Choosing a Flange Face Type

Raised face, flat face and ring type joint — which to specify for a given service, and the face-mixing mistakes that show up as leaks on commissioning.

Faces in B16.5: 6
Default: Raised face (RF)
RF height, Cl 150/300: 1/16 in
RF height, Cl 400+: 1/4 in

Short answer. Pick the face from what you are bolting to and how hot the joint runs. Raised face is the default and covers most service. Flat face is mandatory against cast iron and other brittle castings — a raised face will crack them. Ring type joint takes over at high pressure and temperature, typically Class 600 and above, where a metal seal outlasts a compressed gasket.

Choosing a face

Flange face selection by service.
If the joint is…	Use	Because
Ordinary process or utility, Class 150–600	Raised face (RF)	The B16.5 default. Cheapest, most available, works with sheet and spiral wound gaskets.
Against a cast iron pump, valve or equipment flange	Flat face (FF) with a full-face gasket	A raised face bends the brittle casting inside the bolt circle as the bolts pull it in, and cracks it.
Class 600 and above, or above about 800 °F	Ring type joint (RTJ)	A yielded metal ring survives thermal cycling that relaxes a compressed gasket, and it is pressure-energised.
Hydrocarbon service at high pressure	Ring type joint (RTJ)	The usual specification choice where a leak is a fire or toxic release.
Frequently dismantled, gasket must not blow out	Tongue and groove (T&G)	The gasket is trapped in the groove and cannot be extruded or over-compressed.
Cryogenic or very low temperature	RTJ or RF with a suitable gasket	Differential contraction relaxes bolt load; a metal seal or a spring-energised gasket handles it better.
Behind a stub end (lap joint)	The stub end face governs	The backing flange never contacts the gasket, so the stub end's face is what matters.

Face type must match on both halves of a joint. RF will not seal against RTJ, and RF against FF is the cracked-casting case.

Raised face dimensions

The raised face outside diameter is the gasket contact diameter, and it is common to Classes 150 through 600 in a given size. The tabulated flange thickness in B16.5 excludes the raised face, so a flange always measures thicker than the table by the face height.

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

Raised face gasket contact diameters and face heights, ASME B16.5.
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	0.06251.59 mm	0.2506.35 mm
NPS 3/4	1.6942.93 mm	3.8898.55 mm	0.06251.59 mm	0.2506.35 mm
NPS 1	2.0050.80 mm	4.25107.95 mm	0.06251.59 mm	0.2506.35 mm
NPS 1 1/4	2.5063.50 mm	4.62117.35 mm	0.06251.59 mm	0.2506.35 mm
NPS 1 1/2	2.8873.15 mm	5.00127.00 mm	0.06251.59 mm	0.2506.35 mm
NPS 2	3.6291.95 mm	6.00152.40 mm	0.06251.59 mm	0.2506.35 mm
NPS 2 1/2	4.12104.65 mm	7.00177.80 mm	0.06251.59 mm	0.2506.35 mm
NPS 3	5.00127.00 mm	7.50190.50 mm	0.06251.59 mm	0.2506.35 mm
NPS 3 1/2	5.50139.70 mm	8.50215.90 mm	0.06251.59 mm	0.2506.35 mm
NPS 4	6.19157.23 mm	9.00228.60 mm	0.06251.59 mm	0.2506.35 mm
NPS 5	7.31185.67 mm	10.00254.00 mm	0.06251.59 mm	0.2506.35 mm
NPS 6	8.50215.90 mm	11.00279.40 mm	0.06251.59 mm	0.2506.35 mm
NPS 8	10.62269.75 mm	13.50342.90 mm	0.06251.59 mm	0.2506.35 mm
NPS 10	12.75323.85 mm	16.00406.40 mm	0.06251.59 mm	0.2506.35 mm
NPS 12	15.00381.00 mm	19.00482.60 mm	0.06251.59 mm	0.2506.35 mm
NPS 14	16.25412.75 mm	21.00533.40 mm	0.06251.59 mm	0.2506.35 mm
NPS 16	18.50469.90 mm	23.50596.90 mm	0.06251.59 mm	0.2506.35 mm
NPS 18	21.00533.40 mm	25.00635.00 mm	0.06251.59 mm	0.2506.35 mm
NPS 20	23.00584.20 mm	27.50698.50 mm	0.06251.59 mm	0.2506.35 mm
NPS 24	27.25692.15 mm	32.00812.80 mm	0.06251.59 mm	0.2506.35 mm

Raised face OD is common to Classes 150–600. Classes 900, 1500 and 2500 use different gasket surfaces in several sizes — check B16.5 Table 9.

Surface finish

ASME B16.5 requires a serrated finish on raised and flat faces, concentric or spiral, at 125 to 500 microinches Ra. This is counter-intuitive but important: a face machined smooth seals worse, because the gasket has nothing to key into and can slide radially under pressure. If a face has been skimmed during repair, the serration must be restored.

The mistakes that cause leaks

Mixing faces. RF against RTJ cannot seal. RF against FF cracks castings. Both are found on site more often than they should be.
Ring gasket on a flat face. A flat face needs a full-face gasket covering the bolt holes; a ring gasket lets the flange bend inside the bolt circle.
Reusing an RTJ ring. The ring seals by yielding. Once used it is deformed and will not seal again.
Ignoring the face height in spool dimensions. The 1/4 in face at Class 400 and above adds to the make-up length and to the bolt length required.
Damaged serrations. A radial scratch across the face is a leak path; circumferential marks generally are not.

Common questions

Which flange face should I use as a default?

Raised face. It is the ASME B16.5 default, the most widely available, and suitable for most service through Class 600.

Why must a flat face be used against cast iron?

Because a raised face concentrates load inside the bolt circle and bends the mating flange. Cast iron is brittle and cracks rather than yielding, so a steel flange bolting to a cast iron one should be flat faced with a full-face gasket.

Can I bolt an RF flange to an RTJ flange?

No. The RTJ flange has a machined groove where the RF gasket would need a flat surface, so there is nothing to seal against. Face types must match.

How tall is a 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 B16.5 thickness tables exclude it.

Should flange faces be machined smooth?

No. B16.5 specifies a serrated finish of 125 to 500 microinches Ra. A smooth face gives the gasket nothing to grip and seals worse than a properly serrated one.