In an inorganic chemistry plant, a maintenance operator opened the bottom valve of a settling tank when a leak of hydrofluoric acid (HF, toxic and corrosive) occurred around 6 p.m. Acidic white vapours spread into and around the workshop; two operators fell ill. The operator initiated the internal emergency plan, the alert was announced via loudspeaker, and the site’s employees were confined. The alert was lifted at 7:10 p.m. An estimated 5 litres of acid was released.

Equipment preparation operations that had not gone as planned

The compressed HF condensed in the settling tank after it had cooled down. The settling tank, which had been shut down the day before to prepare for the replacement of a leaky seal on a slip-on flange, should have been drained before opening the piping on which the flange was installed. The investigation showed that the operator had not followed the procedure for preparing the equipment, concerning two points, in particular:

  • The unit’s shutdown procedure includes a phase to stop the circulation of glycolated water, but which had not been carried out;
  • The operator had pressurised the settling tank with nitrogen, whereas this operation was not stipulated by the procedure.

Moreover, the operator had not noticed the frost forming on the piping, which should have alerted him to the evaporation of the leaking liquid HF. Lastly, the operator did not report the difficulties encountered during the procedure to his hierarchy, particularly the difficulties encountered when opening the tank’s bottom valve to drain heavy products located in the bottom of the settling tank.

The valve was partially blocked. This clog explains why the operator, of his own initiative, had performed the pressurisation with nitrogen prior to the dismantling operation. This operation made it possible to operate the valve and continue the preparation operation.

The administration also notes that this preparation procedure does not include formal hold points during the course of the procedure, and that it has not been formally validated. The operator has reminded its staff about the penalties incurred for disregarding procedures. A weekly audit system was set up to identify deviations from the equipment preparation procedures.