A backflow prevention assembly is used to protect potable water supplies from contamination or pollution due to backflow.
In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc. When pressure fails or is reduced, as may happen if a water main bursts, pipes freeze or there is unexpectedly high demand on the water system, then such reduced pressure in the pipe may allow contaminated water
from the ground, from storage or from other sources to be drawn up into the system.
By definition, a backflow is an undesirable reversal of flow of a liquid, gas or solid into the potable water supply. A backflow preventer keeps this from happening.
back-siphonage occurs when higher pressure fluids, gases, or suspended solids move to an area of lower pressure fluids. For example, using a straw to drink a beverage; the suction of drink will make the pressure of fluid inside the straw lower, thus, liquid will move from the cup to inside the straw and then up to your mouth. This is an example of an
If instead, air is blown through the straw and bubbles begin to erupt at the submerged end, this is an example of back-pressure. If instead of air, natural gas had been forced into a potable water tank, it in turn could be carried to your kitchen faucet. This is an example of a direct cross-connection.
Back pressure can force an undesirable contaminant to enter potable water piping. Sources of back pressure may be pumps in the water distribution system, boilers, heat exchanging equipment, or power washing equipment. In these cases there may be an almost constant 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 (boilers) or when bleaches are used for residential power washing. These hazards can be severe, some cases can be fatal.
The simplest, and a very effective way to provide backflow prevention, is to provide an
air gap. An air gap is simply an open space between any device that connects to a plumbing system (like a valve or faucet) and any place where water can collect or pool.
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. A check valve is a common form of backflow prevention, but more complex devices may instead be required.
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.
In many countries. approved backflow prevention assemblies are required by law, and must be installed in accordance with plumbing or building code
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 (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
- Air gap
- Atmospheric vacuum breaker (AVB)
- Check valve (usually not a legally approved method of backflow prevention)
- Chemigation valve (primarily used in agriculture)
- Double check valve, or double check valve assembly (DCVA)
- Pressure vacuum breaker assembly
- (PVB) Reduced Pressure Principle Valve (RPPV or RP Device)
- Reduced pressure zone device (RPZ)
- Spill Resistant Pressure vacuum breaker assembly (SPVB)
- Vacuum breaker