Let’s be Frank: The Start of Process Safety Management: The Flixborough Disaster – June 1, 1974

By Inspector Frank. February 24, 2022

Editor’s Note:  Writing under the pseudonym Inspector Frank, the author of this column offers a first-hand, candid view of what he has witnessed throughout his career. His purpose in sharing these experiences and opinions is to encourage readers to think deeper about what they do, why they do it, and the possible impact of their decisions.

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It was a failure of the cyclohexane plant that led to the explosion that occurred at 1653 hours on Saturday, June 1st, 1974.

A major leak of liquid from the reactor circuit led to the rapid formation of a large cloud of flammable hydrocarbon. When this met an ignition source (probably a furnace at a nearby hydrogen reformer), there was a massive fuel-air explosion. The plant control room collapsed, killing all 18 occupants. Nine other site workers were killed, and a delivery driver died of a heart attack in his cab. 28 were killed onsite, and 36 more were injured. Offsite, 53 additional people were injured. Fires started on-site which were still burning ten days later. Around 1,000 buildings within a mile radius of the site were damaged, as were nearly 800 in Scunthorpe (three miles away); the blast was heard over thirty-five miles away.

I have started using the Flixborough disaster as a safety talk topic, even though the disaster happened almost 48 years ago. Why? Because people forget, and it is still a good set of lessons to learn.

This incident has long been held as one of the big events that drove the concept of process safety management (PSM) forward. First in Europe, and then around the rest of the world. I have been finding Flixborough useful to get people engaged in discussing PSM, and in getting junior inspectors interested in learning why the systems they are using and taking part in even exist.

Plus: being humans, we always turn our heads to look at the car crash, especially if we aren’t involved…

The Flixborough facility was a chemical works owned by Nypro UK, which was a joint venture between Dutch State Mines (DSM) and the British National Coal Board. It had originally been set up to produce fertilizer from by-products of coke ovens in a nearby steelworks. In 1967 it had been reconfigured to produce caprolactam, a chemical used in the manufacture of nylon. On the initial changeover, caprolactam was produced by hydrogenation of phenol. In 1972 there was a push from DSM to use one of their processes in which the caprolactam was produced from cyclohexane. This process was proprietary to DSM.

The process consisted of heating cyclohexane to around 311 degrees Fahrenheit before passing it through a series of six reactors. The reaction itself was the oxidation of cyclohexane with air going over a catalyst, to a mixture of cyclohexanone and cyclohexanol that is usually known as a KA (ketone/alcohol) mixture. The reaction took place in six vessels, each holding about 20 tonnes of material. On leaving the last reactor, the reaction products were removed and the unreacted cyclohexane was then reheated and rerun through again.

Two months prior to the explosion, the No. 5 reactor was discovered to be leaking. When insulation was stripped from it, a crack extending about 6 feet was visible in the mild steel shell of the reactor. Subsequent examination of the crack by DSM determined the cause of the failure to be nitrate stress corrosion cracking of the mild steel cladding. This nitrate stress corrosion cracking was believed to have been due to the practice of spraying nitrate-treated cooling water as a means for diluting and dispersing small leaks.

To maintain production, it was decided to bypass the No. 5 reactor while repairs were being done. A temporary bypass pipe was installed between the No. 4 reactor and the No. 6 reactor. Because the reactors were mounted on a sort of staircase, this pipe was not straight but contained two bends. The pipe was 20 inches in diameter, although the short pipes that were normally used to join the reactors together were 28 inches in diameter. Bellows, also 28 inches in diameter, were installed between each reactor, and these were left at each end of the temporary pipe. This temporary bypass pipe performed satisfactorily for two months after the plant was restarted on April 1.

However, the plant had been shut down for other leaks and during the start-up procedures the process pressure rose slightly, from 125 psi to 129 psi. The bending moment, caused by the action of this slight rise in pressure, was strong enough to tear the bellows. The temporary pipe twisted with this change in the flow, and the bellows were ruptured by shear stress. As a result of the rupture of the bellows, a great amount of cyclohexane escaped from the holes in the bellows and formed a cloud of cyclohexane vapor, which subsequently caused the explosion.

At least that’s the current thought process of what happened, and it took a while to get there. The investigative findings have been debated over the years and most experts now agree that the original government inquiry had some fundamental flaws.

Here are some interesting points to consider in all of this:

  • The workers who designed the temporary pipe were not professional engineers. The only calculations made were for the capacity of the assembly needed to carry the required flow. No calculations were done to ascertain whether the bellows or pipe would withstand the forces that would be exerted.
  • No reference was made to the relevant British Standard or any other accepted standard. No reference was made to the designer's guide issued by the manufacturers of the bellows. No drawing of the pipe was made, other than in chalk on the workshop floor.
  • The support of the temporary pipe was a scaffolding structure upon which the pipe rested, without being fastened down. Therefore, the support structure could not provide enough strength to withstand against bending stresses.
  • No process analysis or management of change type function was performed when it was decided to change process conditions (while PSM did not exist at this time, the idea of performing “what-if” scenario analyses was somewhat common in some petrochemical companies).
  • The source of ignition was probably a natural gas reforming furnace some distance away. It was estimated that 30-50 tonnes of cyclohexane escaped in the 50 seconds that elapsed before ignition occurred.

The original board of inquiry used the above to state the main reason was “human error.” There was some controversy over the exact cause of the failure and whether or not there may have been an external explosion that actually caused the bypass line to fail.

One of the reasons they had such a hard time piecing things together was that all relevant operations staff was killed in the explosion, and all relevant records and instrument history were also destroyed in the explosion.

This was the official board of inquiries summary conclusion statement:

“We believe, however, that if the steps we recommend are carried out, the risk of any similar disaster, already remote, will be lessened. We use the phrase "already remote" advisedly for we wish to make it plain that we found nothing to suggest that the plant as originally designed and constructed created any unacceptable risk. The disaster was caused wholly by the coincidence of a number of unlikely errors in the design and installation of a modification. Such a combination of errors is very unlikely ever to be repeated. Our recommendations should ensure that no similar combination occurs again and that even if it should do so, the errors would be detected before any serious consequences ensued.”

The people of the United Kingdom were basically told the accident was a one-off and should never happen again. However, process safety practitioners around the world felt that the explosion was not the result of basic engineering design errors, but was rather the result of multiple instances of one underlying cause. That cause being a complete failure of plant safety procedures, including the procedural shortcoming of not getting SMEs and/or experienced personnel involved in managing a change.

I still see this as a concern in many facilities I have worked at, which means the lessons that can be learned from Flixborough are still relevant and in need of being remembered.

“Progress, far from consisting in change, depends on retentiveness. When change is absolute there remains no being to improve and no direction is set for possible improvement: and when experience is not retained, as among savages, infancy is perpetual. Those who cannot remember the past are condemned to repeat it.”

– George Santayana (philosopher, poet, novelist)

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