Lasting safety lessons as Flixborough is remembered 30 May 2014

Next Sunday (1 June 2014) will mark the 40th anniversary of the Flixborough disaster – the UK's worst ever mainland process plant explosion, which still influences chemical and process industry safety thinking today.

The explosion at the Flixborough Nypro Chemicals site, near Scunthorpe, killed 28 people and injured 36 others on 1 June 1974.

It resulted in the almost complete destruction of the plant. Further afield, the blast injured another 53 people and caused extensive damage to around 2,000 buildings.

With the exception of the Buncefield fire in 2005, it remains the biggest post war explosion in the UK.

The cause of the disaster, according to the 1974 inquiry, was the installation of a temporary 20 inch reactor bypass pipe, installed to allow production to continue while a large reactor (the fifth in a six-reactor sequence) was removed for repair.

The inquiry determined that the failure of a flexible bellows next to the temporary pipe led to the pipe jacknifing and being ripped off by rising pressure, resulting in a catastrophic release of 30—50 tons of boiling cyclohexane followed by a massive vapour cloud detonation.

However, that view of events was contradicted seven years ago by experts (including Dr john Cox, one of the original investigators), who pointed to crucial evidence either ignored or left unexplained at the time – and postulated that an intense flame jet from a burst elbow on a nearby eight-inch banjo line, also carrying cycohexane, was the initial cause.

Cox and his colleagues agreed that the poorly designed and installed temporary bypass line was a serious contributory factor – having been designed and constructed without input from a competent engineer. Indeed, the only site mechanical engineer had been the works engineer, who had left five months earlier. Further, once installed, the system was pressure checked using nitrogen, not water, as recommended.

However Cox described other salient factors as including that the system used to cool the reactors had been shut down for repair, and water containing nitrates was running instead - potentially causing stress corrosion. Also, at the time of the incident, the plant was not processing. It had undergone a problematic restart so was recirculating cyclohexane, awaiting delivery of nitrogen - also thought by some to have been leaking.

Evidence that convinced Cox and the court at the time that the 20-inch bypass pipe was to blame included the state of the internal baffle and stirrer on the downstream Reactor Six, which had been seriously buckled. The investigators decided that must have been caused by blast damage - meaning that the pipe must have been ripped off at least 20 seconds earlier to allow enough fluid to discharge before the blast. Hence, they said, that must have been the root cause.

However, the 1974 court's own engineering simulation showed inadequate process pressure for the bypass line and its bellows to do anything more than move. "The mechanical engineering experts said it would have needed another 3.5psi to make that pipe jacknife and the bellows rupture," said Cox in a 2007 forensic revisit to the causes of Flixborough. "So in 1974, I accepted that the bellows failed - but why? Had internal pressure drifted up? Was it some kind of process perturbation? Or was there some other external explosion?"

His re-analysis of the events demonstrated convincingly that the additional energy required to rupture the bellows actually came from a prior, completely unrelated explosion and the flame jet from the eight-inch banjo line, also carrying cycohexane, but, crucially, at 9bar.

Cox cited evidence from several witnesses - misinterpreted or omitted at the time, and insisted that this event was the trigger for the 20 inch line's demise and, in turn, the massive cyclohexane ejection, the rapidly expanding vapour cloud and the consequent catastrophic blast.

This was an important finding – both in terms of its further implications for process safety, and associated legislation and engineering and materials guidelines, but also the industry's view of the then discredited bellows and their manufacturer.

For those interested, the evidence for this sequence of events, as laid out seven years ago, is compelling.

Among the most convincing was the remains of a fan rotor assembly from a fin-fan cooler originally above the reactors. That was found on waste ground 50m away in a direction not consistent with the main blast.

Its flight had been witnessed before the main blast, but coincident with a "loud rumbling" sound, which Cox attributed to the, by then, discharging 28-inch diameter reactor nozzles left by the departed 20-inch pipe.

Crucially, the fan rotor had been subject to a brief, but intense, fire and was still covered in soot - unlike the rest of the plant, which was consumed by the subsequent flames - confirming that it must have been flying before the main blast.

That rotor, he explains, has been blown off as a result of another 'mini-explosion' caused by a fire jet from the eight-inch banjo line bathing the fin fan cooler, with its fans still running, and causing practically instantaneous zinc embrittlement and failure of all its galvanised finned steel cooling tubes – which were also carrying cyclohexane.

"At 800—900C, metallurgical simulation shows that zinc will unzip steel in seconds," he explained. Those tubes, he points out, were found in a "neat pile" under the fin fan cooler, indicating that they fell through the running fans before the main blast, causing cyclohexane to pour down onto the flame jet, and hence that mini-explosion – and its flight path.

As for evidence of the initial eight-inch line explosion and flame jet, he cited: 8mm cine film shot by an amateur after the main blast and showing an ongoing 150ft flame column to one side of the large smoke plume; witness observations both on- and off-site of a pre-event fire; detailed metallurgical studies of the failed banjo and associated assemblies; and consequential movement of adjacent heavy separator plant.

Whatever the cause, though, at the time there were no specific UK regulations to control major industrial hazards. So the incident exposed weaknesses in the understanding of hazards, the design of buildings, management systems and organisation.

Commenting on Flixborough, industry safety pioneer, adviser, lecturer and writer, Professor Trevor Kletz (who died last year), said: "Flixborough destroyed the confident feeling that we can always keep large quantities of hazardous chemicals under control – and therefore we should keep the amounts of them in our plants as low as reasonably practicable or use safer materials instead. Inherently, safer design arrived on the chemical industry's agenda."

Forty years on, chemical engineering undergraduates are routinely taught the lessons from Flixborough and the incident continues to influence plant safety in industries ranging from oil and gas to chemicals, pharmaceuticals and other process sectors.

"Flixborough has left a lasting legacy on the chemical and process industries – in the UK, Europe and worldwide," comments Robin Turney, a fellow of the Institution of Chemical Engineers (IChemE), who has studied the accident and its aftermath.

"The accident occurred in an era where early, but concerted efforts were being made to improve safety," he continues.

"Flixborough coincided with the introduction of the Health and Safety at Work Act in the UK, and spurred the development of the European Seveso Directive – named after an accident at a chemical manufacturing plant in Italy in 1976 – strengthening regulation further in 1982."

Turney observes that the last 40 years have seen many improvements in process safety, through regulations, technical measures and better management.

"The avoidance of further disasters will require constant attention to these, together with the active involvement of directors and senior managers to create an organisational safety culture, which promotes open dialogue and seeks to identify and correct weaknesses," he asserts.

Brian Tinham

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