Check valves are generally used in drip irrigation emitters and irrigation sprinklers. Composition disc valves are ideal for applications involving gritty liquids. A clapper valve is a type of check valve which is used in fire fighting. Poppet check valves find use in "macro" flow applications. Arterial check valves are used in applications requiring centrifugal pumps to prevent retrograde flow along the arterial line. Check valves are mechanical valves that permit gases and liquids to flow in only one direction, preventing process flow from reversing. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while backflow forces the valve closed. The mechanics of check valve operation are not complicated. Most check valves contain a ball that sits freely above the seat, which has only one through hole. The ball has a slightly larger diameter than that of the through hole. 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.