Valve is one of the most widely used control components in fluid system, which can be used to switch or control the flow direction and regulation. In terms of the simplest cut-off function, the function of valve sealing in machinery is to prevent medium from leaking out or blocking external substances from entering into the interior along the joint between parts in the cavity where the valve is located, which plays a sealing role The sealing surface is the surface that contacts with and plays the role of sealing. The sealing surface of the valve is the core part of the valve, and its leakage forms can be generally divided into these types, namely, the leakage of the sealing surface, the leakage of the sealing ring connection, the leakage of the sealing part falling off and the leakage of the foreign matters embedded between the sealing surfaces. One of the most widely used valves in pipeline and equipment is to cut off the flow of medium. Therefore, its tightness is the main factor to determine whether internal leakage occurs. The valve sealing surface is generally composed of a pair of sealing pairs, one on the valve body and the other on the valve disc.

The leakage of sealing surface is generally caused by the following reasons:

1. The sealing surface is ground unevenly, and the sealing line cannot be formed;

2. The top center of the connection between the valve rod and the closing part is suspended, incorrect or worn;

3. The valve rod is bent or assembled improperly, so that the closing part is skewed or out of center 4、 Improper selection of sealing surface materials or failure to select valves according to working conditions, the sealing surface is prone to corrosion, erosion and wear; fifthly, overlaying welding and heat treatment are not operated in accordance with regulations, and wear occurs due to low hardness, corrosion due to burning of alloy elements, cracks due to excessive internal stress and other defects

6、 After surface treatment, the sealing surface peeled off or lost its original performance due to excessive grinding;

7. The sealing surface was not closed tightly or the fine seam appeared due to shrinkage after closing resulted in medium corrosion;

8. The shut-off valve was used as throttle valve and pressure reducing valve, and the sealing surface was eroded and damaged

9、 When the valve has reached the fully closed position, the excessive closing force shall be continuously applied, including improper use of long lever operation, the sealing surface being crushed and squeezed out of shape;

10. The sealing surface is excessively worn, resulting in disconnection, that is, the sealing pair cannot be well sealed;

prevention and elimination methods for leakage of valve sealing surface: 1、 When lapping the sealing surface, the lapping tools, abrasives, emery cloth, sandpaper and other objects should be properly selected and the lapping method should be correct. After lapping, the sealing surface should be checked for defects such as indentation, crack, scratch, etc 2、 The connection between the valve rod and the closing piece shall meet the design requirements. If the top center does not meet the requirements, it shall be repaired. The top center shall have a certain movable clearance, especially the axial clearance between the shoulder of the valve rod and the closing piece shall not be less than 2mm. Thirdly, the bending of the valve rod shall be straightened. After the adjustment of the valve rod, the stem nut, the closing piece and the valve seat, they shall be on a common axis 4、 When selecting the valve or replacing the sealing surface, it should meet the working conditions. After the sealing surface is processed, its corrosion resistance, abrasion resistance and other properties are better. Fifthly, the surfacing and heat treatment process should meet the technical requirements of the regulations and specifications. After the sealing surface is processed, it should be accepted without any defects affecting the use 6、 The processes such as flame, nitriding, shed and plating on the sealing surface must be carried out in strict accordance with the technical requirements of the procedures and specifications. If the penetrating layer of the grinding sealing surface is not more than one third of the layer, which causes serious damage to the coating and the penetrating layer, the coating and the penetrating layer shall be removed and the surface treatment shall be carried out again. The sealing surface with high-frequency fire breaking on the surface can be repeatedly broken for repair;

VII. The valve shall be marked for closing or opening, and the sealing surface with loose closing shall be repaired in time. For high-temperature valves, some of the cold contraction cracks after closing shall be closed more than once at a certain time interval after closing; VIII. Valves used as cut-off valves are not allowed to be used as throttle valves and pressure reducing valves, and the closing parts shall be in the fully open or fully closed position. If the medium flow and pressure need to be adjusted, throttle valves and pressure reducing valves shall be set separately 9、 The front opening and closing of the valve shall conform to the section of “operation of the valve”. The valve closing force shall be appropriate. The diameter of the hand wheel shall be less than 320mm, and only one person is allowed to operate. For the hand wheel with the diameter equal to or greater than 320mm, two people are allowed to operate, or one person is allowed to operate with the lever within 500mm 10、 After the water line falls off, it shall be adjusted, and the sealing surface of the sealing surface that cannot be adjusted shall be replaced;