National Academies Press: OpenBook

Design Fires in Road Tunnels (2011)

Chapter: Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size

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Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 106
Page 107
Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 107
Page 108
Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 108
Page 109
Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 109
Page 110
Suggested Citation:"Chapter Twelve - Fixed Fire Suppression and Its Impact on Design Fire Size." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 110

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

105 BACKGROUND PIARC, NFPA, and several European countries are rethink- ing fixed fire suppression application for tunnels. Before the Alpine tunnel fire disasters, Japan and Australia were the only two countries to require and use sprinkler systems in road tun- nels. It is noted that sprinklers were installed in several other tunnels around the world, including the United States. How- ever, those installations were driven by specific requirements and jurisdictions (e.g., Seattle 1952). Based on the literature review, all Japanese class ‘AA’ road tunnels are required to have sprinkler systems. (Class ‘AA’ are tunnels with traffic density of more than 40,000 vehicles per day with a length of more than 1 km or 3,280 ft.) Starting in 1963, a number of full-scale tunnel fire tests have been carried out in Japan. It was concluded that sprinklers are able to reduce fire size and temperature and prevent fire from spreading. In Japan, sprinklers have been used in two or three tunnel fire incidents per year. It shall be noted that the Japanese approach is to activate sprinklers with a 3-min delay. This approach dif- fers from Australia, where sprinklers are activated immedi- ately (it takes 30 s for the deluge system to activate). Lessons from the Burnley Tunnel fire in Australia, where a major disaster was successfully averted by a brand new suc- cessfully working safety system, are currently being studied (69). In March 23, 2007, the fire in the Melbourne City Link Burnley Tunnel started with a road traffic accident involving four cars and three HGVs. The pile-up of trucks and cars inside the 3.4-km (2.1-mi) long Burnley Tunnel that killed three peo- ple burst into a wall of fire that reached temperatures of more than 1,000°C (1,832°F). However, further casualties were avoided. Although, according to the Sydney Morning Herald (70) some witnesses reported that they had not seen any sprin- kler or safety system in operation, Acting Metropolitan Dire Brigade Chief Officer Keith Adamson said that both sprinkler and smoke extraction systems made it much easier to find the source of the fire. Hundreds of motorists were immediately advised to leave their cars with their keys in the ignition and evacuate the tunnel. Most took the emergency exits, which lead to separate pedestrian tunnels, whereas some took the riskiest route by walking back to the tunnel entrance. As a con- sequence, the Burnley Tunnel, which opened in late December 2000, is now widely regarded as an example of a modern safety model. Despite a potentially huge fire and the presence of more than 400 people in the tunnel, only three people died from the traffic accident and none from the subsequent fire. The Burn- ley Tunnel incident demonstrates that fixed fire fighting sys- tems are effective in protecting tunnel infrastructure and delivering human safety (71). Presently, there are several ongoing discussions of the ben- efits of sprinklers. However, there were also some past lessons learned, which are reviewed here. For example, as mentioned earlier, the Ofenegg Tunnel tests (1965) included a 500 L (132 gal) sprinkler test, sprinkler droplets initially evaporated into a high-temperature steam cloud, which caused more damage than the nonsprinklered fires. The open fire was apparently soon extinguished, but was accompanied by a strong odor of gasoline at the portal. The fire then reignited after 17 min (status of sprinkler flow unstated) with pronounced, but nonexplosive, wave-front propagation. However, the ultimate minimum survival dis- tance for an upright subject was judged closer than for the nonsprinkled fires. As noted in the Ofenegg Tunnel test report, during the 1,000 L (264 gal) gasoline burn tests the sprinklers were immediately activated after ignition. The sprinklers reduced the maximum arch temperature significantly. However, the steam apparently pushed burning gases and gasoline vapors into adjacent tunnel sections, where they continued to burn. The fire was apparently extinguished after 10 min, but the tunnel filled with gasoline vapors, which exploded in the nineteenth minute, causing extensive damage to the test setups and injuring three technicians. A lesson learned is that once the sprinkler system is activated, it is not to be turned off until the fire source is completely extinguished and determined safe. A delay in activation produces huge volumes of high tem- perature steam, which can be as dangerous as the combustion products. If all ignition sources cannot be extinguished and the site uniformly cooled below a safe temperature, the fire will reignite, perhaps explosively, when the sprinklers are shut off. Meanwhile, unburned vapors are propelled around the tunnel and ventilation ducts, which can cause another significant hazard to those safely away from the fire, even after the fire is extinguished. CHAPTER TWELVE FIXED FIRE SUPPRESSION AND ITS IMPACT ON DESIGN FIRE SIZE

