June 3, 2011

expert reaction to the explosion reported at the Chevron Oil Refinery near Pembroke

Four workers were killed and several others injured by a blast at an oil refinery near Pembroke, Wales.

Martin Goose, spokesperson for IChemE (the Institution of Chemical Engineers), said:

“Media coverage has understandably focussed on the first response actions of the emergency services and effects on the local community, but less visible actions will already be under way by the operators of the site and the Health and Safety Executive (HSE).

“The first task will be to make the accident scene safe for plant workers and others, whilst preserving damaged plant and equipment and other information such as plant records so as not to prejudice the investigation. Once the site is generally safe to access a multidisciplinary investigation will begin.

“Accident investigation can be a long and detailed process, and ill-informed speculation needs to be avoided. Hazardous installations in the process industries, including refineries such as Chevron’s Pembroke Refinery, provide many of the manufactured products on which 21st Century life is reliant.”

Emeritus Professor Hans Michels, Department of Chemical Engineering and Chemical Technology at Imperial College London, said:

“There is very little technical information coming from the Chevron plant or – as it should be – from the emergency services that have been dealing with the human and material consequences of the explosion. Everything that can be said about the true nature of the explosion or its causes is at this stage therefore speculation.

“This Chevron plant is a broad crude refinery facility and the range of products on its site is therefore vast. The plant is also very large (quote 15% of UK energy needs?), hence even the amounts of most side products will be significant. The tank that blew up was a cylindrical one of some 750 cu. m., (about 8-10 m diameter), not very big therefore, so it probably contained a by-product. Its shape on the photographs and the fact that its contents exploded with a rising fireball, tell us that this was almost certainly a low- pressure volatile fuel.

“We call fuel storage tanks ‘slumbering giants’. Once they explode great quantities of energy are released. For that reason safety precautions are extensive in order not to ‘wake them up’. I am sure the safety regulations at the Chevron plant were excellent and rigorously enforced.

“The most hazardous moments are always when non-routine interference has to be made, such as maintenance work – which was what was happening immediately before the incident. It is then that all hazardous aspects and options have to be kept in focus and under control and maintenance procedures again are very strict. Nonetheless, unforeseen things can happen. Things may break, lines may be opened that were incorrectly assumed to have been purged of combustibles. Vapours may discharge into areas where welding takes place. Electrical spark discharges may unexpectedly happen in local environments that were unexpectedly not purged. The possibilities are endless and safety enforcement under maintenance conditions is an extremely challenging responsibility.

“Somehow the explosion extended to such a large combustible volume that it seems a very large volume of the tank contents blew up. Looking at the aerial photographs it nonetheless appears to me that the main explosion took place outside the tank; if not, the walls would have collapsed. So I would not be surprised to learn in due course that indeed a minor maintenance task led to a ‘loss of containment’ and an unexpected large escape of vapour from the tank and that, after a short while the external cloud found an ignition source and blew up. I note that the fire brigade said that any remaining vapours would quickly have dispersed, i.e. these may have been the rapidly escaping remaining contents from the heavily damaged low pressure tank.”

Dr Tim Fox, Head of Energy at the Institution of Mechanical Engineers, said:

“The type of accident which took place in Pembrokeshire is incredibly rare. The processes and procedures in oil refineries are very rigorous, and the equipment used has to adhere to very strict safety standards. It is too early to say with any certainty what the cause of the accident was, but it is likely that there was some sort of human error involved. The accident is a tragedy for the family and friends of the people killed and injured. We must ensure that once the HSE investigation is complete the knowledge gained from it improves further safety procedures in refineries in the future.”