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3. Everything / Maths, Science & Technology / Engineering

Created: 15th August 2011
The Queen Victoria Blast Furnace Explosion, 1975
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Mary, Bess, Anne and Vic: steelmen the world over know their names. It's unusual for blast furnaces to be designated with anything more than a number, and the Appleby-Frodingham ironworks in the north Lincolnshire town of Scunthorpe is an exceptional place in other ways too. This is where the sintering of iron ore was perfected, and in all probability it will be the last place in Britain where iron is made. The eastern skyline of the town is dominated by the Four Queens, in their iconic file.

The first two furnaces at the northern end of the site date from 1939. When the third and fourth were added in 1954, this elder pair became Bess and Mary. Victoria was and is the southernmost, rising with Anne from a common cast-house structure. These two are the biggest. Operating at her full capacity, Vicky can produce over a million tonnes of liquid iron in a year.

There is no workplace on earth quite like the cast-house of a blast furnace. Thirty miles west of here, the claustrophobic blackness of the coalpit was familiar to thousands. Thirty miles to the east, the trawlermen set out to dare the lashing grey hell of wintry seas. Here between, the hell-pit is almost literal.

How a Blast Furnace Works

The blast furnace converts iron ore (comprising various phases of iron oxide) into liquid iron, at the same time separating out silaceous material and other impurities in a slag. The furnace consists of a conical structure some 20 to 30 metres in height, flaring out to a maximum diameter just above the tuyere level and narrowing in again to a hearth which collects the molten iron at the base. The furnace is charged at the top with a mixture of pulverised ore, coke and flux (commonly dolomitic limestone). These components of the burden have usually been combined and roasted in order to fuse them into a permeable sinter1.

The reduction reaction and the melting of the iron require high temperatures, with the coke providing both the fuel for combustion and the reducing agent. Liquid iron descends through the burden and collects in the hearth, where it is typically drawn off at intervals of about six hours by breaking out a clay-plugged taphole. A liquid slag of lower density floats on the iron, containing much of the silicon, phosphorus and sulphur present in the original ore. Traces of these elements, as well as an excess of carbon, remain in solution in the iron, but can be brought to controlled levels in refining at the steel plant. The slag is tapped off at intervals through notches above the taphole.

The typical tapping temperature is around 1,600°C. Only the tapping is periodic: the charging process is continuous, and once stopped cannot be restarted without costly repair and months of lost production. Allowing the iron to solidify, or chilling to occur to a degree that encourages concretion on the walls, is economically catastrophic. For this reason, the air-blast that maintains combustion is suspended only with reluctance and then only for short periods, and fine judgement between protecting the asset and avoidance of accident is sometimes needed. On 4 November, 1975, an error in that judgement cost the lives of 11 men in the modern British steel industry's worst disaster.

Torpedo Ladles

The molten iron running from the furnace passes down a channel known as the runner before pouring into rail-bound ladles below. The configuration used at Scunthorpe at the time of the accident had a locomotive pulling two ladle bogies with a combined capacity of 500 tonnes, sufficient to receive the entire cast weight of a single furnace. The ladle design had been changed just over two years before, as a result of the Anchor project that had created Europe's biggest basic oxygen melting shop, a mile away down the line.

Before Anchor, iron was shipped in bucket-like 'Jumbo' ladles, open-topped and as high as they were wide. Their replacement, the 'torpedo' ladle had a horizontal tubular geometry, with about the size and aspect ratio of a railway carriage. Torpedoes are thick-walled refractory-lined cylinders into which the iron enters through a narrow aperture in the centre at the top. This port, less than a metre in diameter, is the only opening in the vessel, a feature designed to conserve heat. The small size of this bottleneck and the lack of experience with the torpedo ladle were principal contributing factors to the disaster, since they created a new risk in the event of a major water leak.

The danger of explosion when molten iron and water come into contact has been recognised for hundreds of years, but it used to be thought that the risks were low as long as the water was on top of the iron. If liquid iron is poured onto water, or even onto a wet surface, the chilling effect can lead to the formation of a solidified crust under which pressure builds as the water flashes off to steam. The crust may then fail explosively, flinging out hot metal.

Water on top of iron is usually much less dangerous, because the steam is not then entrapped. Water spraying can be safely used for the cooling of cast iron pigs, for example, in order to induce their solidification. Even if there is a water leak during casting from a blast furnace, therefore, and even if a large flow of water enters the runner, then the explosion risk should be minimal once the iron stream has been staunched. Any water pouring onto the top of a partly filled ladle will flash off.

The disaster at Scunthorpe qualified this piece of empirical knowledge, however. The safety rules had been learned on the old, open-top ladle type. They had never yet been tested under conditions of a high water flow into a laden torpedo.

