Interruption of the melting operation in the cupola furnace.

In case of blower malfunctions due to mains failure or other reasons, it might become necessary to shut down an operating furnace for a shorter or longer time. This is less of a problem for furnaces with intermittent tapping than it is for furnaces with siphon tapping since siphons easily freeze and it is very difficult to burn them open. If the malfunction lasts for a longer period, it is mandatory to completely drain the furnace. It is advantageous if the remaining iron is tapped opposite to the usual tapping with the floor declining towards that side. This is to ensure that the tapping or iron and slag siphon is free from molten residues.

The tapping must be closed with a clay plug as used for new tapping. The blast gate must be closed immediately at the beginning of the downtime in order to avoid the ingress of gases containing carbon monoxide generated due to the lack of oxygen in the furnace which could explode when the blow-in process is started. If each blast nozzle can be locked individually, it is advisable to close them as well. If this is not possible, the nozzles must be sealed with clay.

This is to prevent the hot furnace which acts like a chimney from sucking in air which would slowly melt the iron which in turn would freeze at the bottom. The residual tapping must be kept open as long as possible in order to discharge the molten iron and the molten slag since it is not always possible to avoid the melting of additional iron due to the heat in the furnace. After having removed all the slag from the residual tapping, it must be sealed for further blowing which can then be started. Furnaces which undergo this process can be started without any major problems even after a long period of downtime, except for the initially occurring cold iron.

Additional references:
Cupola furnace network diagram
Blast pressure
Hot blast stove
Blast rate