CHLORINE LEAK DUE TO STRESS CORROSION CRACKING OF SS BOLTS

 Incident
Approximately 150 kg of chlorine gas was released over a period of 140 minutes, due to failure of bolts on a vacuum gas regulator attached to an on-line liquefied chlorine gas drum, used for water treatment. The incident occurred outside work hours, however plant operators were not called out, as the plant alarm system was not correctly programmed. Impact from the release to the surrounding community was limited to chlorine odour being detected by a passing motorist who reported the odour to the operator of the
site. The leak was isolated by a plant operator who arrived at the scene after the release was reported.
Cause
Inspection of the vacuum gas regulator revealed that the release was due to the failure of four bolts on the regulator. Material test results show that the failure of the stainless steel bolts was due to chloride-induced-stress cracking from chlorine attack, which is assumed to have resulted from an extremely small leak over a short duration.
Consequences
As part of the investigation, a number of changes to prevent recurrence of the event have been identified and implemented. These include introduction of a weekly leak check, the company committing to inspection of all vacuum regulators, the replacement of any inappropriate bolts and reprogramming of the chlorine alarm. In addition, the company will investigate the use of different alloy bolts.

Source: Department of Minerals and Energy, Australia