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Illegal carriage of dangerous goods and their effects on tunnel safety
TUNNEL SAFETY
Illegal Carriage of Dangerous Goods and Their Effects on Tunnel Safety J. S. M. Li and W. K. Chow A b s t r a c t --Consequent ~,oa recent fire resulting from a truck carrying illegal diesel outside a tunnel in Hong Kong, people are quite coneerned about the safety aspects of vehicular tunnels. That truck was an ordinary vehicle without appropriate protection against the possible accidental fires due to those liquid fuels. The safety impact on a vehicular tunnel of afire in an unprotected truck carrying illegal diesel is studied in this paper. Empirical results on spill fires and coolingjet expressions were applied to assess the probable tunnel environment. The two-layer zone model C F A S T was also used to verify the results. The smoke temperature in the tunnel within 100 m of the fire might be up to 300°C, and smoke would travel rapidly. It is recommended that a proper safety management scheme should be worked out by the tunnel management authority. If necessary, every truck of enclosed structure should be inspected before entering a vehicular tunnel longer than a specified length, say 230 m. © 2000 Published by Elsevier ScienceLtd. All rights reserved.
1. Introduction " ore and more illegal mobile oil stations are found in Hong Kong [1], selling illicit oil such as un. treated diesel oil. This has caused problems when buildings without proper protection against the possible fire risks are used for storing large quantities of illegal liquid fuel. In addition, trucks carrying those liquid fuels may cause problems while travelling through vehicular tunnels. According to Hong Kong's Fire Services Department (FSD) Regulations [2,3], any vehicle used for conveyance of Category 2 or Category 5 dangerous goods, including diesel oils and other liquid fuels, must have sufficient fire protection and must obtain an approval license issued by the FSD. Vehicles that carry large amounts of inflammable liquid without licenses can greatly endanger the safety of road users, especially when the truck is moving inside a vehicular tunnel. As an example, a fire accident of a lorry carrying about 500 liters of untreated diesel oil occurred just outside the Lion Rock Tunnel [4,5] in July 1999. It did not have an FSD license for carrying dangerous goods; nor did its design protect it against liquid fuel fire. The lorry was driven unusually fast while travelling through the tunnel from Kowloon to Shatin, ~ad it did not stop to pay at the toll counter. The vehicle was observed to be emitting smoke while passing through the counter. The lorry kept going for about 30 m beyond the toll counter before it finally stopped. The driver then went away and left the truck burning. The
M
Present address: Jojo SM. Li and W. K. Chow, Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China
fire was suspected to be started when oil leaked by the hot engine axles was ignited. When such a fire accident occurs in a tunnel, much more severe damage results than in a fire occurring in the open air, as in this case. Aggravating factors include confined space, limited number of escape roads, flashing over and loss of visibility. In view of this, the hazard scenarios for truck fires such as this are assessed in this paper, according to arguments addressed in the literature. In fact, the incident described above is not the first to have occurred in Hong Kong. An explosion once occurred in a container truck carrying motorcycles [6]. The shock waves generated from the explosion were so strong that glass and even some external finishes in nearby buildings were damaged. As reported in the news, the explosion even hurled hot shards of twisted metals up to the 38th floor of a building (nearly 120 m away) adjacent to the road. Fortunately, the explosion happened on an open highway, before the truck entered a tunnel.
2. Fire Scenarios Based on the information reported in the newspaper, the following fire scenario is assumed: A lorry carrying 0.5 m 3 of diesel oil travels in the Lion Rock Tunnel. Oil is spilled from a circular hole at the bottom of the tank, producing a fire. The tank is assumed to be rectangular: length 0.5 m, width 0.1 m, and height 0.5 m. The Lion Rock Tunnel [e.g. 7] was first constructed to provide an economical route for conveyingwater from Shatin to Kowloon, as shown in Figure 1. It was built in the late 1960s, and the tunnel was enlarged later to provide space for an additional road link. One of the main reasons for building the tunnel was because of the new horse-racing ground in Shatin. Shatin was further developed later to become a big urban area, with a population over 1 million.
www.elsevier.com/loca~e/tust ~nnelling and UndergroundSp~e Technology,Vol. 15, No. 2, pp. 167-173, 2000 0886-7798/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. All rights re~erved. PII: S0866-7798(00)00044-4
Pergamon
Key characteristics of the tunnel are as follows: Length, portal to portal: 1,425 m Finished cross-sectional area: 69 m 2 Maximum clear width: 8.5 m Carriageway width: 7m Width of service for each walkway: 0.46 m Minimum headroom for vehicles: 4.7 m Longitudinal gradient: 1 in 400 from Shatin portal
nated air is drawn through the slots in the ceiling and returned to the stations where fresh air is supplied. Carbon monoxide detectors, visibility meters, fire-fighting equipment and emergency telephones are installed. Signals on the detectors are transmitted to the control room so that the speed of the ventilation fan can be changed to supply the desired quantity of fresh air.
