ABS dust explosion

ABS dust explosion in a silo: Unexpected electrostatic ignition hazards

Roald Perbal Manager Industrial Safety SABIC EuroPetrochemicals B.V. P.O. Box 475 6160 AL Geleen, The Netherlands tel: +31 (0)46 4768794 fax: +31 (0)46 4768600 e-mail:

On 10 December 2001 a dust explosion occurred at the industrial site Chemelot of Geleen in the Netherlands, during the pneumatic conveying of warm ABS powder to a silo. At the time of the explosion there were no activities and nobody was injured. A slight permanent deformation of the silo top could be observed due to the presence of a limited volume of explosive dust-air mixture and insufficient pressure relief.

After eliminating all other ignition sources electrostatic discharges, i.e. propagating brush discharges, could be identified as possible ignition source. Inspection of the inside of the pneumatic conveying system showed that during the years insulating layers with a high dielectric strength were formed that gave rise to propagating brush discharges. The silent witnesses of these discharges in the pneumatic conveying line could be clearly observed. The highly charged layer in presence of the earthed wall of the pipe behaves like a parallel plate capacitor with the layer as dielectric. The presence of the earthed conductor causes a redistribution of the electric field in the direction towards the conductor. In this situation a much higher surface charge density and thus energy can be accumulated before field break down occurs. Resistivity measurements and calculations showed also that propagating brush discharges could actually be generated, in spite of the fact that literature concludes from practical experience and experiments in the laboratory that layers of insulating powders formed in metallic pipes during pneumatic transfer are not expected to give rise to propagating brush discharges. The energy released in propagating brush discharges can be high enough to ignite explosive dust-air mixtures even under turbulent conditions.

The main lessons learned from this case study for explosion prevention are:

• Prevention of ignition sources is difficult to achieve. Long-term changes may occur that remain unnoticed and may give rise to potential hazardous situations.
• Insulating layers formed in pipes during pneumatic transport of insulating powders may give rise to propagating brush discharges.
• Inspect and clean periodically the inside of pneumatic conveying systems and silos to prevent built-up of such hazardous insulating layers.