The dangers of dust in the workplace are not limited to the health consequences of inhalation by the workforce. There are many instances in manufacturing industry where the dust produced – either as a by-product or as the end product itself – is potentially flammable.
And while the flammability of dust (or powder) that has settled on the floor or other surface is of minor concern, should that dust become airborne in the form of a dust cloud, then there is a very real danger of a dust explosion.
A dust explosion involves the rapid combustion of dust particles that releases energy and usually generates gaseous reaction products.
While a mass of solid combustible material as a heap or pile will burn relatively slowly due to the limited surface area that is exposed to the oxygen in the air, if that same solid is in the form of a fine powder and it is suspended in air as a dust cloud the result will be quite different.
In this case the surface area exposed to the air is much larger, and if ignition occurs, the whole of the cloud may burn very rapidly, creating a dust explosion.
The most violent explosions usually result from dust/air mixtures that are fuel rich. This means that the oxygen available in the air cannot burn all the dust, and partly burnt, glowing material often remains after the explosion. This can reignite if more air becomes available.
The shape and size of the dust particles, and other factors, strongly affects the force of the explosion and the explosive limits. Only weak explosions are likely where the mean particle size of the dust exceeds 200 microns, or the moisture content exceeds 16%.
Obviously, any explosion needs an ignition source, but these can be surprisingly commonplace, from an electric arc from a machine to an electrostatic discharge from something seemingly safe such as a conveyor roller.
The list of flammable dusts and powders that can be found in manufacturing industry is surprisingly long. Dangerous substances range from obvious item like flour, sugar, and coal dust to pharmaceutical and complimentary compounds and even some metals such as aluminium, magnesium, and titanium.
Controlling dust in such manufacturing operations is not a straightforward task. One option is to maintain a “damp” environment so that the dust is effectively “washed” out of the air. However, this is not practical in applications like flour milling (a sticky situation, indeed).
In the majority of cases, the simplest form of dust control is to clean it up before it becomes a problem. And as any form of sweeping (whether manual or mechanised) comes with the risk of raising the dust into the atmosphere and creating a potentially flammable cloud, the safest solution for dust removal is a vacuum cleaner.
However, not all vacuum cleaners are created equal, particularly when it comes to operating in dust-laden atmospheres.
The Nilfisk Model VHS110 ATEX is an third-party-certified single-phase industrial vacuum cleaner suitable for use in Zone 22 Ex classified areas.
Chris Callicott is Business Development Manager ANZ at Nilfisk, and he has spent many years in the industrial vacuum cleaner market. He feels that a great many companies fail to adequately address their issues with dust and powder in the workplace.
“There’s a lot of complacency about dust issues,” he says. “It’s a difficult and complex issue, and unfortunately for some the reaction is to bury their heads in the sand. And it’s not helped by some poor advice out there in the market – it’s not malicious, it’s genuine misunderstanding of the hazards.”
“Clearly, it is best to capture dust before it becomes airborne. Industrial vacuum cleaners have a role to play here, along with good housekeeping practices.”
The choice of vacuum cleaner is important in this respect, as is its maintenance, keeping in mind that electrical and mechanical equipment frequently provides the ignition source for an explosion. Electrical arcs and sparks and overheated bearings must be avoided at all costs.
Like a number of models in the Nilfisk industrial vacuum cleaner range, the VHS110 is suitable for use in dust-laden atmospheres up to Ex Zone 22, the company also supplies several models suitable for the more stringent requirements of Zone 21.
With its brushless motor, fully conductive construction and L-class antistatic star filter, the unit is designed from the ground up to remove the risk of ignition when used in environments prone to explosive dust/air mixtures.
The ultimate in safe vacuum cleaning comes from the Nilfisk Model CTT40 IECEx, a three-phase cleaner with full IECEx certification.
“In the end,” says Chris Callicott, “safety is all about minimising the risks posed by any dust in the workplace.”
“Asbestos and silica are deadly, but they can take tens of years to kill you. Combustible dust can be instantaneous.”
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