In a chemical nitrogen fertiliser plant, a sump connected to the retention basin of a 70% nitric acid tank was in the process of being pumped out following a rainy period which had damaged the lift pump. Its contents were transferred to a waste truck by a subcontracting company usually used for this type of task. The sump and the retention basin were then cleaned. After the pumping operation had been completed, orange nitrogen oxide vapours were released at approximately 9:30 a.m. from the tank’s bottom and vent. The responders evacuated the area and sounded the alarm. The internal emergency plan was initiated around 10:20 a.m. and the site personnel confined themselves indoors. A first intervention team inspected the tank and discovered two leaks on the lower part at the bottom valve flange and along the circumference of the tank door: 2 m³ of nitric acid had spilled onto the ground. The plant’s internal firefighters put up water curtains to suppress the fumes, and chalk was used to neutralise the acid spilt onto the ground.
Plant personnel remained confined for 1 hour and nearby plants were informed. The two pump technicians were placed under medical supervision. Once the tank had been emptied, it was opened up and rinsed with water. An approved company was called upon to perform the pumping operations and dispose of the puddles on the ground. The internal emergency plan was lifted at 11 a.m.
Material incompatible with the product
The release was caused by a reaction of 31% diluted nitric acid with the carbon steel of the tank. This type of material is incompatible with the storage of a corrosive acid: a circular and perforating corrosion line was discovered on the weld securing the supply pipe to the tank’s removable bottom. In addition, this tank was to be decommissioned in the near future.
Insufficient training and control of the subcontractor
The subcontracting company that had sent the driver felt that, given his experience, he should have been aware of the tank’s incompatibility with an acid load. However, the subsequent investigation showed that it was the only tank vehicle available on that day to empty the sump and that it had no risk markings/panels (diamond or orange plate) which would have reminded the driver of the risk. Furthermore, the standard work permit issued by the operator did not identify the chemical risks and incompatibilities between the product and the tank material. The subcontractor was only given a general oral reminder regarding this fairly common operation when the framework agreement was signed: the company considers that subcontractors who have been approved through the internal qualification process are well aware of specific risks such as incompatibilities with acid. However, the information given to the subcontractor regarding the nature of the product to be pumped was not sufficiently precise and had been misunderstood. Finally, the prevention plan established for pumping actions specified that the responsibility for the choice of equipment according to the characteristics of the product rests with the subcontractor, whereas regulations state that the responsibility lies with the operator.
The operator modified the pumping procedures, taking into account the risk related to incompatibilities between the product to be pumped and the tank material. The training of subcontractors and the information provided on the work permit were reinforced thanks to a systematic characterisation of the nature and concentration of the product to be pumped.
