Failure Knowledge Database / 100 Selected Cases

Disaster of Chemical Plant at FlixboroughJune 1st. 1974, Flixborough UK
TAKEGAWA, Tetsuya (Sumitomo Chemical Co., Ltd. OB)
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
The Flixborough Works of Nypro (UK) Ltd. were demolished at about 4:53 p.m. on Saturday June 1st.
1974, by an explosion of warlike dimensions which was the equivalent of 15 tonnes of TNT.
Of those w orking o n th e si te at t he t ime, 28 w ere ki lled and 36 ot hers suf fered i njuries. Outs ide t heworks in juries an d d amage w ere w idespread. 5 3 people w ere rec orded as cas ualties, w hile 1,821 h ouses
and 167 shops and factories suffered damage to a greater or lesser degree. The plant at which the explosion
occurred was part of a complex for the manufacture of nylon, jointly owned by Dutch State Mines (55%)
and UK National Coal Board (45%).
1. Component
Temporary bypass pipe for reactor and process pipe between separators.
2. Event
The reaction was cyclohexane with air under existence catalysis by 155℃, 125 lb/sq.in. (0.86 MPa), toa mixture of cyclohexanone and cyclohexanol that is usually known as KA (ketone/alcohol) mixture.
The re action took place i n six vessels, each hol ding about 20 tonnes. One of t he rea ctors, t he No. 5
reactor, developed a crack and was removed for repair. In order to maintain production, a temporary bypass
pipe was installed between the No.4 reactor and the No.6 reactor. Because the reactors were mounted on asort of sta ircase, t his p ipe was no t s traight b ut contained t wo bends. The pipe was 2 0in. (508 mm) i n
diameter, although the short pipes that were normally used to join the reactor together were 28in. (711 mm)
in diameter. Bellows also 28 in.(711 mm) in diameter, were installed between each reactor, and these wereleft at each end of the temporary pipe.
This t emporary by pass pi pe perform ed sa tisfactorily fo r tw o months after th e p lant w as rest arted o nApril 1st. However, the process pressure rose slightly, from 125 lb/sq.in. (0.86 MPa) to 129 lb/sq.in. (0.89
MPa ).

The bending moment, caused by the act ion of this sli ght rise in press ure, was strong e nough totear the bellows. The temporary pipe acted to twist the flow, and the bellows were ruptured by shear stress.
As a result of the ru pture of the be llows, a great am ount of cy clohexane escaped from the hole s in th e
bellows and formed a cloud of cyclohexane vapour, which subsequently caused the explosion (Fig. 1).
This ex plosion killed 28 m en w ere and injured 3 6 m en on si te. T he o xidation unit an d ne ighboring
units were destroyed and extensive damage was caused to the rest of the s ite. In particular, the company
office block, about 100 m away, was destroyed.
An initial investigation at the site found a "S" shape 20in. (508 mm) pipe assembly lying on the concrete
1Failure Knowledge Database / 100 Selected Cases

plinth be low its ori ginal position. It h ad jack- knifed c ompletely at t he l ower mitre jo int and a pparently
collided violently with the plinth after being projected forcibly downwards.
Both of the bell ows ha d t orn away fro m the tem porary pipe . They had als o been torn off t he r eactor
nozzles. Eac h of the bel lows had disintegrated into a few lar ge pi eces, all of w hich were found in thevicinity of the failed pipe.
In addition, the initial investigation revealed that a 50 in. (1270 mm) split had occurred in the bend in an
8 in. (203 mm) diameter stainless steel pipe joining two nearby separators.
3. Course
The cycl ohexane oxi dation process was performed i n a seri es of s ix re actors, each holding about 20
tonnes. The reactors were about 12 feet (3658 mm) in diameter and 16 feet (4877 mm) high, and they wereconstructed of 1/2 in. ( 13 mm) mild steel, w ith i nternal cl adding of 1/8 in. ( 3 m m) stainless steel ( type
316L).
After fresh cyclohe xane and recycle cyclohexane were scrubbed with water in a c ooling scrubber, they
were heated in a direct heat exchanger. Subsequently, both forms of c yclohexane were introduced into theNo.1 reac tor and oxidized to a m ixture of cycl ohexanone and cyclohexanol, usual ly kn own as KA(ketone/alcohol), with air under the existence of a catalyst, at 155℃ and 125 lb/sq.in. (0.85 MPa).
At the end of March 1974, the No.5 reactor was found to be leaking, and an inspection showed a crack
over 6 feet (1830 mm) long in the mild steel. Subsequent examination of the crack by Dutch State Mines(DSM) determined the cause of the failure to be nitrate stress corrosion cracking of the mild steel cladding.
This nitrate stress c orrosion crac king was be lieved to have been due to t he pra ctice of spray ing
nitrate-treated cooling water as a means for diluting and dispersing small leaks.
It w as decided to remove the N o.5 reactor, put in a temporary bypass pipe, and restart the plant. The
bypass consisted of a dog-leg pipe from 20 in. (508 mm) diameter 304L stainless steel, installed between
two expansion bellows that were attached to the 28 in. (711 mm) diameter nozzles after the reactor removal.
The bypass was never closely inspected, although at operating temperatures and pressures, it was observed
to lift off the scaffolding supports that were put in during the installation.
After the p lant was rest arted o n Ap ril 1s t.initially this

temporary pipe c onnection fu nctioned

satisfactorily. However, on May 2 9th, a leak w as fo und o n s ight g lass, an d the plant w as sh ut down forrepairs. An attempt to restart the plant was made at 4:00 a.m. on June 1st. More leaks were found, and after
fixing t hese a nd restart ing, the press ure was no ted to rise m ore qui ckly than us ual to 1 25 l b/sq.in. (0.8 6MPa) gauge, well before the first reactor had reached operating temperature. Before any venting took place,
however, another leak developed, the heating was stopped, and the pressure dropped to 6 4 lb/sq.in. (0.44MPa) gauge.
The 7:00 a.m. to 3:00 p.m. shift fixed the leak and began warming the reactors up at 9:30 a.m., needing
to vent only once at 11:30 a.m. when the pressure had reached 129 lb/sq.in. (0.89 MPa) gauge, but did not
vent. The nitrogen stock for purging was found to be insufficient, however, and as a fresh delivery was notexpected before midnight, the system was kept on "dry-cycling", i.e., recycling of hot cyc lohexane under2

Failure Knowledge Database / 100 Selected Cases

pressure but with no adm ission of a ir f or reac tion. At th e e nd of th e 7 :00 a .m. to 3 :00 p .m. shift, t hetemperatures in the reactor system had not leveled out.
The temporary pipe w as d eformed in a " V" shape by ben ding s tress at only sli ghtly abo ve o peratingpressure, and the bellows, the weak link in the chain, were torn away by shear stress. As a result, a massive
vapor cl oud was form ed by the esc ape of cy clohexane fro m the holes of t he ruptured 28 in . (711 mm)
diameter bellows, and subsequent ignition caused the explosion to occur.
The ex plosion, w hich occurred 4:53 p.m., w as est imated t o h ave a n eq uivalent force of at le ast 1 5
tonnes of TNT. What happened on the final shift can never be known because all those in the control room
were killed and all instrumentation and records were destroyed. The explosion also destroyed the oxidation
units and neighboring units and caused extensive damage to the rest of the site.
The initial site investigation had revealed a 50 in. (1270 mm) split had occurred in the bend in an 8 in.
(203 mm) diameter stainless steel pipe joining tw o nearby separators. A leak occurred at a flange on the
non-return valve, located near this 50 in. (1270 mm) split. As a res ult, spontaneous combustion or a s pray
which were ignited by induced electrostatic charge, the result being a flame directed into the inside of an 8
in. (203 mm) bend.
After the explosion, a metallurgical investigation of the 8 in. (203 mm) line y ielded considerable data
concerning t he ef fects of zinc em brittlement an d creep cavitation at h igh t emperatures on austenitic
stainless steel.
At first, it wa s assumed that the assembly failed as a result of a sm all external explosion following theprior rupture of a nearby 8 in. (203 mm) line. However, this theory was rejected by the Court.
4. Cause
After the No.5 reactor was removed for repair, a temporary bypass pipe was installed between the No.4
reactor and the No.6 reactor. The bypass pipe required an "S" shape, because the reactors were mounted ona sort of staircase.
The reas on f or why the bel lows fractur ed by she ar st ress, was that the temporary pipe was i nstalled
without examining what the effect of a slight pressure rise on the bellows would be.
The workers who designed the temporary pipe were not professional engineers. The only calculations
made were of the capacity of the assembly needed to carry the required flow. No calculations were done toascertain whether the bellows or pipe would withstand the forces that would be exerted. No reference wasmade to t he relevant British S tandard or any ot her accepted st andard. No refe rence was m ade t o t he
designer's guide issued by the manufacturers of the bellows. No drawing of the pipe was made, other thanin chalk on the workshop floor.
The support of the temporary pipe was a scaffolding structure upon which the pipe rested, without being
fastened down. Therefore, the support structure could be provide not enough strength to withstand against
bending stress.
The so urce o f ig nition w as pro bably a natural g as r eforming f urnace som e dis tance aw ay. It w as
estimated tha t 30~ 50 tonnes of cyclohexane esca ped in th e 50 s econds t hat e lapsed bef ore i gnition

