venturimeter and it's principles

Venturi meters are instruments for fluid flow measurement. It has a converging section that gives an increase in the flow velocity with a corresponding drop in pressure from which the flow rate can be calculated. 

The reduction in the fluid pressure that occurs when a fluid moves through a constricted passage is known as the Venturi effect. 

Venturi meters are widely used wherever there is a need for fluid flow measurement, specifically in water, chemical, and oil industries. 

Depending on the application and size requirements, industrial venturi meters are constructed in various forms. 

These versatile instruments are known for longevity, long performance, and reliability. In this article, we will discuss about the following:

  • Definition of Venturimeter
  • Venturimeter Diagram and Parts
  • Working Principle of Venturi meter
  • Co-efficient of Discharge for Venturimeter.

What is a Venturimeter? Venturimeter Definition

Venturimeter is a type of flowmeter that works on the principle of Bernoulli’s Equation. This device is widely used in the water, chemical, pharmaceutical, and oil & gas industries to measure the flow rates of fluids inside a pipe. The pipe cross-sectional area is reduced to create a pressure difference which is measured with a manometer to determine the rate of fluid flow. So, the venturi meter is a differential head type flowmeter that converts pressure energy into kinetic energy.

The principle of the Venturimeter was demonstrated by Giovanni Batista Venturi (Hence the name Venturimeter), But it was first used in practical metering applications by Clemens Herschel. 

Venturimeter Diagram and Parts

A venturimeter consists of four parts:

  1. Cylindrical Inlet Section
  2. Conical convergent Section
  3. Cylindrical throat and
  4. Conical divergent outlet
  • Cylindrical Entrance Section: Venturimeter entrance is a straight cylindrical section with a length equal to 5 to 8 times the pipe diameter.
  • Convergence Conical Section: In this section, the venturi meter tube diameter gradually decreases. The conical angle is normally 210 ± 20. While the liquid flows inside the venturimeter, the velocity of fluid increases at the expense of a decrease in pressure.
  • Cylindrical Throat: Throat consists of the minimum venturemeter diameter. In the throat section, the velocity is maximum and pressure is minimum. Normally, throat diameter = 1/3 to 1/4th of inlet pipe diameter.
  • Diverging Conical section: At this section of venturimeter, the tube diameter gradually increases. So, the pressure is built up again to the original inlet pressure. The cone angle is 5-70. British Standard BS-1042 specifies two conical angles, 5–70 and 14–150 for the outlet cone.

Materials for Venturimeter

Small-size venturimeter are made of brass, glass, or bronze and large venturimeters are made of cast iron, steel, or stainless steel.

Working Principle of a Venturimeter | How Does a Venturi Meter Work?

When a fluid flows through a venturimeter, it accelerates in the convergent section and then decelerates in the divergent section. The pressure difference between an upstream section and the throat is measured by a manometer. Using that differential pressure, applying Bernoulli’s Equation and Contininuity Equitation the volumetric flow rate can be estimated. 

Coefficient of Discharge of Venturimeter (Cd)

The coefficient of discharge for Venturimeter, Cd is defined as the ratio of the actual flow rate through the venturi meter tube to the theoretical flow rate. So the venturi meter discharge coefficient is given by:

Cd=Qact/Q

As Qactual will always be less than Qtheoretical due to frictional losses, the value of Cd is always less than 1.0.

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