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Based on their structure, valve positioners can be classified into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners. They are essential accessories for control valves and are usually used in conjunction with pneumatic control valves. These positioners receive the output signal from the controller and use it to control the pneumatic control valve. When the control valve moves, the displacement of its valve stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. Structure of valve positioners: Based on their structural design and working principles, valve positioners can be classified into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce delays in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome frictional forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. Classification of valve positioners: 1. Valve positioners can be classified into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners based on their input signals. (1) The input signal of a pneumatic valve positioner is a standard pneumatic signal, such as a 20–100 kPa pneumatic signal, and its output signal is also a standard pneumatic signal. (2) The input signal for electrical valve positioners is a standard current or voltage signal, such as a 4–20 mA current signal or a 1–5 V voltage signal. Inside the electrical valve positioner, this electrical signal is converted into electromagnetic force, which is then used to generate a pneumatic signal that operates the control valve. (3) The intelligent electrical valve positioner converts the current signal generated in the control room into a pneumatic signal used to drive the control valve. It compensates for the unbalanced forces resulting from fluctuations in medium pressure, due to the frictional force on the valve stem during operation, thereby ensuring that the valve opening corresponds to the current signal sent from the control room. Furthermore, intelligent configuration can be used to set the corresponding parameters, thereby improving the performance of the control valve. 2. Based on the direction of operation, they can be divided into single-direction valve positioners and double-direction valve positioners. When a one-way valve positioner is used with a piston actuator, the valve positioner functions in only one direction; whereas a two-way valve positioner acts on both sides of the cylinder of the piston actuator, functioning in two directions. The common types of actuators used for valve positioners are pneumatic actuators and electric actuators, which can be further divided into straight-stroke and angle-stroke types. Used to automatically or manually open and close various valves, dampers, etc. Functions of valve positioners: 1. Improve the static characteristics of control valves and enhance the linearity of the valve position. 2. Improve the dynamic characteristics of the control valve and reduce the transmission lag of the control signal. 3. Change the flow characteristic of the control valve. 4. Change the response range of the control valve to the signal pressure to achieve step control. 5. Reverse the valve’s operation. Function of electrical valve positioners: 1. When the fluid is viscous or contains solid particles suspended in it, using a positioner can reduce the resistance exerted by the fluid on the movement of the valve stem. 2. Improve the flow characteristics of the control valve. 3. Electrical valve positioners are capable of overcoming the delay in the transmission of control signals, thereby improving the response speed of the valve. 4. For control systems with high requirements for control quality, it is necessary to improve the positioning accuracy of control valves and enhance their reliability. 5. It should be installed in locations where the pressure difference across the valve is greater than 1 MPa; by increasing the air pressure, the output force of the actuator is enhanced, which helps to overcome the opposing forces exerted by the fluid on the valve core and reduces errors during operation. 6. When a single regulator controls two actuators, two positioners can be used – one for low-level operation and the other for high-level operation – to receive low input signals and high input signals respectively, thereby achieving stepped control. 7. To prevent external leakage, when the flowing fluid is at high temperature, high pressure, low temperature, or is toxic, flammable, or explosive, the filler is usually compressed very tightly, thereby increasing the friction between the valve stem and the filler; in such cases, an electrical valve positioner can be used to overcome any delay in response. Function of pneumatic valve positioners: 1. Increase the thrust of the actuator. 2. Increase the speed of movement of the actuator. 3. Improve the flow characteristics of the control valve. 4. Achieve step control. In split-range control, two positioners are operated by one controller, with the operating range of each positioner determined by the split points. The compressed air supplied to the valve positioner must be air that has been filtered and cleaned. Components removed during work must be properly reinstalled after adjustment is complete. Be careful not to subject the magnetic device to vibrations or excessive force, as this can damage the components and reduce its performance. The number of times the nozzle baffle device is removed should be minimized.