Ullage Gas Flammability Diagram

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Lower Flammable Limit;

the concentration of flammable vapour, in air, below which combustion will not occur [at normal atmospheric conditions]. The mixture is too lean. Taken to be 1% for crude oil.

Upper Flammable Limit;

the concentration of flammable vapour, in air, above which combustion will not occur [at normal atmospheric conditions]. The mixture is too rich. Taken to be 10% for crude oil [Oxygen level = 19%].

Flammable Range;

The range where the concentration of flammable vapour and oxygen levels will support combustion. Outside of this ‘envelope’ combustion will not occur. LFL increases with reduction of O₂, UFL reduces until, below 11% O₂, combustion cannot occur.

Vapour Pressure;

pressure exerted by a vapour that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapour and some molecules of vapour are condensing to form liquid. At equilibrium the rates of the two processes are equal and the system appears to be stationary. Once the vapour pressure equals the ambient pressure the liquid boils. Crude oil is has many light factions which produce vapours, the higher the temperature then the more vapour present in the ullage space. Even though the ullage space is inert [less than 8% O₂], the concentration of vapour can be high. If the ullage space is broached the conditions will attempt to revert to normal conditions, passing through the explosive range in the process.

Flash-point;

the lowest temperature at which an ignitable mix with air  can be formed. At this temperature the vapour may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined at which the vapour continues to burn after being ignited. As it is the vapours that initially ignite then fuels with high vapour pressures ignite easier.