The principle and function of valve positioner
The principle and function of valve positioner
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 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 establish the valve One-to-one correspondence between the rod displacement and the controller output signal. Therefore, 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.
(1) It is used in important adjustment systems that require high adjustment quality to improve the positioning accuracy and reliability of the adjustment valve.
(2) Used in occasions where the pressure difference between both ends of the valve is large (△p>1MPa). Increase the output force of the actuator by increasing the air source pressure to overcome the imbalance 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 tightly, so the friction between the valve stem and the packing is relatively large. Use the positioner to overcome the time lag.
(4) When the medium to be adjusted is viscous fluid or contains solid suspended matter, 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 lag 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, and then one actuator will move at low range and the other will move at high range. Process adjustment.
Classification of valve positioners
1. Valve positioners are classified into pneumatic valve positioners, electro-pneumatic valve positioners and intelligent valve positioners according to input signals.
(1) The input signal of the pneumatic valve positioner is a standard gas signal, for example, a 20~100kPa gas signal, and its output signal is also a standard gas signal.
(2) The input signal of the electric valve positioner is a standard current or voltage signal, for example, 4-20mA current signal or 1~5V voltage signal, etc. The electric signal is converted into electromagnetic force inside the electric valve positioner, and then the gas signal is output To toggle the control valve.
(3) The intelligent electric valve positioner converts the current signal output from the control room into the gas signal for driving the regulating valve. According to the friction force of the valve stem when the regulating valve is working, it can offset the unbalanced force generated by the fluctuation of medium pressure and make the valve open. Corresponds to the current signal output from the control room. And can carry on the intelligent configuration to set up the corresponding parameter, reach the goal of improving the control valve performance.
2. According to the direction of action, it can be divided into one-way valve positioner and two-way valve positioner. When the one-way valve positioner is used in a piston type actuator, the valve positioner only works in one direction, and the two-way valve positioner acts on both sides of the cylinder of the piston type actuator and works in two directions.
3. According to the gain symbol of the valve positioner output and input signal, it is divided into positive-acting valve positioner and counter-acting valve positioner. When the input signal of a positive-acting valve positioner increases, the output signal also increases, so the gain is positive. When the input signal of the reaction valve positioner increases, the output signal decreases, so the gain is negative.
4. 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. The input signal of ordinary valve positioner is analog air pressure or current, voltage signal, and the input signal of field bus electrical valve positioner is field bus digital signal.
5. According to whether the valve positioner has CPU or not, it can be divided into ordinary electric valve positioner and intelligent electric valve positioner. Ordinary electric valve positioners do not have a CPU, so they have no intelligence and cannot handle related intelligent calculations. The intelligent electric valve positioner has a CPU, which can process related intelligent calculations, for example, it can perform nonlinear compensation of the forward channel, and the fieldbus electric valve positioner can also be equipped with PID and other functional modules to realize the corresponding calculations.
6. It can also be classified according to the detection method of the feedback signal. For example, valve positioners that use mechanical linkage to detect valve position signals: valve positioners that use Hall effect to detect displacement of valve stems; valve positioners that use electromagnetic induction to detect valve stem displacement.