Fires inside long tunnels are rare but can be catastrophic. Intervention by the fire brigade is difficult due to access problems. Heat and smoke make fire-fighting operations dangerous and slow. Tunnel fires pose not only a life safety threat for car or train passengers, but also cause severe damage to the tunnel lining and mechanical and electrical systems. There is special concern about fires involving heavy goods vehicles, as evidence shows that the most disastrous real fires in tunnels, with the highest death rates, often start at or involve a truck.
The fire protection objective is therefore to suppress any fire that may occur, stop the fire spreading to adjacent vehicles or carriages, and minimise damage to the tunnel and infrastructure. In addition the system should cool down the gases so as to make more tenable conditions for evacuation and access by fire brigades.
State of the art
It is not surprising that up to now only a few active fire protection systems have been installed in tunnels for practical reasons. Tunnel conditions are always windy, due to forced or natural ventilation, and wind speeds of up to five metres / second are typical. Tests and modelling have shown that hot smoke is immediately blown away from the actual fire location so until recently activation by heat detection was not considered reliable. Even when the correct location is found, the suppression agent may be blown away by the ventilation. Computer studies with conventional low pressure sprinklers have shown that so many sprinklers are activated by a large fire that the size of piping and pump unit would be completely impractical.
However recent full-scale tests have shown that high-pressure water mist, due to its strong momentum, can effectively penetrate the windy and turbulent conditions encountered in tunnels, and reduce temperatures sufficiently to prevent fire spread and structural damage.
HI-FOG solution - features and benefits
Marioff has developed a range of HI-FOG water mist fire protection solutions for a wide variety of applications. The company has now developed two practical HI-FOG systems for protection against fires in long tunnels, one using open spray heads and a separate detection system, the other using automatic, pre-activated sprinklers. Spray heads or mist sprinklers are arranged in zones and are designed to operate only at the fire location. This means water use is relatively small, pump units and piping are a practical size, and system costs are reasonable.
The HI-FOG open spray head solution is designed as a zoned deluge system relying on a separate state-of-the-art detection system (e.g.. linear heat detector and CCTV cameras) to set off the whole zone of spray heads. In some applications Marioff recommends that the system incorporate a water mist curtain in each zone to cool down gases, restrict the spread of heat and improve the conditions downwind of the fire. This solution is simple but does require a separate, reliable detection system.
The HI-FOG automatic sprinkler solution is also divided into zones and incorporates sprinklers fitted with special protection caps which prevent false activation. Each zone incorporates a mist curtain which is set off by a sensitive bulb-activated pilot valve and/or a separate detection system. This simultaneously blows the protection caps off the sprinklers so they are ready to operate. If the zone is a false one, the mist curtain cools down the gases and the sprinklers do not open. The system ensures that only the zones closest to the actual fire will be pre-activated. The end result is that a few curtains will spray water, but sprinklers open only in the correct zone. The benefit of this system is that less water is used and the system has the back-up security of self-actuation.
HI-FOG details
A pre-engineered HI-FOG system for protecting tunnels will consist of spray heads and/or automatic sprinklers arranged in zones down the tunnel. These are connected by small bore stainless steel piping to the necessary valve & control units. In the case of the open spray head system these interface with the detection system. An electrically-driven HI-FOG pump unit (SPU), dimensioned for three or four mist curtains and one full zone of sprinklers or spray heads, is located at each end of the tunnel. In case of difficulties of reliable electrical supply these can be diesel-driven units.
For the HI-FOG automatic sprinkler system the choice of sprinkler bulb rating depends on the ambient temperatures, wind speeds, tunnel height and the type and number of vehicles driving in the tunnel.
