Toluene – An Underestimated Fire Hazard
Cause of Fire
As every part was properly earthed, an electrostatically caused ignition first was not in focus. When inspecting the location of fire the metal extension was found as well. It was remarkable that at the end which had been connected to the IBC a charred remnant was found which could not be assigned to the plastic drain cock. Queries brought up that it most likely was the rudiment of the transition piece between the extension and the drain cock. It was made of PVC. At material residues of this part found in the workshop an insulation resistance of R > 1PetaOhm was measured.
Thus the metal extension had been isolated against the earthed IBC by means of the transition piece made of plastic. The extension had been electrostatically charged by the toluene flowing through and so a discharging spark towards the rim of the small metal drum could occur igniting the prevailing optimally ignitable toluene/air-mixture.
Recommendation: Take Extra Care of Earthing
All conductive (metal) parts which can be charged by the process (flowing through pipe) have to be reliably earthed. In general, mixed structures (conductive – isolating) should be avoided in explosion endangered areas.
It would be unlawful to blame the worker for the fact that he did not try hard enough to close the drain cock already in flames to avoid a conflagration, before running away. He had acted absolutely correct to get out of danger because fighting a fire is the task of the fire brigade.
4. How About Earthing?
When toluene is on fire strong soot generation will occur; thus exact determination of causes becomes more or less difficult. This was the way with distilled toluene in a partly filled composite-IBC which had burnt down entirely. In the housing grating entirely blackened by soot, the vague image of the molten down plastic inner container could be seen.
As an ignition source it was considered to be likely that the charge of the flowing toluene had been accumulated in the IBC. This hypothesis proved to be true, when it became known that the pallet of the IBC was made of plastic. And then the »gleaming polished« earthing clamp fixed at the soot blackened housing grating was detected. The reason of the fire was clear to everyone, and to override this embarrassing situation no report was compiled.
In theory and on the basis of case histories it could be depicted why toluene in confirmed electrostatic ignitions of flammable liquids was affected in an above-average degree.