Figure 34 schematically shows the effect of suppression on HRR. With timely activation of a suppression system, the HRR is reduced. With delayed activation, the fire becomes overwhelming and the suppression system is not effective. It is vital to have a clear understanding of the capabilities of the detection system and the lead-in times for activation of the fire life safety systems. It is essential that the detection system be capable of detecting a small fire (in the order of 1–5 MW). If this is not achieved and the fire is not detected until it enters its rapid growth phase, the resultant fire will, in all likelihood, be well beyond the capabilities of a fixed fire suppression sys- tem once it is activated (72). Although a few automatic sprinkler systems have been installed in tunnels, most systems are deluge systems. A del- uge system has a network of open nozzles at the roof of the tunnel, divided into zones, typically of 30 m (100 ft) based on the length of a HGV. When there is a fire, a valve is opened in the zone above the fire and in the zones on either side. Water is sprayed from all the nozzles in the activated zones. Deluge systems have been selected over automatic sprin- kler systems as a result of two concerns. First, the ventilation system in a tunnel could spread heat initially to sprinklers that are not above the fire. Second, a tunnel fire could rapidly develop a considerable amount of heat over a large area so that too many sprinklers would open, overwhelming the water supply. In contrast, a deluge system takes a fixed amount of water and, with suitable detection, it is possible to open only the zones above or next to the fire. Deluge water spray nozzles take water at a typical pressure of 1.5 to 5 bar (21.8 to 72.5 psi) and discharge a pattern of water droplets over the area below. Water spray systems are designed to achieve an even discharge of water over an area, 106 with one specification being the water application density, measured like rainfall in millimeters//minute. Droplets from water spray systems are generally larger than 1 mm (0.04 in.) in diameter. Meanwhile, water mist systems use higher pressures, in some cases more than 100 bar (1450.4 psi), and discharge much finer droplets, 99% of which have a diameter less than 1 mm (0.04 in.). Nozzles with very small orifices are used to create the mist. The smaller droplets are drawn into the fire by its own ventilation and easily evaporate owing to the large- surface area-to-volume ratio. The mist systems may require less water per zone; storage tanks, pumps, and pipes can be smaller, saving on costs. However, to protect the small nozzle orifices higher quality materials, such as stainless steel, are required, which add to the costs. Research projects are investigating to what extent an active fire protection system can limit the maximum HRR and whether an active fire protection system combined with venti- lation offers equal or better life safety. The projects are also investigating how to specify design or performance test crite- ria for tunnel active fire protection systems. Today, more than 100 tunnels are equipped with an active fire protection sys- tem. Fixed fire suppression systems have been successfully used for more than 40 years in Japan’s congested urban road tunnels and, more recently, in all of Australia’s congested urban tunnels. Road tunnel deluge systems require substantial amounts of water, which can have a significant impact on the storage, delivery, and drainage systems (although water mist systems require less water per zone). One study came to the conclu- sion that, although some minimum water application rates would achieve a certain objective, a marginally higher rate would not necessarily improve the situation (79). FIGURE 34 Schematic effect of suppression on heat release rate (71).

