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Introduction to Common Fault Analysis of Intelligent Valve Positioners
Release time:
2026-04-07 15:22
Analysis of Common Faults in Smart Valve Positioners
I. The valve positioner receives a signal input but produces no output pressure signal: for electro-pneumatic positioners, there is foreign matter between the armature and the coil frame; the constant orifice is blocked; the nozzle–flapper assembly is improperly fitted or the nozzle–flapper assembly is damaged; the diaphragm in the amplifier (whether metal or rubber) is damaged; the pneumatic circuit connections are incorrect (including those to the amplifier); the positive and negative terminals of the electro-pneumatic positioner’s input signal cable are reversed; the diode inside the positioner’s input junction box is open-circuited or the wiring is faulty; the air supply pressure does not meet the specified requirements; the amplifier’s air consumption exceeds its rated value by too much; the polarity of the magnet in the electro-pneumatic positioner is installed incorrectly; the preloading force of the amplifier is excessive; the spool in the spool-type amplifier is jammed by foreign matter; the “manual/automatic” selector switch is set in an improper position (i.e., neither in the manual nor in the automatic position); the electro-pneumatic positioner’s input electrical signal is short-circuited; the balance spring has been improperly installed or adjusted.
2. Slow response of the output pressure in the pneumatic positioner: the cone angle of the amplifier’s air-cone valve is too small; the amplifier diaphragm, after prolonged use, exhibits elastic hysteresis; the sensing element of the pneumatic positioner (bellows or diaphragm chamber), after long-term operation, also develops elastic hysteresis; and the feedback spring likewise shows elastic hysteresis.
3. The upper travel positioner indicates a slow pressure change: the amplifier’s inlet ball valve is excessively seated; the amplifier has a high air consumption; the inlet ball valve is contaminated, reducing the flow area; the ratio of the diameter of the constant orifice to the nozzle diameter is less than the specified value (the value stipulated in the technical requirements); the fit between the nozzle and the baffle is poor; and there is slight friction between the armature and the coil frame.
4. Poor positioner linearity: improper selection of the feedback cam or spring; incorrect installation of the feedback mechanism; improper installation of the feedback cam or spring; contamination on the nozzle or baffle; friction between the spool and its mating surfaces in the spool-type amplifier; slight backpressure leakage; improper overall unit installation; jamming of the adjusting valve on the feedback linkage.
5. No input signal but the positioner outputs pressure: the nozzle is blocked; the amplifier’s inlet ball valve is contaminated, causing it to fail to seat properly or damaging the sealing surfaces; the diameter ratio of the constant-orifice to the nozzle diameter exceeds the rated value; there are connection problems among the amplifier’s air circuit boards; the amplifier’s metal diaphragm is deformed or improperly installed, causing the valve stem to push open the inlet ball valve (in amplifiers with non-adjustable preloading); or the baffle has already covered the nozzle.
6. Insufficient stroke (the positioner output pressure fails to reach the maximum value): the contact position between the feedback lever and the actuator pushrod connector is incorrect; the magnetic field strength generated by the permanent magnet is lower than the rated value; the baffle and nozzle are not properly matched; the initial position of the feedback cam is improperly selected; or the main lever balance spring is incorrectly installed.
2020-01-19
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