In the batch-to-continuous transition, it is desirable to fully utilize the surge volume to dampen out flow fluctuations to the continuous operations. This applies to both liquid and solids flow applications. Often the consequences of flow upsets are severe, particularly if downstream loops do not all have flow feedforward (e.g., distillate, bottoms, and steam-to-flow ratios for columns).

The directional velocity limit spreads the flow upset over one half of the surge working volume (e.g., set point=50%). The up- and down-scale velocity limits are set to keep the level near set point by means of a small saw tooth around the average flow for the batch cycle with minimal feedback correction. A head start of the feedforward flow for the first batch is essential to establish a feed-forward signal close to the average flow and to reduce the saw tooth amplitude.

Without a head start and with a constant velocity limit set to achieve the average flow after one batch cycle, the tank level will rise and then continually drop below set point until the full feedback correction is reached.

Proper implementation of the feedforward can reduce the level controller gain requirement to one tenth of its normal value for full feedback correction (2->0.2 for a 50% set point).

Adaptive gain adjustment or gain scheduling can be used to reduce the gain as the error decreases. This is done in a discrete fashion by the use of gain zones. An alternative is error-squared level control for continuous and dramatic gain...