Celsa Manufacturing (UK) Ltd has been fined after an explosion killed two workers and seriously injured another at the Celsa Steelworks site in Cardiff.

A Health and Safety Executive investigation found the explosion would have come without warning to employees Peter O’Brien and Mark Sim, who died at the company’s Rod and Bar Mill. Darren Wood, another employee, also suffered serious injuries.

Cardiff Crown Court heard that on 18 November 2015, electrical engineer Peter, 51, was working with mechanical engineer Mark, 41 in the basement of the site. They were working on an accumulator vessel when it exploded.

A HSE investigation found that a flammable atmosphere developed within the accumulator as hydraulic lubrication oil was being drained from it. The flammable atmosphere 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) Ltd, Castle Works, East Moors Road Cardiff, pleaded guilty to breaching Regulation 3 of the Management of Health and Safety at Work Regulations 1999. The company have been fined £1.8m and ordered to pay costs of £145,771.85.

Speaking after the hearing, 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.”