A pressure rise occurred at 9:45 am in the condensed propylene settling tank (to eliminate the aqueous phase) in a workshop for synthesising chlorinated organic products on an upper-tier, Seveso-rated chemical platform. To reduce pressure in the settling tank, the workshop’s safety flare was activated. The excess product combustion emitted black smoke, visible from afar, for 30 min at the facility’s flare. Unit personnel contained the incident. The plant operator issued a press release and informed local authorities as well as the inspection authorities for classified facilities.

The origin of this pressure rise was traced to a defective, float-type sensor used to measure the very high propylene level. This sensor controlled the safety valve regulating the intake of liquid propylene into the settling tank. An accident analysis revealed that the high-level sensor had also seized subsequent to a mechanical locking of its float beginning at 4:30 pm the day before the accident, yet this problem went undetected by technicians since the unit was operating under stable conditions. An initial pressure rise (14 bar) occurred at 7:30 am following a drop in activity, causing the first round of flaring. At 9:15 am, the supervisor conducted a manual test of high-level sensor operations, which yielded a positive outcome. With automatic regulation reinstated, the settling tank once again began to fill. At 9:45, pressure in the tank exceeded 14 bar, and the degassing valve leading to the safety flare opened, forcing a drop in production. Verifications performed on the float-type, high-level sensor were unable to replicate the mechanical locking, yet the very high-level sensor of the same type (which alone controlled the safety valve) was actually locked despite a detection chain verification 3 months prior.

The operator modified the technology of both these propylene level sensors, with the high-level one being replaced by a microwave (radar) sensor and the very high-level one by a vibrating-blade sensor. Workshop control instructions were revised for technicians to reduce the propylene intake as soon as the pressure alarm surpassed 14 bar.