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FILTRATION & SEPARATION MITIGATING THE DANGERS OF DUST
An introduction to combustible dust and its management, by Koen Oostvogels, application development engineer at Donaldson floor. That would result in 0.025m3
he airborne dust created by many manufacturing facilities that generate, handle, transport, process or store materials that cause or become combustible dust is no mere housekeeping problem. Once those particles have settled in difficult-to- reach places throughout a site, any subsequent disturbance can produce a potentially explosive dust cloud. The creation of a risk mitigation plan is therefore necessary for most facilities - including many that would not traditionally be considered ‘dangerous.’ Along with studying legislation requirements in your region, reducing the likelihood of an incident is the best possible step, beginning with ‘Is my dust actually combustible?’
T
This isn’t as straightforward as it might sound. Few people would smoke a cigarette while refueling their car, yet many wouldn’t think twice about lighting up while taking a break from working on home renovations that have produced an abundance of sawdust. Any fine material that can catch fire when mixed with air is a potential risk. Examples include most solid organic materials (sugar, flour, wood, etc.), metals, and more. In fact, even partly oxidized aluminium dust generated from plasma cutting can be considered explosive under certain circumstances – only a proper risk assessment can provide reassurance. Sending a sample of your dust to a qualified lab is a good place to start. If it’s shown to be combustible, further tests will determine if it’s explosive, how quickly that can happen, and how much force it can carry. This information will help direct the selection of equipment needed to mitigate the hazard.
The Explosion Pentagon – While managing any one or more of the well-known Fire Triangle’s elements of oxygen, heat and fuel can decrease the fire risk, explosion risk-
8 MARCH 2023 | PROCESS & CONTROL
management strategies must consider another two elements. Dust dispersion and dust confinement produce the Explosion Pentagon (see opposite), and may require a separate strategy to address any remaining explosion risks.
The Explosion Pentagon’s fuel element is the finely dispersed dust cloud. As a rule of thumb, a cloud dense enough to screen your hand from view would be considered a risk — or, in more practical terms, a dust layer of just 0.5mm. But, rather like how propane gas is only dangerous between 1.8 – 8.4% concentration volumes in the air — particle size is a key factor. Germany’s Occupational Safety & Health Institute documents outline the results of its testing of a variety of dust samples, including their particle size and concentration required to allow combustion.
Sawdust, for example, is generally only a risk when smaller than 63 microns — the finer the dust, the more reaction surface it has with oxygen — and in a concentration above 30g/m3
room, and there’s a 0.5mm layer of dust on the 50m2 at a typical density of 600kg/m3
of dust, and, , that equates
to 15kg, which, if it became airborne, would produce a dust concentration of 100g/m3 creating an obvious danger.
—
. So, let’s say the smoker we mentioned earlier has been working in a 150m3
Knowing the properties of your dust is paramount. Three main parameters should be tested; the first being Pmax, or the maximum pressure that can be reached on the basis of particle size. Next is the speed with which the pressure rise occurs, which will vary according to room or vessel volume. Multiplying that pressure rise by the volume provides the Kst, which enables us to standardise how fast the pressure rises and define four risk categories: from St0 (does not explode) to St3 (very strong explosion). It should be noted, that a weaker St1 explosion is no less dangerous than an St3 event. The final main parameter concerns knowing the minimum required ignition energy, which will enable safer handling. Other parameters include glowing temperature, whether the dust is conductive, and whether self-ignition is possible. All of these factors are specific to a particular dust
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