Dust Explosions

P. Field

• Publisher: Elsevier Science
• Year: 2012
• Print ISBN: 9780444407467
• eBook ISBN: 9780444596444
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Handbook of Powder Technology, Volume 4: Dust Explosions presents the dust explosion problem in general terms and describes how and why dusts explode. This book discusses the various approaches used to deal with the dust explosion hazards. Organized into five chapters, this volume begins with an overview of the methods used to assess, remove, or minimize the hazard of dust explosions. This text then examines the factors that influence the initiation and severity of an explosion. Other chapters consider the explosion prevention and explosion protection techniques. This book discusses as well the characteristics of a dust explosion and the historical development of the problem. The final chapter deals with the significant concept of explosion protection to reduce the worst effects of an explosion to an acceptable level. This book is a valuable resource for managers, engineers, scientists, safety personnel, and others involved in the handling ad processing of materials in solid particulate form.

CHAPTER 1

INTRODUCTION

Publisher Summary

This chapter discusses the initiation of dust explosions and highlights the typical industries at risk. Dust explosions occur in any industry handling fine particulate combustible material. A dust capable of causing an explosion is regarded as explosible and its explosion characteristics are referred to as its explosibility. The frequency of dust explosions is not in the case of explosions resulting from flammable gas, and this is one of the principal reasons why it has taken so long for the explosion hazards of dusts to become widely recognised. Dust explosions occur when combustible dusts are suspended in air or some other flammable atmosphere and are subjected to a source of ignition. The severity of a dust explosion depends on many factors, but a principal influence is the particle size becausethe consequences of a dust explosion worsen with decrease in particle size. This chapter is guide for managers, scientists, engineers, safety personnel, and others involved in the handling and processing of materials in solid particulate form. It describes the available knowledge, drawn from many countries, on the subject of dust explosions and indicates the methods used to assess, remove, or minimize the hazard.

Dust explosions can occur in any industry handling fine particulate combustible material. An understanding of what is meant by a dust in the context of dust explosions is needed, since a number of terms applied to materials having specific particulate characteristics are often used indiscriminately. A British Standard [1] states that a powder is composed of discrete particles of dry material having a maximum dimension of less than 1000 µm, “grit” is defined as hard particles, usually mineral, of natural or industrial origin, retained on a 75-µm BS test sieve, while a dust is said to be particulate matter which is, or has been airborne and passes a 75-μm BS test sieve. These terms, whilst being technically appropriate in certain industrial situations, can lead to confusion in the context of dust explosions. Since combustible particulate matter in excess of 75 µm is known to be capable of causing an explosion, the term “dust”, in respect of dust explosions and throughout this book, should be regarded as combustible particulate matter capable of being airborne and generally having a particle size less than 500 µm. Another term used to avoid confusion is the term “explosible”, which is preferred to “explosive”, the latter being used in connection with high explosives. A dust capable of causing an explosion is therefore regarded as explosible and its explosion characteristics are referred to as its explosibility. It is also appropriate at this stage to point out that the pressures referred to throughout this book, unless otherwise stated, are gauge pressures, and that the unit of pressure is the bar, (where 1 bar = 10² kN/m² = 14.5 lbf/in²).

The frequency of dust explosions is not as great as in the case of explosions resulting from flammable gas and this may be one of the principal reasons why it has taken so long for the explosion hazards of dusts to become widely recognised. Recent widespread recognition of the industrial explosion hazard probably resulted from several major explosions in the American grain industry towards the end of 1977, which involved large loss of life and widespread destruction of plant. However, some industries still find it difficult to accept that a hazard exists with a material that may have been handled safely over a long period. In other industrial processes, the hazard is not acknowledged either because the main product of the factory is not in particulate form and is in some cases inert, or because it is not recognised through ignorance. A dust explosion has often been initiated because of the careless handling of an unwanted combustible by-product. Typical industries at risk include those involving the manufacture of agricultural products, foodstuffs, pharmaceuticals, chemicals, pigments, polymeric materials, rubbers, metals, and wood. The coal-mining industry recognised that dust explosions contributed to mining tragedies many years ago and since then successful steps have been taken to minimise the hazard. The environment and plant involved in coal mining are specialised and not easily compared with overground industries in which the various industrial plant and handling techniques are fundamentally similar. For this reason, coal dust explosions in the mining industry will not be covered in this book.

Dust explosions occur when combustible dusts are suspended in air or some other flammable atmosphere and are subjected to a source of ignition. The severity of a dust explosion depends on many factors but a principal influence is the particle size, since in general the consequences of a dust explosion worsen with decrease in particle size.

In the industrial situation, the occurrence of ignition sources should never be ruled out; they typically include flames, sparks, hot surfaces, and the results of welding or cutting operations.

Ignition of dusts can result in a fire, particularly if the dust is in a heap or layer, and this may lead to a dust explosion if the burning dust becomes airborne. The hazards of dust fires should not therefore be overlooked when considering dust explosions.

Although dust explosions have been known for nearly two centuries, the complexity of the combustion mechanism has meant that theoretical predictions of the explosion process have been extremely limited. Consequently, dust explosion prevention and protection techniques have to rely to a substantial degree on empirical approaches, which in recent years have proved adequate in the majority of cases. Many countries of the world now have legal standards or codes of practice designed to minimise the risk of dust explosions, specifying explosion prevention and protection requirements.

If a dust explosion hazard can be recognised at the stage when industrial plant is being designed, precautionary measures can be installed without difficulty and at much less cost and inconvenience than in an established plant.

A book by Palmer [2] provides an in-depth study of the fire and explosion hazards presented by dust, as recognised in the early part of the last decade, while a more recent book by Bartknecht [3] includes a study of dust explosions in vessels and pipelines. Both of these books are excellent reference works and are strongly recommended for additional reading on the subject.

