Dust Dangerous Dust

Dust clouds can be ignited by the effects of mechanical friction from overheated bearings or motors, sparks from grinding machinery, static electricity, electrical arcing, welding sparks or naked flames. Here’s Part 1 of our guide to understanding and preventing dust explosions.

BY SIMON FRIDLYAND

* A dust explosion that occurred at a sugar refinery owned by Imperial Sugar in Port Wentworth, GA, on February 7, 2008, killed 13 people and injured 42. The origin of the explosion was narrowed down to the a location at the centre of the factory, and it is believed to have started in a basement beneath the facility’s storage silos.
* In Harbin, China, an explosion of flax dust triggered a chain of explosions that destroyed a whole 13,000 sq m linen plant on March 15, 1987, causing 58 deaths and 177 injuries. The strength and time of each explosion was detected at a nearby earthquake monitoring station.
* At Westwego, LA, a series of explosions on December 23, 1977, destroyed 37 grain silos completely and damaged many others, causing 35 deaths and $100 million worth of damage. The facility consisted of 73 silos with a capacity of 150,000 cu m. It was one of five grain storage explosions that occurred over an eight-day period, and was attributed to unusually dry weather conditions.
* In Bremen, Germany, the greater part of the 400,000 sq ft Roland Mill complex was destroyed by a series of explosions on the February 6, 1979. The complex included a seven-storey flour store, six-storey mill, other silos and an administration building. A pressure wave struck a loaded truck, throwing it against a wall, spreading its load into the air, and this is believed to have caused an open air explosion. No traces were found of seven of the 14 people killed in the fire, who were believed to have been cremated in area of the fire where all traces of combustibles were consumed – meaning the temperature was an estimated 1,000°C.
As you probably noticed, all of these explosions occurred during winter or dry times, when humidity is low. Dust explosions are dangerous because they can set off a chain reaction. The initial explosion is usually small and localized; however it is often sufficient to disturb surrounding dust deposited on floors, roofs, beams and machinery to form a second, much larger cloud, which, in turn, can lead to a far more devastating explosion. Further explosions can follow in other parts of the building or even in neighbouring buildings.
These secondary explosions may occur seconds or even minutes apart, and have been described by those who have survived a dust explosion as sounding like “rolling thunder.” A fire can then follow from scattered burning particles, or from other small dust accumulations that have been ignited.
Any combustible solid material that can be dispersed into the air as a dust cloud is capable of causing a dust explosion. Explosions on record have originated from the following sources:
• Agriculture -- grain dust, flour, sugar, milk powder, wool, paper and wood
• Metals -- aluminum, magnesium, zinc
• Mining -- coal, combustible sulphide ores
• Chemical industry -- sulphur, most plastics, pesticides and pharmaceuticals, including aspirin and vitamin C.
A dust explosion is the very rapid combustion of a dust cloud that produces a flame and a pressure front. There is more explosive energy in the dust from grains, such as wheat, barley and corn, than in an equal amount of TNT. The flame front frequently causes loss of life, while the pressure front often causes extensive damage to buildings.
With some dusts, there is sometimes only a flame front and the pressure front is only minimal; with others, the flame spreads with the effect of an explosion.
An explosion occurs as the flame generates heat and engulfs combustion products, and expansion from both these sources causes an immediate pressure rise, which then moves out as a pressure wave and impacts the surroundings. Pressures of 100 psi to 116 psi can be generated, and if it occurs inside a space such as a building, the effects can be devastating, since most buildings can only withstand 0.5 psi of pressure at the most.
Dust clouds may be ignited by the effects of mechanical friction, such as in overheated bearings, motors overheating from air cooling vents being clogged with dust, particles of steel or stone caught up in grinding machinery to produce sparks, overheated dust-coated light bulbs, static electricity, electrical arcing, welding sparks and naked flames.
As a general rule, dusts require 20 to 50 times more energy from an ignition source compared to a flammable vapour, or they need direct contact with surface temperatures ranging from 300°C to 600°C.
The finer the dust, the greater the hazard. Not only can it be more easily blown into the air, but it will stay suspended in the air much longer. It has a greater surface area per unit volume, so that it can burn all the more rapidly, increasing the intensity of the flame front and the violence of the explosion.
Dusts, like flammable vapours, have lower and upper explosive limits. The lower limit is the concentration of dust in the air to just sustain the flame front. The lower flammability limit ranges from about 10-40 g/cu m, depending on the type of dust. At this concentration it would be quite visible to the naked eye as a fog or cloud.
The upper limit is usually difficult to measure since there appears to be no clear cut-off point. Instead it may -- or may not -- ignite at a given concentration. If it does ignite, it tends to leave behind increasing amounts of charred residue.
Concentrations of dust that are potentially explosive are intolerable for people to remain in and are not likely to be found in the open, however, they can be found around machinery used for crushing, grinding, sanding, milling, filtering, blending, shredding, spray drying or conveying bulk quantities of solid materials.
That’s why dusty environments should be treated with respect, and proper safety measures applied. What’s the solution to the problem? Well, we’re out of space in this issue, so that topic – including methods, products and systems to prevent dust explosions -- will be covered in Part 2 in next issue’s The Safety File column. Be sure to watch for it.

Simon Fridlyand, P.Eng., of SAFE Engineering Inc., specializes in industrial health and safety concerns and PSR compliance. For more information, visit www.safeengineering.ca.
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