Since , it had instead produced caprolactam , a chemical used in the manufacture of nylon 6. This was originally produced by hydrogenation of phenol , but in 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, tpa tons per annum of caprolactam but was reaching a rate of only 47, tpa in early Government controls on the price of caprolactam put further financial pressure on the plant. A major leak of liquid from the reactor circuit caused the rapid formation of a large cloud of flammable hydrocarbon.
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Flixborough disaster explained The Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough , North Lincolnshire , England on Saturday, 1 June It killed 28 people and seriously injured 36 out of a total of 72 people on site at the time.
The casualty figures could have been much higher, if the explosion had occurred on a weekday, when the main office area would have been occupied. The disaster involved and may well have been caused by a hasty modification. There was no on-site senior manager with mechanical engineering expertise virtually all the plant management had chemical engineering qualifications ; mechanical engineering issues with the modification were overlooked by the managers who approved it, nor was the severity of the potential consequences of its failure appreciated.
Flixborough led to a widespread public outcry over process safety. Together with the passage of the UK 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.
Since , 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 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, tpa tons per annum of caprolactam but was reaching a rate of only 47, tpa in early Government controls on the price of caprolactam put further financial pressure on the plant.
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 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, 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 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, filmed by BBC and Yorkshire Television filmstock news crew s 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 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 , 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 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. 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. The reactors were constructed from mild steel with a stainless steel lining; when operating they held in total about tonnes of flammable liquid at a working pressure of 8.
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. 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.
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 than those achieved under automatic control by operator intervention manual operation of vent valves. A pressure-relief valve acting at NaN-0 gauge was also fitted. 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 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 inch nominal bore pipe mm DN , inch nominal bore pipe mm DN 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.
The explosion At about on Saturday 1 June , 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, 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 inch long split in nearby 8-inch nominal bore stainless steel pipework".
Court of Inquiry Immediately after the accident, New Scientist commented presciently on the normal official response to such events, but hoped that the opportunity would be taken to introduce effective government regulation of hazardous process plants. The Secretary of State for Employment set up a Court of Inquiry to establish the causes and circumstances of the disaster and identify any immediate lessons to be learned, and also an expert committee to identify major hazard sites and advise on appropriate measures of control for them.
The inquiry sat for 70 days in the period September — February , and took evidence from over witnesses. In parallel, an Advisory Committee on Major Hazards was set up to look at the longer-term issues associated with hazardous process plants. Circumstances of the disaster The report of the court of inquiry was critical of the installation of the bypass pipework on a number of counts: although plant and senior management were chartered engineer s mostly chemical engineers , the post of Works Engineer which had been occupied by a chartered mechanical engineer had been vacant since January , and at the time of the accident there were no professionally qualified engineers in the works engineering department.
Nypro had recognised this to be a weakness and identified a senior mechanical engineer in an NCB subsidiary as available to provide advice and support if requested. At a meeting of plant and engineering managers to discuss the failure of reactor 5, the external mechanical engineer was not present.
The emphasis was upon prompt restart and — the inquiry felt — although this did not lead to the deliberate acceptance of hazards, it led to the adoption of a course of action whose hazards and indeed engineering practicalities were not adequately considered or understood.
The major problem was thought to be getting reactor 5 moved out of the way. Only the plant engineer was concerned about restarting before the reason for the failure was understood, and the other reactors inspected. The difference in elevation between reactor 4 outlet and reactor 6 inlet was not recognised at the meeting. At a working level the offset was accommodated by a dog-leg in the bypass assembly; a section sloping downwards inserted between and joined with by mitre welds two horizontal lengths of inch pipe abutting the existing inch stubs.
This bypass was supported by scaffolding fitted with supports provided to prevent the bellows having to take the weight of the pipework between them, but with no provision against other loadings. The Inquiry noted on the design of the assembly: The Inquiry noted further that "there was no overall control or planning of the design, construction, testing or fitting of the assembly nor was any check made that the operations had been properly carried out".
The inch hypothesis Tests on replica bypass assemblies showed that deformation of the bellows could occur at pressures below the safety valve setting, but that this deforation 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 this 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.
