Level Control

Let me start by saying I do not have a lot of experience with PID loops and this will be a AB L32E processor. I have a system that moves water. There are 14 well sites that pump into a large tank at a booster station. The booster station needs to control how many of the 14 sites need to be on based on its tank level. My idea is use the PIDE instruction with the tank level as the PV 0-100% and the CV be the number of wells to run 1-14(one well must stay running all the time to keep the line charged). I wrote a little sample program and it seems to work OK but I am not sure the best way to tune it. Once a well is activated it takes anywhere from 2-5 minutes to see the change in the tank level. I am unclear about independent vs dependent and should I just use P or PI or PID. What kind of starting values should I use for those parameters for a slow loop? Just looking for some basic information on what might work best.

Also maybe the IMC function block? I do not know anything about this instruction yet.

AzCtrlEng said:

Once a well is activated it takes anywhere from 2-5 minutes to see the change in the tank level.

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I don't understand. What is happening in that 2-5 minutes? If water is flowing the tank level will be affected.

This does not sound like the proper use of a PID instruction to me.

You could just use the level signal and scale it to 0-100% then use that to turn on the
wells needed. You will need to program in some deadbands of course.

Mickey said:

This does not sound like the proper use of a PID instruction to me.

You could just use the level signal and scale it to 0-100% then use that to turn on the
wells needed. You will need to program in some deadbands of course.

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Yes I have done it that way before using LIM instructions I was just looking for something a little more streamlined.

Yes I have done it that way before using LIM instructions I was just looking for something a little more streamlined.

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It seems that using a PID instruction could result in pumps switching on and off frequently. With a PID, you commonly have overshoot and undershoot before it stabilizes around an output. It seems that you need to control these pumps in step increments, not with a continuously varying output.

It seems that the tank level is only one of the input variables that you need. I assume that water is also being used and lowers the tank level at the same time. If the water-use rate is 0, low, or high, it will affect the number of pumps you need to maintain any given level.

What you could do is use the change in level to control a loss-in-volume function (set up like the common Loss-in-Weight controllers. Then send the Loss-in-Volume PID output through additional logic that makes sure that there has been a Minimum Elapsed time since the last pump start, and also that any running pump has run for at least a Minimum Run Time before it is allowed to stop. It gets complicated in a hurry, so you probably can do as well by using the LIM instructions.