In a refinery, a bleed valve on the line of a reservoir, with an active storage capacity of 61,600 m³ and level measurement capabilities by means of pressure differential, stayed open after an inspection by the operations supervisor. Given the absence of a screw-on plug planned at the end of reservoir tapping, several tens of m³ of heavy fuel oil with a high sulphur concentration spread into the unsealed retention basin. Furthermore, since the sectional valve had stayed open subsequent to the week’s heavy rains, the products flowed towards the stormwater tank that collects from basin low points, where operators detected pollution on December 9 around 11:00 pm.
The hydrocarbon supernatant within the stormwater tank was then pumped and transferred to the hydrocarbon sludge reservoir. Given that the pump associated with the equipment used for skimming was inoperable, a fuel oil film of undetermined height above the water remained in place. The soils from polluted earthen dikes were excavated over a height of 40 cm and those lying below the retention were excavated over 50 to 100 m². The spoiled earth was then removed to a treatment centre. The Hazardous Installations Inspectorate was notified by the operator on December 14.
According to the Inspectorate, the operator would have mistaken the line serving for level measurement using a pressure differential for the line used to reheat the level measurement line and therefore did not screw the plug back on the tap of the bleed valve.
The Inspectorate proceeded with a site visit on December 18, at which time it also observed that the technical gallery through which the hydrocarbon piping passed between refinery reservoirs and units and/or piers was also fouled by hydrocarbons of the viscous fuel oil type. Moreover, the Inspectorate requested the operator to take all measures necessary to clean the polluted technical gallery zone, as well as to recover the supernatant within the stormwater tank and dispose of hydrocarbons from the manholes of retention basin collection networks.
