Flange Dimensions / Reference Tools
Alternate Flange Facings
RF, FF, RTJ, Tongue and Groove, and Male and Female facing types
The Raised Face (RF) is the most common flange facing in general industrial piping. The raised portion concentrates gasket contact pressure on a smaller area, improving the seal. For Class 150 and 300, the raised face height is 1/16 inch; for Class 400 through 2500, it is 1/4 inch. RF flanges use soft gaskets such as spiral wound, compressed fiber, or PTFE. The Flat Face (FF) facing has no raised portion and the gasket covers the full flange face. FF flanges are required when mating with cast iron flanges (ASME B16.1) to prevent bending stress that could crack the brittle casting. FF flanges typically use full-face gaskets made of elastomer or compressed fiber sheet.
Ring Type Joint (RTJ) flanges feature a precision-machined groove in the flange face that accepts a solid metal ring gasket. The ring is slightly softer than the flange material and deforms under bolt load to create a metal-to-metal seal. RTJ facings are used in high-pressure and high-temperature applications (typically Class 600 and above, though they are available in all classes). The three common ring styles are R (oval and octagonal cross-section), RX (pressure-energized design for higher pressures), and BX (designed specifically for API 6BX flanges at 5,000 psi and above). RTJ flanges provide the most reliable seal in critical service but require careful handling to prevent damage to the groove surfaces.
Tongue and Groove (T&G) and Male and Female (M&F) facings are specialty facing types that provide self-aligning gasket retention. In a T&G joint, one flange has a raised ring (tongue) and the mating flange has a matching groove; the gasket sits in the groove and is confined by the tongue. This prevents gasket blowout under pressure surges. M&F facings work on the same principle but with a larger recess (female) and protrusion (male). Both types are used in applications where gasket retention is critical, such as lethal service or vacuum systems. The main disadvantage is that flanges must be separated axially to replace the gasket, which may not be possible in some piping configurations without cutting the line.
