Valve positioner common faults and diagnosis

Valve positioner, according to the structure can be divided into: pneumatic valve positioner, electro-pneumatic valve positioner and intelligent valve positioner, is the main accessory of the regulating valve, usually used in conjunction with the pneumatic regulating valve, it accepts the output signal of the regulator, and then its output signal to control the pneumatic regulating valve, when the regulating valve is actuated, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the status of the valve position is transmitted through the electrical signal to the The status of valve position is transmitted to the upper position system through electric signal.

Valve positioner as the most precise parts of the entire regulating valve, the incidence of failure is also the most, once the failure occurs, it will affect the operation of the entire system, so, for the field maintenance staff, to learn more about some of the knowledge in this area, is very necessary!

Fisher DVC6200 Pneumatic Valve Positioner

1. There is an input signal but the output is very small or no
   • Valve positioner due to travel fine-tuning screws adjusted too much, resulting in torque motor coil desoldering, weld on the lead can be.
   • Torque motor coil internal broken wire or due to overcurrent burned: with a multimeter Ω block to measure the coil resistance, normal should be about 250 Ω, if the deviation from 250 Ω is too large to replace the coil.
   • Signal line contact is poor: check the terminals, eliminate the loose phenomenon.
   • Signal line reversed: check (+) (-) terminal wiring to see if it is reversed.
   • Nozzle stopper position is not correct: readjust the parallelism, see the output changes.
   • Nozzle fixing screws loose: tighten the nozzle fixing screws to meet the stroke requirements.
   • Amplifier failure: check whether the amplifier failure or replacement.
   • Gas resistance blockage: Ф0.12 through the needle to remove the dirt can be.
   • Exhaust hole blockage: positioner base center, there is a nozzle exhaust hole, if you do not pay attention to its blockage, the positioner will stop working.
   • Deformation or breakage of the spring connected to the baffle lever; open the cover of the positioner and replace it.
   • The permanent magnet is placed in the wrong position change the magnetic pole of the permanent magnet, see whether the valve operates.
   • Feedback lever off: readjust the parallelism, see the valve action.
   • Feedback lever range fixed pin deviation: adjust the pin to meet the stroke requirements.
   • Adjusting valve with handwheel: check the position of handwheel and readjust it to the middle position.
   • Loose cam or improper position: tighten the cam or readjust the cam position.
   • Insufficient stiffness of the spring connected to the baffle lever: change the (+) (-) polarity wiring, adjust the distance between the baffle and the nozzle to meet the stroke requirements (this time, it is necessary to change the regulator mode of action).
2. Output pressure oscillation
   • Dirt in the amplifier: Remove dirt from the amplifier.
   • Output pipeline or membrane head air leakage: eliminate air leakage phenomenon, so that the valve action is smooth.
   • Diaphragm head diaphragm aging: replace the aging diaphragm can be.
   • The position of the permanent magnet is not correct: readjust the parallelism of the permanent magnet to eliminate the instability of the magnetic circuit.
   • Fixed feedback lever screws loose: tighten the fixed screws to eliminate valve vibration.
   • Input signal AC component is large: eliminate the AC component or in parallel with the input side of a capacitor. Filter out the AC interference.
   • Back pressure airway has dirt: eliminate dirt, troubleshooting.
   • Valve bar radial loosening: overhaul the regulating valve can be.
3. No input and output
   • Back pressure partially clogged: remove the clogging dirt.
   • Inappropriate position of “automatic and manual switching switch" Turn “automatic and manual switching switch" to “automatic" position in clockwise direction.
4. Poor precision
   • Nozzle, gear plate adjustment is not good: re-adjust the parallelism or nozzle fixing screws to meet the accuracy requirements.
   • Back pressure air leakage; eliminate air leakage.
   • Radial displacement of the regulating valve: overhaul the regulating valve.
   • Improper adjustment of the zero screw: readjust the zero screw to meet the accuracy requirements.
   • Feedback lever and fixed pin position is not correct: re-adjust the pin position according to the travel requirements.

