Boiler fire

There are three main stages of boiler fire:
Stage 1: Normal Soot fire
Stage 2: Hydrogen fire
Stage 3: Iron fire

Stage 1- Normal soot fire:
Soot is deposited in the water tube of the exhaust boiler. When the ship is at slow speed, the exhaust temperature of main engine may vary from 100 to 200 °C . This temperature is enough to ignite “wet soot” whose ignition temperature is around 150 °C .
If the soot is “dry”, it will not get ignited at such low temperature (150 °C) but when the engine is running at higher speed and the temperature of gases reaches to above 300 °C , then in the presence of excess oxygen the deposits of combustible materials will liberate sufficient vapor which can be ignited by a spark or a flame.
The above type soot fire is called small or normal soot fire because the heat energy is conducted away by the circulating boiler water and steam. Also the sparks remain inside the funnel or diminish while passing through the flame arrestor in the funnel top.

Stage 2: Hydrogen fire
Hydrogen fire in an EGB occurs when the chemical reaction of dissociation of water takes place at temperature above 1000 °C. This leads to formation of Hydrogen (H2) and Carbon mono-oxide (CO) which are both combustible in nature.
2H2O= 2H2 + O2
(Dissociation of water leading to formation of hydrogen H2)
H 2 O + C =H 2 + C O
(Reaction of water with carbon deposits leads to formation of carbon monoxide-CO)

Stage 3: Iron fire
At this stage, the chain reaction of oxidation of iron metal starts at a high temperature of 1100 °C. This means at such high temperature the tube will start burning itself, leading to complete meltdown of tube stacks.
2Fe + O2 = 2FeO+ heat