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Design of emergency ventilation system for an underground storage facility
Tunnelling and Underground Space Technology Tunnelling and Underground Space Technology 22 (2007) 293–302
incorporating Trenchless Technology Research
www.elsevier.com/locate/tust
Design of emergency ventilation system for an underground storage facility E.K. Stefopoulos, D.G. Damigos
*
School of Mining Engineering and Metallurgy, Laboratory of Mining and Environmental Technology, National Technical University of Athens, 9 Iroon Polytechniou Street, 15780, Zografou Campus, Athens, Greece Available online 7 September 2006
Abstract Fires have always been a major problem for underground spaces. As the demand for development of new underground structures increases, safety against fire incidents becomes even more important. This paper examines the design of an emergency ventilation system, which will provide the necessary control of smoke and heated gases within an underground warehouse facility, in case a fire occurs. Due to the pattern of the underground area and the variety of products stored, numerous fire scenarios should be examined in order to secure escape routes in every case, which significantly complicates the problem. For this reason, a different approach has been adopted, matching recent developments in fire safety from tunneling projects and ventilation practices from the mining industry. The ventilation design is based on the ‘‘critical velocity’’ theory; however, alternative configurations of the underground space are simulated by means of mine ventilation software. These alternatives affect the direction and velocity of the airflow and, consequently, the air quantity and the fan power required, in order to secure escape routes during fire emergency. The analysis not only determines the ventilation system characteristics, but also indicates the most appropriate design of the facility, in order to come up with a solution that is both secure and economically acceptable. 2006 Elsevier Ltd. All rights reserved. Keywords: Critical velocity; Fire and smoke control; Underground warehouse; Ventilation
1. Introduction Subsurface utilization has proven to be a realistic solution and has been gradually gaining ground during the last decades. Literature provides the reasons leading to the development of underground space (Damigos et al., 2004; Zevgolis et al., 2004). Earlier concerns and concepts for rejecting the option of underground construction, mainly due to economic reasons, are nowadays being re-examined and often decisions for an underground structure, instead of a surface one, may be favored. However, many people may feel uncomfortable driving or dwelling underground. One reason is that the lack of natural light decreases the ability of orientation in time and space. Another major rea-
*
Corresponding author. Tel.: +30 210 7722214; fax: +30 210 7722156. E-mail address: [email protected] (D.G. Damigos).
0886-7798/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2006.07.002
son for negative reactions towards traffic tunnels and underground space is the fear of accidents and fire in an enclosed area (ITA, 1998). Statistically, underground facilities cannot be considered as major hazardous areas. However, in case of a fire accident, much more severe damage occurs compared to a fire in open air. Aggravating factors include confined space, limited number of escape roads, flashing over and loss of visibility (Li and Chow, 2000; Vauquelin and Megret, 2002). A series of fire accidents, which happened worldwide (Channel Tunnel, Mont-Blanc Tunnel, Tauren Tunnel, Gotthard Tunnel) in the last years, have cost many lives and have triggered extensive discussions with regard to the fire safety of underground spaces. Towards this direction, the efforts focus on the prevention of a possible accident and, if this is not attainable, the safe evacuation and rescue of all trapped people. There are several important aspects of fire and life safety (ITA, 1998):
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fire – and collapse – resistant structures; elimination of combustible materials; detection and alarms; compartmentalization; emergency systems (rescue, escape and refuge); fire and smoke control; and maintenance of safety systems.
mine ventilation software. The analysis not only determines the ventilation system characteristics, but also indicates the most appropriate design of the facility, in order to come up with a solution that is both safe and economic. 2. Methodological approach 2.1. Literature review
So far, the measures related to fire fighting in underground spaces (e.g. mines, stations, storage facilities, transportation facilities, etc.) include direct attack with rock dust or water, application of foams or water mist, inertization with nitrogen, carbon dioxide, or exhaust gases from turbine engines and sealing of the affected area. Despite of the wide range of the available fire fighting techniques, it seems that in most cases of large fires the primary concern is the sealing of the fire area (Loomis and McPherson, 1995; Wala et al., 1995; Biffi et al., 1997; Adamus et al., 1995). The latter, however, presupposes that people have already evacuated the underground area. When a fire occurs, high temperatures (exceeding 1000 C) and smoke concentration provide conditions where the chance of survival is near zero. The surrounding walls, together with the lack of heat escape in vertical direction, lead to an intensive temperature increase within a few minutes (Modic, 2003). However, the generated smoke and not the fire itself suggests the major threat for the people trapped underground, as it spreads rapidly throughout the area, reducing the visibility and causing deaths by asphyxiation (Vauquelin and Megret, 2002) due to the hazardous gases, like CO, contained. Thus, in case of a fire, the first action should aim at controlling the propagation of combustion smoke by means of ventilation systems (Vauquelin and Megret, 2002; Modic, 2003; Chen, 2000; Gao et al., 2004). Unless a strong airflow is created and maintained, hot gases and smoke migrate in all directions. With a weak airflow, smoke forms a layer along the tunnel ceiling and can flow against the direction of forced ventilation (‘‘backlayering’’), interfering with personnel evacuation (Modic, 2003). For most fire and safety officials, however, the prospect of employing ventilation to contend with a subway fire emergency goes against their instincts (Cheng et al., 2001). This paper presents the design of an emergency ventilation system, which will provide the necessary control of smoke and heated gases within an underground warehouse facility, if a fire occurs. The ventilation system will provide the conditions for emergency evacuation and rescue. The storage facility will be developed using the room-and-pillar mining method. For certain reasons (e.g. the pattern of the underground area, the variety of products stored, etc.) a different approach has been adopted in this research. Due to the large number of the potential fire scenarios involved, the major concern is to prevent ‘‘backlayering’’ phenomenon in order to secure escape routes. Towards this direction, recent developments, adopted by the tunneling fire safety theory, are being utilized and alternative configurations of the underground space are analyzed by means of
During the last decades, research work and case study analyses have been conducted regarding fires in underground structures and how a proper ventilation system can assist to a safe evacuation procedure. Hinkley (1970) studied the flow of hot gases along an enclosed shopping mall. Heselden (1976) considered the fire and smoke behavior in tunnels. Laage et al. (1987) simulated the effects of mine fire, which has induced ventilation disturbances on the performance of a stench fire warning system. Simcox et al. (1992) used computer simulation to study gas flows from fire at King’s Cross Underground Station. Lea (1994) used three computational models to study the effects of fire in an UK mine. The US Bureau of Mines at the Twin Cities Research Center has developed mine fire and ventilation simulators (Laage et al., 1995). Pomroy and Carigiet (1995) analyzed underground coal mine-fire incidents in the United States from 1978 to 1992. Leitner (2001) studied the fire catastrophe in the Tauern Tunnel south-east of Salzburg, Austria. Due to the more intense use of underground space and the changes in technology the study of fire and safety issues has been complicated. In order to deal with the difficulties involved, experiments and large-scale fire tests have been conducted (e.g. Burnett, 1984; Keski-Rahkonen et al., 1986; Chow and Li, 2001; Vauquelin and Megret, 2002; Gao et al., 2004; Hu et al., 2005). In addition, many researchers employed empirical and numerical models to simulate smoke and temperature distribution in underground spaces, as well as to specify the ‘‘critical velocity’’ of the air needed to prevent upstream movement of smoke from a fire in a tunnel (e.g. Laage et al., 1987; Miclea, 1991; Guelzim et al., 1994; Laage et al., 1995a; Kennedy et al., 1996; Oka and Atkinson, 1996; Dziurzynski et al., 1997; Chow, 1998; Edwards and Hwang, 1999; Megret and Vauquelin, 2000; Wu and Bakar, 2000; Cheng et al., 2001; Modic, 2003; Hu et al., 2005). A relatively recent review of tunnel fires (Grant et al., 1998) pointed out that existing experimental data still show an inadequate fundamental understanding of the interaction between buoyancy-driven combustion products and forced ventilation, the influence of slopes on smoke movement, the effect of tunnel geometry, etc. In any case, however, simulation programs have the advantage that several fire scenarios can be analysed and visualized, providing a dynamic representation of the fire progress, in terms of the spread of combustion products (e.g. CO) and temperature throughout the underground space. Especially with the increasing power of personal
