Page 98 - ebook fluid mechanics_finalize

FLUID MECHANICS

e. Small Orifice

The Venturi Meter described earlier is a reliable flow measuring device.
Furthermore, it causes little pressure loss. For these reasons it is widely used,

particularly for large-volume liquid and gas flows. However, this meter is relatively

complex to construct and hence expensive especially for small pipelines. The cost

of the Venturi Meter seems prohibitive, so simpler device such as Orifice Meter is
used.

The Orifice Meter consists of a flat orifice plate with a circular hole drilled in
it. There is a pressure tap upstream from the orifice plate and another just

downstream. There are three recognized methods of placing the taps and the

coefficient of the meter will depend upon the position of the taps.

The principle of the orifice meter is identical with that of the venturi

meter. The reduction at the cross section of the flowing stream in passing through

the orifice increases the velocity head at the expense of the pressure head, and the
reduction in pressure between the taps is measured by a manometer. Bernoulli's

equation provides a basis for correlating the increase in velocity head with the

decrease in pressure head.

From Figure 4.13 the orifice meter is attached to the manometer. There are
Section 1 (entrance of the orifice) and Section 2 (exit of the orifice also known as

vena contracta).

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