Flixborough disaster By: Paradigma Carlo Giovanni
Introduction• Largest peacetime explosion ever to occur in the UK• Date: Saturday, 1 June 1974• Location: Flixborough chemical plant owned by Nypro (UK) Ltd• Deaths of 28 workers on the site• Widespread damage to property within a 6 mile radius around the plant
• 1967: 20,000 TPA caprolactam plant built by DSM at the Flixborough site using process involving the hydrogenation of phenol C6H5OH + 2 H2 → (CH2)5CO• 1972: 70,000 TPA ; new process used• New process based on oxidation of cyclohexane• Posed much greater hazard than phenol process
Description of the cyclohexane process• Process operates by injecting compressed air into liquid cyclohexane at a working pressure of about 9 bar and temperature of 155°C• Cyclohexanone and cyclohexanol produced• Conversion is low and it is necessary to recirculate the cyclohexane continuously through a train of six large SS-lined reactors
Simplified flow diagram of cyclohexane oxidation plant before March 1974 (Whittingham, 2005)
Events leading to the accident2. Miners overtime ban of Nov, 1973• Resulted in the government passing legislation to restrict the use of electricity by industry to 3 days a week• Was not possible to operate the process on this basis• Was decided to utilize existing emergency power generation on-site
• Major electricity user: 6 stirrers in the cyclohexane reactors• Primary purpose: disperse compressed air that was injected into each reactor via a sparger• Also ensured that droplets of water formed within the reactor system were dispersed into the cyclohexane
1. The No. 5 reactor problem• Jan, 1974:normal electricity supply resumed• Was found that the drive mechanism for the stirrer in the No. 4 reactor had been subject to severe mechanical damage• No reason was found for this. It was therefore decided to continue to operate the plant with the No. 4 reactor stirrer shutdown.
• Cyclohexane reactors were MS vessels fitted with an inner SS lining to resist corrosion• March,1974: Cyclohexane found leaking from 6 feet long vertical crack in the MS shell of the No. 5 reactor• Due to technical problems experienced earlier and the effects of the 3-day week, the plant owners were keen to make up lost production• Therefore decided to remove No. 5 reactor for inspection and continue operation with the remaining five reactors
1. Installation of 20” bypass pipe• This pipe connected together the existing 28 inch bellows on the outlet of reactor No. 4 and the inlet of reactor No. 6• Dog-leg shape of pipe• Company did not have qualified mechanical engineer on site to oversee design and construction• No hydraulic pressure testing of pipe carried out, except for a leakage test using compressed air.
Simplified flow diagram of cyclohexane oxidation plant after March 1974 (Whittingham, 2005)
1. Resumption of production• Plant restarted and operated normally, with occasional stoppages, up until the afternoon of Saturday, 1 June 1974• Previous day: plant had been shut down for minor repairs• Early hours of 1 June: plant in process of being restarted• Start-up involved charging system with liquid cyclohexane to normal level and then recirculating this liquid through a heat exchanger to raise the temperature.
• The pressure in the system was maintained with nitrogen at about 4 bar until the heating process began to raise the pressure due to evaporation of cyclohexane.• The pressure was then allowed to rise to about 8 or 9 bar, venting off nitrogen to relieve any excess pressure. The temperature in the reactors by then was about 150°C.
• On 1 June this procedure was followed except it was noted by the morning shift that by 06.00 hours the pressure had reached 8.5 bar even though the temperature in the No. 1 reactor had only reached 110°C• Was not realized at the time that this discrepancy might have indicated the presence of water in the system• The start-up continued until, at about 16.50 hours, a shift chemist working in the laboratory close to the reactors heard the sound of escaping gas and saw a haze typically associated with a hydrocarbon vapour cloud.
