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Protection of equipment engulfed in a pool fire
J. Loss Prev. Process Ind. Vol. 9. No. 3. pp. 231-240,
19%
Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0950-4230/96 $15.00 + 0.00
SO950-4230(96)00014-9 ELSEVIER
Protection pool fire
of equipment
engulfed
in a
Eul&lia Planas-Cuchi*, Joaquim Casal*, Antonio Lancia? and Leo Bordignon? *Centre d’Estudis de Rise Tecnolbgic (CERTEC), Department of Chemical Engineering of the Universitat Politkcnica de Catalunya-Institut d’Estudis Catalans, Diagonal 647, 08028 Barcelona, Catalonia, Spain i_Tecsa Ricerca & Innovazzione, via Aldo Moro 1, Scanzorosciate (BG), Italy Received 1 I February 1996 When process equipment is engulfed in a fire, the protection system should extinguish the fire and simultaneously cool the equipment. Therefore, the behaviour of equipment engulfed in a fire, as well as its eventual influence on the fire, should be known. In this article, experimental data from pool fires of hexane (4 m2) and kerosene (12 mZ) are discussed. The evolution of the fire up to its full engulfment is studied, as is the evolution of temperatures at different points of a tank engulfed in it. The heat release rates under the different operating conditions are studied. A method is developed to calculate the maximum combustion rate, both for the developing stage and for the fully developed fire. The hindering effect of the tank on the fire is discussed and quantified in terms of ‘efficiency of hindered combustion’. Copyright 0 1996 Elsevier Science Ltd Keywords: pool fire; foam and water; cooling; extinguishing;
Certain industrial installations are designed according to an extremely compact lay-out. Off-shore oil platforms and some sections of chemical process plants exhibit a high density of equipment which, in the event of certain incidents, can lead to dangerous situations and can significantly complicate the management of emergency procedures’. In such installations, moreover, there are usually large inventories of hazardous materials, and protection systems with a high degree of reliability and efficiency must be provided. If flammable materials are processed or stored, a good fire protection system must be installed. Such a system should be able to cover two objectives simultaneously: extinguishing or at least controlling the fire and, furthermore, cooling the structures and the equipment. This second scope is very important, in some cases even more than the first one, as the effect of fire on certain equipment can result in further loss of containment of flammable substances and hence in the escalation of the accident scenario. In order to cover these two objectives there are various possibilities. In the conventional approach, cooling and extinguishing are addressed by two different systems: water deluge systems and foam systems, the specific features of water and foam thus being used separately. Water is the most commonly used fire extinguishing agent; it has a high specific heat and very high latent heat of vaporization, and furthermore it is usually available at
modelling
a low cost. However, the use of water also has several disadvantages, the main one resulting from its relatively high density, higher than that of most hydrocarbon fuels. Furthermore, hydrocarbons are also immiscible with water; therefore, in the event of a pool fire water will neither cover the burning fuel - thus extinguishing or reducing the fire - nor mix and dilute it. The hydrocarbon will maintain a lighter burning layer, and in some cases water may even flash (if it enters a hot mass of fuel) and spread fuel. If a cover for the burning surface is required - and this will help significantly in the control of the fire - foam solutions must be used. However, foam agents are much more expensive than water and, moreover, they are available in much smaller amounts. One interesting approach consists, therefore, in using a single deluge system which discharges foaming solution for a given minimum time and then continues discharging water. Such systems, known as ‘foam and water deluge systems’, should discharge a kind of foam which is fluid enough to behave like water for cooling purposes but stable enough to possess useful extinguishing properties in spill fires. Foam and water deluge systems are quite advantageous, especially for off-shore oil platforms and processing plants, due to the possibility of achieving fast fire control, the simplification of plant structure and operating procedures and the resulting reduction in the costs of the system. This work discusses the behaviour of a typical piece of equipment (a tank) during the development and
231
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Protection
of equipment
engulfed
in a pool fire: E. Planas-Cuchi et al.
233
Table 1 Description of the selected experiments
Test number
(m*)
Pool area Fuel
Extinction agent
Sprinklers configuration
S.A.R3* (Lmin-‘.m-*)
Deluge flow (I.min-‘)
12 13 14 15 88 89 90 91
4 4 4 4 12 12 12 12
Hexane Hexane Hexane Hexane Kerosene Kerosene Kerosene Kerosene
Foam Foam Water Water Foam Foam Foam Foam
two two two two two two two two
14.2 14.2+ 14.2+ 14.2 14.2+ 14.2+ 6.5 6.5
246 296 296 246 296 296 112 112
‘Surface
application
rate
speaking, the results for the hexane pools (4 m2) showed a practically linear trend, essentially identical for the first three tests; run number 15 showed slightly higher values with respect to the rest of the results, although the trend was similar. For the kerosene pools the results were parabolic rather than linear; this can be attributed to the fact that the development of fire over the whole surface of the pool (in this case 12 m2) was slower. The data obtained from the radiometers have been used to plot the variation of radiation intensity as a function of time (Figure 3). As can be observed, after approximately 40 s the fire can be considered to be fully developed, and with a practically stationary regime (the time required to reach this condition was more than 60 s for the 12 m2 kerosene pool fires); typical oscillations exist, which must be related to the turbulence of the fire and to the variability of the shape, size and orientation of flames as a function of time. Later on, the decrease of radiation intensity shows the effects of the start-up of the extinction system. The action of the foam-water mixture caused a significant decrease in the size of the flames, which finally led to the complete extinction of the pool fire as it became covered with a stable layer of foam.
Evolution of temperature As the fire proceeds, the equipment engulfed in it - a tank in this work - will undergo an increase in temperature which will be a function of the evolution and size 4.5 4 3.5 T3 E 2.5 Radiometer 26
j2
-Radiometer
27
z! 1.5 1 0.5 0 0
50
SPKOl125” SPK0/125”+SPY0/90” SPK0/125”+SPY0/90” SPKOf125” SPK0/125”+SPY0/90” SPK1/90”+SPY0/90” SPK0/125” SPKl/l ?O”
100
150
200
250
time (s) Figure3 Variation of the radiation intensity as a function time for test number 13 (4 m*, hexane)
of
of the flames, and also of the location of the measuring point; those zones exposed to flame impingement will undergo stronger heating than those which are not in direct contact with them. On the other hand, after the start-up of the cooling/extinguishing system, different behaviour will be observed depending on the existence or otherwise of a water/foam layer on that particular zone of the equipment surface. The analysis of the experimental results indicates the existence of five different situations or steps4, which are discussed in the following paragraphs.
Initial step (development of the jire) The first step corresponds to the development of the fire and takes place over the first minute (approximately). Figure 4 shows the variation of temperature as a function of time, at diverse measuring points, for two of the tests. Temperature increases approximately linearly with time, the slope depending on the location of the point and on the impingement of the flames on that point. In one minute, the temperature at the bottom of the tank increases from room temperature to more than 350°C while in the lower zone of the sides it does not exceed 250°C and at the top the maximum temperature is 125°C. From the data plotted in Figure 4(b) it can be seen that the temperatures registered by the thermocouples located in the upper sides of the tank show a degree of scattering. This must be attributed to the fact that the fire was not exactly symmetrical owing to turbulences and the arrangement of the module walls. Thus, although theoretically all the thermocouples should show the same behaviour (as in Figure 4(a)), the induced wind and the radiation from the heated walls caused these differences between the measurement points. For the 12 m2 pool fires (Figure 4(c)), the scattering is less noticeable and the temperatures are higher, because in this case the engulfment of the tank in the fire is more complete, and therefore the impingement of the flames on its walls has a greater influence. As in Figure 4(a), the bottom of the tank (Figure 4(d)) for the 12 mz pool fires shows the same behaviour for all the thermocouples. The evolution of the temperature at different points of the tank as a function of time has also been plotted in Figure 5. The experimental points show clearly how the bottom of the tank undergoes the most intense heating action, due to the greater exposure to the flames.