Preparations for the Cast

On the night of the disaster, the loco drew up below the cast-house floor at 1.05am. The runner spout was set in position by crane a few minutes later. This component weighs nearly a tonne, and its function is to direct the stream of molten iron from the end of the runner down into the torpedo mouth.

In 1975, Vicky was in her seventh campaign since her original blowing-in. After three years or so of operation, the lining of a blast furnace needs to be replaced, and the furnace is taken out of use for several months to carry out this work. Her latest campaign had begun in May 1974, and the condition of the furnace was generally good, except that there had been a spate of cooling water leaks in recent weeks.

Blast furnaces need a lot of cooling, and not only to the casing itself. The air-blast that drives combustion is preheated to around 1,000°C using stoves, and enters the furnace via a girth-ring of around 30 large nozzles called tuyeres. The tuyeres incorporate cast copper cooling jackets and each one is fed with pressurised water at around ten litres per second flow. In two weeks during September there had been half a dozen tuyere cooler failures, probably exacerbated by a recently-adopted maintenance practice of replacing worn copper blanking plugs with steel ones.

There were no signs of leaks on this shift, however. At 1.15am, the taphole drill was moved into position, in readiness to begin the cast. By 1.25am the cast was proceeding without any problems, filling the first torpedo shortly before 2.00am. The iron notch was isolated while the loco pulled forward to present the second torpedo, and the metal flow was restored. Everything remained normal around the furnace at this time, and the loco was decoupled and moved off to an adjacent furnace to shuttle more ladles.

Precursors to the Disaster

Until 18 months before, the Borough of Scunthorpe had boasted an exemplary industrial safety record, in spite of there being several highly energetic and potentially explosive plants within its boundaries. The copybook was blotted by what was then Britain's most expensive accident, at Nypro's Flixborough plant some seven miles away on the Trent bank, on 1 June, 1974. Designed to manufacture caprolactam for processing into nylon, the plant's safety was compromised by a jury-rigged pipeline temporarily joining two reactor vessels. The resulting blast killed 28 men, flattened surrounding houses and set a fire that burned for ten days.

The letters of congratulation to the town council, sent to mark the inception of the new Health and Safety Executive just three months previously, didn't read so well after that. Now Scunthorpe's record was about to switch from one of the safest industrial towns in Britain to one of the most blighted.

It was 2.15am when an alarm sounded in the cast-house control cabin, indicating excessive heating somewhere among the exterior equipment of the furnace. An inspection identified a burndown in the blowpipe of the No.3 tuyere, the pipework that directs the pressurised and preheated air from the stoves to the tuyere itself. Flames were soon visible in the area and a crack opened, allowing a jet of hot blast air to escape. It played tangentially along the furnace wall, projecting a shower of debris some five metres forward of the furnace. Without the pressure of the blast to contain it, material from within the furnace began to be forced back through the tuyere by the weight of the burden, where it was drawn into the jet and sprayed out like the paint from an airbrush.

This was a dangerous situation, but not yet a critical one. The foreman gave the instruction to reduce the blast pressure. The furnace keeper attempted to play a hose onto the damaged pipe, with the intent of freezing the leaking debris and effecting a temporary seal. Getting close to such a leak is far too dangerous to be risked, however, since the escaping jet can change direction in a instant and would incinerate anyone in its path. The burndown worsened, and a cooler leak on the adjacent No.2 tuyere was soon detected. Now the furnace was going to have to come off blast altogether to replace the damaged tuyere stocks, and completing the cast and removing as much molten metal and slag from the furnace as possible became a matter of necessity and expediency.

Hot Metal

Few people guess what molten iron is like. They expect something like the familiar depiction of lava, angry red and viscous. In reality, the metal is a blistering yellow-white, too bright to look at, and it flows with the mobility of water, only with seven times the density. When the tap-hole is opened, it streams down the runner and thunders into the ladle below with an energy that demands respect.

The ironworks shift manager arrived in the cast-house at 2.30am and immediately assumed control of the recovery from the emergency. There was by now a persistent and unquantified flow of water from the No.2 tuyere cooler passing down the runner and away over the spout. Steam was copious and filled the void below the floor where the ladles stood.

There was no panic, however, and indeed no cause for it. Situations like this one, though infrequent, are within the experience of every seasoned furnace crew and training ensures their careful negotiation, with priority at all times on the safety of the men. In the last minutes before the blast, the incident appeared to be under control. There was no safe approach to the iron notch area, however, and so the taphole could not be sealed and iron was still running out of the furnace and into the ladle. Fortunately, the natural end of the cast was approaching.

