R, RX and BX ring joint gaskets: which ring fits which flange

A ring joint seals by crushing a metal ring into a groove machined in each flange face, so the ring and the groove have to match exactly. There are three families of ring, each tied to a family of flange, and the ring number alone does not tell you which family you are in. Get the family right first; the number and the material follow.

/01Three families, three flange types.

/02R rings: oval or octagonal.

R rings are made to ASME B16.20 and API 6A and fit ASME B16.5 ring-joint flanges from Class 150 to Class 2500 and API 6B flanges at 2,000, 3,000 and 5,000 psi. They come in two sections. The octagonal ring seats on its angled faces in a modern flat-bottomed groove; the oval ring can sit in either a flat-bottomed groove or the older round-bottomed type. One ring number can serve several flanges — R-24 is listed for NPS 2 Class 900 and 1500 and for API 2-1/16" at 3K and 5K — which is why the number, not the flange size, is what to order.

/03RX rings: same groove, pressure-energised.

An RX ring is taller and has an asymmetric section that the line pressure pushes harder against the groove, so the seal improves as pressure rises. It is designed for the same groove as the R ring of the same number in an API 6B flange: RX-24 and R-24 share a 3.750 in pitch diameter, but RX-24 is 1.000 in tall against 0.630 in for an octagonal R-24. Because the RX ring is taller, the gap left between the flange faces after make-up differs from an R ring's, so check it against the flange data rather than assuming.

/04BX rings: the 6BX family.

BX rings are for API 6BX flanges: every size at 10,000, 15,000 and 20,000 psi, and the large sizes at lower pressure — on our listings BX-160 for 13-5/8" 5K, BX-163 for 18-3/4" 5K and BX-165 for 21-1/4" 5K. Each BX ring carries a small pressure-passage hole (0.060 in on BX-152, 0.120 in on BX-160) so pressure cannot be trapped between the ring and the groove. BX numbers start at 150 and share nothing with the R and RX series; there is no substitute across the families.

/05Material, hardness and reuse.

Our rings are listed in soft iron (D), low carbon steel (S), F5, 304, 316, 321, 347 and 410 stainless, Alloy 625 and Alloy 825. The ring has to be softer than the groove so that the ring deforms and the flange does not; a ring harder than the groove marks it and spoils the next seal. Match the material to the flange and the service, including any sour-service requirement, and check the ring's stamped number and material code before fitting. A ring is crushed into shape when the joint is made up, so treat ring gaskets as single-use.

/06Ordering the right ring.

Give the ring number and the material, and if the number is unknown, the flange standard (ASME B16.5, API 6B or API 6BX), the nominal size and the pressure class. For a full nipple-up, rings are best ordered with the studs and nuts for the same flange. Where the joint is on a wellhead, a valve or treating iron, see reading an API 6A nameplate for the material and temperature classes that apply to the whole assembly.