Main cause of boiler furnace blow back is accumulation of hydrocarbons inside the furnace. Hydrocarbons may exist as vapours or liquid form inside the boiler furnace. Accumulated fuel vapors inside furnace catches fire and develops high pressure inside the furnace thereby resulting in dangerous explosion. It is a usual practice that when there is frequent flame failure, operator try to fire the boiler in manual mode. In manual mode pre purging is not automatically controlled. Hence an attempt is made to fire boiler with insufficient pre purging time and eventuality may be catastrophic explosion.