Flow meters-Venturi tube







Venturi effect

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity.

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. In choked flow the mass flow rate will not increase with a further decrease in the downstream pressure environment.

However, mass flow rate for a compressible fluid can increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a convergent-divergent nozzle.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible flows (such as the flow of water or other fluid, or low speed flow of gas), the theoretical pressure drop (p1p2) at the constriction would be given by:

where ρ is the density of the fluid, v1 is the (slower) fluid velocity where the pipe is wider, v2 is the (faster) fluid velocity where the pipe is narrower (as seen in the figure). This assumes the flowing fluid (or other substance) is not significantly compressible - even though pressure varies, the density is assumed to remain approximately constant.

Venturi Tube

The venturi tube, illustrated in Figure 3, is the most accurate flow-sensing element when properly

calibrated.   The venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone.   It has no projections into the fluid, no sharp corners, and no sudden changes in contour. Figure 3   Venturi Tube The inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease.  The low pressure is measured in the center of the cylindrical throat since the  pressure  will  be  at  its  lowest  value,  and  neither  the  pressure  nor  the  velocity  is  changing. The recovery cone allows for the recovery of pressure such that total pressure loss is only 10% to 25%.  The high pressure is measured upstream of the entrance cone.  The major disadvantages of this type of flow detection are the high initial costs for installation and difficulty in installation and inspection.

 

Working

In the venturi meter the fluid is accelerated through a converging cone of angle 15-20o and the pressure difference between the upstream side of the cone and the throat is measured and provides a signal for the rate of flow.

 

The fluid slows down in a cone with smaller angle (5 - 7o) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no "Vena Contracta". The flow area is at a minimum at the throat.

High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available.

A discharge coefficient cd = 0.975 can be indicated as standard, but the value varies noticeably at low values of the Reynolds number.

The pressure recovery is much better for the venturi meter than for the orifice plate.

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The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services.

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The rangeability is 4 to 1

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Pressure loss is low

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Typical accuracy is 1% of full range

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Required upstream pipe length 5 to 20 diameters

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Viscosity effect is high

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Relative cost is medium

References

www.tpub.com

www.engineeringtoolbox.com

 

 

 


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Wed, 05/20/2009 - 11:08

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