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For those persons involved in fire prevention, the fire triangle is the starting point for any approach to protection or prevention. However, barely two decades ago, a fire was tackled from the perspective of limiting the probability of ignition, separating the fuel from the comburent through dust, foams, gas or water or saturating the rooms with inert gases.

There is actually another approach to prevention: reducing oxygen below a threshold that does not allow combustion to sustain itself. The theory is simple, and was known since Lavoisier, at the end of the 18th century, discovered the role of oxygen in combustion processes. However, the development required not only theoretical concepts but technologies, materials and management techniques that have only recently become available.

In particular it was in Germany, in the 2000s, that the ignition thresholds for the different materials finally began to be identified. A substantial difference between caution and prevention is that the second must act continuously. Maintaining a constant hypoxic atmosphere in an environment, i.e. one in which oxygen does not exceed 15% of the mixture, requires many aspects to be addressed simultaneously.

Firstly, it is necessary to have devices that act on the gases present, segregating some and increasing the concentration of others; in turn these systems must be guided by analysis and continuous control of the air parameters. But this is not enough; building waterproofing techniques are required for the premises together with air exchange systems that allow "perimeter control".

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How an oxygen-reducing system works

To preserve biodegradable materials, whether they are fruit and vegetable products, the cellulose of ancient texts or the canvas of works of art, a by now consolidated technique is that of preserving them within environments where the oxygen content is reduced thereby slowing down metabolism and oxidative reactions. In theory the principle is simple.

Given a room where gas exchange with the outside is controlled in quantity and quality - through sealing and analysis - the air is passed through a "filter" that separates oxygen from nitrogen. The different separation technologies generally exploit the difference in speed with which the different gases pass through the permeable walls of a duct or the capacity of filters - such as those with activated carbon - which in particular pressure conditions are able to absorb or release the oxygen; the gases that pass through the membrane more quickly separate, reducing their contribution to the mixture that flows in the main duct. Thus extracting oxygen, and expelling it from the closed environment, the nitrogen rises within the controlled space to values close to 85%.

Risk-based thinking

All this would be enough to prevent a combustion from starting; however, an oxygen reduction system is not, in itself, an oxygen-reducing fire-fighting system. The latter is not limited to preventing the ignition of flames but does something much more complex: it manages the risk through a monitoring, alarm and danger containment system, in the context of risk scenarios that the rules and experience accumulated in the fight against fires have shown to be possible. The characteristic of continuity, linked to the concept of prevention, makes indissoluble the process of controlling the atmosphere from the logical flow of the activities taking place in the protected premises .

Hybridisations

In a room protected with an ORS system, as at high altitudes - the hypoxic atmosphere is comparable to that of 3000 meters a.s.l. - the percentage of residual oxygen determines the residence time of a subject, the activities that can be performed based on the related effort and the physical conditions that can limit the suitability to operate.

The control of the atmosphere, the analysis of the risk and the creation of infrastructures protected from fires that are safe for the persons who work there require, as is evident, a hybridisation between different skills.

Application fields

ORS fire-fighting systems find their application in environments with little or no traffic, such as temperature-controlled cells, data centres and archives, especially if these contain artistic heritage. There are materials, such as polyurethane, that represent true "black beasts" of the fire-prevention professional. Extinguishing them is very difficult; therefore, preventing their ignition becomes the main objective. In order to neutralise an environment there is not only the method of removing oxygen; extinguishing gases are not however always compatible with the environment, the materials stored, the activities being carried out in the premises or the volumes to be protected. An ORS, operating continuously, once the environment is brought into operating conditions, requires a relatively modest supply of hypoxic air that is enough to maintain the design concentration. In the case of large volumes, gas saturation would require large quantities (not to mention water reserves); an ORS does not require invasive storage - producing nitrogen locally - or large technical rooms (with lower costs associated with civil works compared to alternative systems). Low oxygen content also inhibits the oxidative degradation reactions of protected materials and nitrogen is environmentally friendly and inert.

Dynamic equilibrium

An oxygen-reduction system has the task of maintaining a dynamic balance between the gases present in the atmosphere of the environment to be protected and therefore those who create it out must have the ability to control and/or eliminate the losses that occur in the protected volume (determinable by leak tests such as the Door Fan Test according to UNI EN 15004/1 or the Blower Test according to EN ISO 9972). The system must in turn have its own system of continuous monitoring of the operation and of the presence of high sensitivity fumes.

Risk classes

Based on the oxygen concentration, the room subject to atmosphere control can be placed in one of four different risk classes, according to which the safety measures to be adopted are established (safety signs, electronic oxygen concentration indicators, alarm sirens and high signalling devices).

Clearly this is a complex system and, once developed, an ORS system is subject to maintenance, carried out by qualified personnel; being a "proprietary technology" it typically requires the involvement of whoever designed and built the system.

From an operational point of view, it is an easy-to-manage type of system and even annual maintenance for some components and energy consumption does not differ much from that of a normal air handling unit.

System documentation

The file that accompanies the system is enhanced as the creation follows the design and then moves on to management. The first risk assessment defines the system specification that will be submitted for approval by the Fire Brigade; Once the guidelines are defined, it will be the task of the engineers to translate them into a project. Finally, after a successful test, like any system, the installer must issue the declaration of conformity, pursuant to art. 7 of Italian decree no. 37/2008 and a Certification of compliance and correct functioning must be issued by the relevant fire prevention professionals.

The use and maintenance manual of the system, as required by Italian Decrees 07.08.2012 and 20.12.2012, will guide the operator in management of the system while the emergency plan and safety procedures will regulate access to the premises and the alarm procedures in case of system malfunction or anomalies detected in monitoring oxygen concentration thresholds. All this, however, would be futile if all the personnel accessing the room protected by the ORS system were not properly trained.

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