Four people were killed and another seriously injured when an explosion took place in the Amine Recovery Unit at the Pembroke Refinery on 2nd June 2011.
Valero Energy UK Limited and B&A Contracts Limited are to face charges under Sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974.
At the time of the incident the refinery was operated by Chevron but ownership changed in August 2011.
The defendants are due to appear at Haverfordwest Magistrates’ Court on 24th September 2018.
HSE’s deputy director field operations, Jane Lassey, said: “Following a painstaking and extremely thorough investigation, much of which was conducted jointly with Dyfed Powys Police, we have concluded that there is sufficient evidence to bring criminal charges.”
While seven years may seem a long time for the HSE to reach a decision on pressing charges, it is not unprecedented. When Falcon Crane Hire was fined £750,000 in March 2016 for safety breaches that led to a tower crane falling over in Battersea and killing two men, it was nearly 10 years after the incident took place.