GPM, which side to favor?

Discussion in 'Irrigation' started by j_nolesfan, Mar 24, 2007.

  1. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    Hello All,
    I'm excited to find such a great source of information. I'm in Pensacola, Florida (Northwest tip of Florida near Mobile, AL). I'm currently installing a DIY lawn irrigation system. I plan to use Hunter PGP rotors and probably Rain Bird rotor/spray heads (can't remember the model) as well as some bubblers for my flower beds. The system will have about 34 rotor and or spray heads. I have a well that's 160 ft deep with a 1.5 HP pump and a 50 gal. pressure tank. I intend to use 1.25" or 1.5" sched 40 for my mains and 1" or 3/4" for my laterals. I plan to have all valves (1" valves, not sure which brand yet) in a single manifold near the well tank. With that being said, I measured the GPM by running the well tank at full open via the 1.25" valve outlet for about five minutes, then catching the water via 1.25" 10' pipe into a 5 gallon bucket. The bucket filled in about 10 seconds on several tests. I don't think well pump water recovery will be an issue. Here's where I get into trouble. When calculating my zone sizes based on about a 30 GPM flow rate at an honest 30 PSI on the pressure guage when the 1.25" bypass valve is at full open, I'm not sure if I should favor being under 30 GPM or slightly over. The static pressure of the tank is around 80 PSI. If I run a garden hose to an ordinary oscillating Melnor sprinkler from the spigot on the same leg as my pressure guage, the pressure stays at 85 PSI all day long. I know other things need to be considered like pressure loss in the pipe and general PSI but I'm mainly worried that even with a 50 gal pressure tank, I may put undue stress on my well pump if I don't maintain a 30 GPM flow rate.

    One final question, if I feed the zones in basically a "goal post" patter where I run a main line to the zone and "T" it off with one direction serving half of the heads and the other serving the rest, should my "lateral" (everything past the "T") be 1" (1.25" maybe)? I know that the last two heads on each leg should be connected with 3/4". I plan to have around 8-10 heads per zone of Hunter's PGP rotor heads with an output between 2.8 and 3.4 GPM.

    Thanks in advance,
    Jeff
     
  2. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    I don't see the edit button but I need to add that the longest main line run will probably be 150 feet with no more than two 90 degree elbows in it. It will also flow down a grade of about a total 18" drop over the 150' length.
    Thanks again,
    Jeff
     
  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,444

    You have to do your 'bucket' test with a pressure gauge upstream of the discharge valve, and with the pump running continuously (no cycling) - only then can you collect useful data, combining flow and pressure.

    By the way, in some areas (probably not Florida) it can pay to run a pump for an hour or so while testing, in case it's one that can run dry.
     
  4. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    The well running out of water is the least of my worries. I'm in a subdivision where the well driller has installed several wells. He knows the area and none of the others have complained about running dry for their systems. The bucket test was done with the pressure guage immediately upstream of the discharge valve. It's part of a metal T that has one leg running into the pressure tank and the other running to two ball valved outlets.
    Thanks,
    Jeff
     
  5. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    Again, is it better to exceed the capacity of the pump flow or to keep the output at less than pump capacity? Also, are 1.5" Sched 40 mains good from the valve (located at the pressure tank, centrally) and running as much as 140' before splitting via a T to two different legs roughly forming the shape of a football goal post. After the T, are 1" lines ok with the exception of the leg between the last two heads of each side of the "uprights"? Or, should I stay with 1.25" and drop from 1.25" down to 3/4" when I get to the last two on each leg?
    Thanks again,
    Jeff
     
  6. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    In other words, as for capacity, would it be better for a 30 GPM system to feed a 33-35 GPM capable zone or to feed a 23-25 GPM capable zone?
    Thanks,
    Jeff
     
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,444

    You have to match the flow of the pump. There isn't one side or another to lean on. This is where interchangeable sprinkler-head nozzles come into play.

    I suspect your 'drawdown level' isn't anywhere near 160 feet, if you're actually getting 30 gpm from a horse-and-a-half pump, while sustaining at least 50 psi at the pressure tank. (you have not yet supplied any "Y gpm at X psi" numbers yet - do make sure you have them, since they will guide you in pipe and valve sizes) ~ A quick look at a performance curve for a high-capacity submersible pump makes the 30 gpm figure suspect, unless the actual water level in that well is close to the surface. You really want to take the time to build your own performance curve. In the absense of experience, you do better to go by the numbers.
     
  8. LCPullman

    LCPullman LawnSite Senior Member
    Posts: 589

    Check out this link http://www.irrigationtutorials.com/sprinkler04.htm If you read it through and follow the directions, it should answer your flow question.

    The wet test method would carry on from the info you have, You could do both the dry and wet method just to verify your results.

    You should go with the bigger lines, 1.5/1 as opposed to 1.25/.75 The larger pipe combination with enable you to minimize pressure loss through the pipe.

    At 30gpm you should either run 1.5" globe valves or 1" angle valves. The standard 1" globe valve will have too much pressure loss.
     
  9. j_nolesfan

    j_nolesfan LawnSite Member
    Posts: 81

    Does my pressure tank not serve as a buffer to allow the pump to cycle on and off and run at full output at all times? Does the pressure tank not smooth out the pressure delivered from the system so that it's consistent? I'm asking honest (maybe they're dumb?) questions. It was the only reason I had a tank installed in the first place.
    Thanks again,
    Jeff
     
  10. Duekster

    Duekster LawnSite Fanatic
    from DFW, TX
    Posts: 7,961

    Even as a DIY project I would get a professional design even if I did it through Rain Bird. Check on-line for this service on their website.

    Also, many reputable Irrigation guys get pretty decent discounts so they might actually do the job for slightly more than what you can do it. Lot less headache and maybe better performance.

    I think you should do your home work, I love informed customers because they weed out the guys who do shoddy work.
     

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