- 1 Overview
- 2 The background
- 3 The accident
- 4 The local and corporate aftermath
- 5 Cause of the disaster
- 6 Responce to the report
- 7 The 3 then political conspiracy theories
- 8 Memorials
- 9 Also see
- 10 Links
The Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough, North Lincolnshire, England, on 1 June 1974. It killed 28 people and seriously injured 36 out of a total of only 72 people on site at the time; from the devastation on site it was clear that had the explosion happened in normal office hours the casualty figures could have been much higher. A contemporary campaigner on process safety wrote "the shock waves rattled the confidence of every chemical engineer in the country". The disaster involved (and may well have been caused by) a hasty modification. Mechanical engineering issues with the modification were overlooked by the managers (chemical engineers) who approved it, and the severity of the potential consequences of its failure was not appreciated. Flixborough led to a widespread public outcry over process plant safety. Together with the passage of the Health and Safety at Work Act in the same year it led to (and is often quoted in justification of) a more systematic approach to process safety in UK process industries, and – in conjunction with the Seveso disaster and the consequent EU 'Seveso directives' – to explicit UK government regulation of plant processing or storing large inventories of hazardous materials, currently (2014) by the Control of Major Accident Hazards Regulations 1999 (COMAH).
The chemical works, owned by Nypro UK (a joint venture between Dutch State Mines (DSM) and the British National Coal Board (NCB)) had originally produced fertiliser from by-products of the coke ovens of a nearby steelworks. Since 1967, it had instead produced caprolactam, a chemical used in the manufacture of nylon 6. The caprolactam was produced from cyclohexanone. This was originally produced by hydrogenation of phenol, but in 1972 additional capacity was added built to a DSM design in which hot liquid cyclohexane was partially oxidised by compressed air. The plant was intended to produce 70,000 tpa (tons per annum) of caprolactam but was reaching a rate of only 47,000 tpa in early 1974. Government controls on the price of caprolactam put further financial pressure on the plant
It was a failure of this plant that led to the disaster. A major leak of liquid from the reactor circuit caused the rapid formation of a large cloud of flammable hydrocarbon. When this met an ignition source (probably a furnace at a nearby hydrogen production plant[B]) 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. Fires started on-site which were still burning ten days later. Around 1,000 buildings within a mile radius of the site (in Flixborough itself and in the neighbouring villages of Burton upon Stather and Amcotts) were damaged, as were nearly 800 in Scunthorpe (three miles away); the blast was heard over thirty miles away in Grimsby and Hull. Images of the disaster were soon shown on television thanks to BBC and Yorkshire Television filmstock news crews who had been covering the Appleby-Frodingham Gala in Scunthorpe that afternoon.
The plant was re-built but cyclohexanone was now produced by hydrogenation of phenol (Nypro proposed to produce the hydrogen from LPG; in the absence of timely advice from the Health and Safety Executive (HSE) planning permission for storage of 1200 te LPG at Flixborough was initially granted subject to HSE approval, but HSE objected); as a result of a subsequent collapse in the price of nylon it closed down a few years later. The site was demolished in 1981, although the administration block still remains. The site today is home to the Flixborough Industrial Estate, occupied by various businesses and Glanford Power Station.
The foundations of properties severely damaged by the blast and subsequently demolished can be found on land between the estate and the village, on the route known as Stather Road. A memorial to those who died was erected in front of offices at the rebuilt site in 1977. Cast in bronze, it showed mallards alighting on water: When the plant was closed the statue was moved to the pond at the parish church in Flixborough. During the early hours of New Year's Day 1984 the sculpture was stolen. It has never been recovered but the plinth it stood on, with a plaque listing all those who died that day, can still be found outside the church.
The cyclohexane oxidation process is still operated in much the same plant design in the Far East.
It was a major Nypro (UK) chemical factory exploitation in Lincolnshire during 1974. The huge blast at the Nypro (UK) plant caused by a vapor cloud suddenly igniting. The factory was
Workers had discovered that a vertical crack in chemical reactor No.5 was leaking cyclohexaneon on the 27th of March, 1974. The plant was subsequently shutdown later that day for an investigation, which decided they should shut reactor No.5 down install a bypass assembly to connect reactors No.4 and No.6 so that the plant could continue limited production.
