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The flange face is the surface area that hosts the gasket. The 6 available types of flange face are flat (FF), raised (RF), ring joint (RTJ), lap joint, male and female (M&F), tongue and groove (T&G). Flanges with different faces require different gaskets and shall never be mated to prevent leakage of the joint. RF and FF flanges may have different types of “finish” (i.e. roughness on top the surface): smooth, stock and serrated.
A ring joint flanges (RTJ) is used when a metal-to-metal seal between the mating flanges is required (which is a condition for high-pressure and high-temperature applications, i.e. above 700/800 C°).
A ring joint flange features a circular groove to accommodate a ring joint gasket (oval, or rectangular).
As the two ring joint flanges are bolted together and then tightened, the applied bolting force deforms the gaskets inside the flange groove creating a very tight metal-to-metal seal. To make this happen, the material of the ring joint gasket has to be softer (more ductile) than the material of the flange.
RTJ flanges can be sealed by RTJ gaskets of different styles (R, RX, BX) and profiles (example: octagonal/oval for the R style).
The most common RTJ gasket is the R style with an octagonal section, as it ensures a very strong seal (the oval section is an older type). A “flat groove” design, however, accepts both RTJ gaskets having an octagonal or oval section.
API requires the inspection of ring groove dimensions on the flange face. To pass API requirements, these dimensions must be met. For BX Grooves, the groove’s major diameter and width are specified. For R and RX Grooves, API specifies the groove’s pitch diameter and width.
RTJ measuring kits are available for measuring RTJ flange grooves. This may be required when you need to record the existing dimensions prior to machining with a flange facer, or to check the groove dimensions after any machining operations.