FLANGE FACE TYPES
The most common types of flange face finish (a term that refers to the shape of the surface area or a flange for pipelines) are the flat face (FF), the raised face (RFQ) and ring joint (RTJ). Other, less common, types are the lap joint, the male and female (M&F) and the tongue and groove flange surface area (or flange finish). In this article, we illustrate these concepts, providing pictures and explaining the key differences.
WHAT IS THE FLANGE SURFACE AREA (“FACE”)?
The frequently used term “flange face type” refers to the shape of the flange surface area (i.e., the area where the gaskets are positioned and the two mating flanges are tightened). When making a flanged connection, operators should use two flanges with the same type of face, and never mate flanges with different surface areas, such as a flat face with a raised face or a ring joint with a male and female (in this case, the joint would not seal properly and leak). The ASME B16.5 and ASME B16.47 norms mention a few different types of flange face types:
- Flat face (“FF”)
- Raised face (“RF”)
- Ring joint (“RTJ”)
- Lap joint
- Male and female (“M&F”)
- Large and small tongue-and-groove (“T&G”)
TYPES OF FLANGE FACE FINISH
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 to RF flanges according to ASME B31.1. Generally, soft gaskets are used for this type of flange surface area (flat, full face gaskets).
RAISED FACE FLANGE (RF)
Raised face flanges (RF flanges) are widely used for oil & gas pipelines and are easy to recognize as the gasket surface is positioned 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 flange pressure rating and is defined by ASME B16.5 (for pressure classes 150 and 300, the height is 1.6 mm or 1/16 inch, wheare as for classes from 400 to 2500, the raised face height is approximately 6.4 mm, or 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) are used when a metal-to-metal seal between the mating flanges is required (high-pressure, as 600# and higher, and high-temperature, from 800 F° upward, applications). RTJ faced flanges have circular grooves where the ring joint gaskets (RTJ type) is accommodated. As the two RTJ flanges are bolted together and tightened, the applied bolting force compresses and deforms the metallic gaskets inside the grooves of the flanges and creates a very tight metal-to-metal seal (obviously, for this to happen, 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 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 with an octagonal and oval section.
TONGUE & GROOVE FLANGES (T&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). T&G faced flanges are standardized in both large and small types.
MALE & FEMALE FLANGES (M&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, this is the male flange, whereas the other flange has a matching depression machined on the facing surface, and is 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.