During a transfer between tanks and reservoir filling pumps for a transport company’s bus fleet, 6 m³ of diesel spilled from a 30-m³ tank into its concreted pit.

A oil tanker was regularly supplying with crude diesel two of the three 30-m³ double-walled tanks that over the previous few months had been replacing the 3 single-walled reservoirs. Each evening, four 5 m3/hour distribution stations were supplying 160 buses. Depending on the level of demand, the crude diesel was being normally channelled to a centrifuge / transfer pump room; afterwards, once cleansed of its impurities and water traces, it was routed to either the distribution pumps/guns or the 3rd 30-m³ crude diesel tank. This latter tank was filled to 95%, with a “high level safety” stopping the centrifuge (5 to 10-l capacity) and diesel transfer. With the centrifuge once again operable, the pressure activated when 1 gun was triggered. The “loop” circuit maintained constant pump suction (60 l/min – 2 bar) as long as the gun was off its base, in order to feed the bus’ 240-l tank via the distributor. With the diesel tank full and the gun off its base, the diesel of the loop circuit overflowed upon return into the “clean diesel” tank.

The installation had been placed in degraded mode at the time of this incident, with the centrifuge shut off and isolated as outlined in the specific operating procedure. The 2 crude diesel tanks were then connected via the transfer pumps to the distribution tanks and clean diesel tank. The diesel was not purified and, in isolated safety mode, the operator was no longer being notified of the clean diesel level in the tank. Lastly, since the regulations did not require it, installers of new tanks did not insert a probe with a low point alarm into the pit.

Alerted by the presence of odours, the operator opened the pit and discovered the overflow via an extraction plug and a “connection exposed to the open air”. A subcontractor cleaned the tank and sealed pit; in indicating on this topic that the environment had not been impacted, the operator adopted several measures, namely: daily monitoring of the pits; installation of the probe / alarm in the pit to detect a diesel leak; active safety in normal or degraded mode with a high level / solenoid valve automatically stopping the tank supply at 29 m³; modification of the diesel loop return; and routing of the surplus, with the gun no longer supplying the buses, to the crude diesel tanks (60 m³) in order to avoid an overflow of the clean diesel tank.

The Classified Facilities Inspectorate requested with a 2-week turnaround: a report on the causes of this incident and measures adopted to be adopted to avoid a repeat occurrence; a status report on the site’s safety devices (leak / level detectors) either already installed or planned; centrifuge-related equipment operating in normal / degraded mode; a quantification of the diesel eventually emitted through the terrace vent (though no trace had been detected along the wall or on the ground during the Inspectorate’s visit); and the official receipt of disposal of the pumped diesel evacuated by the subcontractor.