Issues with air-open and air-close operation of control valves, as well as the forward and reverse action shown on the screen
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There is a question regarding Zhejiang University’s ZHKD DCS system: during configuration, the control valves can be set to air-open/air-close or positive output/negative output, depending on the actual conditions of the valves on site. I would like to know what purpose the terms \"positive action\" and \"negative action\" serve in the flow diagram and on the circuit display. What do they do? If the signal sent differs from that required by the valve on site, can we directly change the positive/negative action setting for that circuit?
The principle of action and reaction is related to automatic control, and it has nothing to do with whether a valve operates on air opening or air closing principle. Positive output means that 100% corresponds to 20MA, while negative output means that 100% corresponds to 4MA. Generally, positive output is chosen for valves that operate on air opening principle, and negative output for those that operate on air closing principle. Of course, the positioner also needs to be taken into consideration; the Logic 3200 can be set so that 4MA corresponds to full open or 20MA also corresponds to full open
This damn issue of action and reaction has been elevated to various theoretical levels, creating a lot of unnecessary complications. It’s easy enough to determine whether it’s positive or negative action – just search on Baidu; I’m too lazy to explain it. There’s no need for any concepts like negative feedback; things only get more complicated. All you need to do is bring the process parameters back to their original values. Here’s something I’ve written myself: PV: Instantaneous measurement value; SV: Set value; MV: Valve opening degree. When PV > SV, for FC/FLC valves, the valve needs to open more – this is positive action; it needs to close more – this is negative action. For FO/FLO valves, when PV > SV, the valve needs to open more – this is negative action; it needs to close more – this is positive action. In flow control circuits, when the sensing element and the valve are in the same pipeline, an increase in flow leads to a decrease in valve opening degree. In temperature control circuits, when the sensing element and the valve are in the same pipeline, during heating, an increase in temperature leads to a decrease in valve opening degree; during cooling, an increase in temperature leads to an increase in valve opening degree. In level control circuits, when the sensing element and the valve are in the same pipeline, for bottom outlet pipelines, an increase in level leads to an increase in valve opening degree; for upper inlet pipelines, an increase in level leads to a decrease in valve opening degree. In pressure control circuits, when the sensing element is located before the valve, an increase in pressure leads to an increase in valve opening degree; when the sensing element is located after the valve, an increase in pressure leads to a decrease in valve opening degree. Also, I’d like to tell you that when choosing between positive and negative action for regulators, always opt for the FC type. If you use FO, add an inverter at the output terminal, so that 4 mA corresponds to the minimum valve opening degree and 20 mA corresponds to the maximum opening degree. This post can now be closed – give it points, lots of points! ! ! ! ! ! ! !
Hehe, accidentally posted 2 identical posts; sb latency