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An Introduction to Dust Explosions
Understanding the Myths and Realities of Dust Explosions for a Safer Workplace
By Paul Amyotte Elsevier
Copyright © 2013 Elsevier Inc.
All rights reserved.
ISBN: 978-0-12-397263-7
<h2>CHAPTER 1</h2>
<b>Introduction: Dust Explosions—Myth or Reality?</b></p>
<i>Unicorn: a mythical animal generally depicted with ... a single horn in the middle of the forehead.</i></p>
—Merriam-Webster's Online Dictionary</p>
There is a problem, the nature of which is not well understood, in communicating the results of dust explosion testing and research to stakeholders in industry, government, and the public. In a recent article on dust explosions, I was quoted as follows (p. 47):</p>
<i>When I hear about yet another dust explosion, I hang my head. When someone who has been in the industry for a certain number of years says that they didn't know sugar or flour or aluminum could explode because they'd never seen it happen before—that's just wrong.</i></p>
The answer to this problem is neither as trivial nor as obvious as it may seem. A partial answer—or at least the idea for the paper providing the basis for this book—came from Professor Trevor Kletz during his workshop held as part of the Hazards XXI symposium in Manchester, UK (November 2009). Professor Kletz commented that when poor or impracticable designs are examined, some people may not question the intention of the designers, whereas others may speak up because they see technical oversights and hazards that were not seen before. To illustrate his point, he showed a slide of an animal with what appeared to be a single horn in the center of its forehead (see <b>Figure 1.1</b>). Was it a unicorn? No; the next slide showed the same animal (an oryx) from a different angle, and now it was clear there were two horns on its head (see <b>Figure 1.2</b>). <i>What we see depends on the way we look</i>.</p>
So perhaps dust explosions do occur, in part, because we believe in unicorns—in myths that lack appropriate elements of the natural, management, and social sciences and engineering principles associated with dust explosion prevention and mitigation. This book explores 20 such myths drawn from my research activities and experience in providing dust explosibility test results to industry. Other practitioners and authors would undoubtedly come up with a different set of attitudes and beliefs needing closer examination, but the ones presented here form a useful starting point for a dialogue structured around the explosion pentagon shown in <b>Figure 1.3.</p>
1.1 EXPLOSION PENTAGON</b></p>
In many respects the explosion pentagon affords us everything we need to know on a fundamental level about dust explosion causation. When the requirements of the pentagon are satisfied, the risk of a dust explosion arises. These requirements include the familiar need for a fuel, an oxidant, and an ignition source, augmented by mixing of the fuel and oxidant, as well as confinement of the resulting mixture. The first of these additional components illustrates a key difference between dust and gas explosions—a solid rather than a gaseous fuel. In a dust/air mixture, the dust particles are strongly influenced by gravity; an essential prerequisite for a dust explosion is therefore the formation of a dust/oxidant suspension. Once combustion of this mixture occurs, confinement (partial or complete) permits an overpressure to develop, thus enabling a fast-burning dust flame to transition to a dust explosion.</p>
As helpful as the explosion pentagon may be in understanding why dust explosions occur, it is neutral in terms of how individuals interpret, prevent, and mitigate these requirements. The ensuing chapters illustrate connections between the various pentagon elements and <i>myth</i> typified by erroneous belief (the <i>unicorn</i>), as well as <i>reality</i> expressed through scientific and engineering fact (the <i>oryx</i>). The book concludes with a set of 20 facts to counterbalance the 20 myths identified throughout.</p>
<b>1.2 DUST EXPLOSION MYTHS</b></p>
The myths associated with dust explosions and which are explored in this book are the following [with the applicable pentagon element(s) shown in italics]:</p>
• Dust does not explode (<i>fuel</i>).</p>
• Dust explosions happen only in coal mines and grain elevators (<i>fuel</i>).</p>
• A lot of dust is needed to have an explosion (<i>fuel</i>).</p>
• Gas explosions are much worse than dust explosions (<i>fuel</i>).</p>
• It's up to the testing lab to specify which particle size to test (<i>fuel</i>).</p>
• Any amount of suppressant is better than none (<i>fuel/ignition source</i>).</p>
• Dusts ignite only with a high-energy ignition source (<i>ignition source</i>).</p>
• Only dust clouds—not dust layers𔃀will ignite (<i>ignition source</i>).</p>
• Oxygen removal must be complete to be effective (<i>oxidant</i>).</p>
• Taking away the oxygen makes things safe (<i>oxidant</i>).</p>
• There's no problem if dust is not visible in the air (<i>mixing</i>).</p>
• Once airborne, a dust will quickly settle out of suspension (<i>mixing</i>).</p>
• Mixing is mixing; there are no degrees (<i>mixing</i>).</p>
• Venting is the only/best solution to the dust explosion problem (<i>confinement</i>).</p>
• Total confinement is required to have an explosion (<i>confinement</i>).</p>
• Confinement means four walls, a roof, and a floor (<i>confinement</i>).</p>
• The vocabulary of dust explosions is difficult to understand (<i>pentagon</i>).</p>
• Dust explosion parameters are fundamental material properties (<i>pentagon</i>).</p>
• It makes sense to combine explosion parameters in a single index (<i>pentagon</i>).</p>
• It won't happen to me (<i>pentagon</i>).</p>
<b>1.3 WHY THIS BOOK?</b></p>
