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Old 04-27-2017, 01:48 PM   #1
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Lawn irrigation- PSI vs GPM


I'm planning to install a shallow well and an irrigation system for my lawn. I'm confused about the flow rate (GPM) needed versus the PSI. Technically, the PSI is going to be zero when the system isn't calling for water. Any PSI generated will be a product of the pump running.

When the pump turns on it will have a flow rate determined by the lesser of the pump's rate or the well's actual flow rate. So if the pump can produce 20 GPM and the well can only produce 15 GPM, then it will produce the 15.

If I have 5 heads per zone and each requires 2.5 GPM, then that's a total GPM of 12.5 GPM per zone and I should be fine because I can produce 15 GPM from the pump.

Am I looking at this correctly? Does the PSI come in to play when the piping runs are so long that friction reduces the PSI at the sprinkler heads?

Or is it the excess GPM that affects the PSI? If the pump is trying to supply 15 GPM and the sprinkler heads are only using 12.5 does the excess 2.5 GPM translate to some PSI value?
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Old 04-27-2017, 03:46 PM   #2
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Re: Lawn irrigation- PSI vs GPM


wow you really thought a lot about this. I'm not an engineer but I have some general understanding of this. look at any pump curve and you'll see that as GPM increases PSI decreases. To answer your question, yes length of run and friction loss will affect PSI at the head. I do not believe the extra GPM translates to additional PSI. What PSI is your pump rated for at 15 GPM?
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Old 04-27-2017, 04:08 PM   #3
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Re: Lawn irrigation- PSI vs GPM


I don't have a pump or a well yet. I want to understand what kind of GPM I'll need to get from the well to drive the sprinkler heads. I also need to get a properly sized pump for the well. If it ends up being 25 feet to water and I need 15 GPM then there's no need to spend more for a pump that can lift higher or output at 50 GPM. This stuff seems counterintuitive to me but I'll check out some pump specs and do a little reading. Thanks for the tips.
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Old 04-28-2017, 07:19 AM   #4
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Re: Lawn irrigation- PSI vs GPM


You'll need a certain pressure @ the sprinkler head for it to create an effective spray while distributing the 2.5 gpm. If you cant supply that you need bigger pipe or higher pressure from the pump.
Never had a sprinkler, but I'm a fireman and should be the same thing.

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Old 04-28-2017, 01:45 PM   #5
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Re: Lawn irrigation- PSI vs GPM


PSI and GPM are two different things but they work together to provide an end result. Since a sprinkler system runs on pressure, I'd focus less on GPM and more on PSI. Like Tommy stated above you'll need a certain PSI to make each head pop up. Typically no less than 40 psi.
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