Article
The facing is the part of the flange the gasket sits on. It decides how the joint seals, which gasket it takes, and how much bolt load is needed. Two flanges bolted together must have matching facings.
Raised face (RF)
The gasket sits on a raised ring around the bore. Concentrating the bolt load on a smaller area gives a higher gasket stress and a better seal, which is why raised face is the most common facing in process plants. The raised face height is 1.6 mm (1/16 inch) for Classes 150 and 300, and 6.4 mm (1/4 inch) for Class 400 and above.
Raised faces normally have a serrated, spiral or concentric finish, specified as a surface roughness in the range the gasket manufacturer recommends. The serrations bite into a soft or spiral wound gasket and keep it in place.
Flat face (FF)
The gasket covers the whole face of the flange, out to the bolt circle or beyond. Flat face flanges are used against cast iron, plastic or other brittle equipment, such as some pumps and valves. The full-face gasket spreads the load so the brittle flange is not bent as the bolts are tightened.
Never bolt a raised face flange to a flat face cast iron flange. The raised face acts as a fulcrum, and tightening the bolts can crack the cast iron. Where a steel flange must mate to a flat face, specify a flat face on the steel flange too, or machine the raised face off.
Ring type joint (RTJ)
A groove is machined in each flange face and a solid metal ring, oval or octagonal in section, sits in both grooves. Tightening the bolts deforms the ring into the grooves and forms a metal-to-metal seal. RTJ is used for high pressure and high temperature: many Class 600 and above lines, oil and gas wellhead equipment, and hydrogen service.
The ring must be softer than the flange groove so that the ring deforms, not the flange. Soft iron, low-carbon steel, 304 and 316 stainless rings are common, each with a maximum hardness.
Which gasket for which facing
| Facing | Usual gaskets |
|---|---|
| Raised face | Spiral wound (ASME B16.20), compressed fibre, graphite, PTFE |
| Flat face | Full-face soft gasket: rubber, compressed fibre |
| Ring type joint | Metal ring gasket, R, RX or BX type |
Other facings
Tongue and groove and male and female facings hold the gasket in a recess, which stops it blowing out. They are less common, used mainly on heat exchangers and some valves, and the two flanges are not interchangeable with each other.
What to put on the enquiry
The facing (RF, FF or RTJ), and for RTJ the ring number. If a particular surface finish is required, give it in micrometres Ra.
See flanges and spiral wound gaskets.
Frequently asked questions
What is the most common flange facing?
Raised face (RF), used with spiral wound and sheet gaskets in most process piping.
Can an RF flange be bolted to an FF flange?
It is not recommended, especially against cast iron, because bolting can crack the FF flange. The RF can be machined flat if needed.
What surface finish is used on RF flanges?
Typically a serrated finish of 125–250 µin Ra (about 3.2–6.3 µm).
Key takeaways
- RF is standard; FF for cast iron; RTJ for high pressure.
- Facing must match on both flanges.
- Protect facings from scratches during handling.
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