November 4, 2024
Valve positioner, according to the structure of the pneumatic valve positioner, electric - pneumatic valve positioner and intelligent valve positioner, is the main accessory of the regulating valve, usually used in conjunction with the pneumatic regulating valve, it accepts the output signal of the regulator, and then its output signal to control the pneumatic regulating valve, 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 status of the valve position is transmitted through the electrical signal to the upper position. system.
1. The working principle of the valve positioner
The working principle of the valve positioner is to control the pneumatic control valve by receiving the output signal from the regulator and utilizing the principle of torque balance to realize the precise control of the valve position.
Valve positioner is the main accessory of the regulator valve, usually used in conjunction with the pneumatic control valve. It accepts the output signal of the regulator and then controls the pneumatic control valve with its output signal. When the regulating valve action, the displacement of the valve stem and through the mechanical device feedback to the valve positioner, the valve position status through the electrical signal to
the upper system. The working principle of the valve positioner mainly includes the following steps:
a. Signal conversion: the electrical signal output from the controller (4 ~ 20mA) is converted to gas signal (20Kpa ~ 100Kpa) through the electrical converter, which acts on the bellows.
b. torque balance: the signal pressure acts on the bellows, resulting in lever movement, which in turn affects the nozzle back pressure. After amplified by the pneumatic amplifier, a part of the air source is sent to the air chamber of the pneumatic membrane, which drives the valve stem and spool to move downward, gradually reducing the valve opening.
c. Feedback adjustment: valve stem displacement through the feedback lever feedback to the valve positioner, connected to the eccentric cam and the feedback spring for stretching or compression, in order to achieve torque balance, so that the valve is fixed in a certain position.
This process forms a closed-loop control system, ensuring precise control and stabilization of the valve position. Valve positioners play an important role in various industrial automation process control fields, improving the positioning accuracy and reliability of control valves.
2. The role of the valve positioner
The main role of the valve positioner includes improving the static and dynamic characteristics of the regulating valve, improving the linearity of the valve position, reducing the transmission hysteresis of the regulating signal, changing the flow characteristics of the regulating valve, realizing the split-range control, and reversing the feedback of the valve action.
Increase the action speed of the actuator, improve the dynamic characteristics of the control system, correct the flow characteristics of the regulating valve, make the piston actuator and the two-position action of the long stroke actuator into a proportional action, the use of electric/pneumatic valve positioner, can be used to operate the pneumatic actuator with a current signal of 0-10mADC or 4-20mADC, an electric/pneumatic valve positioner has an electric/pneumatic converter, which can be used for the operation of the pneumatic actuator. An electric/pneumatic valve positioner has the dual role of electric/pneumatic converter and pneumatic valve positioner.
Valve positioner is the main accessory of control valve, usually used with pneumatic control valve. It accepts the output signal from the regulator and then controls the pneumatic control valve with the output signal. When the regulating valve action, the valve stem displacement through the mechanical device feedback to the valve positioner, the valve position status through the electrical signal to the upper system. This design enables the valve positioner to increase the output power of the regulating valve, reduce the transmission hysteresis, accelerate the speed of the valve stem movement, improve the linearity, and overcome the friction and the influence of the unbalance force, thus ensuring the correct positioning of the regulating valve.
Specifically, the role of the valve positioner includes:
a. Improve the static characteristics of the control valve, improve the linearity of the valve position.
b. Improve the dynamic characteristics of the control valve, reduce the transmission lag of the adjustment signal.
c. Change the flow characteristics of the control valve.
d. To realize the split-range control, that is, a control instrument to operate two control valves.
e. So that the valve action reverse feedback.
f. Improve the dynamic characteristics of the control system by increasing the action speed of the actuator.
g. Correct the flow characteristics of the control valve.
h. So that the piston actuator and long-stroke actuator two-position action into proportional action.
i. The use of electric / gas valve positioner, you can use 0 ~ 10mADC or 4 ~ 20mADC current signal to operate the pneumatic actuator, an electric / gas valve positioner with electric / gas converter and pneumatic valve positioner dual role.
3. The classification of the valve positioner
Valve positioner can be divided into pneumatic valve positioner, electrical valve positioner and intelligent valve positioner.
a. Pneumatic valve positioner receives a standard gas signal, such as 20 ~ 100kPa gas signal, its output signal is also a standard gas signal.
This kind of positioner works according to the principle of moment balance, i.e., the pressure on the signal is adjusted by the spring.
b. Electrical valve positioner receives the standard current signal of 4~20mA, converts the input signal into electromagnetic force internally, and then outputs the gas signal to drive. The pneumatic control part of the electrical valve positioner is mechanical, relying on the given signal to control the solenoid and spring together to control the valve opening.
c. Intelligent valve positioner does not require manual correction, can be intelligently set various parameters, automatic adjustment. It is through the microprocessor to receive signals and send instructions to the piezoelectric valve, the piezoelectric valve based on the instructions to drive the amplifier, so that the valve action until the signal and feedback balance. Advantages of intelligent valve positioner include high adjustment accuracy, sensitive response, can be automatically adjusted, debugging is simple, but the price is relatively high, the cleanliness of the gas source requires a high degree of cleanliness, and the low temperature environment may cause the display screen is not sensitive.
These different types of valve positioners differ in structure, working principle and application, but the common goal is to improve the precision, reliability and accuracy of the valve, improve the performance of the valve, control the deviation generated by the work of the mechanical device, and calibration, so as to make the work more smoothly and accurately.
4. The advantages and disadvantages of the valve positioner
The advantages of the valve positioner mainly include:
a. Improve the output power: valve positioner can increase the output power of the control valve, reduce the transmission lag of the adjustment signal, accelerate the movement speed of the valve stem, thereby improving the linearity of the valve.
b. To overcome the impact of friction and unbalance: valve positioner can overcome the friction of the valve stem and eliminate the impact of unbalance, thus ensuring the correct positioning of the control valve.
c. Simplify the choice of control valves: compensation through the flow characteristics of the control valve, intelligent valve positioner or digital controller series nonlinear compensation link with soft connection function, you can easily change the nonlinear characteristics, so that the choice of control valves become simple.
d. Reduce commissioning and maintenance costs: intelligent diagnostic maintenance circuit through the software diagnostics is the direction of development of valve troubleshooting, improve process control by reducing process deviations, automatic verification and configuration, significant time savings, reduce commissioning and maintenance costs.
e. Improve the performance of the control circuit: valve positioner valve position detection using non-contact mode, can greatly improve the performance of the control circuit.
f. Increase the hysteresis link and remote communication capabilities: intelligent valve positioner can increase the advanced hysteresis link, increase the series resonance when the secondary ring time constant, staggered resonance frequency. With remote communication capability, it can realize valve monitoring and report the equipment situation to the user.
The disadvantages of the valve positioner mainly include:
a. higher prices: the price of intelligent valve positioner is relatively high, the cleanliness of the gas source requires a high degree of low-temperature environment may cause the display is not sensitive.
b. long commissioning time: the need for manual commissioning and commissioning time is long, poor environmental resistance, susceptible to temperature, external vibration, easy to wear.
c. not for all applications: due to its sensitivity to the degree of cleanliness of the gas source and low-temperature environment, intelligent valve positioner may not be suitable for all application scenarios.
In summary, the valve positioner has significant advantages in improving the performance of the control valve and the stability of the control system, but there are some limitations, which need to be selected and used according to specific application scenarios and needs.
