To the editor: Comment on the article 'Dust explosions - a rigorous approach to risk reduction'
Author : Alan Tyldesley
30 July 2018
I feel I must comment on the article 'Dust explosions - a rigorous approach to risk reduction' in the July copy of Hazardex. In particular, the observation 'process hazards, not covered by ATEX or IEC standards' leads me to want to fill the gap.
We need to remember that the vast proportion of dust explosions start within the process, because that is where dust clouds form. A rigorous analysis of what happens outside the process may reduce the incidence of fires, but it will not significantly reduce the number of primary explosions.
Next we need to recognise that a very large number of explosions start as a consequence of three types of ignition sources which may be inherent in the system.
The first of these is self-heating materials, of which coal and biomass are handled in the largest quantity, but many more agricultural and food industry products have similar properties. Some self heat from close to room temperature, many more self heat from temperatures within the process or storage unit. There are tests to understand these characteristics, and theoretical ways to extrapolate small scale results, but not all are published by CEN, or the IEC.
The second common process source is size reduction , where large amounts of energy are poured into the fuel, often creating fines in the form of a cloud. Any process disturbance is liable to create the conditions for an explosion.
The third big process category is drying processes, where again energy is poured in to some sort of combustible product. In many cases, to keep drying times short, the fuel is in powder form, and dispersed in air.
In most cases, if any of these products or processes apply, the plant needs to be designed on the basis that an explosion will start, and the consequences must be controlled. A large range of standards cover the design of different means of controlling the consequences of an incipient explosion , mostly from the work of CEN TC305.
Alan Tyldesley, Tyldesley Explosion Consultancy Ltd
(Formerly HSE topic specialist on dust explosions, and formerly chair of BSI committee EXL/23 and CEN TC 305 WG2)