The Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough, North Lincolnshire, England, on Saturday, 1 June 1974. It killed 28 and seriously injured 36 of the 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. 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 equipment modification. Although virtually all of the plant management personnel had chemical engineering qualifications, there was no on-site senior manager with mechanical engineering expertise. Mechanical engineering issues with the modification were overlooked by the managers who approved it, and the severity of potential consequences due to its failure were not taken into account.
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. UK government regulation of plants processing or storing large inventories of hazardous materials is currently under the Control of Major Accident Hazards Regulations 1999 (COMAH). In Europe, the Flixborough disaster and the Seveso disaster in 1976 led to development of the Seveso Directive in 1982 (currently Directive 2012/18/EU issued in 2012).
History of the chemical plant A neighbouring nitrogen plant was built in the late 1930s. On Monday 12 May 1941 German aircraft bombed the nitrogen plant, with eight HE bombs, which killed five workers and injured six others. This was the most serious Luftwaffe raid in Scunthorpe.Scunthorpe Evening Telegraph Friday 8 September 1989, page 8Scunthorpe Evening Telegraph Tuesday 8 January 1991, page 18Scunthorpe Evening Telegraph Tuesday 31 August 1999, page 4
It was decided to build the plant in July 1964. The plant would cost £5.5m. Nypro was formed in December 1964; Fisons, of Suffolk, owned 24%, with Dutch State Mines the rest.Lincolnshire Echo Thursday 17 December 1964, page 5Scunthorpe Evening Telegraph Thursday 17 December 1964, page 1
The plant was built by Simon Carves from early 1965 to 1967, with Stamicarbon, of the Netherlands, a subsidiary of DSM, and Humphreys & Glasgow (which went into liquidation in January 1990), run by Ambrose Congreve. Simon Carves, Humphreys & Glasgow and the Power-Gas Corporation had formed the joint-company Chemical Works Projects Ltd, in 1961, to make large nitrogen plants.Times Friday 26 May 1961, page 22 Humphreys & Glasgow also worked with Sofregaz, of France, in the late 1960s.
It had several miles of aluminium piping.Grimsby Evening Telegraph Wednesday 5 October 1966, page 14Grimsby Evening Telegraph Wednesday 27 September 1967, page 9 The plant opened in early November 1967.Grimsby Evening Telegraph Tuesday 7 November 1967, page 9 It sold products in conjunction with Société Nationale des Pétroles d'Aquitaine (SNPA) of France. The chemical plant was built next door to a nitrogen plant, of Fisons, built from 1938, which opened around 1939, and employed 300 men.Scunthorpe Evening Telegraph Friday 10 March 1939 Products were shipped from Gunness on the Trent.
Another similar nitrogen fertiliser plant opened in Immingham in 1951.The Nypro plant was built at Flixborough, due to the neighbouring nitrogen plant.Gainsborough Evening News Wednesday 7 October 1981, page 11 The nitrogen plant itself had a fire on 30 July 1966.Grimsby Evening Telegraph Thursday 27 March 1986 The chemical plant had a multi-million-pound expansion from 1970.Grimsby Evening Telegraph Monday 19 October 1970, page 10
The chemical works were owned by Nypro UK (later a joint venture between DSM and the British National Coal Board (NCB)) had originally produced fertiliser from by-products of the coke ovens of a nearby steelworks.
Background
Since 1967, the plant 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 tons per annum (tpa) 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 the cyclohexane plant that led to the disaster. One of the cost saving measures was the stoppage of the agitators in the reactors to save electricity. This could lead to stratification of water and organics and steam hammer in the reactor with consequential pressure peaks in the reactors. 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 1,000 buildings within a 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 away; the blast was heard over away in Grimsby, Hull and Saltfleet. Images of the disaster were soon shown on television, filmed by BBC and Yorkshire Television filmstock news crews who had been covering the Appleby-Frodingham Gala in Scunthorpe that afternoon.
The plant was rebuilt 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 1,200 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.
The disaster The plant In the DSM process, cyclohexane was heated to about 155 °C (311 °F) at a pressure of 8.6 Bar 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. 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 (700mm DN) 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 than those achieved under automatic control) by operator intervention (manual operation of vent valves). A pressure-relief valve acting at 11 bar was also fitted.
Reactor 5 leaks and is bypassed Two months prior to the explosion, the number 5 reactor was discovered to be leaking. The most likely cause of the crack in the mild steel was Corrosion Under Insulation (CUI) initiated by the concentration of nitrates which were in the cooling water that was used to cool the seal of the agitator and leaked under the insulation whereof the cladding was in a bad condition. 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 whilst repairs were made. In the absence of 28-inch nominal bore pipe (700mm DN), 20-inch nominal bore pipe (500mm DN) was used to fabricate the bypass pipe for linking reactor 4 outlet to reactor 6 inlet. This by-pass was supported by scaffolding. 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 16:53 on 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 huge cloud of flammable vapour and a massive explosion in the plant. The explosion virtually demolished the site. As it was a weekend, there were relatively few people on site: of the 72 people on-site at the time, 28 were killed and 36 injured. Fires burned 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".
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.