On a Sunday at 5:30 p.m., an explosion occurred on the burner of a hydrochloric acid manufacturing unit at a chemical site. The unit was immediately shut down and the gas alarm was triggered. The only damage was material, caused by the bursting of the pipes.

The explosion was caused by the formation of an explosive mixture of hydrogen and unreacted chlorine in the burner that may have been ignited by static electricity or sunlight. The unit, which operates on the basis of the calculation of a ratio of hydrogen to chlorine, had been running since the preceding day. The flow of hydrogen gradually decreased during the night until, at 9:00 a.m. the following day, the significant drop in the hydrogen feed rate made the burner turn off. This may have been caused by an ice plug formed by condensation inside the line and/or blockage of the flame arrestor. Maintenance technicians were called in to blow out the H2/Cl2 burner’s flame arrestor. The flow meter was also purged between each restart attempt because its values varied. At 5:00 p.m., the burner was restarted and H2 peaks were observed, as if the line was being blown out. At 5:23 p.m., a sharp increase in the H2 value caused the reader to partially close the H2 feed valve. This drop in H2 is believed to have created the conditions allowing the unreacted chlorine to be in the burner, thus creating an explosive mixture upon return of the excess H2 or in contact with a hydrogen pocket.

The operator increased the reliability of its hydrogen supply network by fitting heat insulation and using its H2 flow meters to reduce condensation in the line. The safety studies on the facility’s two burners were reviewed with the burner supplier. The safety threshold of the hydrogen to chlorine ratio was raised.