Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
All the fuel burns instantaneously producing high energy release
Explosion in furnace
Fuel burns with less air

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Steam Boilers, Engines, Nozzles and Turbines
Blow off cock in a boiler is used to

Increase the temperature of saturated steam without raising its pressure
Empty the boiler when required and to discharge the mud, scale or sediments which are accumulated at the bottom of the boiler
Control the flow of steam from the boiler to the main pipe and to shut off the steam completely when required
Put off fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit

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