Abstract: A pipe line has a check valve therein, operable by an actuator. The actuator is connected to be operated by fluid pressure from the pipe line, and is connected with the pipe line by a control circuit including valve means. A differential pressure valve is connected in the control circuit, and is constructed to sense a drop in pressure within the pipe line downstream of the control valve. Upon sensing such a pressure drop, the differential pressure valve is effective to cause connection of the actuator to pipe line pressure upstream of the check valve, whereby the latter is automatically closed.