Mechanical ventilation is provided in the tunnel through two stations, located at each end. Each station is equipped with four supply fans and four exhaust fans, supplying an air flow rate of 142 m3/s. Fresh air is forced through the supply ducts under the carriageway and led into the tunnel through the grilles provided in the walkways. Contami-
Heat and smoke production rates from such a fire are estimated for fire hazard assessment. There are many parameters to be considered when determining the fire size of a road tanker. The heat release rate Qs for the spillage fire can be calculated [8] in terms of the burning rate m", the burning area A, the heat of combustion AHc for diesel oil of value 39.7 MJ/kg [9], and combustion efficiency ~ [8], as:
3. Estimation of Heat Release Rate
N f
NORTHERN APPROACH ROAD
l \ ROCK TUNNEL
APPROACH ROAD
KOWI.OON BAY
TSIM SHA TSUI Figure 1. The Lion Rock Tunnel.
168 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 15, Number 2, 2 0 0 0
Q,, = ~m" AAH (1) As proposed in the literature [e.g. 8], the initial spillage size on the roadway, the burning rate of the spill fire, and the combustion efficiency are important. They are considered in this paper.
q = 2000Awk (V~l - kt)
(3)
where k is given by: (4)
k - Cv ~ D2 v/-~ 8 AT
3.1 Initial Spillage SAze Calculation of the fire spillage size is usually based on the radial spreading of liquid spill along the horizontal surface. The maximum spillage area A ~ can be obtained [8,9] from the volumetric flow rate q (6's) of fuel leaking from a tank, the mass spillage burning rate m" (kg/s mS), and the density of the fuel oil p, (940 to 1000 kg/m3): qp~ (2)
Theoretically, the simplified spillage area A cannot be larger than Am a x . Its value can be less, since spilled fuel will be removed by the drainage gutters.
3.2 Burning Rate of Spill Fires The burning rate of pool fires is usually calculated based on tests using relatively deep fuel beds of thickness greater than 50 ram. However, the thickness of the spillage film on roadways is expected to be less than 7 mm. Based on the literature [e.g. 8] and the tests that determined the burning rate for thin fuel beds (of thickness 7 mm), the burning rate can be taken as 0.036 kg/s m 2 in the analysis of the initial spill fire. Note that the effects of radiation from tunnel walls on the burning rate are not considered in this study.
A~a~ - 1000 m" In the literature 1:8],q at time t (s) from a circular hole at the bottom of the tanker can be calculated in terms of the horizontal surface area of the leaking compartment AT(mS), the hole diameter D (m), the initial height of gasoline h~ (m), the flow contraction coefficient C (0.7), and the acceleration due to gravity g (mL,s2):
70
60
50
40
o f f
30
P f
~3
# ¢
B
20
S J f b ~
o o,
lO
/
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Hole Diameter (m)
-
-
at c o m b u s t i o n e f f i c i e n c y 0 . 8
......
at c o m b u s t i o n e f f i c i e n c y 0.5
Figure 2. Maximum heat release rates for initial spill fires for different hole diameters.
Volume 15, Number 2, 2000
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY169
3.3 Combustion Efficiency The combustion efficiency q will vary with the ventilation conditions. For longitudinal ventilation with air speed higher than 3 m/s, q is assumed to be 0.8; for no ventilation, q is taken as 0.5. The maximum heat release rates for the initial spill fires for hole diameters up to 0.08 m at combustion efficiency 0.5 and 0.8 were calculated using equation (1). The results are presented in Figure 2. The value of Am~ was 53.3 m 2, giving quite a big fire. For small hole diameters resulting from small leakage rates, the heat release rate of the fire is only slightly affected by the combustion efficiency. A possible explanation is that all diesel would be burned up quickly. For the above case, as shown in Figure 2, the heat release rate varies between 2 MW and 61 MW. The heat release rate also depends on the ventilation conditions in the tunnel, and will increase with the ventilation velocity.
4. Fire Environment Fire zone models are well-developed and might be suitable [10] for simulating a tunnel fire with the multi-cell concept. For the Lion Rock Tunnel, the ceiling height is 8.5 m. The equations for the fire plume and ceiling jet employed in most zone models would be valid for this ceiling height. Because the tunnel is long, the horizontal movement of the ceiling jet is unconfined. However, care must be taken on the horizontal ceiling jets when there are no vertical ceiling screens (soffits) at the doorway.
Maximum gas temperature Tm~ near the ceiling at a given radial position r away from the fire plume axis can be described by equations proposed by Alpert [11]: f Tm~- T =
5.38 (Q~/r)~3 H 16.9Qs2;3
(5)
H5/3 where H is the height of the ceiling above fire of 4.7 m, and T is the ambient temperature taken as 25°C. Variations of the temperature rise (Tm~ - T®) along r for a spillage fire of hole diameter 0.08 m, combustion efficiency 0.5 and 0.8 (Q8 was then 38 MW and 61 MW, respectively), are shown in Figure 3. It is observed that the temperature above the fire can be above 350°C, which then decreases to slightly above 100°C when r is 30 m away. The values of (Tm~ - T ) then drops fairly rapidly with r increased, but would still be close to 50°C at 700 m away from the fire. The horizontal component of the smoke velocity can be estimated by the filled ceiling jet equation: 0.197 Qsl/3 HI/2 r~/6
r>0.18H
0.946(-~) ~3
r < 0.18H
V=
(6)
400
350
. ~ 150. ~D I00-
50.
o
0
I
I
100
200
I 300
I 400
I 500
I 600
I 700
800
Distam¢ fi'omphn-~axis (m) ......
at c o n t x ~ o n efficiency 0.5 at ~ d x ~ o n diicier~y 0.8
Figure 3. Gas temperatures near ceiling.