Protection
234
of equipment
engulfed
8) 400 I
in a pool b) 300 I
350 -300 --
JiE& 17 11
A
5 .
250 A
A Themwouple
a
8
I
0 Thermocouple 5
50
0 Thermocouple 11
A htmocouple
17 04
O-1 0
10
20
40 ti:
50
!FA 5
0
0 A A 0
f j
A
o
0 8
40
50
90
8
l
A
a 0
q
n
P
a
i
4
0
10
20
30 time(s)
during the first minute. (a) Test number 13 (4m *, hexane), tank bottom; (b) test number 13 (4 m*, (c) test number 91 (12 m*, kerosene), tank bottom; (d) test number 91 (12 m*, kerosene), upper tank lat-
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150..
-o-t=9or
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k
im.. 50 ..
0
0
8
0
04 60
im
150
200
HeisM (cm) Figure5 Evolution of instants as a function kerosene)
n
04
Operation of the extinguishing
h
90
0 llwrrmcoupb 15
50
time (8) Tank temperature upper tank laterals;
50
150 100
0
30
40
0 Thermocouple 13
”
20
60
A A
1
10
50
0 Ttwmpcoupb 7
-200
04
0
40
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0
O 0
30 time (8)
250
A
8
20
0
0
’
4
I 10
0
0
a Therwcoupb 11
A
f
8
(*)
1
A
I
0
0
60
3501 17 11
8
0 Thermocouple 15
e
100 --
19
. Thermocouple 9
4
150 --
Figure4 hexane), erals
0
m Thermocouple 3
0
l-
d
A
0 Thermocouple 13
200 --
50 --
0
0 Thermocouple 7
g250 --
t
et al.
fire: E. Planas-Cuchi
tank temperature at different time of height. Test number 89 (12 m*,
system
The evolution of the fire during its development and during the extinguishing process can be seen schematically in Figure 6, obtained from a film. In the initial steps the fire increases rapidly, soon engulfing the whole tank. The shape of the flames is variable, as usually happens with relatively large fires. As flame length increases, so do the turbulence and mobility of the flames. From the moment at which the extinction system is started, a significant increase in the turbulence of the flames is observed, together with the existence of wind from the west side; as a consequence, the flames move towards the east side (see Figure 6)4. Therefore, the west side of the tank is less influenced by the flames (the surface area undergoing flame impingement decreases significantly) and consequently the temperatures measured here are lower. After reaching a maximum, the temperature at the various measuring points of the tank starts to decrease; in the plot of temperature versus time, a negative slope is obtained. This situation corresponds to the moment at which the cooling effect of water/foam starts, i.e. when
Protection of equipment Test no12 Time = 10s
235
engulfed in a pool fire: E. Planas-Cuchi et al.
Test no12 Time = 20 8
8 A 300 2 250 a [ 200 j Test no12 Time = 30 s
150
Test no12 Time = 40 s
0
50
100
150
200
250
300
350
400
tlrne (s) Figure 7 Stages of the temperature evolution at the lower tank laterals (thermocouple 6). Test number 12 (4 m2, hexane) Test no12 Time = 50 s n4
Test no12 Time = 60 s
Test no12 Time = 120s
Test no12 Time = 1802
Figure 6 Evolution of the fire during its development and during the extinguishing process. Test number 12 (4 m*, hexane)
the mixture reaches that measuring point. In any event, while the tank surface continues to be exposed to flame impingement, temperature will decrease slowly, although the size of the fire decreases significantly after the start-up of the deluge system. Later on, as the pool gradually gets covered by the foam, the height of the flames gets less and less and they no longer impinge on the tank wall. Now the cooling action of the water is stronger and the temperature decreases dramatically, with a steep negative slope in the plot of temperature versus time. Finally, when the fire is extinguished and the extinguishing system is stopped, a slight increase in temperature is again observed due to the heating caused by the radiation from the hot walls of the experimental module. This process establishes in a relatively clear way the existence of five stages in the evolution of temperature at the various points of the tank wall (Figure 7)4. The first one, which corresponds to the initial development of the fire, is characterized by a rapid increase in temperature; in the plot of temperature versus time a practically
straight line is obtained with a steep slope (the heating rate depends on the location - at the bottom of the tank values of the order of 3.6”C’s were registered). In the second stage, which corresponds to a fully developed pool fire, temperature continues to increase but at a slower rate and with a trend which is no longer linear; in this stage a considerable wind appeared in all the tests. The third stage initiates with the start-up of the cooling/extinguishing system. The temperature starts to decrease - its maximum value having been reached at the transition between these two steps - gradually, at an approximately constant rate. The fourth stage corresponds to a situation in which, due to the blanketing action of the foam over the pool, the magnitude of the flames has decreased considerably and they no longer reach the point in the tank where the temperature is being measured. Now the heat balance is clearly influenced by the dominant mechanism of the foam/water cooling, with a relatively weak heat input by radiation; thus, temperature decreases abruptly, following an approximately linear trend. Finally, once the fire is extinguished, gentle heating is observed. The existence of these five stages was observed only in certain conditions during the experimental tests. In other cases some of the intermediate stages were not found, depending on the point at which the temperature was being measured. This happened, for example, when the influence of flames on the tank wall ceased at the same moment that the cooling mixture reached that particular point. Stage 3 does not exist and just after reaching its maximum value the temperature decreases abruptly to very low values, and afterwards continues to fall very gradually (see Figure 8). For the whole system, this situation would correspond to an immediate extinction of the pool fire. This case, which would be the best one from the point of view of controlling the emergency, in practice implies conditions which can only be found at certain points of real installations. At the bottom of the tank, however, flame impingement existed practically throughout the experimental test, finishing only when the fire was extinguished. Furthermore, this zone was never reached by the cooling mixture in the various tests that were carried out. This
236
engulfed in a pool fire: E. Planas-Cuchi et al.
Protection of equipment 350
overall heat release rate and of the heat released by convection are discussed.
300 250
Overall heat released The heat released during each test was measured as a
1
g B e! 200 i
150
I-” 100
50 _
5 01 0
50
100
150
200
250
300
350
400
time (s) Figure 8 Stages of temperature evolution at the lower tank laterals (thermocouple 12). Test number 12 (4 m2, hexane)
is why slope changes are smoother and therefore the temperature values are higher up to the extinction of the fire (Figure 9).
Heat released from the pool fire Probably the best way to follow the development and evolution of the pool fire is to measure the heat released by the combustion. For a pool fire of a given fuel, the heat release rate is a function of the combustion rate and of the combustion efficiency. The heat release rate will rise rapidly in the initial moments, while the fire is developing and the size of the flames is increasing, and after that it will have an approximately constant value once the fire is completely developed. Nevertheless, for pool fires of a relatively large size, this second stage which would correspond to a stationary state if no changes or external actions were imposed on the system - is characterized by significant variations in flame size and shape. The evolution of the heat release rate as a function of time will be therefore a way to identify the moment at which the pool fire is completely developed. In the following paragraphs the experimental results of the 700
Heat released by convection The heat released from a fire is essentially divided into
two fractions, one transferred by convection and the other transmitted by radiation. Both contributions can be determined separately and independently from Q,,. The heat released by convection can be obtained from the flow rate and temperature of the gases produced in the combustion:
. Q
I, =
c-cm”
iv*moke*
cpx AT,,, .
