An investigation is underway after four people were killed in an explosion at Wessex Water’s Bristol water recycling centre in Avonmouth.
At approximately 11.20 am on Thursday, December 3, fire service, ambulance and police were called to the scene and a major incident was declared. It is believed that an anaerobic digester containing biosolids was the cause of the explosion. A fifth person suffered injuries but is not thought to be in a life-threatening condition.
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Photographs from the scene showed a silo with extensive damage to its upper section and roof. Anaerobic digesters use bacteria to break down waste, producing methane gas in the process. As the name suggests, however, oxygen is not meant to be present, reducing the risk of an explosion from the flammable methane, which is generally siphoned off and used to generate electricity. The remaining biosolids can then be used as fertiliser.
“Waste is generally fed in at the top of the reactor and methane drawn off at the top,” explained Tony Ennis, director of risk assessment firm Haztech Consultants. “The spent waste can be used as soil improver in agriculture, and the methane is often used to drive a gas engine to generate electricity.
“Methane is flammable and explosive between 5 – 15 per cent by volume in air. It is possible that personnel on the top of the digester were carrying out hot work e.g. drilling or other work which caused a spark and ignition of the methane gas in the top of the digester.”
Eyewitness accounts from the scene at Avonmouth reported a ‘boom and an echo’ that lasted around two or three seconds, with no smoke or fire visible in the aftermath. According to Prof Clifford Jones, chemical engineer and visiting professor at the University of Chester, the witness statements may indicate an overpressure explosion, where the blast is the main source of damage and injury.
“When I first read of the incident and the tragic multiple fatalities the word ‘overpressure’ entered my mind immediately, and the word ‘blast’ has featured in several of the media statements so far,” said Prof Jones.
“Whether an explosion has an overpressure depends on conditions at ignition including fuel-air contacting. One has to be extremely careful in arguing from the particular to the general or vice versa. It has often been noted however that when in a hydrocarbon explosion heat is the lethal factor there are comparable numbers of fatal and non-fatal injuries, whereas when pressure is the lethal factor the numbers of fatal and non-fatal injuries differ more widely.
“Up to now we have been informed of four deaths and one non-fatal injury at the Avonmouth explosion. I want to say no more than that that is consistent with there having been an overpressure. Whether there was an overpressure can probably be ascertained from the damage profile at the accident scene.”
I am afraid this was not an anaerobic digester as suggested in this article but a digested cake silo. It is also mentioned that the product may have been limed. The two flammable substances that could be released in those silos are methane and ammonia.
From the photos available, the silos do not appear to have active ventilation fitted suggesting an accumulation of flammable gases would have been possible within them, likely making them a zoned area.
This is definitely a milestone for the industry and its approach to area classification of its assets. It is regrettable that as often happens, an accident is required to stir things up.
My thoughts and prayers are with the works, their families and colleagues.
We last year had to re-design some piping on a similar accident in the UK
Once the investigations have been completed it will be important to see where the risk assessments and method statements (RAMS) went wrong: i.e. was there a “unknown-unknown”, or was the planning incompetent? All too often, these RAMS are still standard formats with little hazard analysis made by engineers.
Photos suggest to me the silo – clearly not a pressure vessel – had a frangible roof designed to fail as it did, ironically with the intention of minimising harm in the event of an explosion. Tragically the victims were on the roof at that moment and suffered the full force of the explosion
I do hope that if hot work played a part in the incident it does not let the company off the hook, as there were doubtless other failings of management, safety procedures and possibly maintenance
When looking at the Pictures, it is obvious that it is a Silo and not a Bio-digester as it has a flat top and bolted sections for the sides [note the conveyor which is used for the Solids feeding, A biodigester is a liquid pump-fed system and normally “Egg-Shaped”]
Hope there were PR valves of rupture discs in the top for any overpressure?
If the contractor was doing any “Hot Work” was the system fully vented and where too? [Hot work includes Welding/Plasma Cutting and hole cutting with Drills, all requiring needed cooling fluids ad fire protection] All workers would also need appropriate PPE and Safety harness’ for working at height?
Hope all persons were certified and a fully organized Protocol in place?
The roof failed catastrophically in the significant overpressure scenario. Rupture or burst discs are not normally designed to relief explosion . In these atmospheric tanks, explosion relief panels would have been required to deal with an internal explosion, which would have protected the tank roof from detaching itself from the tank, projecting the poor workers into the air and denting the conveyor.