At 10:46 pm during a thunderstorm, an electrical outage interrupted polystyrene (PS) production at a Seveso-classified site. A safety disc broke and styrene was released. To minimise the effects of micro-outages (due to thunderstorms) on PS output quality, the site operator typically switched shop power supply onto the 4 electric generating sets of the facility’s Peak Day Withdrawal (PDW) unit. This manoeuvre was performed at 10:20 pm, with 3 sets still available. At 10:43, the thunderstorm knocked out the 1st set. Since the 2 remaining sets were no longer sufficient, the unit entered into safety mode at 10:46, closing all utilities. An employee tried to restart the PDW unit; the on-call electrical maintenance operator was called at 10:53 pm. By 11:05 pm, pressure on the 1st synthesis reactor had begun to rise. As per emergency procedures, gyro monitors started up at 11:15 to remove eventual vapours at the reactor line vent. The site was connected to the grid at 11:18 but the units were only allowed to resume operations a short time later. At 11:20, the disc on the 1st reactor burst at 5.8 bar, spraying a liquid mix containing 10 tonnes of PS and 3 tonnes of styrene.The runaway reactor was caused by the loss of utility service. The control room operator opened the vent too late, given all the actions required to put the 3 polystyrene lines into safe mode, in accordance with procedures.
Source: Aria ACCIDENT ANALYSIS OF INDUSTRIAL AUTOMATION
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