# Determining Sprinkler GPM



## Hoosier (Jun 12, 2018)

This is hopefully my final question regarding my irrigation re-design. I am planning to use all rotator nozzles, and am deciding which ones to use based on the distance I need, and then determining the associated GPM. As of now, I am using a mix of R-VANs and MPs (in different zones), in order to be as precise as possible on the distance, but I think the answer to this question is the same for both.

Here is the MP spec sheet for 90 degrees (again, answer applies to all arc sizes):



So as an example, let's say I have a radius of 23 feet. I look at the specs, and subtract 2 feet from each of these, to account for the real world distance as opposed to the manufacturer's stated distances that are measured in zero wind conditions. Based on that, I see that the MP3000 at 25 PSI is what I need. I will use the PRS40 to regulate the pressure down to 40PSI (it's 55PSI unregulated, measured with the Hunter rotor pressure gauge), and then I will adjust the radius down to 23 feet.

So based on that, a few questions:
1) By adjusting the radius down, am I essentially reducing the PSI (I believe this is what irrigation tutorials is saying, but wasn't 100% sure)? And if this is the case, and Hunter states that these perform best at 40PSI, is this going to create a problem?
2) What is my GPM for that head? The specs say .71 for the _distance _that I will adjust it to, but that nozzle at a _pressure _of 40PSI has a .86 GPM flow.

What I'm not sure of is if I adjust the radius, am I reducing the GPM by essentially reducing the PSI, or retaining the GPM of the regulated PSI, and just reducing the distance that it throws?

And final question - if I have an available 18GPM flow, what would be the highest flow that I should plan for each zone? I assume 18 is too high, in case I adjust things later or miscalculated the flow by a bit, but is it ok to plan for 90% of that? 80% of that?

Thanks again!


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## g-man (Jun 15, 2017)

1) No. Pressure is not being regulated. You are reducing flow. You should use 40PSI from the table and use the 40PSI regulated bodies OR use 30PSI and the 30PSI regulated bodies.

2) I would use the flow that matches the pressure. If you are using the MP3000 at 40PSI, then use 0.86gpm in your calculations. If you are using 30PSI, then use 0.76.

Yes, adjusting the radius via the top screw will reduce the GPM by a little bit, but for your calculations, error on the side that your zone GPM does not exceed your system design GPM.

if you have 18GPM, then you can design to 18GPM or less. A 18GPM zone/pipe does force you into larger pipe size to avoid the other part of the problem, pressure losses. The higher the flow, the higher the pressure loss times the length of pipe.


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## Hoosier (Jun 12, 2018)

Thanks @g-man

This is what confused me from the tutorial (https://www.irrigationtutorials.com/sprinkler-coverage-nozzle-selection-sprinkler-spacings/):

"For spray heads you just use the manufacturer's chart. When you use the radius adjustment on a spray you are simply reducing the water pressure by closing a small valve in the nozzle. As the pressure drops, so does the radius. Just look at the manufacturer's chart for the radius you plan to reduce the sprinkler down to. Then read the GPM for that radius! For example, you're designing for 30 PSI. The radius at 30 PSI of the sprinkler you selected is 15 feet with 1.85 GPM according to the manufacturer's chart. But you want the radius to be 14 feet. Looking at the manufacturer's chart you see that the radius of the same sprinkler is 14′ at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you reduce the radius to 14′ will be 1.65 GPM. That's because when turn the radius adjustment screw to reduce the radius to 14′ what you REALLY did was reduce the pressure to 25 PSI!"

But I think you're suggesting to use the flow of the PSI that I will regulate to, not the distance... When shortening the radius, there will be less flow, but better to design using the flow for the starting pressure to prevent designing in too much flow for that zone, since you can always reduce the flow, but you can't add more, without a lot of re-work in the future. Let me know if I didn't understand your suggestion, otherwise I think I'm all set! Thanks again, you've been very helpful with these questions...


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## g-man (Jun 15, 2017)

:thumbup: that's i would do.

It is better to have margin between zone and design GPM. Sometimes in the peak of summer at 6am, water pressure from the city drops a bit with everyone trying to water their lawns. You still want a working system.

My system design GPM is 7 (3/4 PEX). I only have 1 zone at 5.7 and all the others at 5 or less.


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