170 TUNNELLINGANDUNDERGROUNDSPACE TECHNOLOGY
Volume 15, Number 2, 2000
these cases are very close to 3.1 m. Note t h a t the length limit of a room in CFAST 3.13 was 500 m. For a longer tunnel segment, the multi-cell concept was used. A three-cell configuration with the fire at the central cell was considered. For each segment of length 300 m, the average smoke temperature over the three cells was 132°C and the smoke layer interface height was kept at 2.8 m. If each tunnel segment is taken as 500 m long, the average smoke temperature was 93°C with the smoke layer interface height kept at 2.6 m. The results are more consistent with those shown in Figure 3.
The results of V along r for t h a t spillage fire are shown in Figure 4. The velocity of the ceiling j e t would be above 5 ms 1 at positions above the fire, but decreases rapidly when r is increasecl. The values of V decreases due to volume contraction :resulting from the falling t e m p e r a t u r e as r is increased. Mixing with the e n t r a i n e d cool air would lower the t e m p e r a t u r e further. From the estimation of the horizontal component of gas velocity, the time for smoke propagation along the ceiling can be calculated, with the results shown in Figure 5. It can be seen t h a t smoke ,:an propagate about 100 m in the first minute, and t h a t the propagation of smoke can be fast in such a case of spillage fire. The zone model C FAST version 3.13 [e.g. 12] was applied to verify the above results. A tunnel segment 100 m long with a fire located at the centre was considered. With an assumed NFPA ultra-fast t2-fire burning, with the cutoff heat release rate of 30 MW at 410 s to 1500 s, and then decayed to zero at 2000 s, the average smoke temperature was 320°C with the smoke layer interface height kept at 3.1 m above the floor during the burning period (from 410 s to 1500 s). The results are not consistent with those estimated as shown in Figure 3, as the t e m p e r a t u r e was much lower in that figure. However, the smoke t e m p e r a t u r e decreased to 255°C when the length of the tunnel segment increased to 200 m; 225°C when segment length was 300 m; 195°C when the segment length was 400 m; and 175°C for the length of 500 m. The smoke layer interface heights for all
5. Discussion There are several points to note from the above analysis: • Empirical expressions on the spillage fire and ceiling jets are useful in estimating the probable fire environment in the tunnel. In using the fire zone model CFAST 3.13 [e.g. 12], there is a limit on the tunnel length. The multi-cell concept is useful in this case, but the length of the tunnel segment has to be considered carefully. • As computed from empirical expressions on ceiling jets, smoke with temperature above 100°C will be encountered within 30 m away from the plume and fast horizontal velocity will be generated during fires. This suggests t h a t smoke should be extracted in places as near to the fire source as possible.
7.m
6.
5.
O 0
3,
ii
0
ii
I
I
I
I
I
I
I
100
20O
3O0
400
500
6O0
700
o
8O0
Distance fmmpltam axis (m)
...... at 'rtx .............. at ccmta
on efficient 0.5 on eflici -'y 0.8
Figure 4. Gas velocities near ceiling.
Volume 15, Number 2, 2000
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY171
• Extracting fans and the associated equipment must be able to withstand high temperatures. It is risky to use equipment that is not capable of resisting for a longer time under at least 250°C, according to the FSD Regulations. Note that for places that carry a risk of having large fires, equipment temperature rating at 400°C for 1.5 hours might be recommended elsewhere [e.g. 13]. Temperature rating for fans in air release and depressurization system is 300°C for buildings with sprinkler protection, and 600°C for buildings without sprinkler protection, as specified in BS5588: Part 4 1998 [14]. It certainly guarantees a higher reliability, which is the most important characteristic that equipment must have in this situation. • To cope with the fast propagation of smoke along the tunnel ceiling, it is crucial to start up or boost the ventilation as fast as possible. However, stability of the hot smoke layer might be disturbed if air was blown into the tunnel too early. For a "push-pull" ventilation system, there may be a time delay to allow passengers to evacuate to one side of the fire prior to the actuation of the system. However, the fire size might grow to a large value if the "delay time" is too long. A more detailed calculation must be carried out to determine a suitable operation time. • Smoke temperature and velocity are greatly reduced at a distance further than 100 m from the fire. For a short tunnel, e.g., less than 200 m, smoke can propagate out rapidly and yet remain hot enough to keep its buoyancy. With reference to local regulation [3], dynamic smoke extraction system is not necessary for tunnels of 230 m long or less. This is quite
comparable to the results here. Moreover, for tunnels longer than 230 m, it implies a greater hazard. Therefore, inspection of trucks with enclosed structure, if necessary, should be carried out at tunnels exceeding 230 m in length.
