Common faults and causes of control valves

a) The regulating valve does not operate. The failure phenomena and reasons are as follows: 1. No signal, no gas source. ①The gas source is not turned on, ②The air pipe is blocked or the filter and the pressure reducing valve are blocked due to the water in the gas source and freezes in winter, ③The compressor is malfunctioning; ④The gas source manifold is leaking. 2. There is air source, but no signal. ①The regulator is faulty, ②the signal tube is leaking; ③the bellows of the positioner is leaking; ④the regulating network a) the regulating valve does not operate. The failure phenomena and reasons are as follows: 1. No signal, no gas source. ①The gas source is not turned on, ②The air pipe is blocked or the filter and the pressure reducing valve are blocked due to the water in the gas source and freezes in winter, ③The compressor is malfunctioning; ④The gas source manifold is leaking. 2. There is air source, but no signal. ①The regulator is faulty, ②the signal tube is leaking; ③the bellows of the positioner is leaking; ④the regulating mesh is damaged. 3. The positioner has no air source. ①The filter is blocked; ②The pressure reducing valve is malfunctioning; ③The pipeline is leaking or blocked. 4. The positioner has air source but no output. The orifice of the positioner is blocked. 5. There is signal, no action. ① The valve core falls off, ② the valve core and the valve seat are stuck; ③ the valve stem is bent or broken; ④ the valve seat and the valve core are frozen or burnt dirt; ⑤ the actuator spring is rusted due to long-term use. (B) The operation of the regulating valve is unstable. The failure phenomena and reasons are as follows: 1. The pressure of the air source is unstable. ①The compressor capacity is too small; ②The pressure reducing valve is faulty. 2. The signal pressure is unstable. ①The time constant of the control system (Tu003dRC) is not appropriate; ②The output of the regulator is unstable. 3. The air source pressure is stable and the signal pressure is also stable, but the action of the regulating valve is still unstable. ①The ball valve of the amplifier in the positioner is not tightly closed due to dirt and wear, and the output will oscillate when the air consumption is particularly increased; ②The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle; ③The output tube , Line leaks; ④The rigidity of the actuator is too small; ⑤The frictional resistance in the movement of the valve stem is large, and the contact part is blocked. (C) The control valve vibrates. The failure phenomena and reasons are as follows: 1. The regulating valve vibrates at any opening. ①The support is unstable; ②There is a vibration source nearby; ③The valve core and bushing are seriously worn. 2. The regulating valve vibrates when approaching the fully closed position. ①The regulating valve is larger, and it is often used at a small opening; ②The medium flow direction of the single-seat valve is opposite to the closing direction. (D) The action of the regulating valve is slow. The phenomena and reasons of inactivity are as follows: 1. The valve stem is only sluggish when it moves in one direction. ①The diaphragm is damaged and leaked in the pneumatic diaphragm actuator; ②The 'O' seal in the actuator leaks. 2. The valve stem is sluggish during reciprocating action. ①There is sticky blockage in the valve body; ②The PTFE packing has deteriorated and hardened or the graphite-asbestos packing lubricant is dry; ③The packing is too tight and the friction resistance increases; ④The friction resistance is large because the valve stem is not straight; ⑤No The pneumatic control valve of the positioner can also cause sluggish movement. (E) The leakage of the regulating valve increases. The reasons for the leakage are as follows: 1. Large leakage when the valve is fully closed. ①The valve core is worn out and the internal leakage is serious. ②The valve is not properly adjusted and closed tightly. 2. The valve cannot reach the fully closed position. ①The medium pressure difference is too large, the rigidity of the actuator is small, and the valve is not closed tightly; ②There is foreign matter in the valve; ③The bush is sintered. (F) The adjustable range of flow becomes smaller. The main reason is that the valve core is corroded and becomes smaller, so that the adjustable minimum flow becomes larger.