Hydrogen explosion in chemical plant (H2SO4)

In a chemical plant (SEVESO), corrosion on the water side of a cooling exchanger caused water to enter the sulphuric acid and attack the steel in the tank. This reaction produced hydrogen that accumulated in the dome of the tank and caused the explosion.
The explosion causes the cover of a 99.2% sulphuric acid tank to open partially. The tank was filled to 300 t of its total 1500 t capacity. The cover opened along the circumference of the tank at the dome / shell fitting.

The POI (internal emergency plan) was started. No emission or leak was reported except for the emission of fume at the opening. No environmental or human consequences were reported. Material damage was limited to the equipment located above the tank (acid supply pipes, air pipe going to the dryer, walkway, steam pipe, etc.).

The presence of hydrogen (a few dozen grams), generated by the corrosion of steel by the weak acid was responsible for the explosion.
Before the explosion the tank contained a heterogeneous mixture with a weaker concentration at the surface. The explosion occurred 1 hour and 15 minutes after the tank had been agitated. The ignition of H2 was probably electrostatic.
The two piercing incidents of the heat exchanger were due to a phenomenon of corrosion by pitting near the seam welds on the side of the cooling water.

The INITIATING cause was the production of hydrogen from sulphuric acid and water by corrosion of metals.
The presence of water had been caused by corrosion in a heat exchanger.
Possible root or contributing cause was the modification to the operational procedures which enhanced metal corrosion in the heat exchanger.

sulphuric acid tank

The tank was filled with sulphuric acid to 300 t of its total 1500 t capacity. The acid was at a 99.2% concentration, not homogeneous.
The explosion occurred 1 hour and 15 minutes after the tank had been agitated

No environmental or human consequences were reported.
Material damage was limited to the equipment located above the tank (acid supply pipes, air pipe going to the dryer, walkway, steam pipe, etc.).

1. water control and treatment (temporary use of a non-oxidising biocide and study of a continuous chlorine-bromine treatment),
2. detection of any abnormal arrival of water in the process (temperature detector with alarm, titrators),
3. operating procedures (prohibition on adding weak acids to storage tanks).

There are many examples of similar accident (see for example HIAD event no. 887)