Ask Doctor Bob: What is level control? (PART 1)

Video Transcript:

Hi, I’m Bob Rice. And today, we’re going to be doing part one of a three-part installment on level control, today’s part is going to be focused on the control objective and the basis for what level control is.

So we’re gonna start with a tank. So inside of our tank, we’re going to have some sort of liquid. And we’re going to control the liquid level with a level controller, that is outputted, to just say, a pump. Okay, and so we’re going to control the speed of this pump, to try to maintain the liquid level, the disturbance that we have is we have flows coming into this tank, from multiple streams, they all accumulate in this tank, and then this tank outflow goes to some downstream system, this downstream system is the thing that we’re actually really concerned with, we want to make sure that the flow rate out of this is as smooth as possible, right, we don’t want this flow to be fluctuating up and down a lot. We want this flow to be nice and smooth. So the reason we put a tank in this process is that these flows up here, they move all over the place, right, each of these flows are constantly going up and going down. And if we just pipe all three of these together, and send it to our downstream unit, that downstream unit would get a highly variable flow, which would disrupt its stability in its operation.

So what we do is we put a tank in here, in this case, this would be called a surge tank, right? A surge tank is where we want to control the liquid level between some sort of constraints or tolerances, right, it’s not necessary that we hold the tank perfect at the setpoint of 50%, or whatever value that we have, we just need to make sure the tank level doesn’t get too high or too low that we either trip the tank, we flood the tank, or we cause some other problems associated with it, right. And so the control objective that we’re after, is to protect the downstream unit by minimizing the movement of this pump, right? We don’t want that pump speed to be ramping up and down chasing every single disturbance that’s in the tank. So the control objective is to protect the downstream system by minimizing the output flow fluctuations and allowing the tank level to deviate and actually hit its constraints. All right. So that’s the control objective. Let’s think a little bit about the process itself. Right. So if we take a look at the setpoint, and we’re trending this process, we’ve got the setpoint. And we’ve got some sort of controller output, that’s controlling the process variable, and we get hit with a disturbance, right, the controller is going to say, hey, look, the liquid level is too high. So I need to open this tank, open this pump to go ahead, and bring some more liquid out to dry to drive this back again. And we want to make sure that we open up this speed of this pump, we open up the flow rate a little bit more. So we open up fast enough that we don’t cross a constraint, we don’t want to just instantly move the flow. So we get a little bit of deviation, because now what we’re doing is we’re moving that controller output excessively, which causes more downstream fluctuations.

So what we need to understand for this process is what the control objective is. And in this case, it’s not tight setpoint tracking, it’s to maintain the tolerances of that process variable between the upper and lower constraints. And what we want to do to help better define the objective is starting to understand the flow, the flow rates of the system, and the volumes of the tank, right, if we have between these three flows, say 20 liters per minute of combined flow, and this tank here is 100 liters. And we have a setpoint in the middle at 50 liters. And we have constrained conditions at say 70 liters and at 30 liters. That means that when we’re sitting at setpoint right here, we have about 20 liters on either side of that volume kind of capacity. Right? And if we’re running 20 liters per minute, and everything is balanced, we got 20 liters per minute out of 20 liters per minute in our process variable is going to be perfectly flat. Let’s say that the disturbance flow can vary up to say 40 liters per minute.

So the disturbance goes from 20 to 40 liters per minute. If we did not I think at all, if we didn’t change the pump speed, we left the pump speed just where it was at. And we did nothing. Because there’s a flow imbalance now 40 liters per minute coming in 20 liters per minute coming out, the level is going to rise. And it’s going to take one minute, until we hit this constraint, one minute, that means that our arrest time that we’re looking for needs to be faster than one minute, right, our closed loop time constant, our arrest time that we’re looking for, for this process needs to be faster than one minute. That’s the speed, we’ve set the objective, we got the volume of the tank, we know the flow rates, we know how long it’s going to take, before we hit a constraint, we need to be faster than that we need to be faster than one minute.

Now, you may be saying Hey, Bob, yeah, that’s, that’s great. That’s a surge tank, but I have to do tight level control. The same theory applies. If you tighten your tolerances down here to say, hey, look, I only have plus or minus one liter per minute, or plus or minus five liters, or one liter of five-minute leaders across my setpoint, you still do the same calculation. But what you’re going to end up with is an arrest time that can be drastically smaller can be 10 seconds or five seconds. But you’ve defined it, you’ve laid out the objective, you didn’t just say, hey, I need to track the setpoint. You said look, given the disturbances and the swings that I expect in my process, this is the worst thing that can happen. These are my tolerances. Here’s how I want the process to respond. So when you’re looking at a level controller, you really do want to spend the time and define the objective, how tight of control do you need? Is this a surge tank? Or is this a steam drum?

Right, those are going to have different objectives and different tolerances that are allowed. How big is this tank is this tank 50,000 gallons, or 50 gallons, right you’re going to approach a 50 gallon tank with high flows a little bit more aggressively than a 50,000-gallon tank, right? So understand the size the scope the flows, so you can start to understand the time requirements for the speed of the process. In this video installment, we talked about the control objectives for level controllers, we reviewed the idea of arrest time, and how to define the proper control objective by understanding the physics of the system, the volumes and the flows of the tank.

By understanding the arrest time and the volumes, we can better tune the loop and understand the behavior of the system. Thank you for your time.

If you have a particular topic or an idea that you would like us to cover, please email us at Thank you, and I hope you enjoy this video series.