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What are the common causes of control valve failures? During the use of control valves, issues such as abnormal operation and unstable performance may occur from time to time, or problems like delayed response may arise after prolonged use. So what are the common causes for such situations? Let’s discuss them together. (1) The control valve does not operate. The fault symptoms and causes are as follows: 1. No signal, no air supply. ①The gas supply is not turned on. ② Water present in the gas supply freezes in winter, causing blockages in the air ducts or malfunctions in the filters and pressure regulators. ③ Compressor failure ； ④The main gas supply pipe is leaking. 2. There is air supply, but no signal. ①Regulator failure, ② Signal tube leakage ； ③Leak in the locator bellows ； ④Damage to the regulatory retinal sheet. 3. The locator has no air supply. ①Filter clogged ； ②Pressure relief valve failure I③: Pipeline leakage or blockage. 4. The locator has an air supply but no output. The throttle orifice of the locator is blocked. 5. There is a signal, but no action. ①The valve core has fallen off; ② The valve core is stuck to the surrounding structure or to the valve seat ； ③The valve stem is bent or broken ； ④Frozen valve seat and spool, or fouling in the form of coke deposits ； ⑤The actuator spring has rusted due to long-term disuse. (II) The actuation of the control valve is unstable. The fault symptoms and causes are as follows: 1. Unstable air supply pressure. ①The compressor capacity is too small ； ②The pressure relief valve is faulty. 2. The signal pressure is unstable. ①The time constant of the control system (T=RC) is inappropriate ； ②The regulator output is unstable. 3. The gas supply pressure is stable, and the signal pressure is also stable, but the operation of the control valve remains unstable. ①The ball valve of the amplifier in the positioner becomes poorly sealed due to dirt and wear, which causes output oscillations when air consumption increases significantly ； ②In the locator, the nozzles of the amplifiers are not parallel; the shields do not cover the nozzles ； ③Leakage in output pipes and wires ； ④The actuator has too low stiffness ； ⑤There is high frictional resistance during the movement of the valve stem, and there is obstruction at the contact points. (III) Vibration of the control valve. The fault symptoms and causes are as follows: 1. The control valve vibrates at any opening degree. ①Unstable support ； ②There is a vibration source nearby ； ③The valve core and bushing are severely worn. 2. The control valve vibrates when it is near the fully closed position. ①If the control valve is selected to be too large, it is often used at a low opening degree ； ②In a single-seat valve, the flow direction of the medium is opposite to the closing direction. (IV) The control valve responds slowly. The phenomena and causes of sluggishness are as follows: 1. The valve stem is sluggish only when moving in one direction. ①Diaphragm rupture and leakage in pneumatic diaphragm actuators ； ②Leakage in the “O”-ring of the actuator. 2. The valve stem exhibits sluggishness during its reciprocating motion. ①There is residue blocking inside the valve body ； ②The polytetrafluoroethylene filler hardens due to deterioration, or the lubricant in the graphite-asbestos filler dries out ； ③The filler is packed too tightly, increasing frictional resistance ； ④High frictional resistance due to the non-rectangular valve stem ； ⑤Pneumatic control valves without positioners can also result in sluggish operation. (5) The leakage rate of the control valve increases. The reasons for the leakage are as follows: 1. The leakage is severe when the valve is fully closed. ①The valve core is worn, resulting in severe internal leakage; ② the valve is not properly adjusted and does not close tightly. 2. The valve cannot reach the fully closed position. ①The medium pressure difference is too high, the actuator has low stiffness, and the valve does not close properly ； ②There is a foreign object inside the valve ； ③Bushing sintering. (VI) The adjustable range of flow rate becomes smaller. The main reason is that the valve core corrodes and shrinks, thereby increasing the adjustable minimum flow rate. When a control valve malfunctions, we first need to understand the symptoms and causes of such failures in pneumatic control valves, so that we can take appropriate measures to resolve them. If you have any other experience in dealing with such issues, please feel free to share it.