Is 40psi pressure too low for rotors?

18 years ago

I'm still debating about installing an inground system.

I have a 1" city water line and the pressure is 40psi.

With the pressure drop thru the meter, backflow preventer, automatic valve, line, and sprinker head, will the rotors provide a decent spray pattern? I'm told they may function will will only provide a stream of water and it develop a decent spray pattern to cover the area evenly.

I know I can get a pump but every time I hear about a new issue it ends up costing me.

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Comments (6)

The_Mohave__Kid
18 years ago
Hello ..

That depends ... ofcourse ... on your design.

"I'm told they may function will will only provide a stream of water and it develop a decent spray pattern to cover the area evenly."

This by itself does not make much sense to me.

Rainbird makes at least two rotors designed to work with pressures as low as 25 PSI at the head or more precisely the nozzle ( their 3500 and 5000 series for example ) .. these rotors are designed to spray anywhere from 15 to 35 feet depending on your nozzle selection. They will work fine at these pressures.

Are you certain you have 40 PSI ?? That sounds low for a 1 inch line supplied by a water supllier. By the way .. Why rotors ??

I'm glad to here you like to chat on gardenweb ... do visit the irrigation forum this winter it gets rather quiet around here at that time but it is a good time to plan.

Good Day ...
daddylonglegs
Original Author
18 years ago
thanks for the feedback. Yes, 40 psi is it.
New subdivision, new water tower. I'm on the top of a hill and tied into the folks down the hill so the city tied us over to a different line to avoid overpressurizing the downhillers.
Regarding my choice for rotors, I guess it's the size of my backyard about 65' by 65', I thought about 6 rotors on about 30' head to head would handle the task better than spray heads but I'm open to suggestions. Another designer said go with rotors for anything over 15'. I spoke with a designer several days ago to start getting a few bids, and he said 40 psi is too low and I'll need a pump. I may just go ahead without one and leave room in the basement in case I need one later.

Fall is now in full swing now so it looks like this may be a spring project. I would like to try trenching a zone this fall just to see how hard it is.
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The_Mohave__Kid
18 years ago
Hello daddylonglegs ...

I think you can design your system without a pump ... the trade off will be a bigger watering window .. that is the time your system turns on at night and the time it turns off ...

I'm not trying to design your system via computer but for conversation sake only here are some numbers ... lets say you use Rainbird's 3500 series Rotor with a BIG 4.0 nozzle.

Running 20 gallons per minute through a 1 inch water meter will cost you about - 3 PSI. A Rainbird 3500 4.0 nozzle needs 25 PSI to spray 31 feet and uses about 3 GPM.

Lets say your system needed 12 heads .. that is lets over figure for now since you say you need only six heads ... that would be a total flow of 36 GPM if you ran all the heads at once. Not what we want to do ..

Now if you used three valves each to supply 4 heads then each valve would supply 12 GPM. The pressure loss through 100 feet of schedule 40 PVC would be 4 PSI at a flow of 12 GPM. Lets assume you need 150 feet of pipe per valve then your pressure loss would be 6 PSI.

So ..- 3 PSI at the meter at 20 GPM
- 6 PSI loss at 12 GPM through 150' Sch 40 PVC
- 6 PSI Misc pressure loss ( an overly high estimate )
----------
Thats - 15 PSI... which still leaves you 25 PSI that you need but remember we overestimated here so lets back up ..

Lets assume you have a square lawn 65 x 65 feet. It would be better to go with 9 heads BUT a smaller nozzle .. lets say the 1.5 nozzle that will spray 23 feet at 25 PSI and use only 1.06 GPM instead of the 3.0 GPM of a big 4.0 nozzle. This would give you more overlap in the middle where you will need it .. better then six heads which would only just meet in the middle and thats with no wind.

At 4 gallons per minute per valve your pressure loss will be .75 for 150 feet of pipe. The meter loss will be much less about 0.1 PSI. So that gives us ...

- 0.1 PSI at the meter at 20 GPM
- 0.75 PSI loss at 12 GPM through 150' Sch 40 PVC
- 6. PSI Misc pressure loss ( an overly high estimate )
----------
- 7 PSI and leaves 33 PSI more then enough and even room for future pressure loss.

Good Day ...
daddylonglegs
Original Author
18 years ago
Hello Mohave Kid,

Thanks for the time and effort, I appreciate that.
I'm in agreement, for the most part. I may design each zone for 10 gpm but reserve the right to change my mind after I get the plumbers tap and deduct meter installed to see what kind of flow I can achieve. Right now the best I can do is 5 gpm at a hose bib but I know I have restriced flow there because of the 1/2" copper pipe feeding it.
I expect to have a combination master valve/backflow preventer after the meter which will give me an additional 5 psi drop, then run out thru the basement with 1" pipe then another 3 psi drop thru each anti-siphon auto zone vlv, then 1-2 psi drop thru 100' of 1" polypipe.
I didn't see the 3500 series at Menards but they have 32SA and 42SA which will provide 29' at 2.17gpm or 35' at 2.6 gpm respectively at 25 psi. Maybe 5 rotors per zone, use two zones for the backyard giving me 9 or 10 heads.
Using round numbers
- 1 PSI at the meter at 10 GPM
- 1 PSI at 10 GPM through 100' of 1" polypipe
- 5 psi backflow preventer
- 3 psi automatic valve
I still end up with 30 psi

Keep the faith...
mtorzews
18 years ago
Daddy,

I think you could make it work to, but you'll need to be carefull in the design. If your into this kind of thing go to www.irrigationtutorials.com. Its an excellent site that teaches you how to design the system. And will cover all of the pressure losses. Plus its free.

Next goto www.sprinklerwarehouse.com. You'll find a much better selection of rotors and nozzles here. Plus its cheaper. Don't kick stuck on Rainbird. They are good no doubt, but so is Hunter, Weathermatic, Toro ect.
egghead2004
18 years ago
Daddy,
I just installed a 10 zone system at my house. 17 heads/4 zones for the front, and 22 heads/6 zones in the back.

I'm on a well, that pushes 50-60 pounds. When the sprinklers are on my well pump is always running and the pressure is anywhere from 30-40 pounds, depending on which zone is running.

I have Hunter I-20 rotors. Using the 3.0 nozzle will send the water over 30' at 30 pounds of pressure.

Mine just turned on, I took a pic by my internet provider's FTP is currently down, so I'll post the pic later. These 4 heads are on 1 zone. They are all using 3.0 nozzles, I need to change the last one to 2.0, the arc 180 degrees instead of 360 so it's dumping twice as much water.

The heads are 30 feet apart. The 3.0 nozzles are spraying just about 28-29 feet, almost 100% head to head coverage.

The pressure at the supply pipe is 33 pounds, I'm not sure what it is at the head itself, 25 maybe?

The other thing, as metioned by the mojave kid, for 65x65, I would use at least 9 heads, don't skimp. Since you are worried about pressure, you may be better off with 12 heads on 3 zones.The head would only be 22 feet apart, as opposed to over 32 feet.

I also bought all the heads/timer/valves/risers at sprinkler warehouse, a great deal.

Is 40psi pressure too low for rotors?

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The_Mohave__Kid

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Is 40psi pressure too low for rotors?

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The_Mohave__Kid

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