Application of fixed fire fighting systems in road tunnels29 March 2017
An examination of the 2015 technical report on fixed fire fighting systems (FFFS) in road tunnels. This article was edited in collaboration by the World Road Association working group for tunnel operations, led at that time by LBA’s Les Fielding, which produced the document. The main authors were Bruce Dandie director at RRT Pty Ltd, Australia Norris Harvey practice leader: Fire & Life Safety, Hatch Mott MacDonald
THE TERM FFFS REFERS to a range of technologies that use water as the suppression agent, or water with an additive or some other extinguishing agent. These systems are installed as part of the tunnel infrastructure and require no additional elements to be added when called upon to fight fires. As such, these systems are part of the “fixed” installation, having been installed for the specific purpose of controlling a fire incident over a specific area and are activated automatically, semi-automatically, or manually from a remote location. FFFS are now recognised as one of a range of active technologies which, when appropriately designed, integrated, installed, operated and maintained can positively contribute to asset protection and life safety.
FFFS have the potential to reduce the rates of fire growth and spread, thereby assisting the safety of motorists and the emergency services during the self-rescue and assisted-rescue phases of a fire. Other potential benefits of FFFS relate to minimising fire growth rates and fire spread, thereby reducing the risks to tunnel assets from fire damage, and thereby to avoid or reduce the road network interruptions that may occur while a tunnel is being repaired following a fire incident. A systematic approach is recommended to support the decision as to whether such a system should be installed:
FFFS must be considered in the context of other critical safety systems such as ventilation. Rapid and accurate incident detection and FFFS response are essential components to achieve the best possible performance. The operational performance of FFFS can best be assessed through a system engineering approach, including appropriate regimes for maintenance, testing and training. Careful consideration must be made with respect to the effects of such systems on operational procedures and maintenance budgets.
FFFS have been reliably used in tunnels since the 1950s however their use remains the exception rather than the rule in road tunnels worldwide. While such systems reduce the rates of fire growth and spread, they also demand ongoing maintenance and operational integration to ensure they function in an optimal manner. Like all active life safety and asset tunnel protection systems a decision to use FFFS must be coupled with a willingness and capacity to design, integrate, install, operate and maintain the system.
TYPES AND FEATURES
Deluge systems are typified by a zoned water application, characterised by a significant proportion by volume of relatively large water droplets. The exact performance of these systems varies from tunnel to tunnel as their performance is usually specified as an application rate over a discrete section of tunnel, or as a delivered density application rate in mm/ min or l/min/m2, and not on the basis of droplet size distribution.
Water mist systems
Water mist systems can be either low or high pressure, however, the pressures used are typically higher than that used for deluge systems. Water mist systems are typically used where the volume of water, spatial considerations, or weight restrictions, are issues. Mist systems are characterised by relatively fine water droplets, which assist cooling by the evaporative process. Systems are specified based on the volume of the tunnel in the application zone in l/min/m3.
Common features and variances
Both systems are characterised by:
¦ A water supply with sufficient reliability, quality, quantity and pressure for application at the required rate over the designed tunnel area;
¦ An activation valve (typically a section valve or a solenoid) that controls the flow of water to the distribution network and hence does not rely on localised fusible link sprinkler heads;
¦ A water distribution network between the activation valve and the spray nozzle;
¦ The ability to deliver a predefined volume of water over the designated fire zone for a predetermined period of time.
Water mist systems vary from deluge systems in that water mist systems typically:
¦ Use higher pressures than deluge systems;
¦ Use smaller diameter pipework than deluge systems;
¦ Uses less water volumes and flow rates for the same area of coverage;
¦ Use more specialised material and equipment such as for pumps, pipes and nozzles due to the higher operating pressure, and the requirement to keep the fine spray nozzles clear of any particles that may occur in the pipe network and block the nozzle openings. The need to eliminate blockages may also require the addition of filtration systems.
DECISION FACTORS AND DESIGN CONSIDERATIONS:
When deciding whether or not to install any type of FFFS, the following must be examined:
¦ Compliance with local regulations and guidelines, including legal considerations;
¦ Global guidelines and safety standards;
¦ Life safety;
¦ Asset protection and the protection required to assure the availability of the transport link;
¦ Flexibility for additional traffic regimes such as dangerous goods vehicles;
¦ Fire-fighting response;
¦ The ability to adequately operate and maintain the system, including the roles, positions, and responsibilities of the stakeholders and training of operators;
¦ The installation capital cost and or life cycle cost, as well as the cost benefit from installing FFFS;
¦ System reliability and redundancy; and
¦ Sustainability, as this may also be a factor in the decision. Once it has been determined to install the FFFS, the designer must establish a viable working design, and consider the following design issues in the process of developing the design of the FFFS:
¦ Design fire;
¦ Type of system;
¦ Water suppression characteristics (mist or deluge);
¦ Water supply including possible hydrant and or standpipe systems;
¦ Tunnel drainage;
¦ Space considerations;
¦ Fire detection/activation strategy;
¦ System integration;
¦ Interaction of FFFS with ventilation; and
¦ Other factors.
