How to set up a Split Range Controller

The following is an example of how to use the standard split range controller in CADSIM Plus. The split range controller is often used in the following situations:

• Makeup to tanks: when one stream that is used for level control is insufficient, another stream is substituted.
• The level of a tank is controlled by sending excess flow to another part of the process, but if conditions change, a makeup has to be applied instead.

Example: the excess water from a Machine White Water Chest is sent to another part of the process, but if there is insufficient water, mill water will be used as makeup.

Two controllers that both control level would not work well in this example because each controller would fight the other. A split range controller is a single controller, but its output is split into two ranges.

During the first half of the split range controller's output, the level control will be done by sending excess flow away from the tank through valve 028 as shown in the figure. At half way point, the flow through valve 028 will be shut off, and the mill water valve 011 starts to open.

A split range controller has two outputs for the two valves, which must be mapped to the overall output of the controller. In order to program the split range controller , it is helpful to make a table mapping Out1 and Out2 to the overall Output.

A table appropriate for the split range level control for this example appears below. In the table, maximum will mean that the valve position when fully open (100%). Alternatively, substitute an appropriate maximum flow rate, if the flow is being adjusted directly in a simplified simulation.

The CADSIM Plus split range controller describes the limits of Out1 and Out2 with four variables (fourth column) shown with the appropriate settings if controlling valve position from 0 through 100%. The first part of these variable names (before the underscore) refers to the overall Output and the second part refers to the limit of one or the other split outputs.

The tank level controller is tuned the way it would be if it were a single output controller (hint use TuneLev.xls provided with CADSIM Plus for efficient Lambda tuning).

The simulation charts show the seamless level control as dilution demand increases. Valve 028 first shuts off, then the mill water makeup starts and is increased with demand.