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Water volume, flow and speed question.

  1. Sep 13, 2008 #1
    First I would like to apologise for my lack of understanding in this area, this is all new for me.

    I am designing a water feature (hopefully a working one).
    I have a pump that will deliver 31.8 L/min through a 19mm pipe at 1 bar.

    If 31.8 litre of water = 31,800 cm³ and if
    the volume of pipe to hold 31,800 cm³ would need to be 112.215 metres long.
    r .95² x π = 2.83385 cm³
    31,800 cm³ ÷ 2.83385 cm³ = 11,221.483 cm
    11,221.483 cm ÷ 100 = 112.21483 metres

    Q. Does this mean that 31.8 L/min would be equivalent to the speed of 112.21483 m/min.

    Q. If I had the pipe from the pump (obviously not 112 metres long) flowing on to a 20 cm diameter water wheel, would the wheel speed be approximately 112.21483 m/min.

    Q. Is it the volume of water flow or the pressure that generates the power.
    Would a larger pipe providing more volume but less pressure increase the wheel speed or
    Would it be a smaller pipe providing less volume but a higher pressure.

    Q. Is it possible to calculate the Horse Power or Kilowatts generated by the water flow or pressure on the wheel.

    Thank you in advance.
  2. jcsd
  3. Sep 13, 2008 #2


    User Avatar

    Staff: Mentor

    Welcome to PF!
    I didn't check your math, but the logic looked right. So yes.
    Yes, assuming there wasn't any resistance on it to slow it down.
    Both. Power is pressure times flow rate.
    Higher velocity means more wheel speed, but only if there isn't any resistance on the wheel. Once you attach a generator to it, all this goes out the window. But....
    A good pump or turbine is about 50% efficient, so the power you can generate from a setup like this would be about half of the power of the pump used to push the water (unless there are other losses...). That begs the question: why would you want to do such a thing?
  4. Sep 13, 2008 #3
    Its only for a water feature in a garden setting, I was intending to have a small generator connected to the wheel to power a few 12v low watt garden lights with the generator in a small house and the wheel on the side. I know there is other ways to achieve this, I am not trying to save power it was for more of a novelty type scaled down generator house look. So you switch on the pump and the wheel and generator power the lighting around the water feature.

    The submersible water pump only uses 5 Amps so its not to bad.

    Thank you very much for your reply, a least I know I am sort of on the right track.

    Last edited: Sep 13, 2008
  5. Sep 14, 2008 #4
    Another question regarding the above post.

    Which would have the higher velocity.
    A pump delivering 31.8 l/min at 1 bar or

    A pump delivering 6.5 l/min at 3.5 bar

    or if you can tell me have to work this out I will.
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