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computers, considerable attention has been given towards this direction. 2.2. Methodology description Regarding the ventilation system, fire emergency planning involves simulation of the interaction of the fire and the ventilation system. Thus, the ventilation paths, along which hazardous combustion gases pass through, are determined in order to design safe escape routes and fire fighting activities. This procedure provides useful results for decision-making, in case the underground space pattern is relatively simple and potential fire hazards are limited (e.g. maintenance shops and fuel bays). Dealing, however, with underground storage facilities, i.e. warehouses, the calculation of smoke propagation under different fire scenarios becomes practically impossible, due to the complex layout of the facility and the numerous potential locations of the fire. To overcome this problem, the procedure presented utilizes the ‘‘critical velocity’’ theory and passive fire protection measures (e.g. fire safety departments) so as to establish the air quantity required in every single point of the underground space. Further, different layouts of the underground facility are examined to minimize fan horsepower. 2.2.1. Necessary steps for designing the ventilation system The design of an appropriate ventilation system for underground structures, in general, must take into consideration, mainly, two issues: The supply of the necessary air flow rate that provides adequate ventilation under normal conditions. The smoke and high temperatures management in case of a fire emergency, to establish safe evacuation conditions. According to the methodology presented, the design steps of the ventilation system, for both normal and emergency operation, are: 1. Estimation of the required quantity of the incoming air under normal operating conditions. 2. Determination of the critical air velocity for each branch to control smoke propagation. Air velocity lower than the critical can be considered inadequate, whereas larger than the critical velocity could result in possible increase of the fire size and contribute to its rapid spread throughout the area. 3. Estimation of the required quantity of the total incoming air in order to attain critical velocity and avoid ‘‘backlayering’’. 4. Configuration of the facility layout alternatives, so as to minimize air quantity requirements. 5. Re-estimation of the required air quantity for each alternative. 6. Selection of the final scheme.
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2.2.2. The longitudinal ventilation system during a fire emergency All underground facilities, except from small tunnels, require the application of a ventilation system to dilute the contaminants produced by vehicle engines during normal operation, such that a complete air exchange is succeeded within a reasonable time frame (Tarada, 2000; Modic, 2003). In addition, emergency ventilation is required during a fire to control smoke and gases and provide a safe evacuation path (Modic, 2003). The standard procedure consists of blowing the whole smoke towards a specific exit, while avoiding the occurrence of backlayering (Vauquelin and Megret, 2002; Gao et al., 2004). System performance is not effective if the ventilation rate is low and the buoyancy of smoke is strong enough. The fire plume would impinge on the ceiling and spread against the discharged ventilation flow. It is very important to understand when a smoke backflow will occur (Gao et al., 2004; Wu and Bakar, 2000). In order to prevent smoke backflow, the speed of air discharged by the ventilation system has to be higher than a critical value Vc, known as ‘‘critical velocity’’. This value has become one of the prime criteria for the design of ventilation systems, especially in tunnel section (Chow, 1998; Carvel, 1999; Tarada, 2000). Empirical equations, based on the so-called Froude number preservation theory, are provided in the literature in order to estimate the critical velocity (Chow, 1998; Wu and Bakar, 2000; Chen, 2000; Tarada, 2000). These equations have been modified by several researchers. The critical velocity, in the analysis presented, is determined by means of the following equations, which are the most resent and widely used (Li and Chow, 2003; Modic, 2003; Chen, 2000; Wu and Bakar, 2000): 13 gH Q V c ¼ K Kg ð1Þ q Cp A T f Q þ T0 ð2Þ Tf ¼ q Cp A V c K g ¼ 1 þ 0:037 a0:8 where Vc Tf K Kg a g H Q q Cp A T0
ð3Þ
critical air velocity (m/s); average temperature of fire site gases (K); 0.606 or 0.61; grade factor; tunnel grade (%); acceleration caused by gravity (9.81 m/s2); height of tunnel at the fire site (m); heat release rate (kW); average air density (1.2 kg/m3); specific heat of air (1.1 kJ/K kg); area perpendicular to the flow (m2); temperature of fresh air (K).
Most recommendations for tunnel ventilation system operation concern themselves mainly with control and extraction of smoke in the event of a fire. The heat release
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rate (HRR) of a fire, which is generally expressed in MW, is considered to be an important factor contributing to the severity of a fire and a good measure of its size. Chen (2000) argues that the HRR of tunnel fire is one of the most important parameters in determining the characteristics of smoke propagation in tunnels. The HRR may not only affect the smoke production, but is also related to the impact the fire will have on the structure, the likelihood of fire spread and the difficulty fire fighters will face when tackling the blaze (Carvel, 1999; Carvel et al., 2001). Many researchers (e.g. Haack, 1998; ITA, 1998; Megret and Vauquelin, 2000; Vauquelin and Megret, 2002; Chen, 2000; Li and Chow, 2003) have estimated the size of heat release rate of vehicle fires considering different scenarios, i.e. 3–5 MW HRR corresponds to a large car on fire, 10– 20 MW corresponds to a bus, while a truck on fire releases 50–100 MW.
Fig. 2. Cross-section of the central corridor. The figure displays the ‘‘back-to-back’’ system leaving a 5.40 m wide airway.
3. Case study 3.1. Description and configuration of the underground warehouse The study under investigation involves an underground structure excavated using the room and pillar mining method. It includes one entrance and one exit. Fresh air enters the warehouse by a 50 m long horizontal gallery with a square cross-section of 36 m2, while the air comes out from a 30 m long circular upcast shaft with diameter and cross-sectional area of 4 m and 12.56 m2, respectively. The underground space follows a room-and-pillar layout. The rooms will be 8 m wide and the pillars will have dimensions of 8 · 8 · 8 m. The characteristics and more details of the area are illustrated in Figs. 1–3. The storage system of such a warehouse is given by Zevgolis et al. (2004). In this case the utilization of the euro-pallet (0.80 · 1.20 · 0.144 m) as the storage unit and the ‘‘back to back’’ storage system were preferred. Fig. 1 shows the plan view of the storage facility, where the remaining width of the corridors and the location of the storage shelves are demonstrated. The utilization of the storage space is made taking into consideration the need for movement and circulation of the forklift trucks. The
Fig. 1. Plan view of the storage facility.
Fig. 3. Three-dimensional view of the inside of the underground warehouse (central corridor).
creation of the exit protection zone was adopted in order to protect the evacuation procedure in case of fire, although it reduces the storage capacity. 3.2. Ventilation requirements 3.2.1. Normal conditions In the case of underground storage facilities, air requirements are usually diminished. According to ANSI/ASHRAE Standard 62-2001 about ventilation for acceptable indoor quality, an air entrainment rate of 0.3 l/s m2 is believed to be satisfying for this kind of activity (ASHRAE, 2003). Hence, an air entrainment rate of 0.3 l/s m2 · 1920 m2 = 0.6 m3/s is required. 3.2.2. Emergency conditions The critical air velocity and temperature in relation with the heat release rate in case of a fire in a corridor are presented in the diagrams of Figs. 4 and 5, considering a range of heat release rate from 1 to 100 MW and rectangular cross-sections of 43.2 m2 and 21.6 m2 of central and extreme corridor respectively, based on the warehouse
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Fig. 4. Diagram of critical velocity vs. heat release rate.
