SOME FACTORS AFFECTING FLANGE SEALING

Flange seal is commonly used in chemical containers and pipelines. The utility model has the advantages of simple structure, low cost, convenient installation and maintenance, etc. However, the problems of leaking of flange sealing face have long plagued the production of factories. There are numerous factors affecting the flange seal. From the analysis of the sealing structure and its environment, there are mainly the following aspects.

Bolt Preload

Bolt preload is an important factor affecting the seal. The preload must compress the gasket and achieve initial sealing conditions. At the same time, the preload should not be too large, otherwise the gasket will be crushed or extruded.

As the preloading force is transmitted to the gasket through the flange sealing surface, in order to achieve a good seal, the preloading force must be uniformly applied to the gasket. Therefore, when the preloading force required for the sealing is constant, it is effective to seal by increasing the number of bolts and reducing the diameter of the bolt.

Sealing Surface

The sealing performance of the flange connection is a direct result of the sealing surface type, so the shape of the sealing surface should be properly selected. The choice of flange sealing surface type mainly considers pressure, temperature and medium.

(1)RF

The surface of the threaded flange is smooth and has a dense waterline inside. The surfaces of the slip on flange and plate flange are also smooth. Lap joint flange features a flat face and is always used in conjunction with a stub end. This kind of sealing surface is simple in structure, convenience in processing, and easy to carry out anti-corrosion lining. However, the contact area of the sealing surface gasket is large, and the gasket is easy to be squeezed on both sides when pre-tightening, and it is not easy to be compressed.

(2)MFM

The sealing surface is composed of a convex surface and a concave surface. The gasket is placed on the concave surface to prevent the gasket from being extruded, so it can be implemented to a high pressure.

(3)TG

The sealing surface is composed of a tenon and a groove. The gasket is positioned in the groove and will not be squeezed.  When the pressure is not large, even if the diameter is larger, it can be well sealed. Even for higher pressure, the bolt size is not too large. Therefore, it is easier to obtain a satisfactory sealing effect than the above two sealing faces. The disadvantage of the sealing surface is that the structure and manufacture are complicated, and it is difficult to replace the gasket squeezed in the groove. In addition, the tenon part is easily damaged and should be taken care of during disassembly and transportation. The TG sealing surface is suitable for flammable, explosive, toxic media and high pressure applications.

(4)FF & RJ

The FF seal is suitable for occasions with low pressure (PN≤1.6MPa). The RJ surface is mainly used on the long weld neck flange and the blind flange. The applicable pressure range is 6.3MPa≤PN≤25.0MPa.

(5)Other Types of Sealing Surfaces

For the sealing of high pressure vessels and high pressure pipelines, the sealing surfaces may be tapered or trapezoidal groove sealing surfaces, which are respectively matched with spherical metal gaskets and metal gaskets with elliptical or octagonal sections. Sealing surface can be applied to high pressure occasions, but it needs high dimensional accuracy and high surface finish, and is not easy to process.

Gasket Performance

Gasket is an important component of seal. Proper deformation and resilience of gasket are the necessary conditions for forming seal. The most commonly used gaskets can be divided into non-metallic gasket, metallic, metallic and non-metallic mixed gaskets.

(1)Non-metallic gasket materials include rubber asbestos board, polytetrafluoroethylene and so on. The advantages of these materials are softness. The performance of temperature and pressure resistance is worse than that of metal gaskets. Usually it is only suitable for flange sealing of normal, medium and low pressure equipment and pipelines.

(2)Metal and non-metal mixed gaskets include metal-clad gaskets and wound gaskets. Metal-clad gaskets are made by wrapping non-metals in the thin metal sheets.The metal-wound gasket is a thin low-carbon steel belt that is wound with an asbestos tape. This type of wound gasket has two types: one without positioning ring and the other with positioning ring.  Metal-clad gaskets and wound gaskets have better performance than simple metal gaskets, and suitable in the higher temperature and pressure range.

(3)Metal gasket materials generally do not require high strength, but require flexibility. Commonly used are soft aluminium, copper, iron, Monel alloy steel, etc. Metal gaskets are mainly used for socket weld flanges at medium or high temperatures or medium and high pressure.

The choice of gasket material should be determined according to the temperature, pressure and corrosion of the medium, and also consider the form of the sealing surface, the size of the bolt face and the loading and unloading requirements.

Finding out the reasons that affect the flange sealing is of positive significance to control the leakage of the flange sealing point and increase the economic benefits of the factory.