Raised Face vs Ring Type Joint
Two ways to seal a flanged joint — a compressed gasket or a yielded metal ring — and the pressure and temperature at which the second becomes necessary.
- RF height, Cl 150/300
- 1/16 in (1.6 mm)
- RF height, Cl 400+
- 1/4 in (6.4 mm)
- RTJ seal
- Metal ring, plastic deformation
- Typical RTJ use
- Class 600 and above
Short answer. A raised face seals a compressed gasket between two flat annular surfaces; a ring type joint seals a solid metal ring crushed into a machined groove. RF is the B16.5 default and covers most service. RTJ takes over at high pressure and temperature — usually Class 600 upward — because a metal seal survives thermal cycling that flattens a sheet or spiral wound gasket.
How each one actually seals
A raised face concentrates the bolt load onto a smaller annulus than a flat face would, raising the gasket seating stress for the same bolt torque. The gasket — sheet, spiral wound, or kammprofile — deforms into the serrated finish and seals by compression. It relaxes over time and with temperature, which is why RF joints need retorquing after a thermal cycle.
An RTJ groove takes a solid metal ring, oval or octagonal, softer than the flange. Tightening the bolts yields the ring into the groove flanks, so the seal is a metal-to-metal interference fit rather than a compressed pad. The flange faces stand apart and never touch, and internal pressure acting on the ring tends to push it harder against the groove — the seal gets tighter as pressure rises.
RF: gasket compressed between faces · RTJ: metal ring yielded into a groove
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Raised face OD | Class 150 flange OD | RF height, Cl 150/300 | RF height, Cl 400+ |
|---|---|---|---|---|
| NPS 1/2 | 1.3835.05 mm | 3.5088.90 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 3/4 | 1.6942.93 mm | 3.8898.55 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 1 | 2.0050.80 mm | 4.25107.95 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 1 1/4 | 2.5063.50 mm | 4.62117.35 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 1 1/2 | 2.8873.15 mm | 5.00127.00 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 2 | 3.6291.95 mm | 6.00152.40 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 2 1/2 | 4.12104.65 mm | 7.00177.80 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 3 | 5.00127.00 mm | 7.50190.50 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 3 1/2 | 5.50139.70 mm | 8.50215.90 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 4 | 6.19157.23 mm | 9.00228.60 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 5 | 7.31185.67 mm | 10.00254.00 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 6 | 8.50215.90 mm | 11.00279.40 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 8 | 10.62269.75 mm | 13.50342.90 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 10 | 12.75323.85 mm | 16.00406.40 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 12 | 15.00381.00 mm | 19.00482.60 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 14 | 16.25412.75 mm | 21.00533.40 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 16 | 18.50469.90 mm | 23.50596.90 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 18 | 21.00533.40 mm | 25.00635.00 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 20 | 23.00584.20 mm | 27.50698.50 mm | 0.061.52 mm | 0.256.35 mm |
| NPS 24 | 27.25692.15 mm | 32.00812.80 mm | 0.061.52 mm | 0.256.35 mm |
Raised face outside diameter is common to Classes 150 through 600. Classes 900, 1500 and 2500 use different gasket surfaces in several sizes — check B16.5 Table 9 before ordering a gasket.
Raised face (RF)
- The B16.5 default — widest availability and lowest cost
- Works with cheap sheet and spiral wound gaskets
- Tolerant of modest face damage and misalignment
- Easy to inspect: face contact is visible
- Relaxes with thermal cycling and needs retorquing
Ring type joint (RTJ)
- Metal seal survives high temperature and thermal cycling
- Pressure-energised — tighter as pressure rises
- Standard for Class 600 and above in hydrocarbon service
- Groove must be machined true and undamaged; repair is costly
- Ring is a single-use consumable and adds standoff to the spool length
Flat face is the third option
A flat face flange has no raised area at all and takes a full-face gasket. It exists for one reason: bolting to a brittle casting. Against a cast iron pump or valve flange, a raised face bends the mating flange as the bolts pull it in and cracks it. Any time steel meets cast iron, the steel flange should be flat faced with a full-face gasket. See the full face type reference.
Never mix faces
An RF flange will not seal against an RTJ flange, and a raised face against a flat face is the cracked-casting scenario above. Face type is part of the flange specification and must match on both halves of every joint — it is worth checking on site rather than trusting the isometric.
See also: all six face types · choosing a face · bolt torque · flange dimensions.
Common questions
Can an RF flange bolt to an RTJ flange?
No. The RTJ flange has a machined groove and no flat gasket surface at the right diameter, so there is nothing for an RF gasket to seal against. The faces must match.
At what class should I switch to RTJ?
There is no code threshold. In practice most specs use RF through Class 300 or 600 and RTJ above, with the switch driven by temperature and thermal cycling as much as by pressure.
How tall is the raised face?
1/16 in (1.6 mm) for Classes 150 and 300, and 1/4 in (6.4 mm) for Class 400 and above. The tabulated flange thickness excludes it, so the measured flange is thicker than the table.
Are RTJ rings reusable?
No. The ring seals by yielding into the groove, so it is permanently deformed once made up. Fit a new ring every time the joint is broken.
What gasket does a flat face flange need?
A full-face gasket that covers the entire face including the bolt holes. A ring gasket on a flat face lets the flange bend inside the bolt circle.