There are a number of factors that can lead to boiler explosion. Out of which three most common reasons are excessive boiler pressure, very low water level and improper purging before starting burner. An excessive boiler pressure can be an effect of runaway firing condition in which the burner failed to cut off with failure of one or more safety device. In second condition with a result of low water level cutoff failure with subsequent failure of the duty officer to acknowledge alarm and take action; low water level can lead to localized over heating resulting into deformation of boiler shell even explosion. Some times localized overheating can also be due to heavy scale formation as a result of poor boiler water treatment. The last but not the least is the explosion due to sudden combustion of air fuel mixture in the burner firebox. ……….. I hope thats answer your question. If you still have doubts feel free to request your topic!