EMPTY SULPHURIC ACID TANK EXPLODES DURING HOT WORK AT TOP OF TANK

An empty 100-m³ tank sulphuric acid (H2SO4) exploded at 9:15 am at a chemical site. The tank was projected and fell nearby, toppling the scaffolding set up for reservoir maintenance and causing 3 workers (2 subcontracted personnel) to fall. Two of the three were seriously hurt: the plant employee sustained cranial trauma after falling some ten metres; and a subcontractor was pinned between the tank and the scaffolding, injured to the face by the grinder he had been using. The internal emergency plan was activated, bringing both internal and external responders to the site. The injured were taken to hospital. 

The tank had been temporarily plugged on 18th July using a sealant box after discovering a leak on 17th July. Scheduled for repair at the beginning of August following its drainage, the tank was rinsed with water throughout the weekend of 2nd August, then a subcontractor installed the scaffolding for the specific intervention: reservoir access, process insulation plating, etc. A plant employee, accompanied by 2 subcontractors, then climbed up the tank in order to open the top manway when the explosion occurred.

The accident appears to have followed a hydrogen (H2) accumulation at the top of the tank due to ignition of the inflammable mixture formed with air when cutting corroded bolts from the manway with a grinder. The tank was torn over half the circumference of the shell/bottom junction, and its anchorages were stripped. 
Insufficient rinsing of the tank (just a single rinsing cycle was performed) combined with the presence of a low concentration of sulphuric acid caused an acid attack of the metal, leading to the formation and accumulation of hydrogen at the top of the tank (dome-shaped tank roof). The explosion occurred by means of igniting the flammable mix created with air at the time of splitting the corroded bolts on the dome manhole with a grinder. The metallurgical assessment performed on this tank indicated the presence of extensive internal corrosion over the lower part of the structure. This observation confirmed the sudden onset of corrosion at the tank sidewall due to diluted acid, thus generating a source of hydrogen production.

Source:Aria database