I have a few basic questions regarding control valves, electrical converters, and valve positioners
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This post was last edited by oiloper on 2016-3-15 at 12:18. 1. Electrical converter: According to available information, in control systems using control valves, electrical converters are generally used in conjunction with pneumatic valve positioners; however, they can also be connected directly to the control valves. Since the electrical converter outputs a standard air signal (20-100 KPa), should there be a one-to-one relationship between the spring force of the valve and the air signal output by the electrical converter? Moreover, the spring inside such a control valve must have a preset tension (is there any requirement regarding the preset tension of the spring in control valves equipped with valve positioners?) ), and at 20 KPa the pressure generated by the film (or cylinder) is exactly equal to the preloading force of the spring, while at 100 KPa it is exactly equal to the elastic force generated when the spring is compressed with the valve fully closed (or open). Are there any applications today where valves are controlled directly by electrical converters? Could you give an example? 2. Mechanical valve positioners: Mechanical valve positioners adjust the air pressure entering the control valve by changing the distance between the baffle and the nozzle. So, in a stable state, does the nozzle continuously exhaust air outward? If the input signal increases suddenly by a large amount, won’t the baffle end up pressing directly on the nozzle? 3. Where exactly is the actual cam inside the locator, and how is it installed?
Electrical converters are generally used in conjunction with pneumatic valve positioners; it’s not usually the case that they are used this way – electrical converters directly control the actuator
This post was last edited by jujiangliu on 2016-3-15 at 14:21. The input signal for pneumatic diaphragm control valves is typically a pneumatic signal of 20–100 kPa. Modern control instruments are electric in nature and generate 4–20 mA current signals; therefore, it is necessary to convert these electric signals into pneumatic signals in order to control the control valves. For smaller control valves, an electric/pneumatic converter can be used to convert electrical signals into pneumatic signals in order to control the valve (if stable control cannot be achieved using a valve positioner, an electric/pneumatic converter may be considered) ； Most pneumatic control valves require an electric/pneumatic valve positioner for control. The spring of the control valve needs to have its preload adjusted to meet the requirements of the 20–100 kPa control signal. The second point you mentioned is correct.
Electrical converters are generally used in conjunction with pneumatic valve positioners; they convert electrical signals into pneumatic signals of 0.2–1.0 BAR to control the pneumatic valve positioner, which then outputs the corresponding pneumatic signal to control the control valve. I have come across electrical converters that directly control cylinders, which are used in centrifuges to control the filter cake detector of those centrifuges. That cylinder operates at 0.2–1 BAR. The cam of the mechanical positioner can control the forward and reverse action of the positioner.
1. Electrical converters are generally used in conjunction with pneumatic valve positioners; nowadays, valve positioners that operate using electrical signals are commonly used. 2. Mechanical positioners consume air supply, while the nozzle is always supplied with air. 3. The cam of the mechanical positioner is connected to the valve’s feedback rod, and the installation of the cam is adjusted according to the way the valve operates!
I have the same question. As far as I understand it now, if only EP is used without a locator, it’s similar to open-loop control
It’s possible to do without a positioner; the EP signal can directly control the actuator, though the effect may be slightly inferior.