The Principle Operation of a Dual Check Valve with Intermediate Atmospheric Vent – Backflow Prevention Device – Backflow Backpressure
As water is flowing through the DCVwIV, the check valves are open and the intermediate atmospheric vent is closed. If a backpressure condition were to occur in the outlet or downstream piping, the force of the backpressure with the assistance of the second check valve spring would close the second check valve. With the second check valve closed tight against the seat due to the backpressure condition, a static state would exist within the DCVwIV. Since the DCVwIV is in a static state, the first check spring will close the first check valve against the seat. With the DCVwIV in a static state, line pressure upstream of the first check valve is positive. In a static state, the inlet water pressure is pushing against the diaphragm closing the intermediate atmospheric vent. With the check valves closed during a backflow backpressure conditions the DCVwIV is providing protection.