This book is intended as a guide for managers, scientists, engineers, safety personnel, and others involved in the handling and processing of materials in solid particulate form. It describes the available knowledge, drawn from many countries, on the subject of dust explosions and indicates the methods used to assess, remove or minimise the hazard.

Following this introduction, the second chapter deals with the hazard of dust explosions, looking at the historical development of the problem and considering the available statistics of incidents. The characteristics of a dust explosion are considered in terms of flame propagation, primary and secondary explosions, combustion mechanism, and the numerous, important variables that influence the degree to which a dust explodes. A general account is given of the range of industries generally at risk and a number of specific explosions are described which indicate the typical events surrounding a dust explosion. The third chapter describes the experimental methods used in a number of countries to assess the explosion hazards of dusts and the way in which the data obtained are interpreted and applied to industrial situations. Dust explosion prevention methods including the avoidance of dust suspensions and ignition sources, the use of ignition detection, extinction and inerting techniques are discussed in Chapter four, while the very important concepts of dust explosion protection, including explosion relief and automatic supression techniques are considered in Chapter five. Finally, the appendices include additional information that has become available after the preparation of the main text, a list of international organisations having expertise in the study of the dust explosion hazard, a simplified approach to the assessment of a dust explosion hazard, a list of known explosible and non-explosible dusts, and a substantial amount of explosibility data, determined in both traditional and new apparatus, for a wide range of combustible dusts.

CHAPTER 2

THE DUST EXPLOSION HAZARD

Publisher Summary

This chapter discusses the historical development and characteristics of dust explosions, together with the account of industries at risk and dust explosion incidents. A dust explosion results when finely divided combustible matter is dispersed into an atmosphere containing sufficient oxygen to permit combustion and a source of ignition of appropriate energy. It presents a serious hazard to the safety of personnel and is capable of causing severe damage to plant and buildings. The chapter explains that once the possibility of a hazard is recognised, explosion prevention and protection methods need to be established. The necessary data are obtained from a range of tests designed to determine the explosion parameters of dusts. Dust explosion statistics are difficult to obtain, and their accuracy reflect the incidents that are notified to the appropriate authorities, namely those that generally result in injury to personnel or serious damage to plant.

2.1 AN HISTORICAL VIEW OF DUST EXPLOSIONS

Dust explosions have probably been with us since the time that man developed the windmill for grinding grain. In fact, the first known report of a dust explosion dates back to the eighteenth century; Count Morrozo [4] gives an account of a violent explosion of flour dust in a Turin mill in 1785. At the time, it was thought that the dispersed dust had produced an inflammable gas which had subsequently ignited.

Explosions in coal mines were common during this period, but again it was assumed that flammable gas was the cause, no thought being given to the possible effects of airborne coal dust. It was not until the latter part of the nineteenth century that it was realised that coal dust could ignite and explode in the absense of gas, or that flour dust alone was responsible for the increasing number of explosions in mills.

During the twentieth century, the frequency of dust explosions increased and a wider variety of materials became involved as industry developed. Probably the most devastating dust explosion, in terms of loss of life, outside the coal industry occurred in 1919 in Iowa, USA, when a starch factory exploded, killing 43 people. Major dust explosions, like any other disaster involving serious loss of life, have often initiated research into the problem. An early instance of this was the independent research carried out in the USA, Great Britain, and France following 18 deaths in a Scottish mill in 1872 and six deaths in a French dye factory in 1878. Coal-mining tradgedies in the USA during 1907 resulted in 1148 deaths, and dust explosions contributed to this total. As a result, the US Bureau of Mines was established in 1908 to study the causes of coal dust explosions and was to continue for over 60 years to study the dust explosion properties of hundreds of different materials. In more recent times, a sugar dust explosion in a Swedish silo in 1966 resulted in the Dust Research Institute in Bonn undertaking an investigation of silos [5]; an explosion in a Dutch fodder plant led to research being sponsored by the Dutch government and industries [6], while Norwegian research [7] was stimulated by an explosion during 1970 in the country’s largest and most modern import grain silo.

In December 1977, two of the most serious dust explosions for several decades occurred. In Galveston, Texas, 18 people died, while 36 died in Westwego, Louisiana, as a result of grain dust explosions; the fatalities during that month were significantly higher than the total killed as a direct result of dust explosions in the American grain industry during the previous d...

Table of contents

• Cover image
• Title page
• Table of Contents
• HANDBOOK OF POWDER TECHNOLOGY
• Copyright
• Dedication
• PREFACE
• ACKNOWLEDGEMENTS
• LIST OF TABLES
• LIST OF FIGURES
• LIST OF PHOTOGRAPHS
• Chapter 1: INTRODUCTION
• Chapter 2: THE DUST EXPLOSION HAZARD
• Chapter 3: DETERMINATION OF DUST EXPLOSION PARAMETERS AND THEIR INTERPRETATION
• Chapter 4: PREVENTION OF DUST EXPLOSIONS
• Chapter 5: EXPLOSION PROTECTION
• APPENDIX A: ADDITIONAL INFORMATION
• APPENDIX B: SELECTED LIST OF ORGANISATIONS HAVING EXPERTISE IN THE STUDY OF DUST EXPLOSIONS
• APPENDIX C: SIMPLIFIED ASSESSMENT OF A DUST EXPLOSION HAZARD
• APPENDIX D: EXPLOSIBLE DUSTS
• APPENDIX E: NON-EXPLOSIBLE DUSTS
• APPENDIX F: EXPLOSION PARAMETERS OF COMBUSTIBLE DUSTS
• REFERENCES
• AUTHOR INDEX
• SUBJECT INDEX