Ball check valves contain a ball that sits on the seat, which has only one through-hole. The ball has a slightly larger diameter than that of the through-hole (seat). When the pressure behind the seat exceeds that above the ball, liquid is allowed to flow through the valve. But once the pressure above the ball exceeds the pressure below the seat, the ball returns to rest in the seat, forming a seal that prevents backflow. The ball moves up and down inside the valve depending on the flow and seals against the machined seat when no flow or reverse flow occurs and seals against the seat to stop the reverse flow. Ball check valves are typically preferred for use in pumping stations as they are self-cleaning as the ball rotates during the operation and are vertically maintenance-free.