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Gate valve is the most common kind of industrial valve, widely used in petroleum, petrochemical, power station, long-distance pipeline and shipbuilding and other industrial fields. As a fluid control device, the gate valve opening and closing part is the gate plate, which switches the opening and closing state through the up and down movement of the gate plate, so as to achieve the function of cutting off or connecting the pipeline medium. If the gate valve leaks, it is easy to bring hidden dangers to the pipeline and equipment facilities, and even affect the safety of personnel and equipment. Understanding the structure and operating principle of the gate valve can help us to operate the gate valve correctly and better grasp the operating state of the gate valve. Once the gate valve leaks, it can take the correct disposal method in the first time to reduce economic losses and reduce the probability of accidents. Gate valve leakage is generally divided into internal leakage and external leakage, resulting in more reasons for gate valve leakage, need in-depth analysis and research, in order to take the corresponding disposal measures.
1. The cause of gate valve leakage
1.1 The machining accuracy of the wedge seal ring is low, resulting in internal leakage of the valve wedge seal can be divided into two categories, the first hard seal, also known as the metal wedge ring; The second is a soft seal, also known as a composite graphite wedge ring. If the precision of the wedge sealing ring is low, it is easy to cause internal leakage of the gate valve.
1.2 Unstable operating conditions result in internal leakage of gate valve
Valve production operation in the process of unstable conditions, its pressure and ambient temperature may change greatly, which makes the seal ring to withstand greater impact pressure, erosion force and deformation, resulting in the gap between the seal ring and the valve body more and more large, resulting in valve leakage.
1.3 Valve maintenance quality is not up to standard resulting in gate valve leakage in the process of the staff to open the valve for maintenance, the installation of wedge-shaped sealing ring did not clean the sealing surface, residual some fine particles and impurities, these fine impurities may scratch the sealing surface during the operation of the valve, once the sealing surface forms a penetrating scratch, it will cause valve leakage.
1.4 Medium corrosion wedge sealing ring resulting in internal leakage of gate valve
When designing the wedge-shaped sealing ring, it is necessary to pay more attention to the strength and corrosion resistance of the sealing ring, the surface hardness of the sealing ring is smaller than the hardness of the sealing layer of the valve cover, and the plastic deformation can occur under the action of external conditions, and the medium may also corroded the seal ring during long-term operation, resulting in gate valve leakage.
1.5 Gate valve actuator switch is not in place resulting in gate valve leakage valve switch is controlled by the actuator, the actuator relies on liquid, gas, electricity or other energy and through the motor, cylinder or other devices to convert it into a driving role, driving the valve to the fully open or fully closed position. If the actuator is not opened or closed in place, the gate valve cannot be fully opened or closed, resulting in internal leakage of the valve.
1.6 Valve body defect causes valve leakage
Casting valve body often appear casting defects, such as: porosity, slag inclusion, cracks, sand holes, etc. If these defects are not found in the factory, or found that the correct repair measures are not taken, there will be valve leakage during field use.
2. Handling measures for gate valve leakage
2.1 Fix the valve body pressure plate and increase the bolt preload
When there is a leak or a small amount of leakage inside the gate valve, the first job is to tighten and fix the pressure plate of the valve body, and strengthen the pre-tightening between the pressure plate and the wedge sealing ring by raising the bolt, which can help the plastic deformation of the sealing ring, so as to better reduce the size of the gap between the valve body, the pressure plate and the sealing ring, and avoid the leakage phenomenon of the valve self-sealing. If the amount of leakage does not change greatly when tightening and fixing the bolts, the fastening measures should be stopped immediately and other treatment measures should be taken.
2.2 Inject sealant through grease injection holes
When there is a small internal leakage of the valve, the sealing surface or packing can be injected with grease through the grease injection hole to temporarily relieve the internal leakage of the valve. When it is convenient to repair and maintain, the valve can be removed and returned to the factory for maintenance.
2.3 Weld and strengthen the gap between the end face of the valve body, the stuffing box and the pressure plate
The treatment of valve self-sealing leakage needs to do a good job of disassembling the valve handwheel and valve bracket. First of all, we must cooperate with good technicians to grasp and adjust the working conditions on the site, and then remove the bolts fixed on the valve bracket after opening the valve completely. After the rotating valve bracket is clear that the valve stem does not rise and fall, the valve bracket begins to remove the valve bracket. After removing the valve bracket, the valve body should be exposed in the valve pressure plate part, and the gap between the pressure plate and the valve body should be sealed and welded. In order to consider the valve bracket back assembly process after sealing the leak position, the welding gap size should be between 5 mm and 8 mm, and the welding Angle and the position of the pressure plate is 45 degrees. And grinding according to the actual assembly situation.
2.4 Check the zero position of the actuator
The zero setting of the actuator is not accurate, and the full closing position of the valve is not reached. By manually opening and closing the valve, and then adjusting the actuator zero position, to ensure that the actuator can accurately open or close the valve.
2.5 Valve body repair welding or return to factory repair
When the valve body leaks, the valve body defect can be repaired by the valve manufacturer, or returned to the factory for repair and replacement of the valve body. First of all, the defects of the valve body must be judged, the emergence of penetrating cracks, penetrating defects, and more than a certain area of shrinkage is not allowed to repair, in this case, only to return to the factory to replace the valve body. For the valve body defects that can be repaired, it is necessary to develop the appropriate welding process, determine the welding method, electrode material, post-welding stress relief method, and carry out appropriate non-destructive testing on the welding position to ensure that the repair meets the requirements.
3. Closing remarks
By analyzing the causes and treatment measures of gate valve leakage, we can take some measures to alleviate or eliminate valve leakage, such as increasing bolt pre-tightening force, on-line sealing with pressure and injecting sealant. With pressure sealing to professional staff with a solid foundation to operate, through welding to strengthen the sealing valve end face, stuffing box and pressure plate gap, injection sealant and other ways can better avoid the occurrence of leakage accidents, so as to solve the similar valve leakage problem has brought reference and guidance.