(1)
During the experimental tests, these data were measured by the calorimeter and treated in such a way as to give continuously the value of Q,,,,. The trend of the evolution of the heat released by convection as a function of time is similar to that corresponding to the overall heat release rate: for the pool fire of 4 m* a gradual increase in the heat released, with a steep slope during the first 30 s; later on, a more gradual increase is observed - with some scattering caused by the turbulence and irregularity associated with the phenomenon - until the maximum value (steady state) is reached at approximately 60 s.
‘Ooo I
l-
!Gi4 5
600
function of time by using the adiabatic calorimeter. The plot of overall heat release rate versus time (Figure 10) shows a steep rise at first, with a characteristic change in slope at approximately 30 s; from this moment the heat release rate still increases, but more gradually, up to a maximum-value (fully developed fire). For a 4 m* pool fire of hexane, the heat release rate increased from zero at ignition (t = 0) to rates of the order of 5 100 kW after 30 s. For the kerosene pool fires (12 m*), the heat release rate increased to rates of the order of 16 600 after 60 s. When the deluge system was activated, the heat release rate decreased rapidly until the extinction of the fire. In those cases in which only water was used for the extinction (tests 14 and 15), the heat release rate also decreased after the activation of the deluge system, but a few seconds later the fire revived.
6000 +
500
E
g! 400
E
B B
p 300 8 200 100 0
hod ,
c -F
0
50
7
100
150
200
250
3o 50
100
150
200
250
time (s)
time (s) Figure9 Temperature evolution at the tank (thermocouple 5). Test number 91 (23 m2, hexane)
0
bottom
Figure 10 Total heat release rate evolution time for test number 13 (4 ma, hexane)
as a function
of
Protection
of equipment
engulfed
After this moment, a sudden fall can be observed due to the start-up of the extinguishing system. The 12 m2 pool fires (Figure 11) show an increase in the heat released, with a steep slope, during the first 60 s, after which the extinction system is activated. In this case the steady state was not reached, i.e. the fire was still developing when the sprinklers were activated. It can be observed that after that moment the heat release rate displays an approximately constant value for a certain period, with a steep fall afterwards; this can be attributed to the difficulties in extinguishing the fire. For hexane pool fires of 4 m*, the heat released by convection was approximately 57% of the overall heat released. In the case of kerosene, this fraction was approximately 65%. The kerosene fires gave rise to larger amounts of dark smoke, i.e. the fire was less bright and the amount of heat transmitted by radiation was correspondingly lower.
in a pool
fire: E. Planas-Cuchi
et al.
237
In this equation, the first term on the right-hand side corresponds to the heat transferred by conduction; this is a boundary effect, significant only for pools with a few centimetres in diameter and, therefore, negligible here. The second term is the heat transferred by convection, also negligible for the large pools studied in this work. For pools with a diameter of 1 m or more, the radiation mechanism is the only one which is really important. Therefore, in this work the rate at which heat is transferred to the pool is given by:
ri = -(n&/4)
D
F(q - c)(l
- eekd) .
The theoretical combustion rate can be obtained from this expression and from the heat required to evaporate the fuel. As combustion rate increases with pool size, for a pool of infinite diameter, the maximum combustion rate is obtained:
Discussion Most communications published on hydrocarbon pool fires deal with fully developed fires, and very little information is to be found on the initial unsteady state, when the fire is developing. This is why there is no equation available in the literature to predict the evolution of flame temperature during the initial development of a fire. Moreover, when relatively large equipment is engulfed in a pool fire, it should have a disturbing effect on the flames as compared to a free surface pool fire; again, this aspect has not been studied by those authors who have worked in this field5-8. In the following paragraphs this effect is studied. In order to calculate the theoretical (maximum) combustion rate in both an unsteady and a steady state, a semiempirical expression is derived from the treatment of experimental data. Accordingly, the heat transfer rate from the fire to the pool is given by the following expression: rd 214
Heat bv convection
and for a pool fire of finite diameter: . m f=h’;max N
(1 -emM).
From equation (4) it can be inferred that during the initial development of the fire (unsteady state) the combustion rate will change with time, as will flame temperature. Moreover, flame temperature will also change with height. The experimental data have shown that during the development of a pool fire the flame temperature changes with time according to an expression of this type: Tf
@A=
t (b
+
(f-5)
&)
where a and b are parameters which can depend on height. The values obtained for the different operating conditions are given in Table 2.
Heat by conduction +
oF(G
-
Table 2
1 - emkd).
c)(
Values a and b for different operating conditions
keat by radiation
7000 0 0
t)C b 6000 --
0"
08
coo
%O
a
0
Q,
b
0
80°
Hexane (4 m2)
Kerosene (12 m*)
0.000851 ~0.0034+0.021 h)
(0.000465-0.000188h~ (0.0347+0.0114h)
5000 --
0
Equation (5) was used to calculate the flame temperature just above the pool surface (h = 0). However, it must be taken into account that this temperature corresponds to the central axis of the flame. Flame temperature decreases over the radius, from flame axis to periphery; at the flame boundary, the temperature is approximately 50% (in terms of temperature increase with respect to room temperature*) of the temperature
0
4000 --
00 0
3000 --
0 0
2000 -0 1000
--
rprinklers diwUon
0 0
ocr
:
0
30
60
00
120
150
180
210
240
time (s) Figure 11 Heat released by convection for test number 90 (12 m2, kerosene)
as a function of time
* rR - T, = 0.5 x (Tr- TJ, then r, = 0.5 x CT,+ T,). The average temperature at a given height can be determined, as a function of temperature at flame boundary and room temperature, from the following expression: T,,= (T, + T,)/Z. In this work, a room temperature of 293K has been taken.
238
Protection of equipment
engulfed in a pool fire: E. Planas-Cuchi et al.
