Around a dozen employees working the night shift at Flexitallic's factory in Cleckheaton were taken to hospital after the explosion on 4 March 2013. All were allowed home after being checked over.
The incident, which blew the 'sinter' oven to pieces and caused extensive damage to the roof, was investigated by the Health and Safety Executive (HSE). It found a series of failures had contributed to the sudden explosion.
Huddersfield Magistrates were today (10 March) told that oven was used to 'bake' gasket sheets as part of the production process. This required components to be mixed with a flammable paraffin, formed into sheets and then thoroughly dried to drive off the flammable solvent before being put into a 360 degree oven.
On the evening of the incident, night shift workers had followed instructions left by the day shift manager to transfer a pack of the sheets that had been left in the dryer into the sinter oven.
One operative opened the dryer doors and his colleague used a forklift to load the pack and transfer it to the oven. He then started the oven's cycle and the two men left the oven room.
Ten minutes later, the oven exploded violently, causing extensive damage to the room and punching a massive hole through the roof above, part of which collapsed.
The court was told that the night shift operators believed the drier had completed its work, and loaded the pack into a still-hot oven unaware the sheets were only partially dry. The flammable vapours ignited as they made contact with the electrical heating elements, causing a flashover and then a flashback into the main oven chamber, which exploded with the pressure.
HSE's investigation identified Flexitallic had developed a process with a separate drying operation to ensure the sheets were fully dry before going into the oven. However critical instrumentation on the drier was allowed to fail and was not repaired.
When the firm moved premises in 2009, the drying operation was outsourced and streamlined by a new operator to remove a manual stage after drying. When Flexitallic took back the process in 2012 it had caused some disruption. This combined with the fact the dryer machine had been known to cut out and shut itself down, provided "the ingredients of an incident waiting to happen".
Flexitallic Ltd of Scandinavia Mill, Hunsworth Lane, Cleckheaton, West Yorkshire, pleaded guilty to a breach of Section 2(1) of the Health and Safety at Work etc Act 1974.
HSE inspector John Micklethwaite said: "This was a catastrophic explosion which blew the sinter oven into pieces. Had anyone been in the vicinity, we could have been investigating fatalities and serious injuries.
"Flexitallic knew it was critical the gasket sheets were thoroughly dried out. There was a two-stage drying process in place and a chart giving times depending on thickness of the sheets. But the dryer's temperature gauge and instrumentation had broken down years before and were never replaced, so it was effectively manually operated.
"That night the dryer cut out, as it had occasionally before, but it wasn't spotted because the night shift workers did not normally work in the oven room.
"In addition the company hadn't identified the risk of an explosion from the flammable vapour if sheets were not totally dry so no effective safeguards had been implemented.
"It was an incident that could have, and should have, been avoided. With any such industrial process, the fire and explosion risks must be fully assessed and then controls put in place."