107 Japan and Australia each have their own specified water application rates to use for road tunnel fixed fire suppres- sion system design, which are 6 mm/min (0.15 gpm/ft2) and 10 mm/min (0.25 gpm/ft2), respectively. In full-scale tunnel sprinkler tests conducted in Europe (2nd Benelux), a water application rate of 14 mm/min (0.35 gpm/ft2) has been tested. These values have been added to Figure 35 to demonstrate the significant variation in prescribed water application rates for which little research has been done to compare their effec- tiveness when applied under similar conditions. Fire point theory shows that there are optimum rates of water application that can control a fire and are signifi- cantly less than the rates generally prescribed. Furthermore, this theory suggests that there are minimum water applica- tion rates that can reduce the heat flux below certain criti- cal limits required to sustain combustion and, once these limits are reached, more water offers little or no benefit. The results of the comparative analyses suggest that water application rates as low as 2 mm/min (0.05 gpm/ft2) can offer some benefits by cooling exposed surfaces and assist- ing in limiting the spread of fire from the initiating point (see Figure 35). Although the conclusions are interesting, they need to be further evaluated by answering these questions: • Does the water requirement depend on the design HRR? Typically, the higher the FHRR, the more the water evaporates. • Does the water requirement depend on fire size at the time of fixed fire suppression system activation? It appears that the earlier the system will be activated, the lower the FHRR will be and the less water may be required. Some previous works have already demon- strated that late fixed fire suppression system activa- tion resulted in an inability to take the fire under control, which caused the FHRR to continue to increase. • Does water requirement depend on ventilation and lon- gitudinal air velocity? Ventilation may have a dual effect. Ventilation may blow away or exhaust water particles from the fire site. Ventilation may also increase the speed of evaporation. The blow away effect may result in the need for activation of additional fire zones. The intense evaporation needs additional studies. The following conclusions were drawn in the UPTUN proj- ect on the basis of the fire tests with the fire mitigation systems: • Validation of the performance of fire safety equip- ment, such as water spraying systems, requires full- scale fire testing and cannot be trusted from model simulations. • The efficiency of the water mist systems was satis- factory. • However, the efficiency was strongly dependent on the size of the fire (or heat generation rate), nozzle type, loca- tion, and the water discharge rates. • For the smallest fires (less than or equal to 5 MW or 17 MBtu/hr) the mitigation effect was minor. • The best results were achieved for the largest fires (i.e., a HRR at or above 20 MW or 68 MBtu/hr). The maxi- mum reduction of the HRR was 80%. • A rapid reduction of the temperatures downstream of the fire was noticed after activation of the suppression system. The efficiency of both water mist systems was satisfactory with respect to heat stresses as well as the toxicity of the fire effluents on human beings. • The visibility was not improved downstream of the fire during the first minutes after activation of the suppression systems. However, the visibility was generally increased as the fire size and the HRR were reduced during fire suppression. • The problem of backlayering (i.e., smoke spread upstream of the fire) and the visibility upstream were also signifi- cantly improved after activation of the water mist systems. FIGURE 35 NFPA 13, NFPA 15, and other International Water Application Rates (79).

• High pressure water mist systems are using less water and suppress the fire to a higher degree in the gas phase of the flames. However, for the low pressure systems, the fire extinguishing effect is mainly cooling of the fuel surfaces. Figure 36 shows Type A fires where mitigation action is provided, whereas Type B often represents fires out of control and may provide significant heat exposure to the structure. Type A fires are assumed to be significantly less severe than Type B fires, which may result in unbearable conditions for humans and significant thermal exposure to construc- tions. Small fires, Type A, are often limited to the first object burning and can be ranked by the HRR measured in megawatts or 1,000 Btu/hr, although more severe fires, after signifi- cant flame spread, can also be measured in terms of time– temperature curves. For the UPTUN fire mitigation test program, the main focus has been on Type A fires to protect human beings, to avoid flame spread, and to provide conditions for unhindered escape and rescue. Type A fires can be considered as fires with a HRR of up to 30 MW (102 MBtu/hr), whereas higher HRR can be considered as Type B fires. To operate effectively, the fixed fire suppression system has to be properly maintained. The operator must be able to activate it correctly, and it must survive the events that have resulted in the incident requiring its activation. Automatic activation of the sprinklers by active detectors may need to be delayed because even a light spray could star- tle unaware drivers and make the roadway slippery. Water squirting from the ceiling of a subaqueous tunnel would sug- gest tunnel failure and can induce panic in motorists. 108 Accidental activation of the system with the cause un- known, which happened in Boston, is not acceptable (see Figure 37). A malfunctioning activation of the sprinkler sys- tem drenched the tunnel under City Square in Charlestown, converting the 1,100-ft (335.3-m)-long tunnel into a tempo- rary car wash. The activation was inadvertent and the source of the activation unconfirmed. The malfunction activation forced State Police to close the three main ramps that lead traf- fic from Storrow Drive, Interstate 93 north, and Rutherford Avenue into the tunnel. It is recognized that active fire protection systems can limit the size and growth of a fire and prevent the fire from spread- ing. It is also recognized that active fire protection systems will limit damage to the tunnel in the event of a fire, so that even a fire involving several HGVs will not close the tunnel for long. It could also protect tunnel lining, possibly reducing the amount of passive structural fire protection and providing sig- nificant construction savings. At the control level there are a range of opportunities to fully integrate such systems with the ventilation, operate them FIGURE 36 UPTUN Fire Heat Release—Temperature curve for classifications of ventilation and fixed fire suppression systems (73). FIGURE 37 Accidental activation of the sprinkler fire suppression system in Boston CANA (Central Artery North Area Tunnel) for 45 min on May 15, 2005, at 2 p.m. (54).

