A backflow preventer is a means or mechanism to prevent backflow.
A cross-connection is any temporary or permanent connection between a public water system or consumer’s potable (i.e., drinking) water system and any source of system containing nonpotable water or other substances.
Backflow is the undesirable reversal of flow of nonpotable water or other substances the through a cross-connection and into the piping of a public water system or consumer’s potable water system. There are two types of backflow. Backpressure backflow and backsiphonage.
Backpressure backflow is backflow caused by a downstream pressure that is greater than the upstream or supply pressure in a public water system or consumer’s potable water system. Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure, or a combination of both. Increases in downstream pressure can be created by pumps, temperature increases in boilers, etc. Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting, or breaks in water mains.
Negative pressure is backflow caused by a negative pressure in a Public water system or consumer’s potable water system. The effect is similar to drinking water through a straw. Negative pressure can occur when there is a stoppage of water supply due to nearby fire fighting, a break in a water main, etc.
The basic means of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow.
The basic mechanism for preventing backflow is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal types of mechanical backflow preventer are the reduced pressure principle assembly, the pressure vacuum breaker assembly, and the double check valve assembly. A secondary type of mechanical backflow preventer is the residential dual check valve.
Mechanical backflow preventers have internal seals, springs, and moving parts that are subject to fouling, wear, or fatigue. Additionally, mechanical backflow preventers and air gaps can be bypassed. Therefore, all backflow preventers have to be tested periodically to ensure that they are functioning properly.
Because backflow into a public water system can pollute or contaminate the water in that system, making it unusable or unsafe to drink. Each water supplier has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances.
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