Mechanical Engineering Principles

22 Fluid flow

22.1 Introduction

The measurement of fluid flow is of great impor-
tance in many industrial processes, some examples
including air flow in the ventilating ducts of a coal
mine, the flow rate of water in a condenser at a
power station, the flow rate of liquids in chemical
processes, the control and monitoring of the fuel,
lubricating and cooling fluids of ships and aircraft
engines, and so on. Fluid flow is one of the most dif-
ficult of industrial measurements to carry out, since
flow behaviour depends on a great many variables
concerning the physical properties of a fluid.
There are available a large number of fluid flow
measuring instruments generally calledflowmeters,
which can measure the flow rate of liquids (in m^3 /s)
or the mass flow rate of gaseous fluids (in kg/s). The
two main categories of flowmeters are differential
pressure flowmeters and mechanical flowmeters.

22.2 Differential pressure flowmeters

When certain flowmeters are installed in pipelines
they often cause an obstruction to the fluid flowing
in the pipe by reducing the cross-sectional area of
the pipeline. This causes a change in the velocity
of the fluid, with a related change in pressure.
Figure 22.1 shows a section through a pipeline into
which a flowmeter has been inserted. The flow rate
of the fluid may be determined from a measurement
of the difference between the pressures on the walls
of the pipe at specified distances upstream and
downstream of the flowmeter. Such devices are
known asdifferential pressure flowmeters.
The pressure difference in Figure 22.1 is mea-
sured using a manometer connected to appropriate
pressure tapping points. The pressure is seen to be
greater upstream of the flowmeter than downstream,
the pressure difference being shown ash. Calibra-
tion of the manometer depends on the shape of the
obstruction, the positions of the pressure tapping
points and the physical properties of the fluid.
In industrial applications the pressure difference
is detected by a differential pressure cell, the output

h

Downstream

Flow meter causing obstruction in fluid flow

Direction of Upstream

fluid flow

Manometer

Figure 22.1

from which is either an amplified pressure signal or

an electrical signal.

Examples of differential pressure flowmeters

commonly used include:

(a) Orifice plate (see Section 22.3)

(b) Venturi tube (see Section 22.4)

(c) Flow nozzles (see Section 22.5)

(d) Pitot-static tube (see Section 22.6)

British Standard reference BS 1042: Part 1: 1964

and Part 2A: 1973 ‘Methods for the measurement

of fluid flow in pipes’ gives specifications for mea-

surement, manufacture, tolerances, accuracy, sizes,

choice, and so on, of differential flowmeters.

22.3 Orifice plate

Construction

An orifice plate consists of a circular, thin, flat plate

with a hole (or orifice) machined through its centre

to fine limits of accuracy. The orifice has a diameter

less than the pipeline into which the plate is installed

and a typical section of an installation is shown in

Figure 22.2(a). Orifice plates are manufactured in

stainless steel, monel metal, polyester glass fibre,