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A Reverse Rotating Pump

  1. Aug 4, 2016 #1
    Hi
    What happens when a centrifugal pump rotate inversely? Does it add Head to fluid?
     
  2. jcsd
  3. Aug 4, 2016 #2

    A.T.

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    For radial blades and a symmetric casing, it wouldn't make a difference which way it turns. For curved blades and asymmetric casing the below comparison of forward vs backward curved blades might help (around 4:00):

     
    Last edited: Aug 4, 2016
  4. Aug 5, 2016 #3
    A.T.
    Thank you very much.
    What I can understand from your explanation and the video is, a centrifugal pump NEVER pump fluid inversely, even though it's impeller rotates inversely. It means that a centrifugal pump ALWAYS transfer fluid from suction side to discharge side and direction of rotation of impeller can just affect on amount of flow not direction of flow.
    Am I right? If so, Replacing Phases of a 3 phase pump JUST affect on amount of flow and never affect on direction of flow. Yes?
     
  5. Aug 6, 2016 #4

    David Lewis

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    If fluid is fed into the discharge outlet of a pump, the direction of rotation of the impeller may reverse (depending on type of pump) and the fluid comes out of the suction inlet. In this situation, mechanical power is available at the impeller shaft.

    The advantages of operating a pump as a turbine are convenience, simplicity and low cost. The disadvantages are low efficiency and lack of versatility.
     
    Last edited: Aug 6, 2016
  6. Aug 6, 2016 #5

    CWatters

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    That's NOT true of centrifugal pumps with straight vanes. They pump in the same direction regardless of which way they rotate. Centrifugal pumps with curved blades will be more efficient rotating in one direction than the other but may still pump in the same direction (eg flow not reversed).
     
  7. Aug 6, 2016 #6

    David Lewis

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    Good catch. I stand corrected.
     
  8. Aug 6, 2016 #7
    David & CWatters;
    Your explanations made me a little confused. please correct my thought, if it is not true:
    Both straight blade and curved blades pumps,transfer fluid in ONE direction, regardless of which way they (both of them) rotate. But curved blade pumps have to rotate in just one direction to have more efficient, and if it rotates in reverse direction, then it would has less efficiency. AM I right?
     
  9. Aug 6, 2016 #8

    russ_watters

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    Just to make sure it is clear though; an awful lot of pumps (like the one pictured on the YouTube video cover) don't have symmetrical casings, they have an outlet on one side or the other. For those, if you spin them backwards, it has a huge impact on performance.
     
  10. Aug 6, 2016 #9
    A.T.
    What do you mean with 'Huge Impact'. More explanation is needed. Does it mean the majority of C.F. pump does not transfer fluid to suction side, if they rotate inversely? Or fluid is transferred but with low efficiency?
    Which one?
    Another question is which C.F. pumps can rotate inversely without impact mentioned?
     
    Last edited: Aug 6, 2016
  11. Aug 7, 2016 #10

    russ_watters

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    I think you were responding to me....

    It will vary from one pump to another, so it is tough to put an exact number on it; maybe 50%, maybe 90% reduction in performance (by any way you wish to measure it).
     
  12. Aug 7, 2016 #11
    Good catch.
    Which pumps can behave as both pump and turbine? It is my own thought that axial flow pumps with symmetric casing can operate as pump and turbine (with normal efficiency).
    What do you think?
     
  13. Aug 7, 2016 #12

    Nidum

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    I thought that this question was quite well dealt with in your other threads where we discussed the interpretation of this diagram ?

    2-jpg.102099.jpg

    I certainly don't want to be unhelpful but I am having some difficulty in understanding what the problem is .

    A pump like the one shown in A.T.'s video has the four possible operating conditions shown in the diagram .
     
  14. Aug 7, 2016 #13

    David Lewis

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    I suppose we could extend the plot of flow versus impeller speed to cover turbine operation.
     

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  15. Aug 7, 2016 #14
    Nidum
    russ_watters believes that the pump above will has huge impact if rotates inversely. and you say that pump has four quadrants. Which one is correct? I exactly mean which CF pumps works in both direction without huge impacts (huge headloss or decrease in efficiency)?
     
  16. Aug 7, 2016 #15
    Nidum
    russ_watters believes that the pump above will has huge impact if rotates inversely. and you say that pump has four quadrants. Which one is correct?
    Meanwhile let me use the oppertunity to ask question about Quadrant Up/Left of the diagram: Regarding to https://www.physicsforums.com/threads/four-quadrants-of-pumps.875380/#post-5500916 please explain both H<0 and H>0 in this quadrant. How these two conditions occurs?
     
  17. Aug 8, 2016 #16

    David Lewis

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    If H represents the fluid pressure then I think H>0 means the flow is not freely leaving the discharge outlet. Rather it's impeded when it tries to leave the discharge outlet, for example if you are pumping fluid to an elevated tank.
     
  18. Aug 8, 2016 #17

    David Lewis

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    Probably not many because of the asymmetric casing.

    In the graph, the horizontal axis represents impeller rotational speed. The vertical axis is volume flow rate. When the pump operates normally, you obtain the greatest efficiency (blue line).

    When the impeller rotates in the opposite direction, you get less efficiency (green line). You don't get as much flow even though the impeller spins fast. Hence the line slope is less.

    When the pump operates as a turbine, fluid flow is reversed, and efficiency is again poor (red line). The impeller doesn't spin as fast even though you have lots of flow. So the line slope is steep.

    That's a good point. If you have an axial pump that uses (what looks like) a ship's propeller, it can also work efficiently as a pump. That's helpful if you want to apply power to the turbine in order to add water to the reservoir during times when demand for electricity is low.
     

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