I was under the assumption that boiler explosions were not so much an
EXPLOSION as the boiler turning itself into a rocket. As I understand
it, the water level would fall under the level of the crown sheet of the
firebox, letting the heat soften it enough that the steam pressure could
break through. This pretty much acted as a rocket nozzle and would lift
the boiler off the frame. It also would have the unfortunate
coincidence of roasting everything and everyone unlucky enough to be in
or near the cab at that instance.
This was pretty much solved by putting in a plug with a low melting
point so as the water level approached the crown, the plug would melt
and the water would extinguish the flames in the firebox.
For a boiler to actually EXPLODE I would think would require a
structural flaw or metal fatigue.
