Advantages and disadvantages of flange connection: flange connection has good strength and tightness  it is applicable to a wide range of sizes, and can be applied to both equipment and pipelines, so it is most widely used. But when the flange is connected, it can not be assembled and disassembled quickly, and the manufacturing cost is high.

1、 Flange connection structure and sealing principle

Flange connection structure is an assembly, which is composed of a pair of flanges, a number of bolts, nuts and a gasket. Therefore, the main problems to be solved in the design of flange connection are as follows

It is to prevent medium leakage.

Principle of flange sealing:Under the action of bolt preload, the flange compresses the gasket between the sealing surfaces. When the compression force on the gasket per unit area reaches a certain value, the gasket will be deformed and compacted, and the uneven parts on the sealing surface will be filled, thus forming the initial sealing condition to prevent medium leakage. The required pressing force is called the pre tightening sealing specific pressure of gasket, and the unit is MPa. The specific pressure of pre tightening seal mainly depends on the gasket material. In the working state, the axial force of internal pressure is stretched to reduce the pressing force. The gasket has enough resilience to compensate deformation, and the specific pressure value of pre tightening seal is not less than a certain value (working seal specific pressure - the minimum specific pressure that must be kept between the sealing surface and gasket to prevent medium leakage) Keep a good seal. On the contrary, if the rebound is insufficient, the specific pressure of the pre tightening seal drops below the specific pressure of the working seal, and even the gap appears again at the seal, the seal will fail.

Conditions to ensure tight connection of spiral flange

It can be seen that in order to ensure the flange sealing, it is necessary to make the actual specific pressure on the flange sealing surface not lower than that of the gasket when preloading, but higher than that of the working seal when working.

Flat welding flange——Flange welding is easy to manufacture and widely used in equipment barrel or pipeline, but its rigidity is poor.

After the flange is stressed, the rectangular section of the flange turns slightly. The tube wall or tube wall connected with the flange is bent and deformed along with the flange. Therefore, the additional bending stress will be produced on the section of the cylinder wall near the flange. So the pressure range of flat welding flange is low (PN < 4.0Mpa).

Butt welding flange——Butt welding flange is also called high neck flange or long neck flange. The existence of neck improves the rigidity of flange and reduces the bending stress at the root because the root of neck is thicker than cylinder. In addition, the connection between flange and cylinder or pipe wall is butt weld. The strength of fillet weld of butt welding flange is better than that of flat welding flange, so butt welding flange is suitable for high pressure, high temperature or large equipment diameter.

Loose flange——Flange is not directly fixed on the shell, or although it is fixed, it can not guarantee that the flange and shell as a whole bear bolt load  are classified as loose flange, such as looper flange, threaded flange and lap flange.

loose flange——Flange plate can be made of different materials from equipment or pipeline. It can be used for copper, aluminum, ceramics, graphite and non-metallic materials

On the equipment or piping of the material.

There is no additional bending stress after stress, which is only suitable for low pressure occasions. Threaded flange is widely used in high pressure pipeline, and the additional stress produced by flange on pipe wall is small. But this kind of flange stiffness is small, its thickness is thick, generally only suitable for the lower pressure vessels. The arbitrary flange is connected with the shell as a whole, and its rigidity is worse than that of the whole flange, such as the welding flange with incomplete penetration.

Type a flat welding flange——t is directly welded with the cylinder or head of the vessel. During tightening and working, the additional bending moment will act on the vessel wall

And the rigidity of the flange itself is small. Therefore, it is suitable for the range of low pressure grade and small cylinder diameter.

Flat welding flange——The type B flange has a cylindrical short joint with a wall thickness of not less than 16mm. With this short joint, the rigidity of the whole flange can be increased, and the vessel wall can avoid bearing additional bending moment. So it is suitable for larger diameter and higher pressure. It can be used in the larger diameter range of pn0.25-1.6 pressure rating and the smaller diameter range of 2.5 and 4.0 pressure rating. The suitable diameter range is 300-3000mm and the temperature range is - 20 ℃ - 350 ℃.
Long neck butt welding flange——The neck with thickened root is used to replace the short joint in the type B flat welding flange, so as to increase the overall rigidity of the flange more effectively. At the same time, the flange and equipment are connected by butt welding, so it is used in higher pressure range (PN 0.6MPa  6.4Mpa) and diameter range (dn300mm  2000mm) and applicable temperature range of - 20 ℃ - 450 ℃. The specification of type B flat welding flange with DN less than 2000mm has been included in the scope of long neck butt welding flange. The connection size and flange thickness of the two kinds of flange are exactly the same. Therefore, the type B flat welding flange below dn2000mm can be replaced by rolled long neck butt welding flange to reduce the production cost of flange.

The above describes the classification of several kinds of flange, some of which are commonly used, some are not commonly used. According to the requirements of the equipment, choose the form of flange to make the connection between the two equipment more secure.