109 separately, fully automate them, automate them with manual override, and manually operate them with auto override. How- ever, each of these options must be carefully evaluated. Dif- ferent tunnels will require different approaches. One important lesson learned from the Ofenegg and other tunnel tests is that it is dangerous to turn the fire suppression system off while surfaces are still hot and fuel vapors are pres- ent, because they can ignite and cause an explosion. A well- thought out operation of the fixed fire suppression system is important because fire sizes can be very large. For an active fire protection system to be effective it is essential that fires are quickly and accurately detected. Sprin- kler systems are to be designed to prevent a fire from reaching its peak; however, the droplets will be affected by ventilation. Longitudinal airflow must be selected to ensure an appropriate droplet spread and mass flow performance for given water pressures. No doubt ventilation system performance is also affected by sprinkler operation. However, the main idea is to get a well-designed system with a reliable quick fire-detection system to start these systems before the fire gets too large. For an effective deluge operation, activation must be rapid and accurate. If discharged in this way, fire growth rates are likely controlled, the risk of rapid fire spread min- imized, and, thereby, toxic gas and smoke generation vol- umes contained. The undesirable consequences of its activation, such as smoke de-stratification, increased humidity, and decreased visibility, are hopefully outweighed by their other positive out- comes of fire growth rate control, containment of fire spread, and reduced temperatures. The Runehamar tests brought up the question: Is it possi- ble to manage a 200 MW (682 MBtu/hr) fire? A fire sup- pression industry offers to control the fire size, reducing the maximum HRR by applying a fixed fire suppression system. Once a fire is early detected by a reliable fire-detection sys- tem, the fire protection system could be activated within several minutes, taking the fire in the order of about 10 MW (34 MBtu/hr) and under control, or suppress a small fire. However, the question is what this will do for the tunnel safety (Table 33). With the longitudinal ventilation system, it appears reason- able to activate both systems simultaneously. As a result, the wet fixed fire suppression system will initially start before the longitudinal ventilation reaches full speed. Note that it takes 30 s to discharge water if it is a wet system, whereas it takes 3 min to achieve a full operational ventilation mode. This TABLE 33 IMPACT OF A FIXED FIRE SUPPRESSION SYSTEM (FFSS) ON TUNNEL FIRE SAFETY SSFFfosegnellahCSSFFfosegatnavdA General A sprinkler is designed to react at an early stage of the fire. Takes fire under control, not allowing it to further grow, or grow slowly, or extinguishes a small fire before the fire department arrives. Possible loss of visibility (reduced visibility) especially at an early stage when people evacuate. When the sprinkler system is activated on an already large fire, a large amount of water will be evaporated and, thus, the visibility will be further diminished. Protection of tunnel users and structure. Duration of the fire can be limited and the structure of the tunnel will be subjected to less harsh conditions. Incomplete combustion creates smoke, gases, and steam. Studies needed on critical time to activate the FFSS to protect the tunnel structures. Reaching the fire: help rescue team and firefighters to reach the fire source. Creates slippery environment when water applied. May create panic when it malfunctions with an accidental water release. If a system (a normal wet sprinkler) is activated by a defect such as breaking of the glass, water will be sprinklered into a tunnel with a possibility of causing an accident. Transverse ventilation based on smoke extraction (including single-point extraction) Reduced fire size, see also general Destroys stratification of hot air, which makes ceiling extraction inefficient and evacuation difficult. Reduced fire duration Increases mass of air/water mixture to move, results in increased vent rate for sidewall extraction system. Longitudinal ventilation—unidirectional tunnel with manageable traffic Reduced fire size may result in reduced ventilation rate Increases mass of air/water mixture to move—increases vent rate Cools environment and protects fan units from high temperature Overcomes water curtains created by the FFSS—increases vent rate essaercni—erifehtmorfyawasecnatsbusSFFehtswolBlarenegeeS number of FFS zones for activation. Longitudinal ventilation—unidirectional tunnel with unmanageable traffic or bidirectional tunnel Protects tunnel structure Destroys stratification making evacuation difficult (maybe impossible) to both sides of the fire once the FFSS is activated. Traffic control for low traffic tunnels is imperative.

