Flixborough revisited


NOTE: This webpage was first posted in January 2005 and, slightly abridged, appears in The Chemical Engineer (April 2005). It has been modified for consistency with the published version - save for Figure Nos. 7-10 that continue the Figs 1-6 sequence of Flixborough – Some Additional Lessons ("SAL") and that two additional illustrations in the published version appear in Flixborough – FAQs.

In June 1974, a 2-year-old caprolactam plant near Flixborough was destroyed by a major explosion. Its shock waves echoed all around Britain as neighbours of similar or older plants deluged politicians with fears about nearby installations. Within days, the government decided that a Public Inquiry was necessary to allay public concern.

Fig. 7 Photograph of Section 25A at a late stage in its construction

Its outcome seemed a foregone conclusion. It was obvious that a temporary by-pass line had jack-knifed and ripped away its bellows connections to the 28" nozzles at both ends - and never any doubt that the explosion was fuelled by a major escape of superheated cyclohexane through these two open nozzles.

Officially the Public Inquiry was asked to “establish the causes and circumstances of the disaster.” However, as the circumstances seemed unique, it was anticipated that the Court of Inquiry would reassure the public that there was nothing to fear from all other chemical plants. At that time (June 1974), there was near-universal agreement that the 20" line was the sole cause of the disaster (the so-called 20" line hypothesis).

Fig. 8 Court Plate 8 and a photograph of the 20" line after explosion

To demonstrate their hypothesis, the Court commissioned a replica 20" line to be fabricated and connected by replica bellows between two of the remaining reactors, to simulate what was thought to have happened. It was announced, in advance of the opening of the Public Inquiry, that the film of this simulation would be made available publicly and would demonstrate what had caused the explosion.

There was, however, a dissenting voice. Dr. Keith Gugan, an experienced accident investigator from Burgoynes, had found an 8" line that, from its petal shaped fracture, he deduced had burst through creep failure under pressure. He argued that this would have occurred before the 20" line had jack-knifed (and relieved the system pressure) and that that this pre-event must have been the initiating cause of the disaster. In the jargon of the Court, his event sequence became known as the 8" line hypothesis.

When the Court convened and heard eye-witness evidence from the laboratory, it quickly became apparent that no-one had seen the 20" line fail (although it was visible from the laboratory) and that all witnessed something other than the massive escape of gas before the main explosion. Their accounts suggested that there must have been one or more pre-events that activated the fire siren at least 10 seconds before the jack-knife failure of the 20" line and at least 30 seconds before the main explosion.

Then, a few days into the hearings, the Court conducted their simulation test. As predicted, the 20" line did fail when extra internal pressure was applied – but not in the way expected. Instead of tearing away from the 28" nozzles, the line merely distorted (‘squirmed’) with both bellows connections left intact (Fig. 3a in "SAL"). Their simulation, specifically designed to show that an over-pressurised 20" line initiated the train of events, actually showed that an external force, applied from above, was needed to cause the jack-knife. Scientific objectivity then went on hold as the Court desperately tried to rescue its preferred hypothesis. 

This was not their only concern. By then it was known that something unusual had happened in an overhead bank of fin-fan coolers. Whilst most were crushed in situ when the plant structure was felled by the main explosion, two rotors had flown upwards and at right-angles to the main blast (Fig. 5A in "SAL"), landed outside the structure (Fig. 7) and then were covered with soot from the flash fire (that occurred concurrent with the main explosion).

Fan Rotor 8 (landed past roadway)             Fan Rotor 9 (landed on roadway)

Fig. 9 Fan rotors 8 and 9 after explosion, covered in soot from flash fire

So, two weeks into the public hearings, the Court’s single event hypothesis faced a major challenge. It had to explain more than a dozen eye-witness accounts of at least two events before the main explosion – and, even more difficult, how creep failure in the 8" line and an explosion in the fin-fan coolers could have followed afterwards.

