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Radial Flow Water Pump-How to calculate Watts required to power it

  1. Mar 25, 2014 #1
    Radial Flow Water Pump--How to calculate Watts required to power it

    Hi all,
    I'm currently working on an independent project where I'm building a radial flow water pump to be sent to a village in Senegal. I'm having trouble figuring out what motor to use to power this water pump and how much power is actually required.

    Project specifications:
    -Well is ~12 meters deep
    -Impeller blades are 5.08 cm long
    -Impeller blades are 1.27 cm in width
    -discharge pipe is 0.6cm inner diameter

    So far I have figured that the torque required for the impeller is τ = ρghAR/2, where A is the area of the impeller blade, and R is the distance from the axis to the far end of the impeller blade. ρ is 1000kg/m^2, g is gravity, and h is the height, 12 meters. Is this correct?

    IF so, I desperately need help trying to figure out what motor to use and how many Watts are needed to power it. I know that I can find Watts from torque and rpm, but I don't know how exactly to find the rpm either. If someone could point me in the right direction I would be EXTREMELY grateful.
  2. jcsd
  3. Mar 25, 2014 #2


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    Staff: Mentor

    Welcome to PF!

    You haven't stated what performance you want out of this pump (flow rate and pressure), so motor size can't be calculated until you do. And is there any reason why you are building one from scratch instead of buying an off-the-shelf pump? This isn't an easy thing to do or calculate from scratch.

    Your equation doesn't look useful - where did you get it? Looks to me like it is the torque caused by a stationary column of water sitting on a cantilevered object.
    Last edited: Mar 25, 2014
  4. Mar 25, 2014 #3

    first, I just want to apologize for my elementary understanding of all this, I'm still a junior in high school and am just working on this project in one of my shop classes. I'm interested in all the math and physics involved, I just don't have all the tools yet. So thank you so much for your patients.

    right right of course. Sorry. i'm flexible on the flow rate... although because this is an isolated village, and the pump will be powered with a car battery, I suppose the flow rate would have to be quite small. At this point I'm just trying to figure out how to successfully get water moving up the tube, and I just need to figure out the minimum rpm and wattage required. Is there a way to express those values with flow rate and pressure left as variables?

    As for your question, yes it would be much easier to do that, but I mostly am doing this project for the challenge and to learn by doing. I hope to be an engineer in the future and really think I could learn a lot from this project.

  5. Mar 25, 2014 #4


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    Staff: Mentor

    No problem -- good to know what level the question is coming from.

    The equation for pump power isn't too far off from what you had except the water has to be in motion and it doesn't involve the area of the blades. You can find it and a calculator for it here: http://www.engineeringtoolbox.com/pumps-power-d_505.html

    That gives you the energy needed to be imparted on the water to make it move. Combined with the efficiency of the pump gives you the motor power.

    Knowing your pump's efficiency is much more difficult. If you are making this from scratch, I would assume no more than 50% efficiency.
  6. Mar 25, 2014 #5
    WOW THANKS SO MUCH! that's so helpful. You're a life saver, really.

    Just for my own understanding... an annoying question, perhaps: How does it calculate that? Angular velocity must appear somewhere in those equations, no? Although I suppose the calculator can figure out RPMs based off of the information it asks for...

    I guess what I'm getting at is: are you sure I don't need to know the required RMPs and other variables when choosing a motor? I put my numbers into the calculator, with 50% efficiency and a flow capacity of 1m^3/h and it spit out "Hydraulic Power: 0.03 (kW)" and "Shaft Power: 0.07 (kW)"
    OR are these the types of things I need to consider when managing the circuitry of the system?
  7. Mar 26, 2014 #6
    You apparently already have a pump, and are just trying to size a motor, correct? Did the pump come with an owner's manual that included a pump characteristic graph? If so, you can make good use of this graph.

  8. Mar 26, 2014 #7


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    Staff: Mentor

    Sounds to me like the pump impeller is being made from scratch...

    The equations I linked tackle the problem from the perspective of the water. They tell you how much energy you've imparted on the water based on your chosen requirements. Making that happen from the pump's perspective (for example, calculating the required rpm to get that performance - or even if that pump is capable of that flow/pressure mix) is a whole 'nother story. Calculating a pump's behavior from nothing cannot be done at this level: there is no simple equation for it. My best suggestion would be to flip through some pump catalogues and find one similar in design to yours, to use as a comparison.
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