Common faults of pneumatic control valve

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Pneumatic control valve uses compressed air as the power source, cylinder as the actuator, and drives the valve with the help of valve positioner, converters and other accessories to realize switching or proportional adjustment, and accepts the control signal of the industrial automation control system to regulate flow, pressure, temperature and other process parameters of the fluid in the pipeline. At present, pneumatic control valves are widely used in petrochemical, electric power, metallurgy and other process industries, and are an indispensable and important tache in the control system. Therefore, it is very important to ensure the stable and reliable operation of the pneumatic control valve. When the pneumatic control valve fails, it can be checked according to the common faults of the control valve, to find the reason of the fault in time.
  1. Air source failure

▶On-site air supply is not opened

▶The air source is watery, the weather is cold and frozen.

▶Supply of purified air is stopped

▶The air pressure is too low due to the leakage of the air source main pipe or the blockage of the air line, and the control valve cannot be fully opened or fully closed, or even does not act.

▶The air filter pressure reducer is used for a long time, and there is too much dirts. The black knob under the pressure reducing valve opens and leaks air, so that the output air pressure is lower than the specified pressure, which results in the regulating valve not being able to be fully opened and fully closed, or even not acting.

▶The on-site air line leaks air, and the joint is loose, which results in insufficient air pressure, and the control valve cannot be fully opened or closed, or even does not act.

▶The failure of the filter pressure reducing valve causes the wind pressure to be unstable and causes the regulating valve to oscillate.


  1. Line failure

▶ The wiring terminals of the power cable are loose, fall off, short-circuited; or open-circuited, too much dust on the circuit board results in poor contact; signal fluctuation and vibration of the control valve.

▶ After heavy rain or typhoon, the equipment is wetted by water which cause wiring to be short-circuited, result in the control valve unable fully opened or fully closed.

▶ Reverse polarity will cause the control valve to not act.

▶The middle section of the power cable is faulty due to the failure of the insulating tape; The wire insulation leather peels off causing a short circuit between wires; due to the on-site vibration, the wire is broken, results in the discontinuous action of the control valve, cannot fully open or fully close or even does not act.

▶ Failure of the regulating valve due to incorrect wiring after the regulating valve has been repaired

▶ The output signal of the control valve is unstable, which causes the operation of the control valve to fluctuate.


  1. Positioner failure

▶ The fixed nut of the feedback rod is loose and falls off, or the spring on the feedback rod falls off, causes the feedback rod to loosen, fall off, and jam, causes the regulating valve to oscillate.

▶ The position sensor in the positioner is faulty. When the vibration reaches a dead point, it will cause the central control room to display overt-ravel, and it will return to normal after a while, which can be solved by replacing the position sensor.

▶ The PID parameter setting of the positioner is not suitable.


  1. Valve body failure

▶ The valve spool or valve seat of the control valve is worn (scouring of the medium, rust, welding slag and other dirt), jamming (be blocked by all kinds of impurities in the medium), and the sealing is not tight (the sealing ring is worn), resulting in the media is still in excess when the valve is fully closed, it cannot be controlled.

▶ The control valve disc is pressed too tightly or too loosely, if too tight, it will make the valve stem move slowly or jump; if too loose, it will cause the medium to leak. if the medium is heavy oil, it is likely to burn and cause a big accident.

▶ When the control valve is installed, the pipeline and the valve body are not concentric, it make that the control valve is subjected to excessive additional stress, results in vibration, the valve can not full open or fully close.

▶ The valve stem of the control valve and the fixing nut of the connecting part are loose, and the valve stem and the valve spool are not concentric, which causes the valve to not fully close, and the subjected stress increases, results in the valve stem high frequency oscillation, or even fracture.

▶ The diaphragm head of the control valve is faulty. Due to the long-term use of the diaphragm, the aging and deterioration, the reduction of elasticity, the poor sealing performance, the leakage of the diaphragm, the aging of the compression spring, the reduction of elasticity and the breakage, result in the control valve cannot fully open and fully close or even out of control. The valve spool of the control valve falls off, the valve spool and the valve seat are stuck, and the valve stem is bent or broken, which will cause the control valve to operate abnormally, can not play a role of regulating.

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