Fig. 5. Diagram of temperature vs. heat release rate.
height (8 m) and each corridor width (5.4 m and 2.7 m, respectively). In order to solve Eqs. (1) and (2) the following data were used: K = 0.606 Kg = 1 g = 9.81 m/s2 H=8m q = 1.2 kg/m3 Cp = 1.1 kJ/K Kg A = 43.2 m2 and 21.6 m2 T0 = 273 + 25 = 298 K In order to obtain the fire simulation and the analysis of different layout scenarios, the estimation of the heat release rate is essential. Because of the variety of the potential storing products, there are many uncertainties regarding this factor. In many similar cases the worst-case scenario is chosen, but this leads to conservative results. In the case studied, the use of probabilistic modeling using Monte-Carlo
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simulation is decided in order to achieve more accurate results. The definition of the statistical distribution describing a parameter behavior is used in order to proceed with the probabilistic modeling. In this case, a triangular distribution is chosen to model the heat release rate behavior, based on the analysis of the literature data. Furthermore, it is chosen because the research is at an initial stage with no specific data regarding the products which are to be stored inside the warehouse. Knowing the exact usage of the warehouse can change the selection of distribution to a more representative having in mind the heat release rate and stored quantity of each product so as to have more accurate rather than indicative results. The triangular distribution is a continuous probability distribution typically used as a subjective description of a population for which there is only limited sample data. It is based on knowledge of the minimum and maximum and an inspired guess as to what the modal value might be. The parameters for the triangular distribution are minimum, maximum, and most probable, which falls between the minimum and maximum values. Hence, a triangular shaped distribution is formed, which shows that values near the minimum and maximum are less likely to occur than those near the most probable one. Despite being a simplistic description of a population, it is a very useful distribution for modeling processes where the relationship between variables is known, but data is scarce (Crystal Ball, 2000). The ‘‘Crystal Ball’’ software was used in order to apply the probabilistic modeling and values 1 MW (minimum), 80 MW (maximum) and 40 MW (likeliest) have been entered as the parameters of the triangular distribution. The calculations for the two cross-sections are shown in Figs. 6 and 7, as output from the Monte-Carlo simulation. The critical velocity values are presented (x-axis), in relation to their probability of occurrence, after 5000 iterations. It can be clearly seen that, with a critical air velocity of 3.13 m/s and 3.48 m/s for the cross-sections of 43.2 m2 and 21.6 m2 respectively, the 95% of the possible fire scenarios are being covered. Thus, the values of 135 m3/s and 75 m3/s are the critical air supply rates in order to prevent backlayering.
Fig. 6. Critical velocity estimation as output of the probabilistic modeling (A = 43.2 m2).
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consists of 30 branches and 21 junctions with one fun. Fig. 8 shows the schematic of the ventilation network and Fig. 9 gives the input data.
Fig. 7. Critical velocity estimation as output of the probabilistic modeling (A = 21.6 m2).
3.2.3. Ventilation simulation The simulation results are given for the emergency scenario only, for reasons of conciseness. Simulations are carried out by means of VnetPC 2003 software, which is briefly described hereinafter. (a) The VnetPC 2003 software VnetPC is a program of the Mine Ventilation Services Inc. that simulates mine ventilation networks. The applicability of VnetPC to subsurface ventilation system design ranges from the initial concept to the system operation phase of a project. Given information that describes the geometry of a ventilation network, airway resistances or dimensions, and the locations and characteristic curves of fans, the code produces listings and visual graphics of many parameters including predicted airflows, frictional pressure drops, air power losses in airways, and fan operating points, using the fundamental equations (3) and (4) of airflow. The software has been developed with the assumption of incompressible flow and is based on Kirchhoff’s laws. The code utilizes an accelerated form of the Hardy Cross iterative technique to converge to a solution (VnetPC, 2003). h ¼ R Q02 R¼
k ðL þ Leq Þ Per S3
where R h Q0 k L Leq Per S
Fig. 8. Schematic of the ventilation network.
ð4Þ ð5Þ
airway resistance (N s2 m8); pressure drop (Pa); flow rate (m3/s); friction factor (kg/m3); length of airway (m); equivalent length of shock loss (m); perimeter of airway (m); cross-section of airway (m2).
A ventilation network is a graphical representation of a ventilation system and consists of a set of junctions and interconnecting lines (branches), which denote major or significant airflow routes. The model of the examined area
Fig. 9. Input data.
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Values of friction factor and equivalent lengths are selected by tables given in the literature (SME, 1996; Hartman et al., 1997). For example, values 0.0100 kg/m3 and 0.0158 kg/m3 are used for unlined rectangular airways with uniform sides and irregular conditions respectively, while for estimating the equivalent length of airway 21 to 7 (shaft) which is 43 m, the sharp bend (20 m), the abrupt contraction (3 m) and the air discharge (20 m) are taken into consideration. (b) Execution of the ventilation simulation in case of fire The primary requirements for the emergency ventilation systems are derived from smoke control considerations, i.e. the smoke can always be driven in the required direction without any significant backlayering (Tarada, 2000). For this reason, three scenarios with different modification of the area were considered (Table 1). In all scenarios examined, the fan is installed outside the underground area in order to be protected in case of fire (McPherson, 1993). According to the first scenario, the area was left as it was excavated with the room and pillar pattern, without any modification. Fresh air entering the area is distributed to the three longitudinal corridors (Fig. 10). An insufficient air supply passes through the cross-cuts. There is no way of succeeding critical velocity in every airway, unless excesTable 1 Characteristics of the examined case-scenarios Case-scenario
Characteristics
First
+Room-and-pillar pattern. +Original layout.
Second
+Regulators installation in cross-cuts and central airway. +Division of the area into three longitudinal entries.
Third
+Stopping installation in cross-cuts. +Split of the area into three independent airways. +No air movement through the cross-cuts.
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sive air quantity is supplied. Hence, the scenario is rejected, as smoke would spread uncontrollably in case of a fire. The second scenario is based on the tunnel crosspassages theory (Tarada, 2000). The area is divided into three longitudinal corridors by placing regulators in all cross-cuts. These modifications result in a regularized airflow, as air is directed from the central airway to the extremes. Furthermore, the critical air supply through the ‘‘cross-passages’’ is reduced to 23.5 m3/s. A similar regulator is installed at the end of the central airway in order to increase the air supply through the cross-cuts. However, the critical values cannot be attained without huge air entrainment ratio and, consequently, smoke circulation cannot be avoided. After applying the calculation procedure (Fig. 11) it can be clearly seen that in the extreme corridors the air supply starts from a small value and ends to a value much larger than the critical one. Furthermore, if the central airway becomes the incident airway, the flow reversal from the extreme corridors to the central one through the cross-cuts is required, which is difficult to be obtained. According to the third scenario, stoppings are installed to all the cross-cuts in order to split the area into three independent airways (Fig. 12). This aims to the separation of the entrained airflow into three different directions, avoiding the movement of the air through the cross-cuts that caused the smoke migration in the previous scenarios. The stoppings installed between the pillars are designed to have self-closing doors. Thus, the communication between the longitudinal corridors is allowed, ensuring the escape of the personnel through the neighbor corridor and entry gallery, while smoke is restricted to the incident airway. In this case, smoke is directed to the exhaust shaft and out of the affected corridor whilst the doors protect the non-affected corridors from the effects of heat and smoke. It is believed that this case-scenario creates suitable conditions for safe evacuation and firefighting.
Fig. 10. Result of calculation procedure for the first scenario.
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Fig. 11. Result of calculation procedure for the second scenario.
