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Homework Help: Hey guys help with Venturi Meter question

  1. Aug 2, 2008 #1
    A venturi meter with a 75 mm diameter throat is installed in a horizontal 150 mm diameter pipeline. The pressure at entry to the meter is 70 kN/m2 gauge and the pressure at the meter throat must not fall below 25 kN/m2 absolute. Calculate the maximum flow for which the meter may be used, given that the density of the flowing fluid is 900 kg/m3 and the coefficient of discharge for the meter is 0.96.


    Answer = [0.079 m3/s]

    ive been trying this question for the past hour and seriously my head is abt to explode coz i am getting a tottaly different answer i even tried all the formulas on the net i am still gettin my answer

    Flow rate (Q) = 0.0456275

    and i got this answer
    0.0438

    soo plzz guys help me out thanks
     
  2. jcsd
  3. Aug 2, 2008 #2
    can you show your attempt, it would be better if you debug it step by step
     
  4. Aug 2, 2008 #3
    Thanks for the reply i used this formula

    Q= Cd x At
    -------- x Sqrt 2 x ( P1 - Pt)
    sqrt ( 1 - (At/A1)^2 ) ------- + G
    row

    Cd drag coff

    At Area throat

    A1 Area pipe

    P1 first pressure

    Pt Preasure at throat

    G as in gravity

    i replaced all the values in and thats it
     
  5. Aug 2, 2008 #4
    Your entry pressure is in guage and throat pressure is absolute. Have you considered that?
     
  6. Aug 2, 2008 #5
    Oh no i havent noticed that at all..... can u explain the difference plz
     
  7. Aug 4, 2008 #6
    any one plz explain for me :)
     
  8. Aug 4, 2008 #7

    FredGarvin

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    Science Advisor

    The difference between gauge and absolute pressure is the value of atmospheric pressure. The gauge value uses atmospheric pressure as the 0 or base for the pressure whereas the absolute value uses 0 pressure as the reference. So there is a 14.7 psi or 101 kPa difference between the two values.

    http://en.wikipedia.org/wiki/Difference_between_gauge_and_absolute_pressure
     
  9. Aug 4, 2008 #8
    Well ive been trying to do what you told me exactly but i still get a lower value plz show me the steps for it ...
     
  10. Aug 5, 2008 #9

    stewartcs

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    Science Advisor

    Since the venturi meter is horizontal and assuming the friction is ignored, the flow rate through the meter (and thus inlet) would be:

    [tex] Q = A_2 \cdot v_2 \cdot C_d [/tex]

    And the energy balance reduces to:

    [tex] \frac{P_1}{\rho} + \frac{v_1^2}{2g} = \frac{P_2}{\rho} + \frac{v_2^2}{2g}[/tex]

    Using the Continuity equation to find v_1 in terms of v_2:

    [tex]A_1v_1 = A_2v_2 [/tex]

    [tex]v_1 = \frac{A_2}{A_1} \cdot v_2 [/tex]

    Using the continuity equation along with an energy balance gives a velocity in the throat of:

    [tex] v_2 = \sqrt{\frac{\frac{2g(P_1 - P_2)}{\rho}}{1 - (\frac{A_2}{A_1})^2}} [/tex]

    Since P_1 is a gauge pressure, add 101.325 to get 171.325 kPa absolute at the entrance (since 1 kN/m^2 = 1 kPa).

    You should be able to plug in the given values now to get the flow rate.

    Hope this helps.

    CS

    PS
    You might want to check the math as I ran through that really quick on my scratch pad!
     
  11. Aug 6, 2008 #10
    Thats Gr8 ... i have found the problem.... when i am calculating i forgot to multiply the pressure by 10^3 thats why its giving me a lower value :)

    thanks
     
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