Gavelli_edit.qxp 28/10/08 1:54 pm Page 80
Modelling Pool Fire Hazards from Large-scale Liquefied Natural Gas Spills
Figure 7: Comparison of Hazard Distances to 5kW/m
2
Using the Flame
The selection of the proper flame height correlation for a large-scale
Height Correlations by Heskestad, Moorhouse, Pritchard and Binding,
LNG pool fire model has been the subject of many discussions in
Thomas and Zukoski
recent years, given its implications for thermal heat flux hazard
2,500
distances. A recent report by Sandia
17
suggests that the Pritchard
Heskestad
and Binding correlation may be the most appropriate for LNG, based
Moorhouse
2,000
on comparison with available data. Raj, on the other hand, argues in
Pritchard and Binding
~600m favour of the Thomas correlation.
8
Both correlations were obtained
2
(m)
Thomas
1,500 Zukoski
from experimental data at F
C
greater than approximately 6*10
-3
and,
therefore, are extrapolated outside their range of validation when
1,000 applied to large-scale pool fires. A review of the literature reveals
Distance to 5kW/m that other correlations, such as those by Heskestad and Zukoski,
500 were obtained by fitting data over a wider range of F
C
, with values
down to approximately 1*10
-3
. Therefore, these correlations would
0
not be extrapolated when applied to LNG pool fires up to
0 100 200 300 400 500 600
Pool diameter (m)
approximately 500m in diameter. Interestingly, the Heskestad and
Zukoski correlations predict very similar flame heights for F
Figure 8: Comparison of Hazard Distances to 5kW/m
2
for Surface
C
greater
Emissive Power Values of 190, 220, 265 and 325kW/m
2
than 1*10
-3
, and their predictions fall approximately in the middle of
the Thomas and the Pritchard and Binding correlations.
2,000
SEP = 325 kW/m
2
The view factor is calculated from the pool flame’s size, shape and height,
SEP = 265 kW/m
2
and the following expression is used in the analysis for two- and three-
1,500
SEP = 220 kW/m
2
2
(m)
SEP = 190 kW/m
2 zone solid flame models, which also account for the shielding effect of
~450m
smoke by calculating the view factor separately for each zone:
1,000
N
VF(x,a)= s
i
. VF(x,a
i
)
i=n
Distance to 5 kW/m
500
where N is the number of zones in the solid flame model, s
i
is the
fraction of emissive power emerging from the smoke for zone i (s
1
=
0
0 100 200 300 400 500 600
1) and a
i
is the height of flame zone i.
Pool diameter (m)
The following flame height correlations are compared to provide the Pool Fire Model Comparison
reader with a sample of the variability associated with the flame The effects of the solid flame-based pool fire model parameters described
height parameter: above were evaluated by varying each parameter individually and
comparing the respective hazard distances at the 5kW/m
2
reference
• Heskestad,
14
which is based on many fire scenarios, including threshold. The predicted hazard distances are plotted versus the burning
burners 5–50cm in diameter and using various fuels: gasoline liquid pool diameter to visualise how results vary with pool size, and they
pool fires 0.3–23m in diameter, square pools of JP-4 fuel with are measured with respect to the centre of the pool fire. Other physical
sides of 1–10m and underventilated wood cribs; parameters that enter into the solid flame model (e.g. LNG properties or
• Moorhouse,
11
which is based on rectangular pool fires of LNG as atmospheric conditions) are maintained constant for all cases, and the
large as 13.7m in diameter;
15
values are summarised in Table 1.
• Pritchard and Binding,
16
which is based on hydrocarbon (primarily
LNG) pool fires 6–22m in diameter; The pool fire flame height affects the heat flux to a target through the
• Thomas,
13
which is based on wood crib fires with sides of view factor parameter. Figure 6 shows that, for any given pool size, the
10–200cm; and predicted flame height can vary by a factor of approximately 2.2 for a pool
• Zukoski,
15
which is based on natural gas burners 10–50cm in diameter. diameter of 500m, depending on the flame height correlation being used.
Figure 7 shows the effect of flame height correlation on the distance to a
The flame height correlations are compared in Figure 6 as a function of 5kW/m
2
hazard. The hazard distance trends are consistent with the
F
C
; flame heights measured during field experiments are also shown. differences in the flame height correlations: as expected, a taller flame
From Equation 3 it can be observed that if the LNG vaporisation rate per corresponds to a longer hazard distance. The ratio between the longest
unit area remains constant (as is often assumed in this type of analysis), (Pritchard and Binding) and the shortest (Thomas) hazard distance is
F
C
decreases as the pool diameter is increased; that is, larger pool approximately 1.4; that is, the hazard distance does not increase
diameters are towards the left of the plot in Figure 6 (e.g. for a pool proportionally with the flame height. However, in absolute terms the
diameter of 300m, F
C
is approximately equal to 3*10
-3
). Figure 6 shows predicted hazard distance can vary by hundreds of metres (e.g.
that while most correlations predict similar flame heights in the range of approximately 600m for a pool diameter of 500m). Therefore, the choice
F
C
for which data are available, they tend to diverge as F
C
decreases (i.e. of flame height correlation can have a significant impact on the
as the pool diameter grows). The ratio between the highest (Pritchard- consequence analysis for large-scale LNG spills. The implications of the
Binding) and lowest (Thomas) flame height predictions in the range of flame height correlation debate become evident from Figure 7: a
pool diameters of interest is approximately 2.2. correlation predicting a ‘low’ flame height could underestimate the heat
80
EXPLORATION & PRODUCTION – OIL & GAS REVIEW 2008 – VOLUME 6 ISSUE II
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