If the charge material gets stuck in the furnace shaft in cupola furnaces.
The most common reason for this phenomenon is the use of scrap or pig iron which is too coarse.
If material gets stuck, the portion of the charge located below the bridge thus formed slides down due to the continued melting process causing a cavity in the furnace. Clogging can be recognized by a rapid decrease of the blast pressure upstream of the tuyeres.
If the furnace continues to blow, the bed coke is burnt (s. Cold blasting), and when the inhibiting charge descends, unmelted material can reach the back of the tuyeres which results in a major decrease of the spout temperature of the iron. This mainly occurs if the furnace has been clogged for an extended period, and the coke split has completely burnt down.
Clogging, which is also indicated by a more intense upper fire and mainly occurs in furnaces of smaller diameters, can often be removed by simply turning on and off the air blast. In case of more severe clogging the only remedy is to turn off the air blast and push a long pole in the charging opening. This task is very time consuming and encumbered by the high heat at the furnace throat. A better option is to make sure from the beginning on that only scrap and pig iron with a length corresponding to the furnace diameter is charged. After the charge material has slid down, it is usually necessary to operate the furnace at a reduced blast rate due to the reduced bed coke level and to implement corresponding measures to replenish the bed coke to the previous level.
Cupola furnace network diagram