Basic level control, please help
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Pure water tank level control. The inlet valve controls the liquid level, so it’s not a major issue. The PID control was set at 100 30 0, with the valve opening ranging from 0 to 0.6; however, an excessive opening affected the use of pure water in the small pipelines in other areas. I changed the PID to 100 70 0; now the opening can be as low as 0.3–0.5, which results in a more stable operation. However, the peak value of 0.5 still affects the water flow rate in the small pipes. How should I make adjustments? Thank you in advance.
If it just can’t be increased much, then setting an upper limit will do
Thank you, I’m not from the instrumentation department. The current situation is that many of our PID parameters aren’t tuned properly; I want to use this simple control method to learn how to adjust the PID parameters
Agreed; the key issue is that there’s a problem with the valve selection – the flow rate is too high. Valves with hand valves at the front and back; hold some pressure. If the requirements for liquid level are not high, a PID controller will allow some fluctuation in the liquid level, depending on the specific process.
Additionally, water in small pipes is affected mainly due to pressure changes; measures should be taken to isolate the pressure.
In a multi-pipe parallel water supply system, the resistance in the main pipes is relatively high (due to small pipe diameters, low opening degrees of manual valves, etc.). If the control valve on one of the branch pipes is opened wider, the flow rate in that branch pipe increases, which leads to a decrease in pressure in the main pipes. This, in turn, affects the flow rates in the other branch pipes. These systems are interrelated, and such issues cannot be resolved by simply adjusting control parameters.
The simplest approach is to set a PID output limit
Who controls the water output of the tank?
With such a low valve opening, there must be a problem with the design – the pipe dimensions and the valve dimensions are incorrect. Without knowing the specifics of the process, it’s pointless to talk about PID settings when the valve is kept at an opening of just a few percent. If a large amount of water is required in an emergency while the normal water intake is low, then this water can be supplied through a bypass pipe. If you want to limit the valve opening, add an output limit; PID control has fast tuning and steady tuning modes – for steady tuning, reduce the proportional gain and increase the integral gain.
First question: The control valve cannot operate at an opening degree of 10%! ! ! It’s essential to understand this: as a instrumentation technician, I’m often asked by process engineers why valves don’t work properly when operated at low opening degrees – it’s simply frustrating. It’s a valve; although it involves delicate work, what is the typical stroke length of straight-stroke valves? It’s probably around 10–20 cm. What does 1% mean? 1~2mm, millimeters, dude. Can this distance be adjusted? The valve closes if it goes a little lower, and it exceeds the limit if it goes a little higher; in such cases, adjustment is useless! ! ! The solution is simple: first, scold the designers; then use the backflow valve to restrict flow by closing it partially, and set the opening degree of the control valve to over 10%. The subsequent PID adjustment is extremely easy – only the proportional parameter needs to be adjusted; both the I and D parameters can be set without any problem.