A backflow prevention device is used to protect potable water supplies from contamination or pollution due to backflow.
Backflow may occur under the following two conditions:
- Back-pressure: whenever the amount of water being used exceeds the amount of water being supplied (such as during water-line flushing or breaks in water mains).
- Back-siphonage: can occur when there is a high velocity in a pipe line, when there is a line repair or break that is lower than a service point, or when there is lowered main pressure due to high-water withdrawal rate (such as water-main flushing).
Atmospheric Vacuum Breakers
Backflow prevention for residences is most commonly accomplished through the use of atmospheric vacuum breakers (AVBs) which are devices used in plumbing to prevent backflow of non-potable liquids into the drinking water system. AVBs are constructed of brass and resemble a 90-degree elbow with a hood on top to allow air to enter the water system if a siphon attempts to form. The design of all vacuum breakers is the same: when a vacuum occurs (negative pressure) the breaker opens to admit air and relieve the vacuum. AVBs are small, simple and inexpensive devices that require little maintenance or testing and have long life spans.