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 thought of as “dangerous.”
Along with studying legislation requirements in your region, reducing the likelihood of an incident is the best possible step, beginning with ascertaining whether your dust is combustible.
This isn’t as straightforward as it might sound. Few people would smoke a cigarette while refuelling 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 oxidised 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 whether it’s explosive, how quickly that can happen and how much force it can carry. This information will help to direct the selection of equipment needed to mitigate the hazard.
The explosion pentagon: risk management strategies
Although 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-management strategies must consider another two elements.
Dust dispersion and dust confinement produce the Explosion Pentagon — 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 a risk (in more practical terms, that’s a dust layer of just 0.5 mm). But, rather like how propane gas is only dangerous at concentrations of 1.8–8.4% 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 greater than 30 g/m3.
So, let’s say the smoker we mentioned earlier has been working in a 150 m3 room and there’s a 0.5mm layer of dust on the 50 m2 floor. That would result in 0.025 m3 of dust and, at a typical density of 600 kg/m3, that equates to 15 kg. If it became airborne, that would produce a dust concentration of 100 g/m3 and an obvious danger.
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 based on particle size.
Next is the speed with which the pressure rise occurs, bearing in mind that this 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, however, 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 these factors are very specific to a particular dust generated in a particular process, so seeking professional advice is always recommended.
Planning for safety
Once you have determined the combustibility of your dust, you can develop a plan to mitigate the risks in your process. First, although good housekeeping is only a partial solution, audit your process to identify where nuisance dust is generated, released or accumulates … and deal with it.
These often include intake and mixing locations, bag dumps, welding or cutting stations, beams and light fixtures. In each location, analyse the production processes, housekeeping practices, dust control measures and potential ignition sources present.
Bear in mind that when dust builds up in several locations, a flame front can create a pressure wave that leads to a chain reaction, dislodging and feeding on more dust as it moves through the building.
Also, a supposedly empty vessel is usually more dangerous than a full one; for example, opening an inspection hatch on a silo may cause lingering deposits on ledges to become airborne and increase risk.
Even the equipment employed to mitigate the problem can be a danger zone, with dust collectors being responsible for 30% of such explosions. Cleaning the filter media or emptying dust bins can also create locally dangerous dust clouds.
Many standards and codes may influence decisions on dust control, including local, state and European regulations. Knowing the regulations that apply to your facility is critical and process owners should always research the regulatory requirements in their area.
As of July 2003, there are two directives issued by the European Union that are related to the protection of employees and equipment from risks related to potentially explosive atmospheres 1999/92/EC and 2014/34/EU (ATEX Directives).
ATEX directives make it clear that the responsibility for evaluating the risks and creating an explosion protection document lies with the employer/process owner; they are responsible for the selection of their combustible material management strategy and to ensure compliance with all applicable codes and standards.
Having decades-long experience in providing high-quality dust collectors that have become an integral part of many combustible dust mitigation strategies, Donaldson can help to review process owners’ mitigation strategies and provide the optimal dust collection solution for their chosen strategy.