Flange Facings (RF, FF, RTJ)

What it is

A flange facing is the part of the flange the gasket sits on. ASME B16.5 covers the common facings for steel pipe flanges, and ASME B16.20 covers the metal gaskets used with them.

• Raised face (RF): a raised ring around the bore. The gasket sits inside the bolt circle on the raised ring. This is the most common facing on steel flanges.
• Flat face (FF): the gasket surface is flush with the rest of the flange face. A full-face gasket covers the whole face, with holes for the bolts.
• Ring-type joint (RTJ): a machined groove holds a solid metal ring gasket. Used for high pressure and high temperature service.

Other facings in B16.5 include tongue and groove and male and female, which come in matched pairs.

How to identify

Lay a straightedge across the face. If it rocks on a raised ring inside the bolts, it is RF. If it sits flat across the whole face, it is FF. If there is a machined groove with no raised ring, or a groove in a raised ring, it is RTJ. The facing is usually stamped on the flange rim, such as "RF" or "RTJ", along with class and size.

Facings compared

Surface finish

RF facings are usually machined with a serrated finish, concentric or spiral, so the gasket can grip. B16.5 gives the finish range for raised faces, commonly stated as 125 to 250 microinches average roughness. RTJ grooves need a smoother finish. Gasket makers may call for a different finish for some gasket types.

Key points

• Bolting an RF flange to an FF cast iron flange can crack the cast iron. Machine the RF off the steel flange or use the method the spec calls for.
• Raised face height is not counted in some length dimensions. Check B16.5 for how face-to-face and hub lengths are measured.
• RTJ ring numbers (R-, RX-, BX-) must match the groove. See ASME B16.20 and API 6A tables.
• See the spiral wound gasket color code chart and the flange class identification chart.

Common mistakes

• Pairing RF and FF flanges on cast iron equipment.
• Reusing an RTJ ring. Rings are made to deform once.
• Scratching radial marks across a serrated face.
• Ordering a gasket by size and class without the facing.

Governing standards: ASME B16.5 (pipe flanges and flanged fittings); ASME B16.20 (metallic gaskets); ASME B16.47 (large diameter flanges); API 6A (RX and BX rings).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.