6. Conclusion Statistical data showed that the occurrence of tunnel fires due to trucks carrying liquid fuel is low if the vehicles are equipped with a proper fire protection system. Even so, consequences of such fires should not be disregarded. However, with the increased cases found of transporting illegal liquid fuel, the probability of having such a fire in the tunnels might not be so low as expected. Large tunnel fires with heat release rates up to 60 MW might result. A preliminary hazard assessment with empirical expressions available in the literature and fire zone model was reported. The fire environment due to a liquid spillage fire might be hazardous. Hot smoke would be generated rapidly, and the smoke front would travel with high speed. This would be very dangerous during traffic congestion. Furthermore, the tunnel cannot be used immediately after such a fire occurs. Blocking of a tunnel such as the cited case of the Lion Rock Tunnel implies a huge economic loss. There may be significant damage to the roadway pavement and slabs. Tunnel equipment may be destroyed or severely damaged by the high temperature and fire byproducts. The approximate cost of damage of every tunnel fire incident is different from case to case [15]. In addition, the extent of damage determines how long the tunnel will be closed for investigation and maintenance. For example,
800.r -
~700w ~1)
~g)Ju)'m i) m ~ l ) l ) w ~ a m
O ,=~ soo. O ¢~4oo
o~
,oo
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
(rrm.)
.....
at cvn,b
vn effid "y 0.8
at ¢vntx
icn eiliciency 0.5
Figure 5. Distance travelled of hot gas versus time.
172 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY
Volume 15, Number 2, 2000
t h e 11 k m long M o n t Blanc t u n n e l h a s b e e n closed for n e a r l y one y e a r after t h e 1999 fire [16]. Therefore, a good fire safety m a n a g e m e n t scheme m u s t be worked out by t h e t u n n e l a u t h o r i t y . A possible solution is to inspect every t r u c k of enclosed s t r u c t u r e before it e n t e r s the tunnel. T h a t w a s done y e a r s ago w h e n t h e t u n n e l s were first built, b u t t h e practice was a b a n d o n e d l a t e r because of t h e h e a v y traffic conditions. It is very i m p o r t a n t t h a t safety be provided to the t u n n e l users, since t h e r e m i g h t be m a n y crowded buses p a s s i n g t h r o u g h the tunnels. I f necessary, all t r u c k s of enclosed s t r u c t u r e should be ;prohibited from using the vehicular t u n n e l s d u r i n g r u s h h,~urs. An a l t e r n a t i v e solution is for the t r u c k drivers to sign a s t a t e m e n t g u a r a n t e e i n g t h a t no dangerous goods would be carried. Very h e a v y penalties would t h e n be imposed on drivers c a u g h t doing so.
References [1] South China Morning Post, 28 September 1999. [2] Fire Protection Notice No. 4, Dangerous Goods General, Government of HO~LgKong, 1992. [3] Code of Practice for Minimum Fire Service Installations and Equipment and Inspection and Testing of Installations and Equipment, Gover~.ment of Hong Kong, 1998. [4} Sing Tao Daily, http://www.singtao.com/news/24/ 0724ao21.html. [5] Ming Pao, 24 July :[999.
Volume 15, N u m b e r 2, 2000
[6] South China Morning Post, 25 February 1997. {7] Commemorative Booklet on the Opening of the Tunnel. Public Works Department, Hong Kong, 1967. [8] Ingason, H. 1994. Small Scale Test of a Road Tanker Fire, Proceedingsofthe International Conferenceon Fires in Tunnels, Boras, Sweden, October 10.11, 1994. Compiled by E. Ivarson, Swedish National Testing and Research Institute, Sweden. [9] Babrauskas, V. 1997. Burning Rates, Section 2/Chapter 1, FireProtectionHandbook. National Fire Protection Association. [10] Chow, W,K. 1996. Simulation of tunnel fire using a zone model, Tunnelling and Underground Space Technology 11(2), 221-236. [11] Alpert, R.L. 1972. Calculation of Response Time of CeilingMounted Fire Detectors. Fire Technology 8, 181-195. [12] Peacock, R.D. ; Forney, G.P.; Reneke, P.; Portier R.; and Jones, W.W. 1993. CFAST, the consolidated model of fire growth and smoke transport. NIST Technical Note 1299. Gaithersburg, Md., U.S.A.:National Institute of Standards and -Technology. {13] E. Jacques, E. and Seynhaeve, J.M. 1995. Fire in the Cointe Tunnel: A Design Case Study, Department of Mechanical Engineering, University Catholique de Louvain, Belgium. [14] BS5588 Fire Precautions in the Design, Construction and Use of Buildings, Part 4: Code of Practice for Smoke Control in Protected Escape Routes using Pressurization, 1998. [15] Haack, A. 1992. Fire Protection in Traffic Tunnels - - Initial Findings from Large-ScaleTests, Tunnelling and Underground Space Technology 7(4), 363-375. [16] S.F. Luchian, Task Force for Technical Investigation on the 24 March 1999 Fire in the Mont Blanc Vehicular Tunnel - Status Report of April 13, 1999, available from http:// www.mrtunnel.com]frame3.htm.
TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY173
Illegal Carriage of Dangerous Goods and Their Effects on Tunnel Safety J. S. M. Li and W. K. Chow A b s t r a c t --Consequent ~,oa recent fire resulting from a truck carrying illegal diesel outside a tunnel in Hong Kong, people are quite coneerned about the safety aspects of vehicular tunnels. That truck was an ordinary vehicle without appropriate protection against the possible accidental fires due to those liquid fuels. The safety impact on a vehicular tunnel of afire in an unprotected truck carrying illegal diesel is studied in this paper. Empirical results on spill fires and coolingjet expressions were applied to assess the probable tunnel environment. The two-layer zone model C F A S T was also used to verify the results. The smoke temperature in the tunnel within 100 m of the fire might be up to 300°C, and smoke would travel rapidly. It is recommended that a proper safety management scheme should be worked out by the tunnel management authority. If necessary, every truck of enclosed structure should be inspected before entering a vehicular tunnel longer than a specified length, say 230 m. © 2000 Published by Elsevier ScienceLtd. All rights reserved.
1. Introduction " ore and more illegal mobile oil stations are found in Hong Kong [1], selling illicit oil such as un. treated diesel oil. This has caused problems when buildings without proper protection against the possible fire risks are used for storing large quantities of illegal liquid fuel. In addition, trucks carrying those liquid fuels may cause problems while travelling through vehicular tunnels. According to Hong Kong's Fire Services Department (FSD) Regulations [2,3], any vehicle used for conveyance of Category 2 or Category 5 dangerous goods, including diesel oils and other liquid fuels, must have sufficient fire protection and must obtain an approval license issued by the FSD. Vehicles that carry large amounts of inflammable liquid without licenses can greatly endanger the safety of road users, especially when the truck is moving inside a vehicular tunnel. As an example, a fire accident of a lorry carrying about 500 liters of untreated diesel oil occurred just outside the Lion Rock Tunnel [4,5] in July 1999. It did not have an FSD license for carrying dangerous goods; nor did its design protect it against liquid fuel fire. The lorry was driven unusually fast while travelling through the tunnel from Kowloon to Shatin, ~ad it did not stop to pay at the toll counter. The vehicle was observed to be emitting smoke while passing through the counter. The lorry kept going for about 30 m beyond the toll counter before it finally stopped. The driver then went away and left the truck burning. The
M
Present address: Jojo SM. Li and W. K. Chow, Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China
fire was suspected to be started when oil leaked by the hot engine axles was ignited. When such a fire accident occurs in a tunnel, much more severe damage results than in a fire occurring in the open air, as in this case. Aggravating factors include confined space, limited number of escape roads, flashing over and loss of visibility. In view of this, the hazard scenarios for truck fires such as this are assessed in this paper, according to arguments addressed in the literature. In fact, the incident described above is not the first to have occurred in Hong Kong. An explosion once occurred in a container truck carrying motorcycles [6]. The shock waves generated from the explosion were so strong that glass and even some external finishes in nearby buildings were damaged. As reported in the news, the explosion even hurled hot shards of twisted metals up to the 38th floor of a building (nearly 120 m away) adjacent to the road. Fortunately, the explosion happened on an open highway, before the truck entered a tunnel.
2. Fire Scenarios Based on the information reported in the newspaper, the following fire scenario is assumed: A lorry carrying 0.5 m 3 of diesel oil travels in the Lion Rock Tunnel. Oil is spilled from a circular hole at the bottom of the tank, producing a fire. The tank is assumed to be rectangular: length 0.5 m, width 0.1 m, and height 0.5 m. The Lion Rock Tunnel [e.g. 7] was first constructed to provide an economical route for conveyingwater from Shatin to Kowloon, as shown in Figure 1. It was built in the late 1960s, and the tunnel was enlarged later to provide space for an additional road link. One of the main reasons for building the tunnel was because of the new horse-racing ground in Shatin. Shatin was further developed later to become a big urban area, with a population over 1 million.
www.elsevier.com/loca~e/tust ~nnelling and UndergroundSp~e Technology,Vol. 15, No. 2, pp. 167-173, 2000 0886-7798/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. All rights re~erved. PII: S0866-7798(00)00044-4
Pergamon
Key characteristics of the tunnel are as follows: Length, portal to portal: 1,425 m Finished cross-sectional area: 69 m 2 Maximum clear width: 8.5 m Carriageway width: 7m Width of service for each walkway: 0.46 m Minimum headroom for vehicles: 4.7 m Longitudinal gradient: 1 in 400 from Shatin portal
nated air is drawn through the slots in the ceiling and returned to the stations where fresh air is supplied. Carbon monoxide detectors, visibility meters, fire-fighting equipment and emergency telephones are installed. Signals on the detectors are transmitted to the control room so that the speed of the ventilation fan can be changed to supply the desired quantity of fresh air.
