Analysis of the causes of internal leakage in control valves and solutions

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The zero setting of the actuator is inaccurate, resulting in the valve not reaching its fully closed position. Adjustment methods: 1) Manually close the valve completely (make sure it is fully shut); 2) Close the valve manually with a bit more force, until it can no longer be turned further; 3) Turn it back half a turn in the direction of opening; 4) Then adjust the limit mechanism. 2) The thrust of the actuator is not sufficient. The valve operates by being pushed downward to close, and if the thrust of the actuator is not enough, it is easy to reach the fully closed position when there is no pressure. However, when there is a downward force, it is not possible to overcome the upward force exerted by the liquid, so the valve cannot be closed properly. Solution: Replace the actuator with a higher thrust one, or use a balanced spool to reduce the unbalanced force of the medium. 3 Internal leakage caused by the manufacturing quality of electric control valves: Valve manufacturers fail to exercise strict control over aspects such as valve material, processing techniques, and assembly processes during production; as a result, the sealing surfaces are not properly polished, and products with defects such as pitting and sand inclusions are not thoroughly removed, which leads to internal leakage in electric control valves. Solution: Reprocess the sealing surface. 4. The control mechanism of electric control valves affects the internal leakage of these valves. The traditional control methods for electric control valves involve mechanical components such as valve limit switches and over-torque switches; however, due to factors such as environmental temperature, pressure, and humidity, these control elements can cause inaccurate valve positioning, spring fatigue, and uneven thermal expansion coefficients, all of which lead to internal leakage in electric control valves. Solution: Readjust the limit. 5 Internal leakage caused by issues during the commissioning of electric control valves: Due to factors related to processing and assembly techniques, it is common for electric control valves to fail to open when attempted to be closed manually. If the travel distance of the electric control valve is reduced by adjusting the operating positions of the upper and lower limit switches, it results in an undesirable situation where the valve cannot close properly or cannot open fully. If the travel distance of the electric control valve is increased, it causes the over-torque switch to activate as a protection mechanism. If the setting value of the over-torque switch is increased further, it can lead to damage to the reduction gear mechanism or to the valve itself, and in severe cases, even damage to the motor. To address this issue, during the calibration of the electric control valve, it is common to manually turn the valve all the way closed, then turn it one full turn in the opening direction to set the position of the lower limit switch. After that, the valve is turned to its fully open position to set the position of the upper limit switch. In this way, the valve cannot remain completely closed even after being manually shut off, allowing the door to open and close smoothly. However, this approach inadvertently causes leakage inside the door. Even if the electric control valve is adjusted ideally, since the operating position of the limit switch is relatively fixed, the continuous erosion and wear of the valve by the fluid it controls during operation can also lead to poor sealing of the valve and thus internal leakage. Click to view--Summary of Chemical Engineering Skill Training Courses for 2023. Solution: Adjust the limits again. 6. Incorrect selection can lead to cavitation corrosion of the valve, resulting in internal leakage in electric control valves. Cavitation is related to pressure difference; when the actual pressure difference ΔP across the valve exceeds the critical pressure difference ΔPc for cavitation to occur, cavitation takes place. During this process, the breakdown of bubbles releases enormous amounts of energy, causing severe damage to components such as the valve seat and valve stem. Under cavitation conditions, ordinary valves can suffer serious cavitation corrosion even after just three months or less of operation, leading to a leakage rate at the valve seat that can exceed 30% of the rated flow rate – a loss that cannot be compensated for. Therefore, electric valves designed for different applications have specific technical requirements, and it is crucial to select the appropriate electric control valve based on the system’s operational parameters. Solution: Implement process improvements by using multi-stage pressure reduction or sleeve control valves. 7. Erosion by the medium and internal leakage caused by the aging of electric control valves: After an electric control valve has been adjusted and operated for a certain period of time, issues such as cavitation, erosion by the medium, wear between the valve core and seat, and aging of internal components can occur. As a result, the valve’s stroke may become excessive and it may not close properly, leading to increased leakage. Over time, the internal leakage problem in the electric control valve will worsen. Solution: Readjust the actuator and carry out regular maintenance and calibration.