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Thread: McDonald Irrigation Pumps

  1. #1

    Default McDonald Irrigation Pumps

    Does anyone have any experience/feelings about McDonald Pumps? I am getting ready to have a 1 hp installed to replace old 1/2 hp at home lawn system???

  2. #2
    Moderator valveman's Avatar
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    McDonald pumps are as good as any. However, if you are replacing a 1/2 HP pump with a 1 HP, you will need to use twice as much water when irrigating to keep the pump from cycling.

  3. #3
    Porky Cutter,MGWC Porky's Avatar
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    Default Will the well supply a 1hp pump?

    Do you know that the well will supply a 1hp pump?

    Also as valveman suggests, to prevent the pump from cycling you will need to run more sprinklers or install a Cycle Stop Valve www.cyclestopvalves.com.

  4. #4
    In the Trades Bob NH's Avatar
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    If a 1/2 HP pump is enough to run the sprinklers it would not seem to be a great idea to install a 1 HP pump that needs to be throttled so it delivers the flow of the 1/2 HP pump.

    Throttling a centrifugal pump (any pump you are likely to use is a centrifugal pump of some kind) will cause it to use less power than at rated flow, but a 1 HP pump delivering the flow that you can get from a 1/2 HP pump will use at least 50% more electricity than the 1/2 HP pump.

    EXAMPLE:
    I calculated the power per GPM for a Goulds 33GS20, for which efficiency curves are available. I compared performance per unit of water delivered at the rated GPM and at 50% of the rated GPM.

    A Goulds 33GS20 pump has 67% efficiency at 33 GPM and 195 ft of head. It uses 2.427 HP (motor has a Service Factor of 1.25) and the Franklin 3-wire motor is at 73% efficiency at service factor load. It is using 2.481 kW and 0.0752 kW per GPM.
    http://www.goulds.com/pdf/7312.pdf
    http://www.franklin-electric.com/manual/AIM_13.htm

    The same pump throttled to 50% flow (16.5 GPM) has 53% efficiency and 260 ft of head. It is using 2.046 HP (16% less than at rated flow) and the same motor at rated load is 71% efficient. It is using 2.149 kW at that condition which is 0.1303 kW per GPM.

    The effect of throttling the pump causes the ratio of electrical power per GPM at half-flow to power per GPM at full flow to be 0.1303/0.0752 = 1.733. That is 73% more power per GPM (or kWhr per gallon) at half-flow than if the pump is operated at rated flow.

    Conclusion:
    Throttling the flow does indeed reduce the power used by the pump. In this example the power was reduced by 16% from the power at full flow.

    However, the effect on power per amount of water pumped is significantly greater when the pump is operated at half-flow. In this case the pump uses 73% more energy per gallon delivered at half flow than it does at full flow.

    The conclusion is that pumps should be selected and operated near the point of maximum efficiency, which is usually near the rated flow. Oversizing the pump and throttling the flow imposes a severe energy penalty.

  5. #5
    Moderator valveman's Avatar
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    Yeah Bob, we know all of that. In a perfect world, we would all just size our pumps exactly correct for the application. However, if you need 1 HP worth of water some of the time, and you only need HP water at other times. Saving 16% in energy while you are throttling the pump is a good thing.

    Your other options are to allow the pump to cycle on and off at 100% power, plus the 6 to 9 times the power required for every start of the motor. Or you can use a Variable Speed Drive, which will also reduce the power consumed by about 16%, just like throttling with a Valve.

    Both of these other options are extremely hard on the motor. Running at 100% power and cycling on and off into a pressure tank is one of the biggest killers of motors. The Variable speed causes harmonics, voltage spikes, resonance frequency vibration, skin effect corrosion, and other problems that will also kill a motor.

    Throttling with a valve eliminates cycling and all the problems caused by VFD controls. In the real world, a pump can never be sized exactly to the demand, because demands always vary. The way people use water in this day and age requires the pump to deliver different flow rates at different times. Throttling with a valve is the best way to accomplish this task and make the pump system last longer. 73% more energy per gallon sounds like a lot but, depending on how much water is actually being used, may only be $5 per month. $5 per month is a small price to pay to triple the life of an expensive pump system, while being able to use water anyway you want.

  6. #6
    In the Trades Bob NH's Avatar
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    Quote Originally Posted by valveman View Post
    Yeah Bob, we know all of that. In a perfect world, we would all just size our pumps exactly correct for the application. However, if you need 1 HP worth of water some of the time, and you only need HP water at other times. Saving 16% in energy while you are throttling the pump is a good thing.
    It is using 16% less power to pump 50% less water. That, combined with the fact that the motor is less efficient at that operating point, results in 73% more electricity (kWhr) per quantity of water pumped.

    I accept that the CSV does what is said it does with respect to the way it regulates flow and tank pressure at low demand.

    But there is no free lunch. There is a cost if the pump is operated significantly below the most efficient flow range. I made the calculation at 50% of the design flow. The difference is even greater if the pump is operated at lower percentages of the rated flow.

  7. #7
    Moderator valveman's Avatar
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    And the difference is even less, when running at higher percentages of rated flow.

    Again Bob, yeah we know, there is no free lunch. And since you CANNOT size the pump to exactly match all the demands, you need an economical way to make a pump do several different jobs.

    There is many times the water used for irrigation, than for household use. So it would be more efficient to keep the irrigation demands sized closer to the upper limit of the pumps flow rate. Then the CSV can be used to reduce the flow when water is needed for the house, or other small flow uses. Because average house demands are only about 300 gallons per day, the power consumed will only increase by maybe $2 per month. In many cases, the CSV actually lowers the electric bill by eliminating rapid cycling, which also makes the pump system last many times longer than normal.


    It seems to be a fair trade off that the multiple benefits of using a CSV can save hundreds or thousands of dollars on equipment but, it might possibly cost and extra $2 per month in electricity.
    Last edited by valveman; 09-03-2008 at 06:12 PM.

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