Abstract: A sliding plate of a sliding closure unit is moved to selected throttling positions thereof to relatively restrict the size of a discharge passage through the sliding closure unit. The control of movement of the sliding plate is achieved in a level control operation by a processor. Periodically this level control operation is interrupted and the sliding plate is moved in a throttling position change operation from one throttling position through a completely open position of the discharge passage to another throttling position, such two throttling positions employing throttling surfaces of the sliding closure unit on opposite sides of the discharge passage. The throttling position change operation includes regulating at least the throttling position at which the sliding plate is located at the conclusion of the throttling position change operation by a sequence control program included in the processor that controls the level control operation.