Pump Mathematics

Discussion in 'Irrigation' started by bobw, Aug 18, 2007.

  1. bobw

    bobw LawnSite Senior Member
    Messages: 807

    I'm working on an estimate for an acreage and I just need some validation on my math work for the pump side of things. The local irrigation district is installing the pump and line for the customer and have recommended a 1hp pump, the homeowner is thinking that he will upgrade to a 1 1/2 hp.

    The distance from the pump to his property is 1300 feet. The endpoint will be about 50 feet above the pump. The line will be 4" PVC.

    So... looking at the 1 1/2hp pump...

    we have 50' head, I'd LIKE about 45psi for the rotors, which would be 104' head... so... 154' head

    Does this seem right? The line of pumps that the irrigation district are recommending (Myers Quick Primes) don't even come close to that much head regardless of the motor size.

    What says some of you with more pump experience?
  2. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    The short answer is, size the pump so it will operate at it's "optimal" efficiency. Every pump has it's efficiency sweet spot. You want to try to get it to work as close to that as possible.
  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,369

    Forget the Myers pumps. Go for a jet pump, if the flow will suffice. A 1-1/2 HP Goulds will give you 50-60 psi (albeit at a flow that doesn't justify a 4-inch main)
  4. Johnson_inc

    Johnson_inc LawnSite Member
    Messages: 5

    Close, but i dont think so.

    I always find it easier to work backwards.
    45 psi at rotors + max 10% loss in pipes would be about 6-10 psi + 2-5 psi lost in a filter + 2-5 psi lost in your valves and 22 psi lost in elevation. Your pump will need to develope about 90 psi at what ever flow you are working with. With a 4" main i am assuming well over 100 gpm.

    I dont know of any 1-1/2 hp pump that can put out that flow at that pressure.

    Messages: 18,668

    Maybe you can work a deal with IRRIMASTER and get the 5hp pump from his screwed up pump system in the thread next to yours.

    Messages: 18,668

    If the zones off the 4" main are running 30gpm? Assuming only one zone is running at a time.
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,369

  8. drmiller100

    drmiller100 LawnSite Senior Member
    Messages: 562

    you might double check the price of 4 inches of pvc for 1300 feet. you might be shocked.
    then consider 2 inch as it will have no restrictions for the pump volumes you are talking.
    the next most important question. how big is the area you wish to water? how many zones were you planning? what kind of gpm do you need to finish in a nigiht???
  9. bobw

    bobw LawnSite Senior Member
    Messages: 807

    I don't have very much say in the pump side of all this. This is being driven by the homeowner and the irrigation district. The ID is making the decision on pipe and pump.

    I'm looking at 1 1/2 acres to be irrigated. I haven't even tried to lay out zones until I can get a good grip on what water supply I will have.

    Given that I am not getting a good vibe over the water supply, I will likely pass on this job. Nobody wins if the system doesn't work in the end.
  10. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    Based on my calcs, it would appear the ID is only concerned with providing a certain flow & pressure suitable for residential use. Since you don't have control over what the ID does, you may be stuck with having to add a booster pump to get the pressure up to a decent level.

    Using the GOULD IRRI-GATOR Self-Priming Centrifugal Pump curves, with a TDH of 57.5 (100 GPM at your other specs), a 1 HP pump will give you ~ 50 GPM at the end of your 4" pipe run. The TDH used above does not take into consideration head required after water is delivered.

    My guess is they are looking at providing something between 20-30 GPM at 30-40 PSI with a 1 HP pump, and that is pushing it's capabilities assuming the above pump curves. I'd try to push the HO to size up to 1.5 HP and see where that takes you.

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