Venturi tubes are one of the primary type flow measuring device with lowest permanent pressure loss.It provides low susceptibility to erosion, highest pressure recovery after measurement and it can install at any angle.
Maintenance free operation, lowest pressure loss and easy installation are the major features of the venturi.
Classical Venturi Tube
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 that the decreased pressure head can be measured ;
and a diverging recovery cone where the velocity decreases and almost all of the original pressure head is recovered.
The classical venturi is always manufactured by fabrication with plates or machined from bar stock/forged bar.
Generally, annular chamber/piezometer is used to take the pressure points from the venturi. Since there is no movement of fluid in the annular chamber,
the pressure sensed is strictly static pressure.
Usually 4 taps from the external surface of the venturi into the annular chamber are made.
These are offset from the internal pressure taps. It is through these taps that throat pressure is measured.
Venturis with piezometer connections are unsuitable for use with purge systems used for slurries and dirty fluids
since the purging fluid tends to short circuit to the nearest tap holes.
Piezometer connections are normally used only on very large tubes or where the most accurate average pressure is desired
to compensate for variations in the hydraulic profile of the flowing fluid.
Therefore, when it is necessary to meter dirty fluids and use piezometer taps, sealed sensors which mount flush with the pipe
and throat inside wall should be used.