The plant it's self
In the DSM process, cyclohexane was heated to about 155 °C before passing into a series of six reactors. The reactors were constructed from mild steel with a stainless steel lining; when operating they held in total about 145 tonnes of flammable liquid at a working pressure of 8.8 kg/cm2 gauge (0.86 MPa gauge). In each of the reactors, compressed air was passed through the cyclohexane, causing a small percentage of the cyclohexane to oxidise and produce cyclohexanone, some cyclohexanol also being produced. Each reactor was slightly (approximately 14 inches, 350 mm) lower than the previous one, so that the reaction mixture flowed from one to the next by gravity through nominal 28-inch bore (DN 700 mm) stub pipes with inset bellows. The inlet to each reactor was baffled so that liquid entered the reactors at a low level; the exiting liquid flowed over a weir whose crest was somewhat higher than the top of the outlet pipe. The mixture exiting reactor 6 was processed to remove reaction products, and the unreacted cyclohexane (only about 6% was reacted in each pass) then returned to the start of the reactor loop.
Although the operating pressure was maintained by an automatically controlled bleed valve once the plant had reached steady state, the valve could not be used during start-up, when there was no air feed, the plant being pressurised with nitrogen. During start-up the bleed valve was normally isolated and there was no route for excess pressure to escape; pressure was kept within acceptable limits (slightly wider that those achieved under automatic control) by operator intervention (manual operation of vent valves). A pressure-relief valve acting at 11 kg/cm2 gauge was also fitted.
As the gas pressure built up in the bypass pipes in the late afternoon on 1 June 1974. A 20 inch bypass system ruptured due to either the high pressure or by a fire on a already recently ruptured nearby 8 inch pipe, causing a large vapor leakage, which then ignited and blew the place up in a massive vapor explosion incident.
Reactor 5 leaks and is bypassed
Two months prior to the explosion, the number 5 reactor was discovered to be leaking. When lagging was stripped from it, a crack extending about 6 feet (1.8 m) was visible in the mild steel shell of the reactor. It was decided to install a temporary pipe to bypass the leaking reactor to allow continued operation of the plant while repairs were made. In the absence of 28-inch nominal bore pipe (DN 700 mm), 20-inch nominal bore pipe (DN 500 mm) was used to fabricate the bypass pipe for linking reactor 4 outlet to reactor 6 inlet. The new configuration was tested for leak-tightness at working pressure by pressurisation with nitrogen. For two months after fitting the bypass was operated continuously at temperature and pressure and gave no trouble. At the end of May (by which time the bypass had been lagged) the reactors had to be depressurised and allowed to cool in order to deal with leaks elsewhere. The leaks having been dealt with, early on 1 June attempts began to bring the plant back up to pressure and temperature.
At about 16:53 on Saturday 1 June 1974, there was a massive release of hot cyclohexane in the area of the missing reactor 5, followed shortly by ignition of the resulting cloud of flammable vapour and a massive explosion in the plant. It virtually demolished the site. Since the accident took place at a weekend there were relatively few people on site: of those on-site at the time, 28 were killed and 36 injured. Fires continued on-site for more than ten days. Off-site there were no fatalities, but 50 injuries were reported and about 2,000 properties damaged.
The occupants of the works laboratory had seen the release and evacuated the building before the release ignited; most survived. None of the 18 occupants of the plant control room survived, nor did any records of plant readings. The explosion appeared to have been in the general area of the reactors and after the accident only two possible sites for leaks before the explosion were identified: "the 20 inch bypass assembly with the bellows at both ends torn asunder was found jack-knifed on the plinth beneath" and there was a 50-inch long split in nearby 8-inch nominal bore stainless steel pipework".
The local and corporate aftermath
About 40 tonnes of cyclohexane escaped in about one minute so the a 1 mile zone was evacuated and a 10 mile zone was told to stay indoors. the would be many concerns in many of the places near chemical plants.
1,000 buildings within a mile radius of the site (in Flixborough itself and in the neighbouring villages of Burton upon Stather and Amcotts) were damaged, as were nearly 800 in Scunthorpe (three miles away); the blast was heard over thirty miles away in Grimsby and Hull. Images of the disaster were soon shown on television thanks to BBC and Yorkshire Television filmstock news crews who had been covering the Appleby-Frodingham Gala in Scunthorpe that afternoon.
ICI response to the accident
They were concerned by the bad practices at the plant.
Cause of the disaster
The 20-inch bypass was therefore clearly not what would have been produced or accepted by a more considered process but controversy developed (and became acrimonious) as to whether its failure was the initiating fault in the disaster (the 20-inch hypothesis, argued by the plant designers (DSM) and the plant constructors; and favoured by the court's technical advisers), or had been triggered by an external explosion resulting from a previous failure of the 8-inch line (argued by experts retained by Nypro and their insurers).
