Abstract: An automatic control valve has a main valve member moveable between open and closed positions thereof in response to pressure applied to a first piston or diaphragm in a first control chamber. The valve member moves to modulate flow through the valve to maintain desired flow conditions. Pilot pressure signals are generated to reflect the flow conditions, usually upstream and downstream from the valve, which signals are processed to generate a control signal which is fed to the first control chamber to control pressure therein. Normal movement of the valve produces wear which causes leakage of the diaphragm or seals oft he piston causing loss of primary control of the valve member which tends to fail "open". This causes an initial rise in flow through the valve which is detected by the pilot pressure signal but because of the leakage, is unable to correct.

Abstract: A valve apparatus has a valve body with an inlet, an outlet and a main valve seat disposed therebetween. A complementary main valve member can be moved by an actuating piston/diaphragm between open and closed positions, or located at an intermediate position therebetween. The actuating piston/diaphragm has one side exposed to an actuating chamber which receives fluid from the inlet through a metering orifice on one side, and an opposite side exposed to outlet pressure. A servo valve assembly controls flow from, and pressure within, the actuating chamber and controls movement of the main valve member. The servo valve assembly can be positioned by a relatively low force actuator in any position between extremes thereof, and acts as a master for the main valve member which follows it as a slave between open and closed positions, and any intermediate positions therebetween. The actuator is protected from high fluid forces imposed on the main valve member.