Question
QUESTION: We have a 2" water line from the City and a 1 1/2" line into the irrigation system. Should we install an 1 1/2" RP backflow or does a 2" RP backflow provide more pressure to the irrigation system?

Thanks

ANSWER: Sam,

The answer for this question really depends on the requirements for the rest of the system.

As a general rule pressure losses for reduced pressure backflow preventers range from 8 and 12 PSI. So using 12 PSI for your 2" would be a fairly safe figure. Pressure losses for reduced pressure backflow preventers are very high compared to other backflow preventers due to the method they use to prevent backflow.

You will will be able to move more water (at a lower downstream pressure)with the 2" backflow, but if you don't need it, then don't pay the money for the bigger size and just go with the 1 1/2".

Hope this helps. - BC

---------- FOLLOW-UP ----------

QUESTION: Thanks for your response. So even if the main irrigation lines are 1 1/2", we will get more PSI (and better sprinkler coverage) in the system by using a 2" back flow vs the 1 1/2" back flow?

The commercial property previously had a old non-working vacuum back flow device and it got changed to a 1 1/2 inch RP Backflow (City required?). The owner thinks the PSI has been reduced (less coverage)and maybe a 2" RP deviced should have been installed instead of matching the backflow size to the 1 1/2 pipe. Again the City main is 2", the irrigation system lines are 1 1/2".

Thanks, Sam

Answer
Sam,

Adding a 2" backflow would allow more volume through, but as flow increases, pressure decreases downstream. Is the reduced pressure that the owner perceives even noticable in the coverage pattern. We are only talking about 4-6 PSI.

When your system was designed and installed, all points of friction loss (pressure loss) would have been calculated (backflow, valves, fittings, pipe size, etc.). Pressure and flow are separate principles, but are related at the same time. If you are having a pressure problem, then most likely you will want to stick with the 1 1/2". If you don't have enough water to fulfill the nozzle requirements of the heads on each vavle, you have a flow problem.

The best solution here is to check the flow/calculations of the proposed backflow (zurn possibly?). This info will tell you how much pressure loss you will have by moving to a 2" backflow. You can then determine if that loss will further effect your head performance.

Let me know if you have other questions. - BC