Control Level with a Cascade

This requires different thinking about master and slave loops.

By Cecil L. Smith, Cecil L. Smith, Inc.

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• The output of the drum level controller is the set point for a flow controller whose PV is the measured total flow from the reflux drum.

• If a value for reflux flow is required, it must be computed by subtracting distillate flow from the measured total flow from the reflux drum.

The level control configuration is a simple level-to-flow cascade.

 

Still Another Approach
Figure 9.
This control scheme requires a measurement of total flow from the reflux drum.

With the configurations shown in Figures 7–9, we can say the following about the performance of the upper-stage temperature controller:

• The flow controller that drives the control valve on the reflux must be functioning. For the configuration in Figure 7, it must be using the remote set point computed by the summer.

• Level controller tuning doesn't impact the performance of the upper-stage temperature controller.

• Switching the level controller to manual doesn't affect the upper-stage temperature controller.

However, these advantages come at a price — two flow measurements are required. Older towers likely lack the necessary flow meters. Newer installations probably have them, as the trend is to provide flow measurements wherever practical.

A disadvantage of the configurations in Figures 8 and 9 is that you can't impose the minimum reflux flow simply by specifying a lower set-point limit for the flow controller. Adding logic to impose the minimum increases the complexity of these configurations.

In these configurations as well as the simple feedback scheme in Figure 6, problems arise when reflux flow is much smaller than distillate flow (external reflux ratio L/D «1). Regardless of the configuration, you are controlling the level with a minor discharge flow and, thus, making large changes (on a percentage basis) in a small flow. In most cases, the control valve repeatedly will be driven fully open or to the opening corresponding to the minimum reflux flow. Fortunately, composition control analysis rarely recommends the configuration in Figure 6 when reflux flow is small compared to distillate flow.


CECIL L. SMITH is president of Cecil L. Smith, Inc., Houston. E-mail him at cecilsmith@cox.net. This article is based on concepts from his book "Practical Process Control," published by John Wiley & Sons.

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