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A fire needs fuel, heat and comburent. The water mist systems act on the first two elements. They are able to generate a very fine mist of water, with drops of tens or hundreds of mm in diameter. By absorbing heat in proportion to the contact surface, the drops, expanding by heating, remove energy from the fire, hinder the contribution of comburent and absorb radiant heat. The water mists have been used in fire-fighting since the 1880s but it was only a hundred years later that water mist technology became established and was used in fixed extinguishing systems. Perfectly eco-compatible, it is a technology that economises on the use of extinguishing agents (using flow rates from one tenth to one hundredth less than sprinklers) and, in proportion to the nebulisation rate, minimises wetting (and related damage). This means that water mist systems are suitable for situations in which the available water reserves are limited (for example on the top floors of very tall buildings) or where, for reasons of weight and compatibility, the distribution system must have a minimal impact (pipes of just 15 -20 millimetres make the system particularly suitable for ships and areas of artistic interest). Terrestrially, the main uses are the protection of engine rooms, ordinary risk residential areas and data centres.

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Watermist. A story that comes from afar

Although, as mentioned, water mist is one of the most recent extinguishing technologies, as early as 1880, the American FE Myers produced a portable system with a lance that sprayed atomised water to fight small forest fires. But the same Grinnell at the end of the nineteenth century developed his “pepper pot” nozzle which obtained similar jets.

By passing water from a nozzle to multiple orifices, in 1930 the German Lechler produced the so-called "water powder". Only another ten years passed before the Factory Mutuals engineering division began the first tests on these systems.

Until then, however, it was a solution adopted to fight fires manually, through appropriate lances. The fire on the "Scandinavian Star" passenger ferry on 7 April 1990 (which cost the lives of 158 people) prompted the International Maritime Organisation (IMO) to issue new guidelines for more effective naval fire systems; this regulatory drive, combined with the banning of gases that damaged the ozone layer, which at the time had been in force for a decade, stimulated a notable development of fire-fighting technologies based on high-pressure water mists.

This happened in particular in Sweden with UltraFog and in Finland with Marioff.

The evolution of water mist technology

The evolution of water mist technology is bringing developments in the saturation of environments and the ability of mist penetration in closed cabinets (such as electrical panels) bringing the performance of these systems closer to gas systems (with respect to which they have the undoubted advantage of not being dangerous to humans). Water Mist systems must only be designed, installed and maintained by qualified companies and their use is limited to applications that have been specifically tested or for which the extrapolation of results from tests on other applications is deemed acceptable by the supervisory authority.

water mist: regulations and specifications for a proprietary technology

Currently, water mist is considered a "proprietary technology" and, in addition to the manufacturers' specifications, the reference standards are NFPA 750 and TS 14972. The definition of the project specifications is delegated to the approval bodies which, after having accepted the test protocol, carry out tests on the various systems for the different fire classes and application scenarios. The naval sector, instead, is more regulated by the IMO.

Types of water mist systems

The main types of water mist systems are with open nozzles, "flood-type", used for the intervention in enclosed spaces and can act by room saturation or localised application (with limitations related to the volume to be protected), and closed head systems, like a sprinkler, in which the system intervention is regulated by the individual heads, suitable for large areas, open with slight or ordinary fire danger characteristics.

A further distinction of water mist systems can be made according to the pressure ranging from 5 to 12 bar in low pressure systems, 30-35 bar in medium pressure systems and up to 100-200 bar in high pressure systems.

Nebulisation can typically be obtained by the combined action of the nozzle geometry pressure or by the supply of a fluid - air or nitrogen - and acts as a nebuliser (the gas is stored in cylinders, connected to the water supply).

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