2.3.1 When an inert gas is added to a hydrocarbon
gas/air mixture the result is to increase the lower flammable limit
concentration and to decrease the upper flammable limit concentration.
These effects are illustrated in figure 1,
which should be regarded only as a guide to the principles involved.
2.3.2 Any point on the diagram represents a hydrocarbon
gas/air/inert gas mixture, specified in terms of its hydrocarbon and
oxygen content. Hydrocarbon/air mixtures without inert gas lie on
the line AB, the slope of which shows the reduction in oxygen content
as the hydrocarbon content increases. Points to the left of AB represent
mixtures with their oxygen content further reduced by the addition
of inert gas. It is evident from figure 1 that
as inert gas is added to hydrocarbon/air mixtures the flammable range
progressively decreases until the oxygen content reaches a level generally
taken to be about 11% by volume, at which no mixture can burn. The
figure of 8% by volume specified in these Guidelines for a safely
inerted gas mixture allows some margin beyond this value.
2.3.3 The lower and upper flammability limit mixtures
for hydrocarbon gas in air are represented by the points C and D.
As the inert gas content increases, the flammable limit mixtures change.
This is indicated by the lines CE and DE, which finally converge at
the point E. Only those mixtures represented by points in the shaded
area within the loop CED are capable of burning. Changes of composition,
due to the addition of either air or inert gas, are represented by
movements along straight lines. These lines are directed either towards
the point A (pure air), or towards a point on the oxygen content axis
corresponding to the composition of the added inert gas. Such lines
are shown for the gas mixture represented by the point F.
2.3.4 When an inert mixture, such as that represented
by the point F, is diluted by air its composition moves along the
line FA and therefore enters the shaded area of flammable mixtures.
This means that all inert mixtures in the region above the line GA
(critical dilution line) pass through a flammable condition as they
are mixed with air (for example during a gas-freeing operation). Those
below the line GA, such as that represented by point H, do not become
flammable on dilution. It will be noted that it is possible to move
from a mixture, such as that represented by F, to one such as that
represented by H, by dilution with additional inert gas, i.e. purging.