June 18, 2018

19/06/2018: Dust explosions: A deadly problem—causes and cures

by Vaughn Entwistle, Features Editor, Milling and Grain

Dust explosions have posed a long-standing threat to the grain and milling industry.

One of the earliest documented accounts hearkens back to the 18th century in Turin, Italy. An explosion occurred at Mr Giacomelli’s Bakery Warehouse, and was recorded by a Count Morozzo, who described how flour dust was ignited by an ignition source in the form of a lamp mounted to help flour handlers see. The resulting explosion propagated in multiple directions injuring two employees working in the warehouse. We learn in Morozzo’s report that the flour was extremely dry, and therefore had less moisture than usual.

This tragic scenario has been repeated time and again right up to the modern day. And even though the causes of are now readily understood, dust explosions keep happening.

Causes of dust explosions
There were seven reported grain dust explosions at US food and agricultural facilities in 2017, two more than in 2016, but still below the 10-year average of 9.3 explosions per year.

According to an annual report issued by Purdue University’s Department of Agricultural and Biological engineering. The number of incidents has steadily declined over the years, thanks largely to observing good prevention practices such as keeping facilities clean, training employees, keeping equipment in good working order and more recently by the use of dust detection systems, and dust explosion suppression systems.

Experts in the field created the “Dust Explosion Pentagram” to describe the five constituent elements common to dust explosions:

1. Oxygen. (a major component of the air we breathe)
2. Ignition. A lit cigarette, a spark from static electricity, malfunctioning machinery, or even overheated equipment—all can trigger a fireball.
3. Confinement. When dust particles are contained, such as in a grain silo or inside a mill, they can accumulate. Dust particles can remain suspended in confined spaces for days. When it finally combusts, the confinement will cause intense pressure to build and push the explosion through the entire facility. Confined dust explosions have the power to lift roofs from buildings or buckle solid concrete floors.
4. Fuel. Flour dust explosions are well-known, but almost anything in the form of dust can burn: dried milk powder dust, fishmeal dust, sugar dust, flavourings dust, wheat dust, corn dust, and even non-food-stuffs such as phenolic resin dust, and metal dusts, to provide just a partial list.
5. Dispersion. Dust explosions typically occur when the amount of dust suspended in the air reaches a critical threshold. Too little dust in the air and nothing happens. Similarly, too much dust with too little air might actually suppress an explosion. However, when the critical ratio of dust to air is reached, a simple spark can create a huge fireball. As the explosion propagates through a mill or similar facility, it will often raise more clouds of confined dust, causing a chain reaction of explosions.

Read the full article, HERE.

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine Milling and Grain
which is published by Perendale Publishers Limited.

For additional daily news from milling around the world: global-milling.com

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