Backflow prevention is taking measures to ensure that water does not reverse flow in the distribution system. Since it's not feasible to make Northglenn's entire water system “backflow proof,” the city focuses its efforts on the parts of the system where a backflow event would do the most harm. Often this is where a cross-connection exists. If a cross-connection cannot be eliminated, the city uses a device or mechanical assembly to prevent water from reversing direction in the distribution system. As long as the device is working properly, the system should remain free of the harmful substances that are present at the cross-connection, even during a backflow event.
Common Backflow Preventers
- Air gap - A physical separation between the discharge end of a water pipe and an open or receiving vessel. Example: Each sink in your home has an air gap under the faucet so water can never be siphoned out of the sink.
- Hose bibb backflow preventer - Consists of two check valves spring-loaded to a closed position, and an atmospheric vent. This device screws onto your outside hose spigot and prevents backsiphonage through the hose.
- Pressure vacuum breaker (PVB) - Consists of a spring-loaded (normally closed) check valve and an independently acting air inlet valve spring-loaded to a normally open position. As water flows, it opens the check valve and closes the air inlet valve. When the assembly experiences a vacuum condition, the air inlet valve opens in order to break the vacuum and stop backflow. Pressure vacuum breakers are often used on lawn sprinkler systems. Atmospheric vacuum breakers (AVB) are similar though the air inlet is not spring-loaded and they lack a check valve.
- Double check valve assembly (DC) - Consists of two independently acting check valves spring-loaded to a normally closed position. Commonly used on fire protection system water lines, this assembly prevents backflow by only allowing water to move in one direction.
- Reduced pressure principle assembly (RP) – Next to the air gap, the RP is the most fail-safe device that can be used. It functions similarly to the DC, but has an additional relief valve that allows contaminated water to exit the system in case one of the check valves fails. RP assemblies are used on “high hazard” systems, such as lawn sprinklers, boiler feed lines and connections to other industrial fluid systems.
Why So Many Types?
All of these devices are uniquely suited to different applications and hazard levels. Each device has its own strengths and weaknesses. The RP, for example, is the most fail-safe mechanical backflow preventer, but as such it is also the most expensive device on the market. Given these two factors, the city only installs RPs where the risks to public health are extremely high. Where other devices can be used (ie., where the risk is lower), the city uses the less expensive options.
If you are personally installing a backflow prevention assembly on your property, it is important to make the proper decision. Contact the city for advice on choosing the proper backflow preventer. If you are having a plumbing contractor do the work, they should know which device is appropriate for your uses.