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 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; 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 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 report also commented on matters to be covered by the Advisory Committee on Major Hazards. General observation 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.
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 branches of engineering other 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. 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. The proponent of the 8-inch gasket failure hypothesis responded by arguing that the inch hypothesis had its share of defects which the inquiry report had chosen to overlook, that the 8-inch hypothesis had more in its favour than the report suggested, and that there were important lessons that the inquiry had failed to identify: The HSE website currently says "During the late afternoon on 1 June a 20 inch bypass system ruptured, which may have been caused by a fire on a nearby 8-inch pipe".
Post-enquiry forensic engineering — two-stage rupture of bypass The enquiry noted the existence of a small tear in a bellows fragment, and therefore considered the possibility of a small leak from the bypass having led to an explosion bringing the bypass down.
It noted this to be not inconsistent with eyewitness evidence, but ruled out the scenario because pressure tests showed the bellows did not develop tears until well above the safety valve pressure. This hypothesis has however been revived, with the tears being caused by fatigue failure at the top of the reactor 4 outlet bellows because of flow-induced vibration of the unsupported bypass line.
Finite element analysis has been carried out and suitable eyewitness evidence adduced to support this hypothesis. It was postulated that there had been bulk water in reactor 4 and a disruptive boiling event had occurred when the interface between it and the reaction mixture reached operating temperature.
Abnormal pressures and liquor displacement resulting from this it was argued could have triggered failure of the inch bypass. Dissatisfaction with other aspects of the Inquiry Report The plant design had assumed that the worst consequence of a major leak would be a plant fire and to protect against this a fire detection system had been installed.
Tests by the Fire Research Establishment had shown this to be less effective than intended. Moreover, fire detection only worked if the leak ignited at the leak site; it gave no protection against a major leak with delayed ignition, and the disaster had shown this could lead to multiple worker fatalities.
The plant as designed therefore could be destroyed by a single failure and had a much greater risk of killing workers than the designers had intended. More fundamentally, Trevor Kletz saw the plant as symptomatic of a general failure to consider safety early enough in process plant design, so that designs were inherently safe — instead processes and plant were selected on other grounds then safety systems bolted on to a design with avoidable hazards and unnecessarily high inventory.
But before we do so we should ask if a lamb might do. Critics felt that the Flixborough explosion was not the result of multiple basic engineering design errors unlikely to coincide again; the errors were rather multiple instances of one underlying cause: a complete breakdown of plant safety procedures exacerbated by a lack of relevant engineering expertise, but that lack was also a procedural shortcoming.
Their immediate cause was human error but ICI felt that saying that most accidents were caused by human error was no more useful than saying that most falls are caused by gravity.
ICI had not simply reminded operators to be more careful, but issued explicit instructions on the required quality of isolations, and the required quality of its documentation.
These requirements applied not only to changes to equipment, but also to process changes. All modifications were to be supported by a formal safety assessment.
Flixborough (Nypro UK) Explosion 1st June 1974
History[ edit ] Flixborough has had many different spellings through the centuries, from "Flichesburg" in the Domesday Book to Flikesburg, Flyxburgh and Flixburrow. During the two-year programme, an unprecedented Middle to Late Saxon rural settlement sequence was uncovered, dating between the early 7th and early 11th centuries AD. It is particularly exceptional because of the association of 40 buildings, floor surfaces and massive refuse dumps. The register dates from the year Sheffield Bart. Francis Amcotts Jarvis M.
Twenty-eight workers were killed and a further 36 suffered injuries. It is recognised that the number of casualties would have been more if the incident had occurred on a weekday, as the main office block was not occupied. Offsite consequences resulted in fifty-three reported injuries. Property in the surrounding area was damaged to a varying degree. Prior to the explosion, on 27 March , it was discovered that a vertical crack in reactor No. The plant was subsequently shutdown for an investigation. The investigation that followed identified a serious problem with the reactor and the decision was taken to remove it and install a bypass assembly to connect reactors No.
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