Sealing elements and principles of pressure vessel flanges

Pressure vessel flanges are common components of pressure vessels and are the basic components for connecting various pressure vessel components. They are widely used in petrochemical, electric power, atomic energy, light industry and other industrial fields.

76fae20460aa4ccbaacb0c1104681374 - Sealing elements and principles of pressure vessel flanges

For a container, the flange should be a component, including flanges, gaskets and attachment bolts or studs, and nuts that act to combine the different pressure components while ensuring that the joint does not leak.
The main failure mode of the flange connection is the failure of the joint stiffness and its external appearance is leakage. The medium inside the pressure vessel has the characteristics of high temperature and high pressure, and has flammable and explosive characteristics or toxicity. Once a leak occurs, a serious secondary disaster will occur, which seriously threatens the safety of the pressure vessel body and surrounding equipment and personnel. Therefore, the priority factor in the design and selection of the container flange is the overall sealing performance.
China’s pressure vessel flange standards are self-contained. The earliest pressure vessel flange standard can be traced back to the 1970s. At that time, the pressure vessel flange industry standard JB1157~1164-1973 issued by the Ministry of Machinery, the Ministry of Petroleum and the Ministry of Chemical Industry was used. After a long-term trial and continuous revision, the latest standard is JB/T4700~4707-2000 “Pressure Vessel Flange”.
In terms of the structural form and working principle of the flange, there is not much difference between the pressure vessel flange and the pipe flange. The obvious difference is that the pressure vessel flange has a large specification and is used in connection with various pressure vessels; and the pipe flange is often connected directly to the pipe or connected to the inlet and outlet pipes of pipelines such as pumps, valves, and machines.

Pressure vessel flange working principle:

When a pre-tightening force is applied to the flange bolt, the bolt force compresses the gasket through the flange ring, forcing the flange gasket to undergo compression deformation. When the tamping force reaches a certain value, the four convex surfaces on the flange sealing surface and the gaskets are forced to be filled by the deformation of the gasket, which creates an initial sealing condition for preventing leakage of the fluid medium. At this time, the pressing force on the effective sealing area of the gasket unit is referred to as the initial sealing specific pressure of the gasket. When the equipment or pipeline is subjected to the medium pressure, the bolt is stretched by the tensile stress, the flange sealing surface moves in the direction of separation from each other, the pressing force between the sealing surface and the gasket decreases, and the compression of the gasket decreases. The pre-tightening seal drops in specific pressure. If the gasket has sufficient resilience at this time, the deformation of the gasket can compensate the separation value of the bolt and the sealing surface, and the flange of the pre-tightening seal is only not less than the working sealing pressure, and the flange remains good. Sealed state. Conversely, if the rebounding ability of the gasket is insufficient, the sealing specific pressure drops below the working sealing specific pressure, and even if the sealing port reappears, the sealing fails.

Sealing form of pressure vessel flange:

  • (1) Forced sealing. The sealing surface is pressed by a fastening method such as fastening the end cap to the flange of the cylinder flange to achieve the purpose of sealing, such as a flat gasket seal, a kappar seal, and the like.
  • (2) Self-tightening seal. The pressure of the medium in the container is used to cause a pressing force on the sealing surface to achieve the sealing purpose. Its sealing force increases as the pressure of the medium increases. Therefore, it can maintain reliable sealing performance under high pressure, such as combined seal, O-ring seal, C-ring seal, B-ring seal, wedge seal, octagonal pad and oval pad seal, flat pad self-tightening seal , Wood seal, nitrogen seal, etc.
  • (3) Semi-self-tightening seal. The sealing surface is pressed to achieve the purpose of sealing by using the pressure of the medium in the container and the connection of the fasteners, such as a double cone seal.

