On cascade control systems
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Today, I encountered a phenomenon during operation: the liquid level in a distillation column is controlled by a cascade control system that includes a flow control valve. A container is located between the flow meter and the level gauge; in other words, this container is upstream of the flow meter. After the container is cleaned, it is prepared for stamping. The phenomenon occurred: because I was busy with other things at the time, I didn’t go to that screen as soon as the outdoor control unit notified him that pressure needed to be increased. When I checked again after a few minutes, I found that the liquid level in the tower was dropping rapidly, yet the flow rate indicated by the flow meter was at full scale; the opening degree of the control valve was also 84%, when it should normally be around 50%. The experienced technician explained it as follows: Under normal conditions, if the liquid level remains stable at 50%, then an output of 47 t/h serves as the input value for controlling the flow in the secondary circuit. When pressure is applied, the flow rate measured by the flow meter drops suddenly; the flow control system in the secondary circuit responds immediately to this change, while the main circuit has not yet reacted to the drop in liquid level. As a result, even though the liquid level in the tower is dropping, the flow control valve still commands an opening that is higher than the 47 t/h flow rate observed under stable conditions. That’s why, when I checked the display, I saw that the liquid level was dropping, but the flow control valve was open to a degree much greater than that under stable conditions. I don’t know if this experienced teacher’s analysis is correct, but I can’t think of any other way to put it. If his analysis is correct, I would like to ask whether this situation is a characteristic of cascade control systems, or whether it is related to the properties of the DCS system being used I remember that when I was using another unit before, one that used Yokogawa products from Japan, I never encountered this situation in the cascade control system: the liquid level was dropping, yet the flow control valve kept opening wider. I hope all the experts can offer some guidance!
Is the PID reacting too slowly?
During the driving phase, manual mode should be used first, and automatic mode should be engaged once the process is stable.
Your assumption isn’t correct. In theory, each cycle should adjust the target flow rate based on real-time changes in liquid level; it shouldn’t happen that the liquid level changes significantly yet the controlled flow rate remains unchanged. Either the liquid level sensor responds too slowly, or there is an issue with the program logic
I’ve drawn a schematic based on your description. I’m not sure if it’s right. I find your analysis a bit hard to understand. The PV, OP, and SP values of LIC FIC during that period can be combined into a single trend group for analysis; please send the screenshot.
Yeah, I think it shouldn’t be this slow either, but I can’t think of any other explanation
In a cascade control system, if the liquid level in the tower drops rapidly during operation, then the level control loop should take over control, and the flow rate secondary loop would be disconnected. Is it possible that only the secondary ring is in operation, while the primary ring is not?
It must have been poured in; seeing that the liquid level kept dropping, I immediately switched the cascade control to manual mode
Judging from the description, it seems that the main circuit is being adjusted in reverse. What is the historical trend like? Analyze it by looking at the level gauge, flow meter, and control valve