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Common valve positioners need to master the working principle

Valve positioner Valve positioner is divided into structure: pneumatic valve positioner, electric valve positioner and intelligent valve positioner. It is the main accessory of the regulating valve. It is usually used with the pneumatic regulating valve. It accepts the output of the regulator. The signal is then used to control the pneumatic regulating valve with its output signal. 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 valve position condition is transmitted to the upper system through the electrical signal.
(1) Structure
The valve positioner can be divided into a pneumatic valve positioner, an electric-gas valve positioner and an intelligent valve positioner according to its structural form and working principle.
The valve positioner can increase the output power of the regulating valve, reduce the transmission hysteresis of the adjustment signal, speed up the movement speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of the imbalance force. This ensures the correct positioning of the regulating valve.

(2) Locator classification
1. The valve positioner is divided into a pneumatic valve positioner, an electric valve positioner and a smart valve positioner according to the input signal.
(1) The input signal of the pneumatic valve positioner is a standard gas signal, for example, a 20~100kPa air 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, a 4~20mA current signal or a 1~5V voltage signal, etc., the electric signal is converted into an electromagnetic force inside the electric valve positioner, and then the gas signal is output. Go to the toggle control valve.
(3) The intelligent electric valve positioner converts the current signal outputted from the control room into a gas signal that drives the regulating valve. According to the valve stem friction force when the regulating valve is operated, the imbalance force generated by the medium pressure fluctuation is offset to make the valve opening degree. Corresponds to the current signal output from the control room. And the intelligent configuration can be set to the corresponding parameters to achieve the purpose of improving the performance of the control valve.
2. According to the direction of the action, it can be divided into a one-way valve positioner and a two-way valve positioner. When the one-way valve positioner is used in a piston actuator, the valve positioner has only one direction. The two-way valve positioner acts on both sides of the piston actuator cylinder and acts in both directions.
3. According to the gain symbol of the valve positioner output and input signal, it is divided into positive acting valve positioner and reaction valve positioner. When the input signal of the 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 the valve positioner, the input signal is an analog signal or a digital signal, which can be divided into a common valve positioner and a field bus electric valve positioner. The input signal of the ordinary valve positioner is analog air pressure or current, voltage signal, and the input signal of the field bus electric valve positioner is the digital signal of the field bus.
5. According to whether the valve positioner has CPU, 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 are not intelligent and cannot handle related intelligent calculations. The intelligent electric valve positioner with CPU can handle intelligent calculations. For example, it can perform nonlinear compensation of the forward channel. The field bus electric valve positioner can also carry functional modules such as PID to realize the corresponding calculation.
6. The detection method according to the feedback signal can also be classified.
For example, a valve positioner that detects a valve position signal by a mechanical linkage method: a valve positioner that detects a displacement of a valve stem by a Hall effect detecting displacement method: a valve positioner that detects a displacement of a valve stem by an electromagnetic induction method.
(3) Working principle
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 is changed to the output of the actuator. The signal causes the actuator to act, establishing a one-to-one correspondence between the stem displacement and the controller output signal. Therefore, the valve positioner constitutes a feedback control system in which the valve stem displacement is a measurement signal and the controller outputs a set signal. The manipulated variable of the control system is the output signal of the valve positioner to the actuator.
(4) Principle of positioner operation
(1) It is used for an important adjustment system with high adjustment quality to improve the positioning accuracy and reliability of the regulating valve.
(2) For the case where the pressure difference across the valve is large (△p>1MPa). The output force of the actuator is increased by increasing the pressure of the air source to overcome the imbalance force generated by the liquid on the spool, and the stroke error is reduced.
(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. The positioner can be used to overcome the time lag.
(4) When the medium to be conditioned is a viscous fluid or contains a solid suspension, the positioner can overcome the resistance of the medium to the movement of the valve stem.
(5) For large-diameter (Dg>100mm) regulating valves 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 hysteresis of the control signal and improve the reaction speed of the valve.
(7) Used to improve the flow characteristics of the regulating valve.
(8) When a regulator controls two actuators to implement split-range control, two positioners can be used to receive low input signals and high input signals respectively, then one actuator performs low-range action and the other elevation action constitutes points. Process adjustment.
(5) Adaptation varieties
Commonly used actuators are divided into pneumatic actuators and electric actuators, which are divided into straight stroke and angular stroke. It is used to automatically and manually open and close various valves and wind panels.

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