The sealing performance of flanges is largely determined by the type and machining quality of sealing faces. Different sealing face configurations are designed for various pressure classes and sealing criteria. Proper selection of sealing face type is the key to achieving zero leakage for flange joints.
The Flat Face (FF) sealing face is the simplest configuration with a full flat sealing surface. Flat face flanges are generally fitted with soft gaskets covering the entire sealing face, suitable for low-pressure service. They are widely applied to cast iron flanges and Class 150 low-pressure piping systems. Featuring simple structure and easy machining, this type is especially suitable for equipment requiring corrosion-resistant linings.
The Raised Face (RF) sealing face is the most commonly adopted type. Its sealing surface protrudes above the circumferential joint plane enclosed by flange bolt holes. With simple structure, convenient machining and easy installation, RF sealing faces are available for all pressure classes ranging from Class 150 to Class 2500. Raised face flanges are normally matched with spiral wound gaskets, composite gaskets or non-metallic soft gaskets, which serve as the standard option for general process piping.
Male & Female (M&F) sealing faces are used in pairs consisting of one male flange and one female flange. The mating structure accurately positions the gasket and prevents gasket extrusion under high pressure. This configuration delivers high sealing reliability and is applicable to medium and high-pressure working conditions.
Tongue & Groove (T&G) sealing faces consist of one tongue flange and one groove flange used as a pair. The precise matching of tongue and groove provides excellent gasket positioning and sealing performance, ideal for high-pressure services or pipelines conveying hazardous media.
Ring Joint (RJ) sealing faces are manufactured by machining an annular groove on the flange surface, where a metal ring gasket is seated to form a tight seal. Ring joint flanges have a small effective compression area, and the groove structure restricts the displacement and outward turning of metal ring gaskets under high pressure. This type is suitable for high-temperature and high-pressure services, typically adopted for pressure classes from Class 600 to Class 2500.
The surface finish of sealing faces also greatly affects sealing performance. Different pressure classes and gasket types carry distinct requirements for sealing face roughness. In practical engineering, the sealing face type and machining specifications shall be comprehensively determined based on service conditions, medium properties and gasket categories.
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