Page 28 - Instrumentation and Measurement

2.5 Flow Measurement

2.5.1 Differential Pressure Methods

There are a number of forms of differential pressure devices based on the above equation and
involving constant size constrictions, e.g. the venturi tube, nozzles, Dall tube and orifice plate. In
addition, there are other devices involving variable size constrictions, e.g. the rotameter. The
following are discussions of the characteristics of the above devices.

The venturi tube (Figure 2.23) has a gradual tapering of the pipe from the full diameter to the
constricted diameter. The presence of the venturi tube results in a pressure loss occurring in the
system of about 10 to 15%, a comparatively low value. The pressure difference between the flow
prior to the constriction and the constriction can be measured with a simple U-tube manometer or a
differential diaphragm pressure cell. The instrument can be used with liquids containing particles, is
simple in operation, capable of accuracy of about ± 0.5%, has a long-term reliability, but is
comparatively expensive and has a non-linear relationship between pressure and the volume rate of
flow.

FIGURE 2.23 (A) Venturi tube, (B) venturi nozzle, (C) flow nozzle.

A cheaper form of venturi is provided by the nozzle flow meter (Figure 2.23). Two types of nozzle
are used, the venturi nozzle and the flow nozzle. The venturi nozzle (Figure 2.23(A)) is effectively a
venturi tube with an inlet which is considerably shortened. The flow nozzle (Figure 2.23(B)) is even
shorter. Nozzles produce pressure losses of the order of 40 to 60%. Nozzles are cheaper than venturi
tubes, give similar pressure differences, and have an accuracy of about ± 0.5%. They have the same
non-linear relationship between the pressure and the volume rate of flow.

The Dall tube (Figure 2.24) is another variation of the venturi tube. It gives a higher differential
pressure and a lower pressure drop. The Dall tube is only about two pipe diameters long and is often
used where space does not permit the use of a venturi tube. The orifice plate (Figure 2.24) is simply
a disc with a hole. The effect of introducing it is to constrict the flow to the orifice opening and the
flow channel to an even narrower region downstream of the orifice. The narrowest section of the flow
is not through the orifice but downstream of it and is referred to as the vena contracta.

Page | 28