at the axislo. Thus, the average flame temperature at a given height is approximately T,, = 0.75 . Tf + 73.25
(7)
By introducing this equation, together with equation (6), into equation (5), it is possible to obtain a semiempirical expression for the calculation of the combustion rate during the non-stationary regime: riz(t) =
FU
t
AK + c,(T,-T,)
c
-
0.75(b+at) + 73.25 4
[(
-I
(8)
(l-e-“). 1
The heat release rate can be obtained as
*n = m; AH, . Q t,,fa,
(9)
As in this case the view factor value between the fire and the pool is unity and the mass loss rate can be determined from equation (8); the rate at which heat is released can be calculated from the following relationship:
When there is equipment engulfed in the fire, this equipment has a hindering or obstructing effect on the fire, thus a ‘hindering factor’ can be defined as the ratio between the heat released in the combustion of a pool fire with an obstacle (for example, a tank) and that released in the same pool fire with a free surface: Heat released in the combustion of a pool with an obstacle 5= Heat released in the combustion of the same ’ pool without any obstacle (12) 5 would have a maximum value of 1 in those cases in which the influence of the obstacle is negligible, and decreasing values as the hindering effect increases (i.e. as the size of the equipment as compared to that of the pool increases). Finally, the ‘efficiency of hindered combustion’ can be defined as the ratio between the heat released in the combustion of a hindered pool and the maximum tbeoretical heat which could be released if the combustion was complete and non-obstructed: %bst
This expression gives the maximum heat release rate, i.e. that corresponding to the combustion of all the fuel evaporated. However, in an accidental fire, not all the fuel is burnt, i.e. combustion efficiency is not 100%. Figure 12 shows the experimental values for a fire of 4 m2, as well as those calculated by the method proposed in the aforementioned paragraphs. As can be observed, experimental values are fairly lower than the theoretical ones. Taking into account the fact that the combustion can be hindered by the existence of the engulfed equipment, two different combustion efficiencies can be defined. First of all, the combustion efficiency, Heat released in the combustion of a free surface pool ’ = Heat released in the case of complete combustion ’ of all the fuel evaporated (11)
12000
;
6000
(13)
5 =
Heat released in the combustion of a hindered pool Heat released in the complete combustion of all * the fuel evaporated in a free pool Figure 13 shows the value of q&r for the data shown in Figure 12 (pool fire of 4 m2 with a tank engulfed in it); the experimental data corresponding to the initial moments have a certain lack of accuracy due to the variability of fire spreading and increasing in this first step. Therefore, the VdUeS of qObSIare significant only over the region of Figure 13, in which they are approximately constant. For this set of values, the efficiency of hindered combustion is approximately 47%, considerably lower than the values found in the literature for free surface pool fires. The hindering effect of equipment engulfed in the fire can be observed from the data plotted in Figure 14, corresponding to a pool fire of hexane. For this hydro-
0 0
E 10000 s E 0000
77 x
co -I
14000
8 I
=
0 0 0 0 o
+f
0
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E
0
0
0
0
O
0
00°
0 0
2000 0
0
pi
0
I
7
0
lo
20
30
40
50
60
time(s) Figure 12 Total heat release rate, experimental and theoretical, as a function of time for test number 13 (4 m*, hexane)
0
10
20
;5
30
40
50
60
time (s) Figure 13 Efficiency of hindered combustion time for test number 13 (4 m*, hexane)
as a function of
Protection
of equipment
engulfed
12000 E
10000
c t P ;
8000 6000
0 0
10
20
30
40
50
60
time(s) Figure 14 Total heat release rate as a function of time for tests number 13 and 9 (4 m2. hexane. with and without tank respectively). The horizontal line cor;esponds to the theoretical heat release rate at steady-state, calculated according to Babrauskas’s equation
carbon, combustion efficiency in free surface pool fires of 92% has been proposed in the literature”. Figure 14 shows experimental data corresponding to the heat release rate for the pool fire with and without a tank engulfed in it. Furthermore, the maximum theoretical heat release rate (77= 100%) has also been plotted, calculated according to the method explained in the previous paragraphs. To check this method, the following data can be considered: the heat release rate at steady state (pool fire of hexane, 4 m*) calculated from the combustion rate predicted by Babrauskas’s equation” is 13 045 kW; by applying the new method proposed here which can be used even during the unsteady state - a value of 13 807 kW is obtained at f = 60 s (after reaching the stationary state). The small difference (5.5% with respect to the highest value) confirms the validity of the method. In Figure 14 it can be seen that for a free surface pool fire, the steady state would be reached after approximately 45 s, with a combustion efficiency of approximately 88%.
Conclusions The experimental data obtained have allowed the study of the evolution of a pool fire from its beginning up to its complete development. As the sizes of the pool fires were relatively large (4 m* and 12 m*), both the shape and size of the flames were relatively variable. This was even more important after the start-up of the deluge system, as a relatively strong wind was induced. In these conditions, the fire was moved towards one of the sides and this gave rise to an asymmetrical evolution of the temperatures of the equipment engulfed in the pool fire, with flame impingement essentially on the bottom and on one of the sides. Concerning the heating of this equipment, its temperature increased following an approximately linear trend, at a velocity which depended on the location of the measuring point. Thus, the highest temperatures were measured at the bottom of the tank (approximately 300°C in 1 min). In this zone, there was direct contact
in a pool fire: E. Planas-Cuchi
et al.
239
between the flames and the tank wall practically throughout the run, and the cooling fluid never reached this zone. Instead, as height increased the temperature reached was lower and the effect of the cooling fluid (after the start-up of the deluge system) was far greater. At a given point, the complete process followed five different steps. The first one corresponded to strong heating, followed by a second stage in which the temperature still increased but at a slower rate (in this step, induced wind was already noticeable). Immediately after the start-up of the deluge system, the temperature started to decrease, but very slowly (third step); there was still flame impingement, although the height of the flames was already decreasing. Once the cooling fluid reached that particular point, the temperature decreased dramatically (fourth step) up to the extinction of the fire. Afterwards, the temperature increased slightly due to the radiation from the hot walls of the experimental module. In certain zones of the tank only some of these steps were observed, depending on the existence or otherwise of flame impingement and on the contact or lack of it with the cooling fluid. Concerning the heat released by the fire, the experimental results were significantly lower than the theoretical values which should be expected. A method has been developed to calculate the maximum combustion rate, both for the developing stage (unsteady state) and for the fully developed fire. This has allowed comparison between the theoretical values and the experimental data with and without a tank in the fire. It has been found that the heat release rate is significantly lower when there is a tank engulfed in the fire; the existence of this equipment is in fact an obstacle for the flames. Thus, both a ‘hindering factor’ and an ‘efficiency of hindered combustion’ have been defined. The values of qobsr are significantly lower than those corresponding to the combustion efficiency of a free surface pool, this being a clear effect of the hindering consequence of the engulfed equipment. The results obtained show the advantages of a system with the objective - from the first moment - of simultaneously cooling the equipment and extinguishing the fire. Covering these two aspects with a single deluge system can be very useful in providing a rapid solution to certain emergencies which can be encountered in the process industries.
Acknowledgements Two of the authors @P.-C. and J.C.) gratefully acknowledge partial financial support from the Universities and Research Commission of the Generditut (Catalan government).
References 1. Wighus, R. J.
Loss Prev. Process hi. 1994, 7, 305-309 2. Foam and Water Deluge Systems for Off-Shore Oil Platforms’,
Project Summary Report, Ek Contract: TH-X125/89-IT 3. ‘Fixed Water Surav and Deluge Protection for Oil and Chemical Plant’, IRI Info-m&ion IM.12r2.1.2, 3 June, 1991 4. Casal, J., Planas, E., Lancia, A. and Bordignon, L. in ‘Fire engineering and emergency planning. Research and application’, ed: R. Barham, Chapman and Hall, London, 19%. PD 209-217 5. Aydemir, N. k., Magapu, V. K., Sousa, A. k: M. and Venart, J. E. S. J. Hazardous Materials 1988, 20, 239-262
Protection of equipment engulfed in a pool fire: E. Planas-Cuchi et al.