More specific, this scenario is based on the controlled split of the entrained airflow to the three longitudinal airways, so as to achieve air supply rates of 135 m3/s and 75 m3/s to the central and extreme corridors, respectively. From the calculation procedure (Fig. 13) it can be seen that a regulator of 0.00071 N s2 m8 resistance needs to be installed to the central airway. The air entrainment ratio of 285 m3/s gives an air velocity of 7.9 m/s flowing through the entry gallery, which is in accordance with specifications given by international standards (maximum air velocity during evacuation procedure: 11 m/s, according to NFPA 130 and ASHRAE; Chow and Li, 1999). Finally the fan used for the emergency ventilation system should have a power of 160 kW with an average efficiency of 80% given by Eq. (6). Fig. 12. Three-dimensional view of the layout of the underground area after the installation of stoppings.
N0 ¼
Q0 h 285 m3 =s 448:6 Pa ¼ ¼ 159:8 kW 1000 n 1000 0:8
Fig. 13. Result of calculation procedure for the third scenario.
ð6Þ
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4. Conclusions During the last years, there has been a lot of research regarding fires in underground areas and emergency ventilation systems. These cases refer mainly to transportation tunnels, where the critical velocity theory is applied, and fire events in mines, where the problem is examined through the arrangement of the airflow direction. So far, there is a gap in the literature concerning subsurface storage facilities. The paper examines a special case of an underground warehouse center, which is constructed using a classic mining method, namely room-and-pillar. The analysis tries to bridge this gap by developing ideas adopted from cases met in both tunnel and mine-fire events. In general, the installation of a proper ventilation system in combination with the existence of an evacuation plan could be sufficient in order to safely evacuate an underground area in case of a fire, leaving certain airways free of smoke. Towards this direction, fire simulation software has been proven to be a quite useful tool for fire and safety officials. Fire scenarios can be analyzed and personnel can be trained to contend such an emergency. In the case studied, due to the large number of the potential fire scenarios involved, the analysis follows a different approach, dealing not only with the influence of the ventilation system, but also the outline of the underground space. In addition, Monte-Carlo modeling simulation was used to address the uncertainty of the products stored in the warehouse, covering a number of different situations (heat release rates). The requirement for critical airflow in every airway of the area is proved to be not attainable, since enormous air quantities would be required. Towards this direction, alternative configurations of the underground space are analyzed by means of mine ventilation software. The compartmentalization of the area, by installing stoppings and the partition of the warehouse into three parallel corridors, seems to be the most efficient solution. In addition, the installation of regulators is necessary in order to optimize the ventilation system performance. The analysis proved that the control of smoke flow is achieved at any area of the underground structure and safe evacuation is obtained. Hence, the results not only indicate the proper ventilation system characteristics, but also provide the most suitable design of the facility. The approach presented is less complicated and can be easily applied compared to CFD simulations. Due to the pattern of the underground area and the variety of products stored, numerous fire scenarios should be examined in order to secure escape routes in every case, a fact that would significantly have complicated the analysis. Of course, in some cases (e.g. very large underground facilities) the approach proposed may be unfeasible, due to the high investment costs required for achieving critical airflow velocities in each corridor. According to the analysis, the requirements for air supply during normal operation are minimal. This demand could
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be covered via natural ventilation or the temporary operation of a fan. Nevertheless, the design of the emergency ventilation system is crucial, as fresh air requirements are significant. For this reason, emergency ventilation system should be a part of the overall study of the project, as it interacts with the number and the form of the access works as well as the internal layout of the underground facility. References Adamus, A., Hajek, L., Posta, V., 1995. A review of experiences on the use of nitrogen in Czech coalmines. In: Proceedings of the Seventh US Mine Ventilation Symposium, Lexington, KY, USA, June 5–7. ASHRAE, 2003. Ventilation for Acceptable Indoor Air Quality. ANSIASHRAE Standard 62-2001, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Biffi, M., Walters, D.M., deVilliers, L.J., van der Vyver, C.M., 1997. Fire fighting in deep, narrow, tabular, metalliferous mines using the GAG-3A inert gas generator system. In: Proceedings of the Sixth International Mine Ventilation Congress, Pittsburgh, PA, USA, May 17–22. Burnett, J., 1984. Fire safety concerns for rail rapid transit systems. Fire Safety Journal 8 (1), 3–7. Carvel, R.O., 1999. The effect of ventilation on fires in tunnels. In: International Tunnel Fire and safety Conference, Rotterdam, The Netherlands, 2–3 December. Carvel, R.O., Beard, A.N., Jowitt, P.W., 2001. A method for making realistic estimates of the heat release rate of a fire in a tunnel. In: Third International Conference on Tunnel Fires, Gaithersburg, MD, USA, 9–11 October. Chen, F., 2000. Smoke propagation in road tunnels. ASME Applied Mechanics Review 53 (8), 207–218. Cheng, L.H., Ueng, T.H., Liu, C.W., 2001. Simulation of ventilation and fire in the underground facilities. Fire Safety Journal 36 (6), 597–619. Chow, W.K., 1998. On smoke control for tunnels by longitudinal ventilation. Tunnelling and Underground Space Technology 13 (3), 271–275. Chow, W.K., Li, J.S.M., 1999. Safety requirement and regulations reviews on ventilation and fire for tunnels in the Hong Kong special administrative region. Tunnelling and Underground Space Technology 14 (1), 13–21. Chow, W.K., Li, J.S.M., 2001. Case study: vehicle fire in a cross-harbour tunnel in Hong Kong. Tunnelling and Underground Space Technology 16 (1), 23–30. Crystal Ball, 2000. Crystal Ball 2000 User Manual. Damigos, D., Benardos, A., Kaliampakos, D., 2004. The space beneath: developing the new human-friendly cities. In: First International Conference on Advances in Mineral Resources Management and Environmental Geotechnology, Chania, Crete, Greece, 7–9 June. Dziurzynski, W., Nawrat, S., Roszkowski, J., Trutwin, W., 1997. Computer simulation of mine ventilation disturbed by fires and the use of fire extinguishers. In: Proceedings of the Sixth International Mine Ventilation Congress, Pittsburgh, PA, USA, May 17–22. Edwards, J.C., Hwang, C.C., 1999. CFD analysis of mine-fire smoke spread and reverse flow conditions. In: Proceedings of the Eighth US Mine Ventilation Symposium, Rolla, MO, USA, June 14–17. Gao, P.Z., Liu, S.L., Chow, W.K., Fong, N.K., 2004. Large eddy simulations for studying tunnel smoke ventilation. Tunnelling and Underground Space Technology 19 (6), 577–586. Grant, G.B., Jagger, S.F., Lea, G.J., 1998. Fires in Tunnels, Philosophical Transactions – Royal Society of London, Series A 356, 2873–2906. Guelzim, A., Souil, J.M., Vantelon, J.P., Sou, D.K., Gabay, D., Dallest, D., 1994. Modelling of a reverse layer of fire-induced smoke in a tunnel, Fire safety science. In: Proceedings of the Fourth International Symposium, pp. 277–288.