The working principle of the pressure vessel flange is:

When a pre-tightening force is applied to the flange bolt, the bolt force compresses the gasket through the flange ring, forcing the flange gasket to undergo compression deformation. When the tamping force reaches a certain value, the four convex surfaces on the flange sealing surface and the gasket are forced to be filled by the deformation of the gasket, which creates an initial sealing condition for preventing leakage of the fluid medium.
At this time, the pressing force on the effective sealing area of the gasket unit is referred to as the initial sealing specific pressure of the gasket. When the equipment or pipeline is subjected to the medium pressure, the bolt is stretched by the tensile stress, the flange sealing surface moves in the direction of separation from each other, the pressing force between the sealing surface and the gasket decreases, and the compression of the gasket decreases. The pre-tightening seal drops in specific pressure.
If the gasket has sufficient resilience at this time, the deformation of the gasket can compensate the separation value of the bolt and the sealing surface, and the flange of the pre-tightening seal is only not less than the working sealing pressure, and the flange remains good. Sealed state.
Conversely, if the rebounding ability of the gasket is insufficient, the sealing specific pressure drops below the working sealing specific pressure, and even if the sealing port reappears, the sealing fails.

Classification of pressure vessel flanges

The flat welding flange used on the pressure vessel is divided into a flat welding flange and a flat welding flange.

  1. A flat welding flange is directly welded to the container, and the size of the unwelded joint is small or not welded, and the flange and the container cannot be simultaneously stressed. The strength calculation refers to the loop flange.
  2. B-type flat welding flange is generally attached with a thick short cylinder. The welding quality of the flange and the short cylinder is reliable, and the flange and the container can be stressed at the same time. The strength calculation can be considered according to the overall flange. Type B flat welded flanges are widely used for the connection of medium and low pressure vessels and pipes

Advantages and Disadvantages of Both: The B-type flange has a short cylinder, so the rigidity is better than that of the A-type flange. It can be used for high pressure and large diameter; the type A is V-shaped groove, the type B is U-shaped. The groove is so easy to penetrate, so the strength and rigidity are higher.

The factors affecting the pressure vessel flange seal are as follows:

1. Preload of bolts

Appropriate pre-tightening force can ensure that the gasket can retain a certain sealing specific pressure when working. If the pre-tightening force is too large, the gasket will be crushed or extruded, thereby destroying the seal.
The distribution of the pre-tightening force on the gasket also affects the sealing performance, and the method of ensuring uniform distribution of the pre-tightening force is to increase the number of bolts in the case of satisfying the space required for fastening and dismounting the bolt.

2. Gasket sealing performance

The gasket is an important component that constitutes the seal. Suitable gasket materials should reflect that the gasket can produce the necessary elastic deformation without being crushed or extruded under the proper pre-tightening force.
When working, the distance between the flange sealing faces is enlarged, and the gasket material should have sufficient resilience to make the gasket surface in close contact with the flange surface to maintain good sealing performance.

3. The sealing surface features of the flange

The type and surface properties of the flange sealing surface play a crucial role in the effect of the sealing effect. If the working conditions are more stringent, the concave and convex surfaces and the groove surface should be used.
The flatness of the flange sealing surface, the sealing surface and the centerline and perpendicularity of the flange directly affect the uniformity of the force of the gasket and the good contact between the gasket and the flange.
The surface roughness of the flange sealing surface should be compatible with the requirements of the gasket. No radial tool marks or scratches are allowed on the surface, and surface cracks are not allowed.

4. The stiffness of the flange

Insufficient stiffness can cause excessive warpage of the flange, resulting in seal failure. There are many factors affecting the stiffness of the flange. The method of increasing the thickness of the flange and increasing the outer diameter of the flange can improve the rigidity of the flange, reduce the deformation, and evenly transmit the bolt force to the gasket to obtain a uniform and sufficient sealing pressure. At the same time, the sealing property can be improved. The force arm that reduces the force of the bolt can reduce the bending moment of the flange and is beneficial to the sealing.

5. Operating conditions

Operating temperature, pressure and chemical and physical properties of the media also affect the reliability of the flange connection. The temperature increases, the viscosity of the medium becomes smaller, and the tendency of leakage increases; the high temperature makes the chemical and physical activity of the medium increase, causing corrosion and dissolution of the gasket and the flange; the high temperature can also cause the plug, the flange and the gasket to occur. Creep and stress relaxation.

Source: China Pressure Vessel Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com
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References:

  • https://www.yaang.com

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