240
6. Bainbridge, B. L. and Keltner, N. R. J. Hazardous Materials 1988, 20,214o 7. Beynon, G. V., Cowley, L. T., Small, L. M. and Williams, I. J. Hazardous Materials 1988,20, 227-238 8. Birk, A. M. J. of Hazardous Materials 1988, 20, 197-225 9. Mudan, K. S. hog. Energy. Combust. Sci. 1984, 10, 59-80 10. Heskestad, G. F. Safety J. 1984, 7, 25
11. Tewarson, A. ‘SFPE handbook of fire protection engineering’, Chap. 13, Section 1, 1.179-1.199, 1st edn, NFPA, 1990 12. Babrauskas, V. Fire Tech. 1983, 19, 251
Nomenclature ah
CP d F H
Ah, AH, k
Parameters in equation 5 Specific heat of the generated gases (kJ kg-’ . K-1) Pool diameter (m) View factor (dimensionless) Convection coefficient (kW me2 . K-‘) Heat of combustion (kJ kg-‘) Heat of vaporization (kJ . kg-‘) Extinction coefficient (m-l)
TL1” Tb Tf TR
0
5
Thermal conductivity of the pool walls (kW . m-r K-r) Combustion rate (kg. m-* s-r) Maximum combustion rate (kg. m-*. s-l) Flow rate of the gases produced in the combustion (kg . m-* s-l) Heat transfer rate (kW) Heat transfer rate per unit surface (kW . m-*) Heat released by convection (kW . mdZ) Total heat release rate emitted during the fuel combustion (kW . m-*) Time (s) Room temperature (K) Average temperature of flames at a given height (K) Boiling temperature of the fuel (K) Centreline flame temperature (K) Temperature at flame boundary (K) Temperature rise of the smoke (K) Combustion efficiency Efficiency of hindered combustion Stefan-Boltzmann constant (= 5.67 . 10-r? kW m-* aK4) Hindering factor
19%
Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0950-4230/96 $15.00 + 0.00
SO950-4230(96)00014-9 ELSEVIER
Protection pool fire
of equipment
engulfed
in a
Eul&lia Planas-Cuchi*, Joaquim Casal*, Antonio Lancia? and Leo Bordignon? *Centre d’Estudis de Rise Tecnolbgic (CERTEC), Department of Chemical Engineering of the Universitat Politkcnica de Catalunya-Institut d’Estudis Catalans, Diagonal 647, 08028 Barcelona, Catalonia, Spain i_Tecsa Ricerca & Innovazzione, via Aldo Moro 1, Scanzorosciate (BG), Italy Received 1 I February 1996 When process equipment is engulfed in a fire, the protection system should extinguish the fire and simultaneously cool the equipment. Therefore, the behaviour of equipment engulfed in a fire, as well as its eventual influence on the fire, should be known. In this article, experimental data from pool fires of hexane (4 m2) and kerosene (12 mZ) are discussed. The evolution of the fire up to its full engulfment is studied, as is the evolution of temperatures at different points of a tank engulfed in it. The heat release rates under the different operating conditions are studied. A method is developed to calculate the maximum combustion rate, both for the developing stage and for the fully developed fire. The hindering effect of the tank on the fire is discussed and quantified in terms of ‘efficiency of hindered combustion’. Copyright 0 1996 Elsevier Science Ltd Keywords: pool fire; foam and water; cooling; extinguishing;
Certain industrial installations are designed according to an extremely compact lay-out. Off-shore oil platforms and some sections of chemical process plants exhibit a high density of equipment which, in the event of certain incidents, can lead to dangerous situations and can significantly complicate the management of emergency procedures’. In such installations, moreover, there are usually large inventories of hazardous materials, and protection systems with a high degree of reliability and efficiency must be provided. If flammable materials are processed or stored, a good fire protection system must be installed. Such a system should be able to cover two objectives simultaneously: extinguishing or at least controlling the fire and, furthermore, cooling the structures and the equipment. This second scope is very important, in some cases even more than the first one, as the effect of fire on certain equipment can result in further loss of containment of flammable substances and hence in the escalation of the accident scenario. In order to cover these two objectives there are various possibilities. In the conventional approach, cooling and extinguishing are addressed by two different systems: water deluge systems and foam systems, the specific features of water and foam thus being used separately. Water is the most commonly used fire extinguishing agent; it has a high specific heat and very high latent heat of vaporization, and furthermore it is usually available at
modelling
a low cost. However, the use of water also has several disadvantages, the main one resulting from its relatively high density, higher than that of most hydrocarbon fuels. Furthermore, hydrocarbons are also immiscible with water; therefore, in the event of a pool fire water will neither cover the burning fuel - thus extinguishing or reducing the fire - nor mix and dilute it. The hydrocarbon will maintain a lighter burning layer, and in some cases water may even flash (if it enters a hot mass of fuel) and spread fuel. If a cover for the burning surface is required - and this will help significantly in the control of the fire - foam solutions must be used. However, foam agents are much more expensive than water and, moreover, they are available in much smaller amounts. One interesting approach consists, therefore, in using a single deluge system which discharges foaming solution for a given minimum time and then continues discharging water. Such systems, known as ‘foam and water deluge systems’, should discharge a kind of foam which is fluid enough to behave like water for cooling purposes but stable enough to possess useful extinguishing properties in spill fires. Foam and water deluge systems are quite advantageous, especially for off-shore oil platforms and processing plants, due to the possibility of achieving fast fire control, the simplification of plant structure and operating procedures and the resulting reduction in the costs of the system. This work discusses the behaviour of a typical piece of equipment (a tank) during the development and
231
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Protection
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233
Table 1 Description of the selected experiments
Test number
(m*)
Pool area Fuel
Extinction agent
Sprinklers configuration
S.A.R3* (Lmin-‘.m-*)
Deluge flow (I.min-‘)
12 13 14 15 88 89 90 91
4 4 4 4 12 12 12 12
Hexane Hexane Hexane Hexane Kerosene Kerosene Kerosene Kerosene
Foam Foam Water Water Foam Foam Foam Foam
two two two two two two two two
14.2 14.2+ 14.2+ 14.2 14.2+ 14.2+ 6.5 6.5
246 296 296 246 296 296 112 112
‘Surface
application
rate
speaking, the results for the hexane pools (4 m2) showed a practically linear trend, essentially identical for the first three tests; run number 15 showed slightly higher values with respect to the rest of the results, although the trend was similar. For the kerosene pools the results were parabolic rather than linear; this can be attributed to the fact that the development of fire over the whole surface of the pool (in this case 12 m2) was slower. The data obtained from the radiometers have been used to plot the variation of radiation intensity as a function of time (Figure 3). As can be observed, after approximately 40 s the fire can be considered to be fully developed, and with a practically stationary regime (the time required to reach this condition was more than 60 s for the 12 m2 kerosene pool fires); typical oscillations exist, which must be related to the turbulence of the fire and to the variability of the shape, size and orientation of flames as a function of time. Later on, the decrease of radiation intensity shows the effects of the start-up of the extinction system. The action of the foam-water mixture caused a significant decrease in the size of the flames, which finally led to the complete extinction of the pool fire as it became covered with a stable layer of foam.
Evolution of temperature As the fire proceeds, the equipment engulfed in it - a tank in this work - will undergo an increase in temperature which will be a function of the evolution and size 4.5 4 3.5 T3 E 2.5 Radiometer 26
j2
-Radiometer
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z! 1.5 1 0.5 0 0
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SPKOl125” SPK0/125”+SPY0/90” SPK0/125”+SPY0/90” SPKOf125” SPK0/125”+SPY0/90” SPK1/90”+SPY0/90” SPK0/125” SPKl/l ?O”
100
150
200
250
time (s) Figure3 Variation of the radiation intensity as a function time for test number 13 (4 m*, hexane)
of
of the flames, and also of the location of the measuring point; those zones exposed to flame impingement will undergo stronger heating than those which are not in direct contact with them. On the other hand, after the start-up of the cooling/extinguishing system, different behaviour will be observed depending on the existence or otherwise of a water/foam layer on that particular zone of the equipment surface. The analysis of the experimental results indicates the existence of five different situations or steps4, which are discussed in the following paragraphs.
