SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Don't guess. Run the numbers. www.irrigationtutorials.com

So if you have only 11 gpm then you can only use 3 of the #8 nozzles. =10.5 gpm

There is no such thing as gpm of a pipe v. gpm of a nozzle.

What you do have is a RainBird supplied specification that says a blue,8 nozzle in a maxi-paw will produce a flow rate of 3.5gpm IF the water pressure at the nozzle is 40psi. If the pressure at the nozzle is NOT 40psi, the flow will be higher or lower depending upon the pressure. As an example, based on the RainBird specs I can find online, if the water pressure is 25psi at the nozzle, the flow will only be 2.8gpm. If the pressure is 60psi, the flow will be 4.2gpm.

But here's the tricky question... what is the pressure AT THE NOZZLE, because as water flows through your system, the pressure drops for everything it passes through. But that isn't easy to solve for because the amount of the water pressure drop is a function of the flow. So as you can see, flow is dependant upon pressure and pressure is dependant on flow. So because of the innerdependance of these two values, designing an irrigation system is more of an art that simple science calculations.

As for the pipe, the gpm of the pipe is just the gpm of all the nozzles that pipe feeds. Otherwise, the only gpm related measurement that I can think of associated with a particular size of pipe is the maximum gpm the pipe can handle without violating the 5ft/sec speed limit. Basically, one of the standards in plumbing is that water should not be allowed to flow any faster than 5 feet per second. The basic reason is that moving water has dynamic energy, and as water speed doubles, the energy increases four fold. And is a water valve is suddenly closed, all that dynamic energy has to go somewhere. That's the basic cause of water hammer... water "crashing" into a suddenly closed valve.

So when it comes to 3/4" Sch 40 PVC pipe, the maximum suggested flow rate is a little over 8gpm to avoid violating the 5ft/sec speed limit. However, it sounds like you are using 3/4" Class 200 PVC. Because of the difference in inside diameters, Class 200 PVC has a flow rate of 5.2ft/sec at 11gpm (perhaps close enough to the 5ft/sec speed limit to keep you from "getting a ticket".

Now the other thing to realize is that higher flow rates equate to higher pressure losses. Pushing 11gpm through 3/4" Class 200 PVC is going to loose a little over 5psi for every 100 feet. And of course just to continue complicating things, the flow through all your pipe is likely not the same... because presumably you have one pipe that feeds the 1st nozzle, then that same pipe continues to the 2nd, the 3rd, and the 4th nozzle. So the pressure loss per every 100' of pipe will be different in the pipe that leads to the 1st nozzle v. the pipe that goes from the 1st nozzle to the 2nd nozzle, etc. This also means that the pressure at each nozzle WILL be different and therefore the flow rate of each nozzle WILL be different.

Hope that gives you a partial answer and gives you some food for thought.