Control valves precisely regulate flow, pressure, and temperature in industrial automation production processes. Any sticking, leakage, or oscillation can directly cause fluctuations in product quality. This article will take you through a comprehensive review of these common control valve “ailments” and provide effective solutions to help you quickly and accurately troubleshoot issues, ensuring continuous, stable, and efficient production processes.
1. Control Valve Leakage
1.1 Internal Leakage
Problem: Internal leakage of control valves mainly refers to the problem that the valves do not close tightly. The valve has already shown as fully closed, but the upstream pressure will continue to drop or the sound of medium flow can be heard.
Reason: There are many reasons for internal leakage in valves. For instance, when welding slag, rust on pipes, and impurities get stuck between the valve core and the valve seat, it may impede the sealing performance of the valve. If the valve core or valve seat is damaged by cavitation of high-speed medium and grooves appear, it can also cause internal leakage problems in the valve. In addition, if the valve closing pressure is insufficient in high-pressure differential situations or the type and material of the valve core are improperly selected, the required sealing degree cannot be achieved either.
Solution: For the cause of foreign object jamming, you can try to repeatedly open and close the valve for flushing several times to use the fluid to wash away the foreign object. In addition, it is necessary to promptly check whether the valve core or valve seat is worn, inspect and adjust the air source pressure. Any substandard parts should be repaired or replaced.
1.2 External Leakage
Problem: External leakage refers to the leakage of medium from the exterior of the valve, primarily occurring at the packing gland, bonnet flange, or valve body connection.
Reason: Packing gland leakage may result from packing aging and wear, loosening of packing gland bolts, or scratches on the valve stem surface. Valve bonnet flange leakage is caused by gasket aging or uneven bolt tightening force. While valve body leakage may stem from defects in the valve body casting.
Solution: When there is a slight leakage, the packing gland bolts can be slightly tightened evenly. For external leaks caused by aged packing or gaskets, you need to release the pressure after isolating the valve, and then replace the packing and gaskets with new ones. If valve body cracks are detected, the entire valve typically needs replacement.
2. Control Valve Sticking
Problem: Control valve jamming is usually manifested as the actuator not driving the valve core to move. When a given signal is received, the valve does not open or close or moves extremely slowly.
Reason: The most common causes of this phenomenon are issues with the air supply or power source. Pneumatic valves may malfunction due to insufficient air pressure, filter regulator failure, or air leaks in the supply line. Electric valves may fail because of power interruptions or motor overheating protection. Additionally, control signal short circuits or poor contacts, foreign objects jamming the valve core or stem, and excessively tight packing can also cause valve response problems.
Solution: To solve the problem of the valve not responding, you first need to check whether the air source and power supply are normal, and use a multimeter to measure whether the signals reaching the positioner and the actuator are normal. For pneumatic valves, you need to check in a timely manner whether the air pressure output to the diaphragm changes with the signal. In addition, you need to isolate the valve and drain the medium, then disassemble to inspect and clean the valve internals, valve seats, and replace the damaged diaphragms, packing or seals.
3. Control Valve Noise
Problem: Control valve noise refers to the shrill vibration produced by the valve during operation, accompanied by pipeline vibration, which implies serious operational problems.
Reason: Mechanical vibration occurs when components become loose, bearings wear out, or actuators are improperly secured. In high-pressure differential situations, high-speed fluids generate turbulence or vortices. Turbulence in control valves can cause severe vibration and noise, and may also damage internal pipelines. Additionally, if the pressure after the valve is lower than the saturated vapor pressure of the liquid, bubbles will form and burst, making a cracking sound similar to the flow of sand and gravel.
Solution: To solve the noise problem of the control valve, you must tighten all the internal and external bolts to eliminate mechanical loosening. You can also choose to use low-noise valve cores such as multi-stage pressure-reducing stone valve cores and labyrinth valve cores. The pressure can be gradually reduced step by step by increasing the number of throttling stages, avoiding a single-stage high pressure drop.
4. Control valve positioning inaccuracy
Problem: Inaccurate positioning of control valves refers to the inconsistency between the valve position and the control signal, with fluctuations always having deviations.
Reason: The main reasons for the inaccurate positioning of the control valve are as follows. Firstly, the positioner issue is the main cause. Among them, calibration drift of the positioner, blockage of the throttle hole, failure of the gas hole amplifier, loosening or wear of the feedback rod are all possible problems that may occur with the positioner. Secondly, if the valve stem packing is too tight or dry, it will generate a huge frictional force, making it impossible for the positioner to overcome. If the air source pressure is insufficient and the diaphragm or piston ring leaks, it may lead to insufficient thrust, thereby resulting in insufficient output force of the actuator. In addition, severe erosion of the valve core and valve seat will alter the current situation, leading to changes in the flow characteristics.
Solution: Recalibrating the positioner is the most crucial step and can be carried out according to the procedures in the manual. For the cause of excessive friction, you can loosen the cover or add high-temperature corrosion-resistant lubricant. In addition, you need to check whether the air source pressure is stable and whether there is any internal leakage in the actuator. You also need to check the connection of the feedback rod to ensure there is no gap.
5. Control Valve Pressure Instability
Problem: Abnormal fluctuations in control valve pressure can directly cause inaccurate flow control, resulting in valve noise and the formation of distinctive honeycomb patterns on internal components, potentially damaging the valve itself.
Reason: Improper valve selection, mismatched valve core characteristics with process requirements and other inherent reasons of the valve can cause unstable pressure. There are also some external process factors, such as fluctuations in the pressure of the upstream pump source and unreasonable pipeline design that increases resistance and leads to unstable pressure. In addition, malfunction of the positioner or incorrect signal transmission of the transmitter can also lead to delayed or incorrect control actions, causing system oscillation.
Solution: To address pressure changes, first you need to stabilize the external process source by checking and adjusting the output of the upstream pump and compressor to coordinate with the downstream and avoid drastic changes. Secondly, you can optimize the control loop by verifying the accuracy of the pressure transmitter signal, and then maintain the internal component jamming or leakage problems of the valve itself.
6. Conclusion
The stable operation of control valves is not a trivial matter; it is the core element that maintains the smooth and efficient functioning of the entire automated process. We hope this article serves as a practical tool to help you proactively identify control valve issues and resolve them promptly, ensuring the safety, quality, and efficiency of your enterprise’s production.
