September 2007
By Jennifer J. Jones, government and public affairs specialist, US Chemical Safety Board
Dust explosions can cause large-scale loss of life and catastrophic damage to industrial facilities. Most solid organic materials, as well as many metals and some nonmetallic inorganic materials, will burn or explode if finely divided and dispersed into the air in sufficient concentrations. For example, polishing, grinding, transporting and shaping many of these materials can produce very small particles, which can easily become airborne and settle on surfaces, crevices, dust collectors and other equipment. When disturbed, the materials can generate potentially explosive dust clouds.
Following three catastrophic dust explosions in 2003 that killed a total of 14 workers, the US Chemical Safety Board (CSB) launched a nationwide study of dust explosions in general industry and what can be done to reduce the risks. The CSB study concluded that combustible dust explosions are a serious hazard in American industry, and that existing efforts inadequately address this hazard.
Small amounts can be lethal
The CSB investigates low probability, high consequence events such as dust explosions. While these tragic accidents caused by combustible dust may not occur regularly, when an explosion does occur it can be devastating. The CSB found that facility management and employees were not aware of the catastrophic explosion risk from the dust accumulations.
The CSB identified 281 combustible dust fires and explosions between 1980 and 2005 that killed 119 workers, injured 718 and extensively damaged industrial facilities. The incidents occurred in 44 states, in many different industries and involved a variety of different materials. Six incidents occurred in the paper industry during this time. The incidents involved paper production powders, paper dust and wood dust.
Dust explosions can be primary or secondary. A primary explosion occurs when a dust suspension within a container, room or piece of equipment is ignited and explodes. A secondary explosion occurs when dust accumulated on floors or other surfaces is lofted and ignited by a primary explosion. The blast wave from the secondary explosion can cause accumulated dust in other areas to become suspended in air, which may generate additional dust explosions in a deadly chain reaction.
Even seemingly small amounts of accumulated dust can cause catastrophic damage. The CSB estimated that the explosion that killed six employees and devastated a pharmaceutical plant in 2003 was caused by dust accumulations mainly under a quarter of an inch. The National Fire Protection Assn (NFPA) warns that more than 1/32 of an inch of dust over 5% of a room's surface area presents a significant explosion hazard. The CSB cautions all industries to be aware of this hazard.
Lack of comprehensive standards
A key finding of the CSB hazard study is that there is no US Occupational Safety and Health Administration (OSHA) standard that comprehensively addresses combustible dust explosion hazards in general industry. Although many states and localities have adopted fire codes that have provisions related to combustible dust, a CSB survey found that fire code officials rarely inspect industrial facilities to enforce the codes. The CSB also reviewed 140 material safety data sheets (MSDSs) for combustible powders and found that in almost half the cases (41%), manufacturers provided no warnings that the powders could explode, and only seven referenced appropriate NFPA standards for preventing dust explosions.
The CSB report also called for OSHA to expand dust warnings under its hazard communication standard, to provide training to inspectors on recognizing and preventing combustible dust explosions and to implement a national Special Emphasis Program on combustible dust hazards in general industry.
Measuring key properties for combustible dust at process conditions (temperature, particle size, moisture content, etc) is the first step in addressing the hazards. The best way to prevent secondary dust explosions is to minimize dust accumulations.
Ensuring good housekeeping, maintaining equipment to prevent dust leaks, using dust collectors, eliminating flat surfaces and other areas where dust accumulates, and sealing hard-to-clean areas like the area above a suspended ceiling can effectively prevent secondary dust explosions. To properly manage workplace hazards, managers and employees need to be aware of the hazards and necessary actions to control them.
The CSB is an independent federal agency charged with investigating industrial chemical accidents. To read the full US Chemical Safety Board Combustible Dust Hazards Study and view reference citations, please visit the "Completed Investigations" web page at www.csb.gov.
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