The blast occurred last June in a batch processing unit which was being used to manufacture the hair dye chemical methyl nitrophenol. All three stages in the chemical process were exothermic, and both the rate of reagent addition and temperature control depended on manual control alone.
Problems began when the process operator added nitrosyl sulphuric acid to the reactor too quickly and at too low a temperature. In an effort to compensate, he switched off the cooling system. He then left the process unattended for more than an hour.
The operator was returning to the building housing the process unit when the explosion occurred. The one-tonne, two-metre wide reactor lid was thrown through the roof, coming to ground 100 metres away. The 500-gallon reactor, situated at the top of a four-storey building, was blasted down through two floors, landing on top of another process plant.
Fortunately, nobody was hurt, in part because the incident occurred on a Sunday morning when few staff were on-site. A cloud of acid gases was released, leading to complaints from nearby residents and businesses of spotting of cars and property.
The Health and Safety Executive (HSE) charged Holliday with failing to ensure the health and safety of its employees, contrary to section 2(1) of the Health and Safety at Work Act 1974. The seriousness of the incident was compounded because ten months earlier a runaway reaction on the same process had also resulted in a release of acid gases. Holliday was required to commission a thermal hazard study of the reaction - but failed fully to implement its consultants' recommendations by the time of the more serious incident.
The company pleaded guilty before Bradford Crown Court on 6 May. It rejected any suggestion that it had adopted a cavalier approach to safety, but said that it had now carried out a review of training procedures and increased the number of supervisory staff. The plant is being rebuilt - at a cost believed to be in excess of £1 million - and will include an automatic temperature control system and quench tank.
Fining Holliday £50,000, Judge Barry observed: "However complicated the chemical processes are, those who choose to carry them out on a commercial basis for profit must ensure the safety of their employees." He ordered the firm to pay costs of £63,000 - a high figure largely to compensate the HSE for extensive laboratory tests of the thermal hazards of the process.