To understand what backflow is, it is helpful to understand some of the main principles first:
Cross connection
A cross connection is any actual or potential connection to potable water from any source that may contain contaminants.
Common cross connections are: underground irrigation systems, soda machines, and each house that is tied into the main water supply system.
Backpressure
Backpressure occurs when the downstream pressure in a water line is higher then the supply pressure.
Backpressure can be caused by:
- elevated piping downstream (for example, multistory buildings)
- a pump downstream that can overcome the system pressure
- thermal expansion created by a boiler/water heater heating water and forcing it to flow back in to the system
Backflow
Backflow is, simply put, when water flows backwards. When backflow occurs, it can pull contaminants from any potential cross connection, and release them into the potable system.
For backflow to occur there must be either backpressure or backsiphonage.
Backsiphonage
Backsiphonage is backflow caused by a partial vacuum in the piping system. Anytime the pressure in a system drops below 14.7 PSI (atmospheric pressure), it creates a negative pressure, (a siphon within the line).
Backsiphonage can be caused by:
- heavy line use (for example, if there is a fire)
- main line break
- pump failure
- water column collapse in elevated piping
- Venturi Effect (when pressure is forced through a restriction, the velocity increases and the pressure decreases, creating a siphon)
Backflow Prevention
It is our job to prevent backflow from happening, to make sure our community's water remains safe.
At the point of any cross connection, there should be a backflow prevention "Assembly" (which are testable, like the Double Check assembly at your meter) or a backflow prevention "Device" (which are not testable, like the vacuum breakers on your hose bibs).
Backflow assemblies and devices all work differently to protect from different hazards and hydraulic conditions, but all keep water from flowing backwards.
Because backflow assemblies are mechanical, they need to be tested and maintained regularly. Just like your car, things wear out and break over time, making your backflow assembly not function properly. Backflow assembly testing is done to ensure that our water remains contamination free. Here in Oregon, much like most other states, testing is required at least annually, as well as when the assembly is installed, repaired or replaced. Tests must be completed by a certified tester. This law is in place to protect the health of your family, and the entire community.
(Read the law HERE)