CHIEF MATE EXAM ORAL QUESTIONS AND ANSWERS - LATEST QUESTIONS | Page 199

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A Boilover type of fire refers to an extremely hazardous situation where a semi-enclosed oil or petrochemical fuelled fire is being attempted to be extinguished with water. This situation is caused by the relative density difference of oil/petrochemicals and water. Oil, being lighter than water, causes it to always want to be the surface layer.
As water is being pumped onto the surface of the fuel (where the flames are evident) it will quickly sink below the fuel, to collect at the bottom of the container, doing little to extinguish the flames. As the temperature of the fire slowly builds, descends down through the fuel, it will eventually reach the water that has collected at the bottom of the container. The descending heatwave will have a temperature sufficient to cause the water to vaporise into steam, expanding to more than 1700 times its volume as compared to in its liquid state. The rapidly expanding (possibly superheated) steam expels the fuel upward to boilover and out of the container discharging the, still ignited, fuel onto a large and uncontrolled area outside of the container
Boil off  : A small amount of LNG evaporates from the tank during storage, cooling the tank and keeping the pressure inside the tank constant and the LNG at its boiling point.