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On my birthday, we have been able to witness a very rare event: a bleeding blast furnace. Under special conditions, the pressure of the recovery gas can exceed a critical point where the equilibrium between the recovery gas travelling from the bottom to the top of the furnace and the burden travelling the opposite direction (counter flow principle) gets lost. As a consequence, either the burden pillar collapses or the furnace's core explodes.

To prevent this, the recovery gas collectors have so-called bleeders mounted at their highest point - spring-loaded pressure relief valves that can be compared to a turbocharger's wastegate. They open autonomously or when manually triggered, and the excess gas evades rapidly with a most deafening noise - in this case for about 20 seconds, until the internal pressure was back to normal.

[Thanks to Oliver of Dillinger Hüttenwerke for the technical explanation]