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The Montreal Protocol, which entered into force on 1 January 1989, banned substances that threaten the ozone layer, including halogenated extinguishers, and prompted the search for new environmentally friendly agents, otherwise known as clean agents. The alternatives that exploit the same principle as the systems previously in use, the so-called "in kind" systems - use halocarbons extinguishing agents (halogenated hydrocarbon gases) or inert gases. Unlike some of those "not in kind” ones, which are based on different approaches (water-mist, foam, aerosol, etc.), they allow a fast and clean action resulting in the immediate resumption of activities after the intervention. All clean agent systems act on the oxygen available for the combustion reaction by diluting and/or "removing" it from the flame due to the difference between the various specific gas weights and by chemical-physical means, increasing the heat capacity of the atmosphere contained in the protected volume, which hinders heat propagation. Unlike inert gas systems, halocarbons also act chemically, capturing oxygen through the free radicals that are released via the decomposition of the extinguishing agent.  Used for the protection of spaces where sufficient gas sealing can be achieved and, also for reasons of cost/benefit, gas systems are in particular used where it is not possible to use water (data centres, archives, electrical substations , libraries, warehouses and technical areas with personnel present).

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Specific applications of inergen systems

However, such systems are not suitable for controlling fires that tend to form embers. Clean agent systems are distinguished between those with localised saturation - infrequent - or total saturation (total flooding).

The effectiveness of the release system, the dynamics of the flows (considering that during a fire convective motions are triggered), the geometry of the premises and the quantity, type (porosity) and location of the combustible materials are all essential factors in the design.

Extinction strategies

These systems can act through a first initial discharge followed by an additional maintenance discharge; the varying architecture of the systems depends on innumerable factors, not least the propensity of the extinguishing gases (and in fact of each agent) to escape through the openings.

There is a balance between concentrations, too low - and therefore ineffective - or too high - dangerous for any occupants, and the significant economic implications of the sizing of the system.

Plant architectures

Plant architectures can differ greatly even depending on the risk and distribution of the premises to be protected; systems can be developed that act on a single volume or on different environments simultaneously, with centralised reserves capable of acting on different volumes and distinct risks (this also thanks to the high pressure - 200/300bar - which allows the supplying of sufficiently extended distribution networks).

The sizing of the system

The quantity of extinguishing agent needed to reach the desired saturation depends on the volume of the environment to be protected but the extinguishing action also depends on the discharge time, in the order of seconds. The gas output of the cylinders is regulated by the laws of dynamic gas (the limits are those of the speed of sound for the specific gas, the geometry of the orifices, the storage pressures, etc.) and this must be calculated so that to the extinguishing concentration - theoretically sufficient for extinguishing and determined by the test protocols for the different extinguishing agents - are added corrective safety factors to define the design concentration and the design factors, specific to the application case in question.

These factors also depend on the architecture of the system (systems with manual implementation, for example, require greater safety margins, due to the intrinsic slowness of intervention). A design objective is to minimise the decomposition products of the extinguishing media; the development of decomposition products is related to temperatures, stoichiometric ratios of reagents and reaction times.

Among these especially hydrofluoric acid, which is generated when the agents based on halocarbons come into contact with the flames - is harmful to the occupants and to the equipment, especially if it is electronic. The most common design factors include environmental pressure, ventilation characteristics, the particular geometries of the protected volume and the obstacles that can influence distribution of the gas, as well as the pressure losses of the distribution system. From the above it is clear that it is not always easy to determine the true volume to be actually saturated (project volume).

The experimentation and the data released by the manufacturers of the systems and the calculation software that they distribute - and the independent laboratories that validate them - play an essential role in the correct construction of a gas system.

The development of an inergen system

Significant aspects are the pressure variations caused by the introduction of extinguishing agents. In the case of halocarbons the volume undergoes an initial reduction due to the rapid reduction in temperature caused by vaporisation of the extinguishing agent, followed by an increase when the gas reaches its maximum expansion; in the case of inert gases the volume only presents an increase (the heat exchange and the consequent effect on the volume is negligible in these cases) but with very high values of overpressure, such as to pose stability problems.

During the design phase (and periodically during the operating life of the system) it will therefore be essential to verify the integrity of the protected volume through the door fan integrity test and to assess the overpressures generated by the extinguishing system to calibrate the compensation systems such as dampers equipped with movable fins with calibrated counterweight, which open by relieving excess pressure, preserving the integrity of the weak elements of the structure (light and glazed structures, typically), and then closing such as to ensure saturation for the envisaged time.

Equipment supplied with an inergen system

In support of gas systems, especially if they are halocarbons, it is advisable to use self-contained breathing apparatus for intervention in protected environments. 

The reference standard for inergen systems

The clean agents recognised from a regulatory perspective are those reported in the EN 15004 standard. The international reference standards for the design of systems using extinguishing gases are ISO 14520, NFPA 2001, EN 15004, EN 12094 UL 2166 and 2127 and the FM Approvals 5600 in addition to the environmental regulations (in particular the EC Regulation 2037/2000 and Italian Presidential Decree 147/2006 on the so-called F-Gas) and UNI 10877 which also deal with toxicological problems for the use of clean agent systems in areas where persons are present.

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