Five common faults of valves – mastering them makes you an expert!

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Introduction: Instrumentation technicians often carry out maintenance on valves. Common issues encountered with control valves on-site include failure to operate, oscillation, vibration, slow response, and high leakage rates. The automation channel below will guide you through analyzing each of these faults one by one. 1. The valve does not operate. The first phenomenon is the absence of air supply and signal; the main reasons for this phenomenon are as follows: the air supply is not turned on or the pressure of the air supply is too low ； Impurities in the gas source cause blockages in the gas supply pipes, filters, or pressure regulators ； The filter pressure reducing valve is clogged or faulty. The second phenomenon is that the system has an air supply but fails to operate; in such cases, corresponding repairs must be carried out based on the symptoms of the fault: http://www.tcwdyb.com/upload/2018-01/30/QQtupian20180129130925-e7477.png (1) Symptom: No output from the DCS command signals; in this case, it is necessary to check the relevant command wires ； (2) Phenomenon: The locator shows no display and produces no output; in this case, the locator needs to be replaced ； (3) Phenomenon: Leakage in the pneumatic circuit of the positioner; welding work is required to eliminate the leakage ； (4) Phenomenon: Sticking or deformation of the valve stem or valve core; in such cases, action must be taken or replacement required based on the actual degree of damage ； (5) Phenomenon: The handwheel is not in the correct position; in this case, the handwheel needs to be adjusted to the release position. 2. Oscillation of the control valve: When the air supply pressure is within the required range and the command signal is stable, but the operation of the control valve remains unstable, it is first necessary to check whether the positioner is in the correct position and take appropriate action ； Next, check whether there is a fault with the locator itself: if the locator is faulty, it needs to be repaired or replaced ； In addition, check the output pipeline of the locator for air leaks and eliminate any such leaks ； Finally, check whether the movement of the valve stem and its contact parts are smooth; if not, lubricant should be added or the valve stem reinstalled. http://www.tcwdyb.com/upload/2018-01/30/QQtupian20180129131400.png 3. Vibration of the control valve: At this point, appropriate actions can be taken based on the symptoms of the fault: Unstable mounting base: Reinforce the base ； Caused by vibrating equipment in the vicinity: Eliminate the source of vibration ； The valve core and bushing are severely worn: replace the bushing ； Incorrect selection of control valve: Replace with an appropriate valve ； The flow direction of the medium through the valve is opposite to its closing direction: change the installation orientation of the valve. http://www.tcwdyb.com/upload/2018-01/30/QQtupian20180129131458.png4. The control valve responds slowly; in such cases, appropriate actions can be taken based on the symptoms of the fault: If the diaphragm in the pneumatic diaphragm actuator is damaged and leaking, replace the diaphragm ； Broken O-ring in the actuator: Replace the O-ring seal ； There is debris blocking inside the valve body: Remove the blockage ； An uneven valve stem results in high frictional resistance: address the valve stem. http://www.tcwdyb.com/upload/2018-01/30/QQtupian20180129131612.png 5. Excessive leakage from the control valve: In such cases, appropriate actions can be taken based on the symptoms of the fault – if the valve core is worn and there is severe internal leakage, the internal leakage must be eliminated ； The length of the valve stem is not appropriate, resulting in the valve not closing properly: Adjust the valve stem, adjust the valve ； The sealing ring inside the valve body is damaged: Replace the sealing ring ； The pressure difference across the medium is too high, causing the actuator to not close properly: increase the air supply and improve the actuator ； Foreign object inside the valve: Remove the foreign object ； Low air supply pressure or air leak in the connection tube: Adjust the air supply and eliminate the leak. http://www.tcwdyb.com/upload/2018-01/30/QQtupian20180129131710.png Understanding the causes of on-site failures in control valves quickly can help you achieve better results with less effort!