Views: 241 Author: Site Editor Publish Time: 2021-02-26 Origin: Site
The valve positioner is the main accessory of the control valve. It uses the valve stem displacement signal as the input feedback measurement signal and the controller output signal as the setting signal for comparison. When there is a deviation between the two, change its output signal to the actuator to make the actuator act. Then establish a one-to-one correspondence between the valve stem displacement and the controller output signal. Control valves are mainly divided into air actuated control valve and electrically operated control valve. This article focuses on explaining common locator debugging steps to help instrument people easily master various locators.
The valve positioner constitutes a feedback control system that uses the displacement of the valve stem as the measurement signal and the output of the controller as the setting signal. The manipulated variable of the control system is the output signal from the valve positioner to the actuator.
It is used in important adjustment systems that require high adjustment quality to improve the positioning accuracy and reliability of the adjustment valve.
It is used in occasions where the pressure difference between the two ends of the valve is large. By increasing the air source pressure, the output force of the actuator is increased, the unbalanced force generated by the liquid on the valve core is overcome, and the stroke error is reduced.
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 tightly, so the friction between the butterfly valve lever or ball valve stem and the packing is relatively large, and the time lag can be overcome with a positioner at this time.
When the adjusted medium is viscous fluid or contains solid suspended matter, the positioner can overcome the resistance of the medium to the movement of the valve stem.
It is used for large-diameter (Dg>100mm) regulating valves to increase the output thrust of the actuator.
When the distance between the regulator and rack & pinion actuator is more than 60m, the positioner can overcome the transmission delay of the control signal and improve the response speed of the valve.
It is used to improve the flow characteristics of the regulating valve.
When one regulator controls two actuators to implement split-range control, two positioners can be used to accept low and high input signals respectively. One electric pneumatic actuator performs low-range action, and the other performs high-range action, which constitutes split-range adjustment.
Valve positioners are classified into pneumatic valve positioners, electrical valve positioners and intelligent valve positioners according to input signals.
According to the direction of action, it can be divided into one-way valve positioner and two-way valve positioner.
According to the gain sign of the valve positioner output and input signal, it is divided into positive acting valve positioner and reverse acting valve positioner.
According to whether the input signal of the valve positioner is analog signal or digital signal, it can be divided into ordinary valve positioner and fieldbus electric valve positioner.
According to whether the valve positioner has CPU or not, it can be divided into ordinary electric valve positioner and intelligent electric valve positioner.
It can also be classified according to the detection method of the feedback signal. For example, a valve positioner that uses a mechanical linkage to detect valve position signals. A valve positioner that uses the Hall effect to detect the displacement of the valve stem. In addition, an electromagnetic induction method is used to detect the valve stem displacement of the valve positioner.
