Industrial Explosions – More than Dust in the Wind

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OSHAOmahaOn January 20th, the news reported an explosion in an industrial plant in Omaha, Nebraska. The explosion caused at least two fatalities and ten injuries, two of which were serious. The reports said that the explosion occurred at a plant that manufactured animal feed and animal feed products.

CAVEAT – I do not know the cause of the explosion. The Fire Chief has said that the cause of the explosion (as this is written) is unknown and he has declined to speculate on the cause. Firestorm applauds his refusal to rush to a conclusion. OSHA is investigating and they are including (at this time) a dust explosion as one of the possible causes. Industrial explosions can have any number of causes, but in this post, I am going to discuss dust explosions, which is what I believe happened, and need for awareness and mitigation of that risk.

Explosions similar to this one have occurred numerous times throughout the country at various plants and grain storage facilities. Many times, the cause is the same – fine, dry, combustible particles become suspended in the air in a closed space and a random event causes a spark of some kind. Again, that may not be the cause in this case, and no one should leap to that conclusion, but it is important for people involved in certain agricultural and manufacturing activities to understand this specific risk and include it as needed in their mitigation plans.

While wheat or oats or similar cereal grains can burn, they are not highly combustible. Similarly, the dry components of animal feed are not inherently dangerously flammable. In fact, there is a wide range of industries involving relatively benign material from paper manufacture (the cutting and packaging) to lumber mills to flour mills to grain storage that can, in certain circumstances pose a serious risk of explosion.

The paper or wood or grain is not flammable, but the industrial activities that move, process and package them create very fine dust. In most cases, this dust is drawn into fine filters and removed from the air. But any failure of that air filtration system can create a major risk – quickly.

These fine dust particles possess a significant characteristic that their larger “parent” particles do not – they have a high surface-area-to-mass ratio (almost no mass but a measurable surface area) – that means that each dust particle can heat up very quickly and burn. When few dust particles are in the air, this flammability is not a problem since there is an extremely low probability that a particle will be subject to a source of ignition, and, even if it is, little energy can be released from the combustion of a single dust particle.

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However, when the density of these dust particles in the air reaches a critical point, the situation changes. In a manner akin to a chain reaction, any particle that ignites is close enough to neighboring particles that even the low energy released by the single burning particle is adequate to ignite those neighboring particles which, in turn, ignite other neighboring particles. The result is a deflagration – the propagation of a flame front traveling at sonic velocity through the dust suspended in the air. To any naked-eye observer, the result is an explosion (for anyone interested, the flame front in an explosion travels at supersonic velocity).

More important, the total release of energy can be nearly as destructive as an actual explosion. In a closed space such as an industrial facility or a grain storage silo, this energy release can be devastating, causing serious damage to the facility and posing life-threatening injury to people in the facility.

From the perspective of preparedness, it is important to recognize these kinds of explosions as a risk. The material may be benign, OSHA may prescribe air quality standards that preclude such an explosion, and organizational processes may mitigate the risk, but systems and processes can fail. A failure of an air filtration system can be a seriously disruptive incident. Organizations involved in these kinds of activities should include responses to an evolving explosive environment in their risk mitigation and emergency response plans.

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