Flange Face Types
Raised face, flat face, ring type joint and the rest — what each face does, which gasket it needs, and why two flanges with different faces will never seal against each other.
- Faces in ASME B16.5
- 6
- Most common
- Raised face (RF)
- RF height, Class 150/300
- 1/16 in (1.6 mm)
- RF height, Class 400+
- 1/4 in (6.4 mm)
The face is the part of the flange that actually seals. It does not change the flange outside diameter, bolt circle or bolt pattern — a Class 300 NPS 6 flange has the same bolting whatever its face — but it does dictate the gasket, and two flanges with different faces cannot be bolted together.
Raised Face (RF)
The default in ASME B16.5 and by far the most common. A circular raised area carries the gasket, concentrating bolt load onto a smaller area than a flat face. Height is 1/16 in for Classes 150 and 300, and 1/4 in for Class 400 and above.
Flat Face (FF)
The gasket surface is the full flange face. Used against cast iron equipment flanges and other brittle castings, where a raised face would bend the mating flange and crack it. Needs a full-face gasket.
Ring Type Joint (RTJ)
A machined groove takes a solid metal ring that seals by plastic deformation as the bolts are tightened. The flange faces stand apart and never touch the gasket seating area. Standard for high-pressure and high-temperature service, especially Class 900 and above.
Tongue and Groove (T&G)
A projecting tongue on one flange mates with a matching groove on the other, trapping the gasket so it cannot blow out or be over-compressed. Supplied as a matched pair — a tongue flange is useless without its groove.
Male and Female (M&F)
Like tongue and groove but with a deeper male projection that locates in a female recess. Also a matched pair. Uncommon in new work; found in older plant and in some heat exchanger designs.
Lap Joint (LJ)
Not a face in its own right — the stub end provides the gasket surface and the backing flange only supplies bolt load. The stub end face may itself be flat or serrated.
Surface finish matters as much as the face
ASME B16.5 specifies a serrated finish on raised and flat faces, either concentric or spiral, at 125 to 500 microinches Ra. The serrations bite into a soft gasket and hold it in place. A face machined smooth — which looks better — seals worse, because there is nothing for the gasket to key into.
Flange thickness and the raised face
The thickness tabulated in B16.5 excludes the raised face. A Class 300 NPS 6 flange is listed at 1.44 in and measures 1.50 in overall with its 1/16 in raised face. Class 400 and above add 1/4 in. This matters when calculating stud bolt length.
Common questions
Can I bolt a raised face flange to a flat face flange?
Not properly. The raised face concentrates load on a narrow band and will bend the flat face flange — which is usually cast iron, and will crack. The correct fix is to machine the raised face off the steel flange and use a full-face gasket.
What is the raised face height?
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 in B16.5 excludes it, so the overall thickness is the table figure plus the face.
When do I need a ring type joint flange?
High pressure and high temperature, typically Class 900 and above, and wherever a metal-to-metal seal is required — hydrogen service, wellhead equipment and high-temperature hydrocarbon lines. RTJ flanges only mate with other RTJ flanges of the same ring number.