Page 39 - International safety guide for oil tankers and terminals

BASIC PROPERTIES OF PETROLEUM

Flammable limits % volume Number of dilutions with
---ใช้เพื่อการศึกษาเท่านั้น---
hydrocarbon in air same volume of air to
Gas reduce a mixture of 50% by
Upper Lower volume to LFL
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Propane 9.5 2.2 23
Butane 8.5 1.9 26
Pentane 7.8 1.5 33

Table 1.1 – Flammable limits of propane, butane and pentane

1.2.3 Effect of Inert Gas on Flammability
When an inert gas, typically flue gas, is added to a hydrocarbon gas/air
mixture, the result is to increase the Lower Flammable Limit hydrocarbon
concentration and to decrease the Upper Flammable Limit concentration.
These effects are illustrated in Figure 1.1, which should be regarded only as a
guide to the principles involved.

Every point on the diagram represents a hydrocarbon gas/air/inert gas mixture,
specified in terms of its hydrocarbon and oxygen content. Hydrocarbon gas/air
mixtures without inert gas lie on the line AB, the slope of which reflects the
ึ
reduction in oxygen content as the hydrocarbon content increases. Points to
the left of the line AB represent mixtures with their oxygen content further
reduced by the addition of inert gas.

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 as 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.

On this diagram, changes of composition due to the addition of either air or
inert gas are represented by movements along straight lines 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.

It is evident from Figure 1.1 that, as inert gas is added to hydrocarbon gas/air
mixtures, the flammable range progressively decreases until the oxygen
content reaches a level, generally taken to be about 11% by volume, when no
mixture can burn. The figure of 8% by volume of oxygen, specified in this
guide for a safely inerted gas mixture, allows a margin beyond this value.

When an inerted 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 inerted mixtures in the region
above the line GA go through a flammable condition as they are mixed with air,
for example, during a gas freeing operation.

Those mixtures below the line GA, such as that represented by point H, do not
become flammable on dilution. It should be noted that it is possible to move
from a mixture such as F to one such as H by dilution with additional inert gas
(i.e. purging to remove hydrocarbon gas).

© ICS/OCIMF/IAPH 2006 5