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I'm trying to build a sprinkler system for my large yard. I've set up 2 zones each one with 8 heads being supplied by a 2HP well pump.

I'm using 1" pipe everywhere.

The first zone works fine and the well maintains about 70PSI; the second zone, which is about 20 yards longer than the first and also has 8 heads, works but the well pump is cycling.

I'm guessing that the problem is that with the longer line, I don't have enough pressure to push enough water through.

If I split the 2nd zone line into two with 4 heads on each line, would that work? Each line would be shorter but I'm not sure that would make any difference.

Any advice?

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How is the pump being controlled (is there a pressure switch)? How is the irrigation system controlled? Is there anything else the pump is connected to besides irrigation? Is there a pressure tank in the system, and if so, how big is it? Is the well drilled (how deep?) or dug? –  gregmac Apr 22 '11 at 19:02
    
the pump is attached to an 'small building' and is attached to a tank (150 gal) and a pressure switch. I have the switch set to on at 45PSI and off at 75PSI. I'm not sure how deep the well is. –  Iunknown Apr 22 '11 at 20:02
    
Can you more clearly define how it is 'cycling'? How long is it on for, and how long is it off for, both when it's working "normally", and when you're experiencing the problem? –  gregmac Apr 22 '11 at 21:02
    
I turn on the water, the water pressure starts goes down from 75 PSI down to 45, the pump kicks on and stays on until the pressure goes back to 75 PSI at which point the the pump kicks off. –  Iunknown Apr 22 '11 at 21:29
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4 Answers

"Cycling" in a pressure system is generally when the pump is turning on and off too quickly. Pressure systems are specifically designed so that the pump runs up to its maximum pressure, then you use water from the pressure tank, until it reaches the minimum pressure, at which point the pump turns back on and runs until it reaches the maximum.

The recommended minimum run-time for a 1-2HP pump is usually 2 minutes. How you get to that is based on the flow rate of the pump, and the size of the tank.

Based on your 150gallon tank, and not knowing your flow (in gallons per minute, GPM), based on this tool here's some example flows/times:

  • 16GPM: 3 minutes
  • 20GPM: 2.5 minutes
  • 24GPM: 2 minutes

As you can see, the higher the flow rate, the shorter the pump will run.

In your case, it sounds like one zone is basically matching the flow rate of the pump (you could verify this by watching the pressure gauge while it's running, it should be fairly steady). While this may reduce pump starts, if you use water elsewhere at the same time, your pressure will drop and the pump will not be able to keep the pressure as high.

Now, the flow rate the pump puts out depends on a couple things, most importantly: the pump itself (and now many stages it has), and the depth to the water level. Each submersible pump will have a chart showing the flow rates at different depths, so you'll need to find your model and the water level to figure this out. Here's an example though, using a 2HP pump I found while running at 60psi (depth to water: flow rate):

  • 20ft: 23.9 GPM
  • 60ft: 22.7 GPM
  • 100ft: 21.3 GPM

At first glance, this seems fine. My guess right now is that your first zone matches the flow rate of the pump, while the second zone is simply a lower flow rate than the pump. It's no problem, things are designed to work this way.

If you really want to reduce the cycling, you can hook up a bigger or a second pressure tank. This will give you more capacity, and so the pump will run longer, but it will also stay off longer while you use water stored in the tanks.


Now, if you had fast cycling (eg, <2 minutes run time) or "rapid cycling" (a couple seconds of runtime), then that would indicate a problem with the pressure tank. Normally the tank should be pre-pressurized at 2psi below your cut-off (low) pressure point - so in your case 43psi. To check this, you'll have to turn off your pump and let the water drain out (open a tap somewhere). You can top it up with a normal air compressor.

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My goal is to not have the pump cycle at all. I'm trying to reduce wear and tear on the pump and save money. :) –  Iunknown Apr 22 '11 at 22:55
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Thank you everyone for all the help.

Since I couldn't get my mind around how adding another line would help without increasing the size of the main pipe; I decided to read the manual for the sprinkler head and found out that I could change the nozzle sizes.

So, I replaced the nozzles with highest flow ones I could find and after 3 sprinklers, the well stayed at 70PSI and didn't cycle.

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Here's a good step-by-step tutorial on how to calculate losses in an irrigation system. It's targeted toward wastewater dispersion, but the principles should hold true for regular lawn irrigation.

And here's more lookup tables for losses through pipe and fittings.

EDIT:

If you look at the values in the second reference, you'll see that reducing the flowrate greatly reduces the pressure loss per length of pipe. If you have 200 ft of pipe with eight 2.5 GPM sprinkler heads, you'll need a 20GPM flowrate initially. The pressure loss per 100 ft for that flowrate is 8.66 psi.

On the other hand, if you use two branches of 4 sprinklers each, you only need 10GPM per branch. The pressure loss per 100 ft for that flowrate is 2.40 psi.

This ignores losses due to additional fittings required for the two branches, but it should be negligible.

So in short, splitting your lines should increase your pressure at the end of the lines, but won't give you any advantage in overall flowrate.

DISCLAIMER: I've never done irrigation work before, I'm just using lookup tables - you may want to wait for someone with actual experience to answer and back me up on this one. (Or trash my answer if it's wrong.)

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I'm more trying to salvage what I've done already. Would splitting an existing line into 2 separate lines allow me to push more water –  Iunknown Apr 22 '11 at 18:00
    
Yes, but you'll need to do some calculation of the number of gallons per min over each length of pipe to determine how much of a psi loss you'll experience. Then you'll have to see what the requirements are for your sprinkler heads. This requires research and math on your part, preferably before you try to change things around. If you take the measure 0 times approach, be surprised if you only have to cut twice. –  BMitch Apr 22 '11 at 19:06
    
@B Mitch: You're absolutely right. Each sprinkler head will reduce the pressure by it's GPM rating at that pressure. So when calculating losses, the pressure and losses on runs of pipe between sprinkler heads will be different for each one. That's why I could only give an example 'answer' - I didn't have the sprinkler head specs or the specific layout. –  Doresoom Apr 22 '11 at 19:15
    
It's my understanding that the problem isn't PSI at each head but that I'm not moving enough water thru the line. Right now, everything sprays fine with good distance (about 40' radius) and I'm just trying to keep the pump from cycling. I know I'm close since the line is only a little be longer than one that works correctly with the same number of heads. –  Iunknown Apr 22 '11 at 20:05
    
This article identifies excess pump flowrate as a possible cause for pump cycling. If you're running one zone at 70 psi, and your cutoff is 75 psi, this could be your problem. If your sprinklers don't need the flowrate you're putting out, then you'd build up excess pressure. Is there any grade difference between the two zones? –  Doresoom Apr 22 '11 at 20:26
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The simple way to stop the cycling without much math is to install a Cycle Stop Valve. I have one on my well and it helps with the low volume zones. You can read about it here http://www.cyclestopvalves.com. I bought mine from http://www.pumpsandtanks.com. The guys at pumps and tanks also run a forum that has been extremly helpful. I know it's not stackexchange but the depth of knowledge has been great. http://www.pumpsandtanks.com/Forum/

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