The 20-inch hypothesis
Tests on replica bypass assemblies showed that bellows squirm could occur at pressures below the safety valve setting, but that squirm did not lead to a leak (either from damage to the bellows or from damage to the pipe at the mitre welds) until well above the safety valve setting. However theoretical modelling suggested that the expansion of the bellows as a result of squirm would lead to a significant amount of work being done on them by the reactor contents, and there would be considerable shock loading on the bellows when they reached the end of their travel. If the bellows were 'stiff' (resistant to squirm), the shock loading could cause the bellows to tear at pressures below the safety valve setting; it was not impossible that this could occur at pressures experienced during start-up, when pressure was less tightly controlled. (Plant pressures at the time of the accident were unknown since all relevant instruments and records had been destroyed, and all relevant operators killed). The Inquiry concluded that this ("the 20-inch hypothesis") was 'a probability' but one 'which would readily be displaced if some greater probability' could be found.
The 8-inch hypothesis
Detailed analysis suggested that the 8-inch pipe had failed due to creep cavitation at a high temperature while the pipe was under pressure. Failure had been accelerated by contact with molten zinc and there were indications that an elbow in the pipe had been at significantly higher temperature than the rest of the pipe. The hot elbow led to a non-return valve held between two pipe flanges by twelve bolts. After the disaster, two of the twelve bolts were found to be loose; the inquiry concluded that they were probably loose before the disaster. Nypro argued that the bolts had been loose, there had consequently been a slow leak of process fluid onto lagging leading eventually to a lagging fire, which had worsened the leak to the point where a flame had played undetected upon the elbow, burnt away its lagging and exposed the line to molten zinc, the line then failing with a bulk release of process fluid which extinguished the original fire, but subsequently ignited giving a small explosion which had caused failure of the bypass, a second larger release and a larger explosion. Tests failed to produce a lagging fire with leaked process fluid at process temperatures; one advocate of the 8-inch hypothesis then argued instead that there had been a gasket failure giving a leak with sufficient velocity to induce static charges whose discharge had then ignited the leak.
The inquiry's conclusion
The 8-inch hypothesis was claimed to be supported by eyewitness accounts and by the apparently anomalous position of some debris post-disaster. The inquiry report took the view that explosions frequently throw debris in unexpected directions and eyewitnesses often have confused recollections. The inquiry identified difficulties at various stages of the accident development in the 8-inch hypothesis, their cumulative effect being considered to be such that the report concluded that overall the 20-inch hypothesis involving 'a single event of low probability' was more credible than the 8-inch hypothesis depending upon 'a succession of events, most of which are improbable'.
Lessons to be learned
The inquiry report identified 'lessons to be learned' which it presented under various headings; 'General observation' (relating to cultural issues underlying the disaster), 'specific lessons' (directly relevant to the disaster, but of general applicability) are reported below; there were also 'general' and 'miscellaneous lessons' of less relevance to the disaster. The report also commented on matters to be covered by the Advisory Committee on Major Hazards.
- Plant – where possible – should be designed so that failure does not lead to disaster on a timescale too short to permit corrective action.
- Plant should be designed and run to minimise the rate at which critical management decisions arise (particularly those in which production and safety conflict).
- Feedback within the management structure should ensure that top management understand the responsibilities of individuals and can ensure that their workload, capacity and competence allow them to effectively deal with those responsibilities
- The disaster was caused by 'a well designed and constructed plant' undergoing a modification that destroyed its technical integrity.
- Modifications should be designed, constructed, tested and maintained to the same standards as the original plant
- When the bypass was installed, there was no works engineer in post and company senior personnel (all chemical engineers) were incapable of recognising the existence of a simple engineering problem, let alone solving it
- When an important post is vacant special care should be taken when decisions have to be taken which would normally be taken by or on the advice of the holder of the vacant post
All engineers should learn at least the elements of other branches of engineering than their own.
Matters to be referred to the Advisory Committee
No one concerned in the design or construction of the plant envisaged the possibility of a major disaster happening instantaneously. It was now apparent that such a possibility exists where large amounts of potentially explosive material are processed or stored. It was 'of the greatest importance that plants at which there is a risk of instant as opposed to escalating disaster be identified.
Once identified measures should be taken both to prevent such a disaster so far as is possible and to minimise its consequences should it occur despite all precautions.' There should be coordination between planning authorities and the Health and Safety Executive, so that planning authorities could be advised on safety issues before granting planning permission; similarly the emergency services should have information to draw up a disaster plan.
Offical accident report inclusion
The inquiry summarised its findings as follows:
- "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."
Responce to the report
ICI response to the report
The 3 then political conspiracy theories
- 1984 Bhopal disaster
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- Lees' Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control (3rd Edition), ed Sam Mannan, Butterworth-Heinemann, 2004 ISBN 0750675551, 9780750675550