Page 84 - ebook fluid mechanics_finalize

FLUID MECHANICS

Throat

Entry Converging Diverging section

Direction of
flow
Section 2
Leads gauge filled
Section 1 with liquid in 2 / v2 / a2
pipeline, spec.wt = 
1 / v1 / a1 x

Spec.wt. of gauge liquid= g

Figure 4.9 : Mechanism of Ventury Meter

4.5.3 Mechanism of a Venturi Meter

The arrangement and mode of action of a Venturi Meter ( Figure 4.9 )
▪ The venture meter consists of a short converging conical tube leading to a
cylindrical portion called “throat” which is followed by a diverging section.

▪ The entrance and exit diameter is the same as that of the pipe line into
o
which it is inserted. The angle of the convergent cone is usually 21 , the

length of throat is equal to the throat diameter, and the angle of the
o
o
divergent cone is 5 to 7 to ensure the minimum loss of energy.

▪ Pressure tappings are taken at the entrance and at the throat, either from
the single holes or from a number of holes around the circumference
connecting to an annular chamber or Piezometer ring, and the pressure
difference is measured by a suitable gauge.

▪ For continuity of flow velocity v1 at the entry (section 1) will be less than the

velocity v2 at the throat (section 2) since a1v1 = a2v2 and a1 is greater than
a2. The kinetic energy in the throat will be greater than at the entrance and
since by Bernoulli’s theorem the total energy at the two sections is the

same, the pressure energy at the throat will be less than that at the
entrance. The pressure difference thus created is dependent on the rate of

flow through the meter.

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