Causes of internal leakage of pneumatic control valve and Solutions

Pneumatic control valve has been widely used in various industrial control systems because of its high control accuracy, convenient installation and debugging. However, in the process of use, there are also some problems puzzling the field instrument personnel, that is, the valve leakage. Here we will discuss the causes and solutions of the common internal leakage of pneumatic control valve, hoping to help the site maintenance personnel of the factory.

1. The zero setting of the actuator is not accurate, and the valve is not fully closed. Adjustment method: 1) manually close the valve (must be confirmed that it has been completely closed); 2) manually switch the valve manually and slightly tighten it with force; 3) turn back again (opening the valve direction) half circle; 4) then adjust the limit.

2. The valve is a downward pushing and closing type. The thrust of the actuator is not large enough. When there is no pressure, it is easy to reach the full closing position when debugging. When there is downward thrust, it cannot overcome the upward thrust of the liquid, so it cannot be closed in place. Solution: replace the actuator with high thrust or balance valve core to reduce the unbalanced force of medium.

3. Internal leakage caused by manufacturing quality of pneumatic control valve

During the production process, the valve manufacturer did not strictly control the valve material, processing technology and assembly technology, resulting in unqualified grinding of the sealing surface, incomplete removal of the products with defects such as pockmarks and sand holes, and internal leakage of the pneumatic control valve. Solution: rework the sealing surface.

4. The control part of the pneumatic control valve affects the internal leakage of the valve

The traditional control mode of pneumatic control valve is the mechanical control mode such as valve limit switch and over torque switch. Due to the influence of ambient temperature, pressure and humidity, these control elements cause the valve positioning misalignment, spring fatigue, uneven thermal expansion coefficient and other objective factors, resulting in the internal leakage of pneumatic control valve. Solution: readjust the limit.

5. Internal leakage caused by commissioning of pneumatic control valve

Due to the influence of processing and assembly technology, pneumatic control valves are generally unable to open after being closed manually. For example, if the stroke of pneumatic control valve is adjusted slightly by the action position of the upper and lower limit switches, there will be an irrational state that the pneumatic control valve is not closed tightly or the valve is not opened; if the stroke of pneumatic control valve is adjusted slightly, the protection action of over torque switch will be caused If the action value of the over torque switch is adjusted larger, the deceleration mechanism or valve will be damaged or even the motor will be burnt out. In order to solve this problem, usually, when the pneumatic control valve is debugged, manually swing the pneumatic control valve to the bottom, and then swing one circle to the open direction to fix the lower limit switch position of the pneumatic valve, and then open the pneumatic control valve to the full open position to fix the upper limit switch position, so that the pneumatic control valve will not have the phenomenon that the pneumatic control valve cannot be opened after the manual closing, so that the pneumatic valve can be opened and closed freely, But it causes the leakage of the pneumatic door. Even if the adjustment of the pneumatic control valve is ideal, because the action position of the limit switch is relatively fixed, the constant erosion and wear of the valve by the medium controlled by the valve during the operation will also cause the internal leakage caused by the valve not being closed tightly. Solution: readjust the limit.

6. Cavitation corrosion of valve caused by wrong type selection causes internal leakage of pneumatic control valve

Cavitation is related to the pressure difference. When the actual pressure difference △ P of the valve is greater than the critical pressure difference △ PC, cavitation will occur. When the bubble breaks in the process of cavitation, huge energy will be released, which will cause great damage to the valve seat, valve core and other throttling elements. In general, the valve can operate for up to three months or even less under cavitation conditions, that is, the valve suffers serious cavitation corrosion, As a result, the leakage of valve seat can reach more than 30% of the rated flow, which is irreparable. Therefore, different pneumatic valves have different specific technical requirements, so it is very important to select the pneumatic control valve reasonably according to the system process flow. Solution: improve the process, select multi-stage pressure reducing or sleeve control valve.

7. Flushing of medium and internal leakage caused by aging of pneumatic control valve

After the pneumatic control valve is adjusted and operated for a certain period of time, due to the cavitation of the valve, the erosion of the medium, the abrasion of the valve core and the valve seat, the aging of the internal components and other reasons, the pneumatic control valve stroke is too large, and the pneumatic control valve is not closed tightly, which causes the leakage of the pneumatic control valve to increase. With the passage of time, the internal leakage of the pneumatic control valve will become more and more serious. Solution: readjust the actuator, maintain and calibrate it regularly.