At around 3:30 a.m., (highly corrosive) concentrated sulphuric acid began leaking along the lower generating line of a 20 m³ tanker truck at the centre of a refinery. The leak consisted of a continuous trickle of greenish liquid. The refinery’s workers and those of a neighbouring site were ordered to stay indoors. The internal emergency plan was implemented at 4:40 a.m. The prefecture implemented the external emergency plan at 9:35 a.m. A 50 m cordon was set up around the tank. Traffic on a railway line running through the refinery and located 250 m from the tank was halted. Two schools in the municipality were locked down. Internal and external emergency services placed the tank on an unloading station located over a secondary containment. The acid leakage rate was estimated at 1 m³/h. An earthen bund was formed to contain the leak. An exothermic reaction occurred in the tank (temperature of 120 °C). The tank was gravity drained to another tank. Pumps were used to complete the transfer. This second tank was moved away from the facilities and the railway line. Traffic resumed at 2:30 p.m. The external emergency plan was lifted at 4:30 p.m.
The second tank was placed on a 36 m³ secondary containment located 1.4 km away. At 2:00 p.m., the temperature of the acid in it was 90 °C. Its concentration was halved. The initial option to neutralise the acid using carbonate was abandoned due to the risk of losing control over the reaction. The next day, the acid’s temperature remained stable at 60–70 °C. It was emptied into a third tank specially designed for the transport of highly corrosive substances (dilute acids). This operation ended at 5:00 a.m. the next day.
Outcome of mismanagement of a tank leak
The accident was caused by a hazardous situation that got out of hand. Two days earlier, a 1 t/h leak was found on the drain valve of a tank containing 18 m³ of 96% sulphuric acid. Uneven tightening of the valve’s bonnet screws caused the valve’s seal to become partially unseated, allowing the acid to corrode the nuts and screws. The emergency response unit was called into action at 7:30 a.m. The operator set up a water mist to knock down the acid. The operator then pumped the acid/water mixture to the tank that would subsequently spring a leak. Many difficulties were encountered during this operation:
- it was impossible to isolate the leak;
- efforts to pump directly from the tank failed;
- transfer equipment broke.
The safety department found that many warning signs had not been heeded: gases and vapours emanating from the secondary containment, a 20 °C rise in the acid’s temperature, a greenish-coloured leak at the discharge end of the transfer pump, vapours emanating from the tank’s manholes. The operation ended at 6:30 p.m. the day before the leak on the tank. The risks of uncontrolled reaction and tank corrosion were not identified and no special monitoring was set up.
Improper sealing was found on the pipes connecting the sulphuric acid tank’s secondary containment and the tank unloading area to the neutralisation pit. For three days, the emission limit value (ELV) for suspended solids at the refinery’s wastewater treatment plant exceeded the authorised limits (40 mg/l instead of 30 mg/l). Quarterly monitoring of the groundwater downstream of the facilities was put in place.
The prefect issued a formal notice requiring the operator to demonstrate that the design and operation of the facilities include the risk of incompatibility between substances and with the environment.
Action plan implemented
The operator:
- replaced the drain valve on the sulphuric acid tank and revised the maintenance plan for all valves that may come into contact with similar acids;
- repaired the pipe between the secondary containments and the neutralisation pit;
- increased his workers’ knowledge of the risks of acid-induced corrosion and the risks of incompatibility with water;
- identified which tanks are to be used based on the compatibility of their construction with the acid concentration of substances.
