove on the loaded side.
- Expect the flange faces not to touch on R and RX joints. That gap is normal. On a BX joint the faces do contact, and that is also normal.
See ring type joint flanges for the flange side, and machining and inspecting ring joint grooves for what the groove has to be.