A Health and Safety Executive investigation found the explosion would have come without warning to engineers Peter O’Brien and Mark Sim, who died at the rod and bar mill. Darren Wood, another employee, also suffered serious injuries.

Cardiff Crown Court heard that on 18 November 2015, electrical engineer O’Brien, 51, was working with mechanical engineer Sim, 41 in the basement of the steelworks site on an accumulator vessel.

Investigators found that a flammable atmosphere developed within the accumulator as hydraulic lubrication oil was being drained from it. This was ignited by an electric heater within the accumulator.

The investigation found that the company failed to assess the risks to which its employees were exposed when draining lubrication oil from the accumulator.

Manually draining hydraulic lubrication oil from the accumulator by a procedure referred as a ‘blow down’ had developed through the company employees’ local custom and practice; this “procedure” was not fully understood or consistently carried out by the Company’s employees, exposing them to the risk of explosion.

Celsa Manufacturing (UK) pleaded guilty to breaching Management of Health and Safety at Work Regulations. The company was also ordered to pay costs of £145,771.85.

HSE inspector Lee Schilling said: “This incident, which had devastating consequences for all of those involved, was entirely preventable.

“The company failed to assess the risks of the maintenance work and identify suitable control measures to prevent an explosion.”