company news about RTJ Flanges: Ring Joint Groove Geometry, R, RX and BX Gaskets, and Field Inspection

A ring type joint is the one bolted connection in a process plant where the gasket is not asked to compress. It is asked to yield. A machined metal ring is forced into a groove cut in each flange face until ring and groove meet metal to metal, and the seal that forms is a narrow band of controlled plastic deformation made in two flanges at once.

That single decision explains why RTJ connections hold where a fibre gasket would be extruded, why they are unforgiving of a scratched groove, and why the ring that sealed on the test stand should not go back into the joint after the line is broken.

Why a Ring Type Joint Exists

A raised-face joint spreads bolt load across an annular seating area. The gasket has thickness, and that thickness absorbs misalignment, surface imperfections and the small movements that appear when a line heats up. The trade-off is a modest unit load at the sealing surface, which makes the gasket material the limiting factor for pressure, temperature and chemistry.

An RTJ joint concentrates the same load into a narrow contact band, so stress at the sealing line is far higher than any flat gasket could survive. The metal ring moves the service limits into the metal itself, and because it has already deformed into the groove, the joint behaves almost like a welded connection.

That stiffness is also the weakness: no gasket thickness is left to absorb an error, so a worn groove, an incorrect ring number, non-parallel flanges or a single-pass bolt load each produce a leak that retightening cannot cure. Any raised face on an RTJ flange is a locating surface, not a seal.

The Three Ring Styles: R, RX and BX

All three families seal by metal-to-metal contact but are not freely interchangeable. Rings made to ASME B16.20 and API 6A are identified by an R, RX or BX number, and that number is the only reliable way to match a ring to a groove.

Style R: the baseline ring

Style R is the original ring joint and still the most common. It is made in oval and octagonal cross-sections, both interchangeable in modern flat-bottom octagonal grooves: the octagonal ring contacts 23-degree angled faces, the oval ring radiused ends. Sealing faces are finished to 1.6 micrometre Ra, or 63 micro-inch RMS, and that ring finish, not the flange face, governs tightness. R rings cover ASME B16.5 flanges from NPS 1/2 to NPS 24 and ASME B16.47 Series A above that.

Style RX: pressure-energized in an R groove

An RX ring is taller and heavier than the R ring of the same size. Its outside bevels touch the outer groove surfaces first, so a seal exists before the line is pressurized, and rising pressure pushes the ring outward to raise seating stress. That self-energizing action resists shock loads, test-pressure spikes and vibration. RX rings fit standard R grooves, but the extra height changes assembled length, which matters on a spool dimensioned around an R ring.

Style BX: the API 6A geometry

BX rings have a pitch diameter slightly larger than the groove pitch diameter, so contact starts on the outside of the ring and pre-loads it before pressure arrives. Each ring carries a pressure passage that lets trapped pressure equalize. BX rings belong to API 6A wellhead equipment, fit only BX grooves and are not interchangeable with RX.

Groove Geometry: Where Leak-Tightness Is Decided

A groove is defined by four features that matter in the field: flank angle, width, depth and pitch diameter. Tolerances are measured in hundredths of a millimetre and the ring is machined to match, which is why rings are ordered by number rather than by pipe size. The flat bottom does not seal; it provides a datum and space for the small deformation that occurs at make-up.

Damage is the practical risk. A circumferential scratch across a flank is a leak path no bolt load will close, because the ring cannot deform into a groove that no longer matches its profile; dents, burrs and pitting rank with it. Re-machining removes the defect but changes depth or width, so the flange must be re-qualified against the new groove dimensions and re-paired with a ring that fits them.

Ring Material and Hardness: A Pair That Cannot Be Reversed

The governing rule is simple and often broken: the ring must be softer than the flange it seals against. The ring is the part designed to yield; if it is harder than the groove, the groove yields and the flange becomes the consumable item. Published values are ceilings, not targets.

The problem case is stainless on stainless. F304 or F316 flanges forged to ASTM A182/A182M are commonly supplied annealed, and a 316 ring at the 160 HB ceiling leaves little margin against a groove of similar hardness. Specify the ring so its permitted maximum sits clearly below the measured groove hardness, and confirm both from mill certificates; a single hardness figure on a purchase order is not a material check.

