The
valve positioner is the key accessory of the
control valve, which plays a vital role in the
control valve system. The core function of the valve positioner is to establish the precise correspondence between the displacement of the valve stem and the output signal of the controller. Specifically, the valve positioner uses the displacement signal from the valve stem as a feedback measurement signal and compares the set signal from the controller as a target signal. When there is a deviation between the two, the positioner will adjust its output signal to the actuator, and drive the actuator to make the corresponding action, so that the position of the valve matches the set signal.
Main Functions of the Valve Positioner
Valve positioners are suitable for systems that require high quality regulation. By precisely controlling the position of the valve, the positioner can significantly improve the positioning accuracy and reliability of the regulator, and ensure the stability and control effect of the system. Overcome high pressure difference
In the case of large pressure difference between the two ends of the valve (Δp > 1MPa), the positioner increases the output force of the actuator by increasing the air source pressure, so as to overcome the unbalanced force generated by the liquid on the valve core, reduce the stroke error, and ensure the normal operation of the valve.
When the medium to be adjusted has the characteristics of high temperature, high pressure, low temperature, toxic, flammable or explosive, in order to prevent external leakage, the packing is often pressed tightly, increasing the friction between the valve stem and the packing. At this time, the positioner can effectively overcome this friction and reduce the time delay of the system.
When the medium is a viscous fluid or contains a solid suspension, the medium will cause additional resistance to stem movement. The positioner can overcome these resistance, ensuring smooth and precise valve action.
For large diameter (Dg > 100mm) regulating valves, the positioner can improve the control performance of the valve by increasing the output thrust of the actuator to ensure the stable operation of the large diameter valve.
When the distance between the regulator and the actuator exceeds 60 meters, the control signal may lag during transmission. The positioner can effectively overcome this hysteresis phenomenon and improve the response speed and control accuracy of the valve.
The valve positioner can optimize the flow characteristics of the regulating valve by adjusting the opening of the valve, thereby improving the overall performance of the system.
In the case of two actuators controlled by one regulator, two positioners can be used to receive low input signals and high input signals respectively. One actuator is responsible for the low range action and the other is responsible for the height action, so that the split range adjustment can be adapted to more complex control needs.
Input and output signals: The input and output signals of the pneumatic valve positioner are pressure signals, and the common range is 20~100kPa. The pneumatic signal is controlled by the pressure signal provided by the air source. This type of positioner is usually used in the pneumatic control system, suitable for the application of the pneumatic actuator, such as the regulator system in the petrochemical, chemical and other industries.
Electrical valve positioners accept current or voltage signals, such as a 420mA current signal or a 15V voltage signal. The interior converts electrical signals into pneumatic signals. An electromagnetic device inside the positioner converts the electrical signal into a pneumatic signal, which drives the action of the control valve. This conversion usually relies on a solenoid valve or electromagnetic actuator to complete. It is widely used in systems that require electrical signal control, such as electrical automation systems and industrial process control.
Intelligent electric valve positioners not only convert current signals into gas signals, but also dynamically adjust them based on real-time feedback. It utilizes built-in intelligent algorithms to optimize valve performance, adjusting for deviations due to stem friction and medium pressure fluctuations. This type of locator usually has user configuration and self-learning functions, and can be adjusted and optimized by software to improve the control accuracy. It is suitable for applications requiring complex control and optimization, such as high-precision industrial process control and advanced manufacturing automation systems.
In piston actuators, one-way valve positioners are controlled in only one direction and are suitable for simple adjustment requirements, often used in simpler control systems such as basic flow control applications or simple on/off control systems.
The bidirectional valve positioner can be controlled in both directions of the piston actuator, providing bidirectional adjustment capability. The positioner can adjust the actuator in both directions and is suitable for control systems that require two-way action. It is used in systems that require complex regulation, such as industrial process control with forward and reverse control requirements, to provide higher accuracy.
When the input signal increases, the output signal also increases, and the gain is positive. This kind of positioner shows good performance when responding to the demand of forward control. Positive acting valve positioners are suitable for control systems that require a linear forward response, such as process control applications that require consistent regulation.
When the input signal increases, the output signal decreases and the gain is negative. Suitable for systems requiring negative response. Reverse valve positioners are suitable for applications where reverse control is required, such as certain reverse regulated process controls, providing reverse regulation characteristics.
Without a central processing unit (CPU), its function is relatively simple, unable to carry out complex intelligent computing and processing. Suitable for applications where basic control functions are required, such as traditional process control systems.
Equipped with CPU, it can carry out complex intelligent calculation and data processing, such as nonlinear compensation of forward channel, PID control, etc. It has strong processing capacity and flexibility. Intelligent electrical valve positioners are suitable for applications that require advanced control and optimization, such as complex industrial process control that requires real-time adjustment and nonlinear compensation.
By understanding the classification, function and application of different types of positioners, users can choose the most suitable valve positioners according to actual needs to optimize control performance and improve the stability of the system. Whether pneumatic, electrical or intelligent electrical valve positioners, their respective characteristics and advantages make them play an important role in specific applications. Mastering this knowledge can not only improve the accuracy of valve regulation, but also effectively respond to various working conditions, so as to ensure the smooth progress of industrial processes and efficient operation of the system. I hope that through the detailed introduction of this article, you can better understand and apply the valve positioner, and improve the performance of the overall control system.
