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A backflow prevention device is used to protect
potable water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
supplies from contamination or pollution due to
backflow Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its wate ...
. In
water distribution system A water distribution system is a part of water supply network with components that carry potable water from a centralized treatment plant or wells to consumers to satisfy residential, commercial, industrial and fire fighting requirements. Definit ...
s, water is normally maintained at a significant pressure to enable water to flow from the tap, shower, or other fixture. Water pressure may fail or be reduced when a water main bursts, pipes freeze, or there is unexpectedly high demand on the water system (for example, when several
fire hydrant A fire hydrant, waterplug, or firecock (archaic) is a connection point by which firefighters can tap into a water supply. It is a component of active fire protection. Underground fire hydrants have been used in Europe and Asia since at least t ...
s are opened). Reduced pressure in the pipe may allow contaminated water from the soil, from storage, or from other sources to be drawn up into the system.


Terminology

Backflow means the undesirable reversal of flow of a liquid, gas, or suspended solid into the potable water supply; a backflow preventer is designed to keep this from happening. Points at which a potable water system connects with a non-potable water system are called cross connections. Such connections occur naturally in appliances such as clothes washers and
dishwasher A dishwasher is a machine that is used to clean dishware, cookware, and cutlery automatically. Unlike manual dishwashing, which relies heavily on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, ...
s, but they must be carefully designed and installed to prevent backflow. Another common location for a backflow preventer is the connection of a
fire sprinkler A fire sprinkler or sprinkler head is the component of a fire sprinkler system that discharges water when the effects of a fire have been detected, such as when a predetermined temperature has been exceeded. Fire sprinklers are extensively used ...
system to a water main, to prevent pressurized water from flowing from the fire suppression system into the public water supply. Back-siphonage occurs when higher pressure fluids, gases, or suspended solids move to an area of lower pressure fluids. For example, when a
drinking straw A drinking straw is a utensil that is intended to carry the contents of a beverage to one's mouth. Straws are commonly made from plastics but environmental concerns and new regulation have led to rise in reusable and biodegradable straws. These ...
is used to consume a beverage, suction reduces the pressure of fluid inside the straw, causing liquid to move from the cup to inside the straw and then into the drinker's mouth. A significant drop of pressure in a water delivery system creates a similar suction, ''pulling'' possibly undesirable material into the system. This is an example of an indirect cross-connection. Back-pressure occurs for example when air is blown through the straw and bubbles begin to erupt at the submerged end. If instead of air,
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...
had been forced into a potable water tank, the gas in turn could be carried to a kitchen faucet. This is an example of a direct cross-connection, with undesirable material being ''pushed'' into the system. Back pressure can force an undesirable contaminant to enter potable water piping. Sources of back pressure may be boilers, heat exchanging equipment, power washing equipment, fire sprinklers, or pumps in the water distribution system. In some cases there may be an almost continuous risk of overcoming the static water pressure in the piping. To reduce the risk of contamination, a backflow preventer can be fitted. A backflow preventer is also important when potentially toxic chemicals are used, for instance for commercial/industrial
descaling A descaling agent or chemical descaler is a liquid chemical substance used to remove limescale from metal surfaces in contact with hot water, such as in boilers, water heaters, and kettles. Limescale is either white or brown in colour due to ...
of boilers, or when chemical
bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...
es are used for residential power washing. A closely related device is the
backwater valve A backwater valve is a backflow prevention device used to prevent outbound water through a dwelling's drain pipes from re-entering -- " back flowing"—into a home. The valve contains a flap that allows water to exit the home, but closes to preven ...
, which is designed to prevent sewage from backing up into a building and causing basement flooding.


Preventive devices

The simplest, most reliable way to provide backflow prevention is to provide an air gap. An air gap is simply an open vertical space between any device that connects to a plumbing system (like a valve or faucet) and any place where contaminated water can collect or pool. A simple air gap has no moving parts, other than flowing water. Many
plumbing code A plumbing code is a code that provides regulations for the design, installation and inspection of building plumbing and sanitary systems. In the United States, jurisdictions enact their own codes, some of which are based upon model plumbing codes. ...
s specify a minimum air gap distance required for various circumstances, such as a drain connection for a dishwasher, e.g., BS 6282. Alternatively, a specialized backflow preventer valve may be installed at strategic locations in the plumbing system wherever there is a risk of contaminated fluids entering the water supply pipes. These valves are used where there is not sufficient vertical clearance or physical space to install an air gap, or when pressurized operation or other factors rule out use of an air gap. Because these valves use moving parts, they are often required to be inspected or tested periodically.


Regulatory requirements

To prevent contamination due to back pressure, many health regulatory regimes require an air gap or mechanical backflow prevention assembly between the delivery point of mains water and local storage or use. Where submerged mains inflow is permitted, a backflow prevention assembly is required, which protects the potable water system from contamination hazards. A
check valve A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. Check valves are two-port valves, meaning they have t ...
is a basic form of backflow prevention, but often more complex devices are required because check valves are not considered to be reliable, when compared to more sophisticated devices with redundancies and reduced-pressure zones. In many countries. approved backflow prevention assemblies are required by law, and must be installed in accordance with plumbing or
building code A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permiss ...
s. A typical backflow assembly has test cocks and shut-off valves, and must be tested when installed, if relocated or repaired, and also on a periodic basis. In the US, the
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scal ...
(EPA) holds local water suppliers responsible for maintaining a certain amount of purity in potable water systems. Many states and/or local municipalities require annual testing of backflow prevention assemblies. In most cases, the law requires a double check (DC), reduced pressure principle device ( RP) device, or an air gap when backflow prevention is mandated.


Partial list of backflow prevention devices and assemblies

* Air gap (AG the purest form of backflow prevention) * Atmospheric vacuum breaker (AVB) *
Check valve A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. Check valves are two-port valves, meaning they have t ...
(usually not a legally approved method of backflow prevention.) * Chemigation valve (primarily used in agriculture) * Double check valve assembly (DCVA) * Dual check valve (A dual check is not a testable device and is mainly used on residential customers) * Pressure vacuum breaker assembly (PVB) *
Reduced pressure zone device A reduced pressure zone device (RPZD, RPZ, or RPZ valve) is a type of backflow prevention device used to protect water supplies from contamination. RPZDs may also be known as reduced pressure principle (RP), reduced pressure principle backflow p ...
(RPZ) * Spill resistant pressure vacuum breaker assembly (SPVB) * Vacuum breaker


See also

*
Backflow (plumbing) Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water ...


References


External links


International Association of Plumbing and Mechanical OfficialsDrinking Water & Backflow Prevention magazineThe American Backflow Prevention AssociationFoundation for Cross-Connection Control and Hydraulic Research
{{Backflow Plumbing Irrigation Backflow