Debugging steps and principles of the valve positioners

The valve positioner is the main accessory of the control valve. It takes the valve stem displacement signal as the input feedback measurement signal, and uses the controller output signal as the setting signal for comparison. When there is a deviation between the two, the output to the actuator is changed. The signal makes the actuator act, and the one-to-one correspondence between the displacement of the valve stem and the output signal of the controller is established. This article focuses on explaining common locator debugging steps to help instrument personnel easily master various locators.

The principle and function of valve positioner

The valve positioner is the main accessory of the control valve. It takes the valve stem displacement signal as the input feedback measurement signal, and uses the controller output signal as the setting signal for comparison. When there is a deviation between the two, the output signal to the actuator is changed to make the actuator act, and the valve is established. The one-to-one correspondence between the rod displacement and the controller output signal. Therefore, the valve positioner constitutes a feedback control system with the stem displacement as the measurement signal and the controller output as the set signal. The manipulated variable of the control system is the output signal of the valve positioner to the actuator.

(1) It is used for important adjustment systems with high requirements for adjustment quality to improve the positioning accuracy and reliability of the adjustment valve.

(2) It is used in the occasions where the pressure difference between the two ends of the valve is large (△p>1MPa). By increasing the air source pressure, the output force of the actuator is increased to overcome the unbalanced force generated by the liquid on the valve core and reduce the stroke error.

(3) When the medium to be adjusted is high temperature, high pressure, low temperature, toxic, flammable and explosive, in order to prevent external leakage, the packing is often pressed very tightly, so the friction between the valve stem and the packing is large. Time lag can be overcome with a locator.

(4) When the medium to be adjusted is a viscous fluid or contains suspended solids, the positioner can overcome the resistance of the medium to the movement of the valve stem.

(5) It is used for regulating valve with large diameter (Dg>100mm) to increase the output thrust of the actuator.

(6) When the distance between the regulator and the actuator is more than 60m, the positioner can overcome the transmission delay of the control signal and improve the response speed of the valve.

(7) Used to improve the flow characteristics of the regulating valve.

(8) When one regulator controls two actuators to implement split-range control, two positioners can be used to accept low input signals and high input signals respectively, then one actuator operates at low range and the other moves at high level, which constitutes a split program adjustment.