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Thread: Water head- Which way is right?!

  1. #1
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    Water head- Which way is right?!

    We have an ongoing debate about water head in a simple system, maybe someone here can help clarify:

    head.jpg

    Water supply from a resevoir feeds an upstand with a flexible hose outlet attached. Is the water head measured from the top of the resevoir supply to the top of the upstand or to the point where the flexible hose outlet is located?

    Which distance indicates water head? 1 or 2?

    I would love a clear explanation!

  2. #2
    Technical Fellow Kelly Bramble's Avatar
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    I think the most correct terms are "Hydraulic Pressure Head" as the fluid could be anything liquid.

    So, if you're trying to determine the pressure at the flexible hose OUTLET or discharge then number 1.

    If you're trying to determine the pressure at the flexible hose INLET then number 2.

    See: http://www.engineersedge.com/hydraul...sure_10054.htm

  3. #3
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    Providing that's a closed system, mentally move the hose horizontally back to the vertical wall of the reservoir and ask yourself the questions again.

    Number 2 appears to be to the top of the upstand tank, but that has no function or influence in this system. Number 2 should be from the top of the reservoir level to the inlet point of the flexible hose as Kelly mentioned.

  4. #4
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    Neither is technically correct.
    Head is not exactly distance but only makes sense if you write the Bernoulli equation for flow. For a frictionless system between any 2 points

    H1+p1/rho +V1^2/2g=H2+p2/rho+V2^2/2g

    where
    H1 ,P1,V1 are the height(measured from some arbitrary datum), pressure and velocity at point 1

    H2,p2, V2 are the downstream values at any point 2

    If point 1 is the top of the reservoir
    p1(gauge Pressure) and V1 are about 0 and the equation becomes

    H1=H2 +p2/rho+V2^2/2g

    H2 +p2/rho+V2^2/2g is called the total pressure head and is the same throughout this frictionless system.
    At the outlet, p2=atmospheric= 0 gauge and then
    H1=H2+v2^2/2g
    Some call H2-H1 the "water head" and in that context the velocity head, v2^2/2g at he outlet is
    equal to the "water head"

    So flow can be characterized as

    H2-H1=water head=pressure Head + velocity head

    Hope this doesn't make the confusion worse.

  5. #5
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    While Zeke, may be correct and I say "maybe" as I have not checked anything, but he usauly spot on with his stuff, but the question was "water head" as in pressure at specific points. And, in fact, if we wanted to chew up a few more helpless or hapless Nits, the flow characteristics (losses) of the flexible hose, the temperature and density of the fluid should also be drawn into Zeke's process, even though he quotes a "frictionless" and thus purely theoretical system.

    Soooo, if you had a pressure gauge at the inlet and outlets of the flexible hose and zero flow, then Kelly's and my previous comments still hold true.

    Next !!

  6. #6
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    ok maybe I should rephrase the question!

    Will changing the height of the upstand (z) affect the rate of water flow coming out of the flexible hose?

    Scan_Pic0030.jpg

  7. #7
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    No, only the vertical distance between the reservoir surface and the outlet at the hose determines the flow rate; any intermediate positioning of the hose is mostly irrelevant.

  8. #8
    Technical Fellow Kelly Bramble's Avatar
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    Quote Originally Posted by zeke View Post
    No, only the vertical distance between the reservoir surface and the outlet at the hose determines the flow rate; any intermediate positioning of the hose is mostly irrelevant.
    Not to overcomplicate the system concept but pipe length, fittings and pipe diameter will affect flow rate as well.

  9. #9
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    What about if it is Heavy Water? Would that make a difference?

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    How about if the upstand is higher than the tank water level? still no effect? Forgive my ignorance!

  11. #11
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    Quote Originally Posted by Kelly Bramble View Post
    Not to overcomplicate the system concept but pipe length, fittings and pipe diameter will affect flow rate as well.
    True- but only if we change the fittings between the two systems- surely if everything else is identical except the height of the upstand all we need to decide is if the upstand height will affect the output.

  12. #12
    Chief Engineeer jboggs's Avatar
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    catastrafi - There are only two values that have any significant effect on your output: the elevation of the highest point of water storage wherever it is, and the elevation of the hose outlet. Theoretically everything in between is insignificant.

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