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FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974

FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974. The NYPRO PLANT operated by the ROYAL DUTCH SHELL COMPANY at FLIXBOROUGH. Produced caprolactam H 2 N(CH 2 ) 5 COOH a raw material for the manufacture of Nylon 6 from benzene . PRODUCTION OF nylon 6 from BENZENE.

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FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974

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  1. FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974

  2. The NYPRO PLANT operated by the ROYAL DUTCH SHELL COMPANYat FLIXBOROUGH

  3. Produced caprolactam H2N(CH2)5COOH a raw material for the manufacture of Nylon 6 from benzene

  4. PRODUCTION OF nylon 6 from BENZENE • Benzene C6H6 + H2-------------> Cyclohexane C6H12under high pressure • Cyclohexane + NH3 + O2 ---------> Cyclohexane oxide 02needs to be carefully controlled • Cyclohexane oxide + oleum -------> Caprolactam • Caprolactam + H20 ---> HOOC(CH2)5NH2 + Δ 260o ------------------------------> Nylon 6 • --[C(CH2)5NC(CH2)5N--] n +nH20 0 HO H

  5. PRE ACCIDENT • Brine system used to cool the reactors was shutdown for repair • Water containing nitrates used instead of brine • Reactor 5 (out of 6 in series) taken out of system as it was leaking cyclohexane • A temporary by-pass put in a ‘dog leg’ with bellows was used to connect reactors 4 & 6 • By pass inadequately supported • Repair was rushed and not built under the supervision of an engineer. • System checked using N2 instead of H20 the STANDARD SAFETY TEST

  6. ACCIDENT • 1st June pressure built up quickly • Cause was not found -almost certainly a high presure N2 leak into the system • Fire detected on an 8’’ section of pipe • By pass pipe ruptured - under the strain of high prssure, high temperature and stress fracture caused by the nitrates in the cooling water • 60 seconds later the H2 plant caught light and detonation of 30 tons of cylohexane occurred • Fires lasted 10 days

  7. Violent Explosion • 28 men died - no one survived from the control room • Only 8 bodies recovered • 53 other workers needed hospitalised treatment • Blast waves felt 4 miles away • £18 million in todays terms £200 million asset totally destroyed • 1821 houses, 167 shops/factories damaged • Plant was a mound of ash and fused metal • Plant declared safe 13 days after the accident

  8. What went wrong? • Changing the cooling procedure using water instead of brine caused stress corrosion in reactor 5. Nitrate induced cracking was known by METALLURGISTS but was less well known by CHEMICAL ENGINEERS MODIFICATION 1 • Pressure of cooling water higher than that used with brine SIGNIFICANCE NOT KNOWN • Inadequate design of ‘dog leg ‘ by-pass,connecting reactors 4 & 6.The men responsible for building the 20” temporary by -pass were not engineers.‘THEY DID NOT KNOW THAT THEY DID NOT KNOW’ MODIFICATION 2

  9. The modified system was tested using pressurised N2 for leaks the STANDARD SAFETY TEST uses H20, had water been used the by-pass would have been found to be defective MODIFICATION 3 • Inadequate support of by-pass, bellows free to rotate when pressure rose • By-pass pipe and bellows were not inspected • Design of plant inadequate not enough thought to the potentials of an accident • Lack of N2 for inerting - putting out of fires • Lack of provision for releasing gas build ups .

  10. There was no professional engineer in the plant • at the time of the accident - and there were • critical decisions to be made • Huge quantities of highly inflammable • cyclohexane stored on site. • Modification approval not taken seriously, • British Standards neither consulted nor • adhered to, no body consulted the British • Standards Handbook

  11. LESSONS DRAWN • Modifications to be designed, constructed, tested and maintained to the same standards as the original plant - there needs to be a full assessment of the potential consequences • All managers to visibly inspect the plant • A full assessment of potential consequences if there is a failure in the plant • Companies to ensure they employ enough qualified and experienced staff to make critical decisions • Plants to be laid out to avoid the ‘domino’ effect • Occupied buildings close to hazardous plants to be blast resistant

  12. The storage of hazardous compounds to be kept to a minimum • Sufficient inerting material on site. • Any modifications, permanernt or temporary are the source of a potential disaster, which may change the validity and the results of HAZOP and Risk Analysis

  13. HIGH COST ACCIDENTS SINCE • FLIXBOROUGH • Since Flixborough there have been 24 UK high cost accidents conservative estimate £500 million (1996 values). • In most high cost accidents there were no deaths (two had one death and one had five deaths). Several accidents with a high death toll eg Bhopal around 4000 deaths and Mexico City >500 deaths were not high cost • COMMON FEATURES • Isolation valves - problems with isolation leaks of ignited flammablesubstances was a major factor in 2 accidents. In each case isolating valves could not be closed either because they were not remotely operable or couldn’t be accessed as too close to the fire/damaged by the fire

  14. Fire water -in several accidents the water supplies for fire fighting were inadequate • Storage protocol a lack of understanding and poor management of storage/process/chemical segregation was a factor in 3 accidents • Perceived LOWrisk at least 2 accidents involved activites which were not considered to be the main high risk activity on site • Escalation potential- in 2 accidents the intense heat generated by the fire posed a real threat of escalation to involve adjacent storage tanks. The fire services set up water coooling monitors • Routine inspection & maintenance of critical equipment -3 accidents revealed deficiences in the inspection & maintenance procedures

  15. Unreliable & inadequate controlsystems-4 accidents revealed a combination of inadequate and unreliable process control equipment eg pump failed for several hours, in another level indicators were KNOWN to give false readings, in another the process system failed to identify the correct position of valve equipment and in the fourth vital pressure information was not displayed on the VDU used by the operators • Location of key buildings 2 accidents resulted in the total destruction of or severe damage to the control room buildings • Loss of process control- 2 of the accidents resulted in runaway reactions • Information to the public- 2 accidents revealed that information to the public was inadequate- great public concern was generated!

  16. MAJOR ACCIDENTS 1999-2002 • COMAH (Control of Major Accident Hazards) apply to 1200 establishments that have the potential to cause major accidents. The general duty of the regulationsis that ‘Every Operator shall take all measures necessary to prevent major accidents and limit their consequencesto persons and the environment’ • 1999 - 2002 there were 21 COMAH major accidents • 4 prosecutions + 5 investigations ongoing • Employee error- failure to follow procedures, runaway chemical reaction cause unknown, ruptured bellows, corrosion of defective welds/ tanks, pipework failure, ruptured valves

  17. Inadequate maintenance, pipe fatigue, incorrrect setting of valves all contributed to these accidents • FAILURES • Check pipework not corroded • Bellows not to correct specification • Lack of back up isolation valves • Incompatible substances stored in the same warehouse • Inadequate smoke detection and fire fighting facilities • Time delays 50 minutes before accident reported to emergencies services • Inadequate identification of potentially hazardous imurities • Hazard markings not checked on tankers, driver documentation not checked allcauses of accidents

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