Initial step (development of the jire) The first step corresponds to the development of the fire and takes place over the first minute (approximately). Figure 4 shows the variation of temperature as a function of time, at diverse measuring points, for two of the tests. Temperature increases approximately linearly with time, the slope depending on the location of the point and on the impingement of the flames on that point. In one minute, the temperature at the bottom of the tank increases from room temperature to more than 350°C while in the lower zone of the sides it does not exceed 250°C and at the top the maximum temperature is 125°C. From the data plotted in Figure 4(b) it can be seen that the temperatures registered by the thermocouples located in the upper sides of the tank show a degree of scattering. This must be attributed to the fact that the fire was not exactly symmetrical owing to turbulences and the arrangement of the module walls. Thus, although theoretically all the thermocouples should show the same behaviour (as in Figure 4(a)), the induced wind and the radiation from the heated walls caused these differences between the measurement points. For the 12 m2 pool fires (Figure 4(c)), the scattering is less noticeable and the temperatures are higher, because in this case the engulfment of the tank in the fire is more complete, and therefore the impingement of the flames on its walls has a greater influence. As in Figure 4(a), the bottom of the tank (Figure 4(d)) for the 12 mz pool fires shows the same behaviour for all the thermocouples. The evolution of the temperature at different points of the tank as a function of time has also been plotted in Figure 5. The experimental points show clearly how the bottom of the tank undergoes the most intense heating action, due to the greater exposure to the flames.
Protection
234
of equipment
engulfed
8) 400 I
in a pool b) 300 I
350 -300 --
JiE& 17 11
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A Themwouple
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Operation of the extinguishing
h
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time (8) Tank temperature upper tank laterals;
50
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19
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0 Thermocouple 7
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t
et al.
fire: E. Planas-Cuchi
tank temperature at different time of height. Test number 89 (12 m*,
system
The evolution of the fire during its development and during the extinguishing process can be seen schematically in Figure 6, obtained from a film. In the initial steps the fire increases rapidly, soon engulfing the whole tank. The shape of the flames is variable, as usually happens with relatively large fires. As flame length increases, so do the turbulence and mobility of the flames. From the moment at which the extinction system is started, a significant increase in the turbulence of the flames is observed, together with the existence of wind from the west side; as a consequence, the flames move towards the east side (see Figure 6)4. Therefore, the west side of the tank is less influenced by the flames (the surface area undergoing flame impingement decreases significantly) and consequently the temperatures measured here are lower. After reaching a maximum, the temperature at the various measuring points of the tank starts to decrease; in the plot of temperature versus time, a negative slope is obtained. This situation corresponds to the moment at which the cooling effect of water/foam starts, i.e. when
Protection of equipment Test no12 Time = 10s
235
engulfed in a pool fire: E. Planas-Cuchi et al.
Test no12 Time = 20 8
8 A 300 2 250 a [ 200 j Test no12 Time = 30 s
150
Test no12 Time = 40 s
0
50
100
150
200
250
300
350
400
tlrne (s) Figure 7 Stages of the temperature evolution at the lower tank laterals (thermocouple 6). Test number 12 (4 m2, hexane) Test no12 Time = 50 s n4
Test no12 Time = 60 s
Test no12 Time = 120s
Test no12 Time = 1802
Figure 6 Evolution of the fire during its development and during the extinguishing process. Test number 12 (4 m*, hexane)
the mixture reaches that measuring point. In any event, while the tank surface continues to be exposed to flame impingement, temperature will decrease slowly, although the size of the fire decreases significantly after the start-up of the deluge system. Later on, as the pool gradually gets covered by the foam, the height of the flames gets less and less and they no longer impinge on the tank wall. Now the cooling action of the water is stronger and the temperature decreases dramatically, with a steep negative slope in the plot of temperature versus time. Finally, when the fire is extinguished and the extinguishing system is stopped, a slight increase in temperature is again observed due to the heating caused by the radiation from the hot walls of the experimental module. This process establishes in a relatively clear way the existence of five stages in the evolution of temperature at the various points of the tank wall (Figure 7)4. The first one, which corresponds to the initial development of the fire, is characterized by a rapid increase in temperature; in the plot of temperature versus time a practically
straight line is obtained with a steep slope (the heating rate depends on the location - at the bottom of the tank values of the order of 3.6”C’s were registered). In the second stage, which corresponds to a fully developed pool fire, temperature continues to increase but at a slower rate and with a trend which is no longer linear; in this stage a considerable wind appeared in all the tests. The third stage initiates with the start-up of the cooling/extinguishing system. The temperature starts to decrease - its maximum value having been reached at the transition between these two steps - gradually, at an approximately constant rate. The fourth stage corresponds to a situation in which, due to the blanketing action of the foam over the pool, the magnitude of the flames has decreased considerably and they no longer reach the point in the tank where the temperature is being measured. Now the heat balance is clearly influenced by the dominant mechanism of the foam/water cooling, with a relatively weak heat input by radiation; thus, temperature decreases abruptly, following an approximately linear trend. Finally, once the fire is extinguished, gentle heating is observed. The existence of these five stages was observed only in certain conditions during the experimental tests. In other cases some of the intermediate stages were not found, depending on the point at which the temperature was being measured. This happened, for example, when the influence of flames on the tank wall ceased at the same moment that the cooling mixture reached that particular point. Stage 3 does not exist and just after reaching its maximum value the temperature decreases abruptly to very low values, and afterwards continues to fall very gradually (see Figure 8). For the whole system, this situation would correspond to an immediate extinction of the pool fire. This case, which would be the best one from the point of view of controlling the emergency, in practice implies conditions which can only be found at certain points of real installations. At the bottom of the tank, however, flame impingement existed practically throughout the experimental test, finishing only when the fire was extinguished. Furthermore, this zone was never reached by the cooling mixture in the various tests that were carried out. This
236
engulfed in a pool fire: E. Planas-Cuchi et al.
Protection of equipment 350
overall heat release rate and of the heat released by convection are discussed.
300 250
Overall heat released The heat released during each test was measured as a
1
g B e! 200 i
150
I-” 100
50 _
5 01 0
50
100
150
200
250
300
350
400
time (s) Figure 8 Stages of temperature evolution at the lower tank laterals (thermocouple 12). Test number 12 (4 m2, hexane)
is why slope changes are smoother and therefore the temperature values are higher up to the extinction of the fire (Figure 9).
Heat released from the pool fire Probably the best way to follow the development and evolution of the pool fire is to measure the heat released by the combustion. For a pool fire of a given fuel, the heat release rate is a function of the combustion rate and of the combustion efficiency. The heat release rate will rise rapidly in the initial moments, while the fire is developing and the size of the flames is increasing, and after that it will have an approximately constant value once the fire is completely developed. Nevertheless, for pool fires of a relatively large size, this second stage which would correspond to a stationary state if no changes or external actions were imposed on the system - is characterized by significant variations in flame size and shape. The evolution of the heat release rate as a function of time will be therefore a way to identify the moment at which the pool fire is completely developed. In the following paragraphs the experimental results of the 700
Heat released by convection The heat released from a fire is essentially divided into
two fractions, one transferred by convection and the other transmitted by radiation. Both contributions can be determined separately and independently from Q,,. The heat released by convection can be obtained from the flow rate and temperature of the gases produced in the combustion:
. Q
I, =
c-cm”
iv*moke*
cpx AT,,, .
(1)
During the experimental tests, these data were measured by the calorimeter and treated in such a way as to give continuously the value of Q,,,,. The trend of the evolution of the heat released by convection as a function of time is similar to that corresponding to the overall heat release rate: for the pool fire of 4 m* a gradual increase in the heat released, with a steep slope during the first 30 s; later on, a more gradual increase is observed - with some scattering caused by the turbulence and irregularity associated with the phenomenon - until the maximum value (steady state) is reached at approximately 60 s.
‘Ooo I
l-
!Gi4 5
600
function of time by using the adiabatic calorimeter. The plot of overall heat release rate versus time (Figure 10) shows a steep rise at first, with a characteristic change in slope at approximately 30 s; from this moment the heat release rate still increases, but more gradually, up to a maximum-value (fully developed fire). For a 4 m* pool fire of hexane, the heat release rate increased from zero at ignition (t = 0) to rates of the order of 5 100 kW after 30 s. For the kerosene pool fires (12 m*), the heat release rate increased to rates of the order of 16 600 after 60 s. When the deluge system was activated, the heat release rate decreased rapidly until the extinction of the fire. In those cases in which only water was used for the extinction (tests 14 and 15), the heat release rate also decreased after the activation of the deluge system, but a few seconds later the fire revived.
6000 +
500
E
g! 400
E
B B
p 300 8 200 100 0
hod ,
c -F
0
50
7
100
150
200
250
3o 50
100
150
200
250
time (s)
time (s) Figure9 Temperature evolution at the tank (thermocouple 5). Test number 91 (23 m2, hexane)
0
bottom
Figure 10 Total heat release rate evolution time for test number 13 (4 ma, hexane)
as a function
of
Protection
of equipment
engulfed
After this moment, a sudden fall can be observed due to the start-up of the extinguishing system. The 12 m2 pool fires (Figure 11) show an increase in the heat released, with a steep slope, during the first 60 s, after which the extinction system is activated. In this case the steady state was not reached, i.e. the fire was still developing when the sprinklers were activated. It can be observed that after that moment the heat release rate displays an approximately constant value for a certain period, with a steep fall afterwards; this can be attributed to the difficulties in extinguishing the fire. For hexane pool fires of 4 m*, the heat released by convection was approximately 57% of the overall heat released. In the case of kerosene, this fraction was approximately 65%. The kerosene fires gave rise to larger amounts of dark smoke, i.e. the fire was less bright and the amount of heat transmitted by radiation was correspondingly lower.
in a pool
fire: E. Planas-Cuchi
et al.
237
In this equation, the first term on the right-hand side corresponds to the heat transferred by conduction; this is a boundary effect, significant only for pools with a few centimetres in diameter and, therefore, negligible here. The second term is the heat transferred by convection, also negligible for the large pools studied in this work. For pools with a diameter of 1 m or more, the radiation mechanism is the only one which is really important. Therefore, in this work the rate at which heat is transferred to the pool is given by:
ri = -(n&/4)
D
F(q - c)(l
- eekd) .
The theoretical combustion rate can be obtained from this expression and from the heat required to evaporate the fuel. As combustion rate increases with pool size, for a pool of infinite diameter, the maximum combustion rate is obtained:
Discussion Most communications published on hydrocarbon pool fires deal with fully developed fires, and very little information is to be found on the initial unsteady state, when the fire is developing. This is why there is no equation available in the literature to predict the evolution of flame temperature during the initial development of a fire. Moreover, when relatively large equipment is engulfed in a pool fire, it should have a disturbing effect on the flames as compared to a free surface pool fire; again, this aspect has not been studied by those authors who have worked in this field5-8. In the following paragraphs this effect is studied. In order to calculate the theoretical (maximum) combustion rate in both an unsteady and a steady state, a semiempirical expression is derived from the treatment of experimental data. Accordingly, the heat transfer rate from the fire to the pool is given by the following expression: rd 214
Heat bv convection
and for a pool fire of finite diameter: . m f=h’;max N
(1 -emM).
From equation (4) it can be inferred that during the initial development of the fire (unsteady state) the combustion rate will change with time, as will flame temperature. Moreover, flame temperature will also change with height. The experimental data have shown that during the development of a pool fire the flame temperature changes with time according to an expression of this type: Tf
@A=
t (b
+
(f-5)
&)
where a and b are parameters which can depend on height. The values obtained for the different operating conditions are given in Table 2.
Heat by conduction +
oF(G
-
Table 2
1 - emkd).
c)(
Values a and b for different operating conditions
keat by radiation
7000 0 0
t)C b 6000 --
0"
08
coo
%O
a
0
Q,
b
0
80°
Hexane (4 m2)
Kerosene (12 m*)
0.000851 ~0.0034+0.021 h)
(0.000465-0.000188h~ (0.0347+0.0114h)
5000 --
0
Equation (5) was used to calculate the flame temperature just above the pool surface (h = 0). However, it must be taken into account that this temperature corresponds to the central axis of the flame. Flame temperature decreases over the radius, from flame axis to periphery; at the flame boundary, the temperature is approximately 50% (in terms of temperature increase with respect to room temperature*) of the temperature
0
4000 --
00 0
3000 --
0 0
2000 -0 1000
--
rprinklers diwUon
0 0
ocr
:
0
30
60
00
120
150
180
210
240
time (s) Figure 11 Heat released by convection for test number 90 (12 m2, kerosene)
as a function of time
* rR - T, = 0.5 x (Tr- TJ, then r, = 0.5 x CT,+ T,). The average temperature at a given height can be determined, as a function of temperature at flame boundary and room temperature, from the following expression: T,,= (T, + T,)/Z. In this work, a room temperature of 293K has been taken.
238
Protection of equipment
engulfed in a pool fire: E. Planas-Cuchi et al.
at the axislo. Thus, the average flame temperature at a given height is approximately T,, = 0.75 . Tf + 73.25
(7)
By introducing this equation, together with equation (6), into equation (5), it is possible to obtain a semiempirical expression for the calculation of the combustion rate during the non-stationary regime: riz(t) =
FU
t
AK + c,(T,-T,)
c
-
0.75(b+at) + 73.25 4
[(
-I
(8)
(l-e-“). 1
The heat release rate can be obtained as
*n = m; AH, . Q t,,fa,
(9)
As in this case the view factor value between the fire and the pool is unity and the mass loss rate can be determined from equation (8); the rate at which heat is released can be calculated from the following relationship:
When there is equipment engulfed in the fire, this equipment has a hindering or obstructing effect on the fire, thus a ‘hindering factor’ can be defined as the ratio between the heat released in the combustion of a pool fire with an obstacle (for example, a tank) and that released in the same pool fire with a free surface: Heat released in the combustion of a pool with an obstacle 5= Heat released in the combustion of the same ’ pool without any obstacle (12) 5 would have a maximum value of 1 in those cases in which the influence of the obstacle is negligible, and decreasing values as the hindering effect increases (i.e. as the size of the equipment as compared to that of the pool increases). Finally, the ‘efficiency of hindered combustion’ can be defined as the ratio between the heat released in the combustion of a hindered pool and the maximum tbeoretical heat which could be released if the combustion was complete and non-obstructed: %bst
This expression gives the maximum heat release rate, i.e. that corresponding to the combustion of all the fuel evaporated. However, in an accidental fire, not all the fuel is burnt, i.e. combustion efficiency is not 100%. Figure 12 shows the experimental values for a fire of 4 m2, as well as those calculated by the method proposed in the aforementioned paragraphs. As can be observed, experimental values are fairly lower than the theoretical ones. Taking into account the fact that the combustion can be hindered by the existence of the engulfed equipment, two different combustion efficiencies can be defined. First of all, the combustion efficiency, Heat released in the combustion of a free surface pool ’ = Heat released in the case of complete combustion ’ of all the fuel evaporated (11)
12000
;
6000
(13)
5 =
Heat released in the combustion of a hindered pool Heat released in the complete combustion of all * the fuel evaporated in a free pool Figure 13 shows the value of q&r for the data shown in Figure 12 (pool fire of 4 m2 with a tank engulfed in it); the experimental data corresponding to the initial moments have a certain lack of accuracy due to the variability of fire spreading and increasing in this first step. Therefore, the VdUeS of qObSIare significant only over the region of Figure 13, in which they are approximately constant. For this set of values, the efficiency of hindered combustion is approximately 47%, considerably lower than the values found in the literature for free surface pool fires. The hindering effect of equipment engulfed in the fire can be observed from the data plotted in Figure 14, corresponding to a pool fire of hexane. For this hydro-
0 0
E 10000 s E 0000
77 x
co -I
14000
8 I
=
0 0 0 0 o
+f
0
4000
E
0
0
0
0
O
0
00°
0 0
2000 0
0
pi
0
I
7
0
lo
20
30
40
50
60
time(s) Figure 12 Total heat release rate, experimental and theoretical, as a function of time for test number 13 (4 m*, hexane)
0
10
20
;5
30
40
50
60
time (s) Figure 13 Efficiency of hindered combustion time for test number 13 (4 m*, hexane)
as a function of
Protection
of equipment
engulfed
12000 E
10000
c t P ;
8000 6000
0 0
10
20
30
40
50
60
time(s) Figure 14 Total heat release rate as a function of time for tests number 13 and 9 (4 m2. hexane. with and without tank respectively). The horizontal line cor;esponds to the theoretical heat release rate at steady-state, calculated according to Babrauskas’s equation
carbon, combustion efficiency in free surface pool fires of 92% has been proposed in the literature”. Figure 14 shows experimental data corresponding to the heat release rate for the pool fire with and without a tank engulfed in it. Furthermore, the maximum theoretical heat release rate (77= 100%) has also been plotted, calculated according to the method explained in the previous paragraphs. To check this method, the following data can be considered: the heat release rate at steady state (pool fire of hexane, 4 m*) calculated from the combustion rate predicted by Babrauskas’s equation” is 13 045 kW; by applying the new method proposed here which can be used even during the unsteady state - a value of 13 807 kW is obtained at f = 60 s (after reaching the stationary state). The small difference (5.5% with respect to the highest value) confirms the validity of the method. In Figure 14 it can be seen that for a free surface pool fire, the steady state would be reached after approximately 45 s, with a combustion efficiency of approximately 88%.
