Recent severe road tunnel fires emphasize the need for improvement in tunnel fire management - sprinkler application is one technique being promoted.
Introduction
Historically, the disappointing results of the Ofenegg Tunnel fire tests (1965, Switzerland) had a negative impact on sprinkler application in tunnels. The tests, which employed pools of aircraft fuel, led to the view that visibility was much reduced by the sprinkler systems and hot steam was generated that could cause scalding at long distances from the fire. The steam production also displaced smoke more quickly causing temperatures to be higher than without sprinklers. After extinguishment the fuel continued to evaporate, reaching critical concentrations within about 20 minutes. Subsequent deflagrations occurred that created air velocities of up to 30 meters per second.
It was the impact of this experience that was reflected in the World Road Association (PIARC) recommendations which, between 1983 (World Road Congress in Sydney) and 2004, consistently advised against the installation of fixed fire fighting systems (FFFS) in road tunnels, and this position was reflected in U.S. standards.
One of the factors that maintained this attitude against the application of FFFS in tunnels was the fire sizes generally used. The fire sizes chosen on which to base the design were relatively small—20 to 30 MW—typical of a bus or truck fire. Such fires were regarded as manageable and ventilation systems were sized to control smoke for such events.
Several severe road tunnel fires - the Mont Blanc Tunnel (France/Italy, 1999), the Tauern Tunnel (Austria, 1999), the St. Gotthard Tunnel (Switzerland, 2001), and the Frejus Tunnel (France/Italy, 2005) - resulted in loss of life, injury, and infrastructure damage that were far more extensive than if they had occurred on surface roadways. These fire incidents demonstrated that fire sizes could be much larger than 20-30 MW and completely changed the perception of the design fire size. Since then the maximum design fires utilized in tunnel design have increased as much as tenfold in some cases. These recent incidents have emphasized the need for further improvement to be made in tunnel fire management; the FFFS is one technique that is actively being promoted.
Types of FFFS
Several types of FFFS have been used in road tunnels worldwide:
- Sprinkler/spray (water deluge) systems, based on dense water jets consisting of large-size droplets;
- Water mist systems, based on very fine water droplets; and
- Foam water suppression systems.
Water sprinkler type FFFS have been installed in road tunnels of significant length for many years in Japan and Australia. Tunnels that have water deluge fixed fire fighting systems installed can also be found in the United States, Norway, Canada, and Sweden. These have been found to be effective in preventing fire spread and enhancing cooling of the tunnel structure. In 1999, two fires occurred in the underwater tunnels of the Tokyo Metropolitan Expressway and the FFFS helped control the fires so firefighters could approach and eventually extinguish the fires. The deluge system in Sydney Harbor Tunnel in Australia is reported to have worked well during a van fire in 2004. Another example is the Burnley Tunnel fire in 2007; the deluge system was activated quickly and this was deemed by firefighters to have kept the fire under control. Based on this experience, and the development of alternative types of FFFS, PIARC re-evaluated its position with regard to FFFS and at the same time the European Community undertook research programs to examine fire suppression and the impact of larger design fires.
Several relevant European research programs, including UPTUN (Multinational European Research Project) and the SOLIT (Safety of Life in Tunnels) Project, have demonstrated through independent tunnel fire tests that, with early activation, high pressure water mist systems can be effective in controlling potential 200 MW solid fuel fires and 200 MW diesel oil pool fires. The water mist systems have been installed in the A86 tunnel in Paris, the M30 tunnel in Madrid, the Roertunnel and the Tunnel Swalmen in the Netherlands, and other tunnels in Europe.
Therefore, FFFS are now increasingly being considered in the design of tunnel systems worldwide. This position is also reflected in changes to the recent NFPA 502 and PIARC documentation.
Choosing a Fire Suppression System
Choosing the type of fire suppression system for a road tunnel is not an easy decision to make. Some of the different aspects of the systems are as follows:
Water Sprinkler Fire Protection System
The water sprinkler fire protection system (see Figure 1) has existed for over 100 years and is a commonly used and reliable technology; deluge water sprinkler systems are the common FFFS in Australia and Japan. The system performs very well for Class A (solid fuel) fires, but is considered to be less suited for Class B (liquid fuel, oil) fires or where "splashing" of the fuel is to be avoided.
Water Mist Fire Protection System
Compared to the water sprinkler system, the water mist system (see Figure 2) generates much smaller water droplets and therefore has advantages in promoting more efficient gas-phase cooling and uses 2 to 3 times less water for road tunnels (depending on the system used). Both the water mist and water vapor system can measurably reduce radiant heat flux to objects near the fire - this helps firefighters approach the fire and provides better conditions for evacuation. However, because the system contains fine water particles, it may be less efficient in cooling or wetting the fuel surfaces; therefore, the system is less efficient to combat solid fuel fires compared with the water sprinkler system.
