Sulphuric acid tank explosion in a chemical plant

An explosion occurred in an empty 100-m³ reservoir of sulphuric acid (H2SO4) of a chemical plant. This happened when three workers opened the dome of the tank (a grinder was used to unscrew the bolts).
The tank was torn over half the circumference of the shell/bottom junction, and its anchorages were stripped.
Installed directly above its retention basin at a height of several metres, the tank was projected and fell nearby, toppling the scaffolding set up for access and maintenance and injuring three workers.

The internal emergency plan was activated, bringing both internal and external responders to the site.

Approximately one 15 days before the incident, a leak of sulphuric acid had been detected from the reservoir tank. The leak was temporary closed, and the tank was emptied and rinsed with water. After which scaffolding were installed for the repair of the tank.

INITIATING CAUSE appears to be hydrogen accumulation at the top of the tank. When the worker opened the tank, air mixed to hydrogen forming a flammable mixture ignited by the spark of the grinder. The tank had only been rinsed once, which strengthened the hypothesis of insufficient tank rinsing causing the acid attack of metal with formation of hydrogen as reaction product. The post-incident inspection confirmed the presence of corrosion in the lower part of the tank internal surface, which is the part which remained always in contact with the acid.

ROOT CAUSE was the failure to recognise 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.

The fall of the tank caused 3 workers (2 subcontracted personnel) present on the scaffolding, to fall. Two of the three were seriously hurt. The injured were taken to hospital.

The classified facilities inspectorate initiated an investigation.

The internal emergency plan was activated, bringing both internal and external responders to the site. The operator notified the Prefecture, local city halls and the general public.

It is well known that many metals are attacked by acids, even in a diluted form, according to the formula: Fe + 2H+ <=> Fe2+ + H2.
If this reaction happens in enclosed areas, such as closed tanks, hydrogen will cumulate at the top. Therefore, rinsing tanks with water is not an effective preventing measure ensuring formation of flammable atmosphere when the tanks are then open to air. This is the cause of several incidents with serious consequences (see for example HIAD_209 occurred in 1985, HIAD_ 906 occurred in 1989, HIAD_019 occurred in 1994, HIAD_713 occurred in 2001, HIAD_888 occurred in 2005).

A review and analysis of these occurrences would allow a more realistic and 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.