Control Notes

While I was recently helping a chemical company optimize several of their critical control loops, I noticed they had a ratio controller in manual control mode. I asked about the loop and they told me it has never worked in automatic control.

I occasionally come across loops that never have worked in automatic control. Sometimes it’s a tuning problem, sometimes it’s an issue with the measurement or control valve, one time the control direction was wrong – go figure. However, when it is a ratio control loop that’s not working, most often the problem lies with the design of the control strategy.

Ratio Control Explained

Process design and operations often calls for keeping a certain ratio two or more flow rates. One of the flows in a ratio-control scenario, sometimes called the master flow or wild flow, is set according to an external objective like production rate. The ratio controller manipulates the other flow to maintain the desired ratio between the two flows. The flow controlled by the ratio controller is called the controlled flow. For example, when treating drinking water with chlorine, the water is the wild flow, and the chlorine is the controlled flow.

There are two fundamentally different designs for ratio control. One of them is the correct design, the other one does not work in practice.

The Intuitive but Incorrect Design

In this design, the ratio is calculated by dividing the one flow by the other. This calculated ratio is then used as the process variable for a ratio controller. This design creates a highly nonlinear control loop of which the process gain is inversely proportional to the flow rate in the denominator. Ratio controllers like this are frequently dead (virtually no control) or unstable. This design should be avoided at all times, and if you have one of these, correct the design!

The Correct Design

The wild flow should be multiplied by the desired ratio to calculate a set point for the controlled flow. A standard flow controller then controls the flow according to this set point. If required, you can divide the controlled flow by the wild flow to display the actual ratio to the operator, but don’t use it to control the ratio.

The figure below shows the two ratio-control designs. Ratio control should not be based on a division of flow rates as shown on the left, but rather on calculating a flow set point, as shown on the right.

Two designs for ratio control. Left: The incorrect design. Right: The correct design.

For example, the correct design for a ratio controller of fuel and air is shown below. Fuel is the wild flow and air is the controlled flow.

Controlling the ratio of combustion air to fuel flow.

I have seen several ratio controllers running in manual control mode because of “tuning problems.” After further investigation it often turns out that the incorrect design is in use. Now that you know the difference, you know what to look for and how to correct it.

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