At around 6:20 a.m. in a chemical plant, a tanker was in the process of being loaded with 34% hydrochloric acid (HCl, corrosive acid) when a low pH alarm (pH 4) was triggered in an effluent storage pit. The operator used another alkaline effluent (pH 12) to neutralise the acidity in the pit, which allowed the pH of the pit’s contents to be maintained close to 7. There was therefore no effect on the pH of the effluent released into the natural environment.

The accident was attributed to work performed on a pump at the loading station which required the personnel to open the purge valve on the loading station’s HCl loading line. These operations were carried out on the day prior to the accident in preparation of an intervention scheduled for the following day. The truck driver knew that the pump was to be replaced in the morning and was not supposed to be used. Having arrived early, he decided to begin the HCl loading operation from another tank and with another loading pump. As he was unaware that the purge valve was open, the product discharged into the pit for 2 hours. This went undetected by the personnel until the pit’s alarm was triggered.

After having scheduled the work to be carried out on the pump, the plant’s logistics department informed the central services that the loading station was unavailable, but the internal management software did not block the driver’s registration formalities upon his arrival at the depot. Also, on the morning of the accident, the crew that had conducted the preparatory operations the day before had not formally informed (in writing) the incoming crew that the loading station was unavailable. And finally, there was no local signalling/signage alerting the driver that the purge valve was open.

The operator introduced the following corrective actions:

• it is no longer possible to purge two non-continuous loading stations;
• modification of the pump preparation procedure, including the identification of the valves that are not in their normal position;
• a warning sign is placed on open purge valves;
• feedback was sent to central services regarding the consequences of their forgetting to block the loading station in the management software.