A valve has a first duct, a second duct into which the first duct opens, thereby defining an intersection that forms a seat at right angles to the first duct for a valve disk that slides in the second duct at right angles to the seat between a closed position and an open position, and an actuator for operating the valve disk between its two positions. For the open position, the actuator is controlled to bring the valve disk into close proximity to a wall of the second duct facing the seat.