The pressure in the bustle main now fell to a level low enough for the whole furnace to be taken off-blast. The foreman telephoned traffic control and asked them to return the loco. As the flow of iron in the runner ceased and the danger receded with it, there must have been a sense of relief among the crew. There was still a lot of work to be done, though, and it needed to be done quickly in order to preserve the furnace. Men began entering the area in preparation for repairs to the damaged tuyere stocks.

The shunter below kicked out the wooden scotches that chocked the torpedo bogies in position on the rails. He returned to his cab, and acknowledged the signal to pull away. The couplings tightened. It was 2.47am.

The Inquiry

The Court of Inquiry and the subsequent report of the Health and Safety Executive described the scene at dawn the next day. The loco and the ladles, all derailed, remained where they had come to rest, some five metres away from the chocking position. The cast-house floor above and substantial parts of adjacent structures had collapsed into the trackside area. The exterior cast-house wall had been demolished and the control cabin was extensively damaged. The floor area that was still intact was strewn with fused slag and iron, and with broken bricks and glass. Everything was covered with a thin plating of iron.

Four men were killed outright. Seven others died later in hospital as a result of their injuries. Ninety tons of iron were thrown from the ladle, some half of which entered the cast-house area. The runner spout was recovered from the cast-house roof.

The authorities also drew conclusions about the causes of the explosion and about measures to prevent its repetition. In the confines of a torpedo ladle, they observed, water on the surface of molten iron is not safe after all. When applied in high volume over several minutes, a chilled crust will form on top of the iron bath. Boiling is slowed by the insulating effects of this crust, so that water will then fill the void above it. If the ladle is moved, the crust may fracture, bringing a large volume of molten iron and water into sudden contact in a confined space. There is evidence that, in this case, water flowed under the still largely intact crust, and that steam pressure then lifted this crust to block the mouth and effectively seal the torpedo. A catastrophic escalation of pressure is believed to have ensued, with the crust finally bursting explosively a few seconds later.

The steel plug in the No.2 tuyere cooler was also found to be badly corroded. It experienced significantly different thermal expansion from the surrounding copper, when subjected to forced cooling through the attempts to contain the burndown at the No.3 position. For both these reasons, the original copper component would have stood a better chance of constraining the leak, perhaps to a degree that would have prevented the disaster. The inquiry recommended that the practice of replacement using steel plugs should be suspended with immediate effect. It further proposed that some easily-actuated means of diverting water flow away from the torpedo mouth should be designed and implemented in the spout area.

Notwithstanding these recommendations, the direct cause of the accident was found to be the moving of the locomotive. There would probably have been no explosion if the torpedo had been left to freeze. The decisions taken by the men present were nonetheless rational and blameless. All of their experience up until that time, as well as all the emergency procedures, suggested that the ladle could be safely moved away from the still-flowing water stream, and indeed that leaving it in place was the more dangerous and ultimately very costly alternative.

The Queen Victoria Blast Furnace Disaster provides a telling case study for a particular kind of industrial hazard, and the inquiry report concludes with an observation that explains it. Wherever and whenever the introduction of new technology is considered, it is imperative that the established norms of operational safety are reviewed. A safe method for dealing with water in a Jumbo ladle turned out to be unsafe with the new torpedoes. The risk could have been foreseen based on existing knowledge if the scenario had been analysed. A tragedy occurred because nobody realised the need to do so.

Queens' Approach

The memorial plaque was made, of course, of iron from the Queen Victoria furnace. You can see it up there on Prospect Avenue, still diligently cleaned, and lit at all times. There is dignity and calm in this place, a haven amid what will always be a truly terrible environment. The towering furnaces, and the tempest of energy they contain, might once have distracted us from the humanity of the enterprise. We might have overlooked the men like us who work here, but never after this.

There were 23 men in the direct vicinity when the explosion occurred. Of the survivors, six were both able and minded to return to work at British Steel. For them as for others, the ironworks was for a while a subdued and frightening place.

The spirit will out, though. There is something irrepressible and heroic about blastfurnacemen. There is a pride and privilege in working there, even if only for a little time. Iron, and the men who make it, will always be special.


1 Sinter is a porous agglomeration of metallic particles fused together by heating to an elevated temperature.


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ENTRY DATA
Written and Researched by:

Pinniped

Edited by:

Lanzababy - h2g2Guide Editor - write for the Future Guide - we need your contributions :) Lanzababy

Referenced Entries:

An Unreliable History of Steelmaking
Scunthorpe, North Lincolnshire, UK
Crucible Steel-making
Henry Bessemer and the Development of Bulk Steelmaking



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