The Principle Operation of an Atmospheric Vacuum Breaker Backflow Prevention Device – No-Flow Static State
If the downstream shut-off valve is closed, no water is flowing through the AVB and a static state exists within the body of the AVB. In a static state, the AVB is not pressurized downstream of the upstream shut-off valve. Since there is no water pressure keeping the float disc open and the air inlet port closed, atmospheric pressure is entering the open air inlet port breaking the vacuum affect. The atmospheric pressure is pushing down on the float disc keeping the check valve closed and the air inlet port open eliminating a siphon condition. Atmospheric pressure keeps the float disc closed until the upstream shut-off valve is opened which, moves the float disc upward closing the air inlet port when the air inlet port is open, the PVB is providing protection and preventing non-potable from entering the potable water system. A shut-off valve installed downstream of the AVB would not allow the introduction of atmospheric pressure into the air inlet port since the AVB would be pressurized.
