4 replies to this topic

[CS Kang]

I have this misconception about pressure drop of venturi meter and orifice meter. I have read about pressure recovery for both meters and found that there is always a permanent pressure/head loss dependent on the area ratio for orifice while for venturi, fluid slows down in a cone with smaller angle where kinetic energy is converted back to pressure energy. If that is so, then why is the pressure drop of venturi meter higher than that of the orifice? Also, since venturi has a greater pressure drop than orifice, does it means that orifice is an inferior meter as compared to venturi? Thanks for enlightening!

[Padmakar Katre]

Dear C S Kang,
I think even this time also you are confused.As you told the pressure drop in venturi is more compared to the orifice is absolutely wrong.Venturimeters have better pressure recovery (Cd~0.94 to 0.99) than the orifice meters (Cd~0.6).
In case of the orifice meters the if you apply the Bernaulli and Equation of continuity we will come to know that the velocty is the maximum and pressure at the lowest and downstream of the vena-contracta the velocity starts to decrease to the extent like what it was there in upstream of orifice and the pressure at vena contracta is less and starts increasing d/s of vena-contracta and reaches to a value which is lower than than what it is in u/s of orifice so there is net peremanant head loss which you have to made up by pumping machine.
While as in case of the venturimeter the converging section the angle is almost 15-20 degree where pressure starts lowering and velosity starts increasing and both reaches to its extreme at the throat but if we find the diverging section the angle is of say 5-7 degree and here you find your most of the kinetic energy is converted back to pressure keeping the minimum head loss i.e. high pressure recovery or the energy recovery.

I think the above two paragraphs will clear your confusion and now the difference between the orifice and the venturi is
1.Pressure recovery (high in Venturi)
2.Cost of orifice is less
3.Accuraccy in case of orifice is 2-4 % of the full scale/range while as in case of venturi its 1% of full scale.
For more deatils you can have readings of any of instrumentation book.
I hope it will solve your confusion.

[JoeWong]

CS Kang,

If that is so, then why is the pressure drop of venturi meter higher than that of the orifice?

Please advise where you got the statement stating that pressure drop of venturi meter higher than that of the orifice
.


Please read the following charts.


Source : Engineering Toolbox



Source : OMEGA

They show that permanent / unrecoverable pressure drop for orifice is higher than venturi.

I am NOT INST. specialist...
My speculation (or can be silly statement) :

Permanent pressure drop (DPp)= Pressure drop across fitting (DPf) + Recoverable pressure drop (DPr)

As venturi is having "longer" travel length and unique construction, pressure drop across fitting (DPf) is higher compare to orifice. However, smooth expansion of venturi generate high recoverable pressure drop (DPr). The net effect is Permanent pressure drop (DPp) for venturi is lower than orifice.

Typical real world example is expander at the outlet of PSV. The inlet pressure to expander is lower than the outlet pressure of expander.

"Pressure drop" in your statement refer to pressure drop across fitting (DPf).

You may reconfirm with your lecturer who are specialize in this area and advise us accordingly.



JoeWong

[Dr_IQ]

If the throat dia of venturi meter is smaller than the vena contracta dia of the orifice then the pressure drop from up-stream to throat would be more in venturi meter. Recovery is a separate issue.

[djack77494]

Assuming both venturi and orifice are generating the same differential pressure (at the point of minimum pressure), then there is no reason the venturi meter would have a smaller throat than the orifice vena contracta. I would assume that one would want a certain dP for effective flow measurement. That dP is generated by the fluid's velocity in proportion to the throat diameter. For equal dP, the permanent pressure loss is significantly higher for the orifice meter, as is shown in the preceeding materials. As a minor point, the venturi meter is much longer than the orifice plate, so it has additional frictional loss associated with it. I would assume that the orifice arrangement would have a similar frictional loss due to straight pipe that would be substituted for the venturi.