Raised Face Flange (RF)
The Raised Face flange is the most common type used in process plant applications, and is easily to identify. It is referred to as a raised face because the gasket surfaces are raised above the bolting circle face. This face type allows the use of a wide combination of gasket designs, including flat ring sheet types and metallic composites such as spiral wound and double jacketed types. The purpose of a RF flange is to concentrate more pressure on a smaller gasket area and thereby increase the pressure containment capability of the joint. For 150# and 300# flanges, the raised face is of 1.6 mm (1/16 inch) and is included in the thickness specified. For higher rating, the flange thickness does not include the raised face thickness. The raised face thickness for higher rating is 6.4 mm (1/4 inch). The typical flange face finish for ASME B16.5 RF flanges is 125 to 250 ?in Ra (3 to 6 ?m Ra).
Flat Face Flange (FF)
The Flat Face flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting. Flat face flanges are never to be bolted to a raised face flange. ASME B31.1 says that when connecting flat face cast iron flanges to carbon steel flanges, the raised face on the carbon steel flange must be removed, and that a full face gasket is required. This is to keep the thin, brittle cast iron flange from being sprung into the gap caused by the raised face of the carbon steel flange.
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