Dust explosions–Cases, causes, consequences, and control

In spite of extensive research and development for more than 100 years to prevent and mitigate dust explosions in the process industries, this hazard continues to threaten industries that manufacture, use and/or handle powders and dusts of combustible materials. Lack of methods for predicting real dust cloud structures and flame propagation processes has been a major obstacle to prediction of course and consequences of dust explosions in practice. However, work at developing comprehensive numerical simulation models for solving these problems is now on its way. This requires detailed experimental and theoretical studies of the physics and chemistry of dust cloud generation and combustion. The present paper discusses how this kind of work will promote the development of means for prevention and mitigation of dust explosions in practice. However, progress in other areas will also be discussed, e.g. ignition prevention. The importance of using inherently safe process design, building on knowledge in powder science and technology, and of systematic education/training of personnel, is also emphasized.

Current status and expected future trends in dust explosion research

Dust explosions: A threat to the process industries

To reveal the flame-propagation behavior and the thermal-radiation effects during coal-dust explosions, two coal-dust clouds were tested in a semi-enclosed vertical combustion tube. A high-speed video camera and a thermal infrared imaging device were used to record the flame-propagation process and the thermal-radiation effects of the fireball at the combustion-tube outlet. The flame propagated more quickly and with a higher temperature in the more volatile coal-dust cloud. The coal-dust concentration also significantly affected the propagation behavior of the combustion zone. When the coal-dust concentration was increased, the flame-propagation velocity and the fireball temperature increased before decreasing overall. Based on the experimental results, a dynamic model of the thermal radiation was employed to describe the changes in the fireballs quantitatively and to estimate the thermal-radiation effects during coal-dust explosions.

Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions

Right from the early days of the process industries, continuous efforts have been made to develop and improve measures for prevention and mitigation of dust explosions in these industries. Nevertheless this hazard continues to threaten industries that manufacture, use and/or handle powders and dusts of a wide range of combustible materials. To improve methods for predicting explosion development in real industrial plant has been one major challenge. Hence, during the last years comprehensive numerical simulation codes, for addressing this problem, have been developed. Progress has also been made in other areas, for example, ignition source prevention. The importance of adopting inherently safer process design, by building on firm knowledge in powder science and technology, and of systematic education/training of personnel, is also emphasized.

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Dust Explosion Prevention and Mitigation, Status and Developments in Basic Knowledge and in Practical Application

The fireball from a vented dust explosion presents a danger to personnel who may be within the vicinity of the event. The risk of serious injury to people caught within the fireball is great, and anyone just outside the fireball may be at risk from thermal radiation. This report describes a project in which the effects of thermal radiation from vented dust explosions was studied. The aim was to establish the areas around a fireball in which people would be at risk from thermal radiation. Six dusts were tested in a large vented vessel and external fireballs were generated under a range of conditions. The fireball geometry and the heat flux from the fireball were studied. A range of material samples were exposed to the fireball. The safe areas around the fireballs were established for each of the six dusts. Generally, the larger vent areas resulted in the larger fireballs and high heat pulse values. However, the fireball was usually too brief to ignite fabric samples unless they were very close to the fireball. The work has shown that in most cases the safe area was relatively close to the surface of the largest fireball.

Thermal radiation from vented dust explosions

Much industrial dust-handling plant consists of vessels connected by pipelines. If a dust explosion propagates through such a system, the overall explosion event can be more violent than if a single vessel only is involved, due to a combination of increased turbulence, pressure piling and a jet flame ignition source in the second vessel. This paper gives guidance, based on an extensive experimental programme reported previously, on two aspects of the protection of interconnected vessels: containment and venting.

Dust explosion protection in linked vessels: guidance for containment and venting

This paper is a report of an experimental programme of work to develop inexpensive dust explosion propagation prevention devices. Two types of device were investigated: one is intended for use with charge chutes attached to process vessels and one is intended for use with pipes attached to process vessels. Three dusts were used with KSt values up to 308 bar m s-1. Both devices were found to be effective and resulted in the elimination or significant reduction in the flame from the charge chute or the pipeline.

Simple devices to prevent dust explosion propagation in charge chutes and pipes

This paper is a report of an experimental programme on the explosion protection of bucket elevators by venting. The project was a collaborative effort with funding by the Health and Safety Executive and manufacturers and users of bucket elevators through the British Materials Handling Board. Two bucket elevators were used in the project—a single leg elevator and a twin-leg elevator. Four dusts were used with KSt values up to 211 bar m s−1 and dust clouds were produced by dust injection and by normal operation. Reduced explosion pressures were measured and guidance has been derived from the results. This guidance is in terms of vent spacing as a function of the KSt value of the dust.

Explosion venting of bucket elevators

Equipment manufacturers supplying specialist equipment into the process industries for use where flammable liquids, gases or dusts are handled have responsibilities for the safety and suitability of their equipment under European single market legislation specifically directive 94/9/EC. Operators of process plants handling flammables have responsibilities under European harmonised health and safety law, specifically directive 99/92/EC. The legal responsibilities of those who design and build such plants, who are likely to be neither an equipment manufacturer nor the final plant operator are quite unclear, unless that plant comes under the major hazards directive 96/82/EC (Seveso 2). This paper describes some examples where the uncertainty has contributed to bad design and serious consequences, and suggests questions the design contractor and final user need to address to minimise these problems.

A Tyldesley

Papers

Thermal radiation from vented dust explosions

Dust explosion protection in linked vessels: guidance for containment and venting

Simple devices to prevent dust explosion propagation in charge chutes and pipes

Explosion venting of bucket elevators

The design and build contractor, a responsible role, not properly recognised by health and safety law