FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974

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FLIXBOROUGH A MODIFICATION ACCIDENT JUNE 1974
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The NYPRO PLANT operated by the ROYAL DUTCH
SHELL COMPANYat FLIXBOROUGH
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Produced caprolactam H2N(CH2)5COOH a raw material
for the manufacture of Nylon 6 from benzene
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PRODUCTION OF nylon 6 from BENZENE

• Benzene C6H6 H2-------------gt Cyclohexane
  C6H12 under high pressure
• Cyclohexane NH3 O2 ---------gt Cyclohexane
  oxide 02 needs to be carefully controlled
• Cyclohexane oxide oleum -------gt Caprolactam
• Caprolactam H20 ---gt HOOC(CH2)5NH2 ? 260o
  ------------------------------gt Nylon 6
• --C(CH2)5NC(CH2)5N-- n nH20
• 0 HO H
  

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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

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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

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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

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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

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• 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
• .

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• 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

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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

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• 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

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• 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 gt500
  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 couldnt be accessed as too
  close to the fire/damaged by the fire

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• 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 LOW risk 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

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• Unreliable inadequate control systems-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!

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• 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

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• 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