Pneumatic control valve is compressed air as the power source, with the cylinder as the actuator, and with the valve positioner, converter and other accessories to drive the valve, to achieve switching or proportional adjustment, accept the control signal of the industrial automation control system to complete the adjustment of the flow, pressure, temperature and other process parameters of the pipeline medium.

At present, pneumatic control valve is widely used in petrochemical, electric power, metallurgy and other process industries, and is an essential and important link 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 investigated according to the common faults of the control valve to find out the cause of the failure in time.

1. Air source failure

(1) The site air source is not opened;

(2) Water source, cold and icy weather;

(3) Stop supply of purified wind;

(4) Air main pipe leakage or wind line blockage leads to low wind pressure, and the regulating valve can not be fully opened or fully closed, or even no action;

(5) The air filter pressure reducer is used for a long time, there is too much dirt, the black knob under the pressure reducing valve opens the air leakage, so that the output wind pressure is less than the specified pressure, resulting in the regulator can not be fully opened and closed, or even no action;

(6) Air leakage in the field wind line, loose joints, resulting in insufficient wind pressure, regulating valve can not be fully opened and closed, or even no action;

(7) Filter pressure reducing valve failure, resulting in unstable wind pressure, resulting in oscillation of the regulating valve.

2. Line failure

(1) The wiring end of the power line is loose, falling off, short circuit, open circuit, too much dust on the circuit board leads to poor contact, signal fluctuation, vibration of the regulator;

(2) After heavy rain or typhoon, the equipment water is damp and the wiring is short circuit, resulting in the regulator can not be fully opened or fully closed;

(3) Polarity reverse will cause the regulating valve does not operate;

(4) The middle section of the power line is faulty. Due to the failure of the insulation tape, the insulation skin of the wire falls off, resulting in short circuit between the wire and the wire. Due to the on-site vibration, the wire breaks, resulting in discontinuous oscillation of the regulating valve, which cannot be fully opened or fully closed or even no action;

(5) Due to the wiring failure of the regulating valve after maintenance, the regulating valve fails;

(6) The output signal of the regulating valve is unstable, resulting in fluctuations in the operation of the regulating valve.

3. The locator is faulty

(1) The fixing nut of the feedback rod is loose and falls off, and the spring on the feedback rod falls off, causing the feedback rod to loose, fall off, and jam, which makes the regulating valve oscillate;

(2) Position sensor failure in the positioner, when the vibration to the bad point will cause the central control room display overrange, after a while and return to normal, can be solved by replacing; ? Positioner PID parameter setting is not appropriate;

4. The regulating valve body is faulty

(1) Control valve spool or seat wear (media erosion, rust, welding slag and other dirt scratches and wear), jammed (various impurities in the media blocked), seal is not strict (seal ring wear), resulting in the valve is still too much media, can not be controlled;

(2) The regulating valve disc is too tight or too loose, too tight makes the regulating valve stem slow or jump, too loose will make the medium leak, if the heavy oil is likely to burn, resulting in a great accident;

(3) When the regulating valve is installed, the pipe and the valve body are different, so that the regulating valve is subjected to excessive additional stress, causing oscillation, and can not be fully opened or fully closed;

(4) The fixing nut of the regulating valve stem and connector is loose, the valve stem is different from the valve core, resulting in the valve not closing, the stress is increased, resulting in high frequency oscillation of the valve stem, and even fracture;

(5) Regulator valve film head failure, due to the long-term use of the film, aging deterioration, elasticity becomes smaller, sealing becomes worse, diaphragm leakage, compression spring aging, elasticity becomes smaller, fracture, resulting in the regulator can not be fully opened and fully closed or even lose control of the regulator valve spool off, valve spool and seat stuck, valve stem bent or broken will lead to normal valve operation, but can not play a regulating role.