Which of the following is NOT considered a cause of furnace explosions?

The reason why the choice that mentions "correct fuel flow before ignition source" is not considered a cause of furnace explosions lies in the fundamental safety procedures required in furnace operation. Properly regulating fuel flow before introducing an ignition source is a critical safety measure. It ensures that the right amount of fuel is present to achieve combustion efficiently without creating dangerous conditions.

In contrast, the other options represent scenarios that can lead to unsafe situations. For instance, having the main fuel valve closed when the burner flame is lost can lead to an accumulation of unburned fuel, which poses a significant explosion risk if ignited. Similarly, insufficient purge can prevent harmful gases from being adequately cleared from the furnace, increasing the risk of explosion when an ignition source is introduced. Lastly, burners lit from hot refractory can also be hazardous, as it may ignite residual fuel that has not been adequately purged. Therefore, maintaining a controlled and safe fuel flow is fundamental to preventing explosions in furnace operations.