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Haack, A., 1998. Fire protection in traffic tunnels: general aspects and results of the EUREKA Project. Tunnelling and Underground Space Technology 13 (4), 377–381. Hartman, H.L., Mutmansky, J.M., Ramani, R.V., Wang, Y.J., 1997. Mine Ventilation and Air Conditioning, third ed. John Wiley & Sons, Inc. Heselden, A.J.M., 1976. Studies of fire and smoke behavior relevant to tunnels. In: Proceedings of Second International Symposium on the Aerodynamics and Ventilation of Vehicle Tunnels, BHRA Fluid Engineering, Cambridge, UK, pp. 23–25. Hinkley, P.L., 1970. The flow of hot gases along an enclosed shopping mall: a tentative theory, Fire Research Note No. 807, Fire Research Station, Borehamwood, Herts, UK. Hu, L.H., Huo, R., Li, Y.Z., Wang, H.B., Chow, W.K., 2005. Full-scale burning tests on studying smoke temperature and velocity along a corridor. Tunnelling and Underground Space Technology 20 (3), 223– 229. International Tunneling Association—ITA, 1998. Fire and life safety for underground facilities: present status of fire and life safety principles related to underground facilities. Tunnelling and Underground Space Technology, 13 (3), 217–221. Kennedy, W.D., Gonzales, J.A., Sanchez, J.G., 1996. Derivation and application of the SES critical velocity equations. ASHRAE Transactions: Research 102 (2), 40–44. Keski-Rahkonen, O., Holmlund, C., Loikkanen, P., Ludvigsen, H., Mikkola, E., 1986. Two full scale pilot fire experiments in a tu´nel, Valtion Teknillinen Tutkimuskeskus, VTT/RR-453, Palotekniikan Lab, Espoo, Finland. Laage, L., Pomroy, W., Weber, T., 1987. Computer modeling of the effect of mine-fire-induced ventilation disturbances on stench fire warning system performance. US Bureau of Mines, BUMINESIC-9154, Twin Cities Research Center, Minneapolis, MN, USA. Laage, L., Greuer, R., Pomroy, W., 1995. MFIRE Users Manual ver. 2.20, Twin Cities Research Center, US Bureau of Mines, Minneapolis, MN, USA, US Bureau of Mines Training Workshop on the ‘‘MFIRE’’ mine fire and ventilation simulator. Laage, L.W., Yang, H., 1995. Mine-fire experiments and simulation with MFIRE. In: Proceedings of the Seventh US Mine Ventilation Symposium, Lexington, KY, USA, June 5–7. Lea, C.J., 1994. Computational modeling of mine fires. Mining Engineer (London) 154 (394), 17–21. Leitner, A., 2001. The fire catastrophe in the Tauern tunnel: experience and conclusions for the Austrian guidelines. Tunnelling and Underground Space Technology 16 (3), 217–223. Li, J.S.M., Chow, W.K., 2000. Illegal carriage of dangerous goods and their effects on tunnel safety. Tunneling and Underground Space Technology 15 (2), 167–173.
Li, J.S.M., Chow, W.K., 2003. Numerical studies on performance evaluation of tunnel ventilation safety systems. Tunnelling and Underground Space Technology 18 (5), 435–452. Loomis, I.M., McPherson, M.J., 1995. Application of water mist for the control of fuel-rich fires in model coal mine entries. In: Proceedings of the Seventh US Mine Ventilation Symposium, Lexington, KY, USA, June 5–7. McPherson, M.J., 1993. Subsurface Ventilation and Environmental Engineering. Chapman & Hall. Megret, O., Vauquelin, O., 2000. A model to evaluate tunnel fire characteristics. Fire Safety Journal 34 (4), 393–401. Miclea, P.C., 1991. Application of SES and MFIRE computer simulation programs for tunnel fire modeling—a comparative study. ICF Kaiser Engineers Inc., Oakland, CA, USA. Modic, J., 2003. Fire simulation in road tunnels. Tunnelling and Underground Space Technology 18 (5), 525–530. Oka, Y., Atkinson, G.T., 1996. Control of smoke flow in tunnel fires. Fire Safety Journal 25 (4), 305–322. Pomroy, W.H., Carigiet, A.M., 1995. An analysis of underground coal mine-fire incidents in the United States from 1978 through 1992. In: Proceedings of the Seventh US Mine Ventilation Symposium, Lexington, KY, USA, June 5–7. Simcox, S., Wilkes, N.S., Jones, I.P., 1992. Computer simulation of the flows of hot gases from fire at King’s Cross Underground Station. Fire Safety Journal 18 (1), 49–82. SME, 1996. In: Hartman, H.L. (Ed.), second ed., Mining Engineering Handbook, vols. I & II Society for Mining, Metallurgy, and Exploration, Inc., Littleton, CO. Tarada, F., 2000. Critical velocities for smoke control in tunnel crosspassages. In: First International Conference on Major Tunnel and Infrastructure Projects, Taipei, Taiwan, 22–24 May. Vauquelin, O., Megret, O., 2002. Smoke extraction experiments in case of fire in a tunnel. Fire Safety Journal 37 (5), 525–533. VnetPC, 2003. User’s Manual and Tutorial. Mine Ventilation Services, Inc. Wala, A.M., Dziurzynski, W., Tracz, J., Wooton, D., 1995. Validation study of the mine-fire simulation model. In: Proceedings of the Seventh US Mine Ventilation Symposium, Lexington, KY, USA, June 5–7. Wu, Y., Bakar, M.Z.A., 2000. Control of smoke flow in tunnel fires using longitudinal ventilation systems—a study of the critical velocity. Fire Safety Journal 35 (4), 363–390. Zevgolis, I.E., Mavrikos, A.A., Kaliampakos, D.C., 2004. Construction, storage capacity and economics of an underground warehousinglogistics center in Athens, Greece. Tunnelling and Underground Space Technology 19 (2), 165–173.
incorporating Trenchless Technology Research
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Design of emergency ventilation system for an underground storage facility E.K. Stefopoulos, D.G. Damigos
*
School of Mining Engineering and Metallurgy, Laboratory of Mining and Environmental Technology, National Technical University of Athens, 9 Iroon Polytechniou Street, 15780, Zografou Campus, Athens, Greece Available online 7 September 2006
Abstract Fires have always been a major problem for underground spaces. As the demand for development of new underground structures increases, safety against fire incidents becomes even more important. This paper examines the design of an emergency ventilation system, which will provide the necessary control of smoke and heated gases within an underground warehouse facility, in case a fire occurs. Due to the pattern of the underground area and the variety of products stored, numerous fire scenarios should be examined in order to secure escape routes in every case, which significantly complicates the problem. For this reason, a different approach has been adopted, matching recent developments in fire safety from tunneling projects and ventilation practices from the mining industry. The ventilation design is based on the ‘‘critical velocity’’ theory; however, alternative configurations of the underground space are simulated by means of mine ventilation software. These alternatives affect the direction and velocity of the airflow and, consequently, the air quantity and the fan power required, in order to secure escape routes during fire emergency. The analysis not only determines the ventilation system characteristics, but also indicates the most appropriate design of the facility, in order to come up with a solution that is both secure and economically acceptable. 2006 Elsevier Ltd. All rights reserved. Keywords: Critical velocity; Fire and smoke control; Underground warehouse; Ventilation
1. Introduction Subsurface utilization has proven to be a realistic solution and has been gradually gaining ground during the last decades. Literature provides the reasons leading to the development of underground space (Damigos et al., 2004; Zevgolis et al., 2004). Earlier concerns and concepts for rejecting the option of underground construction, mainly due to economic reasons, are nowadays being re-examined and often decisions for an underground structure, instead of a surface one, may be favored. However, many people may feel uncomfortable driving or dwelling underground. One reason is that the lack of natural light decreases the ability of orientation in time and space. Another major rea-
*
Corresponding author. Tel.: +30 210 7722214; fax: +30 210 7722156. E-mail address: [email protected] (D.G. Damigos).
