Flow meters-Venturi tube
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 (p1 − p2) 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.
The venturi tube, illustrated in Figure 3, is the most accurate flow-sensing element when properly
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.
The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services.
The rangeability is 4 to 1
Pressure loss is low
Typical accuracy is 1% of full range
Required upstream pipe length 5 to 20 diameters
Viscosity effect is high
Relative cost is medium