Venturi tubes, flow nozzles, and flow tubes, like all differential pressure producers, are based on Bernoulli’s theorem. General performance and calculations are similar to those for orifice plates. Modern precision manufacturing techniques allow much greater accuracy of the coefficient for venturi tubes and flow nozzles computed from dimensions, and the coefficients are only moderately less reliable than those for orifice plates.
THE CLASSIC VENTURI TUBE
The venturi tube, as designed by Clemens Herschel in 1887.
It consists of
- A cylindrical inlet section equal to the pipe diameter
- A converging conical section in which the cross sectional area decreases, causing the velocity to increase with a corresponding increase in the velocity head and a decrease in the pressure head
- A cylindrical throat section where the velocity is constant so the decreased pressure head can be measured
- A diverging recovery cone where the velocity decreases and almost all of the original pressure head is recovered
The classic venturi is manufactured with a cast iron body and a bronze or stainless-steel throat section. At the midpoint of the throat, six to eight pressure taps connect the throat to an annular chamber so that the throat pressure is averaged. The cross-sectional area of the chamber is 1.5 times the cross-sectional area of the taps. Because there is no movement of fluid in the annular chamber, the pressure sensed is strictly static pressure.
Usually, four taps from the external surface of the venturi into the annular chamber are made. These are offset from the internal pressure taps. Throat pressure is measured through these taps. This flow meter is limited to use on clean, noncorrosive liquids and gases, because it is impossible to clean out or flush out the pressure taps if they clog up with dirt or debris.
SHORT-FORM VENTURI TUBE
In the 1950s, in an effort to reduce costs and laying length, manufacturers developed the second-generation or short-form venturi shown in Figure. There were two major differences in this design
The internal annular chamber was replaced by a single pressure tap or, in some cases, an external pressure averaging chamber, and the recovery cone angle was increased from 7 to 21°
The pressure taps are located one-quarter to one-half pipe diameter upstream of the inlet cone and at the middle of the throat section. A piezometer ring is sometimes used for differential pressure measurement. This consists of several holes in the plane of the tap locations. Each set of holes is connected in an annulus ring to give an average pressure. Venturies with piezometer connections are unsuitable for use with purge systems used for slurries and dirty fluids, because the purging fluid tends to short circuit to the nearest tap holes.
Venturies are built in several forms.
1.These include the standard long-form or classic venturi
2. A modified short form where the outlet cone is shortened , an eccentric form
3.To handle mixed phases or to minimize buildup of heavy materials, and a rectangular form
A venturi tube may be installed in any position to suit the requirements of the application and piping. The only limitation is that, with liquids, the venturi is always full. In most cases, the valved pressure taps will follow the same installation guidelines as for orifice plates.
- Calculated calibration figures are less accurate than for orifice plates. For greater accuracy, each individual Venturi tube has to be flow calibrated by passing known flows through the Venturi and recording the resulting differential pressures.
- The differential pressure generated by a venturi tube is lower than for an orifice plate and, therefore, a high sensitivity flow transmitter is needed.
- It is more bulky and more expensive.