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Backflow is the reversal of the normal and intended direction of water flow in a water system. Devices and assemblies
known as backflow preventers are installed to prevent backflow, which can contaminate potable water supplies.
Why is backflow a problem?
Backflow is a potential problem in a water system because it can spread contaminated water back through a
distribution system. For example, backflow at uncontrolled cross connections (cross-connections are any actual or
potential connection between the public water supply and a source of contamination or pollution) can allow pollutants
or contaminants to enter the potable water system. Sickness can result from ingesting water that has been
contaminated due to backflow.
Backflow may occur under the following two conditions:
Back-pressure is the reverse from normal flow direction within a piping system as the result of the downstream
pressure being higher than the supply pressure. This reduction in supply pressure occurs 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).
Back-siphonage is the reverse from normal flow direction within a piping system that is caused by negative pressure
in the supply piping (i.e., the reversal of normal flow in a system caused by a vacuum or partial vacuum within the
water supply piping). 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 during fire-fighting or water-main flushing).
Atmospheric Vacuum Breakers
Backflow prevention for residences is most commonly accomplished through the use of atmospheric vacuum
breakers (AVBs). AVBs operate by allowing the entry of air into a pipe so that a siphon cannot form. AVBs are bent
at 90 degrees and are usually composed of brass. Compared with backflow preventer assembles, AVBs are small,
simple and inexpensive devices that require little maintenance or testing. They have long life spans and are suitable
for residential purposes such as sprinkler systems.
- The AVB must be at least 6 inches above any higher point downstream of the device. For this reason, they
can never be installed below grade. Even if they are installed 6 inches above grade, inspectors should make
sure that they are not installed less than 6 inches above some other point in the system downstream of the
- The AVB cannot be installed in an enclosure containing air contaminants. If contaminated air enters the water
piping, it can poison the potable water supply.
- A shut-off valve should never be placed downstream of any AVB, as this would result in continuous pressure
on the AVB.
- AVBs cannot be subject to continuous pressure for 12 hours in any 24-hour period or they may malfunction.
- Spillage of water from the top of the AVB is an indication that the device has failed and needs to be replaced.
Types of Backflow Preventer Assemblies
Some types of assemblies are common in commercial and agricultural applications but are rare for residential uses.
The appropriate type of backflow preventer for any given application will depend on the degree of potential hazard.
The primary types of backflow preventers appropriate for use at municipalities and utilities are:
- Double check valves: These are commonly used in elevated tanks and non-toxic boilers. Double check-valve
assemblies are effective against backflow caused by back-pressure and back-siphonage and are used to
protect the potable water system from low-hazard substances. Double-checks consist of two positive-seating
check valves installed as a unit between two tightly closing shut-off valves, and are fitted with testcocks.
- Reduced pressure principle assemblies: These are commonly used in industrial plants, hospitals, morgues,
chemical plants, irrigation systems, boilers, and fire sprinkler systems. Reduced pressure principle assemblies
(RPs) protect against back-pressure and back-siphonage of pollutants and contaminants. The assembly is
comprised of two internally loaded, independently operating check valves with a mechanically independent,
hydraulically dependent relief valve between them.
- Pressure vacuum breakers: These are commonly used in industrial plants, cooling towers, laboratories,
laundries, swimming pools, lawn sprinkler systems, and fire sprinkler systems. Pressure vacuum breakers use
a check valve designed to close with the aid of a spring when water flow stops. Its air-inlet valve opens when
the internal pressure is one psi above atmospheric pressure, preventing non-potable water from being
siphoned back into the potable system. The assembly includes resilient, seated shut-off valves and testcocks.
Requirements for Testers and Inspectors
A number of organizations, such as the American Water Works Association (AWWA) and the American Backflow
Prevention Association (ABPA) offer certification courses designed to train professionals to test backflow preventers.
Requirements for training vary by jurisdiction. Inspection of backflow preventers requires knowledge of installation
requirements, although inspectors are not required to become certified.
Hose faucet screw on type
Vacuum breaker made on top