Use Of Venturimeter for Flow Measurement and its Advantages

Use Of Venturimeter for Flow Measurement and its Advantages 

A venturimeter is a device used for measuring the rate of a flow of a fluid flowing through a pipe.

It consists of three parts.
  1.  A short converging part
  2.  Throat
  3.  Diverging part
It is based on the principle of Bernoulli’s equation. Inside of the venturimeter pressure difference is created by reducing the cross-sectional area of the flow passage. The pressure difference is measured by using a differential U-tube manometer. This pressure difference helps in the determination of rate of flow of fluid or discharge through the pipe line. As the inlet area of the venturi is large than at the throat, the velocity at the throat increases resulting in decrease of pressure. By this, a pressure difference is created between the inlet and the throat of the venturi.

Basic principle:

When a venture meter is placed in apipe carrying the fluid whose flow rate is to be measured, a pressure drop occurs between the entrance and throat of the venturimeter. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.
Construction of Venturi meter

The following are the main parts and areas of venture meter:
  1. The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.
  2. After the entry, there is a converging conical section with an included angle of 19’ to 23’.
  3. Following the converging section, there is a cylindrical section with minimum area called as the throat.
  4. After the throat, there is a diverging conical section with an included angle of 5’ to 15’.
  5. Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venture meter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge, etc) as shown in diagram.
Operation of venturi meter:
  • The fluid whose flow rate is to be measured enters the entry section of the venturi meter with a pressure P1.
  • As the fluid from the entry section of venturi meter flows into the converging section, its pressure keeps on reducing and attains a minimum value P2 when it enters the throat. That is, in the throat, the fluid pressure P2 will be minimum.
  • The differential pressure sensor attached between the entry and throat section of the venturi meter records the pressure difference(P1-P2) which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
  • The diverging section has been provided to enable the fluid to regain its pressure and hence its kinetic energy. Lesser the angle of the diverging section, greater is the recovery.
  • It is used where high pressure recovery is required.
  • Can be used for measuring flow rates of water,gases,suspended solids, slurries and dirty liquids.
  • Can be used to measure high flow rates in pipes having diameters in a few meters.
Advantages of venturi meters
  1. Less changes of getting clogged with sediments
  2. Coefficient of discharge is high.
  3. Its behavior can be predicted perfectly.
  4. Can be installed vertically, horizontally or inclined.
  1. They are large in size and hence where space is limited, they cannot be used.
  2. Expensive initial cost, installation and maintenance.
  3. Require long laying length. That is, the veturimeter has ti be proceeded by a straight pipe which is free from fittings and misalignments to avoid turbulence in flow, for satisfactory operation. Therefore, straightening vanes are a must.
  4. Cannot be used in pipes below 7.5 cm diameter.

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