Flange & valve end machining
Safety and liability. Flanges, valves and pipe ends hold pressure. A wrongly machined face, groove or weld prep can leak or fail without warning, causing serious injury, death, fire or environmental damage. This page is a general overview for orientation only. It is not a standard, a specification or engineering advice. It shows only a few typical figures and does not reproduce the ASME, API, EN or MSS dimension tables. Always work from the current licensed standard, the purchase order and drawing, and your quality plan. Where the code requires it, get the work approved by a competent engineer or inspector. LatheCalculator accepts no liability for any loss or damage arising from use of this information.
The main face types, finishes and end preparations you’ll meet when machining or modifying flanges and valve ends, plus a calculator for weld bevels and counterbores.
Open the bevel & counterbore calculator →
Facings
| Face | What it is | Key figures (typical) |
|---|---|---|
| RF – raised face | A raised ring inside the bolt holes that the gasket sits on. The most common face. | Height 1.6 mm (1/16") for Class 150 and 300; 6.4 mm (1/4") for Class 400 and above.1 Height tolerance: check the standard est. |
| FF – flat face | The whole face is flat, often to mate with cast-iron or other brittle flanges. Uses a full-face gasket. | Don’t bolt an RF flange to a brittle FF flange: machine the raised face off, or use the correct gasket per the spec. |
| RTJ – ring-type joint | An octagonal or oval metal ring sits in a machined groove. | R-ring groove sidewall angle 23° ±0.5°.3 Groove finish 1.6 µm Ra maximum.2 Groove sizes are in ASME B16.5 / API 6A. |
| Tongue & groove / male & female | Matched pairs that trap the gasket. | Finish 3.2 µm Ra maximum.2 |
Surface finish
- Serrated (spiral or concentric) RF: 3.2–6.3 µm Ra, 45–55 grooves per inch, cut with a tool radius of at least 1.5 mm (1/16").2 This finish grips spiral-wound and soft gaskets. Don’t polish it smooth.
- RTJ groove: 1.6 µm Ra maximum, free of radial scratches.2 BX (API 6A) grooves are usually finer, around 0.8 µm est.
- On any sealing face, radial scratches or tool marks across the face are leak paths. Mark them as rejects or dress them per the spec.
Grooves per inch → feed: 50 grooves/in ≈ 0.51 mm/rev on a facing cut. Ra can be checked with a comparator plate or a profilometer.
Weld and threaded ends
- Butt weld: typical bevel 37.5° ±2.5° with a 1.6 ±0.8 mm (1/16" ±1/32") root face. Heavy walls (over 22 mm) use a compound bevel: 10° above 19 mm from the bore.4 Some pipeline work uses 30° (API 1104).4 Calculate the setback →
- Unequal walls: taper the thicker side no steeper than 1:4 (about 14°) or 30°, depending on the code. A counterbore must never take the wall below the design minimum.5
- Socket weld: leave about a 1.6 mm (1/16") gap at the bottom of the socket before welding, so the pipe can expand. Minimum fillet leg is about 1.09 × the nominal pipe wall (ASME B31.3, commonly published). Socket bore and depth are set by ASME B16.11.
- NPT: taper 1 in 16 on diameter, a half-angle of 1.7899°, 60° thread form.6 Gauge with L1 ring/plug gauges, not by eye. BSP taper (R/Rc) has the same 1:16 taper but a 55° form, so the two don’t interchange.
Common modifications
- Re-facing: skim the minimum needed to clean up, then re-cut the serration. Check that the RF height and flange thickness still meet the standard minimum afterwards.
- Converting RF to FF: machine the raised face flush with the bolt-circle face. Only do it where the purchaser specifies it, and check the remaining thickness.
- Re-cutting RTJ grooves: re-cutting oversize changes the ring size. Record it and use the matching ring. Many specs require a weld-overlay repair instead.
- Weld overlay (for example Inconel 625 on sealing faces and grooves): usually specified as about 3 mm minimum finished thickness after machining est.. The purchase spec governs. Allow for it when setting the pre-overlay cut depth.
- Valve ends: converting flanged to butt-weld or socket-weld ends, or changing the bore, may affect the pressure rating and certification. Check with the valve manufacturer and the inspector first.
Sources
- Exal, flange facings datasheet (RF heights).
- Enerpac, flange surface finish guidance (serrated RF, T&G and RTJ finishes).
- Munaco Sealing, ring-joint gasket groove information (23° groove angle).
- ESCO Tool, pipe weld preparation guide.
- Wermac, unequal wall thickness.
- Engineering ToolBox, NPT thread dimensions.
Governing standards (not reproduced here): ASME B16.5, B16.47, B16.25, B16.11, B1.20.1, B31.3; API 6A; EN 1092-1. Figures marked est. have not been confirmed against a published source.