The smell of chlorine (Cl2, toxic gas) was detected at around 8:50 a.m. in the building housing the chlorine and soda production unit of a chemical plant. The unit’s sensors detected the leak though it did not reach the automatic trip threshold. The operating personnel shut down the electrolysis room and degassed the facility. The products still in the manufacturing process were emptied into the unit’s treatment columns. The facility operator initiated the internal contingency plan. The site’s gas warning siren was also activated. The internal contingency plan was cancelled at 9:25 a.m. A local resident called to inquire about the origin of the siren’s activation. The facility operator sent out a press release.

The chlorine emission (4.3 kg) resulted from a leak on a flange of the 98% sulphuric acid supply line of the chlorine drying column. A micro-leak of sulphuric acid had occurred a few hours earlier on the flange seal. To reduce the chemical risk, a trickle of water was put in place to dilute the acid. Diluted sulphuric acid is corrosive to steel and flowed onto one of the flange’s steel bolts, causing its deterioration. The flange was then held in place by only 3 bolts. The leak worsened when the pressure increased to 2/3 bar during the discharge of the H2SO4 feed pump. It lifted up, releasing sulphuric acid and a small amount of chlorine from the drying column.

The operator checked all the flanges and their retaining bolts on the following days to ensure that there were no further micro-leaks. The staff was also given a refresher course on the risks of diluting sulphuric acid.