If you are installing a new sprinkler system, maintaining your current system, or retro-fitting an existing system you need to know how many gallons per minute are available for you to use. The easiest way to find this number is to do a google search for a flow chart for your particular type of sprinkler pipe. To see a flow chart for Schedule 40 PVC pipe see my blog post on Flow, Velocity, & Friction Loss.
Other factors that influence how many gallons per minute you can safely use in your sprinkler system are pressure, pipe size, and type of pipe used in the sprinkler system.
To make things more clear, let’s try an example. In a yard I recently worked on the water meter size was 3/4″, the static water pressure was 65 psi, and the pipe size of the sprinkler system was 3/4″ PVC Schedule 40. If I go to the flow chart and look up 3/4″ PVC Schedule 40 IPS Plastic Pipe I find that if I want to use 10 gallons per minute on a sprinkler zone the velocity in feet per second is 6.00 and the friction loss over 100 feet of pipe is 7.77 psi. All these numbers are within recommended ranges. That would make the dynamic (working) pressure of the system somewhere around 58 psi at 10 gallons per minute. If however, I want to use 18 gallons per minute on the same sprinkler zone, the velocity increases to 10.81 feet per second and the friction loss increases to 23.07 psi. Not a good situation.
Flow problems in sprinkler systems are the most common problems I encounter where I live. How flow in your sprinkler pipes affects the sprinkler heads in a zone works like this. If you have a sprinkler zone with 10 sprinkler heads on it, each sprinkler head uses “x” gallons per minute. If the sprinkler heads are trying to use 18-20 gallons per minute and the sprinkler is only safely capable of delivering 10-12 gallons per minute, you can see where problems would arise. It’s like the bucket of water you see at the top of the page, there is only a certain amount of water the bucket can deliver. Sprinkler systems work in much the same way, your sprinkler piping can only deliver a certain amount “volume of flow” of water before problems start occurring.
When I design or work on sprinkler systems, I always work within safe limits of what the system will deliver in flow. It often means that a few extra sprinkler valves need to be installed to keep within these limits, but I never have to go back and work on the system because it has performance and coverage problems.