Flange Facing Types: Raised Face (RF), Flat Face (FF) and Ring Type Joint (RTJ)
The facing is the sealing surface of a flange. It decides which gasket is used, how the bolt load is transferred to the gasket and how much pressure the joint can hold.
The three facings below cover almost all industrial piping. ASME B16.5 and B16.47 define them for Class 150 to 2500 flanges, and EN 1092-1 defines the European equivalents (Type A flat face, Type B raised face, plus tongue/groove, spigot/recess and O-ring variants).
Comparison
| Facing | How it seals | Gasket | Typical use |
|---|---|---|---|
| RF (raised face) | A raised ring inside the bolt circle concentrates bolt load on a smaller gasket area. | Ring gasket inside the bolts: compressed fibre, PTFE, spiral wound | Most process, oil, gas, steam and chemical lines |
| FF (flat face) | The whole face is flat. The gasket covers the full face and the bolts pass through it. | Full-face gasket: rubber (EPDM, NBR), compressed fibre | Low-pressure water lines, cast iron and ductile iron valves and pumps, plastic and GRP pipe, AWWA C207 flanges |
| RTJ (ring type joint) | A metal ring sits in a machined groove and deforms under bolt load, giving a metal-to-metal seal. | Oval or octagonal metal ring (soft iron, low carbon steel, stainless steel) | Oil and gas, high pressure (typically Class 600 and above), high temperature |
Raised face (RF)
The most common facing in industrial piping. In ASME B16.5 the raised face is 1.6 mm (1/16 in.) high for Class 150 and 300, and 6.4 mm (1/4 in.) for Class 400 and above. The face is usually given a concentric or spiral serrated finish so that soft gaskets grip it. Because the bolt load acts on a small area, RF joints seal well at moderate and high pressure.
Flat face (FF)
The flange face is flat across its whole width and a full-face gasket is used. The bolt load is spread over a larger area, which avoids bending the flange. This is why flat face flanges are required when connecting to brittle or flexible materials such as cast iron, ductile iron, plastic or GRP.
Do not bolt an RF flange to a cast iron FF flange. When the bolts are tightened, the raised face acts as a fulcrum and the outer part of the cast iron flange bends, which can crack it. Use an FF steel flange (or machine the raised face off) with a full-face gasket.
Ring type joint (RTJ)
A trapezoidal groove is machined into each flange face and a solid metal ring is compressed between them. The ring must be softer than the groove so that it deforms and not the flange. Ring numbers (for example R23 or R45) are selected from the ASME B16.5 tables for each size and class, and ring dimensions are given in ASME B16.20. RTJ flanges are common on wellhead, pipeline and refinery equipment at high pressure and temperature.
Other facings
Tongue-and-groove and male-female (spigot and recess) facings hold the gasket in place and prevent it from blowing out. They are used where gasket position must be guaranteed, but the two mating flanges are not interchangeable, so spare parts must be planned.
How to choose
- Match the facing of the equipment you connect to (valve, pump, vessel nozzle).
- Against cast iron, ductile iron, plastic or GRP, use flat face with a full-face gasket.
- For general process lines up to Class 300, raised face is the default.
- For high pressure and temperature, or where the specification requires a metal seal, use RTJ.
Flanges from Yazıcı Endüstri
Yazıcı Endüstri has manufactured flanges and pipe fittings in Istanbul since 1998 and exports to more than 37 countries. We machine RF, FF and RTJ facings to EN 1092-1 and ASME B16.5 / B16.47 on CNC lathes, in carbon and stainless steel. See also CNC machining and facing and the AWWA C207 flange classes guide.
Frequently asked questions
Can a raised face flange be bolted to a flat face flange?
Between two steel flanges it is possible but not recommended. Against a cast iron, ductile iron or plastic flat face flange it should be avoided, because the bolt load on the raised face can crack the mating flange.
How is the RTJ ring number selected?
From the ASME B16.5 dimension tables for the flange size and pressure class. Ring dimensions are defined in ASME B16.20.
Which gasket is used with a flat face flange?
A full-face gasket that covers the whole face and has holes for the bolts, typically rubber (EPDM, NBR) or compressed non-asbestos fibre.
This guide is for general information. For project decisions, follow the governing standard and the project specification.