Regarding valve positioners

http://bbs.hcbbs.com/viewthread.php?tid=289179 This one is good

Thank you so much! There’s one more thing I don’t understand: what kind of device is the pneumatic remote control he mentioned? Is it operated manually on-site? ?

Currently, the control signals from the control room to the on-site control valves are all 4-20mA electrical signals; therefore, electrical valve positioners are used to convert these electrical signals into pneumatic signals in order to control the pneumatic control valves. In the past (before the 1970s), control instruments were pneumatic, and the control signals were also pneumatic; therefore, valve positioners were pneumatic valve positioners. The function of a valve positioner is not limited to signal conversion; its more important roles include altering the dynamic characteristics of the control valve, overcoming friction in the valve stem, increasing the speed at which the control valve operates, and enabling precise positioning of the control valve.

Apart from the different signals they receive, electrical valve positioners also have the function of performing electrical conversion; regardless of whether they are pneumatic or electric, their functions are the same； Additionally, the remote control is mounted on the dial; generally, the output of pneumatic control instruments is sent to the field via the manual and automatic switching knob on the remote control, allowing the control valve to be directly operated using the remote control to bypass the control gauge.

Use the manual/automatic switch knob on the pneumatic remote control to send signals to the site in order to control the control valve.

It is generally a gas signal of 20-100 KPA. From the electrical converter. Although it is a pneumatic signal, a pneumatic positioner still needs to be installed. Many functions can be achieved through positioners... they improve the dynamic and static characteristics of valves, enhance their operating efficiency, and it is also possible to perform functions such as split-range control using positioners

Working principle of electrical valve positioners: Electrical valve positioners are designed to operate based on the principle of force balance. They are valve positioners that control valves using electrical signal, and were developed on the basis of pneumatic valve positioners. 　When the current signal from the electric regulator is applied to the coils of the torque motor assembly, a magnetic field is generated in the air gap of the torque motor. This magnetic field acts together with the magnetic field produced by the permanent magnets, resulting in a force on the armature that pushes it to the left. The main lever (armature) rotates around pivot point 14, causing the baffle to move closer to the nozzle. The back pressure at the nozzle is amplified by an amplifier and then sent to the air chamber of the diaphragm actuator, causing the valve stem to move downward. This in turn causes the feedback rod to rotate around its pivot point. The feedback cam, which is connected to the same axis, rotates in a counter-clockwise direction. Through the rollers, this causes the secondary lever to rotate around its pivot point, stretching the feedback spring 19. When the pulling force of the spring on the main lever equals the torque exerted by the torque motor on the main lever, the mechanical lever system reaches an equilibrium state. At this point, a certain signal current corresponds to a certain valve position. The spring is used to adjust the zero position. The above mode of operation is positive action; to change the mode of operation, simply flip the cam, changing from direction A to direction B, etc. A positive-action positioner is one in which an increase in the signal current results in an increase in the output pressure ； A reactive positioner is one in which an increase in signal current results in a decrease in output pressure. A direct-acting actuator can achieve the operation of a reverse-acting actuator simply by installing a reverse-acting positioner ； On the contrary, a reaction actuator can achieve the same functionality as a direct-acting actuator simply by being equipped with a reaction positioner. When a 4–20 mA DC current is supplied to the coil by a regulator or an output safety barrier, it magnetizes the armature (i.e., the lever) located inside the coil. Since the moving arm is located within the magnetic field generated by the permanent magnet, the two magnetic fields interact to exert a deflection torque on the moving arm (lever), causing it to deflect around the center of the pivot. If the signal increases, the left side of the lever should move downward. At this point, the baffle fixed to the lever moves closer to the nozzle, increasing the back pressure of the amplifier. The amplified output air pressure acts on the diaphragm of the control valve, causing its valve stem to move downward. The displacement of the valve stem is converted into the angular displacement of the feedback shaft and feedback plate through a tie rod, and then the feedback mechanism is moved via an adjustment fulcrum. The feedback spring fixed at the other end of the lever is stretched, generating a negative feedback torque (in the opposite direction to the torque produced by the input signal), which balances the lever; simultaneously, the valve stem is stabilized in a corresponding fixed position, thereby establishing a proportional relationship between the signal current and the position of the valve stem.