Using any type of Fixed Fire Fighting Systems (FFFS) in road tunnels has, in many parts of the world, been a controversial issue and still is today. An overview is presented regarding the current research, standards, debates, and attitudes regarding FFFS in road tunnels. This overview shows that there is a general negative viewpoint at using FFFS from the decision makers, which to a large part contradicts from the findings of research, results and experience from actual tunnel fires.


INTRODUCTION

The purpose of a FFFS in a road tunnel is determined by the fire protection objectives, which can be simplified to either fire suppression or fire control. The differences between the two objectives are the amount of water needed to be used, the possible use of a foam agent, the positioning of the sprinklers and the principle of the activation system.1

 

The fixed fire fighting systems used in road tunnels are generally water based. The different kinds of water-based FFFS can be divided into three groups: traditional sprinklers/water spray systems, water mist systems and systems with an added foam agent. In principle, it can be said that smaller droplets will attack the flames and the smoke plume while larger droplets will attack the fire base and solid objects. However, adding a foam agent to the FFFS will create a film of foam which will separate the burning fuel from the oxygen and thereby suffocate the fire.

 

USEFUL STANDARDS

Two standards that can be used when engineering FFFS inroad tunnels are NFPA 5022 and Engineering Guidance for Water-Based Fire Fighting Systems for the Protection of Tunnels and Subsurface Facilities.3 As the title implies, the NFPA 502 is not exclusively intended as a standard for road tunnels and FFFS.


During the European Research Project UPTUN, the Engineering Guidance for Water-Based Fire Fighting Systems for the Protection of Tunnels and Subsurface Facilities was written. It provides information on design, installation and maintenance of water-based Fixed Fire Fighting Systems to be used in tunnels.

 

ROAD TUNNELS WITH FFFS

Japan has suffered from accidents involving tunnel fires, and this has resulted in a unique experience using FFFS for a period that has extended over more than four decades.4 They have approximately 80 road tunnels equipped with FFFS.5

 

 

 Some road tunnels in the rest of the world equipped with FFFS are shown in Tables 1 to Table 3.

 

LARGE-SCALE FIRE TESTS

The main objective of the UPTUN project5 was to find new methods for fire safety in existing tunnels, and during this project, tests with water mist systems were performed. The large-scale fire tests were focused on fire control rather than fire suppression, and they were conducted with low pressure and high pressure water mist systems. Two different fire scenarios were used in the tests: pool fires and solid fuel fires in the form of a stack of wood pallets. The fires had a potential severity on the order of 10 MW to 20 MW under free burning conditions. Some results from the UPTUN project were as follows:

 

  • Both types of systems tested were able to reduce the heat release rates of the fires in the range of 40% to 70%. It was not possible to determine whether or not one type of system performed better than the other.
  • After the activation of the systems, gas temperatures were reduced rapidly downstream the fire.
  • Back layering was reduced after the activation of the system, which resulted in better visibility upstream the fire.
  • The efficiency of the system was dependent on the fire size, nozzle type, water discharge density and the location of the fire.
  • The visibility downstream the fire did not initially improve; however, when the fire size was reduced by the water mist system, the visibility also increased.

In 2005, a series of tests was conducted in the Runahamar Tunnel in Norway in order to evaluate the effectiveness of FFFS using compressed air foam (CAF).6 The first fire test consisted of solid fuel in the form of wood pallets with a volume of 100 m3 and with a heat release rate (HRR) up to 300 MW. The second test fire was a diesel pool fire with an area of 100 m2 and a HRR of 200 MW. The FFFS successfully extinguished the large pool fire and controlled the solid fuel fire, but did not extinguish it. Upstream of the fire, the air temperature was cooled down to 50˚C and downstream the temperature was cooled to below 100˚C. Thus, preventing fire spread and generating an acceptable working environment for firefighters. The visibility in the tunnel was completely lost before the discharge of the FFFS.

 

 

RESERVATIONS AGAINST THE USAGE OF FFFS IN ROAD TUNNELS

There are numerous reservations against the usage of FFFS in road tunnels. These reservations have been made in the past and are still used today.

 

One concern has been that using FFFS in road tunnels could worsen the conditions for the evacuating tunnel users. This is because the FFFS will generate steam that may injure them or that the cooling effect of the FFFS could cause destratification of the smoke.

 

Other concerns have focused on the effectiveness of FFFS in road tunnels. Fires that start in the engine or in the compartment may be difficult to protect using a FFFS. Another concern has been that petroleum fires may continue to produce combustible gases after they have been extinguished by a FFFS, and an explosive environment will be created.

 

Current research shows that FFFS lowers temperatures in the tunnel considerably, even close to the fire. A free burning fire can reach 1000˚C at relatively long distances from the fire source. Steam can, in some cases,be found in the close proximity of the fire but the cooling effect of a FFFS outweighs any danger that might be caused by this.3


Concerning the potential destratification of the smoke, research shows that the smoke will reach the tunnel floor when using these kinds of systems. However, tests conducted within the UPTUN project have shown that the smoke remains stratified only for relative short distances even when FFFS are not used due to thermal effects and ventilation.

 

The FFFS also reduce the production of smoke, and the water droplet itself can bind particles -thereby reducing the toxic effects and increasing visibility conditions.3

 

Concerning the effectiveness of FFFS, the general view today on FFFS is that they are used to suppressor control the fire, stopping it from spreading, and to reduce temperatures in the tunnel. The main reason for installing a FFFS is, therefore, not to extinguish a fire. The intention is that the fire is later extinguished by the rescue services.


If a fire in a vehicle is not suppressed or controlled, it will in a very short time over tax the vehicle and start spreading itself to adjacent vehicles. Furthermore, it is more vital to protect the evacuees from the high temperatures and the high HRRs generated than a potential explosive environment after a petroleum fire has been extinguished by the rescue services. In most cases, people who can escape have done so.

 

Andreas Hggkvist is with Lule University of Technology.

 

References:

  1. Arvidson, M., Alternative fire sprinkler systems for roadway tunnels. Proceedings of the International Symposium on Catastrophic Tunnel Fires, Bors, Sweden, 2003.
  2. NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, National Fire Protection Association, Quincy, MA, 2008.
  3. The Research Project UPTUN. Engineering Guidance for Water Based Fire Fighting Systems for the Protection of Tunnels and Subsurface Facilities, R251. The European Commission, Brussels, 2007.
  4. Road Tunnels: An Assessment Of Fixed Fire Fighting Systems. World Road Association, La Defence, France, 2008.
  5. The Research Project UPTUN. Fire development and mitigation measures D241 - Development of new innovative technologies. The European Commission, Brussels, 2008
  6. Liu, Z. et al. Challenges for Use of Fixed Fire Suppression Systems in Road Tunnel Fire Protection. NRCC-49232, National Research Council of Canada, Ottawa, 2007.