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A brief look from de control system failures point of view.

The Flixborough Works of Nypro (UK) Ltd. was a plant designed toproduce 70,000 tons per year of Caprolactam, a basic raw materialfor the production of Nylon.

In one almighty explosion the overall complex was demolished atabout 4:53 p.m, on Saturday June 1st. 1974. The explosion, of warlikedimensions, was the equivalent of 15 tonnes of TNT and let 28 mendead.

All of this, largely due to negligence on the use of control systemsand performing changes in the plant process design without take intoaccount the necessary modifications in the control system.

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The process where the accident occurred consisted of six reactors inseries. In these reactors Cyclohexane, highly flammable, was oxidizedwith air to a mixture of Cyclohexanone and Cyclohexanol.

The reaction was slow and the conversion had to be kept low to avoid theproduction of unwanted by-products, so the inventory in the plant waslarge, about 400 tonnes.

The reaction took place in the liquid phase and each reactor used tohold about 20 tonnes.

The reaction was highly exotermic. The temperature was controlledtrough evaporation of cyclohexane and the pressure by nitrogeninjection.

One of the reactors developed a crack andwas removed for repair. In order to maintainproduction a temporary bypass pipe wasinstalled in its place.

The bypass (the only one available at theplant) consisted of a dog-leg pipe from 20inand this was used although the vesselapertures were of 28-inch diameter.

Calculation showed that this size would beadequate for the flow rates required,however the pipe was not design bypersonal professionally qualified or underappropriate security settings.

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The pipe, which was supposed to be temporary, performed for twomonths until a slight rise in pressure occurred. The nitrogen stock forpurging was found to be deficient, so there was insufficient provision forthe venting of off-gas as a method of pressure control.

A fresh delivery of Nitrogen was not expected before midnight. However,the production was not stopped.

The slight rise in pressure perhaps did not cause damage in the reactors,however it was enough to make the pipe to twist. The bending momentwas strong enough to tear the bellows and two 28 inch holes appeared inthe plant.

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As a result, a massive vapor cloud was formed by the escape ofcyclohexane from the holes of the ruptured.

It was estimated that about 30–50 tonnes escaped in the 50 seconds thatelapsed before ignition occurred. The source of ignition was probably afurnace some distance away.

In addition to this, the plant site contained excessively large inventoriesof dangerous compounds. This included 330,000 gallons of cyclohexane,66,000 gallons of naphtha, 11,000 gallons of toluene, 26,400 gallons ofbenzene, and 450 gallons of gasoline.

The resulting explosion, one of the worst vapour cloud explosions thathas ever occurred, destroyed the oxidation unit and neighbouring unitsand caused extensive damage to the rest of the site.

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In all 54 casualties, 28 workers were killed and a further 36 sufferedserious injuries. 18 of these fatalities occurred in the main control whenno one escaped from the control room before the ceiling collapsed.

In the surroundings, 1,821 houses and 167 shops and factories suffereddamage, adding more economic losses to the ones had from the plant.

Loss of life would have been substantially greater if the accident hadoccurred on a weekday when the administrative offices were filled withemployees

In the UK, the government set up an Advisory Committee on MajorHazards, which had a huge influence on accidents regulation anddangerous compounds management.

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1. Performing changes in the plant process design without take intoaccount the necessary modifications in the control system. Thebypass was never closely inspected; it was attached to the processand no pressure-testing was carried out on it in order to resettle newset points. Due to the fact that there was not pressure control in thebypass, this was easily damaged by the pressure increase.

2. Negligence and improper use of the existing control system. Theslight rise in pressure must have been controlled by nitrogeninjection. Despite the lack of this compound, they decided to continuewith the process instead of shutting down the plant. If the pressure hadbeen controlled, the accident had not been happened.

1. The importance of understanding and managing the potentialconflict of priorities between safety and production.

2. The importance not only of a good process control implemented inthe plant, but also of accepting and take in consideration theinformation given by this.

3. The need to take steps to limit exposure of personnel to potentialhazards, for instance, protecting the control room and building itblast-proof.

4. The importance of reducing the inventory, mainly when hazardouschemical area used. If we set out to reduce inventories, theresulting plants will be cheaper as well as safer.

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Kletz, Trevor A., Learning from accidents, 3rd edition, Butterworth-Heinemann, Oxford, UK, 2001, Chapter 8.

Cox, S., Tait, R., Safety, Reliability and Risk Management: an integrated approach, 2nd edition, Butterworth-Heinemann, Oxford, UK, 1998, Chapter 16, p. 300-303.

Health and Safety Executive, ‘The Flixborough Disaster : Report of the Court of Inquiry’, HMSO, ISBN 0113610750, 1975.

Kletz, Trevor A., What Went Wrong? – Case Histories of Process Plant Disasters, 4th edition,Gulf, Houston, Texas, 1998, Sections 9 and12.

Video links:

https://www.youtube.com/watch?v=8A1xSCUtB-M

https://www.youtube.com/watch?v=tCsTlvCQmBY