The Principle Operation of an Atmospheric Vacuum Breaker Backflow Prevention Device – Flowing Dynamic State

As water flows through the AVB with a demand downstream, the water pressure will force the float disc upward opening the check valve and closing the air inlet port. The top of the float disc acts as an air inlet valve. If the air inlet valve is not functioning and does not properly close the air inlet port, water will discharge out of the air inlet port. As the inlet rate of flow decreased to a static state, the float disc starts to drop downward and the air inlet port opens. When the upstream shut-off valve is closed, the float disc drops down due to the force of atmospheric pressure pushing down on the float disc. The closed float disc, if operating properly, functions as a closed check valve and air inlet valve. The float disc will stay closed until there is a demand. When the inlet water pressure is terminated, atmospheric pressure enters the air inlet port closing the check vale and preventing water downstream of the float disc from entering the potable water system. Because the check valve is closed and the air inlet port is open breaking the vacuum affect, the AVB is providing protection. If the check valve was not closed tight and air inlet port was not opened to the atmosphere, protection would not be provided. Because the float disc may fail, a certified tester should inspect the internal parts the AVB at least annually or more frequently if necessary.