0886-7798/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tust.2006.07.002
son for negative reactions towards traffic tunnels and underground space is the fear of accidents and fire in an enclosed area (ITA, 1998). Statistically, underground facilities cannot be considered as major hazardous areas. However, in case of a fire accident, much more severe damage occurs compared to a fire in open air. Aggravating factors include confined space, limited number of escape roads, flashing over and loss of visibility (Li and Chow, 2000; Vauquelin and Megret, 2002). A series of fire accidents, which happened worldwide (Channel Tunnel, Mont-Blanc Tunnel, Tauren Tunnel, Gotthard Tunnel) in the last years, have cost many lives and have triggered extensive discussions with regard to the fire safety of underground spaces. Towards this direction, the efforts focus on the prevention of a possible accident and, if this is not attainable, the safe evacuation and rescue of all trapped people. There are several important aspects of fire and life safety (ITA, 1998):
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fire – and collapse – resistant structures; elimination of combustible materials; detection and alarms; compartmentalization; emergency systems (rescue, escape and refuge); fire and smoke control; and maintenance of safety systems.
mine ventilation software. The analysis not only determines the ventilation system characteristics, but also indicates the most appropriate design of the facility, in order to come up with a solution that is both safe and economic. 2. Methodological approach 2.1. Literature review
So far, the measures related to fire fighting in underground spaces (e.g. mines, stations, storage facilities, transportation facilities, etc.) include direct attack with rock dust or water, application of foams or water mist, inertization with nitrogen, carbon dioxide, or exhaust gases from turbine engines and sealing of the affected area. Despite of the wide range of the available fire fighting techniques, it seems that in most cases of large fires the primary concern is the sealing of the fire area (Loomis and McPherson, 1995; Wala et al., 1995; Biffi et al., 1997; Adamus et al., 1995). The latter, however, presupposes that people have already evacuated the underground area. When a fire occurs, high temperatures (exceeding 1000 C) and smoke concentration provide conditions where the chance of survival is near zero. The surrounding walls, together with the lack of heat escape in vertical direction, lead to an intensive temperature increase within a few minutes (Modic, 2003). However, the generated smoke and not the fire itself suggests the major threat for the people trapped underground, as it spreads rapidly throughout the area, reducing the visibility and causing deaths by asphyxiation (Vauquelin and Megret, 2002) due to the hazardous gases, like CO, contained. Thus, in case of a fire, the first action should aim at controlling the propagation of combustion smoke by means of ventilation systems (Vauquelin and Megret, 2002; Modic, 2003; Chen, 2000; Gao et al., 2004). Unless a strong airflow is created and maintained, hot gases and smoke migrate in all directions. With a weak airflow, smoke forms a layer along the tunnel ceiling and can flow against the direction of forced ventilation (‘‘backlayering’’), interfering with personnel evacuation (Modic, 2003). For most fire and safety officials, however, the prospect of employing ventilation to contend with a subway fire emergency goes against their instincts (Cheng et al., 2001). This paper presents the design of an emergency ventilation system, which will provide the necessary control of smoke and heated gases within an underground warehouse facility, if a fire occurs. The ventilation system will provide the conditions for emergency evacuation and rescue. The storage facility will be developed using the room-and-pillar mining method. For certain reasons (e.g. the pattern of the underground area, the variety of products stored, etc.) a different approach has been adopted in this research. Due to the large number of the potential fire scenarios involved, the major concern is to prevent ‘‘backlayering’’ phenomenon in order to secure escape routes. Towards this direction, recent developments, adopted by the tunneling fire safety theory, are being utilized and alternative configurations of the underground space are analyzed by means of
During the last decades, research work and case study analyses have been conducted regarding fires in underground structures and how a proper ventilation system can assist to a safe evacuation procedure. Hinkley (1970) studied the flow of hot gases along an enclosed shopping mall. Heselden (1976) considered the fire and smoke behavior in tunnels. Laage et al. (1987) simulated the effects of mine fire, which has induced ventilation disturbances on the performance of a stench fire warning system. Simcox et al. (1992) used computer simulation to study gas flows from fire at King’s Cross Underground Station. Lea (1994) used three computational models to study the effects of fire in an UK mine. The US Bureau of Mines at the Twin Cities Research Center has developed mine fire and ventilation simulators (Laage et al., 1995). Pomroy and Carigiet (1995) analyzed underground coal mine-fire incidents in the United States from 1978 to 1992. Leitner (2001) studied the fire catastrophe in the Tauern Tunnel south-east of Salzburg, Austria. Due to the more intense use of underground space and the changes in technology the study of fire and safety issues has been complicated. In order to deal with the difficulties involved, experiments and large-scale fire tests have been conducted (e.g. Burnett, 1984; Keski-Rahkonen et al., 1986; Chow and Li, 2001; Vauquelin and Megret, 2002; Gao et al., 2004; Hu et al., 2005). In addition, many researchers employed empirical and numerical models to simulate smoke and temperature distribution in underground spaces, as well as to specify the ‘‘critical velocity’’ of the air needed to prevent upstream movement of smoke from a fire in a tunnel (e.g. Laage et al., 1987; Miclea, 1991; Guelzim et al., 1994; Laage et al., 1995a; Kennedy et al., 1996; Oka and Atkinson, 1996; Dziurzynski et al., 1997; Chow, 1998; Edwards and Hwang, 1999; Megret and Vauquelin, 2000; Wu and Bakar, 2000; Cheng et al., 2001; Modic, 2003; Hu et al., 2005). A relatively recent review of tunnel fires (Grant et al., 1998) pointed out that existing experimental data still show an inadequate fundamental understanding of the interaction between buoyancy-driven combustion products and forced ventilation, the influence of slopes on smoke movement, the effect of tunnel geometry, etc. In any case, however, simulation programs have the advantage that several fire scenarios can be analysed and visualized, providing a dynamic representation of the fire progress, in terms of the spread of combustion products (e.g. CO) and temperature throughout the underground space. Especially with the increasing power of personal
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computers, considerable attention has been given towards this direction. 2.2. Methodology description Regarding the ventilation system, fire emergency planning involves simulation of the interaction of the fire and the ventilation system. Thus, the ventilation paths, along which hazardous combustion gases pass through, are determined in order to design safe escape routes and fire fighting activities. This procedure provides useful results for decision-making, in case the underground space pattern is relatively simple and potential fire hazards are limited (e.g. maintenance shops and fuel bays). Dealing, however, with underground storage facilities, i.e. warehouses, the calculation of smoke propagation under different fire scenarios becomes practically impossible, due to the complex layout of the facility and the numerous potential locations of the fire. To overcome this problem, the procedure presented utilizes the ‘‘critical velocity’’ theory and passive fire protection measures (e.g. fire safety departments) so as to establish the air quantity required in every single point of the underground space. Further, different layouts of the underground facility are examined to minimize fan horsepower. 2.2.1. Necessary steps for designing the ventilation system The design of an appropriate ventilation system for underground structures, in general, must take into consideration, mainly, two issues: The supply of the necessary air flow rate that provides adequate ventilation under normal conditions. The smoke and high temperatures management in case of a fire emergency, to establish safe evacuation conditions. According to the methodology presented, the design steps of the ventilation system, for both normal and emergency operation, are: 1. Estimation of the required quantity of the incoming air under normal operating conditions. 2. Determination of the critical air velocity for each branch to control smoke propagation. Air velocity lower than the critical can be considered inadequate, whereas larger than the critical velocity could result in possible increase of the fire size and contribute to its rapid spread throughout the area. 3. Estimation of the required quantity of the total incoming air in order to attain critical velocity and avoid ‘‘backlayering’’. 4. Configuration of the facility layout alternatives, so as to minimize air quantity requirements. 5. Re-estimation of the required air quantity for each alternative. 6. Selection of the final scheme.
