# Homework Help: Hey guys help with Venturi Meter question

1. Aug 2, 2008

### vip_uae

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. Aug 2, 2008

### ank_gl

can you show your attempt, it would be better if you debug it step by step

3. Aug 2, 2008

### vip_uae

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

4. Aug 2, 2008

### nucleus

Your entry pressure is in guage and throat pressure is absolute. Have you considered that?

5. Aug 2, 2008

### vip_uae

Oh no i havent noticed that at all..... can u explain the difference plz

6. Aug 4, 2008

### vip_uae

any one plz explain for me :)

7. Aug 4, 2008

### FredGarvin

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

8. Aug 4, 2008

### vip_uae

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 ...

9. Aug 5, 2008

### stewartcs

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

$$Q = A_2 \cdot v_2 \cdot C_d$$

And the energy balance reduces to:

$$\frac{P_1}{\rho} + \frac{v_1^2}{2g} = \frac{P_2}{\rho} + \frac{v_2^2}{2g}$$

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

$$A_1v_1 = A_2v_2$$

$$v_1 = \frac{A_2}{A_1} \cdot v_2$$

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

$$v_2 = \sqrt{\frac{\frac{2g(P_1 - P_2)}{\rho}}{1 - (\frac{A_2}{A_1})^2}}$$

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!

10. Aug 6, 2008

### vip_uae

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