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Hydraulic grade line (HGL) vs energy line (EL)

  1. Mar 20, 2016 #1
    1. The problem statement, all variables and given/known data
    i was told that the major loss causes EL and HGl to decrease gradually , while minor loss causes EL and HGL to decreses rapidly ... Why ?

    2. Relevant equations


    3. The attempt at a solution
    is it because the minor loss(energy loss-( which is caused by the friction loss in the bent of pipes) )is higher than those major loss(due to friction in pipes) ?
     
  2. jcsd
  3. Mar 21, 2016 #2

    berkeman

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    Staff: Mentor

    Can you provide much more detail for this question? "I was told" does not help. Can you post links to sources that make this statement, and provide more information about what types of lines you are referring to? What kind of bends are you referring to? What size pipes, and carrying what gas/liquid?
     
  4. Mar 21, 2016 #3
    @foo9008
    "I was told" I agree is somewhat vague.

    And the answer is it is true, not true, sometimes true and never true, depending upon your piping system.
    At least for the HGL, which can increase in value as well as decrease.
    The EL never increases.

    You should realize that the energy line is the total energy of the fluid.
    If you look at the Bernouilli equation, there it is for you - the EL is the addition of pressure energy, velocity energy, elevation energy, although these can be stated as "head".

    At any location along the pipe, from the EL line, subtract the velocity head and you get the HGL.

    An entrance loss for example will decrease the EL and HGL.

    But what about going from a smaller pipe to a larger pipe diameter. The EL can decrease, but the HGL can increase in the the larger diameter section due to the slower velocity head.
    So your statement abount EL and HGL is not quite correct if you think they always both decrease in tandem.
    Certainly if EL decreases, the HGL cannot recover to where it once was.

    It is much better in keeping losses to a minimum, to have gradual bends, gradual changes in diameters,
    Do you think the reason is friction against the side walls of the pipe for minor losses, or something else that could cause energy loss in sharp corners and changes in diameters. How does the fluid behave with sharp corners and changes in diameters?
     
  5. Mar 22, 2016 #4
    EL decreases due to the loss of energy due to friction such as heat ?

    the fluid loss energy when they encounter 'sharp corner and edges' . is the minor loss(energy loss-( which is caused by the friction loss in the bent of pipes) )is higher than those major loss(due to friction in pipes) ?
     
  6. Mar 22, 2016 #5
    The losses will end up as heat, yes.

    The minor losses stem more from the turbulence, or secondary flows, induced into the fluid from fittings, bends, entrances and exits.

    One way to find your losses is to use the equivalent length method.
    Here is a site that has a list of typical fittings, and their loss in equivalent lengths of pipe in terms of Le/D.

    http://roymech.co.uk/Related/Fluids/Fluids_Pipe.html

    The Darcy-Weisbach formula for head loss is usually used in the following form:

    minor_loss_eq01.png
    With the equivalent length method, the factor now becomes ƒ(L+Le)/D.
    And the equation, if we add up all the minor losses, looks like,
    minor_loss_eq02.png

    In some situations I believe the minor losses can become a significant percentage of the total loss.

    have to run, so ..
     
  7. Mar 22, 2016 #6
    due to this reason , the minor loss is normally higher than major loss?
     
  8. Mar 22, 2016 #7
    If you are comparing the actual length of the fitting to same length of pipe, then one could reach that conclusion. But it doesn't tell you much.

    Try a sample system of just a hole in a tank draining through a length of straight pipe? What if the pipe is short and oversized - short and oversized being in relation to the ratio L/D in equation ( 1). What is the ratio of minor/major losses? I suggest some calculations on your own for that simple case, changing pipe diameter and length, with different entrance and exit types ( nozzle, exit into another tank, square, rounded, embedded entrance ) to find out when and if the minor entrance and exit losses overcome the pipe friction losses. Add a valve or a bend to see what that does to the flow. It should give you a more intuition and feel of the losses.
     
  9. Mar 22, 2016 #8
    The Darcy-Weisbach formula for head loss is for sum of minor loss ? or total head loss?
     
  10. Mar 22, 2016 #9
    L/ D will become small , leading to the sum of minor loss become small ? the minor /major loss become small , am i right ?
     
  11. Mar 22, 2016 #10
    For this case,
    The minor loss is the pipe entrance and pipe exit.
    The major loss is the pipe friction.

    Would you like to revise your answer?

    I can only tell you that for long straight piping systems the minor losses can become negligible.
    For short piping systems, the minor losses become more significant.
     
  12. Mar 22, 2016 #11
    i'm confused now , can you states the formula of major and minor loss?
     
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