IMPULSE STROM vs FIRE IN TUNNELS
Modern techniques for designing and constructing tunnels permit a detailed analysis of the different fire risks and the possible means for detecting and extinguishing these fires.
This allows creating an ideal symbiosis in tunnel construction: involving existing power structure and fire detection and extinguishing at initial stages; raising considerably the tunnel’s safety level. However, unfortunately, there are several tunnels where with enormous quantities of electric cables, with different purposes and occasionally belonging to different owners. Voltage and current strength in these cables are not always known and they cannot be cut off from a single command panel. As a result, is not possible to extinguish fires in such tunnels with conventional water or foam systems, nor the use of volumetric extinguishing methods can be applied, due to the big volumes involved and the existence of ventilation.
Subways are such tunnels.
In modern times, when negative factors of anthropogenic, environmental and terrorist nature aggravate, it becomes particular important to enhance subway fire protection, once big fires may cause mass mortality and transport collapse of a city.
Fires in underground transportation structures and tunnels are characterized by quick spreading of dangerous substances, fast temperature raising, blocking of escape routes, contamination of adjacent surface areas and buildings with toxic products result of the fire, damage of tunnels’ structures, and interruption of important traffic artery.
Fire development in subway train cars, on passenger platforms, escalators, tunnels and subway objects, which are critically important for safety, has essential peculiarities, such as:
• semi-confined areas with significant concentration of people, associated with the complexities of passenger evacuation;
• high possibility of burning of materials that discharge toxic components when burning;
• presence of a power cables with voltage of up to 10 kW;
• presence of a considerable number of electric installations under voltage, including the train cars;
• high air exchange speed causing fast increase of temperature to the values of over one thousand degrees.
After three or five minutes the fire starts, the situation becomes critical, due to the usual concentration of people in the tunnel. In such situations, people must be immediately evacuated from the tunnel, and the following situations may occur:
• possibility of dense smoke on escape ways;
• great difficulties, and in several occasions, it may be virtually impossibility to supply required volumes and flows of fire extinguishing substances to fire centers;
• likelihood of panic among passengers.
The specific characteristics of the fires on underground objects and in tunnels, demand strict and specific requirements to fire extinguishing systems, in particular:
• fire extinguishing substances used in rail wagons, on passenger platforms and escalators must be safe for people;
• the system should secure quick performance, including cases in which maneuvering is restricted in the tunnel;
• the fire extinguishing system should secure safety for the people present in rail wagons, on platforms and escalators,
• the fire liquidation task should not lead to flooding of subway tunnels, cable trays, and engineer premises.
The impulse powder fire extinguishing technology “Impulse Storm” allows solving most of the problems involved on fire extinguishing in specific conditions on subways. This article briefly presents the advantages of the Impulse Storm technology in two particular examples.
CASE 1: UNEXPECTED IGNITION OF ELECTRIC CABLES OR CONTACT RAILS
Old subway tunnels, built several of dozens of years, have several energized cables with a variety of purposes
such as communication and electric power supply cables. Furthermore, there are occasions in which cables interconnect objects belonging to third parties, with unknown electric current and voltage intensity, and without fast possibilities for disconnecting in case of urgent requirements. Subway train conductors are usually the first persons who detect a fire that emerged in the result of a short circuit in a cable or in a contact rail. One may think that the initial, simplest and easiest action should be having the conductor stop the train, take a fire extinguisher, and extinguish the burning cable before the neighboring cables insulation catches on fire and the fire evolves to an uncontrolled stage. However, in reality, this is impossible to do, due to several reasons:
• one cannot approach the area of a short circuit in less than two meters, in order to avoid electric shock.
• it is prohibited to use water or water-and-foam fire extinguishers.
• Due to the distance from the ignition center, the use a carbon dioxide or other gas fire extinguishers is inefficient. Some energized cables can be disposed at heights reaching up to 5 meters from the floor level.
These three points are also applicable for cases when a fire happens under train cars and in a contact rail.