Mechanical ventilation is provided in the tunnel through two stations, located at each end. Each station is equipped with four supply fans and four exhaust fans, supplying an air flow rate of 142 m3/s. Fresh air is forced through the supply ducts under the carriageway and led into the tunnel through the grilles provided in the walkways. Contami-
Heat and smoke production rates from such a fire are estimated for fire hazard assessment. There are many parameters to be considered when determining the fire size of a road tanker. The heat release rate Qs for the spillage fire can be calculated [8] in terms of the burning rate m", the burning area A, the heat of combustion AHc for diesel oil of value 39.7 MJ/kg [9], and combustion efficiency ~ [8], as:
3. Estimation of Heat Release Rate
N f
NORTHERN APPROACH ROAD
l \ ROCK TUNNEL
APPROACH ROAD
KOWI.OON BAY
TSIM SHA TSUI Figure 1. The Lion Rock Tunnel.
168 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 15, Number 2, 2 0 0 0
Q,, = ~m" AAH (1) As proposed in the literature [e.g. 8], the initial spillage size on the roadway, the burning rate of the spill fire, and the combustion efficiency are important. They are considered in this paper.
q = 2000Awk (V~l - kt)
(3)
where k is given by: (4)
k - Cv ~ D2 v/-~ 8 AT
3.1 Initial Spillage SAze Calculation of the fire spillage size is usually based on the radial spreading of liquid spill along the horizontal surface. The maximum spillage area A ~ can be obtained [8,9] from the volumetric flow rate q (6's) of fuel leaking from a tank, the mass spillage burning rate m" (kg/s mS), and the density of the fuel oil p, (940 to 1000 kg/m3): qp~ (2)
Theoretically, the simplified spillage area A cannot be larger than Am a x . Its value can be less, since spilled fuel will be removed by the drainage gutters.
3.2 Burning Rate of Spill Fires The burning rate of pool fires is usually calculated based on tests using relatively deep fuel beds of thickness greater than 50 ram. However, the thickness of the spillage film on roadways is expected to be less than 7 mm. Based on the literature [e.g. 8] and the tests that determined the burning rate for thin fuel beds (of thickness 7 mm), the burning rate can be taken as 0.036 kg/s m 2 in the analysis of the initial spill fire. Note that the effects of radiation from tunnel walls on the burning rate are not considered in this study.
A~a~ - 1000 m" In the literature 1:8],q at time t (s) from a circular hole at the bottom of the tanker can be calculated in terms of the horizontal surface area of the leaking compartment AT(mS), the hole diameter D (m), the initial height of gasoline h~ (m), the flow contraction coefficient C (0.7), and the acceleration due to gravity g (mL,s2):
70
60
50
40
o f f
30
P f
~3
# ¢
B
20
S J f b ~
o o,
lO
/
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Hole Diameter (m)
-
-
at c o m b u s t i o n e f f i c i e n c y 0 . 8
......
at c o m b u s t i o n e f f i c i e n c y 0.5
Figure 2. Maximum heat release rates for initial spill fires for different hole diameters.
Volume 15, Number 2, 2000
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY169
3.3 Combustion Efficiency The combustion efficiency q will vary with the ventilation conditions. For longitudinal ventilation with air speed higher than 3 m/s, q is assumed to be 0.8; for no ventilation, q is taken as 0.5. The maximum heat release rates for the initial spill fires for hole diameters up to 0.08 m at combustion efficiency 0.5 and 0.8 were calculated using equation (1). The results are presented in Figure 2. The value of Am~ was 53.3 m 2, giving quite a big fire. For small hole diameters resulting from small leakage rates, the heat release rate of the fire is only slightly affected by the combustion efficiency. A possible explanation is that all diesel would be burned up quickly. For the above case, as shown in Figure 2, the heat release rate varies between 2 MW and 61 MW. The heat release rate also depends on the ventilation conditions in the tunnel, and will increase with the ventilation velocity.
4. Fire Environment Fire zone models are well-developed and might be suitable [10] for simulating a tunnel fire with the multi-cell concept. For the Lion Rock Tunnel, the ceiling height is 8.5 m. The equations for the fire plume and ceiling jet employed in most zone models would be valid for this ceiling height. Because the tunnel is long, the horizontal movement of the ceiling jet is unconfined. However, care must be taken on the horizontal ceiling jets when there are no vertical ceiling screens (soffits) at the doorway.
Maximum gas temperature Tm~ near the ceiling at a given radial position r away from the fire plume axis can be described by equations proposed by Alpert [11]: f Tm~- T =
5.38 (Q~/r)~3 H 16.9Qs2;3
(5)
H5/3 where H is the height of the ceiling above fire of 4.7 m, and T is the ambient temperature taken as 25°C. Variations of the temperature rise (Tm~ - T®) along r for a spillage fire of hole diameter 0.08 m, combustion efficiency 0.5 and 0.8 (Q8 was then 38 MW and 61 MW, respectively), are shown in Figure 3. It is observed that the temperature above the fire can be above 350°C, which then decreases to slightly above 100°C when r is 30 m away. The values of (Tm~ - T ) then drops fairly rapidly with r increased, but would still be close to 50°C at 700 m away from the fire. The horizontal component of the smoke velocity can be estimated by the filled ceiling jet equation: 0.197 Qsl/3 HI/2 r~/6
r>0.18H
0.946(-~) ~3
r < 0.18H
V=
(6)
400
350
. ~ 150. ~D I00-
50.
o
0
I
I
100
200
I 300
I 400
I 500
I 600
I 700
800
Distam¢ fi'omphn-~axis (m) ......
at c o n t x ~ o n efficiency 0.5 at ~ d x ~ o n diicier~y 0.8
Figure 3. Gas temperatures near ceiling.
