Tube / nozzle Venturi are mainly used to measure flow of liquids, eg water distribution systems or biogas plants from wastewater treatment. These primary elements originate much lower pressure drop than orifice plates or nozzles, so they require less pumping pressure. However, its uncertainty is slightly higher than the orifice plate differential pressure which often involves lower. Venturi Tubes and Venturi nozzles have an outlet diffuser divergent constant fluid velocity decreases as the pressure increases.

Model Fig. Venturi (top) and venturi model (below).

Advantages:

- Load losses lower than with orifice plates

- Wear resistant internal

- Less sensitive to disturbances originating upstream

- Good performance for values ​​of | 3 and higher for larger sizes

Disadvantages:

The disadvantages compared to orifice plates are:

- Increased overall lengths

- Installation and maintenance costs higher

- Nominal diameters have problems handling large (greater deadweight transport)

The nozzles are a good compromise between the Venturi tubes and orifice plates because they combine a compact design with few losses. reasonably low pressure. There are two types of nozzles according to ISA 1932 and long radius nozzles ISO 5167-1/A1 (Figure below).

Because of its rounded entrance section with radio, the nozzles can tolerate the fluid flow rates are also very high and a good choice for abrasive fluids. The pressure differential is lower than orifice plates, and therefore also the pressure losses, but uncertainty in the measurement is slightly higher.

The pressure is detected in tubes or annular grooves, much like the case of orifice plates.

Models Fig. nozzle. Left: ISA 1932 nozzle. Right: long radius nozzle ISO 5167-1/A1

Measuring principle - Pitot Tubes

Pitot tubes are also based on pressure differences. Pitot tube piping is installed transversely in the direction of fluid flow or rectangular ducts.

The measuring element, a rod-shaped detector has one or more slits in the front and in back, connected to a differential pressure transmitter for two independent channels.

The grooves in the front load recorded total pressure (dynamic pressure = more static pressure). The grooves on the back only detect static pressure. Consequently, the pressure differential between the front and the back corresponds to the dynamic pressure in the pipe, where flow can be calculated directly from the equations previously discussed. The detector traverses the diameter of the pipe and the grooves are arranged so as to register a representative of the average fluid velocity. This does not mean, however, these detectors are completely immune to the effects of the velocity profile and turbulence. Pitot tubes originate a small differential pressure differential pressure transmitters are capable of detecting modern with a high degree of accuracy. Consequently, the use of Pitot tubes is becoming increasingly more common.

Figure: The principle of measurement of Pitot tubes (multi-port model).

The equations to calculate the mass flow rate (Qm) and the volumetric flow (Qv) can be written in simplified form:

Symbols (of the flow equation and Figure above):

The parameter K depends on the design features of the detector and the inside diameter of the pipe. The value of this parameter is obtained by empirical methods using a calibration process.

The losses in the Pitot tubes are significantly lower than in the methods by narrowing, particularly in large diameter pipes. However, the effects due to velocity profile and turbulence may be more significant for larger diameter of 1,000 mm (40 "). The effect of wear on the accuracy of the measurement is completely negligible (<1.5% v.1 .).

Figure: Pitot tube "Deltatop" E + H.

Henry Pitot (1695-1771) described this method of measurement for the first time in 1732 and used it to determine the speed of sailing ships. Today, any aircraft equipped with two or more Pitot tubes to measure air velocity.

Ludwig Prandtl (1875-1953) combined with a pitot static pressure tap. The sensors are based on these principles have up to four of these tubes Prandtl arranged by the entire cross section of the pipe (multi-port model). Pitoty Prandtl tubes are widely used in laboratories and are also a popular choice in temporary point velocity measurements of fluids in piping systems to determine fluid velocities relative.

Fig: Measurement principle of a Pitot tube (A) and a Prandtl tube (B).

PDIN = dynamic pressure, static pressure = pestat, Ptot = total pressure