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Question about pump theory

  1. Oct 19, 2006 #1
    Hello people,
    I have a question regarding operation of a water pump.
    Say, I have a large hot water pump (for purpose of heating) and that pump "pushes" (don't know correct term) water into system. Thers is one big pipeline consist of two pipes (one for sending and one for receiving water).
    I wonder if pressure in the pipeline is determined with pipe's cross section or pressure is constant and depends only on pump?
    Also what if paralel to this pipeline is connected another on (pump push water into one big pipeline which branches to two pipelines). First pipeline maintan same cross section, but second has half diameter of first pipeline, what pressure will be in first pipeline, and what pressure will be in second?
    In other words, will pump keep pressure same in both pipelines no matter if their cross sections are are not same, or pressure will differ?
    I know that pressure is Force/cross section, does pump give same force that produce two different pressure, or give same pressure?

    Thanks for help
    Last edited: Oct 19, 2006
  2. jcsd
  3. Oct 19, 2006 #2
    The total pressure in the pipe is a constant (provided viscous losses are negligible). I.e. static, dynamic, and hydrostatic pressures will add up to a constant value (the value of the pump pressure).

    Let's assume the pipes are all level, then the static and dynamic pressure will change as the cross section of the pipe changes.

    Because mass is conserved, the pipe with smaller cross section will have higher velocity (i.e. higher dynamic pressure), and lower static pressure.

    The opposite is true for the larger pipe.

    But in both cases the sum of the static and dynamic pressures are equal.

    Warning: You are using your definitions very loosely and its going to get you into trouble. I would look online for what static and dynamic pressure are. This Force/cross section is going to lead you to the wrong conclusions.
    Last edited: Oct 19, 2006
  4. Oct 20, 2006 #3
    Thanks for the reply.
    Yes, I'm aware I didn't use precise definitions and terms. According to the Bernoulli principle:
    rho*velocity^2/2+rho*g*h+p = const
    which means that sum of dynamic, hydrostatic and static pressure is constant. That's OK and I understand that. If I have manometer sticked into pipe and it shows, for example 15 bar, which pressure is it? Is it static pressure p?
    Also I'm not really sure how water pumps are described. Does water pump behave like source of constat water pressure (static) or what?

  5. Oct 20, 2006 #4
    It depends on how you stick the manometer in there. If it is directly into the flow, then it is the stagnation pressure. If it is normal to the flow it is the static pressure.

    A pump does work onto the system, so you put in a work term when you do your energy balance using the bernoulli equation.
    Last edited: Oct 20, 2006
  6. Oct 21, 2006 #5


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    No, a pump does not generate pressure. A pump generates volume flux. The pressure of the facility is given by the actuators at the end of the pipeline (i.e. hydraulic actuators) or the vessel from what the pump is pumping.
  7. Oct 21, 2006 #6
    Thanks for replies.
    I never learned anything about flow and pipeline theory, and trying to learn something fast.
    What is exactly a volume flux, do you mean mass or volumetric flow?
    Do you know any good links online where I can learn basic things about fluid dynamcis, especially how to calculate pressure in pipeline.
    Actually I'm electrical technician and I'm interesting to make analogy between electric and fluid quantities.
  8. Oct 22, 2006 #7
    To be sure I understand story about static and dynamic pressure in a fluid, I need to check few things.
    Bernoulli equation is this:
    rho*vel^2/2+rho*g*h+p = const.

    1. rho*vel^2/2 is called dynamic pressure, right?
    2. p is static pressure exerted by fluid, right?

    Look at the picture in attachment, there is a fluid flow and two manometers.
    I assume that B will show grater pressure than manometer A, because of it's position in pipe.
    Now if I understood correctly, Pressure that will show manometer B will be total pressure i.e. sum of both static and dynamic pressures (rho*v^2/2+p),
    but what pressure will show manometer A? I think it will show static pressure p since this pressure is same in all directions.
    Please confirm this, or tell me what I'm doing wrong?
    Thank you

    Attached Files:

  9. Oct 23, 2006 #8


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    I cannot see the attachment, but I think you're right.
  10. Oct 24, 2006 #9
    Just click on it and it will be opened in another window...
  11. Oct 24, 2006 #10


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    You are correct. Station B is measuring total pressure and A is measuring static pressure.

    As a note, pressure is not created by a pump. Pressure is a result to the resistance to flow. The pump creates the flow, the piping attached to the pump creates the pressure.
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