Measurement and Control Basics Fourth Edition

Chapter 9 - Flow Measurement

Introduction

We begin this chapter by discussing the basic principles of flow, then
move on to derive the basic equations for flow velocity and volumetric
flow. The chapter concludes with a discussion of the common types of
flow measuring devices and instruments, such as orifice plates, venturi
tubes, flow nozzles, wedge flow elements, pitot tubes, annubars, turbine
flowmeters, vortex shedding devices, magnetic flowmeters, ultrasonic
flowmeters, positive-displacement flowmeters, mass flowmeters, and
rotameters.

The study of fluids in motion, or flow, is one of the most complex branches
of engineering. This complexity is reflected in such familiar examples as
the flow of a river during a flood or a swirling cloud of smoke from a process
plant smokestack. Each drop of water or each smoke particle is governed
by Newton's laws, but the equations for the entire flow are very
complicated. Fortunately, idealized models that are simple enough to permit
detailed analysis can represent most situations in process control.

Flow Principles

Our discussion in this chapter will consider only a so-called ideal fluid, that
is, a liquid that is incompressible and has no internal friction or viscosity.
The assumption of incompressibility is usually a good approximation for
liquids. A gas can also be treated as incompressible if the differential pressure
driving it is low. Internal friction in a fluid gives rise to shearing
stresses when two adjacent layers of fluid move relative to each other, or
when the fluid flows inside a tube or around an obstacle. In most cases in
process control, these shearing forces can be ignored in contrast to gravitational
forces or forces from differential pressures.

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