This article was originally published in the April 1907 edition of “Electrochemical and Metallurgical Industry” magazine. The publication of this material aims to provide historical insight on the subject and its place in industry.
When a furnace is working normally the blast pressure drops gradually from the level of the tuyeres up to the zone of fusion, or say 6 feet, above them, where there is a rapid drop for a short distance and then a gradual drop from this point to the top of the furnace. When a furnace is hanging, especially if it stops moving for a considerable time, the stock burns out below, leaving a cavity. In the case of a blast furnace hanging stubbornly, the interior of the arch formed may look quite smooth, due to fusion going on at this point.
After the cavity reaches a certain size a portion of the stock is liable to drop, breaking the arch and causing the stock above to come down with a rush. At the same time the compressed air in the cavity at the bottom of the furnace is released, and goes up through the falling stock with a tremendous rush. If the top of the blast furnace is constructed with its hopper and cone set on the brickwork and held almost entirely by its own weight, this rush of gases is liable to lift the entire top up, or perhaps throw it entirely off the furnace.
To prevent the upheaval of this bell and hopper apparatus, so-called explosion doors were placed on the down-comers, and in some cases extra openings were carried out through the casings and additional relief doors were fitted to these openings. In the case of heavy slips, where heavy pressures and large volumes of blast are used, the upward rush of the blast picks up the ore and coke and carries it up with it and hurls it out of the relief doors in great quantities, it being not unusual to see from 30 to 50 tons of ore, coke and limestone thrown out of a furnace in less than as many seconds at the same time that a vast cloud of gases and ore dust envelopes the top of the furnace, and some time these gases light as they come out, making a tremendous flame.
Mr. Julian Kennedy took up, several years ago, the problem of preventing these so-called "explosions" with resulting loss of life and other damage. The fundamental point of his solution is that we have not to do here really with an "explosion." What really happens is that it is simply the compressed air from the blast in the bottom of the furnace rushes upwards and does the damage.
By examining the construction of blast furnaces as used at that time in the United States, Mr. Kennedy was unable to find a single blast furnace the top of which could not be lifted by a pressure of 254 pounds per square inch. The throwing off of the top is therefore fully accounted for by the pressure of the compressed air accumulated in the cavity below a handling charge.
Photo Credit: Powerhouse Museum