1. Definition
A valve positioner is a device used to precisely control the position of a valve in an industrial process control system. It is an important component that works in conjunction with a control valve to ensure precise regulation of fluid flow, gas flow, or other process variables.
2. Working Principle
Receiving Signals
A valve positioner receives an input signal from a controller, such as a pneumatic signal of 3 to 15 psi in a pneumatic system or an electrical signal of 4 to 20 mA in an electrical system. This signal represents the desired position of the valve.
Compare and Adjust
It then compares the received signal with the actual position of the valve. This is generally done with position sensors on the valve stem that detect the exact position of the valve.
If there is a difference between the desired position indicated by the input signal and the actual position of the valve, the valve positioner takes corrective action. For pneumatic valves, it may adjust the air pressure supplied to the valve actuator to move the valve to the correct position; for electric valves, it may adjust the current or voltage supplied to the motor or actuator to achieve the appropriate valve position.
3. Functions and Benefits
Accuracy
It can significantly improve the accuracy of valve positioning. This is critical in processes that require precise flow control, such as chemical manufacturing, pharmaceutical production, and power generation. For example, in chemical reactors, the exact amount of feed reactants needs to be precisely controlled, and the valve positioner ensures that the valve regulating the flow of these reactants is opened or closed with the required accuracy.
Response Speed
The valve positioner can improve the speed of the valve's response to changes in the control signal. When the desired flow or other process variables change suddenly, the positioner can quickly move the valve to the new desired position, minimizing the lag time in the process control loop. This is beneficial in applications that require rapid adjustments, such as certain types of automated manufacturing processes.
Overcoming Friction and External Forces
In industrial environments, the moving parts of the valve may encounter friction or be affected by external forces such as vibration and pressure fluctuations. The valve positioner can continuously adjust the force of the actuator to overcome these factors and maintain the correct valve position. For example, if the valve is located in an area with significant vibration, the positioner will work hard to keep the valve stable and in the proper position despite the interference.
4. Types
Pneumatic Valve Positioner
It is used in conjunction with pneumatic control valves. Working on a pneumatic signal, using air pressure to adjust the position of a valve. Pneumatic valve positioners are often used in applications where pneumatic systems are already present, such as in some older industrial plants, or in processes where using compressed air is convenient and cost-effective.
Electrical valve positioner
Electrical valve positioners receive an electrical signal (usually 4 to 20 mA) and use electrical means such as a motor or solenoid to adjust the position of a valve. In modern industrial applications, they are often preferred when precise electrical control is possible and integration with digital control systems is easier. Electrical valve positioners are often used in applications such as advanced manufacturing plants, where high-tech equipment requires precise valve control in order to operate efficiently.
Smart valve positioner
This is a more advanced version of the valve positioner. Having digital communication capabilities, such as fieldbus protocols (e.g. Profibus, Foundation Fieldbus), allows them to communicate with other devices in the control system (such as controllers and sensors). Smart valve positioners can provide additional features such as diagnostics, self-calibration, and remote monitoring and control. They are increasingly used in modern industrial processes that want increased automation and intelligent control.
