United States Fisher Pneumatic Valve Positioner is the primary accessory to the Pneumatic Control Valve, which is a single-output valve positioner. It describes the main features and common faults of the valve positioner, as well as the treatment methods and protection methods.
1. The main features of the locator
(1) The large-diameter pilot-operated relay eliminates the blockage of the air path and makes the control valve move fast.
(2) Do not replace the parts when changing the mode of action, just change the installation position of the relay.
(3) The flow characteristics of the control valve can be changed by replacing the cam, and there are three characteristics: linear, equal percentage and quick opening.
(4) Sensitive and reliable, even if the working conditions often change, the performance of the control valve is still stable.
(5) A bypass component is installed, and the control valve can be used to maintain the positioner without stopping
2. Common failure causes of Fisher pneumatic positioner failure in United States
2.1 There is a loss signal, no output force
(1) The signal gas circuit of the relay is blocked
(1) The casting hole is not cast.
(2) The position of the rubber pad changes to block the signal hole.
(2) The parts that make up the relay signal cavity leak
1) Broken diaphragm.
2) The sealing surface is uneven.
3) Aging of the gasket.
(3) The relay air supply port baffle is not opened
1) The gap between the diaphragm disc and the baffle plate connected to the central shaft is too large.
2) The thickness of the diaphragm tray is too small.
3) The baffle clamp is too elastic.
(4) A large amount of air leakage from the actuator and pipeline.
2.2 The output pressure is not reduced
(1) The relay exhaust port baffle is not opened
1) The depth of the screw hole of the diaphragm disc is shallow, so that the central shaft cannot be tightened to the predetermined position.
2) The exhaust port baffle clamp is too elastic.
(2) The compression of the feedback spring is too small or the stiffness is too low.
2.3 The basic error (linearity deviation) is unqualified
(1) The accuracy of the cam is low
1) There are burrs or dirt on the cam surface.
2) The cam mounting hole is not properly positioned.
(2) Poor linear accuracy of feedback springs
1) The reed wire material is not suitable.
2) Improper heat treatment.
3) It has not been established.
(3) The zero position of the locator is not adjusted.
(4) The feedback lever at the midpoint of the stroke is not leveled, and the position of the stroke pin is inconsistent with the position of the actuator.
(5) Leakage of relay output air circuit
1) The rubber pad is aging and failing.
2) The paper gasket above the central shaft is damaged and cannot be sealed
3) The small diaphragm of the relay is not compressed
(6) The back pressure of the relay is not adjusted
1) The spacing between the two baffles is improper.
2) The baffle and nozzle cannot be sealed.
(7) The actual air supply pressure is too different from the design requirements.
(8) Air leakage from the actuator
1) Pipe joints.
2) Diaphragm.
3) The O-ring of the reaction actuator.
(9) The cam installation position is wrong, the cam installation position on page 1 of the product manual is 50-100mm stroke, and the cam position of 12-50mm stroke should rotate 180 degrees.
2.4 The return difference (variation) is unqualified
(1) The two end faces of the feedback spring are not parallel, and the spring rotates during the working process.
(2) The cam fastening screws are loose, and they should be checked frequently in the vibration environment.
(3) The feedback spring rate is too low.
1) The material is inappropriate.
2) Not heat-treated.
(4) The radial clearance and axial clearance between the shaft and the shaft sleeve are large.
(5) The riveting part of the shaft and the feedback lever hole is loose and should be welded instead.
(6) The spot welding between the rotating shaft and the cam fixing plate is loose.
(7) The gap at the rotating fulcrum of the U-shaped plate is too large.
(8) The air supply pressure is unstable.
(9) The back pressure of the relay is not suitable.
(10) The stroke pin lock nut at the feedback lever is not tightened.
2.5 The error of the locator stroke is too large
(1) The cam installation position does not meet the requirements of the manual.
(2) The position of the stroke pin in the long groove of the feedback lever is incorrect.
(3) When the feedback spring selects the wrong input signal 20-100kPa, the feedback spring with Ф2.9mm spring wire diameter is selected; when the feedback spring is 20-60kPa, the feedback spring with a length of Ф2.6mm and 42mm is selected; when the feedback spring is selected from 60-100kPa, the feedback spring with a length of 52mm is selected by Ф2.6mm; when the signal is 20-100kPa for a small stroke (6-12mm), the feedback spring with a spring wire Фmm is selected.
2.6 The base station is operating incorrectly
(1) The relay is installed in the wrong position, such as the positive acting type is installed in the RA position.
(2) The characteristics of the cam are selected incorrectly, for example, the equal percentage characteristic cam is selected for the linear flow characteristics.
(3) The bypass switch is in the wrong position, for example, the switch plate is in the bypass position when using the positioner.
There are also some faults, such as the slow action speed of the locator, no input signal, etc., because the relationship with the locator itself is not very close, so this article omits the explanation.
Protection method of Fisher valve positioner:
(1) Completely remove the dust and moisture of the locator with clean gas.
(2) If you leave the locator in the workplace for a long time and do not use it, in order to avoid rainwater entering the locator, please cover it with a sleeve. If the environment is high temperature and humidity, measures should be taken to prevent condensation in the locator. The product must be protected from condensation until it is shipped for export.
(3) The locator must be taken care of during operation and transportation. Do not subject the base station to strong shocks and vibrations.
(4) If the positioner is used at a temperature other than the specified temperature, the sealing material will quickly degrade and the positioner cannot work normally, so it is recommended that the temperature should be used in strict accordance with the instructions and avoid using it at a temperature outside the regulations.
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