Forensics

Electric Arc Steelmaking Furnace Accidents

Forensic Services Newsletter

The proportion of the world's steel produced by the electric arc furnace process has steadily increased in recent decades and now accounts for 40% of total steel production. It is one of the most demanding applications in industry, with typically over 100 tonne of scrap metal being melted, refined and discharged from a furnace up to 24 times a day. The melting point of steel is 1500°C, higher than other common metals, and when things go wrong the results can be spectacular, with much damage and possible loss of life. We are seeing more breakdowns, even to the extent that we have one consultant earmarked for breakouts, another for conveyors and a third for electrical problems. It is appropriate that we describe some of these failures. The design of an electric arc furnace is shown on the right, together with workers holding an oxygen lance and flinching from sparks from the furnace.

Explosions

Following an explosion, the refractory of the furnace had to be removed to allow the distortion of the shell to be evaluated. The photograph shows the shell after extensive weld repair.

The two main causes of explosions are 'explosive' items in the scrap and the 'mixing' of water and steel. Considering that the raw material is scrap, there is a potential for explosives, pressure vessels and pockets of water to be accidentally introduced into the furnace. The roofs and sides of electrical furnaces are water-cooled and the accidental introduction of water onto the surface of the pool of molten slag/steel happens from time to time, usually without damage. It is when water somehow becomes trapped beneath molten steel or slag that damage and injury can occur.

Breakouts

Inside and outside views of the steel shell of a furnace, damaged by breakout. The refractory was removed to allow the shell to be repaired.

Another of our newsletters describes breakouts from glass and metal furnaces. The location of the breakout is crucial to the extent of damage to the furnace and surrounding equipment.

Conveyors

Conveyor belts are made of vulcanized rubber (see left) and can themselves catch on fire or accidentally introduce water into the furnace. The natural movement of the belt and the inclination are factors to be taken into account in fire investigation.

Transformers

Arc furnace transformer assembly, as seen from the end opposite from the OLTC.

Being one of the major components in an arc furnace system, the failure of the furnace transformer can often represent a long period of production downtime.

The arc furnace transformer either directly or indirectly by way of a voltage step-down transformer, accepts power from the electrical utility company and supplies power to the electrodes of the arc furnace at relatively low voltages and very high current values.

Arc furnace transformers have tap-changers to enable them to cope with the frequent variation of the voltage/current requirements at the electrodes of the furnace. Modern arc furnace transformers employ OLTC's (on-load, tap-changers) to carry out this task, thus avoiding the necessity to take the furnace 'off-line' while the tapping positions on the high voltage coils of the transformer are changed. There is inevitable wear and tear of critical components.

Air Handling

Eroded water tube-lined duct above an arc furnace. The question was whether damage was a result of an accident or was long term.

Arc furnaces produce vast quantities of dirty air. At the steelmaking plant where the author worked, particularly dirty melts would be made at 2am in the hope that no one would notice. A more modern approach is to have large air handling units that dwarf the furnace itself. When there is an incident, there can be inquiry as to whether the water-cooled air ducts are actually damaged by the event or years of service.

Miscellaneous

We have two miscellaneous accidents to describe, each of which a consultant is unlikely to re-experience in a lifetime. The first, on the left, shows a brand-new furnace being stripped down for repair after the first melt. This was because the water channels were only tack welded during manufacture, allowing water to flow into the furnace.

The second accident began with a new furnace and an inexperienced crane driver. After the liquid steel was tapped into a ladle, the stopper on the ladle jammed. The crane driver lost communication with the foreman and decided for himself where to dump the now-unusable molten steel, with spectacular results, as can been seen on the right.

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