Huge pressure drop with 1 sprinkler

Discussion in 'Irrigation / Sprinkler Forum' started by h22lude, Aug 30, 2020.

  1. h22lude

    h22lude Member

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    Sep 23, 2018
    Location:
    Rhode Island
    When I was testing it right now, itstayed steady because I was only running the water for a minute. I just had a larger tank installed so it takes a little while for the pump to cycle. But if I let the sprinklers run like normal, the pump will cycle. I haven't tested the cycle time with two MP Rotators. I would like to get the sprinklers to use as much water needed to keep the pump running but I can't because of the pressure issue. I'd most likely have to run both spigots wide open to keep the pump running. I can also install another spigot right near the pressure tank. That would allow me to run a few sprinklers at each spigot. But back to my original issue, not sure if the pressure is there to handle that.

    The well won't run dry, I know that. There is about 650 gallons of water in the well from static level to pump.

    1.5 GPM recovery. That's probably why they put the pump down so deep.
     
  2. wwhitney

    wwhitney Active Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    OK, per https://www.hunterindustries.com/irrigation-product/mp-rotator/mp-rotator that flows 0.86 GPM at 40 psi, the design pressure, 30' radius. At 30 psi, it flows 0.76 GPM, 27' radius.

    Those numbers show you have some huge obstruction between your tank and your spigot (does it currently go through your filter?) No way that 1.5 GPM should drop 20 PSI.

    So if you run a properly sized dedicated line to the irrigation from the pre-filter tee that you mentioned, that should take care of that pressure drop, and there's no reason you can't run a bunch of those MP rotators, depending on what your well can provide. You can probably design the piping from the tank to the sprinklers to give you, say, 10 or 15 psi drop at 5 to 10 GPM.

    Now you want to characterize the behavior of your well/tank/pump. To do this, set up a constant 3 GPM draw (for example), and monitor the pressure at the tank through a full cycle (until the behavior becomes periodic or steady state). You could also repeat at higher draw rates.

    If the long term behavior is that the pump cycles, the pressure at the tank will be cycling from 40 psi to 60 psi, so your sprinklers will be cycling in their coverage and output. You'd probably want to limit the piping losses to 10 psi, so you always get at least 30 psi to the sprinklers. Or you could consider changing the pressure tank settings to 45/65 or 50/70, but I'm not at all aware of what other consequences that may have.

    If the long term behavior is that at a desired draw rate (say 5 GPM) you can get the pump to run constantly at a fixed tank pressure (say 50 psi), that's probably desirable. That way you avoid the sprinkler cycling in output rate and coverage. Plus you may have a higher design pressure for the sprinkler piping/system without raising the pressure tank settings.

    The above all assumes you run the sprinklers at a time of day when there are not other water draws.

    Cheers, Wayne
     
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  3. h22lude

    h22lude Member

    Joined:
    Sep 23, 2018
    Location:
    Rhode Island
    I had this issue prior to the new tank and filters being installed which tells me, if there is an obstruction, it would be after the filters. But after the filters, the front spigot is tee'd off about 5ft down the line. So that tells me if there were an obstruction is would have to be that 5 foot run of tubing between the filter and where the front spigot tee is. Unless I have multiple obstructions blocking both spigot tubing.

    I built my own sprinkler stands out of 3/4" PVC. I could attach some of that PVC to the 1" valve and see how many GPM I get. Out of the back spigot, I'm getting 6GPM with nothing attached. I could also route that PVC outside and test the two sprinklers again to see if I get similar results.

    Would using 1" PVC be better for the spinklers?

    I'm not sure what the issue is yet but I agree it seems odd to loss that much pressure with two sprinklers running.
     
  4. wwhitney

    wwhitney Active Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Unless it's just the filters themselves, they certainly have a pressure drop versus flow curve. Do you have a pressure gauge after the filters? If not adding one could be useful. Reach4 suggested that if there's a laundry tub or hose bibb (that you aren't drawing from) near to but after the filters, you could measure the pressure there to get the pressure drop primarily due to the filters.

    OK, is that the spigot at which you tested two sprinklers? And when you are taking pressure measurements, are you doing so upstream of any hoses that go to the sprinklers? I don't think you want to be measuring the pressure drop in the hoses, as your permanent sprinkler system isn't going to be using hoses. [Although an accurate simultaneous pressure measurement just upstream of the sprinkler head would tell you what pressure the sprinkler is getting, so you could look up the flow rate from the manufacturer's chart.]

