A feedback level controller manipulates a makeup control valve to maintain the level in a tank. The liquid flow through the control valve is measured using an existing flowmeter. Which of the following is not a typical attribute of upgrading this single-loop control strategy to a cascade control strategy?

1. Improved level control
2. Large valve movements
3. Operation at steadier flow rates
4. Separate level and flow controller tuning

The feedback level controller maintains the level in the tank by opening the control valve more when the level is lower than its setpoint and opening the valve less when the level is higher than its setpoint.

In this application, cascade control consists of a level controller that manipulates the setpoint of a flow controller. Each controller can be individually tuned. (Answer D) The flow controller can be tuned to react relatively quickly to account for flow noise, valve characteristics and setpoint changes. The level controller can be tuned to react relatively slowly to account for level changes that occur more slowly. The typical net result of proper tuning is improved level control and steadier flow control (Answer A and Answer C). Steadier flow control suggests that there will be less control valve movement, so Answer B is false.

Additional complicating factors

Some years ago, the accuracy of a paper mill effluent flowmeter used for billing purposes was in question. Intermittent flow caused the effluent sump level controller to vary the flow widely during normal operation. Implementing a cascade control strategy (with a deadband) would have steadied the flow at the expense of allowing the level to wander near the level setpoint. Importantly, it would also enable the operator to obtain a reasonable measurement of the average effluent flow rate, which was not possible with the existing control strategy.