A venturi tube contains a throat which is smaller in a diameter to the pipeline, into which it fits. The restriction diameter should not be less than   0.224 D, and not more than 0.742 D where D is the nominal bore diameter of the pipe. When the fluid flows through it, the pressure at the throat is lower than the upstream pressure (because of increased kinetic energy due to increased velocity, ½ mv²) and the consequent reduction in flow energy (E_f = P •. v). The flow rate is proportional to the pressure difference, P1 -– P2.

where,  k is dependent on the pipeline and throat diameter,density and discharge coefficient

The venturi flow meter should always be used for turbulent flow. Its accuracy for a wide range of instruments is about 0.5%. ideal for use in Heat Ventilation Air Cond. (HVAC) applications, or air to furnaces and boilers and for liquids containing particles and slurries.

What is Venturi effect ?

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

In fluid dynamics, a fluid’s velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressuremust decrease in accord with the principle of conservation of mechanical energy. Thus any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is balanced by a drop in pressure.

By measuring the change in pressure, the flow rate can be determined, as in various flow measurement devices such as venturi meters, venturi nozzles and orifice plates.

Main Features of Venturi meter:

• Pressure recovery for venturi tubes is a lot better as compared to the orifice plates.
• Venturi tubes are appropriate for clean, dirty and viscous liquid and few slurry services as well.
• Rangeability of a venturi tube varies from 4 to 1.
• In venturi tubes, pressure loss is low and viscosity effect is high.
• Usually, accuracy of a venturi tube is 1% of its full range
• Essential upstream pipe length of a venturi tube is 5 to 20 diameters
• Venturi tubes exist in sizes up to 72″.
• A venturi tube can pass 25 to 50% extra flow than that of an orifice plate keeping the same drop in pressure.
• Since, the early cost of a venturi tube is quite high, it is mainly engaged on larger flows and in more complicated or challenging flow applications.
• Unlike orifice plates, venturi tubes are almost insensitive to velocity profile effects and consequently call for less straight pipe run than an orifice.
• Venturi tubes are resistant to corrosion, erosion, and internal scale build up, owing to their contoured nature, pooled with the self- scouring action of the flow through the tube.
• Use of venturi tubes involves a lot of savings in installation and operating and maintenance costs.
• Venturi tube flowmeters are frequently used in applications involving higher TurnDown Rates or lower pressure drops, particularly in areas where orifice plates fails to perform.
• By means of right instrumentation and flow calibration, the flowrate of a venturi tube can be brought down to about 10% of its full scale range with correct accuracy which in turn results in a TurnDown Rate of 10:1.

Advantages of Venturi meter:

• They can handle large flow volumes at low pressure drops.
• Venturi tube flowmeters involve no moving parts.
• They can be mounted in large diameter pipes via flanged, welded or threaded-end fittings.
• They can be used with nearly all liquids, as well as those containing extreme solids content.
• Venturi tubes involve no projections into the fluid and no sharp corners. Also there are no rapid changes in contour.

Disadvantages of Venturi meter:

• Highly expensive
• Occupies considerable space
• Cannot be altered for measuring pressure beyond a maximum velocity.