Dust Explosions – Sources of Dispersion and Ignition Not Independent
In the November 2015 issue of the Chemical Engineer I wrote an article (https://www.integpharma.com/white-papers) about the fatal flaws associated Hazardous Area Classification.
In this article I wrote the following with respect to equipment selection.
“Also, the logic behind this selection is that the event that causes a dust cloud to form is independent of the event that leads to an ignition source becoming active. This is probably true for electrical
equipment, less so for non-electrical equipment and could be completely wrong for an electrostatic spark where the creation of the dust cloud could also create the spark.”
The video at the link below shows a corn silo at White Farms Inc in Indiana, USA, toppling over and rupturing. This causes a large dust cloud to be dispersed which ignites immediately to give a flash fire. Possible sources of ignition include static electricity generated by the flow of corn and dust or a spark create by metal to metal contact as the silo ruptured.
Video of corn silo toppling and susequent flash fire
This is an example of an event that simultaneously causes both the formation of a dust cloud and a source of ignition i.e. they are not independent.
Using the approach detailed in IEC 60079-10-2:2015 it is likely that the area outside the corn silo would be considered to be a Zone 2 since the likelihood of a dust cloud forming is low and certainly not expected during normal operation.
However, since the creation of the dust cloud and source of ignition are not independent the risk of an explosion (or at least a flash fire) are higher would be expected for a Zone 2. What the video illustrates is that in event of catastrophic failure of such silos there is a very high probability of a flash fire and this probability should be taken into account during any hazard analysis.
In my opinion, in any situation where the failure of mechanical equipment can lead simultaneously to both the creation of an explosive atmosphere in the form of a dust cloud and the creation of a source of ignition, the probability of an explosion occurring should be considered much greater than would be expected for a Zone 2 hazardous area.