In retrospect it is obvious that the proceedings should have been adjourned to allow time for scientific tests to resolve the technical issues. Unfortunately, the Court had already ‘gone public’ in favour of its 20" line hypothesis. The Court decided to hear evidence on various peripheral issues whilst it conducted new investigations that, they hoped, would restore scientific credibility to their preferred hypothesis.

Simultaneously, various other issues of concern were under study – most of which have been recorded in research reports and/or presented at the IChemE symposium held at the end of 1975. The most relevant of these for the causation issue concerned “zinc embrittlement of stainless steel” – and were conducted by Mr. Orbons of DSM.

His findings explained why fin-fan coolers that were made from stainless steel had shattered (whilst those from mild steel remained intact) and revealed, inter alia, that the explosion in the stainless steel fin-fan coolers (numbers 7-10) could have been caused by a fire of only a few seconds. The Court accepted that this caused the fin-fans’ explosion - but not that it must have occurred before the main explosion.

 Meanwhile, the 8" line had remained in the care of the Court but (as they did not think it relevant to causation) not available for metallurgical examination. Eventually, just before the Christmas recess, when the Court was satisfied it had the evidence it needed to complete its findings, it was released for forensic tests by Sir Alan Cottrell and Professor Peter Swann.

Their creep rupture tests were conclusive. These showed that the metallographic and morphological characteristics of the failure of the 8" line at its elbow were consistent with failure after about 4 minutes between 950°C and 1000°C at its normal working pressure (equivalent to 7.4kg/mm2). This is illustrated in Figure 10 (copied from The Chemical Engineer, April 1976, page 270).

Fig. 10 Examples of w-type creep cavities in steel of 8” line

Given that tens of tonnes of cyclohexane would have escaped from the 28" nozzles within 4 minutes of the 20" line jack knife (more again from equipment damaged by the main explosion within a minute of the first event), it is inconceivable the normal working pressure would have been retained in the 8" line for 4 minutes if, as argued by the Court, the 20" line was first to fail. Moreover, the direction of the applied stress was consistent creep failure in an undistorted 8" line. This was convincing evidence that the 8" line actually failed before the 20" line.

 Nor was this all: Cottrell and Swann also determined the temperature profile of nearby sections of the 8" line (Fig. 6A), showing a rapid fall from 950°C at the intrados of the elbow to less than 750°C only a few feet away. This confirmed that something other than the general fire after the explosion must have been responsible for highly-localised heating at the intrados (hypothesised in Fig. 6B).

Figure 6 - reproduced from tce, May 1976

Lacking further forensic evidence, the missing details of the hypothesis then had to be completed by a sequence of credible conjectures:

ü        There was a gasket leak at the nearby non-return valve (a credible scenario as two bolts were found to have been left untightened).

ü        This leak resulted in an escape of cyclohexane that was directed at the intrados of the elbow from an essentially intact lagging box.

ü        The escape ignited and preferentially heated the sections of lagging over the intrados and nearby.

ü        The lagging disintegrated and the zinc-coated wires (used to keep the lagging in place) contacted (or came very close to) the stainless steel pipe.

ü        The 3" crack (top of Fig. 6A) was created instantly by zinc embrittlement.

ü        With cyclohexane now escaping on either side, the intrados of the elbow became vapour-locked and its temperature rose to 950°C.

ü        After another 4 minutes, creep failure occurred at the petal crack.

Having conjectured this scientifically credible sequence of events that conformed to the unimpeachable hard forensic evidence, the eye-witness statements were reviewed (Figures 4 and 5A of "SAL"). Using the main explosion as the fixed time origin and working back by re-enactments of what each witness remembered seeing, hearing and doing just beforehand, it was easy to establish that:

ü               More than 20 eye-witnesses saw flames up to ¾ minute earlier.

ü               Of these, 8 described a vertical flame consistent with an emission from the ruptured elbow of the 8" line up to 25 seconds earlier.

ü               One eye-witness (at location 15 of Fig. 4 ) reported seeing, at least 25 seconds earlier, an object consistent with the trajectory, shape and size of Fan Rotor 9 in flight.

ü               The 7 laboratory eye-witnesses (Fig. 5A) were alerted at least 32 seconds earlier and were running for safety within 10 seconds (without having seen the 20" line collapse).

