Returning to our jaunt through Unit C, we now come to element C2. This is possibly my favourite subject – fire and explosion. This is largely unchanged from before, mostly clarifying and expanding on what was there. It covers the basic properties of flammables/explosives and the familiar mechanisms of vapour cloud explosions and BLEVEs, the way buildings are affected in fires (structurally) and the prevention of explosions. Of note, is explicit mention of the dust pentagon. It’s tempting to think of this as some magic amulet to ward off fires or a translation of 5-steps to risk assessment for the Ancient Greeks. No, in fact it’s something that was already effectively included before anyway in terms of what you need to have in place for a dust explosion to occur.
In order to totally humiliate the fire triangle (or even the fire tetrahedron if we’re feeling three dimensional) there are five sides to a pentagon – obviously. The first three sides are the same as the fire triangle – fuel (the combustible dust), oxygen (present in sufficient quantities), ignition source (such as a spark). The final two sides of the pentagon are dispersion (the fine dust has to be dispersed in the air as a cloud in sufficient concentration) and confinement (the dispersed cloud has to be enclosed, say, in a container or pipe – otherwise the explosion is rather,…er…, unsatisfying – more a ‘poof’ really). In truth there are other factors too – such as the size of the dust particles (big chunks don’t disperse or burn well – which appears to be the same principle for sewage) and moisture content/humidity (wet stuff doesn’t burn well).
You may, just may, want to try that yourself. Small dust explosions can be conveniently attempted in the garden at home. I have found that Caster sugar (or other similar combustible fine dusts) works well, provided you can get it airborne as you ignite it. A blow torch (or candle), a shower head connected to a length of hose pipe and a semi-enclosed pipe (like a length of soil pipe) is about all you need for a fun afternoon in the garden. Put the sugar on the shower head in the soil pipe with the candle and blow down the other end of the pipe (safe distance) – the smell of molten/burned sugar (and possibly singed hair) is a memory to treasure with the kids after the inevitable visit to the accident and emergency department of the local hospital; but at least you will have learned something and had some fun in the process……. But remember to book your hospital bed in advance (it pays to be prepared). Alternatively, you could just watch a video on YouTube…..