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Thread: Irrigation Pumps

  1. #1
    In the Trades Bob NH's Avatar
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    Default Irrigation Pumps

    Many of the threads on this board about irrigation pumps are related to mis-matching the pump to the system.

    The ideal pump for irrigation would have nearly constant pressure across a wide range of flow. Then the inevitable variations in sprinkler demand would have less effect on the pressure and the sprinklers would all work about the same when there are different demands on different sections of the system.

    Definite purpose "irrigator" pumps such as the Goulds GT series and others are pretty good for systems that get water from ponds or from very shallow wells with a lot of capacity. Shallow well jet pumps can be used for low capacity systems up to 15-20 GPM, but they are much less efficient and should be used only for low flow situations or where the lift from a shallow well is too great for a straight centrifugal pump to operate well.

    Submersible pumps intended for use in wells are terrible choices for irrigating from a pond or shallow well. They have steep head/flow curves and at less than design flow the pressure is usually too much for the system. Stock pumps at "big box" stores are just about the worst possible choice because they stock pumps that are certain to provide high enough pressure from a fairly deep well to a home water system.

    High capacity/low head submersibles are readily available from sources such as Goulds distributors (and I'm sure others) that will provide good flow at the operating point. They are good for providing pressure margin but are not as forgiving of mis-matched pump/system conditions as a good single stage centrifugal.

    You can irrigate with almost any kind of pump if you spend enough for tanks, valves, and controls and if the electric power is not a significant factor. There are ways to waste that energy you are buying over the wires to make just about anything work.

    But if you want an efficient system you should take the trouble to match the pump to the system.

  2. #2

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    Bob NH,

    Great advice! Personally in my experience there has only been one pump manufacturer, did not know there were others.

    Have been dissapointed that Gould has changed their irrigation pumps in the last several years reducing the suction from 2 to 1.5 inches.

    Paul

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    Paul,

    Goulds has changed all of their pumps a lot over the years. They have to have the record for redesigning their pumps.

    Bob,

    I know you will never get off badmouthing the constant pressure valves that are on the market, but in my opinion they are the way to go. With a submersible more so than any other pump.

    I don't know how many self priming centrifugals you have seen hooked up to wells, ponds, lakes etc. But with a pressure switch and tank, they are next to impossible to make work without either cycling the pump to death or setting the pressure where the pump does not cycle but won't shut off. And every foot of water difference with the seasonal changes, the switch has to be reset.

    An end suction is a booster pump. Nothing else. Try priming an end suction sometime.

    bob...

  4. #4
    In the Trades Bob NH's Avatar
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    "I don't know how many self priming centrifugals you have seen hooked up to wells, ponds, lakes etc. But with a pressure switch and tank, they are next to impossible to make work without either cycling the pump to death or setting the pressure where the pump does not cycle but won't shut off. And every foot of water difference with the seasonal changes, the switch has to be reset."

    I have used lots of end suction centrifugals in process equipment and in water supply systems. They need to be matched to the system.

    The problem with end suction centrifugals is that they are often misapplied. They should never be used where there is more than about 10 ft of "lift" because the 3500 RPM pumps require typically require about 15 ft of Net Positive Suction Head (NPSH) at the most efficient flow rate. The inlet pipe should usually be one size larger than the fitting on the pump and should be connected through an eccentric reducer so that air will not be trapped in the inlet pipe.

    They need to be set up to operate near the peak efficiency point, which is not anywhere near shutoff pressure. That means they will be producing high flow and you can't put a dinky little bladder tank at the pump and expect it to work. High flow irrigation pumps should be installed to discharge to the system.

    End suction centrifugals are often misapplied because they are selected for pressure capability instead of flow requirements. I have seen a lot of 5 HP and 7.5 HP centrifugals that were picked to provide 65 psi for a water system. They end up with a pump that supplies 60 to 80 GPM when they need 25 GPM but the seller of the pump didn't recommend a multistage pump that would have met their needs. And if they are going to use a big pump like that they need much more tank capacity just for the pump.

    I just put in a system where a 5HP, 75 GPM pump was used to supply water to a camp that needs about 25 GPM during peak hour and about 15,000 gallons per day. They have two 1000 gallon steel tanks, 600 ft away from the pump, in which they never maintained a proper air cushion. They had an 80 gallon bladder tank at the pump where the pressure switch was located. The pump cycled terribly with about 20 second on times when it was a good day and it was difficult to control because the pipe losses were acting against the pressure switch and disappeared as soon as the pump switched off.

    I installed a 2 HP Goulds 33GS20 submersible and moved the pressure switch to the tanks. The on times are now typically 10 minutes and they can control to a much smaller pressure range.

  5. #5
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    I see the same sort of thing all the time.

    We just recently worked on a sprinkler system for a large subdivision. They drilled one 6" well. Installed a 10 hp submersible pump with a 2 gallon bladder tank and a Cycle Stop Valve. It works as good as can be expected and would have never worked without the CSV because each home owner has his own sprinkler timer. You can imagine this 10 hp pump just idling away producing somewhere around 10-15 gpm to one homeowners lot at a time while the electric meter is just a spinning.

    I guess it's not politically correct any more for each home owner to have his own well and pump. It might be unsightly. Besides, we would never get the rig in the yard to drill, there are no yards anymore.

    bob...

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