Process control systems use a variety of field devices to control and/or monitor process parameters. Field devices, such as air-operated on/off valves, employ a control fluid to move a flow control member relative to an orifice between an open position to allow fluid flow through the valve and a closed position to restrict or prevent fluid flow through the valve. However, such air-operated on/off valves lack precise control of fluid flow through the valve.
To precisely control the flow of fluid through a valve, the valve often employs an electric controller or actuator to actuate or move the flow control member relative to the orifice. However, during a fail condition (e.g., when electric power to the electric controller fails), an electric controller typically fails (i.e., holds) at its last control position. As a result, valves employing electric controllers or actuators often have a fail-safe system upstream from the valve that activates to prevent fluid flow to the valve during a fail condition. However, such fail-safe systems increase complexity, costs and consume additional space.