Sealing surface types and selection of pipe flanges

Flanges, a key component of pipe fittings, have various types and different sealing surface forms. In engineering practice, understanding and mastering these different types of flanges and their sealing surface forms is crucial to ensure the safe and efficient operation of pipeline systems.

Pipe flange sealing surfaces basically include seven types: full-face FF, raised face RF, concave face FM, convex face M, tenon face T, groove face G, and ring joint face RTJ (RJ). The types selected vary depending on the working conditions, media, pressure, specifications, temperature, etc. Let's get to know the types of sealing surfaces of these commonly used flange fittings together!

1. FF - Full Face Sealing Surface

The surface of the full face sealing surface (Flat Face Full Face, FF) is completely flat and is a smooth plane. It is suitable for occasions with low pressure and non-toxic media. It is usually used for connections with low requirements such as water pipes.

Pipe Flange Flat Sealing Face

2. RF - Raised Face Sealing Surface
Raised face sealing surface (RF) is the most widespread and most commonly used type of flange sealing surface, mainly used for narrow face flanges. International standards, European systems and domestic standards are all fixed heights, but in American standard flanges, it should be noted that the height of high pressure will increase the height of the sealing surface, and there are also many types of gaskets.

Gaskets suitable for raised face sealing flanges include: various non-metallic flat gaskets, coated gaskets; metal gaskets; spiral wound gaskets (including those with outer rings or inner and outer rings), etc.

Pipe Flange Sealing Raised Face

ASTM Low and High Pressure Flange

Narrow face flange, that is, the contact surface of the gasket is located within the surrounding area surrounded by the long bolt holes of the flange. During installation, the gasket position can be fixed in the center of the flange surface with the help of bolts. Wide face flange, the contact area between the flange and the gasket is located on both sides of the center net of the flange studs.

3. M/FM - Convex and Concave Sealing Surface

Concave and concave sealing surface, including Male Face (M) and Female Face (FM) designs, they are used in conjunction with each other, easy to align during installation, not only simplifying the installation and centering process, but also effectively preventing the gasket from being squeezed out under high pressure. This design is particularly suitable for high-pressure working conditions, and its gasket selection is also flexible and diverse.

Pipe Flange Sealing Convex and Concave Face
 

The sealing gaskets suitable for the concave and convex sealing surface include: various non-metallic flat gaskets, coated gaskets; metal coated gaskets; basic type or inner ring type winding seals, etc.

4. T/G - Tongue and Groove Sealing Surface

Tongue and groove sealing surface consists of two parts: Tongue Face (T) and Groove Face (G), and also adopts a male and female pairing design. This design achieves excellent sealing effect through the precise matching of the groove and the tenon. The gasket is properly placed in the annular groove. Restricted by the metal walls on both sides, it can be prevented from being compressed and deformed and squeezed into the pipeline, thereby effectively avoiding direct erosion of the medium.

Pipe Flange Sealing Tongue and Groove Face

Since the gasket does not directly contact the fluid medium in the pipe, it is less susceptible to erosion or corrosion by the fluid medium, so it can be used in high pressure, flammable and explosive, toxic media and other occasions with strict sealing requirements. However, it is worth noting that its processing difficulty is relatively high. The sealing surface is easy to damage when replacing the gasket, and the flange must be separated axially during installation and removal. Therefore, it is necessary to fully consider and reserve space for axial separation when designing the pipeline.

The gaskets suitable for tongue-and-groove sealing surfaces include: various metal and non-metallic flat gaskets, metal-clad gaskets and basic spiral wound gaskets.

5. RTJ (RJ) - Ring Joint Sealing Surface

Ring joint sealing surface, also known as Ring Joint Face, referred to as RTJ or RJ, is famous for its unique trapezoidal groove design and is specially designed for cooperation with metal ring gaskets. The flange sealed by the ring joint face is also a narrow face flange, and an annular trapezoidal groove is opened on the raised surface of the flange as the flange sealing surface. Like the tongue-and-groove face flange, this type of flange must be separated axially during installation and removal. Therefore, the possibility of separating the flange axially should be considered when designing the pipeline.

Pipe Flange Sealing Ring Joint Face

This sealing surface is specially used to cooperate with solid metal gaskets processed into octagonal or elliptical shapes of metal materials to achieve a sealed connection. Since metal ring gaskets can be based on the inherent characteristics of various metals, this type of sealing surface has good sealing performance and relatively loose installation requirements. It is suitable for use under high temperature and high pressure conditions, but at the same time, its processing accuracy requirements are also quite strict to ensure the perfect fit of the sealing surface.

Optimization strategy for the matching of pipe flange sealing surface and gasket:

To ensure that the gasket can play an excellent sealing effect, the quality of the pipe flange surface must be strictly controlled. Any surface defects, such as nicks, grooves or scratches, may damage the integrity of the seal. At the same time, the finish of the flange surface, measured by roughness, texture and waviness, must match the characteristics of the gasket.

In terms of roughness, it should be controlled within an appropriate range, neither too rough to cause the gasket to fail to form an effective seal, nor too smooth to lack the necessary friction. The texture design must be consistent with the center line of the gasket to avoid forming a direct leakage path. In special situations, such as the use of glass-lined equipment or warped flanges, the influence of waviness needs special attention, and corresponding measures should be taken, such as selecting highly compressed gaskets or enhancing the stiffness of the flange.

Read more: Pipe flange size specifications and classification or Inspection standards for pipe flanges