The Principle Operation of an Atmospheric Vacuum Breaker Backflow Prevention Device

An Atmospheric Vacuum Breaker (AVB) is a mechanical backflow prevention device designed to prevent the backsiphonage of non-potable water into the potable water system. The AVB is not designed to control backflow backpressure and should not be used where there is a possibility of backpressure in the downstream or outlet piping. The AVB is a 90-degree elbow equipped with a single one-piece disc float not spring loaded that acts as a check valve and air inlet valve. The AVB has a canopy/hood/cover on the top that allows air into the AVB during a backsiphonage situation in the inlet piping system. The disc float check valve is normally closed by gravity when there is no flow through the AVB and open when there is flow. The disc float air inlet valve closes by the inlet pressure when the AVB in flowing and opens by gravity/atmospheric pressure when there is no flow through the AVB. When the air inlet valve is open, an air gap is created eliminating a siphon condition. The AVB can be used in low and high hazard situations but not with continuous pressure. A downstream shut-off valve cannot be installed on a AVB as this would result in continuous pressure on the AVB. The AVB can be equipped with an upstream shut-off valve but it is not designed for continuous pressure and should not be subjected to more than twelve hours of continuous flow. In a continuous flow situation, there is the possibility of the disc float air inlet valve may become stuck or sealed closed and not open when there is a no-flow or backsiphonage situation. The AVB is not an “assembly” because it does not have upstream and downstream shut-off valves or test cocks. The AVB is suitable for cold or hot water services up to 180 degrees F. The maximum working pressure is 125 psi. In general, the AVB is available in ½ inch to 3 inches. The AVB cannot be tested in-line but can be repaired in-line. The AVB shall comply with ASSE Standard 1001 and shall be installed at least 6 inches (152mm) above highest point of downstream piping or hose. Downstream shutoff valves are allowed to be installed on AVB. The AVB can spill water therefore; the AVB shall be installed where spillage is not objectionable. Use only with non-continuous pressure (pressurized a maximum of 12 out of 24 hours).