On a tubular exchanger, a disc broke over ¼ of its cross-section at 4:50 am during a pressure surge in the liquid ammonia (NH3 ) circuit connecting NH3 storage cells to a urea workshop operating under stable conditions. NH3 was partially led to a 100-m high degassing stack. Given stable weather conditions, a foul-smelling cloud drifted towards the city. The release occurred unbeknownst to control room operators, who had incorrectly interpreted several alarms that had tripped. Once the diagnosis rendered, the device was isolated at 6:25 am. The plant operator only became aware of the severity of the event at 8 am; two and a half hours were then needed to fully determine the origin and likely causes. The 10 tonnes of NH3 release was due to a succession of physical, organisational and human malfunctions: - Lack of anomaly detection and automatic safety systems: information made available to control room operators was inadequate; - Poor diagnosis / decision-making process lacking adequate verifications despite several precursors; - Incomplete safety recommendations, insufficient monitoring procedures and inspection plans. This poor diagnosis would explain the delay required to isolate the deficient circuit and the potential impact of this release. Long periods elapsed between the onset of the accident, the alarm and activation of the internal emergency plan, source identification, causes and circumstances of the discharge, and then a definitive quantification.
Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION