It is Saturday the 1st of June 1974 and a 2 year old caprolactam unit at
the flixborough chemical plant in flixborough England, is being placed back into service.
The installation has been somewhat of a disappointment for the site's
operator only being able to achieve full production at the beginning of the year.
Its short service life has been plagued with issues and leaks
and Just a few days before it had been shut down to a suspected leak.
By the late afternoon the site would be wiped off the face of the map
in a massive and disastrous explosion.
Many would be killed, many more injured
and millions of pounds worth of damage would lay in the explosion's wake.
My name is John. This is plainly difficult and today we are looking at the flixborough disaster.
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Background
Flixborough is a village in north Lincolnshire England
which is around here on a map.
Half a mile away from
the sleepy village in 1938 will welcome a new neighbour, nitrogen fertilisers limited.
The company planned to use a 60 acre site
for the production of ammonium nitrate, but 26 years later the area would be repurposed.
Nitrogen fertilisers ltd parent company
fission limited merged with Dutch state mines to create a new company called nypro limited.
This company was going to repurpose the site away from fertiliser production
to the creation of caprolactam, an intermediate compound used in the manufacture of nylon.
An initial unit with an output of 20,000 tons per year was commissioned in 1967 using
phenol hydrogenation, but the company sought out to increase production.
This would be via a cyclohexane oxidation process
and would require nypro to build a new plant onsite; this would be given the name phase 2.
Completed in 1972, the new installation made use of a chain of 6 reactors.
Whilst we are here lets take a look at how the whole process ran.
Each reactor was set 35cm 14 inches below the next in a line,
this was to make use of good old gravity during the production process.
Each reactor had a capacity of 45m3 5 m high with a diameter of 3.5m,
the walls consisted of 13mm steel with 3mm on the inside made of stainless steel.
The reactors also had a centrifugal agitator and had 28inch 711mm diameter
pipes with expansion bellows on each end to connect the 6 together in sequence.
The process of oxidation made use of all 6 reactors, inside
25m3 of cyclohexane is pressurised to 8.8bar and heated to 155oc.
Compressed air was injected and circulated via a perforated ramp
inside the reactors in the presence of a catalyst.
The liquid passed to each reactor via the pipe work and
by the time it reached the outlet of the 6th around 6% of the cyclohexane had reacted.
The remaining material was reprocessed removing unwanted byproducts and sent
back through the reactors, The average flow through the system was 250-300m3 per hour.
To heat up the cyclohexane a heat exchanger is used that employs steam.
The Steam flow to the exchanger is controlled by a valve,
which can be controlled manually by the control room or automatically.
To bypass the valve a block valve is provided, this was closed on start-up meaning the only
way to avoid overpressure is through the safety valves or manual intervention.
During the reactions some gas is given off and this is routed back via an off gas line to the
heat exchanger and then to the atmosphere via a cooling tower, scrubber, absorber and flare.
The off gas line served to equalise pressure in all the reactors, safety valves of each reactor
was also provided which would vent gas to the scrubber and flare, this was set at 11kg/cm2.
The new installation proved to be a little bit of a disappointment with
technical difficulties and leaks leading to lower than anticipated production levels.
A leak is discovered
The lacklustre production figures would be further hindered by the discovery
of a cyclohexane leak around reactor 5 on the 28th March 1974.
An investigation was undertaken the same evening
and a crack was discovered in reactor 5’s steel wall.
Management agreed to shut down and cool off the installation
for a more in-depth investigation in the morning.
In the light of day the situation must have felt like it was going from bad to catastrophe.
A 6ft crack was found in reactor 5, a managers meeting was called to discuss what to do next.
With production now completely halted,
the management team brainstormed what the best way to restart production would look like.
It was agreed that reactor 5 be removed for inspection,
but in Lieu of a replacement, a bit of clever plumbing could be employed.
Instead of reactor 5 they would install an extended
bypass pipe section to link reactors 4 and 6:
This would make use of the already in situ 711mm 28inch pipe and bellows for
the connections between 4-5 and 5-6 but be extended with 500 mm 20 inch welded pipes.
Can you see the potential problem?
Once completed the installation would be able to resume production.
A later court of inquiry report would point out three major concerns that weren’t addressed.
The first being that management still didn’t know the cause of the crack,
by not ascertaining what made it meant the same failure could repeat on the remaining 5 reactors,
they all were pretty much identical after all.
The second being that no one thought to inspect
the other reactors for any similar failures in their steel structures.
And finally the third was that the new bypass pipe would have an inlet and outlet
at significantly different heights to a fully working installation.
As the drop between 4-6 was twice that of between the other reactors.
In order to connect it all together a dog leg pipe was fabricated out
of 20inch diameter pipe welding them together.
This in addition to the restriction provided by
the reduction in pipe width created a pinch point in the installation.
The calculations on the loads the new pipe could take
showed it to be allowable, however the dog leg part and its welds were not checked.
This failed to meet the British standard at the time and would be
unallowable if inspected by a qualified engineer.
You might ask why use a 20inch pipe. Well, it was all they had on site.
After reactor 5 was removed a scaffold was built around where the replacement pipe would go.
Fabrication of the dog leg was completed on the 29th March,
and was then installed into position for pressure testing.
Using nitrogen the pipe work was tested to 4kg/cm2, after no leaks were found
it was then tested to 9kg/cm2 again no leaks presented themselves.
But like the whole design process this testing stage failed to meet British standards
which would have required the pipe work to be hydrostatically tested
at a pressure of no less than 11kg/cm2 i.e. the pressure the safety valve was set to operate.
This meant that the pipe work was never tested up to the maximum pressure
that could be experienced by the system.
Nypro pressed the installation back to work on the 1st of April,
no leaks were detected but no one in particular was keeping an eye on the homemade bypass pipe.
Between the first of April and the 29th of may
the installation was used almost constantly, albeit for two occasions of shut down.
Lead up to disaster another leak
Yet another leak was discovered on the 29th of may,
this resulted in a shut down resulting in repair work being done on the 30th and 31st.
After nitrogen pressure testing the repaired sections seemed leak free,
restarting began on the 31st at 4am, just 1 hour later more leaks were discovered.
After cooling down and inspection these new leaks seemed to have repaired themselves.
Again the reactors were started up and heat was applied, pressure in reactor 1 rose too quickly,
and again leaks were discovered requiring yet another shut down.
In the early hours of the 31st the operators were confronted with a broken
plant and to add further frustration the tools needed to repair it were locked away out of reach.
The reactors would be given the ok for restarting on the 1st June 1974.
Disaster
It’s 7am and a new shift of operators are starting work, the time between the
previous leaks and this teams book on to duty is pretty hazy even the official report states that
“The evidence with regard to the commencement of the shift is unsatisfactory”
Multiple witness accounts contradict what state the state plant was in,
whether it was pressurised or not.
But by around 8am
a reading of 8.5kg/cm2 had been achieved at a low temperature from nitrogen pressurisation.
The previous shift had reported a high pressure event requiring ventilation took
place in the early hours of the 1st but not much else was thought of it.
We also know that no leaks were found or at least reported.
It was decided to begin the startup process by using steam into the heat exchanger
to warm the Installation up to 160c at 0930.
By 12pm the process control technician noticed the
pressure at a point where intervention was required.
Reducing the steam yielded no result by now the pressure was at 9.2kg/cm2,
the technician sent an operator out to open the block valve.
This did get the pressure to below 8.8 kgcm2.
The system was in a pretty difficult state as even though its pressure was now
satisfactory the reduction in steam had reduced the vital temperature needed to start up.
Although we will never know for certain and that will become apparent
shortly it is likely the steam bypass line was opened to allow heating.
But again likely a whole new situation of high pressure requiring venting would present itself.
Needless to say, the 1st of June was proving to be a difficult shift to get started.
But their day would go from frustrating to fatal just a few hours later.
