Venturi Meter–Construction, Working, Equation, Application, Advantages, and Disadvantages

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Construction and specification of venturi meter

Venturi is an example for restriction type flow meter. Its work based on Bernoulli’s principle. In Venturi, Pressure energy (PE) converted into Kinetic energy (KE) to calculate flow rate (discharge) in a closed pipeline. It consists of three parts as shown in following fig; (i) converging inlet or inlet cone, (ii) throat, (iii) divergent cone or outlet cone.
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D, d – Diameter of inlet or pipeline and throat
P1, P2 – Pressure at inlet and throat
hm – Manometric deflection

venturi_meter_basic_dimensions

Above fig shows the general dimensions of a Herschel standers type venturi meter. Venturi meter is usually made of cast iron, bronze or steel. The converging part is made shorter by employing large cone angle (19° - 21°) while diverging section is longer with lower cone angle (5° - 15°). The high-pressure tap is located at starting of the Venturi and low-pressure tap located in the middle of throat sections.
Accuracy of this type flow meter ranges from ±0.25% to ±3%

Pressure variation along the pipeline for Venturi tube

pressure_variation_along_the_pipeline_for_Venturi_tube

Why pressure drop of venturi meter is very low as compared to that of another flow meter?
Venturi provides a definite power improvement in power losses. Venturi tube has a smooth entry as well as a smooth exit. Unlike the orifice meter, the fluid jet after passing through the restriction (throat) of venturi expands to a large area gradually. So this type of flow meter experiences lesser pressure drop as compared to that of the orifice. The expanding section of venturi meter returns the flow near to its original pressure.

Venturi meter flow rate equation


Theoretical flow rate Q_T=  (A_1 A_2)/(√(〖A_1〗^2 〖A_2〗^2 )  )  √2gh

Actual flow rate,
a1, a2 = Area of cross-section at inlet and throat
g – Acceleration due to gravity
h – Pressure head difference
Cd = Coefficient of discharge

Read: Conversion of manometric deflection to Pressure head difference

Applications of venturi meter

Venturi used in a wide variety of applications that includes gas, liquids, slurries, suspended oils and other processes where permanent pressure loss is not tolerable.  It is widely used in large diameter pipes such as found in the waste treatment process. It allows solid particles flow through it because of their gradually sloping smooth design; so they are suitable for measurement of dirty fluid. It also be used to measure fluid velocity.

Advantages and disadvantages of Venturi meter

Advantages

  • High-pressure recovery. Low permanent pressure drop.
  • High coefficient of discharge.
  • Smooth construction and low cone angle help to solid particles flow through it. So it can be used for dirty fluids.
  • It can be installed in any direction horizontal, vertical and inclined.
  • More accurate than orifice and flow nozzle.

Disadvantages

  • Size, as well as cost is high
  • Difficult to inspection due to its construction
  • Nonlinear
  • For satisfactory operation, the venturi must be proceeded by long straight pipes.
  • Its maintenance is not easy
  • It cannot use in pipe that has small diameter (70mm)

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