The former owners of an oil refinery in Pembroke have been fined £5m after four contractors were killed in an explosion.
Dennis Riley, 52, Robert Broome, 48, Andrew Jenkins, 33, and Julie Jones, 54, had been draining a chemical storage tank at the then Chevron refinery in 2011.
But flammable gases inside it ignited.
The judge said no fine could “reflect the value of someone’s life”.
A fifth worker, Andrew Phillips, caught in the resulting fireball survived but with life-changing burns.
Chevron will have to pay the fine and court costs of £1m as part of a deal it struck with Valero Energy UK Limited, which bought the site shortly after the disaster.
Specialist cleaning company B&A Contracts, which employed Mr Broome and Mr Jenkins, was fined £120,000 and ordered to pay cost of £40,000 after admitting health and safety breaches.
Speaking just after the eight year anniversary of the tragedy, Mr Justice Lewis at Swansea Crown Court said no fine could “reflect the value of someone’s life” but it was right the companies were sentenced according to the law.
“The tragedy has had a devastating impact on the families of those who died and on Mr Phillips and his family,” he said.
“Nothing this court says or does can bring back the four people who lost their lives or minimise the suffering of Andrew Phillips.”
Mr Justice Lewis said those working for Chevron at the time had failed to “know of or appreciate” the risk of “flammable vapour,” which had apparently been building up inside the tank over a number of years.
The court heard that just days before the explosion one Chevron worker had carried out a gas test which should have alerted the refinery to the flammable atmosphere, but its results were either “not properly communicated” or “not understood.”
“If the work had have been stopped (at this point) the explosion would not have happened. The four deaths and the injuries would not have happened,” said Mr Justice Lewis.
The court heard it was impossible to say for certain what had caused the explosion, but that experts believed it would have been either a static spark caused by the workers “unearthed hosepipe” or the presence of pyrogenic substances within the tank, which can ignite spontaneously when dry.
Mr Justice Lewis said B&A Contracts had failed to follow its own health and safety protocol by using an unearthed hosepipe to drain the tank.
Both companies involved had previously admitted health and safety offences.
The five workers caught in the explosion had been instructed to pump residue from a tank which normally contained a mix of amine and diesel, but was going through a cleaning process.
Linda Jenkins, mother of Andrew Jenkins told BBC Wales “justice hadn’t been done”.
“They are all going home to their family,” she said. “I’ll never see my son again.”
After the hearing, Health and Safety Executive inspector Andrew Knowles said: “This incident, which had devastating consequences for all of those involved, was entirely preventable. Many opportunities to take action to control risk were missed, that would have prevented the incident from occurring.
“It is important to realise that the incident could have had even more serious consequences had the butane sphere or pipe track been damaged by the flying tank roof.
Det Supt Anthony Griffiths of Dyfed-Powys Police said it had been a “very complex investigation”.
“We hope that the lessons learned ensure that a tragedy of this nature doesn’t happen again,” he said. “Our thoughts remain with all the families involved.”