Fixed Foam-Water Fire Suppression Systems
Fixed foam-water fire suppression systems may be another alternative to combat tunnel fires. A foam agent is especially suited for the control and extinguishment of flammable and combustible liquid-type fires. There are two types of foam-water fire suppression systems proposed for road tunnels:
- the foam-water sprinkler system (see Figure 3); and
- the compressed air foam (CAF) system (see Figure 4).
The use of the foam-water sprinkler system against diesel pool fires was investigated in the Memorial Tunnel in West Virginia by Bechtel and Parsons Brinckerhoff (now part of WSP | Parsons Brinckerhoff). The foam-water sprinkler deluge system has been installed in several tunnels in Seattle, Washington. The compressed air foam (CAF) system has been tested in road tunnels in the Netherlands. For both types of foam-water suppression systems, corrosion protection is required for the storage tanks and the pipe systems, and the system can be costly in the long run because of the corrosion problem associated with the use of foam agents.
For longer tunnels, the use of foam-water fire suppression systems may be challenging:
- for the foam-water sprinkler system, the delivery time of the foam may be too long as the foam tanks have to be installed at the tunnel portals and it may take time for the foam to reach the fire if the fire is located in the middle of the tunnels; and
- for the CAF system, additional mechanical rooms need to be installed at specific intervals of length in the tunnels which increases the initial capital cost of the installation of a CAF system.
Conclusion
The FFFS is also being considered in road tunnels to reduce the size of the ventilation system required. When authorities prepare to permit all types of traffic, such as dangerous goods or heavy goods vehicles, to cope with increasing economic activities, mitigation options that can combat 200 - 300 MW fires would be necessary for tunnels, as recommended by NFPA 502 and most European standards. Without FFFS, large fires (such as 200 - 300 MW) dictate the need for a very powerful ventilation system, increasing space requirements and adding significant cost. In addition, FFFS, unlike a ventilation system, can provide benefits for firefighting, tunnel system protection, and operational continuity.
Although the benefits of FFFS are clear, many design issues remain, such as: the reduction in the design fire size with the inclusion of the FFFS and the subsequent reduction in ventilation requirements; the impact of the FFFS on the structural protection system; the performance of the FFFS under operational conditions that have not been tested in the tunnel fire experiments; and the impact of the FFFS on the overall tunnel safety concept and operation procedures.
The most reliable method available to date for those unsolved design questions is full-scale testing, but that is extremely expensive and impractical for new or existing tunnels. A computational fluid dynamics (CFD) fire modeling approach is an alternative and holds great promise once a reasonable correlation between numerical simulations and full-scale tests has been achieved.
References
- Haerter, “Fire Tests in the Ofenegg-Tunnel in 1965”, International Symposium on Catastrophic Tunnel Fires, Boros, Sweden, November 2003.
- PIARC 2008: Road Tunnels: An Assessment of Fixed Fire Fighting Systems.
- UPTUN, Fire development and mitigation measures, Work Package 2 of the Research Project UPTUN, 2008.
- Starke, H., “Fire Suppression in Road Tunnel Fires by a Water Mist System – Results of the SOLIT Project”, Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010.
- Water Mist Fire Suppression Systems for Road Tunnels, Final Report, The SOLIT Research Project, 2007.
- NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, 2014 Edition, National Fire Protection Association.
- Huijben, Ir. J.W., “Tests On Fire Detection Systems And Sprinkler in a Tunnel,” ITC Conference Basel 2-4, December 2002.
- Liu, Z.G., Kashef, A., Lougheed, G., Kim, A.K., “Challenges for Use of Fixed Fire Suppression Systems in Road Tunnel Fire Protection”, NRCC -49232, Suppression & Detection Research Applications – A Technical Working Conference (SUPDET 2007), Orlando, Florida, 2007.
- Quenneville, R., “The Emergence of CAF Fixed-Pipe Fire Suppression Systems”, Fire & Safety Magazine, Spring, 2006.
- Memorial Tunnel Fire Ventilation Test Program, Test Report (section 8.10), Massachusetts Highway Department, by Bechtel/Parsons Brinckerhoff, Nov. 1995.
- Lemaire, A.D. and Meeussen, V.J.A., “Effects of Water Mist on Real Large Tunnel Fires: Experimental Determination of BLEVE-risk and Tenability during Growth and Suppression”, Rept. 2008-Efectis-R0425, Efectis Nederland BV, June 2008.
- Grant, G., Brenton, J., Drysdale, D., “Fire Suppression by Water Sprays,” Progress in Energy and Combustion Science 26 (2000), 79-130.
- Tunnels Study Center (CETU), "Water Mists in Road Tunnel," State of knowledge and provisional assessment elements regarding their use, June 2010.
- NFPA 15, Standard for Water Spray Fixed System for Fire Protection, 2007 Edition, National Fire Protection Association.