3

Failure Knowledge Database / 100 Selected Cases

occurred.
Besides these details, the initial site investigation revealed that a 50 in.(1270 mm) split had occurred in
the bend in an 8 in. (203 mm) diameter stainless steel pipe joining two nearby separators. The cause of thissplit is as follows. A leak occurred as a result of two loose bolts at a flange on the non-return valve, locatednear the 50 in. (1270 mm) split. This leak gave rise to an accumulation of oxidizable residues in the lagging
and spontaneous combustion or a spray which was ignited by induced elctrostatic charges; the result beinga flame directed into the inside of an 8 i n. (203 mm) bend. It w as assumed that the 50 in. (1270 mm) split
occurred by zinc em brittlement an d cr eep cavitation at high tem peratures. T his assum ption was la ter
confirmed by a metallurgical investigation showing that zinc embrittlement on austenitic stainless steel at atemperature of betwee n 800 and 900 ℃ could o ccur in a pipe u nder a stress of 3.2 1 t on/sq.in. (4 8.8
N/sq.mm) within a few seconds. The possible sources of zinc that could cause such an embrittlement attack
were the galvanized s tairways and t he wire securi ng the l agging. Furthermore, it w as show n tha t rapi dcreep cavitation of stainless steel may occur within minutes at temperatures of 950℃ or more and under astress of 4.7 ton/sq.in. (71.4 N/sq.mm).
In regard t o t he fail ure of t he tem porary pipe an d a n 8 in. (20 3 m m) pipe , it w as supp osed that theassembly failed as a result of a small external explosion following prior rupture of a nearby 8 in. (203 mm)
pipe. H owever, th e C ourt con cluded t hat both phenomena occ urred as a result of a se quence ofimprobabilities and coincidences.
5. Immediate Action
Immediately after this disaster occurred, UK government ordered that a formal investigation be carriedout by a Court of Inquiry, consisted of a panel of experts.
Experimental work, which included a full scale simulation was carried out at Flixborough. These tests
showed that the bellows squired into on "S" shape at a pressure only slightly above the operating pressure.
Calculations of flow through the 28 in. (711mm) diameter open nozzles of the two reactors supported
the view that the release of the cloud of cyclohexane vapour of sufficient size to cause the disaster rapidly
followed the collapse of the temporary pipe. It is almost impossible to prevent ignition of a leak the size ofthat which occurred at Flixborough. However, it is possible to locate and layout a plant so that injuries anddamage are m inimized if an explosion occ urs. It was suggested th at a di agram be made of t he relation
between the distance from the point of release and the size of the cloud (tonnes of hydrocarbon equivalent)at the plant containing materials that might explode. This diagram shows the restriction on design divided
into s ix areas : A〜F. For exam ple, area A, the re gion within 20 m fro m the point of re lease, s hould notcontain any buildings at all. In area B, t here should not be any other hazardous pla nts or site roa ds. In area
C, th ere sh ould n ot be a ny low pressure tan ks. In area D , roofs of bui ldings sh ould b e in dependently
supported and windows protected, and there should not be any public roads. In area E, there should not be
any houses, and as for area F, there are no limitations on design.

