In the calculation, as dense gas dispersion takes
place over a ten minute period the radius of the
dispersed LPG increases by almost a factor of 10.
This causes envelopment of parts of the surround-
ings which were not affected by the initial cloud
and so places them at risk.
One further point might be made in relation to
the equation for dense gas dispersion when
applied to LPG. We have tended in this article to
approximate the composition to pure propane, but
of course LPG can contain significant amounts,
even a preponderance, of butane. If the above
calculation is repeated for butane, for which the
molar mass is 0.058 kg, the radii are respectively
39 and 155m. These differ almost insignificantly
from the values for propane. If the composition, as
proportions of propane and of butane, of a partic-
ular example of LPG were known the effective
molar mass could of course be calculated and
used in the equation, but it is doubtful whether
such a refinement would be of benefit. Equivalently,
one can use the molar mass either of propane or
of butane to represent LPG generically for the
purposes of a dense gas dispersion analysis.
Combustion behaviour
When LPG exiting an orifice ignites, it burns as a
jet fire. Returning to our example where propane
leaks at a rate of 0.6 kg s�1 and noting that the
calorific value of propane is approximately 50 MJ
kg�1, a jet fire resulting from such a leak will
release heat at a rate:
50 MJ kg�1 � 0.6 kg s�1 = 30 MW
A burner for natural gas and/or fuel oil at a power
station would release heat at a rate comparable to
this. There are empirical correlations for predicting
jet fire lengths according to leakage rate,2 and such
a length has only a weak dependence on the orien-
tation. The importance of flame length is that if
there is impingement of the flame on to previously
unaffected hydrocarbon inventory escalation will
occur, as happened at Crescent City (see below).
When there is catastrophic release of LPG a
BLEVE � boiling liquid expanding vapour explo-
sion � is expected whether or not there is ignition.
This is a consequence of the enormous mechanical
energies involved in sudden release, a point noted
more than once previously in this article. A BLEVE
is a physical phenomenon and water can display a
BLEVE: that is what happens when a pressure
cooker or an autoclave blows up. If when LPG
leakage occurs there is ignition it will burn as a
fireball, so the course of events is best described as
a `BLEVE-fireball'.
Case study: Crescent City Illinois 1970
This was the worst ever accident involving LPG and
has already been recounted at some length by the
present author.3 It began when there was derail-
ment of a train pulling 12 tank cars of LPG, which
were distributed amongst other types of car which
the train was pulling. One of the LPG-bearing cars
ruptured immediately on impact and a BLEVE-fire-
ball resulted. Heating caused by the fireball caused
some of the other LPG rail cars to start to discharge
inventory through their safety valves, which were
set to open at 20 bar which is just over twice the
pressure at ordinary temperatures. LPG so released
from one car ignited and burnt as a jet fire which
extended to a nearby one and had a `torch effect',
causing it to burst open and its contents burn as a
BLEVE-fireball. The course of events whereby a jet
flame from one tank car heated another causing it
to burst open was repeated.
Concluding remarks: the non-
interchangeability of LPG and
natural gas
In bringing this article to a close the author will
emphasise that serious consequences can result
when a burner designed and adjusted for natural
gas, for example at a cooker, receives LPG instead.
There have been cases of this in situations where
natural gas supply has been interrupted. Natural gas
and propane/butane have different air requirements.
More importantly, a natural gas flame propagates
more slowly than a propane/butane flame making a
burner for the former unsuitable for the latter. On
no account should such substitution take place.
References
1. Jones J.C. `Hydrocarbons: Physical Properties and
their Relevance to Utilisation' Ventus Publishing,
Fredricksberg, in press.
2. Jones J.C. `Hydrocarbon Process Safety: A Text for
Students and Professionals' Whittles Publishing,
Caithness (2003) US Edition published by Pennwell,
Oklahoma.
3. Jones J.C. `Combustion Science: Principles and
Practice' Millennium Books, Sydney (1993).
IFF
62 INTERNATIONAL FIRE FIGHTER
FIRES HAZARDS WITH LIQUEFIED PETROLEUM GAS (LPG)
INDUSTRIAL
We have tended in this article to approximate the
composition to pure propane, but of course LPG can
contain significant amounts, even a preponderance, of
butane. If the above calculation is repeated for butane,
for which the molar mass is 0.058 kg, the radii are
respectively 39 and 155m. These differ almost
insignificantly from the values for propane.

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