- Raised Face (RF) - Refers to a raised portion on the flange facing that provides a sealing surface. When two flanges are bolted together, the raised face on one flange comes into contact with the flat face of the other flange. This design helps create a better seal between the flanges, improving the connection's leak resistance. The raised face typically has a specific height, which can vary based on industry standards and the flange's size and pressure rating.
- Smooth Finish (SF) - The surface of the raised face is machined or finished to be smooth and flat. The smooth finish ensures proper contact and sealing between the flanges when they are bolted together. A smooth finish is important for preventing leaks and maintaining the integrity of the joint under pressure.
Several years ago, the standard finish on raised face flanges was around 500 microinch roughness. Some users and licensers desired a smoother flange finish for services like hydrogen to get a better flange seal when used with spiral wound gaskets. The term RFSF was used to differentiate these smooth face flanges from those with the rougher standard finish. When an RFSF flange is specified, the required finish should also be defined, for example 63-125Ra or 125-250Ra.
In recent editions of ASME B16.5, the standard finish is defined as 125-250 Microinch average. Because the standard finish is now much smoother than before, the RFSF requirements have pretty much been abandoned. You'll find that the smooth finish requirement for flanges and valves in most old piping specifications are equivalent or close to the standard finish for flanges today.
Flanges with a raised face and smooth finish are used in applications where a reliable and leak resistant connection is required. These flanges are commonly used in industries such as oil and gas, petrochemicals, chemical processing, and power generation. The specific dimensions and design of the raised face and the smooth finish can vary based on industry standards, the type of fluid being transported, and the operational conditions of the piping system. Proper installation, including using the correct gasket material, bolting procedures, and torque specifications, is crucial to ensure a secure and leak free connection.