RESEARCH AND ANALYSIS
Various types of Fixed Fire Fighting Systems (FFFS) have been used in buildings for more than 150 years. These systems are well understood in the building industry and are required by many codes and standards for the protection of life and property. In road tunnels, this has not always been the case.
In 1999 when PIARC “Fire and Smoke Control in Road Tunnels” guidance was published; FFFS were not recommended and were not accepted in many parts of the world due to fears of creating adverse conditions in a road tunnel environment. While these fears have generally been proven to be groundless, and the benefits of FFFS have been validated by fire testing and operating experience, the installation of FFFS is still not considered to be appropriate for all road tunnels.
In 1965 the tests carried out at Offenegg indicated that FFFS were not safe for use in road tunnels. These tests, together with other considerations believed to be valid at the time led to the following beliefs: i. Water can cause explosion in petrol and other chemical substances if not combined with appropriate additives;
ii. There is a risk that the fire is extinguished but flammable gases are still produced and may cause an explosion;
iii. Vaporised steam can hurt people;
iv. The efficiency of extinguishment is low for fires in vehicles;
v. The smoke layer is cooled down and de-stratified, so that it may cover the whole tunnel leading to loss of life;
vi. Maintenance can be costly;
vii. Visibility is reduced.
For the next 40 years, other than Australia and Japan and specific tunnels in the USA, FFFS were typically not installed in road tunnels. Research and testing carried out since the Mont Blanc, St. Gotthard and Tauern Tunnel Fires between 1999 and 2001 has further considered the use and application of FFFS. In the large number of tests carried out, no explosions have been caused by FFFS; flammable gases did not continue to be produced and hence no explosions occurred and vaporised steam was not generated in sufficient quantities to constitute a threat.
The tests did show that shielded fires were not entirely extinguished however thermal management is shown to be achieved; stratified smoke is de-stratified upon activation of the FFFS and visibility is reduced within the FFFS zone of application. Tests have shown that early activation of FFFS limits the fire heat release rate. Early fire control in practice, can be achieved by remotely operating the system before the arrival of the Fire Service.
LESSONS FROM REAL LIFE INCIDENTS
The objectives and benefits of FFFS have been illustrated by the Burnley Tunnel Fire of March 2008. During rush hour in Melbourne, Australia, a heavy goods vehicle swerved and impacted a vehicle in the adjacent lane. There was an immediate explosion and an ensuing fire. The activation of the deluge sprinkler system and ventilation system did not put the fire out, but it did minimise the spread of the fire and allowed time for the fire brigade to arrive at the scene of the incident. The injured people were a result of the accident and not the ensuring fire. There was no damage to the tunnel and traffic was able to use the tunnel shortly thereafter.
Fire events in tunnels continue to show the significant consequences of these types of events in a road tunnel environment to tunnel users, the tunnel infrastructure, as well as the impact to the wider road network on society. This has produced sustained pressure for further improvements to techniques and technologies to manage the risk and consequence of fires in tunnels. FFFS are a method that can deliver user safety and infrastructure protection; however, their use is not widespread for various economic, technical, political and social reasons. This report provides guidance on the decisions required before adopting FFFS and, if FFFS are to be adopted, provides guidance on the required design and implementation considerations.
Extensive testing has demonstrated that while FFFS have the ability to reduce the fire size and prevent the fire load reaching its full potential, high gas temperatures may still be reached that affect the structure or other infrastructure in the immediate vicinity of the fire. This has a direct link to choosing the correct design fire HRR for the design of FFFS to limit fire growth to, and the adoption of procedures to assure early activation of systems in the event of fire.
Where installed, maintained and operated effectively, FFFS have a positive impact on egress by extending the available evacuation time. This benefit applies to vehicles upstream in a longitudinally ventilated tunnel, and to both sides of a fire in a transversely ventilated tunnel. However, while the conditions downstream of a fire in a longitudinally ventilated tunnel are significantly improved, untenable conditions may still exist after activation of the FFFS.
The length of tunnel roadway covered by FFFS is affected by the available water supply and the tunnel width. Operation of FFFS can reduce the visibility for drivers within the area of operation, however, most vehicles within the activated zone(s) should be stopped as a consequence of the fire event.
Nevertheless, procedures should be adopted to manage traffic and operate the tunnel systems without exposing motorists to additional hazards. This also means that FFFS should be reliable and the potential for false activation eliminated.
FFFS are now recognised as a proven active technology for the management of risks to both tunnel assets and tunnel users from fires