Pollution
Humain
Environnement
Economique

In a paper pulp mill, the supply line to a boiler burning wood chips was shut down for maintenance on a feeding screw. The system was drained. Upon service restart, a fouled high level sensor required a technician’s presence on the silo roof to perform cleaning. After announcing that the installation could be restarted, he was climbing down when the silo exploded, killing him on the spot. The silo roof and duckboard were lifted and sheet metal torn off. The expert appraisal suggested that a deposit of wood chip dust had stuck to the silo walls and undergone self-heating, most likely initiated by an influx of hot air from the boiler. When operations resumed, this deposit became detached and created an ignited dust cloud triggering the explosion. The operator implemented a series of remedial measures: access prohibited to the silo roof during a filling phase, suction of dust at the top of the silo, regular cleaning of silo walls, dual stop valves installed on the section between the silo extraction screw and the boiler, a magnetic separator or stone trap introduced to avoid the presence of foreign bodies in the chips, use of temperature probes between the silo bottom and the boiler inlet on both sides of the check valves, a CO analyser placed on the silo, water vapour inerting taps on both the silo’s upper and lower parts, and larger vents (2.4 m²).