The flange face, i.e. the metallic surface area that hosts the gasket and creates the seal, may be flat (flat face flange, “FF”), raised (raised face flange, “RF”), grooved (ring joint, male and female, tongue & groove flanges), or lap joint. Flanges featuring different faces shall never be mated, example a flat face and a raised face flange, to prevent leakage of the joint.
TYPES OF FLANGE FACE
The ASME B16.5 and ASME B16.47 norms mention a few different types of flange faces:
- Flat face flange (FF)
- Raised face flange (RF)
- Ring joint flange (RTJ)
- Lap joint flange
- Male and female flange (M&F)
- Large and small tongue-and-groove flange (T&G)
RAISED FACE FLANGE (RF)
A raised face flange (RF) is easy to recognize as the gasket surface area is positioned above the bolting line of the flange. A raised face flange is compatible with a wide range of gaskets, ranging from flat to semi-metallic and metallic types (as, for example, jacketed gaskets and spiral wound gaskets), either ring or full face.
The main scope of a raised face flange design is to concentrate the pressure of the two mating flanges on a small surface and increase the strength of the seal.
The height of the raised face depends on the flange pressure rating as defined by the ASME B16.5 specification (for pressure classes 150 and 300, the height is 1.6 mm or 1/16 inch, for classes from 400 to 2500, the raised face height is approximately 6.4 mm, or 1/4 inch).
The most common flange finish for ASME B16.5 RF flanges is 125 to 250 micron Ra (3 to 6 micron Ra). The raised face is, according to ASME B16.5, the default flange face finish for manufacturers (this means that buyer shall specify in the order if another flange face is required, as flat face or ring joint).
Raised face flanges are the most sold type of flange, at least for petrochemical applications.
FLAT FACE FLANGE (FF)
Flat face flanges (FF) have a contact surface having the same height of the bolting line of the flange. Full face gaskets, generally of the soft type, are used between two flat face flanges.
According to ASME B31.3, a flat face flange should never be mated with a raised face flange as the resulting flanged joint would definitely leak.
RING JOINT FLANGE (RTJ)
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.
LAP JOINT FLANGE
A lap joint flange has a flat face, which is not used to seal the flanged joint but simply hosts the back of a stub end. The sealing surface is actually on the stub end itself and may be either flat face or raised face.
TONGUE AND GROOVE FLANGE (T AND G)
Two tongue and groove flanges (T&G face) perfectly fit one into the other: one flange has a raised ring, the other a groove and they can be mated easily (the tongue enters the groove and seals the joint). Tongue and groove flanges are standardized in both large and small types.
MALE AND FEMALE FLANGE (M AND F)
Similarly to tongue and groove flanges, male and female flanges (M&F face type) match one to the other as well.
One flange has an area extended beyond its face area, the male flange, the other flange has a matching depression machined on the facing surface, the female flange.
The female face is 3/16” deep, while the male face is 1/4″ high, and both of them are smooth finished.
The outer diameter of the female face retains the gasket.