A clean out operation of a batch still, known as ’60 still base’, was organised in order to remove residues. This vessel had never been cleaned since it was installed in the nitrotoluenes area in 1961.

An operator dipped the sludge to examine it and reported the sludge as gritty with the consistency of soft butter to management. No sample was sent for analysis nor was the atmosphere inside the vessel checked for a flammable vapour. It was mistakenly thought that the material was a thermally stable tar.

In order to soften the sludge, which was estimated to have a depth of 34 cm (14 in), steam was applied to the bottom battery. Advice was given not to exceed 90°C. Employees started the clean out operation using a metal rake. The material was tar-like and had liquid entrained in it. Approximately one hour into the cleaning process a longer rake was used to reach further into the still.

The vessel’s temperature gauge in the control room was reported to be reading 48°C, instructions were given to isolate the steam.

At approximately 13:20 hrs a number of employees involved in the raking left the still base to get on with other tasks. One person left on the scaffold had stopped raking and noticed a blue light, which turned instantly to an orange flame. As he leapt from the scaffold an incandescent conical jet erupted from the manhole. This projected horizontally towards the Meissner control building. A vertical jet of burning vapours shot out of the top rear vent to the height of the distillation column nearby.

The jet fire lasted for approximately one minute before subsiding to localised fires around the manlid and buildings nearby. The force of the jet destroyed the scaffold, in the process, propelling the manhole cover into the centre of the Meissner control building. The jet severely damaged this building and then impacted on the north face of the main office block causing a number of fires to start inside the building.

A total of 22 fire appliances and over 100 fire fighters attended the incident.

KEY ISSUES:
• MAINTENANCE PROCEDURES
• REACTION / PRODUCT TESTING
• RAW MATERIALS CONTROL / SAMPLING
• CONTROL SYSTEMS
• PERMIT TO WORK SYSTEMS
• HAZARDOUS AREA CLASSIFICATION / FLAMEPROOFING
• PLANT LAYOUT

Report: https://www.icheme.org/media/13704/the-fire-at-hickson-and-welch-ltd.pdf

Image Credit: HSE