Mechanical Engineering Principles

248 MECHANICAL ENGINEERING PRINCIPLES

(a)

(b)

AB

Orifice

plate

Direction of flow

Pressure

due to orifice

plate

Figure 22.2

and for large pipes, such as sewers or hot gas mains,
in brick and concrete.

Principles of operation

When a fluid moves through a restriction in a pipe,
the fluid accelerates and a reduction in pressure
occurs, the magnitude of which is related to the flow
rate of the fluid. The variation of pressure near an
orifice plate is shown in Figure 22.2(b). The position
of minimum pressure is located downstream from
the orifice plate where the flow stream is narrowest.
This point of minimum cross-sectional area of the
jet is called the ‘vena contracta’. Beyond this point
the pressure rises but does not return to the original
upstream value and there is a permanent pressure
loss. This loss depends on the size and type of orifice
plate, the positions of the upstream and downstream
pressure tappings and the change in fluid velocity
between the pressure tappings that depends on the
flow rate and the dimensions of the orifice plate.
In Figure 22.2(a) corner pressure tappings are shown
at A andB. Alternatively, with an orifice plate
inserted into a pipeline of diameterd, pressure tap-
pings are often located at distances ofdandd/ 2
from the plate respectively upstream and down-
stream. At distancedupstream the flow pattern is
not influenced by the presence of the orifice plate,
and distanced/2 coincides with the vena contracta.

Advantages of orifice plates

(i) They are relatively inexpensive.

(ii) They are usually thin enough to fit between an
existing pair of pipe flanges.

Disadvantages of orifice plates

(i) The sharpness of the edge of the orifice can

become worn with use, causing calibration

errors.

(ii) The possible build-up of matter against the

plate.

(iii) A considerable loss in the pumping efficiency

due to the pressure loss downstream of the

plate.

Applications

Orifice plates are usually used in medium and large

pipes and are best suited to the indication and control

of essentially constant flow rates. Several applica-

tions are found in the general process industries.

22.4 Venturi tube

Construction

The Venturi tube or venturimeter is an instrument

for measuring with accuracy the flow rate of fluids

in pipes. A typical arrangement of a section through

such a device is shown in Figure 22.3, and consists

of a short converging conical tube called the inlet

or upstream cone, leading to a cylindrical portion

called the throat. A diverging section called the out-

let or recovery cone follows this. The entrance and

exit diameter is the same as that of the pipeline into

which it is installed. Angleβis usually a maximum

of 21°, giving a taper ofβ/2 of 10.5°. The length

of the throat is made equal to the diameter of the

throat. Angleαis about 5°to 7°to ensure a min-

imum loss of energy but where this is unimportant

αcan be as large as 14°to 15°.

b

A

a

B

Entry Throat Recovery cone Exit

Upstream

cone

h

Pressure

chambers

Manometer

Direction of

fluid flow

Figure 22.3