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Backflow is the undesirable reversal of the flow of nonpotable water or other substances 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 and back siphonage.
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A cross-connection is any temporary or permanent connection between a public water supply or consumer's potable (i.e. drinking) water system and any other source containing nonpotable water or other substances. An example is the piping between a public water supply or consumer's potable water system and an auxiliary water source, cooling system, or irrigation supply.
Back pressure backflow is 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 (i.e. downstream pressure that is greater than the potable water supply pressure) can result from several things: an increase in downstream pressure which can be created by pumps, temperature increases in boilers, etc; a reduction in the potable water supply pressure which can occur whenever the amount of water being used exceeds the amount being supplied, such as during water line flushing, fire fighting, or breaks in water mains; or a combination of both.
Back siphonage is backflow caused by a negative pressure (i.e. a vacuum or partial vacuum) in a public water system or consumer's potable water system. The effect is similar to drinking water through a straw and can occur when there is a stoppage of water supply due to nearby firefighting, a break in a water main, etc.
Backflow into a public water system can pollute or contaminate the water in that system (i.e. backflow into a public water system can make it unusable or unsafe to drink), and each water supplier has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances and must take reasonable precautions to protect its public water system against backflow. Furthermore, consumers generally have absolute faith that water delivered to them through a public water system is always safe to drink.
Water suppliers usually do not have the authority or capability to repeatedly inspect every consumer's premises for cross-connections and backflow protection. Alternatively, each water supplier should ensure that a proper backflow preventer is installed and maintained at the water service connection to each system or premises that poses a significant hazard to the public water system. Generally, this would include the water service connection to each dedicated fire protection system or irrigation piping system and the water service connection to each of the following types of premises:
A backflow preventer is a means or mechanism to prevent backflow. The basic means is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. The basic mechanism is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal types of mechanical backflow preventers 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.
An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against backpressure backflow or back siphonage but is not always practical and can easily be bypassed.
An RP is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks. An RP is effective against backpressure backflow and back siphonage and may be used to isolate health or non-health hazards.
A DC is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves. It includes shutoff valves at each end of the assembly and is equipped with test cocks. A DC is effective against backpressure backflow and back siphonage but should be used to isolate only non-health hazards.
A RDC is similar to a DC in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valves. However, it usually does not include shutoff valves, may or may not be equipped with test cocks or ports, and is generally less reliable than a DC. A RDC is effective against backpressure backflow and back siphonage but should be used to isolate only non-health hazards and is intended for use only in water service connections to single-family homes.
Mechanical backflow preventers have internal seals, springs, and moving parts that are subject to fouling, wear, or fatigue. Also, mechanical backflow preventers and air gaps can be bypassed. Therefore, they have to be tested periodically to ensure that they are functioning properly. A visual check of air gaps is sufficient, but mechanical backflow preventers have to be tested with properly calibrated gauge equipment.