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these cases are very close to 3.1 m. Note t h a t the length limit of a room in CFAST 3.13 was 500 m. For a longer tunnel segment, the multi-cell concept was used. A three-cell configuration with the fire at the central cell was considered. For each segment of length 300 m, the average smoke temperature over the three cells was 132°C and the smoke layer interface height was kept at 2.8 m. If each tunnel segment is taken as 500 m long, the average smoke temperature was 93°C with the smoke layer interface height kept at 2.6 m. The results are more consistent with those shown in Figure 3.
The results of V along r for t h a t spillage fire are shown in Figure 4. The velocity of the ceiling j e t would be above 5 ms 1 at positions above the fire, but decreases rapidly when r is increasecl. The values of V decreases due to volume contraction :resulting from the falling t e m p e r a t u r e as r is increased. Mixing with the e n t r a i n e d cool air would lower the t e m p e r a t u r e further. From the estimation of the horizontal component of gas velocity, the time for smoke propagation along the ceiling can be calculated, with the results shown in Figure 5. It can be seen t h a t smoke ,:an propagate about 100 m in the first minute, and t h a t the propagation of smoke can be fast in such a case of spillage fire. The zone model C FAST version 3.13 [e.g. 12] was applied to verify the above results. A tunnel segment 100 m long with a fire located at the centre was considered. With an assumed NFPA ultra-fast t2-fire burning, with the cutoff heat release rate of 30 MW at 410 s to 1500 s, and then decayed to zero at 2000 s, the average smoke temperature was 320°C with the smoke layer interface height kept at 3.1 m above the floor during the burning period (from 410 s to 1500 s). The results are not consistent with those estimated as shown in Figure 3, as the t e m p e r a t u r e was much lower in that figure. However, the smoke t e m p e r a t u r e decreased to 255°C when the length of the tunnel segment increased to 200 m; 225°C when segment length was 300 m; 195°C when the segment length was 400 m; and 175°C for the length of 500 m. The smoke layer interface heights for all
5. Discussion There are several points to note from the above analysis: • Empirical expressions on the spillage fire and ceiling jets are useful in estimating the probable fire environment in the tunnel. In using the fire zone model CFAST 3.13 [e.g. 12], there is a limit on the tunnel length. The multi-cell concept is useful in this case, but the length of the tunnel segment has to be considered carefully. • As computed from empirical expressions on ceiling jets, smoke with temperature above 100°C will be encountered within 30 m away from the plume and fast horizontal velocity will be generated during fires. This suggests t h a t smoke should be extracted in places as near to the fire source as possible.
7.m
6.
5.
O 0
3,
ii
0
ii
I
I
I
I
I
I
I
100
20O
3O0
400
500
6O0
700
o
8O0
Distance fmmpltam axis (m)
...... at 'rtx .............. at ccmta
on efficient 0.5 on eflici -'y 0.8
Figure 4. Gas velocities near ceiling.
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• Extracting fans and the associated equipment must be able to withstand high temperatures. It is risky to use equipment that is not capable of resisting for a longer time under at least 250°C, according to the FSD Regulations. Note that for places that carry a risk of having large fires, equipment temperature rating at 400°C for 1.5 hours might be recommended elsewhere [e.g. 13]. Temperature rating for fans in air release and depressurization system is 300°C for buildings with sprinkler protection, and 600°C for buildings without sprinkler protection, as specified in BS5588: Part 4 1998 [14]. It certainly guarantees a higher reliability, which is the most important characteristic that equipment must have in this situation. • To cope with the fast propagation of smoke along the tunnel ceiling, it is crucial to start up or boost the ventilation as fast as possible. However, stability of the hot smoke layer might be disturbed if air was blown into the tunnel too early. For a "push-pull" ventilation system, there may be a time delay to allow passengers to evacuate to one side of the fire prior to the actuation of the system. However, the fire size might grow to a large value if the "delay time" is too long. A more detailed calculation must be carried out to determine a suitable operation time. • Smoke temperature and velocity are greatly reduced at a distance further than 100 m from the fire. For a short tunnel, e.g., less than 200 m, smoke can propagate out rapidly and yet remain hot enough to keep its buoyancy. With reference to local regulation [3], dynamic smoke extraction system is not necessary for tunnels of 230 m long or less. This is quite
comparable to the results here. Moreover, for tunnels longer than 230 m, it implies a greater hazard. Therefore, inspection of trucks with enclosed structure, if necessary, should be carried out at tunnels exceeding 230 m in length.
