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Suction Lift Question

Discussion in 'Irrigation Pumps & Supplies' started by Hayduke, Jan 2, 2020.

  1. Hayduke

    Hayduke LawnSite Senior Member
    from Oregon
    Messages: 391

    Somebody please explain this to me, and either confirm or deny what a pump guy told me. I do not work with pumps much as most of my systems are on municipal water supplies so I admit ignorance on this.
    My clients have a shallow irrigation well with above ground Goulds centrifugal pump. The footvalve sits about 20 feet down in the well. During the winter the water level is only a couple feet above the footvalve, but in the summer, according to the client, the well fills up due to large scale irrigation of surrounding fields.
    I had to design a sprinkler system for them during the dry/winter season based on the water level being about 20 feet below the pump; therefore 20 feet of suction lift to add to the pressure loss calculations.
    A pump professional told me that when the well fills up, the pressure loss will decrease based on the higher water level. While I kind of get this, it doesn't quite make sense, because the pump intake is still 20 feet down and the pump has to pull the water up 20 feet even though the water level may be only a couple feet below the pump.
    How does this work? Please enlighten me Lawnsite veterans.
  2. Crazy 4 grass

    Crazy 4 grass LawnSite Gold Member
    Messages: 3,449

    Is this a jet pump sucking from the top ? If that is the case the higher the water the level in the well pipe, the less head or resistance to overcome.
  3. OP

    Hayduke LawnSite Senior Member
    from Oregon
    Messages: 391

    I guess I am looking for a little more of an explanation. Because of the foot valve, the suction pipe is always primed and therefore the water level in the suction pipe is always the same.
    The intake point is always the same at 20 feet below the pump no matter what the water level is.
    The suction pipe is closed off to the water in the well except when the foot valve is open.
    So what causes the suction head loss to be less when the water level is higher give the above information? Might be overthinking this one, but I tend to do that! P.S it is a centrifugal pump sucking from the top.
  4. BrandonV

    BrandonV LawnSite Fanatic
    Messages: 5,281

    The pumps are designed to “push” the water... it’s more work to have to “suck” it up to the level of the impeller. Most of the pumps we use are jet pumps and you’ll see a noticeable difference in the way they work on a full well/lake as opposed to when the water is further down
  5. top_gun_de

    top_gun_de LawnSite Member
    from Germany
    Messages: 28

    Suction pumps are actually 2 pumps built on one shaft, one for pushing water and one for suction. They share a common shaft, so their combined performance is limited by the motor performance.

    The power needed by the suction part is really determined by the water level in the well, not by the position of the suction valve. If the water level is 10 feet, the workload to pull the water to ground level is half of what is needed at 20 feet.

    So you get more pressure and also more flow if enough water is available.
  6. Necoflyer

    Necoflyer LawnSite Member
    from CO
    Messages: 5

    The professional is right. It is vertical distance from the water level to the pump that you are lifting. Makes no difference if the foot valve is 20 feet or 2000 feet down.
  7. Andrew H

    Andrew H LawnSite Gold Member
    Messages: 3,148

    Two pumps or two impellers?
  8. TedM

    TedM LawnSite Member
    Messages: 1

    The only thing that matters here is where the water level is above the foot valve

    True, but the only time suction lift is considered is when the pump is running, lifting water. The suction created by the pump running is what makes the foot valve open.

    Lots of variables must be considered for efficient design. But for your application suffice it to say that atmospheric pressure and the weight of the water above the foot valve assists the pump in it's suction lift. If the water level is 2 feet above the foot valve the pump must overcome more suction head than it would if the water level is 20 feet above the foot valve. One way to think about it is to consider how water pressure increases the deeper the water gets. I'm no expert but, this makes sense to me even if it is not the perfect answer. Hope that helps...
    Last edited: Mar 28, 2020
  9. OP

    Hayduke LawnSite Senior Member
    from Oregon
    Messages: 391

    I appreciate the reply.
    Yep I think that is it. It is the additional pressure of the column of water over the the foot valve that counter acts the loss of suction head. Hard to wrap my head around it since the water is pushing down the but the suction is pulling up. How can that additional pressure aid in suction lift?
    I think the answer to that is going to be some very complicated physics equations. I'll stick with the simple answer you provided!!!
  10. cjohn2000

    cjohn2000 LawnSite Senior Member
    Messages: 595

    Something I just learned from one of my suppliers ( we dont dealnwith pumps too much up here), is that if you are specifying a pump and it has to lift 5' ( we draw off of lakes) and needs to produce 10 feet of head, then you need a pump capable of producing 15 feet of head total at least.

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