In a society in which mobility is increasing and traffic volume is growing rapidly, modern tunnels with reliable fire protection are essential. Increasingly, long and complex tunnel systems are being planned and built and existing tunnels are going through modernization processes in order to meet the growing requirements. As a result of the access limitations of a tunnel, tunnel fires present significant risk to people, vehicles and the tunnel structure itself.
For decades, fire protection in tunnels consisted primarily of providing safe escape routes and passive measures to prevent tunnel collapse and protect infrastructure in the event of a tunnel fire. In recent decades, however, human safety concerns have given rise to a trend towards active fire prevention in tunnel environments.
The results of several full-scale tunnel experiments and European Union studies have demonstrated that water-based firefighting systems can sufficiently control the spread of fire and increase the chances of evacuating people in the tunnel.
When considering a tunnel fire protection system, the designer must weigh the pros and cons of potential risk vs. budget constraints. Many tunnels are long enough to necessitate active fire protection. NFPA Standard 502 requires the installation of fixed, water-based fire-fighting systems in tunnels 200 meters or longer. For these tunnels, as well as for shorter ones in which active fire protection is to be installed, the main question is what type of fire water system to use.
Water mist vs. deluge fire protection: which works best in tunnel environments?
An active fire protection system utilizes water or foam to suppress a fire or stop its progress in order to save human life and protect property. Fixed firefighting systems are an active way to fight fires in tunnels. Such systems are often called water-based firefighting systems or simply fire suppression systems. Typically, an active system pumps water through a series of pipes from outside the tunnel to an end point within the tunnel.
There are two types of active fire protection suitable for use in tunnel environments:
- Standard deluge system: This type uses standard fire protection deluge valves and sprinklers that open automatically when triggered to emit a shower of water over the affected area.
- Water mist system: This type of system operates under high pressure (40-100 bar) to distribute a fine mist of water in the event of a fire.
Weighing the options
When considering what type of fire protection to utilize, main parameters such as water flow, pressure rating, the effect on other safety utilities like ventilation and the most important parameter, its effectiveness in suppressing the fire must be considered. The water mist system utilizes less water than the deluge valve. However, the need to maintain high pressure (100 bar) has its drawbacks. A water mist system requires a high pressure pump and special end equipment to create the water mist. In addition, all piping components must be able to handle the high water pressure required. These specialty components are typically much higher in cost than standard deluge system components. As a result, water mist systems can be prohibitively expensive. Ventilation systems used in medium and long tunnels can damage the cooling effect of water mist systems, rendering them less effective.
The standard deluge system uses more water. However, because deluge systems work by cooling and wetting the fuel surfaces, it makes them much more effective at reducing fire size and achieving extinguishment of unshielded fires. Once the surfaces have been wetted, it prevents reignition. In addition, because deluge systems aren’t affected by the presence of ventilation systems, they can be used together with high-power ventilation systems which raises the likelihood of people being able to survive a tunnel fire.
Designers must weigh the pros and cons of installing a water mist system vs. a deluge system for tunnel fire protection on a case-by-case basis, in terms of financial, safety, and sustainability considerations. Some of the things a designer should take into account include:
- Water availability
- Tunnel accessibility and egress
- Availability of firefighting personnel
- Cost of equipment
- Long term maintenance and budget
Fortunately, both types of systems work well in terms of their ultimate intent: saving human lives and property in the event of a tunnel fire. For applications where water accessibility is of less concern, and for situations where budget is a primary consideration, deluge systems remain a sound economic and practical choice.
BERMAD is a leading manufacturer of deluge fire protection equipment. For more information on tunnel fire prevention or to connect with a BERMAD fire protection representative in your region, contact us.