- Public Works
- Water Treatment & Distribution
- Cross Connection Control Program
Cross Connection Control Program
Backflow Preventer Necessity
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, 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, a backflow preventer is designed to keep this from happening.
Cross Connections Defined
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. 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 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.