The accident• 16.53 hours on 1 June 1974: massive aerial explosion occurred with a force later estimated to be about 15 to 45 tonnes of TNT equivalent• Explosion heard up to 30 miles away and damage sustained to property over a radius of about 6 miles around the plant• 28 plant workers killed with no survivors from the control room• All records and charts for the start-up destroyed• Following the explosion, 20 inch bypass assembly was found in a ruptured condition
The Public Inquiry• Following the disaster, public inquiry conducted under the chairmanship of Roger Parker QC• To establish the causes and circumstances of the disaster• To identify lessons to be learnt from the disaster
Conclusions of the inquiry• The immediate cause of the main explosion was the rupture of the 20 inch bypass assembly between the No. 4 and No. 6 reactor• Two main theories to explain
The 20 inch pipe theory• The 20 inch bypass assembly failed due to its unsatisfactory design features• However, the assembly had survived 2 months of normal operation.• A number of independent pressure tests were commissioned to determine unusual conditions
• The normal working pressure = 8 bar• practice during start-up to allow the pressure to build up to about 9 bar.• The safety valves for the system, were set to discharge at a pressure of 11 bar• At pressure above 11 bar, squirming motion which distorted the bellows.
• Even when the assembly squirmed, no rupture until pressure crossed 14.5 bar, a pressure not achievable in reactors.• Inquiry concluded that a rupture of the 20 inch bypass due to pressure, temperature conditions• Report conceded ambiguity in the hypothesis• Simulation tests could not replicate failure at similar conditions
The 8 inch hypothesis• Alternative theory• 50 inch split in an 8 inch line connected to separator below bypass• this failure led to a smaller explosion causing failure of the main 20 inch bypass• Zinc embrittlement had caused the split• Small lagging fire at a leaking flange causing zinc to drip onto the 8 inch pipe• Brittle failure – Vapour release – Explosion – 20 inch failure
• Inquiry Report had devoted discussion of this two-stage theory• Finally dismissed as being too improbable• No other theories considered by them to explain failure of 20 inch bypass pipe
The water theory• Another alternative theory• Not considered by the Inquiry• Much of scientific work after Inquiry closed• Examined the effects of not operating the No. 4 reactor stirrer during the start-up at a time when water may be present• More probable explanation
• Cyclohexane and water are normally immiscible• Azeotrope forms due to the limited solubility of water in cyclohexane.• This azeotrope has lower boiling point than either water or cyclohexane• Unstable interfacial layer may form• Under certain conditions can boil and erupt violently ejecting cyclohexane and superheated water from the reactor.
• Normally impossible for water layer to form due to dispersion of water by air distribution• During start-up, the air to the reactors shut off• If stirrers running during start-up, no water layer• If stirrer stops, a layer of water could form, together with the unstable azeotrope.
• As temperature of reactor increases, boiling point of azeotrope is reached• Possibility of a sudden violent eruption from the reactor and ejection of slugs of liquid reactant• Slugs exert high mechanical forces on the bypass assembly, loosely supported by scaffolding• Causes bypass assembly to fail without the high static pressure in the reactors
Alternative event sequence• Most credible explanation• Explains failure of 20 inch bypass• Also provides an explanation for the whole sequence of events
• Unexplained failure of drive mechanism of No. 4 reactor• Crack developing in the lining and shell of No. 5 reactor• Failure of the 20 inch bypass assembly.• Any/all failures caused by violent eruption of reactor contents due to presence of water• Committee failed to see common thread
• Drive mechanism failure for No. 4 reactor unexplained• Thought to be irrelevant• Crack in the shell of the No. 5 reactor due to stress corrosion• Proposed by plant owners but not credible
• The failure of the bypass concluded by Inquiry due to reactors being over-pressurized• Implies human error, not verifiable• Greatest failing of Inquiry was not taking account of all the events 6 months prior to the disaster• Issue of non-operation of the reactor stirrers ignored
Conclusions• Human error analysis Table gives causes against the different types of error that occurred.• Direct cause Failure of the 20 inch bypass pipe led to huge release of inflammable cyclohexane vapour which ignited• Root causes A badly designed 20 inch bypass pipe installed rather than finding reasons for the crack in the No. 5 reactor Why bypass failed ?
Safety considerations• Learnings• Low inventory especially of flashing fluids• Before modifying process, carry out systematic search for possible cause of problem• Carry out HAZOP analysis• Construct modifications to same standard as original plant• Use blast-resistant control rooms and buildings