J. LDSS Prev. Process Ind. Vol. 9. No. 1. pp. 1-2, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. Al1 rights reserved
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ELSEVIER
Introduction to the Special Issue on Dust Explosions. Theme: Hazard Evaluation, Prevention and Mitigation of Dust Explosions tive tool for assessing the pressure development of dust explosions and suggests a modification of the cubic relationship based on a function of flame thickness and vessel radius; there are thus both fundamental and practical implications of their study. The work of Lunn et al., as described in the first of two papers from the UK Health and Safety Laboratory, deals with the particular problem of dust explosions in systems of linked, enclosed vessels. Hazard evaluation in this case is aimed at gaining a better understanding of the phenomenon known as pressure-piling; such knowledge is critical in applying the mitigation technique of containment, whereby explosion products and overpressure are contained entirely within the process unit. The authors begin with a review of recent research in this area and then describe the results and analysis of their experimentation with vessels having volumes of 2, 4 and 20 m”. While most of the papers in this issue deal with dust/air suspensions, the work of Nakajima and Tanaka offers an interesting look at the potential hazards of dust layers. In this fundamental study, the critical state for spontaneous ignition is classified as being of either the Frank-Kamenetskii type or the oxygen-deficient type. The authors present guidance for type discrimination based largely on the bed material and porosity. Bearing in mind that the primary use of hazard evaluation data is the design of preventive or protective measures, Nakajima and Tanaka illustrate the practica1 significante of their work through the example of aeration cooling an effective technique for Frank-Kamenetskii-type layers, but one that can actually increase the hazard of oxygen-deficient-type layers. Cashdollar’s paper on coal dust explosibility provides a thorough review of research conducted at the US Bureau of Mines. Although the work described is specific to a particular dust, the lessons learned are applicable to many other materials with respect to how hazard evaluation is affected by changes in dust composition, dust particle size and the co-presence of a flammable gas. Of particular note in this paper is the validation of laboratory-scale data through comparison with full-scale test results. The special issue theme of explosion prevention is addressed in Cashdollar’s work by his examination of the requirement in coal mines for admixture of inert rock dust with combustible coal dust.
In the introduction to his paper in this issue of the journal, Eckhoff comments that: The total amount ofexisting knowledge on industrial dust explosions, their origin, propagation, prevention and mitigation, is vast. And yet, further information is continually being generated through on-going research in a large number of countries. The aim of this special issue is to gather in one reference some of the results of this on-going research, consistent with the theme of Huzard Evaluation, Prevention and Mitigation of Dust Explosions. It would, of course, be impossible to Capture in one instant al1 of the excellent research on dust explosions being conducted worldwide. The very definition of research prohibits this ‘freezing of al1 information’. Nevertheless, it is hoped that the current document wil1 provide the reader with a sample of recent work on dust explosions and wil1 perhaps indicate future directions for concerted effort. The first paper, by Eckhoff, gives a thorough review of the author’s own work and that of others worldwide, primarily dealing with the period from 1990 onwards. Al1 elements of the special issue’s theme are addressed, with particular emphasis on existing and potential linkages between fundamental research and industrial practice. In many respects, Eckhoff sets the tone for the remainder of the papers and offers a foundation for future work in the field. Siwek continues with the theme of hazard evaluation by drawing on his extensive work at Ciba-Geigy. He gives practica1 information on laboratory-scale tests commonly used in Europe and elsewhere to determine the flammabilitylexplosibility parameters of dust layers and dust suspensions. An important feature of his paper is the reference to testing standards developed by organizations such as the International Electrotechnical Commission (IEC) and the International Standardization Organization (ISO). Centra1 to the matter of hazard evaluation (and subsequently the mitigation technique of venting) is the cubic relationship by which the K,, parameter is obtained. The fact that K,, is not an invariant property of a material is amply demonstrated in the work of Dahoe et al. They focus on the role of flame thickness in pressure development during a dust/air mixture explosion and present a three-zone model which accounts for this influence. Their work presents a predic-
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Editorial
Explosion prevention can be achieved by use of an inert gas (e.g. nitrogen) to ensure that process operation occurs at oxygen concentrations below the maximum permissible level. This approach is aimed at removing the oxidant criterion for explosion. Alternatively, removal of the heat necessary for combustion can be achieved by use of an inert dust as described in Cashdollar’s paper. This concept, however, has not been applied extensively in industries other than coal mining. The paper by Mintz et al. describes one such application where the inerting dust is already present in the final product; product degradation is not, therefore, a prohibitive factor in the use of explosion-inhibiting additives. The authors provide data which show how the safe preparation of powdered metals (Al/Mg) for refractory materials can be enhanced by early introduction into the manufacturing process of inert Mg0 powder. The theme of explosion mitigation (or protection) is the subject of the remaining four papers in the special issue. The first three deal with pressure relief venting while the final paper covers automatie suppression of explosions. The common feature of these papers is that they al1 address the situation where, despite having evaluated the hazard and taken appropriate preventive measures, an explosion has occurred and the primary concern is to mitigate the damage to personnel and plant. Siwek’s paper on explosion venting technology contains a wealth of practica1 information obtained from industrial-scale testing. Such testing, and the empirical correlations derived from the data, form the backbone of the venting guidelines issued by the Verein Deutscher Ingenieure (VDI) and the Institute of Chemical Engineers (IChemE) in Europe, and the National Fire Protection Association (NFPA) in North America. It is interesting to note the emergence of regression equations in the new VDI guideline, with a decreased emphasis on the use of nomographs. The absente of adequate venting theories, and the subsequent reliance on experimentation, is illustrated further in the paper by Holbrow et al. They describe an experimental study which extends the work of Lunn et aE. (this issue) on interconnected enclosed vessel systems to interconnected vented vessels. As in their previous paper, the authors first give a thorough review of recent research in the field and then describe the results and analysis of their experimentation with vessels having
volumes of 2, 6.3 and 20 m3. Much practica1 guidance is given on the variables affecting vent area requirements for interconnected vessels (e.g. diameter and length of interconnecting pipe); particularly significant is the authors’ attention to the relationship between these variables and vent areas determined by Ks, nomograph guidelines. Further advances in venting technology are likely to arise through a combination of several factors: ongoing programmes of industrial-scale testing, development and use of ‘first-principle’ mathematica1 models, and continued efforts at computational fluid dynamics (CFD) modelling. The paper by Tamanini and Valiulis describes how the first two of these approaches have been incorporated in a comprehensive and long-standing programme at Factory Mutual Research Corporation. The authors draw on their own experimental results and the test data of other researchers, coupled with the use of theoretical models, to present a new approach to the sizing of explosion relief vents. It is significant to note that in their conclusions they cal1 for contributions to the advancement of venting technology from organizations worldwide, with consensus on the conceptual framework for such work. This fits wel1 with Eckhoff’s encouragement (this issue) of current efforts to establish international cooperation in joint research programmes. Automatie explosion suppression is the subject of the final paper, by Moore. The author draws on his extensive industrial experience to provide a practica1 overview of suppressant choices and the basis for making selections. He emphasizes the importante of choosing the best agent for suppression, ‘best’ often being a compromise between effectiveness and practica1 acceptante. The issue of acceptance is particularly important with the phase-out of chlorine- and bromine-containing compounds. As a final note, 1 wish to express my gratitude to both the manuscript authors and the reviewers for the quality of their work and the timeliness of their efforts. 1 would also like to acknowledge with appreciation the dedication to this special issue of the editorial and production staff at Elsevier Science Ltd and the office staff of the Department of Chemical Engineering at the Technical University of Nova Scotia. Paul R. Amyotte Technical University of Nova Scotia