1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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

    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


    User Avatar
    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.

  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


    User Avatar
    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.


    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 :)

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook