Page 28 - Dust Explosions in the Process Industries

apter 1

Dust Explosions-Origin, Propagation,
Prevention, and Mitigation: An Overview

1.
THE NATURE OF DUST EXPLOSIONS

1.1 .1
THE PHENOMENON

1.I .I .I
What Is an Explosion?

The concept of explosion is not unambiguous. Encyclopedias give varying definitions
that mainly fall in two categories. The first focuses on the noise or “bang” due to the
sudden release of a strong pressure wave, or blast wave. The origin of this pressure
wa.ve, whether a chemical or mechanical energy release, is of secondary concern. This
definition of an explosion is in accordance with the basic meaning of the word (“sudden
outburst”).
The second category of definitions is confined to explosions caused by the sudden
release of chemical energy. This includes explosions of gases and dusts and solid explo-
sives. The emphasis is then often put on the chemical energy release itself, and explo-
~ionis defined accordingly. One possible definition could then be “An explosion is an
exothermal chemical process that, when occurring at constant volume, gives rise to a
sudden and significant pressure rise.”
In this text, the definition of an explosion shifts pragmatically between the two alter-
natives, focusing on either cause or effect, depending on the context.

1 .I .I .2
What Is a Dust Explosion?

The phenomenon named dust explosions is in fact quite simple and easy to envisage in
terms of daily life experience. Any solid material that can burn in air will do so with a
violence and speed that increases with increasing degree of subdivision of the material.
Figure l.l(a) illustrates how a piece of wood, once ignited, burns slowly, releasing its
heat over a long period of time. When cut in small pieces, as illustrated in Figure I.l(b),
the combustion rate increases, because the total contact surface area between wood and
air has increased. Also, ignition of the wood has become easier. If the subdivision is con-
tinued right down to the level of small particles of sizes on the order of 0.1 mm or less
and the particles are suspended in a sufficiently large volume of air to give each particle