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Common Fault Types of Valve Positioners
Release time:
2026-04-07 15:22
A valve positioner receives the output signal from the controller and uses this signal to control the pneumatic control valve. Once the control valve actuates, the stem displacement is fed back to the valve positioner via a mechanical linkage, and the valve position is transmitted to the higher-level system in the form of an electrical signal. Also known as a pneumatic valve positioner, the valve positioner is a key accessory for control valves and is typically used in conjunction with pneumatic control valves.
Valve positioners operate on the principle of mechanical force balance, specifically using nozzle-flapper technology, and typically exhibit the following types of failures:
1) Due to its operation based on the mechanical force-balance principle, it has numerous moving parts that are susceptible to temperature and vibration, leading to fluctuations in the control valve.
2) The nozzle-flapper technology is employed; however, due to the extremely small nozzle orifices, they are prone to blockage by dust or an impure air supply, resulting in malfunction of the positioner.
3) Based on the principle of force equilibrium, variations in the spring’s elastic coefficient under harsh field conditions can lead to nonlinearity in the control valve, thereby degrading control performance.
4) The smart positioner consists of a microprocessor (CPU), A/D and D/A converters, and other components; its operating principle differs fundamentally from that of a conventional positioner. The comparison between the setpoint and the actual value is conducted entirely in electrical signals, rather than through force balance. Consequently, it can overcome the force-balance limitations inherent in conventional positioners. However, when used in emergency shutdown applications—such as emergency shut-off valves and emergency vent valves—these valves are required to remain stationary at a specific position under normal conditions and only to act reliably in the event of an emergency. Prolonged dwell at a fixed position can easily cause the electrical transducer to lose control, leading to the dangerous situation where small signals fail to produce any response. Moreover, the potentiometric position sensors employed in these valves operate in the field, where resistance values are prone to drift, which may result in the same hazardous condition: small signals fail to act while large signals drive the valve fully open. Therefore, to ensure the reliability and serviceability of smart positioners, they must be subjected to frequent testing.
2020-01-19
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