Hydrogen explosion in chemical plant (H2SO4)

This event occurred at a plant (classified SEVESO) producing inorganic base products.
An explosion took place in a storage tank containing almost pure sulphuric acid (H2SO4).

The explosion causes the cover the tank to open. The tank was filled to 300 t of its total 1500 t capacity. The cover opened along the circumference of the tank at connection between the dome and the cylindrical shell.

The 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.).

Water had accessed the tank, enabling the attack of the tank. The reaction produced hydrogen (a few dozen grams) that accumulated in the dome and caused the explosion.
Before the explosion the tank contained a heterogeneous mixture with a weaker concentration at the surface. The explosion occurred approximately 1 hour after the tank had been agitated. The ignition was probably due to electrostatics.
The water entering the tank was coming from the water-cooling side of a cooling exchanger, which at been corroded pitting near the seam welds.

The INITIATING CAUSE was the unwanted production of hydrogen from the reaction between sulphuric acid and steel in presence of water. The presence of water had been caused by corrosion in a heat exchanger.

Possible ROOT or CONTRIBUTING CAUSE was a change in some process parameters, which enhanced metal corrosion in the heat exchanger.
Modification of the anti-legionella biocide treatment since the previous year may have favoured the phenomena of corrosion by pitting. The use of liquid bleach instead of bromine greatly increased the oxidising power of water and thus lowered the starting temperature of corrosion pitting.
Another aggravating factor was the decrease in water flow rate due to the drop in the workshop activity for a long period, which contributed to increase the temperature of the medium.

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.).

The POI (internal emergency plan) was activated.

There are several examples of accidents caused by hydrogen formation from a weak acid attach of steel (see for example HIAD event no. 887)

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).