As is notoriously known, the main risk deriving from the flammability of the raw materials and of the finished product in the tissue field is fire. Based on occurrence, the risk of dust explosions is certainly less relevant.
Alessandro Mazzeranghi
If we consider the issue under the aspect of potential damage, the criticality level is reversed. It is true that a good design/execution of firefighting systems and procedures is a reasonable guarantee for the containment of damage to people and things, but to contain an eventual explosion, effective measures are few and far between. So the rule is to prevent explosions from taking place.
CLASSIFICATION OF THE AREAS AND CONSEQUENT ASSESSMENT OF THE RISK OF EXPLOSION. Personally, I am convinced that often when considering the risk of dust explosion we base our reasoning on a rather uncertain foundation, i.e., the classification of the “areas at risk of explosion” pursuant to directive 94/9/EC and corresponding technical norms. Now for gases, classification is a rigorous practice based on clear and repeatable phenomena (for example like methane that oozes from a faulty gasket and disseminates in the surrounding space until it becomes diluted so much that explosion is no longer possible due to lack of the fuel); but for dust, the concept of classification may be, as we said, uncertain.
CONSIDERING FLAMMABLE DUST HAVING A FINE PARTICLE SIZE (dust produced by the paper machine), we could ask ourselves if that dust – of which we have analyzed a sample – is really potentially explosive, and if our production process is capable of generating a concentration greater than the lower limit for explosiveness (for dust clouds). In all probability, the answer would be: yes, the dust is potentially explosive; no, the dust cloud does not present enough dust to make an explosion possible. Of course! Because we consider the production cycle that yields a limited quantity of dust per unit of time and cubic meter of air. We will hence discover that any problems could be had in the suction devices, but above all in the filters. And we sure know (!?) how to make those safe.
NATURALLY, WE ARE COMFORTED BY THIS STUDY; a specialist came, he or she examined our plant and told us that the risk of explosion is limited to some elements located outside the paper mill’s structures, where there are few people, and that in any case these elements (the filter we spoke about) can be made safe in no time.
But are we really so reassured? Some important questions arise:
• The first is banal: what if we change raw materials and products? Do the same considerations still hold true? Of course, some tissue papers produce more dust than others when converted, probably even smaller in particle size, so if we introduce these into our production cycle, the process conditions vary to the point as to modify the classification.
• The second question: are we sure that there are no other situations that can concretely generate hazardous dust clouds? There could be dust accumulations somewhere, mounted in the course of time that, if removed in inappropriate fashion, could generate an explosive atmosphere.
• And also: are we sure that the filters are really safe? In theory they are, but it suffices that in the course of a maintenance process, the ground is removed and not reinstated, and they could become potential bombs. And if someone feels reassured by the fact that this takes place outside the facilities, let’s not forget that in many cases it has come back inside through the suction hoses.
There is also a phenomenon not to underestimate, the so called secondary explosion. For example, the explosion inside a filter, besides the direct destructive effect, also causes a very strong air movement that could raise any dust present in the environment that, due to the effect of the energy available following the first explosion, could, in turn, explode. So the first explosion may be followed by a second, often more devastating and extended, explosion.
RELATIVELY SIMPLE, WELL-KNOWN PHENOMENA that are not, however, immediately easy to assess where laws and norms in force are slavishly followed. For anyone wishing to delve more in depth into this theme, there is a true case in point described in a brief film footage published on YouTube: https://www.youtube.com/watch?v=Jg7mLSG-Yws (the explosion of the Imperial Sugar factory as re-constructed by the investigative bodies).
So in conclusion: classification is a useful instrument, but remains a supporting instrument. The heart of the issue must lie in risk assessment.
ASSESSING THE RISK OF DUST EXPLOSIONS
As we have tried to highlight, the issue is not to neglect consideration based solely on the outcomes of classification, but to retain it incomplete. A broader risk assessment is necessary, and it starts with the complete consideration of the possible scenarios (usual, incidental and also exceptional) that could give rise to “an ATEX dust emergency”. An example is a rather banal incident tied to the han-dling of a big bag of flammable dusts (an additive) from the transport vehicle to the rack. The big bag was either already severed or was damaged upon lifting by the forklift that was unloading it. In any case, it started losing dust particles, creating a sort of cloud. The forklift driver, having received no specific instructions, acted pursuant to his own judgment and in the end caused an explosion, fortunately with minimal consequences for people and things, but only because the event took place in an open area... Would you find such a case in point in your ATEX assessment? And if you can’t find it there, where would it be considered within the risk assessment for occupational health & safety?
WE NEED TO RETURN TO THE BASICS, following an elementary but valid course for all risks:
• Do the starting conditions to create an ATEX dust hazard situation exist? Which, translated, means:
• Are there flammable dusts in my work site?
• Do these dusts have physical characteristics that could generate an ATEX hazard?
• Could these be present in sufficient quantities (also through accumulation) to create - even only in the worst case scenario - an ATEX hazard situation? If the series of answers is affirmative, i.e., if the ATEX dust hazard can exists, then we proceed with a true risk assessment by considering all the possible scenarios that can concretely lead to this problem and by considering whether current measures are suitable for:
• Preventing the formation of explosive atmospheres
• Preventing the start of explosive atmospheres • Protecting goods and people from the consequences of possible explosions
TO CONCLUDE, A LITTLE GAME. During a training course on the issue, we could describe this example: my mother-in-law wants to prepare egg tagliatelle (typical Italian pasta dish) using 2 Kg of flour, eggs (as the name says) and water. She has decided to make the pasta on the kitchen table. In that moment, a dish that requires several hours’ cooking time is on the stove. The flour, of course, is contained in a bag. Is there an ATEX dust problem here?
Banal, almost infantile; but in our factories, it is easier to forget a scenario as simple as this and underestimate the risk of explosion that we certainly have in our boiler room or in the area of our cogenerator! *