Some problems in an assessment of the
consequences of a fire and an explosion during
the multicomponent mixture unknown
composition release.
Melania Pofit-Szczepańska
The Main School of Fire Service, Firefighting and Rescue Operation Department
Słowackiego Street 52/54
01-629 Warsaw, Poland
Consequences of hazardous
substances releases
The important part of the safety report is the analytical
part in which fire, explosion and toxicity hazards are analyzed
as well as the consequences of these releases to the
atmosphere. If the released medium is the substance of a
known composition and the parameters of process are known
too, the calculation of the consequences of these releases is not
difficult. The descriptions of the way of procedure can be
found in the literature.
If however, the mixture of unknown composition leaks through
the rupture of the pipeline or vessel the assessment of the
consequences of these releases is more complicated.
The method of thermodynamic
substitute
The method has been applied to the assessment of fire and
explosion consequences during the rupture of the pipeline
and the release of the slops mixture of unknown
composition to the atmosphere in a Polish refinery.This
method called “the method of thermodynamic substitute”,
is one of the methods used in the calculations different
parameters dealing with release of dangerous substances.
The method of thermodynamic
substitute
The single component as a thermodynamic substitute is
used very often. Of course, the most reliable use of a single
component consequence model results when the single
component simulates the behaviour of the multicomponent
fluid over all potential conditions from storage conditions
to ambient atmospheric conditions. Naturally, this involves
an intimate knowledge of the thermodynamic behaviour of
the mixture.
Single component model
The single component models are used very often because
they are much easier and generally run faster. Considering
D.W. Johnson’s example :
the release of methane – pentane vapour from a large vessel
operating at 3 bars and 65oC and a release of pure propane in
the same conditions. The release will escape through a 15 cm
diameter hole made at the side of the vessel. The release is
angled 45o above horizontal. The release rate will be relatively
constant since pressure and temperature in the large vessel will
change slowly with time. In table 1 the computed results are
given and fig. 1 shows the LFL contours. With many
uncertainties the agreement is good. Thus it appears possible
to model release.
90
75
60
45
30
15
0
- 15
0
15
30
45
60
75
90 105
120
Downwind Distance [m]
Fig. 1 Dispersion of vapour releases to the lower flammable limit
Table 1.
Item
Vapour production [kg/s]
Distance to LFL [m]
Height above grade at LFL [m]
Propane
C1-C5
15,7
30
30
15,8
30
30
3
2
1
Fig. 2 The arrangement of the installation and the location of vessels
Fig. 3 Visible damage on the pipeline which cause the realistic accidental spill
Case study
3
2
1
•Problem:
The refinery, division-slops installation. An arrangement of the
installation and the cylindrical vessels 1, 2, 3 of a division are
shown in the fig 2. The road A, road 1and cross-road A1 can be
seen. Tank cars waiting for the loading of asphalt and their position
when fire and explosion took place are marked in yellow. The area
of an enveloped accident amounted to about 5000m2. The fill of
vessels: 3 – 18%, 2 – 26%, 1 – 80%.
•An assumed course of the incident:
At first it was a small leak through a small hole. It was a long time
before the slops were released through 0.1m diameter hole in the
following conditions: t = 50oC, p = 0.5 bar. The part of slops
according to their density were absorbed by the concrete bottom of
pipelines duct or by the thermo-isolation of pipelines. The lightest
fraction mixed with air generated the cloud of slops. Can be
assumed that the released quantity of slops was larger than 150 tons
(calculations). Data published in a literature indicate that the
generation of cloud during the realistic accidental spill is possible if
the flow out is above 100 kg of relative non-reactive fuel
(hydrocarbons).
Case study
The following conditions were in the time of accidental spill:
• night,
• F - Pasquille class,
• T = - 2,7oC
• Vv = 2 m/s.
In these conditions, the direction of wind had less influence on the
cloud propagation than the buoyant forces. The flammable cloud have
encountered an ignition source probably somewhere in the vicinity
of the tank car 1 /fig. 2/ in the form of low energy source /damaged
electric installation of tank car 1/.In order to analyse the development of
fire and explosion, two variants of the procedure have been discussed.
