On a Top Tier SEVESO chemical site, 30 t of molten para-toluidine (PT) was released from a heated bulk storage tank during the replacement of a transfer pump in a bottom run-off line.

The tank contents (normally heated to 55° C) had been isolated 3 days earlier by a single isolation valve and the removal of steam trace heating around the line to create a frozen plug of PT (PT has a melting point of 44° C).

At 10 am, a maintenance technician had removed the faulty pump and was collecting a replacement from stores when the isolation failed. PT had not solidified as anticipated, probably due to the large quantity of liquid in the storage tank and the high ambient temperature on the day of the accident. Consequently, molten PT flowed from the pipe, over-flowed the spill bund and spread across the site, contaminating a wide area with a solid layer of toxic material.

The alarm was raised and the on-site emergency team attended the scene. Several attempts were made to stem the leak, which eventually slowed after 3 hours, sufficient to allow mechanical isolation. The spill was contained on the site using sand bags.

There were no injuries sustained at the time of the release though 4 workers suffered exposure to PT during the subsequent clean-up operation, 3 of which required hospital treatment. There was no damage to the environment.

The main cause of the accident was an inadequate isolation of the tank contents during a high hazard maintenance operation. Reliance was placed on a single isolation valve and an assumed frozen plug of PT to seal the molten contents of the tank.

The operator upgraded its isolation practices, procedures and personnel training to ensure that safety does not rely only on single valve isolation. He revised the emergency measures to stop leakage as well as the remediation measures (adequacy of personal protective equipment, training of clean-up teams especially on the removal of contaminated personal protective equipment)