Clean agent systems are a type of alternative fire extinguishing system (as opposed to traditional automatic fire sprinklers) that are used to protect property that would be expensive to replace or that is essential to continuity of business operations. For example, a financial services firm would be interested in having their Information and Technology equipment protected with a fire protection system, but would want it done in such a way that system actuation would not damage or ruin equipment; indeed, having a system protect a fire within an IT room that destroys all of the equipment while protecting it would be equivalent to not having any protection within the room at all (notwithstanding the life safety hazards and fire spread issues stemming from not having any protection).
Clean agent systems are essentially discharge nozzles connected to a pressurized tank that releases the contents of the tank upon system activation. The system can be activated by electronic means, such as smoke detectors, or by mechanical means, such as fusible links. Clean agent systems are either inert gases or fluorine based compressed liquids. Common clean agents are (or mixtures of): nitrogen, carbon dioxide, argon; proprietary HFC molecules or fluorinated ketones. All of these compounds are electrically nonconductive and do not destroy equipment or property. Cleanup is typically limited to the dust and debris that is dislodged, during a fairly vigorous discharge. Clean agent systems have alarm safety features to ensure occupants have left the protected enclosure before the system activates.
Clean agents are effective in suppressing and in many cases extinguishing fires through one of three actions:
1) reduction of oxygen, 2) removal of heat, 3) interruption of the chemical reaction.
In the case of inert gases (combinations of N2, CO2 and Ar), the singular methodology is the reduction of oxygen within a room or compartment to levels below which combustion cannot be sustained. Generally, a fire cannot propagate when the oxygen fraction (by volume) is below 13%. This is because the heat released by oxidization is insufficient to heat the combustible material to a temperature above its fire point. Inert gas systems are typically delivered above 40% design concentration by volume.
Fluorine based agents (HFC’s and Fluorinated ketones) suppress fire primarily through the reduction of heat. These compounds are able to absorb significant heat at the fire source, thus limiting the fire’s ability to sustain itself. Fluorine based agents have very little impact on oxygen within the protected space, as they are generally delivered at anywhere between 5-8% by volume to be effective in fire suppression.
More historical clean agents such as Halon 1301 (total flooding) and Halon 1211 (streaming) were very effective fire suppression agents due to their ability to interrupt the chemical chain reaction. The Bromine atom within the ‘halon’ family of compounds is particularly good at this! Unfortunately, Bromine is also a powerful ozone depleting element, thus Halon use for fire suppression applications is strictly limited to critical systems (typically aerospace), and may not be used for standard, new system applications. Halon 1301 systems were typically designed at 3-5% by volume, and thus, also had a negligible impact on oxygen within the protected space.
All clean agents are stored in pressurized containers and could pose some hazard from mishandling or improper service or maintenance. However, the US EPA does include a list of clean agents in the Significant New Alternatives Policy (SNAP) list, which provides guidance for safe use concentrations in normally occupied spaces.
Inert gas systems can be problematic if not designed and installed correctly, due to their inherent purpose of reducing oxygen concentration. OSHA guidelines prohibit oxygen depletion conditions below 19.5%; however, most healthy individuals can tolerate oxygen levels of 15% for an extended period.
Carbon dioxide systems, while not typically considered a true ‘clean agent’, are most dangerous, as carbon dioxide produces a specific respiratory response in the human body, and, as such, can be more lethal than the other agents.
As an alternative to clean agents, special automatic fire sprinkler systems can be installed. These systems are referred to as “pre-action sprinkler systems.” Pre-action sprinkler systems are interlocked with electronic detectors, such as smoke or heat detectors, and activate only when the electronic detection system actuates. As an additional layer of protection from accidental activation, a single sprinkler head and an electronic detector need to activate before water fills the normally dry piping that protects sensitive rooms. When the system is not active, its piping is dry, and no damage would occur if a sprinkler head accidentally or inadvertently discharged (because the electronic detection system also must actuate to fill the sprinkler system with water). This type of sprinkler system is referred to as a double-interlock pre-action sprinkler system.
Condensed aerosol fire protection systems are also an alternative to clean agent systems. Condensed aerosol systems have either a mechanical or electronic detection system. Upon actuation, fine particles with a propellant are expelled into the space. This mixture quickly cools and extinguishes the fire. These systems offer minimal clean up, similar to clean agent systems, but are not designated as “clean agent systems,” as they operate differently and on different principles.