A Ring Type Joint flange does not seal on the face. It seals on the groove — specifically on two thin contact bands inside the groove walls, where the ring gasket bites in under bolt load. Get the groove geometry within API 6A spec and the joint holds 15,000 psi without complaint. Miss the geometry by 0.1 mm on depth or 1° on angle and the flange leaks the day you bring it back into service.
I have been called to enough re-do jobs over the years to know what goes wrong on RTJ grooves in the field. Below is the procedure that holds and the mistakes that do not.
The three groove profiles you will meet
Every RTJ flange is one of three groove forms, defined by API 6A and ASME B16.20:
- R (octagonal or oval) — symmetric 23° walls, flat or radiused bottom. The historical standard, still the majority of installed base in older units.
- RX — pressure-energised. 23° walls, deeper than R, a pressure passage at the OD. Replaces R in higher-rated service without re-machining the flange — same groove, different ring.
- BX — for API 6BX flanges only. 23° walls, very tight tolerance, fully contained gasket. The standard on subsea and high-pressure wellhead service.
Before you cut anything, read the flange stamping and confirm which profile you are restoring. Crews who skip this step have machined R grooves where the gasket was BX and the flange has not sealed since.
What “on tolerance” actually means
API 6A and ASME B16.20 specify, for each ring number, the groove width, depth, OD and ID to ± 0.13 mm (± 0.005 in) on critical dimensions, plus a surface roughness of Ra 1.6 µm (63 µin) maximum, plus the 23° wall angle ± 0.5°.
| Parameter | API 6A tolerance | What it does if wrong |
|---|---|---|
| Wall angle | 23° ± 0.5° | Wrong contact line → ring rolls or fails to seat |
| Groove depth | ± 0.13 mm | Over-deep: ring bottoms before sealing. Under: bolt-up over-stresses ring |
| Groove width | ± 0.13 mm | Ring tilts; line contact lost |
| Surface finish | Ra ≤ 1.6 µm | Above 1.6 µm leaks even at full bolt load |
| Concentricity to bore | ≤ 0.1 mm TIR | Eccentric groove leaks on one side |
The numbers are tight because the seal is on a 0.5 mm contact band. There is nowhere for the geometry to hide.
On-site procedure that holds
This is the way an ID-mounted portable flange facer restores an R or RX groove in the field. BX work needs the same sequence with stricter inspection.
1. Set the reference axis on the bore, not the face
The flange face is what failed — it is the part you are about to machine, so it cannot be the reference. Mount the machine on the bore (ID) with an expanding mandrel and verify concentricity with a dial indicator to ≤ 0.05 mm TIR before any cut. If the bore itself is damaged, switch to OD reference with the same care.
I have lost count of crews who set the machine on the face for speed, re-cut the groove, and produced a beautiful groove that was 0.4 mm eccentric to the bore. The flange leaks. Time saved on setup: 20 minutes. Time lost on the second mobilisation: a day.
2. Lock everything before the first cut
Vibration is the enemy of surface finish. Before you touch the feed:
- Bolt the flange to its mating piece or to a witness flange under partial bolt load. A free-hanging flange will chatter.
- Lock the machine’s radial slide; only the feed slide should move during the cut.
- Check the air or hydraulic supply is steady — feed motors fed off a fluctuating supply produce a wavy groove.
3. Resurface the face first, then cut the groove
Take the face down enough to remove any pitting or wash, then re-cut the groove. This means the groove dimensions are measured from the new face, not the old corroded one. The new face has to remain within the flange thickness tolerance — check the original drawing.
4. Cut the groove with one tool, one pass for the walls
The 23° walls have to be straight and meeting in a defined radius (R) or land (RX). Use a single-point profile tool that cuts both walls in one pass. Two-pass cutting introduces a step at the bottom of the groove that the ring will find.
5. Finish to Ra 1.6 µm with a finishing pass
The roughing pass establishes geometry. The finishing pass — fine feed, sharp tool — gets you to Ra 1.6 µm. Confirm with a roughness comparator or surface roughness tester. Do not approve the work on visual inspection alone. Ra 1.6 µm looks the same as Ra 3.2 µm under worklight, and Ra 3.2 µm leaks.
6. Inspect every dimension before re-installing
- Depth gauge in three positions around the groove.
- Vernier or fitted ring gauge on width.
- Bevel protractor or template on wall angle.
- Roughness comparator on finish.
- TIR check on concentricity to bore.
Record every measurement against the API 6A target. The inspection record is the proof you need when the flange goes back into service.
Four mistakes I keep seeing
1. Trusting the face as the reference
Covered above. Always the bore (or OD on a damaged bore), never the face you are about to cut.
2. Cutting an R groove for a BX flange
The 23° walls are the same. The depth and width are not. An R groove on a BX flange will not contain the gasket properly under pressure. Read the flange stamping. If illegible, measure the original groove before you start — depth and width tell you which profile it is.
3. Ignoring surface finish
Ra is the sealing parameter. A bright shiny groove cut with a worn insert is Ra 6 µm, four times the API limit. The visual quality is unrelated. Use a roughness tester or comparator on every job.
4. Re-using the gasket
This is not a machining mistake, but I will mention it because crews who do excellent groove work routinely undo the work by re-bolting on a used ring. Ring gaskets are single-use. The ring deforms to fit the groove on first compression; it will not re-seal the same way.
What this means for tooling
RTJ work needs an ID-mountable flange facer with enough rigidity to hold the 23° wall angle and the surface finish under field conditions. Most general-purpose facing machines will do an OK R-groove repair. For RX and BX work the rigidity ceiling matters more — go for a heavier machine even if the flange size is modest.
See the flange facing machine range for OD and bore ranges, or the portable flange facer as a typical on-site machine. For squaring the pipe end before re-welding a damaged flange, the pipe facing machine range covers the same work on the pipe side.
For background on related flange sealing geometry, see flange surface finish requirements.
What to do next
If you have an RTJ leak in service:
- Identify the groove profile (R / RX / BX) and ring number from the flange stamping or original spec.
- Confirm the bore is undamaged enough to use as the machining reference.
- Quote with the flange OD, groove profile, ring number and your site’s permit-to-work constraints — send those to the quote form and we will match the right facer to the work.
A field RTJ repair done correctly is a 4 to 8 hour job per flange. A field RTJ repair done incorrectly is the same job again next week.
About the author — Dongmei Guo is founder of Kedes Machine. On the shop floor since 2010, she builds portable on-site machining and cold cutting equipment in Shenzhen and works directly with field crews in oil & gas, power, shipbuilding and high-purity tubing. She writes here when a question comes up on the workshop floor that is worth answering in full.