There are five types of flange faces commonly found. The surface finish of the faces are specified in the flange standards.
Raised Face (RF). The raised face is the most common facing employed with bronze, ductile iron, and steel flanges. The RF is ¹⁄₁₆-in high for Class 150 and Class 300 flanges and ¹⁄₄-in high for all pressure classes, higher than Class 300. The facing on a RF flange has a concentric or phonographic groove with a controlled surface finish. Sealing is achieved by compressing a flat, soft, or semi metallic gasket between mating flanges in contact with the raised face portion of the flange.
Ring-Type Joint (RTJ). This type is typically used in the most severe duties, for example, in high-pressure-gas pipe work. Ring-type metal gaskets must be used on this type of flange facing.
RTJ for API 6A Type 6B, BS 1560 and ASME B16.5 Flanges
The seal is made by plastic deformation of the RTJ gasket into the groove in the flange, resulting in intimate metal-to-metal contact between the gasket and the flange groove. The faces of the two opposing flange faces do not come into contact because a gap is maintained by the presence of the gasket. Such RTJ flanges will normally have raised faces, but flat faces may also be used or specified.
RTJ for API 6A Type 6BX Flanges
API 6A Type 6BX flanges have raised faces. These flanges incorporate special metal ring joint gaskets. The pitch diameter of the ring is slightly greater than the pitch diameter of the flange groove. This factor preloads the gasket and creates a pressure-energized seal. A Type 6BX flange joint that does not achieve face-to-face contact will not seal and, therefore, must not be put into service.
Flat Face (FF). Flat-face flanges are a variant of raised face flanges. Sealing is achieved by compression of a flat nonmetallic gasket (very rarely a flat metallic gasket) between the grooved surfaces of the mating FF flanges. The gasket fits over the entire face of the flange. FF flanges are normally used on the least arduous of duties, such as low pressure water piping having Class 125 and Class 250 flanges and flanged valves and fittings. In this case the large gasket contact area spreads the flange loading and reduces flange stresses. Note: Both ASME B16.5 and BS 1560 specify flat face flanges and raised face flanges as well as RTJ flanges. API 6A is specific to RTJ flanges only.
Male and Female Facings. The female face is ³⁄₁₆-in deep, the male face is ¹⁄₄-in high, and both are smooth finished. The outer diameter of the female face acts to locate and retain the gasket. Custom male and female facings are commonly found on the heat exchanger shell to channel and cover flanges.
Tongue-and-Groove Facings. Tongue-and-groove facings are standardized in both large and small types. They differ from male-and-female in that the inside diameters of the tongue-and-groove do not extend into the flange base, thus retaining the gasket on its inner and outer diameter. These are commonly found on pump covers and valve bonnets.