In the basis on the information received from the rafinery was assumed
that the released mixture had the C1 to C8 composition and components
were in equivalent concentrations.
Case study
For C1 to C5
n-butane could be the substitute C1 to C5
of the fraction
For C6 to C8
n-heksane could be the substitute C6 to C8
of the fraction /liquid fraction/
from the following causes:
•the relative density of n-butane ≈ 2.0
•the relative density of released
gases mixture
≈ 1.68
•Lower flammability limit
of n-butane
≈ 2.21%
•Lower flammability limit
for the mixrure
≈2.36%
•the relative density of n-heksane ≈ 3.0
•the relative density of released
liquid mixture
≈ 3.4
• Lower flammability limit
of n-heksane
≈ 1.52%
• Lower flammability limit
for the mixrure
≈1.61%
Case study
3
2
1
Assumptions applied in calculations:
- fuel-air mixture burns in the way that no
damaging overpressure is generating /flash
fire/
- generated explosion is the deflagration
- dispersion of the released mixture occurs in two
types of surroundings:
a) in obstructed environment, the vapour cloud is
located in space between dike area and
vessels and between the vessels 3 and 2 and
pipelines ducts /fig. 2/, where the ruptured
pipeline /fig. 3/ was situated
b) in open space – the flammable mixture covers
the area about 5000m2: the area of pipelines
duct – cross-road A1 – a space outside of
the cross-road – the space round tank cars
waiting for the loading of asphalt.
Case study
The generated cloud was spreading down from SE direction to NW.
-
The area of the tank car was 162m2 (18m x 3m x 3.5m).
--
The volume: 567m3.
The area of pipelines duct was about 460m2 (46m distance
from the realistic accidental leak to cross-road marked A1/fig 2/.
-
The open area (non-built) of accident was 1500m2.
In tables 2-7 can be seen some results of the calculations: an
overpressure, positive-phase duration of explosion and the energies
of combustion at different
Case study
Distances from accidental leak : 10m, 30m, 48m, 68m and 100m
were considered and they made calculations of the explosion
parameters possible in the following places:-
10m – the nearest distance from an accidental leak to the dike area
-
30m – the distance from an accidental leak to the vessel 3
-
48m – distance from the accidental leak to the cross-road A1
-
68m – distance from the accidental leak to the place where probably
the piloted ignition occurred
-
100m – the distance from the accidental leak to the place faraway
30m outside of the cross-road A1 /near tank car 1/ /fig. 1/
On the basis of the technical documentation the mass of the
accidental leaks was determined:
-
6,000 kg
-
10,000 kg
-
20,000 kg
EXPLOSION PARAMETERS OF RELEASED SLOPS IN A
FUNCTION OF THE DISTANCE FROM AN ACCIDENTAL LEAK
Table 2. N-butane – thermodynamic substitute, obstructed environment.
Real distance from
accidental leak [m]
Overpressure [bar]
Positive-phase duration
of overpressure [s]
Energy of combustion
during
of explosion [MJ]
leak 6,000 kg
10
30
48
68
100
0.20
0.18
0.12
0.08
0.06
0.12
0.13
0.13
0.13
0.13
10,000
10,000
10,000
10,000
10,000
0.11
0.10
0.09
0.09
0.09
16,600
16,600
16,600
16,600
16,600
0.13
0.12
0.11
0.11
0.10
23,320
23,320
23,320
23,320
23,320
leak 10,000 kg
10
30
48
68
100
0.17
0.10
0.08
0.06
0.04
leak 20,000 kg
10
30
48
68
100
0.19
0.19
0.12
0.02
0.01
Table 3. N-hexane – thermodynamic substitute, obstructed environment.
