The CA9C is a backflow preventer for the protection of drinking water systems(main public system or private system). A backflow preventer is a sanitary device which is able to stop backflow of polluted water into the water system. CA9C is designed with 2 check-valves separated with a zone connected to the atmosphere. In case of upstream pressure loss or downstream overpressure(with leaking second check-valve), CA9C discharges by venting the middle zone to the atmosphere, this feature prevents any backflow polluted water in the device.