SPECIAL FIRE FIGHTING SYSTEMS (SPRINKLER, GAS, WATER MIST, FOAM)
WATER MIST LOW PRESSURE
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The flame of a fire burns the combustible gases consuming a large amount of oxygen and producing heat.
The use of water mist for fire fighting is important for the following effects:
- cooling
- inertization
- post-separation effect
Cooling:
As a consequence of the water separation in a myriad of small droplets of microscopic dimensions, a reaction surface is created through which the heat produced by the fire is absorbed. One liter of water requires 335 KJ in order to shift from 20° to 100° C and additional 2257 KJ in order to change into vapour. Water is the extinguishing means with the higher capacity of heat absorption and it can be used in a very efficient way in the water mist technologies due to its extended reaction surface, if compared to the extinguishing techniques conventionally known.
Inertization:
When evaporating, water increases its volume up to 1640 times, which causes a rarefaction of the oxygen present in the air, at the source of fire. In this process, the extinguishing inertization means is not carried to the flame source from the outside, but it is produced only in direct proximity to the fire. Differently from all that occurs with extinguishing gases, the use of water mist systems does not require completely closed rooms.
As a sufficient energetic potential is released, the origin of the fire can be flooded by water mist all at a time, so that a fire can be choked within a few seconds. This is made possible by the combination of different extinguishing effects. The oxygen content is reduced from 16% down to 6 % in volume, near the origin of the flames, while the normal oxygen content (about 21%), is maintained in all of the remaining room.
Post-separation effect:
The droplets of water present between the flames and the combustible surface will strongly reduce the heat irradiation. The combustion rate decreases and the overheating of the surrounding possible flame sources is reduced. A decisive factor for this effect is also the generation of a sufficient quantity of microscopic droplets as the reflecting capacity increases whenever the droplet dimensions decrease.
“Differently from all that occurs with extinguishing gases, the use of water mist systems does not require completely closed rooms.“
The extinguishing effect described above becomes effective – at a different extent according to the scenario produced by the flames – and at the same time the flames can be extinguished by very low quantities of water.
Additional effects:
Smoke/gas washing
It is possible to obtain a non secondary post-separation effect consisting in the washing of the combustion gas particles as the majority of the components of the combustion gases has an absorbent nature and tends to adhere to the smallest droplets. The same can be said about the soot particles that bind themselves to the water droplets.
Shield effect
During the post-separation effect previously described, the tiny water droplets reduce the heat irradiation to the surrounding objects. This prevents the possible spreading of the flames and can be exploited, for example, in order to create fire fighting barriers.
Uniform cooling of the surfaces.
The water mist systems can be designed so that the incandescent surfaces (metallic) are struck by the water mist in a very homogeneous way. In this way it is possible to avoid the deformation of such surfaces due to an irregular cooling.
Thanks to the low conductivity of water mist, especially when non mineralized water is used, it is possible to fight the fires in proximity to the electrical installations.
Protection of the people
The capability of water mist to reduce the temperature more quickly than any other extinguishing agent and to wash away the residual gases is of vital importance in the protection of the people. Moreover, a study carried out by the US Agency for the Environmental Protection and by the German National Association for Fire Protection clearly showed that water, used as an extinguishing agent, does not constitute any hazardous factor for the people.
In normal circumstances, also the reduction of the oxygen present in the air does not represent any danger because it occurs only in proximity to the fire.
Another aspect of the water mist systems is the following: fire is usually extinguished at a harmless oxygen concentration of about 16-18% in volume, because the choking effect is always present in combination with other extinguishing effects.
This means that no hazard for the people can derive from the use of water mist.
Water mist systems can be activated with no notice, apart from particular situations.
The entrance of the Fire Brigades in the areas where a water mist system has just fired, becomes then considerably safer.
Extinguishing additive
The extinguishing effects previously described are furtherly improved by the mixing of water to 12% extinguishing additive ES liquid, in order to obtain a substance with very high extinguishing power which is at the same time completely void of any toxic or irritating effects on the people and biodegradable.
Reference norms:
The low pressure water mist system is designed according to NFPA 750. The same system is in compliance with the international standards IMO 800 (International Maritime Organization) and its components are approved by qualified bodies such as Lloyds, UL, etc.
The low pressure water mist system is identified in the international standard NFPA 750 (National Fire Protection Association) with the installation of WATER MIST extinguishing systems and it is recognized by the same standard NFPA 750 as fit to extinguish Class A fires (combustible solids – paper, wood, etc.) and Class B fires (combustible liquids – petrol, oil, etc.)
Water mist is accepted in Italy in accordance with the circular letter by the Home Office General Direction of Civil Protection and Fire Fighting Services, destined to all the Regional Inspectorates and all the Provincial Headquarters of the Fire Brigades, which invites to take into consideration the adoption of these systems as an alternative to halons. |
HIGH PRESSURE SYSTEMS
FOAM SYSTEMS
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