Soft iron and low carbon steel rings are zinc plated to a maximum of 8 micrometres for atmospheric protection, not sealing. Every ring carries permanent low-stress markings showing style, ring number, material and standard.

Bolting and Assembly Practice

The ring deforms once. Bolt load must be high enough to force it into the groove and still leave preload after short-term relaxation; because the contact area is small, that load is reached quickly and the margin for error in tightening is small too.

• Match bolting to the service. ASTM A193/A193M covers alloy and stainless bolting for high-temperature or high-pressure work; low-temperature service moves to A320/A320M.
• Use the torque values issued for the project, not generic tables that ignore lubricant, coating and flange class. The sequence is covered in the note on flange bolt torque and tightening sequence.
• Tighten in stages in a crossing pattern and watch the gap close evenly; a joint that closes on one side first has already tilted the ring.
• Keep groove and ring clean and dry, since anti-seize or grease on the sealing faces changes the friction the ring needs to deform, and never reuse a ring or fit two gaskets to close a gap.

Bolting is where most post-start-up leaks begin. A joint that passes a cold hydrotest and leaks hot has usually lost preload, not gasket volume: bolt, flange and ring expand at different rates.

Inspection Before the Joint Is Closed

Most RTJ failures are decided before the last bolt is tightened. At make-up, confirm the ring number against both flange markings and the line specification; inspect each groove for circumferential scratches, dents, burrs, pits and paint on the flanks; check that the ring sits flat; verify material and hardness against the certificate; confirm studs enter freely and protrude evenly; and record the final flange gap at four points so a tilted joint is caught before pressure test.

When RTJ Is the Wrong Choice

RTJ is a poor default. It earns its place where pressure, temperature, thermal cycling or media rule out a softer gasket; elsewhere it adds machining cost and maintenance discipline for no sealing benefit. Conversion is not simply a groove cut into an existing face either: flange thickness, groove geometry, stack height and bolting change together.

Conclusion

An RTJ connection is decided by three things fixed before assembly: the groove, the ring and the bolt load. The groove must carry the correct profile and pitch diameter without circumferential damage; the ring must be the right number, material and hardness for that groove; the bolt load must deform the ring once, evenly, and hold it. Get all three right and the joint will hold through thermal cycles that would destroy a flat gasket.

Frequently Asked Questions

Can an oval R ring replace an octagonal R ring in the same groove?

Yes. Both share the same ring number and fit flat-bottom octagonal grooves, but they contact the groove differently and should not be mixed within one flange pair.

Can an RX ring be used in a standard R groove?

Yes, RX rings fit standard R grooves and are used to add shock and vibration resistance. The ring is taller, so assembled length increases and any spool dimensioned around an R ring must be re-checked.

Why does an RTJ joint leak after passing a hydrotest?

Usually because preload was lost, not because the ring failed. Differential expansion between bolts, flanges and ring at operating temperature, or a groove that was marginal from the start, shows up only at service temperature.

Is an RTJ flange stronger than a raised-face flange?

The flange body is not automatically stronger; the sealing system changes. A metal ring tolerates higher pressure and temperature than a soft gasket, and demands a sound groove and adequate bolt load.

Specifying RTJ Flanges for an Inquiry

An inquiry that omits any of these will come back with a question, and sometimes with the wrong part: size, class and dimensional standard (NPS 6 Class 600 to ASME B16.5); flange type and grade (weld neck, ASTM A182 F316L); ring number and style (R45 octagonal, or RX for a retrofit); ring material with maximum hardness and certificate requirement; and groove finish with any witness point.

RTJ flanges in stainless, duplex, super duplex and nickel alloy grades sit within the alloy steel flange range, including weld neck flanges supplied with RF or RTJ facing. Because a ring joint is dimensioned by ring number, the technical enquiry should carry that number with size, class, standard and material, so flange and ring are quoted as one matched assembly.

For the geometry, tolerances and hardness values behind these families, see the ring type joint technical data published by Flexitallic for rings made to ASME B16.20 and API 6A.

This article is general information only. Final selection must be based on the applicable code, the project specification and confirmation by qualified engineering personnel.