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2.2.2. The longitudinal ventilation system during a fire emergency All underground facilities, except from small tunnels, require the application of a ventilation system to dilute the contaminants produced by vehicle engines during normal operation, such that a complete air exchange is succeeded within a reasonable time frame (Tarada, 2000; Modic, 2003). In addition, emergency ventilation is required during a fire to control smoke and gases and provide a safe evacuation path (Modic, 2003). The standard procedure consists of blowing the whole smoke towards a specific exit, while avoiding the occurrence of backlayering (Vauquelin and Megret, 2002; Gao et al., 2004). System performance is not effective if the ventilation rate is low and the buoyancy of smoke is strong enough. The fire plume would impinge on the ceiling and spread against the discharged ventilation flow. It is very important to understand when a smoke backflow will occur (Gao et al., 2004; Wu and Bakar, 2000). In order to prevent smoke backflow, the speed of air discharged by the ventilation system has to be higher than a critical value Vc, known as ‘‘critical velocity’’. This value has become one of the prime criteria for the design of ventilation systems, especially in tunnel section (Chow, 1998; Carvel, 1999; Tarada, 2000). Empirical equations, based on the so-called Froude number preservation theory, are provided in the literature in order to estimate the critical velocity (Chow, 1998; Wu and Bakar, 2000; Chen, 2000; Tarada, 2000). These equations have been modified by several researchers. The critical velocity, in the analysis presented, is determined by means of the following equations, which are the most resent and widely used (Li and Chow, 2003; Modic, 2003; Chen, 2000; Wu and Bakar, 2000): 13 gH Q V c ¼ K Kg ð1Þ q Cp A T f Q þ T0 ð2Þ Tf ¼ q Cp A V c K g ¼ 1 þ 0:037 a0:8 where Vc Tf K Kg a g H Q q Cp A T0
ð3Þ
critical air velocity (m/s); average temperature of fire site gases (K); 0.606 or 0.61; grade factor; tunnel grade (%); acceleration caused by gravity (9.81 m/s2); height of tunnel at the fire site (m); heat release rate (kW); average air density (1.2 kg/m3); specific heat of air (1.1 kJ/K kg); area perpendicular to the flow (m2); temperature of fresh air (K).
Most recommendations for tunnel ventilation system operation concern themselves mainly with control and extraction of smoke in the event of a fire. The heat release
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rate (HRR) of a fire, which is generally expressed in MW, is considered to be an important factor contributing to the severity of a fire and a good measure of its size. Chen (2000) argues that the HRR of tunnel fire is one of the most important parameters in determining the characteristics of smoke propagation in tunnels. The HRR may not only affect the smoke production, but is also related to the impact the fire will have on the structure, the likelihood of fire spread and the difficulty fire fighters will face when tackling the blaze (Carvel, 1999; Carvel et al., 2001). Many researchers (e.g. Haack, 1998; ITA, 1998; Megret and Vauquelin, 2000; Vauquelin and Megret, 2002; Chen, 2000; Li and Chow, 2003) have estimated the size of heat release rate of vehicle fires considering different scenarios, i.e. 3–5 MW HRR corresponds to a large car on fire, 10– 20 MW corresponds to a bus, while a truck on fire releases 50–100 MW.
Fig. 2. Cross-section of the central corridor. The figure displays the ‘‘back-to-back’’ system leaving a 5.40 m wide airway.
3. Case study 3.1. Description and configuration of the underground warehouse The study under investigation involves an underground structure excavated using the room and pillar mining method. It includes one entrance and one exit. Fresh air enters the warehouse by a 50 m long horizontal gallery with a square cross-section of 36 m2, while the air comes out from a 30 m long circular upcast shaft with diameter and cross-sectional area of 4 m and 12.56 m2, respectively. The underground space follows a room-and-pillar layout. The rooms will be 8 m wide and the pillars will have dimensions of 8 · 8 · 8 m. The characteristics and more details of the area are illustrated in Figs. 1–3. The storage system of such a warehouse is given by Zevgolis et al. (2004). In this case the utilization of the euro-pallet (0.80 · 1.20 · 0.144 m) as the storage unit and the ‘‘back to back’’ storage system were preferred. Fig. 1 shows the plan view of the storage facility, where the remaining width of the corridors and the location of the storage shelves are demonstrated. The utilization of the storage space is made taking into consideration the need for movement and circulation of the forklift trucks. The
Fig. 1. Plan view of the storage facility.
Fig. 3. Three-dimensional view of the inside of the underground warehouse (central corridor).
creation of the exit protection zone was adopted in order to protect the evacuation procedure in case of fire, although it reduces the storage capacity. 3.2. Ventilation requirements 3.2.1. Normal conditions In the case of underground storage facilities, air requirements are usually diminished. According to ANSI/ASHRAE Standard 62-2001 about ventilation for acceptable indoor quality, an air entrainment rate of 0.3 l/s m2 is believed to be satisfying for this kind of activity (ASHRAE, 2003). Hence, an air entrainment rate of 0.3 l/s m2 · 1920 m2 = 0.6 m3/s is required. 3.2.2. Emergency conditions The critical air velocity and temperature in relation with the heat release rate in case of a fire in a corridor are presented in the diagrams of Figs. 4 and 5, considering a range of heat release rate from 1 to 100 MW and rectangular cross-sections of 43.2 m2 and 21.6 m2 of central and extreme corridor respectively, based on the warehouse
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Fig. 4. Diagram of critical velocity vs. heat release rate.
Fig. 5. Diagram of temperature vs. heat release rate.
height (8 m) and each corridor width (5.4 m and 2.7 m, respectively). In order to solve Eqs. (1) and (2) the following data were used: K = 0.606 Kg = 1 g = 9.81 m/s2 H=8m q = 1.2 kg/m3 Cp = 1.1 kJ/K Kg A = 43.2 m2 and 21.6 m2 T0 = 273 + 25 = 298 K In order to obtain the fire simulation and the analysis of different layout scenarios, the estimation of the heat release rate is essential. Because of the variety of the potential storing products, there are many uncertainties regarding this factor. In many similar cases the worst-case scenario is chosen, but this leads to conservative results. In the case studied, the use of probabilistic modeling using Monte-Carlo
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simulation is decided in order to achieve more accurate results. The definition of the statistical distribution describing a parameter behavior is used in order to proceed with the probabilistic modeling. In this case, a triangular distribution is chosen to model the heat release rate behavior, based on the analysis of the literature data. Furthermore, it is chosen because the research is at an initial stage with no specific data regarding the products which are to be stored inside the warehouse. Knowing the exact usage of the warehouse can change the selection of distribution to a more representative having in mind the heat release rate and stored quantity of each product so as to have more accurate rather than indicative results. The triangular distribution is a continuous probability distribution typically used as a subjective description of a population for which there is only limited sample data. It is based on knowledge of the minimum and maximum and an inspired guess as to what the modal value might be. The parameters for the triangular distribution are minimum, maximum, and most probable, which falls between the minimum and maximum values. Hence, a triangular shaped distribution is formed, which shows that values near the minimum and maximum are less likely to occur than those near the most probable one. Despite being a simplistic description of a population, it is a very useful distribution for modeling processes where the relationship between variables is known, but data is scarce (Crystal Ball, 2000). The ‘‘Crystal Ball’’ software was used in order to apply the probabilistic modeling and values 1 MW (minimum), 80 MW (maximum) and 40 MW (likeliest) have been entered as the parameters of the triangular distribution. The calculations for the two cross-sections are shown in Figs. 6 and 7, as output from the Monte-Carlo simulation. The critical velocity values are presented (x-axis), in relation to their probability of occurrence, after 5000 iterations. It can be clearly seen that, with a critical air velocity of 3.13 m/s and 3.48 m/s for the cross-sections of 43.2 m2 and 21.6 m2 respectively, the 95% of the possible fire scenarios are being covered. Thus, the values of 135 m3/s and 75 m3/s are the critical air supply rates in order to prevent backlayering.
Fig. 6. Critical velocity estimation as output of the probabilistic modeling (A = 43.2 m2).
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consists of 30 branches and 21 junctions with one fun. Fig. 8 shows the schematic of the ventilation network and Fig. 9 gives the input data.
Fig. 7. Critical velocity estimation as output of the probabilistic modeling (A = 21.6 m2).