allows the sprinkler system to discharge water in a low air velocity environment, thus protecting people and structures by taking control of a fire at an early stage of its growth. Once the ventilation reaches full speed, the sprinkler zones may need to be revisited and either switched or additional activation zones will be required to account for ventilation. With the transverse ventilation system using ceiling exhaust, the sequence of activations may differ. The primary purpose of the fire life safety system is to save lives and allow for safe evacuation. Destruction of the smoke layer, worsening of visibility, and potential generation of hot steam, need to be considered. The Japanese approach for the transverse system may be reasonable, which allows for a minimum of a 3-min delay before the sprinkler activa- tion, so that people can leave the sprinkler zones. However, sprinkler activation delay may be dangerous for the tunnel structure and can lead to fire spread and growth. This con- firms the need for an integrated approach to all fire life safety systems (2). There are a number of questions that need further study: 1. NFPA 502 and other standards allow for a maximum air velocity in a tunnel of 12 m/s (2,200 fpm). Ventilation systems are designed for significantly smaller critical air velocities, but in combination with wind, other natural factors, and the traffic pattern, the resultant air velocities may be that high. What will such velocities do to a fixed fire suppression system’s performance? 2. Once a fixed fire suppression system is activated, it will create a water curtain in the tunnel for longitudinal air velocity. The air velocity will be reduced and could be less than critical for the sprinkler controlled fire HRR. Will smoke be under control or does the ventilation system performance need to be increased? 3. If a sprinkler is activated early enough, can ventilation be reduced or eliminated and what will be the impact on smoke production? 4. A fixed fire suppression system will increase humidity in the tunnel. How will this humidity affect the ventila- tion and fan’s performance? 5. Other questions are related to fire detection and the operator’s control of the situation, low visibility, haz- ardous slippery conditions, system activation malfunc- tion concerns, and optimum systems activation time. Critical factors such as droplet size distribution and trajec- tory modeling of droplets through a range of longitudinal velocities are essential for CFD modeling. NFPA 502 recognizes the benefits of the fixed fire sup- pression system for road tunnels, but is concerned with the available fire-detection technology, with the further visibility reduction, and with the impact of the fixed fire suppression system on the effectiveness of tunnel ventilation. 110 Annex E (the explanatory material) of NFPA 502 (2008 edition) notes that the major concerns expressed in the past by tunnel designers, engineers, and authorities worldwide regard- ing the use and effectiveness of water-based fixed firefighting systems in road tunnels, along with the current assessment of those issues have been revisited as follows: 1. Fires in road tunnels usually occur inside vehicles or inside passenger or engine compartments designed to be waterproof from above; therefore, water-based fixed firefighting systems would not have an extinguishing effect. It is now recognized that the purpose of a water- based fixed firefighting system is not to extinguish the fire but to prevent fire spread to other vehicles so that the fire does not grow to a size that cannot be attacked by the fire service. 2. If any delay occurs between ignition and the water- based fixed firefighting system activation, a thin water spray on a very hot fire could produce large quantities of superheated steam without materially suppressing the fire. Fire tests have shown this not to be a valid concern. A properly designed water-based fixed firefighting sys- tem suppresses the fire and cools the tunnel environ- ment. Because a HGV fire needs only 10 min to exceed 100 MW (341 MBtu/hr) and 1200°C (2192°F), which are fatal conditions, it is important to operate the fixed firefighting system as soon as possible. 3. Tunnels are long and narrow, often sloped laterally and longitudinally, vigorously ventilated, and never sub- divided: therefore, heat normally will not be localized over a fire. Advances in fire-detection technology have made it possible to pinpoint the location of a fire in a tunnel with sufficient accuracy to operate a zoned water-based fixed firefighting system. 4. Because of the stratification of the hot gas plume along the tunnel ceiling, a number of the activated fixed fire suppression systems would not, in all probability, be located over the fire. A large number of the activated water-based fixed firefighting systems would be located away from the fire scene, producing a cooling effect that would tend to draw the stratified layer of smoke down toward the roadway level, thus impeding rescue and firefighting efforts. Independent laboratories have commented that they do not observe smoke stratification. Any activated water-based fixed firefighting system not over the fire would cool the tunnel to help rescue services to inter- vene. Zoned systems are released by a detection sys- tem that is accurate even with forced ventilation. 5. Water spraying from the ceiling of a subaqueous tunnel could suggest tunnel failure and induce panic in motorists. This theoretical concern was not borne out in prac- tice. In the event of a fire, motorists are likely to rec- ognize water spraying from nozzles as a fire safety

