Article
In high pressure and high temperature service, a soft gasket on a raised face is not always enough. Ring type joint (RTJ) flanges use a solid metal ring seated in machined grooves. As the bolts are tightened, the ring is squeezed into the grooves and forms a metal-to-metal seal.
Where RTJ flanges are used
- Class 600 and above in oil and gas, refinery and petrochemical plants
- Wellhead equipment and christmas trees (API 6A)
- High temperature hydrogen and steam service
Ring types
| Ring | Shape | Use |
|---|---|---|
| R oval | Oval cross-section | Older designs; fits flat-bottom and round-bottom grooves |
| R octagonal | Octagonal cross-section | Most common in ASME B16.5 RTJ flanges; better sealing |
| RX | Pressure-energised | Higher pressure; system pressure helps the seal |
| BX | Pressure-energised, face-to-face design | API 6A 6BX flanges up to very high pressure |
Ring numbers
Rings are identified by a number, for example R-37 for a 4" Class 600 flange. The ring number depends on size and class, and the flange groove must match the ring. ASME B16.20 lists the ring dimensions and ASME B16.5 lists which ring suits each flange.
Hardness rule
The ring must be softer than the flange groove so that the ring deforms, not the groove. A common guide is that the ring should be at least 20–30 HB softer. Typical ring materials:
- Soft iron (max 90 HB) – for carbon steel flanges
- Low carbon steel (max 120 HB)
- 304 / 316 stainless (max 160 HB) – for stainless flanges
- Inconel 625 and other alloys for special service
Groove care
The groove surface finish and dimensions are critical. Scratches across the groove create leak paths, so RTJ flanges should be supplied with groove protection and handled carefully during installation. Rings are not reusable; fit a new ring each time the joint is opened.
A complete description
Weld Neck Flange 4" Class 600 RTJ, bore to SCH 80S, ASTM A182 F316L, ASME B16.5 + Ring Gasket R-37 octagonal, SS 316
Frequently asked questions
Can an RTJ ring be reused?
No. The ring deforms when the joint is tightened, so a new ring should be fitted every time the joint is made up.
Why must the ring be softer than the flange?
So the ring deforms into the groove and seals, without damaging the groove surface.
What is the difference between R and RX rings?
R rings are standard oval or octagonal rings; RX rings are pressure-energised and seal better at higher pressure.
Key takeaways
- RTJ flanges seal metal-to-metal with a ring in machined grooves.
- Match ring number to flange size and class.
- Ring must be softer than the groove; never reuse rings.
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