Sulphuric acid tank explosion in a chemical plant

An empty 100-m³ reservoir of sulphuric acid (H2SO4) exploded at 9:15 am at a chemical site. Installed directly above its retention basin at a height of several metres, this reservoir 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 internal emergency plan was activated, bringing both internal and external responders to the site. The injured were taken to hospital. The operator notified the Prefecture, local city halls and the general public. The classified facilities inspectorate initiated an investigation.
17th July, a leak of sulphuric acid was detected from the reservoir.
18th July, the leak is closed as temporary solution.
31st July, the reservoir is emptied and rinsed with water
3rd August, the reservoir is emptied and rinsed with water, after which a subcontractor installed a scaffolding for the specific intervention (reservoir access, process insulation plating, etc.).
4th August, a plant employee, accompanied by 2 subcontractors, opens the dome when the explosion occurred (a grinder was used to unscrew the bolts).

The accident appears to have been causes the release of hydrogen accumulated at the top of the reservoir, ignited by the spark of the grinder. The tank was torn over half the circumference of the shell/bottom junction, and its anchorages were stripped.
The tank had only been rinsed once, which strengthened the hypothesis of insufficient tank rinsing causing the acid attack of metal with an H2 release.

INITIATING cause appears to be hydrogen accumulation at the top of the tank followed by ignition of the inflammable mixture formed by the use of a grinder.

Hydrogen was formed according to the formula: Fe + 2H+ <=> Fe2+ + H2

Root cause was the failing of recognising the hazardous situation, despite various accidents already reported on H2 explosions after steel reservoir attack with diluted acid at other plants.

A reservoir of sulphuric acid (H2SO4)

Maintenance work was being executed at the reservoir affected by the accident, because of a leak detected few days before.

This reservoir had been repaired in 1989 following an incident that caused delamination without failure of its fastenings, due to a CO2 release during sulphuric acid neutralisation; an angle bar had been added and the cleaning procedure had been modified.

DESCRIPTION OF THE FACILITY
The plant, classified SEVESO upper tier, produces intermediates for the plastics industry (ethylene, polypropylene, butane et benzene) from naphtha and butane.

Two of the three were seriously hurt.

The classified facilities inspectorate initiated an investigation.

As generic return of experience, the inspection concluded that several accidents are caused by metals attack by diluted acid forming gaseous hydrogen and started a review of this type of occurrences aiming at a more effective risk assessments.

The investigation issued these specific recommendations to the site:
1. To provide tank with a purge valve
2. To improve the procedure of emptying and rinsing, to ensure the minimisation of the residual acid
3. To perform flammable gas (hydrogen) detection before the start of any maintenance work
4. To improve the tank design to facilitate the unavoidable production of hydrogen.
5. To use dry air to avoid that humidity accelerate the internal corrosion.
6. To ensure the stability of the tank also under out-of-specification forces.