111 measure. Behavioral studies have shown that most people do not panic in a fire, even when they are unable to see. 6. The use of water-based fixed firefighting systems could cause the delamination of the smoke layer and induce turbulence and mixing of the air and smoke, thus fur- ther threatening the safety of persons in the tunnel. This has been shown not to be a valid concern. Fire tests have demonstrated that smoke does not usually form a layer at the top of the tunnel but quickly fills the cross section. Normal air movement in the tunnel accel- erates this process. A water-based fixed firefighting sys- tem reduces temperatures and the risk of fire spread to other vehicles. 7. Testing of a water-based fixed firefighting system on a periodic basis to determine its state of readiness is impractical and costly. A full discharge test is normally performed only at system commissioning. During routine testing, the system can be configured to discharge flow to the drain- age system. SUMMARY PIARC, NFPA, and several European countries are rethink- ing their position on fixed fire suppression system application for their tunnels. It is recognized that sprinklers are able to reduce fire size and temperature and prevent fire from spread- ing. In addition, it is recognized that timely activation of active fire protection systems will limit damage to the tunnel in the event of a fire. However, there were also some earlier lessons, which are to be reviewed when making a decision: • Once the sprinkler system is activated, it shall not be turned off until a fire source is completely extinguished and the tunnel determined to be safe. • With timely activation of a suppression system, the HRR is reduced. With delayed activation fire over- whelms and the suppression system may not be effec- tive. Extended delay with a fixed fire suppression system may result in its inability to control fire, in structural damages, and possible explosions. A reliable automatic fire-detection system is essential. • It is essential that the detection system is capable of detecting a small fire (in the order of 1–5 MW) • Accidental activation of the sprinkler system is un- acceptable. Today, more than 100 tunnels worldwide are equipped with an active fire protection system. Although a few automatic sprinkler systems have been installed in tunnels, most systems are deluge systems. Water mist systems may require less water per zone. Storage tanks, pumps, and pipes can be smaller, sav- ing on costs. However, to protect the small nozzle orifices higher-quality materials, such as stainless steel, are required, which add to the costs. The type of ventilation system influences the type of sprin- kler system and the sprinkler system design impacts the venti- lation system performance. Some of the challenges faced with considering ventilation and fixed fire suppression systems in the tunnel are: • Selection of the type of fixed fire suppression system depends on the type of tunnel ventilation system. • Wet fixed fire suppression systems can be activated before ventilation and can control fire growth rate, fire size, and the overall smoke production rate at an early stage of fire development. • Activation time of a fixed fire suppression system may differ depending on the type of ventilation. • For longitudinal ventilation, the sprinkler zones may need to be switched or additional zones may be required once ventilation mode is in full speed. • With transverse ventilation, a short system activation delay may need to be considered. • Delay with the fixed fire suppression system activation will require additional water supply because of the larger fire size at the time of activation. • Extended delay with a fixed fire suppression system may result in its inability to control fire, in structural damages, and a possible explosion. A reliable automatic fire- detection system is essential. The undesirable consequences of fixed fire suppression system activation, such as smoke destratification, increased humidity, and decreased visibility, are hopefully outweighed by its other positive outcomes of fire growth rate control, containment of fire spread, and reduced temperatures. The questions that need additional investigation are whether the fixed fire suppression system can replace other tunnel fire life safety systems, such as ventilation and passive protection systems, or whether the size and requirements for such systems can be reduced.

Next: Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review »
Design Fires in Road Tunnels Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 415: Design Fires in Road Tunnels information on the state of the practice of design fires in road tunnels, focusing on tunnel fire dynamics and the means of fire management for design guidance.

Note: On September 20, 2011, the following errata was released related to NCHRP Synthesis 415. The electronic version of the publicaiton was changed to reflect the corrections.

On pages 106 and 107, an incorrect reference was cited. In the final paragraph on page 106, the last sentence should read: One study came to the conclusion that, although some minimum water application rates would achieve a certain objective, a marginally higher rate would not necessarily improve the situation (79). The figure caption for Figure 35 at the bottom of page 107 should read: FIGURE 35 NFPA 13, NFPA 15, and other International Water Application Rates (79).

The added reference is as follows:

79. Harris, K., “Water Application Rates for Fixed Fire Fighting Systems in Road Tunnels,” Proceedings from the Fourth International Symposium on Tunnel Safety and Security, A. Lönnermark and H. Ingason, Eds., Frankfurt am Main, Germany, Mar. 17–19, 2010.

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