Of those working on the site at the time, 28 were killed and 36 others suffered injuries. Outside the works injuries and damage were widespread. The reaction took place in six vessels, each holding about 20 tonnes. One of the reactors, the No.

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Of those working on the site at the time, 28 were killed and 36 others suffered injuries. Outside the works injuries and damage were widespread. The reaction took place in six vessels, each holding about 20 tonnes. One of the reactors, the No. In order to maintain production, a temporary bypass pipe was installed between the No. Because the reactors were mounted on a sort of staircase, this pipe was not straight but contained two bends.

The pipe was 20in. Bellows also 28 in. This temporary bypass pipe performed satisfactorily for two months after the plant was restarted on April 1st. The bending moment, caused by the action of this slight rise in pressure, was strong enough to tear the bellows. The temporary pipe acted to twist the flow, and the bellows were ruptured by shear stress. As a result of the rupture of the bellows, a great amount of cyclohexane escaped from the holes in the bellows and formed a cloud of cyclohexane vapour, which subsequently caused the explosion Fig.

This explosion killed 28 men were and injured 36 men on site. The oxidation unit and neighboring units were destroyed and extensive damage was caused to the rest of the site. In particular, the company office block, about m away, was destroyed. An initial investigation at the site found a "S" shape 20in. It had jack-knifed completely at the lower mitre joint and apparently collided violently with the plinth after being projected forcibly downwards.

Both of the bellows had torn away from the temporary pipe. They had also been torn off the reactor nozzles. Each of the bellows had disintegrated into a few large pieces, all of which were found in the vicinity of the failed pipe. In addition, the initial investigation revealed that a 50 in.

Sequence The cyclohexane oxidation process was performed in a series of six reactors, each holding about 20tonnes. After fresh cyclohexane and recycle cyclohexane were scrubbed with water in a cooling scrubber, they were heated in a direct heat exchanger. Subsequently, both forms of cyclohexane were introduced into the No. At the end of March , the No. 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 corrosion cracking was believed to have been due to the practice of spraying nitrate-treated cooling water as a means for diluting and dispersing small leaks. It was decided to remove the No.

The bypass consisted of a dog-leg pipe from 20in. 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 plant was restarted on April 1st. However, on May 29th, a leak was found on sight glass, and the plant was shut down for repairs. An attempt to restart the plant was made at a.

The a. The nitrogen stock for purging was found to be insufficient, however, and as a fresh delivery was not expected before midnight, the system was kept on "dry-cycling", i. At the end of the a. The temporary pipe was deformed in a "V" shape by bending stress at only slightly above operating pressure, and the bellows, the weak link in the chain, were torn away by shear stress.

As a result, a massive vapor cloud was formed by the escape of cyclohexane from the holes of the ruptured 28in. The explosion, which occurred p. 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 50in. A leak occurred at a flange on the non-return valve, located near this 50in. As a result, spontaneous combustion or a spray which were ignited by induced electrostatic charge, the result being a flame directed into the inside of an 8in.

After the explosion, a metallurgical investigation of the 8in. At first, it was assumed that the assembly failed as a result of a small external explosion following the prior rupture of a nearby 8in.

However, this theory was rejected by the Court. Cause After the No. The bypass pipe required an "S" shape, because the reactors were mounted on a sort of staircase. The reason for why the bellows fractured by shear stress, was that the temporary pipe was installed 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 to ascertain whether the bellows or pipe would withstand the forces that would be exerted.

No reference was made to the relevant British Standard or any other accepted standard. No reference was made to the designer's guide issued by the manufacturers of the bellows.

No drawing of the pipe was made, other than in 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 source of ignition was probably a natural gas reforming furnace some distance away.

It was estimated that tonnes of cyclohexane escaped in the 50 seconds that elapsed before ignition occurred. Besides these details, the initial site investigation revealed that a 50in.

The cause of this split is as follows. A leak occurred as a result of two loose bolts at a flange on the non-return valve, located near the 50in. 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 being a flame directed into the inside of an 8in.

It was assumed that the 50in. This assumption was later confirmed by a metallurgical investigation showing that zinc embrittlement on austenitic stainless steel at a temperature of between and degree C could occur in a pipe under a stress of 3. The possible sources of zinc that could cause such an embrittlement attack were the galvanized stairways and the wire securing the lagging.

Furthermore, it was shown that rapid creep cavitation of stainless steel may occur within minutes at temperatures of degree C or more and under a stress of 4. In regard to the failure of the temporary pipe and an 8in.

However, the Court concluded that both phenomena occurred as a result of a sequence of improbabilities and coincidences. Response Immediately after this disaster occurred, UK government ordered that a formal investigation be carried out 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 the28 in. It is almost impossible to prevent ignition of a leak the size of that which occurred at Flixborough. However, it is possible to locate and layout a plant so that injuries and damage are minimized if an explosion occurs.

It was suggested that a diagram be made of the 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 six areas: A-F.

For example, area A, the region within 20m from the point of release, should not contain any buildings at all. In area B, there should not be any other hazardous plants or site roads. In area C, there should not be any low pressure tanks.

In area D, roofs of buildings should be independently 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. Countermeasures Because the plant suffered wide-scale destruction, the first step of the countermeasure was to rebuild the plant according to the lessons learned from the disaster.

In this case, it was necessary to consider the improvement of the production process. Because the production process was a high inventory process, if a leak 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 production process for manufacturing cyclohexanol was changed from the oxidation of cyclohexane to the hydrogeneration of phenol.

However, this process is at least as hazardous. Knowledge Comment It was well known to metallurgists that water contained nitrates causes stress corrosion cracking of mild steel. The decision that nitrate treated cooling water be sprayed on the top of the reactor, was made by persons 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 made until they have been authorised by a professionally qualified manager. After the reactor was removed, the temporary bypass pipe was designed and installed by mechanical engineers 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 not strong enough to withstand the bending stress of the pipe. Consequently, the bending moment caused by a slight 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 to professional engineers.

Occasionally, a large disaster can occur as a result of hot gas spouting to another piece of equipment, such as the gas that leaked from the 50in.


Flixborough (Nypro UK) Explosion 1st June 1974

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Daily coronavirus briefing

Friday 30 May Mark Dunton Records and research 8 comments. It was the biggest explosion to ever occur in Britain during peacetime , until the fire at the Hertfordshire oil storage terminal Buncefield in December At Flixborough, 28 workers were killed and 36 others onsite suffered injuries. Outside the works, injuries and damage occurred on a widespread scale but there were no fatalities. It was recognised that the number of casualties would have been even higher had the incident occurred on a weekday. The explosion was estimated to be equivalent to 16 tonnes of TNT and the subsequent fires raged for ten days.


Flixborough, 1 June 1974

Forty-five years ago today, on June 1, , the Nypro chemical plant at Flixborough, south of the Humber and north of Scunthorpe, exploded. It killed 28 workers, wrecking property over a wide area and sending debris across the Humber to rain down on Hull and its surrounding area. Writing to commemorate its 40th anniversary five years ago, Stuart Russell, working for the Hull Daily Mail at the time of the disaster, looked back on his memories of one of Britain's biggest peacetime explosions. Only two hours earlier, children played here, families gathered for tea and, for some, it was time to relax in well-tended gardens. Now, only their wrecked houses remained, stark and empty against a back-drop of smoke and fire.


The Flixborough disaster that killed 28 people and left debris crashing down on Hull

The Flixborough disaster was an explosion at a chemical plant close to the village of Flixborough , North Lincolnshire , England on Saturday, 1 June It killed 28 people and seriously injured 36 out of a total of 72 people on site at the time. The casualty figures could have been much higher, if the explosion had occurred on a weekday, when the main office area would have been occupied. The disaster involved and may well have been caused by a hasty modification. There was no on-site senior manager with mechanical engineering expertise virtually all the plant management had chemical engineering qualifications ; mechanical engineering issues with the modification were overlooked by the managers who approved it, nor was the severity of the potential consequences of its failure appreciated. Flixborough led to a widespread public outcry over process safety.

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