3.2.3. Ventilation simulation The simulation results are given for the emergency scenario only, for reasons of conciseness. Simulations are carried out by means of VnetPC 2003 software, which is briefly described hereinafter. (a) The VnetPC 2003 software VnetPC is a program of the Mine Ventilation Services Inc. that simulates mine ventilation networks. The applicability of VnetPC to subsurface ventilation system design ranges from the initial concept to the system operation phase of a project. Given information that describes the geometry of a ventilation network, airway resistances or dimensions, and the locations and characteristic curves of fans, the code produces listings and visual graphics of many parameters including predicted airflows, frictional pressure drops, air power losses in airways, and fan operating points, using the fundamental equations (3) and (4) of airflow. The software has been developed with the assumption of incompressible flow and is based on Kirchhoff’s laws. The code utilizes an accelerated form of the Hardy Cross iterative technique to converge to a solution (VnetPC, 2003). h ¼ R Q02 R¼
k ðL þ Leq Þ Per S3
where R h Q0 k L Leq Per S
Fig. 8. Schematic of the ventilation network.
ð4Þ ð5Þ
airway resistance (N s2 m8); pressure drop (Pa); flow rate (m3/s); friction factor (kg/m3); length of airway (m); equivalent length of shock loss (m); perimeter of airway (m); cross-section of airway (m2).
A ventilation network is a graphical representation of a ventilation system and consists of a set of junctions and interconnecting lines (branches), which denote major or significant airflow routes. The model of the examined area
Fig. 9. Input data.
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Values of friction factor and equivalent lengths are selected by tables given in the literature (SME, 1996; Hartman et al., 1997). For example, values 0.0100 kg/m3 and 0.0158 kg/m3 are used for unlined rectangular airways with uniform sides and irregular conditions respectively, while for estimating the equivalent length of airway 21 to 7 (shaft) which is 43 m, the sharp bend (20 m), the abrupt contraction (3 m) and the air discharge (20 m) are taken into consideration. (b) Execution of the ventilation simulation in case of fire The primary requirements for the emergency ventilation systems are derived from smoke control considerations, i.e. the smoke can always be driven in the required direction without any significant backlayering (Tarada, 2000). For this reason, three scenarios with different modification of the area were considered (Table 1). In all scenarios examined, the fan is installed outside the underground area in order to be protected in case of fire (McPherson, 1993). According to the first scenario, the area was left as it was excavated with the room and pillar pattern, without any modification. Fresh air entering the area is distributed to the three longitudinal corridors (Fig. 10). An insufficient air supply passes through the cross-cuts. There is no way of succeeding critical velocity in every airway, unless excesTable 1 Characteristics of the examined case-scenarios Case-scenario
Characteristics
First
+Room-and-pillar pattern. +Original layout.
Second
+Regulators installation in cross-cuts and central airway. +Division of the area into three longitudinal entries.
Third
+Stopping installation in cross-cuts. +Split of the area into three independent airways. +No air movement through the cross-cuts.
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sive air quantity is supplied. Hence, the scenario is rejected, as smoke would spread uncontrollably in case of a fire. The second scenario is based on the tunnel crosspassages theory (Tarada, 2000). The area is divided into three longitudinal corridors by placing regulators in all cross-cuts. These modifications result in a regularized airflow, as air is directed from the central airway to the extremes. Furthermore, the critical air supply through the ‘‘cross-passages’’ is reduced to 23.5 m3/s. A similar regulator is installed at the end of the central airway in order to increase the air supply through the cross-cuts. However, the critical values cannot be attained without huge air entrainment ratio and, consequently, smoke circulation cannot be avoided. After applying the calculation procedure (Fig. 11) it can be clearly seen that in the extreme corridors the air supply starts from a small value and ends to a value much larger than the critical one. Furthermore, if the central airway becomes the incident airway, the flow reversal from the extreme corridors to the central one through the cross-cuts is required, which is difficult to be obtained. According to the third scenario, stoppings are installed to all the cross-cuts in order to split the area into three independent airways (Fig. 12). This aims to the separation of the entrained airflow into three different directions, avoiding the movement of the air through the cross-cuts that caused the smoke migration in the previous scenarios. The stoppings installed between the pillars are designed to have self-closing doors. Thus, the communication between the longitudinal corridors is allowed, ensuring the escape of the personnel through the neighbor corridor and entry gallery, while smoke is restricted to the incident airway. In this case, smoke is directed to the exhaust shaft and out of the affected corridor whilst the doors protect the non-affected corridors from the effects of heat and smoke. It is believed that this case-scenario creates suitable conditions for safe evacuation and firefighting.
Fig. 10. Result of calculation procedure for the first scenario.
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Fig. 11. Result of calculation procedure for the second scenario.
More specific, this scenario is based on the controlled split of the entrained airflow to the three longitudinal airways, so as to achieve air supply rates of 135 m3/s and 75 m3/s to the central and extreme corridors, respectively. From the calculation procedure (Fig. 13) it can be seen that a regulator of 0.00071 N s2 m8 resistance needs to be installed to the central airway. The air entrainment ratio of 285 m3/s gives an air velocity of 7.9 m/s flowing through the entry gallery, which is in accordance with specifications given by international standards (maximum air velocity during evacuation procedure: 11 m/s, according to NFPA 130 and ASHRAE; Chow and Li, 1999). Finally the fan used for the emergency ventilation system should have a power of 160 kW with an average efficiency of 80% given by Eq. (6). Fig. 12. Three-dimensional view of the layout of the underground area after the installation of stoppings.
N0 ¼
Q0 h 285 m3 =s 448:6 Pa ¼ ¼ 159:8 kW 1000 n 1000 0:8
Fig. 13. Result of calculation procedure for the third scenario.
ð6Þ
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4. Conclusions During the last years, there has been a lot of research regarding fires in underground areas and emergency ventilation systems. These cases refer mainly to transportation tunnels, where the critical velocity theory is applied, and fire events in mines, where the problem is examined through the arrangement of the airflow direction. So far, there is a gap in the literature concerning subsurface storage facilities. The paper examines a special case of an underground warehouse center, which is constructed using a classic mining method, namely room-and-pillar. The analysis tries to bridge this gap by developing ideas adopted from cases met in both tunnel and mine-fire events. In general, the installation of a proper ventilation system in combination with the existence of an evacuation plan could be sufficient in order to safely evacuate an underground area in case of a fire, leaving certain airways free of smoke. Towards this direction, fire simulation software has been proven to be a quite useful tool for fire and safety officials. Fire scenarios can be analyzed and personnel can be trained to contend such an emergency. In the case studied, due to the large number of the potential fire scenarios involved, the analysis follows a different approach, dealing not only with the influence of the ventilation system, but also the outline of the underground space. In addition, Monte-Carlo modeling simulation was used to address the uncertainty of the products stored in the warehouse, covering a number of different situations (heat release rates). The requirement for critical airflow in every airway of the area is proved to be not attainable, since enormous air quantities would be required. Towards this direction, alternative configurations of the underground space are analyzed by means of mine ventilation software. The compartmentalization of the area, by installing stoppings and the partition of the warehouse into three parallel corridors, seems to be the most efficient solution. In addition, the installation of regulators is necessary in order to optimize the ventilation system performance. The analysis proved that the control of smoke flow is achieved at any area of the underground structure and safe evacuation is obtained. Hence, the results not only indicate the proper ventilation system characteristics, but also provide the most suitable design of the facility. The approach presented is less complicated and can be easily applied compared to CFD simulations. Due to the pattern of the underground area and the variety of products stored, numerous fire scenarios should be examined in order to secure escape routes in every case, a fact that would significantly have complicated the analysis. Of course, in some cases (e.g. very large underground facilities) the approach proposed may be unfeasible, due to the high investment costs required for achieving critical airflow velocities in each corridor. According to the analysis, the requirements for air supply during normal operation are minimal. This demand could
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