Pollution
Humain
Environnement
Economique

At 10:13 on a chemical intermediates manufacturing plant, the overfilling of a process feed stock tank released 13.8 t of toxic liquid phenol to a bunded area. The released phenol solidified in the low ambient temperatures (5°C) and was chiselled out by staff wearing full personal protective equipment. It was then drummed ready for disposal in accordance with waste disposal legislation. There were no injuries or environmental effects as a result of this incident. The cost of material losses, clean up and lost production amounted to £39,800.

The plant had two phenol stock tanks; a larger storage tank (40 t) and a smaller process feed stock tank (18 t). The larger stock tank received road tanker deliveries and phenol needed for the process was transferred to the smaller tank in 12 tonne lots. Process phenol was then continuously circulated between the manufacturing plant and the smaller tank.

Shortly after a tanker delivery, the smaller tank was topped up and the transfer system was set up by an operator to circulate phenol for the process. However, the circulation was incorrectly set. This resulted in closure of the 18 tonne tank outlet and pumping of phenol from the larger storage tank to the plant then back into the smaller tank, which gradually overflowed into the bund.

Adding to that, since level measurement relied on load cells, the panel operators’ understanding of plant status was flawed ; thay did not notice that the smaller tank was filling up as the high weight alarm, which sounded during the loading phase (it was used to judge when sufficient phenol had been transferred) had been acknowledged.

The investigation found that the mix of automated and manual systems for controlling pump mode, switching and valve settings was vulnerable to human error.

The company implemented improvements to the safety related control systems on the plant as a whole. The control system for the phenol storage and process feeds was redesigned to include direct level measurements and hard-wired interlocking to the transfer pump.