Pressure vessel failure in a chemicals plant

The vessel affected by the incident was a pressurised heat exchanger vessel of the hydrodesulphurisation process. The desulphurisation gas had high hydrogen content. The vessel had cracks in the upper transverse fillet welds at the gas inlet which caused low stress brittle fractures. The cracks had been found during the maintenance, but not fixed. This eventually caused a gas release.
Approximately 2.5 hours after that a leak was detected and reported, a first explosion occurred. As the heat exchanger explosion was facing the desulphurisation pump room, the leaking gas gathered in the pump room, under the effect of the first explosion flame. 7 seconds later, a secondary explosion occurred in the desulphurisation pump room, resulting in 3 deaths. Due to the impact of debris from the first explosion and the high temperature of the open flame, a third explosion occurred in another cracked pressure pipe.

the INITIATING CAUSE was corrosion of a welded joint, with a low brittle fracture resistance. One of the root CAUSES was a lack of maintenance and repair. Another could be attributed to lack of risk management and safety culture. The existence of the crack was known, but no action had been taken.

hydrogen tank, pump building

three fatalities, six injured persons, and a direct economic loss of 100 million Euros.

The cracks had been found during the maintenance, but not fixed. The reasons for omitting to take action are unknown.
The company will have to review inspection/maintenance/repair procedures including assessment of risks from damage and ageing effects, to avoid that a near miss evolve in an incident.