looped mains

Discussion in 'Irrigation' started by skurkp, Dec 5, 2005.

  1. skurkp

    skurkp LawnSite Member
    Posts: 248

    I am trying to understand how to solve the hydraulics for looped mains. If I have a system that has a looped main and two zones are opened and I have to figure the head pressure for a certain head. The head in question is closer to the main and meter that the other zone that is running. Do I figure the total gpm for the farthest zone all the way to the main line that has the zone operating that I need to now the head pressure and then figure the loss with of total gpm and pressure loss to the main? We did not cover this in class and when asked this in the state test I really had no way of knowing how to start. I am taking a refresher class on Dec. 12 and then the retest on Dec. 15 and would like to pass this time. Thanks in advance.
     
  2. jerryrwm

    jerryrwm LawnSite Bronze Member
    Posts: 1,274

    Take the total length of the loop and divide that in half. Then take the total flow for both zones operating at the same time and divide that in half. Then reference the charts for the mainline pipe size at that flow.

    For example:

    The main line is 2" SDR-21 and the total length of the loop is 400'.

    Zone 1 is flowing 22.0 GPM and zone 2 is flowing 33.0 GPM for a total of 55.0 GPM

    Take 400' x .5 = 200'
    take 55.0 GPM x .5 = 27.5 GPM

    so your pressure loss calculations would be for 200' of 2" SDR-21 at 27.5 GPM


    The reason this works is the worst case scenario is that both valves are located at the midway point around the mainline and water takes the path of least resistance. Rather than calculate the proportions and percentages to arrive at the finite answer, if you will remember "Half the distance, Half the flow" and use those figures, you will find that you are going to be within .5 - 1.0 psi either way.

    Who is teaching your referesher course?

    Jerry
     
  3. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    Jerry, I always wondered if cutting the flow loss in half if you had a looped main was accurate enough, seems like it is when you break it down like that.

    That Texas test sounds difficult, you can probably charge well down there since it weeds out the fly by nights?
     
  4. jerryrwm

    jerryrwm LawnSite Bronze Member
    Posts: 1,274

    Jon,

    That test is a *****. They have a 50-55% failure rate. it is tough for sure, but very thorough.

    But just because some are smart enough to pass the test doesn't mean they are smart enough to charge enough.

    To quote the general - "They are stuck on stupid." There are folks out there charging the same price for systems that I did when I started 25 yrs ago! But many of them last about 2-3 yrs before they starve themselves out.

    Jerry
     
  5. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    Jerry is spot on. Theoretically you can also make the looped main one size smaller than otherwise necessary (based on GPM) since you will have no more pressure loss (and possibly less total loss) because of the looped main. This will offset the price of extra main line. However, once the water leaves the loop and travels into the zone the calculations revert back to normal. You won't be able to reduce pipe sizes in the zone itself and it's possible to have the main zone lateral larger than the actual looped main. :p
     
  6. Critical Care

    Critical Care LawnSite Bronze Member
    Posts: 1,654

    Never installed a looped main, but perhaps under the right circumstances it could be an entertaining idea – whatever those right circumstances would be. However, I suppose that in a way looped mains are a bit like electrical parallel circuitry.
     
  7. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    I've installed a few looped mains. Mostly in large commercial jobs were they made a lot of sense.
     
  8. bicmudpuppy

    bicmudpuppy LawnSite Silver Member
    Posts: 2,781

    Hands down the hardest test I have ever taken in my life! That failure rate includes re-takes and when I took mine there was a group that had already failed two or more times. As to it helping and making things more professional and profitable..........not ten years ago, and I would venture not now. It is still just a classroom type test. You can pass it without ever having had any dirt on our hands.
     
  9. skurkp

    skurkp LawnSite Member
    Posts: 248

    Well some say the test is a piece of cake. For me it was not and I do industrial maintenance I thought that with the line of work that I do it would be easy especially after taken the course for 10 weeks. NOT!! I do suppose that a lot had to do with the instructor, he would get off course and talk about his life and other happenings more that the class work, no labs, no hands on, nothing. I suppose with the right instructor I might have passed I did study (something most in my class did not). There were four people in the class for the fourth time trying to get it so they could pass the test and they had retaken the test three times. I really need this, not because I want to start an irrigation company, but because I do commercial mowing and my customers want me to take care of their systems, and I won't touch them until I pass the test. Not to mention that it costs my more money every time. Thanks Jerry for the help , I printed it out so I would have it to help study. December 15 is not far away.
     
  10. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    On your normal residential and smaller commercial properties a looped main is not that viable. It's mostly used for large peroperties.

    BTW... What happened to your swimmimg fish?
     

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