Flange facings explained: RF, FF and RTJ, and the gaskets that match

A flange joint is only as good as the match between the flange faces and the gasket between them. ASME B16.5 defines several facing types, and each one expects a particular style of gasket. Getting the combination right is cheaper than fixing a leaking joint.

The three common facings

Raised face (RF)

The most common facing on steel flanges. A raised ring around the bore concentrates bolt load on a small gasket area, which gives a higher sealing pressure for the same bolting. The raised face carries a machined serrated finish that grips the gasket. RF is the standard on most process piping, valves and pressure vessels in Class 150 and above.

Flat face (FF)

The entire face is flat, with no raised area. FF is used where the mating equipment has a flat face, for example cast iron valves and pump casings, because bolting a raised face flange against a brittle cast flange can bend it and crack it. The gasket in FF joints is a full-face gasket that extends out to the bolts.

Ring type joint (RTJ)

The flange face carries a machined groove, and a metal ring, oval or octagonal, sits in it. As the bolts are tightened, the ring deforms slightly and seals metal to metal. RTJ is used on high pressure and high temperature joints, typically Class 600 and above in oil and gas, where soft gaskets would blow out or creep.

Matching the gasket

Gasket material must also suit the fluid and temperature. Compressed fibre sheet is common for water and low-duty service, graphite for steam and high temperature, PTFE for aggressive chemicals, and metal windings or kammprofile for demanding duties. Make sure the gasket facing and winding material are compatible with the flange material, especially in stainless steel and duplex systems.

Mixing facings: what to avoid

Pressure classes and facing height

ASME B16.5 flanges are made in Classes 150, 300, 400, 600, 900, 1500 and 2500, from NPS 1/2 to NPS 24. The raised face height is small, and for Class 150 and 300 it is included in the flange thickness, while for higher classes it is added on top. For sizes above NPS 24, ASME B16.47 Series A and Series B cover large diameter flanges. A dimensional drawing from the supplier confirms the exact figures for your size and class.

Checklist before ordering

• State the facing on every line: RF, FF or RTJ
• For RTJ, state the ring groove number and the ring type
• Match the gasket to the flange class, facing and fluid
• Specify bolting grade, such as B7 studs with 2H nuts for carbon steel flanges
• Check the mating equipment flange facing before finalising the order

Surface finish matters

The serrated finish on the gasket seating area is not cosmetic. It bites into the gasket and prevents blow-out. Do not polish or machine the face smooth, and protect it from scratches that run radially across the seating area, because those create leak paths.

Looking for RF, FF or RTJ flanges and the right gaskets to go with them? Tell us the size, class and fluid and we will quote the complete set.