A backflow prevention device 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, 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 hydrants 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.
Backflow means the undesirable reversal of flow of a liquid, gas, or suspended solid into the potable water supply. In other words, its a term in plumbing for an unwanted flow of water in the reverse direction. 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 dishwashers, 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 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 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. This is an example of an indirect cross-connection, undesirable material being pulled into the system.
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 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 of boilers, or when chemical bleaches are used for residential power washing.
A closely related device is the backwater valve, which is designed to prevent sewage from backing up into a building and causing basement flooding.
Backflow Prevention 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. Other preventive devices include but not limited to
- Air gap
- Atmospheric vacuum breaker (AVB)
- Chemigation valve (primarily used in agriculture)
- Double check valve, or double check valve assembly (DCVA)
- Pressure vacuum breaker assembly (PVB)
- Reduced pressure zone device (RPZ)
- Spill resistant pressure vacuum breaker assembly (SPVB)
- Vacuum breaker
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 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 codes.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
Risk of contamination
The precise measures required to prevent backflow depend on the risk of contamination, i.e. the condition of the water in the connected system. This is categorized into different risk levels:
- Category 1: No risk. Potable water
- Category 2: Aesthetic quality affected, e.g. water which may have been heated
- Category 3: Slight hazard from substances of low toxicity, e.g. cold water storage tanks
- Category 4: Significant hazard, e.g. pesticides
- Category 5: Serious health risk, e.g. human waste
How Crown can Help
Only licensed plumbers with backflow prevention accreditation issued by a registered training organisation (backflow plumbers) can inspect, commission and test backflow devices. With that said, Crown Plumbing Specialists are accredited in backflow prevention and works with Sydney Water to ensure compliance.
So Call Now, Call Crown Plumbing Specialists on 02 9042 1512