FLANGE FACES TYPES
The term “flange facing” refers to the shape of the contact surface that seals the connection between two flanges (the area that seat the gaskets). The ASME B16.5 and ASME B16.47 norms mention different types of flange faces such as:
- flat face (FF)
- raised face (RF)
- lap joint
- male and female (M&F)
- the large and small tongue-and-groove type (T&G)
- ring joint (RTJ)
RTJ, T&G and M&F flanges should never be coupled, as the contact surfaces of these differently faces flanges would not match and create leakage of the joint. The considerations about facing types apply both to carbon steel and stainless steel flanges.
The types of flange faces are shown in the following image:
FLAT FACE FLANGE (FF)
Flat face flanges (FF) have a contact surface at the same height of the bolting line of the flange. Flat flanges should never be bolted with RF flanges according to ASME B31.1. Flat face is the most simple type of flange facing available.
RAISED FACE FLANGE (RF)
Raised face flanges (RF flanges) are widely used for oil & gas pipelines and are easy to recognize because the gasket surface is located above the bolting line of the flange. RF face flanges are compatible with a wide range of gaskets, from the flat cut types to the metallic and semi-metallic types (jacketed gaskets and spiral wound gaskets). The main goal of raised face flanges is to concentrate the contact pressure on a small area of the flange thus increasing the strength of the seal.
The height of the raised surface depends on the rating of the flange and is defined by ASME B16.5 (for pressure classes 150 and 300, the height of raised face shall be approx. 1.6 mm (1/16 inch), while, for classes from 400 to 2500, the raised face height shall be approximately 6.4 mm (1/4 inch). The most common flange face finish for ASME B16.5 RF flanges is 125 to 250 micron Ra (3 to 6 micron Ra).
RING JOINT FLANGES (RTJ)
Ring joint type flanges (RTJ) create a metal to metal seal between the mating flanges and are therefore used in high-pressure (600# and higher) and high-temperature (800 F° and higher) services. RTJ faced flanges have circular grooves that host metallic ring joint gaskets (RTJ type). As the two RTJ flanges are bolted together and tightened, the applied force compresses and deforms the metallic gaskets inside the grooves and creates a very tight metal-to-metal seal (obviously, the material of the ring joint gaskets has to be softer than the material of the flanges). 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 octagonal section, as it ensures a very strong seal (the oval section is an older type, less and less used). The common "flat groove" design, however, accepts both RTJ gaskets with octagonal and oval section.
TONGUE & GROOVE FLANGES (T&G)
Two T&G flanges shall be matched one to the other (i.e. either of them cannot be mated with flanges with a different face). The male flange has a tongue (which is a raised ring), wherease the female flange has a grrove that match the tongue perfectly. T&G facings are standardized in both large and small types.
MALE & FEMALE FLANGES (M&F)
Male and female flanges (M&F flanges) are manufactured to be matched one to the other as well. Indeed, one flange face has an area extended beyond the normal flange face, i.e. the male, while the other flange has a matching depression machined on the facing surface, i.e. the female. The female face is 3/16'' deep, while the male face is 1/4-inch high, and both of them are smooth finished. The outer diameter of the female face retains the gasket.