6. Conclusion Statistical data showed that the occurrence of tunnel fires due to trucks carrying liquid fuel is low if the vehicles are equipped with a proper fire protection system. Even so, consequences of such fires should not be disregarded. However, with the increased cases found of transporting illegal liquid fuel, the probability of having such a fire in the tunnels might not be so low as expected. Large tunnel fires with heat release rates up to 60 MW might result. A preliminary hazard assessment with empirical expressions available in the literature and fire zone model was reported. The fire environment due to a liquid spillage fire might be hazardous. Hot smoke would be generated rapidly, and the smoke front would travel with high speed. This would be very dangerous during traffic congestion. Furthermore, the tunnel cannot be used immediately after such a fire occurs. Blocking of a tunnel such as the cited case of the Lion Rock Tunnel implies a huge economic loss. There may be significant damage to the roadway pavement and slabs. Tunnel equipment may be destroyed or severely damaged by the high temperature and fire byproducts. The approximate cost of damage of every tunnel fire incident is different from case to case [15]. In addition, the extent of damage determines how long the tunnel will be closed for investigation and maintenance. For example,
800.r -
~700w ~1)
~g)Ju)'m i) m ~ l ) l ) w ~ a m
O ,=~ soo. O ¢~4oo
o~
,oo
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
(rrm.)
.....
at cvn,b
vn effid "y 0.8
at ¢vntx
icn eiliciency 0.5
Figure 5. Distance travelled of hot gas versus time.
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t h e 11 k m long M o n t Blanc t u n n e l h a s b e e n closed for n e a r l y one y e a r after t h e 1999 fire [16]. Therefore, a good fire safety m a n a g e m e n t scheme m u s t be worked out by t h e t u n n e l a u t h o r i t y . A possible solution is to inspect every t r u c k of enclosed s t r u c t u r e before it e n t e r s the tunnel. T h a t w a s done y e a r s ago w h e n t h e t u n n e l s were first built, b u t t h e practice was a b a n d o n e d l a t e r because of t h e h e a v y traffic conditions. It is very i m p o r t a n t t h a t safety be provided to the t u n n e l users, since t h e r e m i g h t be m a n y crowded buses p a s s i n g t h r o u g h the tunnels. I f necessary, all t r u c k s of enclosed s t r u c t u r e should be ;prohibited from using the vehicular t u n n e l s d u r i n g r u s h h,~urs. An a l t e r n a t i v e solution is for the t r u c k drivers to sign a s t a t e m e n t g u a r a n t e e i n g t h a t no dangerous goods would be carried. Very h e a v y penalties would t h e n be imposed on drivers c a u g h t doing so.
References [1] South China Morning Post, 28 September 1999. [2] Fire Protection Notice No. 4, Dangerous Goods General, Government of HO~LgKong, 1992. [3] Code of Practice for Minimum Fire Service Installations and Equipment and Inspection and Testing of Installations and Equipment, Gover~.ment of Hong Kong, 1998. [4} Sing Tao Daily, http://www.singtao.com/news/24/ 0724ao21.html. [5] Ming Pao, 24 July :[999.
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[6] South China Morning Post, 25 February 1997. {7] Commemorative Booklet on the Opening of the Tunnel. Public Works Department, Hong Kong, 1967. [8] Ingason, H. 1994. Small Scale Test of a Road Tanker Fire, Proceedingsofthe International Conferenceon Fires in Tunnels, Boras, Sweden, October 10.11, 1994. Compiled by E. Ivarson, Swedish National Testing and Research Institute, Sweden. [9] Babrauskas, V. 1997. Burning Rates, Section 2/Chapter 1, FireProtectionHandbook. National Fire Protection Association. [10] Chow, W,K. 1996. Simulation of tunnel fire using a zone model, Tunnelling and Underground Space Technology 11(2), 221-236. [11] Alpert, R.L. 1972. Calculation of Response Time of CeilingMounted Fire Detectors. Fire Technology 8, 181-195. [12] Peacock, R.D. ; Forney, G.P.; Reneke, P.; Portier R.; and Jones, W.W. 1993. CFAST, the consolidated model of fire growth and smoke transport. NIST Technical Note 1299. Gaithersburg, Md., U.S.A.:National Institute of Standards and -Technology. {13] E. Jacques, E. and Seynhaeve, J.M. 1995. Fire in the Cointe Tunnel: A Design Case Study, Department of Mechanical Engineering, University Catholique de Louvain, Belgium. [14] BS5588 Fire Precautions in the Design, Construction and Use of Buildings, Part 4: Code of Practice for Smoke Control in Protected Escape Routes using Pressurization, 1998. [15] Haack, A. 1992. Fire Protection in Traffic Tunnels - - Initial Findings from Large-ScaleTests, Tunnelling and Underground Space Technology 7(4), 363-375. [16] S.F. Luchian, Task Force for Technical Investigation on the 24 March 1999 Fire in the Mont Blanc Vehicular Tunnel - Status Report of April 13, 1999, available from http:// www.mrtunnel.com]frame3.htm.
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