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Hi, I’m the Industrial Control Assistant who focuses on serious learning today. I came across some very basic knowledge about valves and couldn’t help but want to share it. Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce the delay in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome the frictional forces acting on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. Common actuators include pneumatic actuators and electric actuators, which can be further divided into linear stroke and rotary stroke types. Pneumatic control valves such as various shut-off valves and dampers, which are used for automatic or manual opening and closing, operate using compressed air as a power source, with cylinders serving as actuators. Accessories such as electrical valve positioners, converters, solenoid valves, and hold-down valves are used to drive these valves, enabling on/off or proportional control. They receive control signals from industrial automation systems in order to regulate various process parameters such as the flow rate, pressure, and temperature of the fluid flowing through the pipes. Pneumatic control valves are characterized by simple control, fast response, and intrinsic safety, eliminating the need for additional explosion-proof measures. Features of pneumatic motors: Pneumatic motors are prime movers that use compressed air as their working medium. They are power devices that convert pressure energy into mechanical energy by utilizing the expansion of compressed gas. The pneumatic motor of the valve positioner allows for stepless speed control. By controlling the opening degree of the intake valve or exhaust valve, that is, by controlling the flow rate of compressed air, it is possible to adjust the motor’s output power and speed. This allows for the adjustment of speed and power. It can rotate forward as well as backward. Most pneumatic motors can achieve forward and reverse rotation of their output shaft, as well as instant reversal, simply by using a control valve to change the direction of air intake and exhaust. During forward and reverse conversion, the impact is minimal. A major advantage of the reversing operation of a pneumatic motor is its ability to reach full speed almost instantly. A vane air motor can reach full speed in one and a half rotations; a piston air motor can reach full speed in less than a second. By using a control valve to change the intake direction, forward and reverse rotation can be achieved. It achieves forward and reverse rotation in a short time, at high speed, with minimal impact, and without the need to unload the load. The pneumatic control section of a mechanical valve positioner is mechanical; it controls the opening degree of the valve through the combined action of an electromagnet and a spring, which are controlled by a given signal. Its working principle is as follows: compressed air enters the backpressure chamber of the nozzle through a fixed throttle, and then exits through the gap between the nozzle and the baffle. When the coil of the torque motor receives an electrical signal of 4–20 mA, the armature rotates from left to right under the influence of the magnetic field, centered around the pivot plate spring; this reduces the gap between the baffle and the nozzle, thereby increasing the backpressure at the nozzle. As a result, the valve element of the control valve moves upward due to the increased pressure in the air supply chamber. The valve opens, and at the same time the valve feedback rod causes the piston diaphragm rod to rotate, increasing the tension in the feedback spring and thus expanding the gap between the baffle and the nozzle, so that the valve opening remains at an equilibrium level. Issues to be noted when using the valve positioner include: due to the use of mechanical mechanisms, its transmission efficiency is poor, which results in delayed valve operation and an inability to effectively regulate the variable being controlled. This results in low sensitivity. Furthermore, the difference between the set value and the feedback value for the valve positioners should be less than 5%. However, after the positioners have been in use for some time, this difference often exceeds 5%, preventing production staff from knowing the actual opening degree of the valves, which may lead to incorrect operations and affect production. There is also the issue of prolonged calibration time. This valve positioner is a non-intelligent type, and it cannot be calibrated automatically; manual calibration is required. The calibration process usually takes about half an hour, or even longer. Additionally, the valve needs to be opened and closed frequently, which delays the resumption of production. That’s all for today. If you have any questions, feel free to leave a message; I will see them. Updates will continue, so please stay tuned.