What are some different types of flange faces?

Raised Face (RF)
RF is the most common flange face in industrial applications. The sealing surface consists of a raised area inside the bolt circle. The raised area concentrates the sealing force provided by the bolting. The force concentration helps provide a better seal than flat face flanges. Raised face flanges can use a wide variety of gasket designs. Flat ring sheet gaskets are common for class 150, and spiral wound gaskets are common for all pressure classes.

Flat Face (FF)
FF flanges are typically used in lower-pressure class applications and are common with class 125 cast iron flanges. RF flanges should not be used in combination with a cast iron FF flange because damage to the cast iron flange may occur. Full-face gaskets should be used.

Ring Type Joint (RTJ)
RTJ flanges are typically used in higher-pressure applications (class 600 and up). A ring groove is machined into the flange’s raised face. The seal is achieved by compressing and metal ring (AKA RTJ gasket) in the groove forming a metal-to-metal seal. The RTJ gaskets can come in four styles/profiles. (Oval, Octagonal, RX, BX) The size of the ring required will be stamped on the side of the flange and will be in the R-XX format.