Backflow Prevention Devices – Why they exist and why you may need to install one
Why do I need a backflow prevention device on my irrigation system?
Because they can save lives and it is in many cases is a legal requirement
95% of the landscape irrigation systems we install or design require a backflow prevention device. So there is a good chance you need one on your irrigation system
Basically many of the commercially available backflow prevention devices contain one-way valves, which only allow the water to travel in one direction. This means that if nasty contaminants enter the water through the irrigation system, your drinking water supply is protected from those contaminants.
When installing a backflow prevention device you need to consider a few things, including:
1. Hazard Levels
2. Pressure Loss
3. Where the backflow device is placed
What Are Hazard Levels?
- Low Hazard – domestic situation, hooked up to the drinking supply and has nothing fancy. We install a Untestable Dual Check Valve Device.
- Medium Hazard – generally used on a commercial project, hooked into a drinking water supply. We install a Testable Double Check Valve Device.
- High Hazard – usually on a commercial project where an alternate water supply exists or some form of chemical injection into the irrigation system is required (fertilisers, chlorine, treflan or another chemical). We install an air gap (which is a method rather than a device) or a RPZ reduced pressure zone device
How does the pressure loss through a backflow device affect irrigation systems?
Pressure loss through the backflow prevention device is not so much an issue with things like drinking fountains, baths, showers or even garden hoses; the water pressure would have to be extremely low for anyone to complain about these sorts of situations.
With pressurised irrigations systems, there is a minimum pressure required to run the emitters, for dripline it’s a much lower pressure, it could be anywhere from 80 – 150 kPa, but for a sprinkler, at the head, you need around 280 – 350 kPa or even more. Irrigation wont work without the right pressure to the outlets
Every fitting, valve or bit of pipe in an irrigation system will cause some form of pressure loss, Backflow prevention devices cause a significant amount of pressure loss. This needs to be considered in the irrigation design.
Where is the backflow prevention device placed?
Individual or Zone Device – the one that protects the water supply within the property is installed at the point of connection to the irrigation system
Containment Protection Device – Placed directly downstream of the water meter, to protect the rest of the water network(Public or town).