The Principle Operation of an Atmospheric Vacuum Breaker Backflow Prevention Device – Check Valve and Air Inlet Valve

The check valve and air inlet valve are in the same unit constructed usually of a polyethylene float also known as poppet, which is free to travel on a shaft and seal in the uppermost position against atmosphere with an elastomeric disc. Water flow lifts the float, which causes the disc to seal the atmospheric vent port. Water pressure keeps the float or poppet in the upward sealed position at all rates of flow, preventing leakage. The float disc will close with water pressure from the inlet or outlet. Therefore, the AVB will not prevent non-potable from entering the potable water system if there is a backpressure condition. The AVB is only approved for controlling a backsiphonage condition. The float will open the air inlet if there is a backsiphonage or no-flow condition. Termination of the inlet water supply will cause the float disc to drop down opening the atmospheric inlet vent port venting the AVB to the atmosphere through the canopy/hood/cover. Introducing air into the AVB breaks the vacuum and prevents the siphoning of non-potable water into the potable water system. If the water pressure in the inlet piping is reduced to sub-atmospheric pressure, the float disc/ poppet drops and allows air to enter the AVB, breaking the vacuum caused by the backsiphonage condition. To determine if the float disc is operating properly, the AVB can be shut off at the inlet valve and visual inspection of the float disc can be made. If the float disc is operation properly, it should drop down freely and a sound should be heard when the float disc closes. An internal inspection of the AVB can be made to assess the working of the float disc by removing the canopy/hood/cover.