Failing to do which of the following can lead to explosions in the firebox?

Purge the firebox refers to the procedure of removing any unburned fuel and combustible gases from the firebox before ignition. This step is crucial because, if flammable gases accumulate, they can create an explosive environment when ignited. Ensuring the firebox is purged helps prevent a buildup of explosive mixtures that can lead to dangerous conditions, including explosions.

Regular maintenance involves checking and servicing the boiler, which is essential for safe operation, but it doesn't specifically address the immediate concern of gas accumulation in the firebox. Monitoring pressure levels and controlling the fuel supply are also critical for safe boiler operations, but they primarily relate to managing the operational processes rather than specifically preventing explosive conditions in the firebox. Thus, purging the firebox stands out as an essential procedure specifically aimed at preventing explosions caused by gas accumulation.