The existing fire extinguishers occupy reasonable areas in the conductor’s cabin and often cannot be applied because of their heavy weight.
The problems involved with the requirements of instantaneous extinguishing of cable ignitions, rail wagons space and contact rail are solved by using the “EDDY extinguishers”, a hand-held Impulse Storm technology fire extinguisher.
With its small weight (up to 1 kilogram) and dimensions (not exceeding a police baton), these extinguishers can be fixed to walls or even on the structure of a subway cab These fire extinguishers have a range of effective action reaching up to 12 meters in vertical or horizontal planes, allowing the extinguishing in any place within a tunnel, irrespective of existence of electricity of any voltage or current strength. Numerous tests carried out in subways of Kiev (Ukraine), Moscow and Saint Petersburg (Russia) have shown that currently the Eddy is the only fire extinguisher that can be effectively and safely used for extinguishing ignitions on electric cables or other electric objects under voltage.
Hand-held fire extinguishers – Eddy can be used in a one-barrel configuration and in a multi-barrel variant, either by conductors, security personnel or firemen
The Eddy fire extinguishers have a long shelf life, requiring little servicing, and can be used even after 10 years of storage. These extinguishers can be simply disposed on tunnel walls in certain intervals and can be left unattended for many years, until a fire happens, and any person who finds himself near can use the extinguisher.
One of the most valuable properties of the Impulse Storm technology upon extinguishing fires in tunnels is that there is no need to turn off the electric power. The Impulse Storm technology has been tested in currents over 50,000 Volts, and can be used in much higher voltages.
As an example, in 1995 during a fire in a subway in the city of Baku, capital of Azerbaijan, 289 people died as a result of power cut off, solution adopted to secure a possibility for fire extinguishing. Unfortunately, the fire was not extinguished, and a train, with its passengers, was caught in a fire trap. Even though the evacuation plan was activated, on the escape route to the nearest station, people died as a result of high temperatures and poisonous products from the burning process. If the Impulse Storm technology was used, there would be no need to cut off the power, allowing to drive the train to the nearest station, and securely evacuate the passengers.
CASE 2: NARROW CABLE TUNNELS
The problem of extinguishing fires on electric cables in narrow tunnels is commonly known. Usually, a man cannot get through these tunnels; water and water-and-foam fire extinguishers cannot be used until electricity is cut off; and the use of volumetric gas or aerosol extinguishing means will be ineffective due to ventilation systems. The Impulse Storm technology in fire extinguishers specially designed for use in narrow tunnels can instantly put out a fire, reaching distances up to 50 meters from the extinguisher. The extinguishing on these tunnels is performed by instantly spraying a mixture of a highly-concentrated cloud of neutral gas and a special one-component powder. The speed of the sprayer cloud on the tunnel’s channel can reach 100 meters per second. The high impact action of the cloud on the flame will immediately knock down the flame, and the powder will immediately extinguish, bind and cover the melting plastic cable casings and prevent re-ignition. Considering that the casings of modern cables either do not burn at all, or burn without smoldering, i.e. only vapors of the melting casing burn, the Impulse Storm technology is the only technology with true possibilities to extinguish fires in inaccessible narrow cable tunnels without having to cut off the electricity.
The fire prevention powder Impulse Storm, specially designed for use in these cases, is an unique powder with bulk weight of 0,2 g/cm³. The powder is not a chemical synthesis product – it consists of very stable natural components which do not chemically react with any other substance. As a result, the powder sprayed by the Impulse Storm technology into the tunnel will play an important role as a fire agent, even after the extinguishing is made. The powder can be stored in a shaft for unlimited number of years without losing its fire prevention properties and causing no harm to cables or environment. This powder does not cohere or turn into a solid mass, and it can be removed anytime, even in 10 or 20 years. It is recommended that the powder is left in the tunnel, serving as neutral filler, and in case of an eventual ignition, it will play an important role in preventing fire development.
In summary, it is possible to fill narrow cable tunnels with the Impulse Storm powder immediately after its construction. The easiness, big volume, low mass, and low cost of the powder allow such application.