Check valves are used to allow fluid flow in a system in only one direction. They are operated by the flow of fluid in the piping. A check valve may be the swing type, lift type, or ball type. Most valves can be classified as being either stop valves or check valves. Some valves, however, function either as stop valves or as check valves-depending on the position of the valve stem. These valves are known as stop-check valves.