An explosion occurred in the vapour space of a storage tank of aluminium effluent from the production units of a chemical plant. Two employees were hurt: one burned on the ankles, the other less severely to the hair and ears.

This accident occurred as a tank lorry finished loading some of the 60 m³ of effluent present in the full tank. The unit’s shop foreman was on the tank to clean the storage level indicator (bimetallic type) with a wash bottle of distilled water when he noticed it was malfunctioning during the transfer; in fact, the pneumatic valve allowing water supply to the production workshop, which was automatically controlled by the tank’s level indicator, had not reopened despite unloading of the effluent.

A flash occurred inside the tank while the loading operator was approaching the transfer valve to close it and right as the shop foreman was removing the bimetallic strip. The storage manhole was opened, releasing the flames and smoke.

The aluminium effluent in question was composed of 90% water, 4% sodium salt, aluminate ions, toluene and benzene despite the presence of upstream devices designed to retain these solvents (settling for toluene and distillation for benzene). These ineffective devices were probably responsible for the formation of explosive atmospheres (ATEX) in the vapour space of the tank, thus causing the explosion. The ignition source could not be clearly established (no electrical records, unscrewing of the bimetallic element).

A misunderstanding of the ATEX risk associated with this storage, despite the presence of solvents in upstream processes, and the inadequate response of the shop foreman (absence of work orders, no lock-out of equipment during maintenance, non-intervention of maintenance staff, etc.) were the causes of this accident.

Various measures were taken to reduce the likelihood of recurrence of such an accident, namely:

• placement of the storage tanks under nitrogen during loading or unloading,
• revision of the procedure for using work orders,
• design of a high-level control system with no malfunction risk due to the physicochemical aspects of stored products,
• creation of a display indicating that the prohibition to perform maintenance on equipment in use without work orders or authorisation.