For many processes, such as blending and boiler combustion, a key objective is to maintain the flow rates of two process streams in some proportion to one another. In such cases, ratio control may be applied. When ratio control is applied, one process input, the dependent input, is proportioned to the other process input, known as the independent input. The independent input may be a process measurement or its setpoint. The proportion that is to be maintained between the inputs is known as the ratio. For example, a ratio of 1:1 would specify that the two inputs are to be maintained in the same proportion. As the value of the independent input changes, through ratio control the other process input is changed to maintain the proportion of the inputs specified by the ratio setpoint. In nearly all ratio control applications, the ratio controller sets the setpoints of the flow controllers rather than the valve positions, as illustrated below. Thus, any non-linearity installed characteristics associated with valves is addressed by the flow controllers and has no impact on the ratio controller being able to maintain the ratio setpoint.

In most cases, the independent input measurement, not the setpoint, is used as the input to the RATIO block. The reason is that for some operating conditions, the independent loop output may become saturated.

The ratio block is used to implement ratio control. The input to the ratio block is the measurement or setpoint of the independent flow input to the process. This independent measurement is also known as the process “wild flow” input. In the RATIO block, this input is multiplied by the ratio to determine the setpoint of the dependent flow input to the process. A typical implementation of ratio control is illustrated below.

Under normal operating conditions, the RATIO block multiplies the independent input flow provided on IN_1 by the RATIO block setpoint to provide an output to the downstream block. The actual ratio is calculated by the block using the IN and IN_1 inputs.

Several ratio control application examples are detailed in chapter 13 of Control Loop Foundation – Batch and Continuous Processes. Also, the ratio control workshop included in that chapter provides several exercises that may be used to further explore ratio control. By accessing the book’s web site, you may complete this ratio control workshop using your web browser. The viewer below may be used to see the solution to this exercise.