What happened at the Hickson & Welch factory in 1992?

On 21 September 1992 an explosion occurred, destroying the centre of the control building and tragically claiming the lives of five people.

Hickson & Welch was a manufacturer of nitrotoluenes with capacity to produce around 30,000 tonnes per year. The container that produced the spontaneous ignition was known as a 60 still base and had been in production since 1962.

After years of it being in service a decision was made to clean out the vessel, which had accumulated a large amount of residue. The sludge being thick in consistency required softening in order to allow for easier removal. The vessel is reported to have contained an estimated depth of 34cm (14 inches).

What caused the Hickson & Welch factory explosion?

Workers applied heat to the bottom of the vessel - the supervisor instructed that the temperature was not to exceed 90°C. Approximately one hour into the clearing the vessel’s temperature gauge in the control room was reported to be reading 48°C - instructions were given to isolate the heat.

At approximately 13:20 pm a number of employees involved in raking the sludge left the base to get on with other tasks. One person left had stopped raking due to noticing a blue light, which turned instantly to an orange flame.

As he leapt from the scaffold a glowing, pointed jet flame erupted from the manhole and  projected horizontally towards the control building. The jet fire lasted for around one minute before subsiding, its force destroying the centre of the control building.

What can the industry learn from the Hickson & Welch factory explosion?

The decision to clean out the vessel for the first time after 30 years of operation resulted in maintenance having no experience of the task. Management failed to guide workers and provide any kind of risk assessment for working in such conditions.

There was also no analysis carried out for the material being removed and inadequate measurement of the slide temperature - this was due to the temperature probe being in the wrong place, and a faulty steam heat valve supplied a higher temperature than anticipated.

Control measures were inadequate and there was no permit to work issued for the task of using a metal rake to remove sludge from a flammable atmosphere.