Real distance from
accidental leak [m]
10
30
48
68
100
10
30
48
68
100
10
30
48
68
100
Positive-phase
Overpressure [bar]
duration of
overpressure [s]
leak 6,000 kg
0.20
0.11
0.17
0.07
0.15
0.07
0.07
0.07
0.04
0.07
leak 10,000 kg
0.20
0.13
0.18
0.09
0.12
0.08
0.05
0.08
0.03
0.07
leak 20,000 kg
0.23
0.17
0.18
0.15
0.12
0.12
0.09
0.11
0.03
0.10
Energy of combustion
during of explosion
[MJ]
7,500
7,500
7,500
7,500
7,500
12,495
12,495
12,495
12,495
12,495
24,990
24,990
24,990
24,990
24,990
Table 4. N-butane – thermodynamic substitute, open, non-built environment.
Positive-phase
duration of
overpressure [s]
leak 6,000 kg
Real distance from
accidental leak [m]
Overpressure
[bar]
10
30
48
68
100
0.001
Energy of
combustion during
of explosion [MJ]
0.94
122,186
1.11
203,728
1.40
407,456
leak 10,000 kg
10
30
48
68
100
0.001
leak 20,000 kg
10
30
48
68
100
0.001
Table 5. N-hexane – thermodynamic substitute, open, non-built environment.
Real distance from
accidental leak [m]
Overpressure [bar]
Positive-phase duration
of overpressure [s]
Energy of combustion
during of explosion
[MJ]
leak 6,000 kg
10
30
48
68
100
0.0015
0.96
91,720
1.20
152,791
1.58
305,582
leak 10,000 kg
10
30
48
68
100
0.002
leak 20,000 kg
10
30
48
68
100
0.019
0.018
0.008
0.008
0.008
Table 6. Summary comparison some results of the calculations
of the range cloud vapour explosion /ST – n-hexane/
Type of environment
Energy of combustion
[MJ]
Leak [kg]
Range of explosion
[m]
obstructed
/ with obstacles/
7,500
12,495
24,990
6,000
10,000
20,000
42.03
49.83
62.79
open
/non-built/
91,720
152,791
305,582
6,000
10,000
20,000
96.84
114.79
144.63
Table 7. Summary comparison some results of the calculations
of the range cloud vapour explosion /ST – n-butane/
Type of environment
obstructed
/ with obstacles/
open
/non-built/
Energy of combustion
[MJ]
Leak [kg]
Range of explosion
[m]
10,000
16,600
23,320
122,186
203,728
407,456
6,000
10,000
20,000
6,000
10,000
20,000
46.26
54.78
61.35
106.55
126.35
159.19
The analysis of results
In the basis of the technological data and the analysis
of the run of fire which had taken place before the
explosion, you can explain the relationship: “fire–
explosion–consequences”.
The fire of slops cloud have started at about two
o’clock at night when the cloud had already propagated
for about 70 m towards the three tank cars. About 2
min after fire the first explosion took place which
almost completely damaged the vessel no.3 filled only
in 18 % /photo 1-2/.
Photo 1. Deformation of vessel
no.3 after the explosion with the
visible displacement
Photo 2. Deformation of vessel no.3,
visible detachment from the foundations
as a result of an explosion
The analysis of results

The vessel no.2 /filled 26 %/ was damaged by the
second explosion after 45s later /photo 3/.
 Probably, this time was needed to form explosive
mixture because the hydrocarbons have the narrow
flammability limits /about 1–10%/. The vessel
no.3 filled with benzol recovery oil in 80 % was
less damaged /photo 4/.
Photo 3. Deformation of vessel no.2, visible
detachment from the foundations and
displacement outside concrete wall
Photo 4. Deformation of vessel no.1
Conclusions
The safety reports and firefighting-rescue plans should
contain:
• characteristics of hazardous materials
• indentification of the threat sources
• the probable scenarios of the accidents
• the quantitative evaluation of the potential results for both
people and the environment
To calculate the results of the hazardous occurrences the
knowledge of the input data is needed
Simulation via the use of pure component consequences
makes it possible to predict more or less accurately conditions
when a liquid or gases is released.
Thank you very much for your
attention