Briefly describe the sealing principles of common electric valves, and further grasp the valve sealing conditions. Several common phenomena of electric valve seal failure, analyze the reasons and give solutions. The analysis of leakage accidents of electric valve sealing highlights the importance of valve sealing to the safe production of the device and calls for more attention to the sealing problem of electric valves. In the piping design and construction process, more appropriate suggestions for valve selection are given. Various electric valves have many uses, and the most direct consequence of unreasonable selection is leakage. The resulting safety production accidents such as environmental pollution, material loss, personal injury, fires, explosions, and even equipment shutdowns will be irreversible. How to correctly design, select, install, and maintain valves is an urgent task in the chemical industry. The rapid development of industrial technology has pushed the sealing structure technology and material requirements of valves to a whole new level. Valves with strong wear resistance, high sealing performance and strong corrosion resistance, which are suitable for high and low temperature and high pressure, are springing up. This article introduces the sealing conditions of the following common typical electric valves. 1 Sealing conditions of common valves 1.1 The sealing conditions of electric ball valves are suitable for various sealing forms under different working conditions. 1.1.1 The soft seal is located upstream and downstream of the valve body of the floating ball valve. A special embedding machine is used to embed the valve seat as completely symmetrically as possible into the valve seat, and the valve seat provides the initial pre-tightening force by its own elasticity. This structure has the advantages of reliable sealing and easy operation. The sealing material is generally polyphthalamide fiber (NYLON), reinforced polytetrafluoroethylene (RPTFE), polyether ether ketone (PEEK), p-polyphenol (PPL), polytetrafluoroethylene (PTFE), polyether ether ketone ( PEEK) and so on. 1.1.2 Metal hard seal is provided on the upstream side of the valve with a valve seat assembly composed of valve seat, valve seat ring and backing ring, which is surfacing on the seal with cemented carbide, and the backing ring fixes the seat ring on the valve seat Inside, the valve seat and the valve body are sealed by a 0-ring, the purpose is to provide the necessary initial sealing pre-tightening force for the valve. The valve body downstream of the electric valve is directly welded with cemented carbide. The cemented carbide port is not only to prevent the valve seat from damaging the valve body, but also to prevent the viscous material containing solid particles from damaging the valve. . The material of cemented carbide is generally satellite. The downstream valve seat mainly plays a sealing role. Such valves are directional in installation. The metal hard-sealed ball valve is suitable for high temperature, high pressure, viscous and unclean slurry and slurry. 1.1.3 Metal composite seal Metal composite seal is a combination of metal seal and soft seal. The metal seal is the first level seal, most of the medium is cut off and the particulate impurities are prevented from entering the second level seal. The secondary seal is a soft seal, which completely blocks the medium. Compared with the metal seal of the electric ball valve, the metal composite seal requires a valve seat assembly on the upstream and downstream of the valve. At this time, the valve is bidirectional and there is no installation direction requirement. Metal composite seal inherits the respective advantages of metal seal and soft seal. Reliable sealing, erosion resistance and abrasion resistance, convenient operation and long service life. 1.2 The sealing condition of the electric gate valve
1.2.1 The sealing form of the electric gate valve is used when the static pressure of the medium on the gate plate at the outlet end of the medium is greater than the sealing force between the valve plate and the valve seat, this sealing form is automatic sealing. When the static pressure of the medium is less than the sealing force, forced sealing is required. The valve stem rotates, and the gate at the outlet end of the medium is forced to seal with the sealing surface of the valve seat driven by the axial force of the valve stem. Single and compulsory sealing means that there is a seal between the gate valve at the medium outlet end and the sealing surface of the valve seat, while the valve plate at the medium inlet end is sealed with the valve seat and there is no seal between the gate plate and the sealing surface of the valve seat at the medium inlet and outlet ends. Sealed is called double-sided forced sealing. 1.2.2 The sealing principle of the wedge gate valve The two sealing surfaces on the valve seat and the valve body are in close contact with the sealing surface of the wedge gate to form a seal, which can increase the auxiliary sealing load. The advantage of the wedge gate is that it can better meet the high requirements. Low pressure medium pressure sealing requirements. 1.2.3 Advantages and disadvantages of wedge gate valves are generally suitable for strict sealing conditions or relatively harsh operating conditions such as low temperature, high temperature, high pressure, and vaporization. The valve plate of the gate valve is not in contact with the valve seat before closing, which reduces the wear and damage to the valve plate. However, since the gate plate of the wedge gate valve is sealed with the valve seat and has an inclination angle, the valve plate is easy to be mixed with large-diameter medium materials, which makes the operation of the gate valve laborious, the valve plate damages the seat seal, and the gate valve sealing effect is not good. Therefore, the wedge gate valve has requirements for the cleanliness of the conveying medium. 1.2.4 The characteristics of the gate valve in parallel are different from the wedge gate valve. The two valve plates of the flat gate valve are sealed and parallel to each other; rotating the hand wheel, the parallel gate slides on the two valve seats to wipe the particles in the medium, so the flat plate The gate valve is suitable for installation in a viscous medium with suspended particles, and has low flow resistance. Flat gate valves have requirements for medium pressure: when the medium pressure is too high, they often open and close, and the sealing surface of the valve plate and the valve seat is worn out. When the medium pressure is too small, the sealing force of the sealing surface of the valve plate and the valve seat cannot easily achieve a proper seal. And the high-speed and high-pressure fluid has a greater impact on the valve plate. 1.2.5 The choice of gate valve type is recommended for viscous and suspended particles. The preferred choice is flat gate valve: such as knife-type flat gate valve; and clean fluid may wish to choose wedge gate valve.
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