Conclusions The experimental data obtained have allowed the study of the evolution of a pool fire from its beginning up to its complete development. As the sizes of the pool fires were relatively large (4 m* and 12 m*), both the shape and size of the flames were relatively variable. This was even more important after the start-up of the deluge system, as a relatively strong wind was induced. In these conditions, the fire was moved towards one of the sides and this gave rise to an asymmetrical evolution of the temperatures of the equipment engulfed in the pool fire, with flame impingement essentially on the bottom and on one of the sides. Concerning the heating of this equipment, its temperature increased following an approximately linear trend, at a velocity which depended on the location of the measuring point. Thus, the highest temperatures were measured at the bottom of the tank (approximately 300°C in 1 min). In this zone, there was direct contact
in a pool fire: E. Planas-Cuchi
et al.
239
between the flames and the tank wall practically throughout the run, and the cooling fluid never reached this zone. Instead, as height increased the temperature reached was lower and the effect of the cooling fluid (after the start-up of the deluge system) was far greater. At a given point, the complete process followed five different steps. The first one corresponded to strong heating, followed by a second stage in which the temperature still increased but at a slower rate (in this step, induced wind was already noticeable). Immediately after the start-up of the deluge system, the temperature started to decrease, but very slowly (third step); there was still flame impingement, although the height of the flames was already decreasing. Once the cooling fluid reached that particular point, the temperature decreased dramatically (fourth step) up to the extinction of the fire. Afterwards, the temperature increased slightly due to the radiation from the hot walls of the experimental module. In certain zones of the tank only some of these steps were observed, depending on the existence or otherwise of flame impingement and on the contact or lack of it with the cooling fluid. Concerning the heat released by the fire, the experimental results were significantly lower than the theoretical values which should be expected. A method has been developed to calculate the maximum combustion rate, both for the developing stage (unsteady state) and for the fully developed fire. This has allowed comparison between the theoretical values and the experimental data with and without a tank in the fire. It has been found that the heat release rate is significantly lower when there is a tank engulfed in the fire; the existence of this equipment is in fact an obstacle for the flames. Thus, both a ‘hindering factor’ and an ‘efficiency of hindered combustion’ have been defined. The values of qobsr are significantly lower than those corresponding to the combustion efficiency of a free surface pool, this being a clear effect of the hindering consequence of the engulfed equipment. The results obtained show the advantages of a system with the objective - from the first moment - of simultaneously cooling the equipment and extinguishing the fire. Covering these two aspects with a single deluge system can be very useful in providing a rapid solution to certain emergencies which can be encountered in the process industries.
Acknowledgements Two of the authors @P.-C. and J.C.) gratefully acknowledge partial financial support from the Universities and Research Commission of the Generditut (Catalan government).
References 1. Wighus, R. J.
Loss Prev. Process hi. 1994, 7, 305-309 2. Foam and Water Deluge Systems for Off-Shore Oil Platforms’,
Project Summary Report, Ek Contract: TH-X125/89-IT 3. ‘Fixed Water Surav and Deluge Protection for Oil and Chemical Plant’, IRI Info-m&ion IM.12r2.1.2, 3 June, 1991 4. Casal, J., Planas, E., Lancia, A. and Bordignon, L. in ‘Fire engineering and emergency planning. Research and application’, ed: R. Barham, Chapman and Hall, London, 19%. PD 209-217 5. Aydemir, N. k., Magapu, V. K., Sousa, A. k: M. and Venart, J. E. S. J. Hazardous Materials 1988, 20, 239-262
Protection of equipment engulfed in a pool fire: E. Planas-Cuchi et al.
240
6. Bainbridge, B. L. and Keltner, N. R. J. Hazardous Materials 1988, 20,214o 7. Beynon, G. V., Cowley, L. T., Small, L. M. and Williams, I. J. Hazardous Materials 1988,20, 227-238 8. Birk, A. M. J. of Hazardous Materials 1988, 20, 197-225 9. Mudan, K. S. hog. Energy. Combust. Sci. 1984, 10, 59-80 10. Heskestad, G. F. Safety J. 1984, 7, 25
11. Tewarson, A. ‘SFPE handbook of fire protection engineering’, Chap. 13, Section 1, 1.179-1.199, 1st edn, NFPA, 1990 12. Babrauskas, V. Fire Tech. 1983, 19, 251
Nomenclature ah
CP d F H
Ah, AH, k
Parameters in equation 5 Specific heat of the generated gases (kJ kg-’ . K-1) Pool diameter (m) View factor (dimensionless) Convection coefficient (kW me2 . K-‘) Heat of combustion (kJ kg-‘) Heat of vaporization (kJ . kg-‘) Extinction coefficient (m-l)
TL1” Tb Tf TR
0
5
Thermal conductivity of the pool walls (kW . m-r K-r) Combustion rate (kg. m-* s-r) Maximum combustion rate (kg. m-*. s-l) Flow rate of the gases produced in the combustion (kg . m-* s-l) Heat transfer rate (kW) Heat transfer rate per unit surface (kW . m-*) Heat released by convection (kW . mdZ) Total heat release rate emitted during the fuel combustion (kW . m-*) Time (s) Room temperature (K) Average temperature of flames at a given height (K) Boiling temperature of the fuel (K) Centreline flame temperature (K) Temperature at flame boundary (K) Temperature rise of the smoke (K) Combustion efficiency Efficiency of hindered combustion Stefan-Boltzmann constant (= 5.67 . 10-r? kW m-* aK4) Hindering factor