A valve used to control the flow of process material through a pipe by controlling the flow area through the valve by way of an applied air pressure signal. (p. 289) The operation of a control valve involves an air supply that positions its movable part (i.e. plug, ball, or vane) relative to the stationary seat of the valve. A valve actuator accurately locates the valve plug in a position determined by the pneumatic control signal and operates to move the valve to either fully open or fully closed positions. The actuators may be either piston or diaphragm types. Air-to-open valves require air to open and therefore automatically close in the event of fail closure. They are therefore used on fuel lines to furnaces. Air-to-close valves fail to open on a loss of air pressure and are used on air lines into fuel burners. In general, fail-to-open and fail-to-close valves operate when the supplied air pressure drops below a minimum value.