Pneumatic control valves use compressed air as their power source and a cylinder as their actuator. With the aid of accessories such as valve positioners and transducers, they drive the valve to achieve on/off or proportional control. By receiving control signals from industrial automation control systems, they regulate various process parameters-such as flow, pressure, and temperature-of the medium in pipelines.
Currently, pneumatic control valves are widely used in process industries such as petrochemicals, power generation, and metallurgy, and serve as an indispensable component in control systems. Therefore, ensuring the stable and reliable operation of pneumatic control valves is of utmost importance. When a pneumatic control valve malfunctions, troubleshooting can be conducted based on common valve failures to promptly identify the cause of the malfunction.
I. Air Supply Issues
The on-site air supply is not turned on;
The air supply contains moisture, which has frozen due to cold weather;
The purified air supply has stopped;
A leak in the main air supply line or a blockage in the air line has caused the air pressure to drop too low, preventing the regulating valve from fully opening or closing, or even causing it to fail to operate;
The air filter-regulator has been in use for an extended period and is heavily contaminated; the black knob below the pressure-reducing valve is open, causing air leakage, which results in an output air pressure lower than the specified pressure, preventing the control valve from fully opening or closing, or even causing it to fail to operate;
Air leaks in the on-site air lines or loose connections result in insufficient air pressure, preventing the control valve from fully opening or closing, or even causing it to fail to operate;
A malfunction in the filter-regulator valve causes unstable air pressure, resulting in oscillation of the control valve.
II. Wiring Faults
Loose, disconnected, short-circuited, or broken power supply wires; excessive dust accumulation on the circuit board causing poor contact and signal fluctuations, resulting in valve vibration;
After heavy rain or a typhoon, water ingress or moisture in the equipment causes wiring short circuits, preventing the control valve from fully opening or closing;
Reversed polarity will cause the control valve to fail to operate;
Failures in the middle section of the power cable-such as insulation tape failure causing the wire insulation to peel off and resulting in short circuits between wires, or wire breaks due to on-site vibrations-can cause the control valve to operate intermittently or oscillate, preventing it from fully opening, fully closing, or even operating at all;
Wiring errors following control valve maintenance can lead to control valve malfunctions;
Unstable control valve output signals cause fluctuations in control valve operation.
III. Positioner Malfunctions
The feedback rod retaining nut becomes loose or falls off, and the spring on the feedback rod detaches, causing the feedback rod to become loose, detach, or jam, resulting in valve oscillation;
A malfunction in the position sensor within the positioner; when vibration reaches a critical point, the control room display may show an overtravel condition, which returns to normal after a while; this can be resolved by replacing the sensor; Inappropriate tuning of the positioner's PID parameters;
IV. Control Valve Body Failures
Wear on the control valve plug or seat (caused by media erosion, rust, welding slag, or other contaminants), sticking (due to blockage by various impurities in the media), or poor sealing (due to worn sealing rings), resulting in excessive media flow even when the valve is fully closed, making control impossible;
The control valve packing is either too tight or too loose. If too tight, the valve stem moves sluggishly or jerks; if too loose, the medium leaks. If the medium is heavy oil, it is highly likely to ignite, causing a major accident;
During installation, the pipeline is not aligned with the valve body, subjecting the control valve to excessive additional stress, causing oscillation and preventing it from fully opening or closing;
Loose fastening nuts on the control valve stem and connecting components cause misalignment between the stem and the valve plug, resulting in the valve failing to close completely. This increases the stress on the valve stem, leading to high-frequency vibration and even fracture;
Failure of the control valve diaphragm assembly: Due to prolonged use, the diaphragm ages and deteriorates, losing elasticity and compromising its seal, resulting in air leaks. Additionally, the compression spring may age, lose elasticity, and break, preventing the valve from fully opening or closing-or even causing it to lose control entirely. Furthermore, if the valve plug falls out, becomes jammed against the seat, or the valve stem bends or breaks, the valve may operate normally but fail to perform its regulating function.