    Bigger is always better for reducing frictional pressure loss. But before you can make any sizing decisions, you need to figure out the available pressure you have and the flow you want to get. Ultimately you're going to want to run all new piping back to that pre-filter tee you mentioned. Indoors could be copper, PVC, PEX, etc. I think buried outdoors is usually PVC, 1" is not out of the question.

    Cheers, Wayne
     
  5. h22lude

    h22lude Member

    Joined:
    Sep 23, 2018
    Location:
    Rhode Island
    I have a spigot right at the pressure switch and another one right after the filter. Same pressure with and without a faucet running.

    Yes the back spigot is the one I'm using for the sprinklers and that's the one I tested at 6GPM.

    Any pressure reading I take is right at the sprinkler. The stands I made have an inlet and two outlets. With two sprinklers, I measured the pressure at the second sprinkler.

    These have the 3/4" PEX going to the spigot. From the spigot there is maybe 5ft of 5/8" garden hose to the sprinkler stand, which is 3/4" PVC. From that first stand to the other stand is 3/4" PVC laying on the ground. So it is mostly 3/4" PVC with very little 5/8" hose.

    It does seem like 3/4" PEX and PVC have a much greater pressure loss value per 100ft than 1". But it is still odd I get that much pressure loss with 3/4". Though I do think sprinkler heads are made for 1" piping in mind.
     
  6. wwhitney

    wwhitney Active Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    I suggest getting a hose y-adapter and using it to measure pressure at the spigot in addition to at the sprinkler.

    It is possible that the orifice in a hose bibb is so small that it is causing significant pressure drop. 6 GPM wide open isn't a lot, and from what I understand of your piping system the hose bibb is likely the controlling factor. [A diagram with devices and pipe sizes/types and lengths would still be helpful for reference, it's hard to put together all the textual descriptions. Or if most of the complexity is in one physical area, then a photo with labels on the lines running out of the picture.]

    There is a theoretical issue with measuring pressure of a flowing fluid (that's the reason for pitot tubes), but the one time I tried to compute the magnitude of the effect, it was only a couple psi, and so I figured it was negligible. Perhaps I'm mistaken and there is a special procedure needed to measure the pressure under flow conditions.

    Cheers, Wayne
     
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  7. h22lude

    h22lude Member

    Joined:
    Sep 23, 2018
    Location:
    Rhode Island
    I can try to take a few pictures and label things. I'll try a wye too.

    Maybe that's the issue. Maybe if I had an actual inground system coming from the 1" PEX valve with no spigot to restrict anything, I'll get much better flow and pressure. But I don't want to put together an irrigation system not fully knowing if the pressure will be ok. That's partly why I'm going through these tests. Might just have to get some 1" PVC and see what that valve can do. If I keep as much 1" as possible, it should simulate an irrigation system. The only section that has to be 1/2" is the tee because the sprinkler head has 1/2" female threading.

    Edit: the back spigot is a frost free one. Not sure if those are more restrictive or not but figured I would mention it
     
    Last edited: Sep 8, 2020
  8. wwhitney

    wwhitney Active Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    If you characterize what your well/pump can do, and you start your irrigation piping at a tee near the pressure tank, I'm confident you can get the capabilities of your well/pump to your sprinkler heads. I'm not sure what the current restriction is caused by, my best guess is the hose bibb/garden hose.

    This random pressure drop calculator: http://www.plasticpipecalculator.com/PressureDropHeadLoss.aspx tells me that if you are running 3.6 GPM through 100' of 1" PEX (that 100' includes the equivalent lengths of all the fittings), the pressure drop is only 1 psi. Double that to 7.2 GPM and the pressure drop goes up to 3.7 psi. [Pressure loss varies with flow Q as Q^(1.85).]

    If what some random guy on the internet says doesn't give you enough confidence, you can run a test with a temporary pipe run from your tee to your sprinkler stand. If you're going to be getting 100' of 1" PEX for the project, I suggest using the whole roll for the test, then you can just cut off the ends and reuse 99% of the roll.

    Cheers, Wayne
     
  9. h22lude

    h22lude Member

    Joined:
    Sep 23, 2018
    Location:
    Rhode Island
    I bought some 1" PVC to attached to the valve right off the pressure tank. I'll test the flow there as soon as I figured out my new issue involving the pressure switch
     
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