4Failure Knowledge Database / 100 Selected Cases

6. Countermeasure
Because the plant suffered wide-scale destruction, the first step of the countermeasure was to rebuild theplant according to the less ons l earned f rom the d isaster. In t his case, it was ne cessary to consider th e
improvement of the production process. Because the production process was a high inventory process, if aleak occurred, the plant would suffer a great amount of damage. Therefore, if the production process were
designed for inventory reduction, then it might be possible to minimize the damage.
In fact, the producti on process fo r m anufacturing cy clohexanol w as cha nged fr om the oxi dation of
cyclohexane to the hydrogeneration of phenol. However, this process is at least as hazardous.
7. Knowledge
It was well known to metallurgists that water contained nitrates causes stress corrosion cracking of mild
steel. The decision that nitr ate tre ated cooli ng wat er be sprayed on the t op of the reactor, was made bypersons who were not very knowledgeable. In fact, the decision was hardly known to the engineers.
No change in the operating condition outside the approved range should be m ade until they have beenauthorised by a professionally qualified manager.
After th e rea ctor w as rem oved, t he t emporary by pass pipe w as des igned and installed by m echanicalengineers who were not professionally qualified because of the desire to quickly restore production.
No calculations were done to ascertain whether or not the bellows or the temporary pipe would be able
to withstand the strains that would occur as a result of a slight rise in pressure. Also, the supports were notstrong enough to w ithstand the bending stress of the pipe. Consequently, the bending moment caused by aslight rise in pressure was sufficient to cause the temporary pipe to tear the bellows.
Especially in plants that are treating hazardous materials, even relatively easy repairs or improvements
of facilities should be made with thorough reference to the standards, and the design should be entrusted toprofessional engineers.
Occasionally, a large disaster can occur as a result of hot gas spouting to another piece of equipment,such as the gas tha t le aked fro m the 50 in. (1 270 mm) split i n t he bend i n an 8 in. (2 03 m m) diam eter
stainless steel pipe joining two nearby separators. So、it is necessary to consider methods for detecting andpreventing leaks.
For plants containing materials that might explode, the safety of the plant layout and equipment location
should be considered. For instance, no buildings should be within 20 m of plan t, and the construction ofoccupied buildings near the plant, should be strengthened.
On acc ount of th e high inventory of t he production proc ess a t Flixborough, a lar ge am ount of
cyclohexane escaped and caused a large scale explosion. If the plant had applied a low inventory process, alarge scale explosion would not occur. It was not known why Flixborough adopted a high inventory process.
Howevewr, when investigating the process operated at Flixborough, the output of the plant was found to beabout 50,000 tonnes/year KA. Assuming a linear velocity of 0.5 m/s, it could all have passed through a pipe
1.6 in. (40.6 mm) in diam eter. The actual pipe sizes ranged up to 28 in. (711 mm), so the cross section of
the pipe, and thus the flow rate, was (28/1.6)2 = 300 times greater than t he theoretical minimum diameter5

Failure Knowledge Database / 100 Selected Cases

required to support the plant output.
8. SequelIn the UK, the governm ent set up a n Advis ory Committee on M ajor Hazar ds t o consi der t he wider
implications of the Flixborough explosion. It took about ten years f or their recommendations to be m ade
and to come into force. The recommendation of the committee resulted in the establishment of the CIMAH(Control of Industrial Major Account Hazards). The regula tions have now been replaced by t he COMAH(Control of Major Accident Hazards).
The plant changed its production process for manufacturing cyclohexanon and cyclohexanol from theoxidation of cyclohexane to the hydrogeneration of phenol. However, the rebuilt plant as closed down, aftera few years, for commercial reasons.
9. Information Source(1) Sir Fredrick Warner: Chemical Engineering Progress, Vol.71, No.9 (1975).
(2) S.J. Gould: Learning from Accident-3 rd Edition Chapter 8 Culf Professional Publishing.
10. Primary Scenario
01. Ignorance
02: Insufficient Knowledge
03: Lacked Experience
04: Non-Regular Action
05: Change
06: Design Change
07: Poor Design
08: Shearing Stress
09: Usage
10: Maintenance/Repair
11: Pipe
12: Stainless Steel
13: Poor Inspection
14: Failure
15: Large-Scale Damage
16: Buckling
17: Explosion
18: Leakage
19: Ignition/Combustion
20: Usage
21: Operation/Use6

Failure Knowledge Database / 100 Selected Cases

22: Operation
23: Bad Event
24: Mechanical Event
25: Looseness of Pipe Flange
26: Internal Gas Leakage
27: Ignition/Combustion
28: Zinc Embrittlement

7Failure Knowledge Database / 100 Selected Cases

Fig. 1

View of fractured temporary pipe.

8