The answer to the question "Why this book?" is obtained by examining what the current book is intended to be, and what it is not intended to be. Starting with the latter point, this book has not been written as a comprehensive treatise on all important aspects of dust explosions. Such an endeavor would require a different focus in reviewing the intensive research on dust explosions that has been conducted in the public and private sectors over the past several decades. This research has led to many advances including improved understanding of dust explosion fundamentals, enhanced mitigation techniques such as venting and suppression, and recognition of the role of inherently safer design in dust explosion prevention and mitigation (Chapter 13 in Kletz and Amyotte).</p>
The preceding text references are available to readers who desire advanced treatment in the areas indicated. Additionally, various archival journal articles have been written for specialists in industrial loss prevention and dust explosion research. As explained by Amyotte and Eckhoff, recent reviews cover in detail case histories, causes, consequences, and control of dust explosions; the role of powder science and technology in understanding dust explosion phenomena; and the status of developments in basic knowledge and practical applications with respect to dust explosion prevention and mitigation.</p>
While the scope of the current book is also related to dust explosion phenomena (specifically the prevention and mitigation of dust explosions), it differs from the other works cited in the preceding paragraph in terms of both motivation and objective. Having said this, I would be remiss in not acknowledging the role played by these resources in writing my own manuscript (as evidenced by the numerous references to them throughout).</p>
The writing of this book has been motivated in equal measure by a desire to aid in the protection of people, business assets, operational production, and the natural environment, and a need to address important communication issues with respect to understanding dust explosions.</p>
More generally, one of the process safety research topics identified in the recently published <i>Process Safety Research Agenda for the 21st Century</i> is easy-to-implement process safety methods for industry. Quoting from this document (p. 42):</p>
<i>Due to the sophistication needed to make progress, the gap in the level of theoretical knowledge between academia and most industry experts tends to widen and become an obstacle to communication. This can cause a decrease both in the flow of industry experience to academia and the implementation of newly acquired knowledge to industry. Special effort should be made to counter this trend. Easy-to-implement methods require the developer to fully master the method and the knowledge it is based on in order to describe complex phenomena in simple terms and make the method transparent and user friendly.</i></p>
These motivational points have led to the objective of exploring the myths and realities associated with dust explosion risk reduction. To achieve this objective, I have attempted to provide extensively referenced facts on dust explosions in a manner that clearly and unambiguously refutes several misconceptions about dust explosions. A key feature in this regard is the closing section of each chapter in which readers are invited to express their own thoughts on questions related to the specific content of the chapter.</p>
<b>1.4 WHAT DO <i>YOU</i> THINK?</b></p>
As noted on its website (<b>www.csb.gov</b>), the U.S. Chemical Safety and Hazard Investigation Board (Chemical Safety Board or CSB) is an independent, non-regulatory federal agency that conducts root cause investigations of chemical accidents at fixed industrial facilities. The reports of its investigations are available on the CSB website for downloading and are often accompanied by video footage and animation of the incident sequence. These reports—incident investigations, case studies, safety bulletins, and urgent recommendations—are an excellent resource for training exercises aimed at learning lessons from previous incidents.</p>
The following excerpt is taken from the CSB document describing the results of a recent investigation effort (p. 2):</p>
<i>This case study examines multiple iron dust flash fires and a hydrogen explosion at the Hoeganaes facility in Gallatin, TN. The first iron dust flash fire incident killed two workers and the second injured an employee. The third incident, a hydrogen explosion and resulting iron dust flash fires, claimed three lives and injured two other workers.</i></p>
This particular Hoeganaes plant manufactures atomized iron powder for the production of metal parts in the automotive and other industries. Hydrogen is used in the plant's continuous annealing furnaces to prevent oxidation of the iron powder. Further details, including the answers to the following questions, can be found in the CSB case study.</p>
Before reading the full report, however, consider the following questions based on your current knowledge and understanding of the explosion pentagon and its various elements:</p>
• <b><i>Fuel:</i></b> Can metal dusts such as iron explode? What range of iron dust particle sizes would be expected to support an explosion?</p>
• <b><i>Ignition Source:</i></b> What is a typical energy required to ignite a cloud of iron dust particles with diameters < 75 µm? What about the temperature of a hot surface required to ignite such a dust cloud?</p>
• <b><i>Oxidant:</i></b> Is it practical to eliminate oxygen-containing air from all plant areas in which a dust explosion might occur?</p>
• <b><i>Mixing:</i></b> How could iron dust deposits such as those shown in <b>Figure 1.4</b> be raised into suspension?</p>
• <b><i>Confinement:</i></b> What would be required for an iron dust flash fire to transition to an explosion?
(Continues...)
Excerpted from An Introduction to Dust Explosions by Paul Amyotte. Copyright © 2013 Elsevier Inc.. Excerpted by permission of Elsevier.
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