The flange face is the surface area that hosts the gasket. The 6 available types of flange face are flat (FF), raised (RF), ring joint (RTJ), lap joint, male and female (M&F), tongue and groove (T&G). Flanges with different faces require different gaskets and shall never be mated to prevent leakage of the joint. RF and FF flanges may have different types of “finish” (i.e. roughness on top the surface): smooth, stock and serrated. See: Flange Face Finish Type
FLANGE FACE TYPES
The ASME B16.5 and ASME B16.47 norms mention a few different types of flange faces:
- Flat face flange (FF)
- Raised face flange (RF)
- Ring joint flange (RTJ)
- Lap joint flange
- Large and small Male and female flange (M&F)
- Large and small tongue-and-groove flange (T&G)
RAISED FACE FLANGE (RF)
A raised face flange (RF) is easy to recognize as the gasket surface area is positioned above the bolting line of the flange.
A raised face flange is compatible with a wide range of flange gaskets, ranging from flat to semi-metallic and metallic types (as, for example, jacketed gaskets and spiral wound gaskets), either ring or full face.
The main scope of a raised face flange design is to concentrate the pressure of the two mating flanges on a small surface and increase the strength of the seal.
The height of the raised face depends on the flange pressure rating as defined by the ASME B16.5 specification (for pressure classes 150 and 300, the height is 1.6 mm or 1/16 inch, for classes from 400 to 2500, the raised face height is approximately 6.4 mm, or 1/4 inch).
The most common flange finish for ASME B16.5 RF flanges is 125 to 250 micron Ra (3 to 6 micron Ra). The raised face is, according to ASME B16.5, the default flange face finish for manufacturers (this means that buyer shall specify in the order if another flange face is required, as flat face or ring joint).
Raised face flanges are the most sold type of flange, at least for petrochemical applications.