Fisher 3582i Electro-Pneumatic Valve Positioner

1. Zero, range is not allowed
   • Due to the valve positioner installation process debugging is not allowed or site vibration, temperature changes and control valve stem stroke changes, feedback lever position changes and other reasons so that the minimum opening and maximum opening of the control valve and the control room signal is not consistent. Resulting in the valve positioner output signal can not make the regulator valve fully open and closed, resulting in large leakage, limited and other phenomena. In the positioner on-site calibration should first ensure that the regulating valve action is good, the feedback system is firmly installed action is good, and then through the standard signal to adjust. So that the stroke of the control valve and the control signal is consistent.
2. Throttle hole blockage
   • Stolen material clogging the throttle hole. No output signal from the positioner, resulting in no action of the control valve.
3. Spoils between nozzle and baffle plate
   • Affected by the field environment, the positioner will be attached to a layer of dust after using for a period of time, which affects the back pressure of the nozzle baffle, thus affecting the output of the positioner. This will cause unstable condition of the regulating valve and generate oscillation.
4. Bad sealing
   • Long-term use of various fastening nuts, sealing gaskets are prone to loosening, aging phenomenon, resulting in locator leakage. So that the regulating valve can not be fully open and closed, the valve position is unstable, resulting in regulation of oscillation.
5. Feedback rod failure
   • Long-term operation of the feedback rod fastening nut gradually loosened or even fall off, resulting in the feedback rod loose, skewed, and fixed parts of the card touch, fall off. So that the regulating valve action is slow, frequent fluctuations, regulating valve limit or even loss of control. The limit spring on the feedback plate falls off, or the feedback rod comes out from it, resulting in poor contact between the feedback rod and the feedback plate, resulting in hysteresis, resulting in frequent action of the control valve. Make it difficult to stabilize the controlled parameters, especially in the control valve action requires accurate temperature control to produce a greater impact.
6. Loose fixing nut
   • Valve positioner fixing nut installation is not secure to produce loose, resulting in the positioner skewed, affecting the feedback lever action, resulting in the phenomenon of card touch. Make the regulating valve action is not stable, resulting in limit and other phenomena. The fastening screws of various springs in the positioner are loosened in the vibration environment, which changes the preload of the spring and affects the tension and state of the spring. So that the zero range of the positioner changes, the positioner is not linear, resulting in the regulator valve can not be fully open and closed, the regulator valve action is not linear.
7. Change of permanent magnet position
   • As a result of external forces, so that the position of the two magnets change, change the position of the magnetic field, is the coil force imbalance, the valve positioner output is not linear, resulting in the adjustment valve action is not linear. Magnet adsorption of impurities such as iron pins, etc., the formation of card contact to impede the movement of the baffle plate, so that the output of the positioner is not allowed, so that the regulator valve action is inconsistent with the control signal.

Valmet Neles ND9206he1t Intelligent Valve Positioner

1. Slow output speed of valve positioner
   • Reason: 1) Input gas chamber leakage; 2) constant throttle hole blocking phenomenon; 3) gas source pipeline leakage; 4) baffle plate has carved or poor position.
   • Measures to be taken: 1) replace the damaged diaphragm; 2) clean the throttle hole; 3) detect the leakage and deal with it; 4) replace the baffle plate or readjust it.
2. Valve positioner no output pressure (input signal after the baffle does not act)
   • Reason: 1) Positive and negative level wiring error; 2) Gas source five pressure; 3) Magnetoelectric component coil disconnection.
   • Measures to be taken: 1) rewire; 2) input the specified pressure of the gas source; 3) connect the coil disconnection.
3. Valve positioner no output pressure (input signal baffle action)
   • Reason: 1) the gap between the nozzle baffle is too large; 2) the baffle surface is scratched; 3) the zero position is not properly adjusted; 4) amplifier airway blockage; 5) nozzle throttle hole blockage; 6) zero spring is too soft.
   • Measures to be taken: 1) loosen the fastening screw, in the input of the maximum signal to adjust the gap between the nozzle baffle, so that the pressure gauge value of 0.1 ~ 0.11Mpa, after fastening the screw, in the negative check once; 2) replace the baffle; 3) input the minimum signal, turn the zero screw to make the towing frame to the left, tighten the zero spring until the actuator begins to act; 4) check the air circuit; 5) A: use a thin steel wire to dredge B: Replace the filter element of the air filter pressure reducing valve; 6) Replace the zeroing spring.
4. The output pressure of the positioner does not decrease after the input current signal changes.
   • Reason: 1) installation position is not appropriate, so that the gap between the nozzle and the baffle is too small; 2) nozzle end face or baffle and nozzle contact parts have stolen goods, resulting in a reduction in the actual gap; 3) adjustment is not tightened after the screw on the nozzle base, so that the gap changes; 4) constant air flow hole parts off, resulting in a throttle hole becomes larger.
   • Measures to be taken: 1) readjust the gap between the nozzle and the baffle; 2) remove the nozzle end face or the baffle and the nozzle contact parts of the spoils; 3) tighten the screws on the base of the nozzle; 4) check the constant throttle orifice, and re-install the off parts.
5. Zero misalignment
   • Reason: 1) Zeroing screws have loosening; 2) input gas chamber diaphragm leakage; 3) travel adjustment after the fasteners loose; 4) cam loosening.
   • Measures: 1) tighten the zero screw and readjust the zero point; 2) tighten the amplifier screw and replace the damaged diaphragm; 3) re-tighten the stroke adjustment piece; 4) tighten the cam.
6. Positioning output misalignment phenomenon
   • Reason: 1) input air chamber leakage; 2) cam loose; 3) main lever pivot plate, spring plate screws loose.
   • Measures to be taken: 1) replace the broken parts or tighten the seal leakage; 2) retighten the cam; 3) retighten the screws.