A check valve is among the simplest valves in the industrial world. Found in practically all systems, these valves allow unidirectional fluid flow through a pipe or aperture. They require no manual adjustments because they are flow-sensitive; they open in response to a certain "upstream" pressure level and close below it, or in response to positive "downstream" pressure. Sump pumps, steam lines, irrigation systems and injection lines all feature check valves, and the valves between the atria and ventricles of your heart are essentially check valves.

Common check valves include the swing check valve, which operates much like any gate, and the ball check valve, in which flow stops in response to the occlusion of the opening by a spherical component. When fluid flow pressure becomes sufficiently great -- and the value of this pressure varies according to the design of the valve, which in turn depends on the needs of the system -- a disc within the valve housing slides forward, drawing the gate or ball open and allowing flow through the opening. The internal sealing of these valves is self-regulating, so minor levels of backflow often occur.