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Flow Control Conundrum

I recently helped a control engineer review the control strategies used in his plant. The company was experiencing some control problems and wanted a second opinion. While most of the controls were designed correctly, we found a few areas requiring design changes. One of these areas had to do with balancing flow rates through banks of filters in their water filtration plant.

System Description

Raw (river) water enters a clarifier to separate the heavy solids. The water flows over a weir and is pumped through two filter banks in series. Each bank has three filters. The flow rates through the individual filters have to be balanced for filter efficiency.

Filtration Plant P&ID

Filtration Plant PFD – click to display full size

Control Design as Found

The clarifier had a level controller of which the output became the set point of the six flow controllers. Actually the flow set point of each bank of flow controllers were divided by the number of filters in service, but I left that detail out of the diagram to keep it simple.

Operational Problems

The site found that the second set of flow controllers would slowly increase their outputs until they were all at 100% and the valves were wide open. Occasionally, when certain filters were out of service, the first set of flow controllers would slowly increase their outputs until they were all at 100% and the valves wide open, while the second set of valves controlled properly.

Equivalent Control Design and Problem

The multiple filters and flow paths obscured the design problem. If we simplify the hydraulic flow paths and control design we end up with two flow controllers in series on the same pipe receiving the same set point.

Equivalent Control Design

Equivalent (simplified) Control Design

And here is the problem: we effectively have only one process variable (the common flow rate), but we have two controllers trying to control it. If one of the flow transmitters measure a slightly lower flow rate than the other, its controller will open its valve to get more flow. Although this might work temporarily, the flow rate in the other control loop will also have increased and that controller will close its control valve to compensate, bringing the flow back down to the original level. Eventually the control loop reading low will saturate with its controller output at 100% while the remaining control loop will do the flow control.

You cannot control the same process variable with two control loops at the same time!

Alternative Design

We had to have a common set point for the second set of flow controllers, but it had to be independent of the set point of the first set. Yet the two sets of controllers must in some way be linked to work in unison. Although there are other ways to solve this problem, here is the way we did it. We used the average position of the valves in the first bank as a set point to a position controller that controls the average position of the valves in the second bank.

Alternative Design

Alternative Design (only simplified version shown)

The position controller generates a flow set point to the second set of flow controllers that is independent of the flow set point of the first set of controllers. Basically, the valve position controller does not care what the actual flow rate is, as long as the valves in the second bank are on average at the same position as those of the first bank. In this way calibration errors has a negligible effect on the system as a whole.

After changing the design, we tuned the flow control loops, then the position controller, then the level controller. After that, the system worked perfectly.

 

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners

 

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