Traditionally, most tunnels are protected against fire by passive safety measures. The term “passive” refers to the use of construction materials that inhibit the spread of fire and safety structures such as special escape tunnels, emergency exits to a neighbouring tunnel or to the surface, and spaces where people can find shelter. Passive safety measures contribute to underground safety in important ways, but can prove insufficient during major fires such as those that occurred in the Mont- Blanc and Gothard tunnels.
In the search for solutions that better mitigate the threat of underground fire, experience drawn from fire testing and field installations shows that high-pressure water mist technology is an excellent solution for preventing direct and consequential damage to people, tunnel infrastructure and traffic. Therefore an increasing number of high-pressure water mist systems are being installed around the world and working to protect underground transportation infrastructure such as road tunnels. In this environment water mist systems beat down fire instantly upon activation, suppressing and controlling it before it can spread, using minimal amounts of water.
As the world’s leading supplier of water mist systems for fire protection, Marioff has delivered a number of HI-FOG® Water Mist Fire Protection Systems to operators of underground facilities. As a result of fullscale fire testing of the system in the tunnel environment, these forward-looking organizations were convinced by HI-FOG’s ability to provide excellent fire protection with minimal water discharge.
A86 Paris
The A86 West tunnel in Paris, protected by a high-pressure water mist system, is probably the world’s largest tunnel protected by an active fire protection system. This tunnel is a feat of engineering and design at 10 km long with two superimposed levels, one in each direction. Each level has two traffic lanes and a hard shoulder. Reserved for light vehicles, the tunnel is equipped with all the latest safety devices, exceeding the requirements of new French regulations governing tunnel safety.
Front-line fire protection is provided by a high-pressure water mist system with the following mission in case of fire: reduce the ambient temperature, facilitating the evacuation of motorists to the emergency exits located at 200-metre intervals; meanwhile, moderate conditions for the emergency services arriving on the scene, allowing them to contain, control and extinguish the fire.
The importance of fire protection is magnified in this type of tunnel, designed to carry very dense traffic. The ceilings are very low: it may be difficult to evacuate even small quantities of smoke. The heat given off by a fire will also quickly affect the low-lying ceiling equipment.
The A86 West water mist system carries out its mission with 24 spray heads per coverage zone, each 33 meters long. The design calls for three zones to trigger simultaneously in order to cover an area about 100 m long around the seat of a fire. HI-FOG® micro-droplets rapidly absorb heat, particularly by evaporation, giving very effective cooling. They greatly reduce the amount of smoke a fire gives off, by limiting the development of its heat intensity. This contributes to safe evacuation and saves lives during the first critical seconds and minutes of a tunnel fire.
M30 Madrid
The Madrid M30 project is the world’s largest urban tunnel project, involving 99km of new road construction with 56km comprising tunnels. The project covers the redesign, rerouting and refurbishing of the inner ring road of the city of Madrid. Over a number of phases the M30 project has been rerouting major sections of the road through new tunnels, freeing up surface areas for redevelopment into green areas, footpaths, bicycle paths and new housing while significantly reducing inter-city travel times.
In the project planning phase, seven tunnel segments were identified as requiring an active fire protection system. A number of different water mist systems were subject to full-scale fire testing. The results convinced the road authority that it was possible to design and build a water mist system to handle the M30’s extra-wide tunnel sections and potentially large fire loads (HGVs) while using modest amounts of water compared to traditional deluge systems. The seven tunnel segments designated as requiring active fire protection are now protected by water mist systems.
Helsinki service tunnel
The Helsinki service tunnel was built in 2007-2009 and is protected by Finland’s first automatic tunnel fire protection system. The total length of this tunnel is 2 km and it connects several commercial centres of the city and e.g. the University of Helsinki including their parking facilities and service tunnels. Service tunnel will allow city to expand its pedestrian zones by guiding especially the service traffic into the tunnel.
The average height of the tunnel is 5.5 metres and the width varies from 7 m to 20 m. The tunnel also contains 4 roundabouts of different dimensions. It was a considerable technical challenge to implement the water mist system zone sizing taking the roundabouts into account while maintaining the required performance together with the ventilation and fire detection systems.
The water mist system is designed to discharge over two 25m long zones simultaneously. The system flow rate is secured by three large pump units driven by diesel engines. The flow rate of each pump is 1200l/min; two of them are required to operate simultaneously with the third designated as the back-up unit. The back-up pump unit can be deployed to add flow rate, covering larger areas if needed. Water for the water mist system pumps is supplied from two separate sources: a main city sprinkler feed and a water reservoir located close to the highpressure pumps. The water is fed from the water reservoir by a diesel pump unit for one hour of operating time.
Water mist’s excellent cooling capability is considered to be an essential advantage. It is notable in tunnels that, if there is no fire suppression system to provide cooling, fire fighters cannot approach the fire close enough to fight the fire effectively using their fire hoses. Furthermore, experience gathered from full-scale tunnel fire tests showed that the velocity of the longitudinal ventilation during a water mist discharge can be less than when water mist is not being discharged. This helps the fire brigade approach the fire scene safely from the upstream side they will not be impeded by a loss of visibility.
A general fire protection principle governing this type of project requires all water used to fight fire to be collected in a separate collection reservoir. This is also a clear cost-cutting benefit of Hi-Fog compared to the more water consuming conventional deluge systems.
The climate in Finland can be extreme cold during the winter and therefore a thermal protection is needed for the water mist system piping: a trace heating system was installed for the main tubes affected by low temperatures. Roads in Finland are treated with salt to prevent icing in temperatures just below zero. All the pipes and components are made of high-grade stainless steel grades in order to give the best protection against corrosion in aggressive tunnel environments. This will naturally prolong the life-time of the water mist system. Marioff’s high pressure water mist technology combined with professional service presented the most economical solution for the Helsinki service tunnel.
System testing during the commissioning was carried out in order to ensure seamless interaction with other safety systems. Training of operating personnel and the local fire brigade were included to Marioff’s turn-key delivery in the Helsinki Service tunnel project.
Worldwide effort
Improving safety in underground infrastructure is a worldwide effort bringing together scientific research, governmental authority and private sector business. As the commercial proponent of a significant technology for underground safety, highpressure water mist as exemplified by Hi- Fog, Marioff has carried out an intensive program of R&D and full-scale fire testing of the system in the tunnel environment. As a result, type approvals for the system performance in this environment have been granted by a number of leading authorities regulating tunnel safety.
Hi-Fog uses plain water and is therefore environmentally safe. The system uses substantially less water than conventional deluge systems, a great advantage in underground environments where water supply can be problematic The smaller the water droplet, the greater the surface area of the water in the volume of space and the greater the system’s fire fighting effectiveness. Hi_Fog water mist attacks two of the three things that fire needs to grow Protected by the world’s largest high-pressure water mist system the A86 West tunnel in Paris is a model of tunnel safety Hi-Fog spray head for tunnel fire protection, with assembly body and protective cap Hi-Fog spray head system installed in roundabouts in Helsinki service tunnel A full-scale tunnel fire test carried out for the M30 Madrid tunnel project
