Backflow preventers are mechanical plumbing devices installed in a plumbing system to prevent water from flowing backward in the system.
When working properly they can reliably prevent the backflow of water of an unknown quality from flowing back into the community water system. There are degrees of protection ranging from a vacuum breaker on a garden hose line to reduced pressure zone backflow preventers commonly used in an industrial setting where chemicals are connected to water lines. Any interconnected fluid systems in which the pressure of one exceeds the pressure of the other may have flow from one to the other as a result of the pressure differential. The flow will occur from the zone of higher pressure tot he zone of lower pressure. Backflow can occur two different ways, by backsiphonage and backpressure.
Backsiphonage occurs when there is a sudden reduction in the water pressure in the distribution system, such as during a water main break or fire fighting, water flow can be reversed. This can create a suction effect, drawing potential harmful contaminates into the potable water distribution system.
Backpressure is created when pressure in a nonpotable system exceeds the pressure in the potable system providing the makeup water. This can force the potable water to reverse its direction of flow through the cross-connection between the two systems.
A backflow device protects drinking water from coming into contact with a questionable contaminant. When installed correctly and operating properly, it will prevent contaminants from entering the drinking water supply, even when low or negative pressure situations occur.
Some Common Backflow Preventer device types
Reduced pressure zone device (Hazard rating – High)
This device has two independent action, non-return valves that are force-loaded to the closed position. A relief valve is positioned between the non-return valves and is arranged to be force-loaded to open to atmosphere. With this design criteria, the reduced pressure principle backflow preventer can provide protec-tion against backsiphonage and backpressure when both the first and second checks become fouled.
Break tank/ air gap (Hazard rating – High)
This device has an air break (physical gap) between the lowest water service pipe outlet and any upstream tank, fitting or container. In general, the air gap must be twice the supply pipe diameter but never less than one inch.
Testable double check valve (Hazard rating – Medium)
This device has two independent action, non-return valves that are force-loaded to the closed position. IT is essentially two single check valves coupled within one body and furnished with test cocks and two tightly closing gate valves.
Pressure vacuum breaker assembly (PVB)
A PVB is a testable mechanical backflow preventer that consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded air inlet valve on the discharge side of the check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks. A PVB may be used to isolate health or non-health hazards, and is to be installed 12-inches above highest downstream water discharge. It is effective against back siphonage only and is most commonly used on lawn sprinkler systems.
Under the provisions of the Safe Drinking Water Act of1974, the Federal Government has established, through the EPA (Environmental Protection Agency), national standards of safe drinking water. The states are responsible for the enforcement of these standards as well as the supervision of public water supply systems and the sources of drinking water. The water purveyor (supplier) is held responsible for compliance to the provisions of the Safe Drinking Water Act, to include a warranty that water quality provided by his operation is in conformance with the EPA standards at the source, and is delivered to the customer without the quality being compromised as a result of its delivery through the distribution system.