At around 4:51pm the 20 inch bypass connection failed at its bellows
releasing upwards of 60 tons of cyclohexane,
this came into contact with the reforming tower of the nearby hydrogen plant and ignition started.
At 4:53 pm an explosion felt up to 50km away enveloped the flixborough plant.
The flames set off secondary fires around the large site up to 100m tall.
The control room which housed most of the vital equipment for monitoring the reactors
was completely destroyed, taking with it 18 staff who worked within.
Luckily due to it being a weekend fewer staff were on site but the death toll was still significant
at 28 with an additional 36 injured and another 50 injured off site by debris.
For context if this disaster happened on a Monday around 500 people would have been onsite.
Over 2,000 buildings would receive varying amounts of damage,
and the emergency response teams would be fighting the fire for over two days.
Aftermath
An investigation would start quickly after the explosion,
but one glaring problem immediately became apparent.
Remember when I said about some parts of the chronology leading up to the explosion,
being a little bit hard to know for certain.
Well the explosion destroyed a lot of the valuable evidence and vital first hand accounts of events
in the last few moments.
The difficulty would result in a bit of a headache for investigators but nevertheless the court of
inquiry sat between September 1974 and February 1975 and examined physical and witness evidence.
The report would be published later on in 1975
and would summarise the official theory of the disaster.
The road to explosion started with the removal of reactor 5,
and its subsequent replacement with the bypass assembly.
The blame falls squarely at the feet of this assembly not being built
and tested to the relevant standards.
Also during the design of the dog leg assembly, which was done on the workshop floor no less,
no one thought to consult the bellows manufacturers design
guide which would have specifically pointed out that it was not suitable for such an application.
For nearly a month the bypass section was pressurised and depressurised
weakening the section, to the point that the pressure
and temperature used in day to day operation caused a rupture.
I can show you in this demonstration,
the more I bend the metal the more it gets weak and eventually will fail.
The report absolved blame of the control room staff
on the day as they had acted within the rules set out in relieving pressure when needed to.
Also the staff to manufacture the dog leg were absolved of blame as they were simply
building what they were told, and the workmanship of the welds was acceptable.
There was at the time a contending theory, it was generally accepted that the 20inch installation
had failed but the initiating events were argued to have come from an 8 inch pipe nearby,
which after a gasket failing caused a fire which then caused the explosion.
Regardless The blame falls on those who decided that this was the right
route to take, instead of stopping production, investigating reactor 5 and then reinstalling.
But there is another all too familiar factor to this event, financial.
You see the UK in the 1970s
could only be described as a mess, the country was in the grips of the miners strike.
Because of the state of emergency enacted by the British government,
industry was limited to just 3 days of electricity per week.
In order to save energy the agitators in the reactors were turned off.
This though was just the cherry on the turd cake for nypro,
as the caprolactam unit had for its short 2 year life never reached the
predicted levels of production which by 1974 was severely financially straining the company.
From a management perspective the plant had to run to be able to try and claw back some money.
Needless to say we can put some of the blame of the disaster and
inadequate reactor 5 bypass down to financial pressures.
Management were not against breaking the rules as
it was found to be holding far more chemicals on site than licenced to.
Hundreds of thousands of gallons more in fact.
The company also undertook an internal investigation into the event
and found that there had been several minor near misses involving onsite fabricated fixes.
The disaster at flixborough and the disaster in seveso Italy in 1976,
video available in the plainly difficult arhicves,
helped push through better awareness and laws regarding industrial safety.
This would result in the seveso directive of 1982.
The plant at flixborough to much protest was actually rebuilt and reopened,
but nypro uk ended up having to dig deep in the pockets in the aftermath.
At least 6,000 public liability insurance claims were filed by 1975,
costing around £24m roughly 150 million today.
Now where would you rate this event on my disaster
and legacy scales I’m going to say about a 6 for both.
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https://www.aria.developpement-durable.gouv.fr/wp-content/files_mf/FD_5611_flixborough_1974_ang.pdf
https://www.icheme.org/media/17752/the-flixborough-disaster-report-of-the-court-of-inquiry.pdf