PREVENTION OF TOXIC CHEMICAL RELEASE 1023
ture of gas and air, ignition takes place at the point of contact,
i.e. incites chemical reaction. Combustion proceeds from the
outside of the mixture towards the center, and thereby forms
a plane of combustion which divides the gaseous mixture
into two parts. On the one side are the highly heated products
of combustion, and on the other, is the still unconsumed gas
mixture.
The velocity at which this plane advances in different
for each gaseous mixture, and depends both on the compo-
sition of the mixture and on the pressure to which it is sub-
jected. The higher the velocity of propagation the greater
the rise in temperature, and this latter, in turn, directly influ-
ences gas expansion and the products of combustion, which
thereby exert such a high pressure on their environment that
any opposing medium, vessels, tanks, piping, walls, etc. is
ruptured.APPLICABLE EXPLOSIBILITY DATANot all explosibility data reported in the literature are appli-
cable to purging problems. From the safety standpoint, it is
desirable to select the widest explosive limits for the purge
operation. In addition, an ample safety factor be applied,
especially to the lower explosive limit. For acetylene-air
mixtures, at atmospheric pressure and temperature, the
published and accepted values for the lower explosive limit
(LEL) and upper explosive limit (UEL) are 2.5 and 80.0%
acetylene in air respectively. These are the widest limits
recording. See Table 1.
In every case involving combustible gas handling or
processing, constant awareness of the inherent dangers to
life and limb as well as property is imperative. In the case
of acetylene, it is a well known fact that it has unstable
characteristic at any pressure and whether or not a decom-
position would take place depends on the intensity of
the initial source of ignition. Higher pressures have the
effect of lowering the initial energy necessary to initiate a
decomposition.ACETYLENE—CASE STUDYAcetylene is inherently an unstable gas at any pressure and
whether or not a decomposition would take place depends
on the intensity of the initial source of ignition. Higher pres-
sures have the effect of lowering the initial energy necessary
to start a decomposition. Higher initial pressures will also
result in an increase in the ratio of the maximum pressure
developed in the decomposition to the initial pressure. As
for acetylene-air mixtures, pretty much the same behavior
is manifested. With decreasing ignition energies, the initial
pressures must be correspondingly increased so as to bring
the total gas mixture volume to decomposition.
The size, shape and orientation of a process vessel as
well as its material of construction have a profound effect
on the ignition limits of combustible gas mixtures. The same
applies to acetylene decompositions. It is a matter of heatbalance during the combustion process. Even the relative
position of the source of ignition. The widest explosive range
is obtained for vertical cylindrical vessels or tanks when the
ignition source is located at its base.
The presence of water vapor and high humidity acts as a
diluent and an inert gas. This effect is typical for all combus-
tible gas–air mixtures.
As for the effect on explosion limits, the ratio of vessel
surface to cross-sectional area from a cooling or heat bal-
ance point of view, long slender vessels (high ratio of surface
to sectional area) narrow the limits of explosibility. It’s all a
matter of the rate at which heat is removed from the gas mix-
ture inside the vessel. As for the temperature effect, higher
temperatures induce convection currents within the vessel
and increase turbulence and widen the limits.
Thus, only tests of actual operating setups can tell the
effects on the widening or narrowing of the explosibility
range. Then, once established, the LEL and UEL determined
experimentally can be used to develop a purging graph here-
inafter discussed and developed.EFFECT OF INERT GASThe effect of an inert gas on explosibility of combustible gases
in air can be shown graphically on Figure 1 for acetylene–
air mixtures. Once the conditions for actual operating condi-
tions and configuration are determined experimentally, the
graph can be set up as we shall see.
As nitrogen is added to mixtures of the gas and air within
the explosive range, a series of new mixtures are formed
each of which has a different UEL and LEL than the pre-
ceding mixture and the explosive range is narrowed along
definite lines of demarcation. As the air and combustible gas
contents are reduced by the addition of nitrogen, the line of
LELs and the line of UELs converge and meet at a point,
i.e., D. Here the range has degenerated to zero. No mixture
of acetylene, air and nitrogen which contains less oxygen
than the lowest point on the line LDU (point D ) is explosive
in itself, but all mixtures within the areas bound by LUD are
within the limits of explosibility and are therefore explosive.
Again, in Figure 1, line ADE is drawn tangent to LDU at D.
Any mixture represented by point X to the right of line DE
and below the upper explosive limits DU is not explosive in
itself. However, on dilution of that mixture with air, a new
series of mixtures will be formed having compositions fall-
ing along line X A which passes through the explosive area.
Similarly, any mixture represented by a point to the left of
line LD and DE will not form explosive mixtures on further
dilution with air.
Note line LD is not vertical but swings to the right
as falls.APPLICATION OF GRAPHAt startup when placing gas equipment into service purg-
ing from air to inert gas, the object is to reduce the oxygenC016_011_r03.indd 1023C016_011_r03.indd 1023 11/18/2005 1:12:55 PM11/18/2005 1:12:55 P