Page 440 - The Mechatronics Handbook

0066_Frame_C19 Page 62 Wednesday, January 9, 2002 5:27 PM

19.5 Flow Measurement

Richard Thorn

Introduction

Flow measurement is something that nearly everyone has experienced of. Everyday examples include the
metering of household utilities such as water and gas. Similarly flowmeters are used in nearly every sector
of industry, from petroleum to food manufacture and processing. It is therefore not surprising that today,
the total world flowmeter market is worth over $3000 million and expected to continue growing steadily
in the future.
However, what is surprising, given the undoubted importance of flow measurement to the economy,
is the accuracy and technology of the most commonly used flowmeters which are poor and relatively old
fashioned in comparison to instruments used to measure other measurands such as pressure and temper-
ature. For example, the orifice plate flowmeter, which is still one of the most frequently used flowmeters
in the process industry, only has a typical accuracy of ±2% of reading and was first used commercially in
the late 1800s. The conservative nature of the flow measurement industry means that traditional techniques
such as the orifice plate, Venturimeter, and variable area flowmeter still dominate, while ultrasonic flow-
meters which were first demonstrated in the 1950s are still considered to be “new” devices by many users.
This article will consider the most commonly used commercially available methods of flow measurement.
For recent research developments in flow measurement see [1].

Terminology

The term flow measurement is a general term, and before selecting a flowmeter it is important to be sure
what type of flow measurement is actually required. For a fluid flowing through a pipe, flow measurement
may mean any one of six different types of measurement.
1. Point velocity measurement—the fluid’s velocity at a fixed point across the pipe’s cross section
(m/s)
2. Mean flow velocity measurement—average fluid velocity across the cross section of the pipe (m/s)
3. Volumetric flowrate measurement—the rate of change in the volume of fluid passing through the
3
pipe with time (m /s)
3
4. Total volume measurement—the total volume of fluid which has passed through the pipe (m )
5. Mass flowrate measurement—the rate of change in the mass of the fluid passing through the pipe
with time (kg/s)
6. Total mass measurement—the total mass of fluid passing through the pipe (kg/s)
Although the most common type of flow measurement is that of a fluid through a closed conduit or
pipe, open channel flow measurements are also regularly needed in applications such as sewage and water
treatment. For further information on open channel flow measurement techniques see [2].

Flow Characteristics
The fluid being metered is usually a liquid or gas, and is known as single phase flow. However, there is
an increasing need for the flowrate of multiphase mixtures to be measured (see the section titled “Two-
Phase Flow”).
There are a number of important principles relating to the characteristic of flow in a pipe, which should
be understood before a flowmeter can be selected and used with confidence. These are the meaning of
Reynolds number, and the importance of the flow’s velocity profile.
v
The Reynolds number Re is the ratio of the inertia forces in the flow (ρ D) to the viscous forces in
the flow (η), and it can be used to determine whether a fluid flow is laminar or turbulent in nature.

©2002 CRC Press LLC