The facing is the part of the flange that seats the gasket. Matching facings, and matching the gasket to the facing, is essential for a leak-free joint.
Raised face (RF)
The most common facing in process plants. A raised ring around the bore concentrates bolt load on a smaller gasket area, improving sealing. Under ASME B16.5 the raised face is 1.6 mm (1/16 in) high on Class 150 and 300 flanges and 6.4 mm (1/4 in) high on Class 400 and above. The face is normally given a serrated (concentric or spiral) finish, typically around 3.2–6.3 µm Ra (125–250 µin), so that soft and spiral-wound gaskets can grip it.
Flat face (FF)
The gasket surface is flush with the rest of the flange face and a full-face gasket covers the whole face, including the bolt holes. Flat face flanges are used to mate with cast iron, plastic or other brittle equipment flanges, where a raised face could bend and crack the mating flange when bolts are tightened. Never bolt an RF flange to an FF cast-iron flange without first machining off the raised face.
Ring type joint (RTJ)
A precisely machined groove holds a solid metal ring gasket (types R, RX, or BX for API 6A). When the bolts are tightened, the ring deforms into the groove to form a metal-to-metal seal. RTJ facings are used for high pressure and high temperature service, typically Class 600 and above, in refineries, oil and gas and wellhead equipment. The ring material should be softer than the flange so that the ring, not the groove, deforms.
Other facings
Tongue-and-groove and male-and-female facings trap the gasket and are used where gasket blow-out must be avoided, such as on heat exchanger channels. They must be ordered in matching pairs.
Quick guide
| Raised face | General process service; spiral-wound or soft gaskets |
|---|---|
| Flat face | Mating with cast iron or non-metallic flanges; full-face gaskets |
| Ring type joint | High pressure and temperature; metal ring gaskets |
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