In all, over forty eye-witnesses provided statements for scrutiny. None contradicted the 8" line hypothesis: several provided strong confirmatory evidence and helped to determine the timing of events. From the standpoint of science, the debate ended with the forensic metallurgical findings. If the sole consideration for the Public Inquiry was to find the truth, it would have endorsed this multi-event and multi-cause explanation. 

But there was a problem. The 20" line hypothesis did not blame anyone outside Nypro and was preferred by the process licensors and plant constructors, the Factory Inspectorate and everyone already on public record in its favour. These were pressing commercial, institutional and personal objections to the 8" line hypothesis. None of the parties represented at the hearings would benefit if it were adopted.

Its adoption would have obliged the Court to highlight a frightening large number of defects in plant design and construction (loose bolts, gasket integrity, vulnerability of stainless steel in contact with zinc, the hugely damaging potential of a jet fire, an excessive inventory of superheated cyclohexane). These would have been unpalatable findings for a public looking to the Court for reassurance.

So the Court held firm. Their report reasserts (Para. 209) that the 20" line was the sole item at fault: “The disaster was caused by the introduction into a well-designed and constructed plant of a modification which destroyed its integrity”. With the hindsight of maturity – though I did not think so at the time – I concede that they may have had some excuse for their technically implausible conclusion.

If they had endorsed the more credible hypothesis, with all its attendant lessons, it could have exacerbated fears about the location of chemical plants. The Nypro plant was ½ mile from the nearest village (Flixborough) yet 72 out of its 79 houses were damaged. It was thought to be one of the most modern plants in Britain at that time and built in accordance with the best practice of the day. If two loose bolts initiated the Flixborough events, was anyone safe living close to a chemical plant?

These, I am sure, are the reasons why the Court reached its scientifically perverse decision. They were impelled to blame the totally unsuitable 20" line as a one-off cause of the disaster - and give the rest of the Nypro plant (and, by implication, all others in Britain) a clean bill of health. This decision was predictable and signalled well in advance of their Report.

Their predicament had been explained to me, weeks before the hearings ended, by the Secretary to the Inquiry, a senior Factory Inspector of high personal integrity who had been present throughout the proceedings. He advised me then to concentrate on the lessons of the investigations and gave a pledge that the Factory Inspectorate (then in transition to becoming the HSE) would publicise all the lessons of Flixborough – irrespective of the Court’s findings.

Bernard O’Reilly kept his word and I took his advice. My main efforts, after the official report was out, were to publicise the lessons from the investigations. Within a few months the Institution of Chemical Engineers had endorsed my personal proposal that it sponsor a symposium on “The Technical Lessons of Flixborough.”

It was held on 16th December 1975 in Nottingham and its papers published in the April/May 1976 issues of The Chemical Engineer. My contribution, “Flixborough – Some Additional Lessons”, reprinted for this website, created so much interest that several hundred extra copies were reprinted as a tce supplement. Although it focused on the lessons for the future and was not meant as a backward-looking review of the causation hypotheses, much of this interest stemmed from its implicit explanation of (what tce termed) the “main alternative theory to that put forward by the Court”.

Today, thanks to the efforts of an indefatigable Ralph King and an unwavering Jim Venart, it remains common knowledge that the Court of Inquiry did not win universal approval. Thankfully, throughout the past thirty years, safety professionals have ignored the central conclusion of the Public Inquiry (Para. 209) and concentrated on its ‘unofficial’ lessons. One example will suffice: the concept of “Inherent Safety”.

This supposed “well-designed” plant had 200 tonnes of superheated cyclohexane circulating without a single ESD isolation valve to reduce an escape. There was no way the plant operators could contain a leak, irrespective of its cause or location. Had the IChemE (and, notably, Trevor Kletz) accepted the Court’s findings, the concept of “Inherent Safety” – now an integral feature of design safety philosophy - would not have seen the light of day.

For further details, see the original “Flixborough – Some Additional Lessons”.

John Cox, January 2005