Clean Agent System Basics

Often times sprinkler systems are the required, go-to solution for protecting people and property against fire hazards. While they do an excellent job at this, sometime there is a need to quickly suppress a fire and protect high value sensitive items and this is where clean agents come into play, they have the ability to protect these assets by extinguishing fires without damaging equipment in the area. By definition a clean agent is a gaseous fire suppressant that is electrically nonconducting and that does not leave a residue upon evaporation. This is ideal when protecting high value items like historical artifacts or sensitive electronic equipment. The umbrella term “clean agents” includes both halocarbon agents and inert gas agents. Carbon dioxide (CO2) is another extinguishing agent with all the properties of a clean agent but is often classified differently due to the dangers associated with it. Here we will review the different types of gaseous fire protection systems and how they work.

How do gaseous suppression agents work?

Gaseous fire suppression agents work fundamentally the way any fire suppression media works; by removing one or more of the components of what was traditionally referred to as the fire triangle and now more appropriately, the fire tetrahedron.

Unlike water, which primarily works by removing heat, most gaseous suppression systems suppress fire primarily by reducing the available oxygen for combustion with a secondary benefit of cooling and inhibiting the chemical chain reaction. A portion of the agents do have a primary mechanism of heat absorption with the secondary benefits being a reduced oxygen concentration and inhibiting the chemical chain reaction. Gaseous fire protection systems usually are supplied by pressurized gas or liquid cylinders. When this pressurized gas is released, it is volume expands and it goes through a process known as adiabatic cooling, which is the reduction of heat through change in air pressure caused by that volumetric expansion. This cooling is the primary mechanism by which heat is removed.

These systems can provide protection through either a “total flooding” or a “local application” approach.

Total flooding

As the name suggests, total flooding systems discharge extinguishing agent throughout an entire space to suppress the fire. To do this, the gaseous agent must be introduced into the space and mix with the air in that space at a concentration that is specific to the particular gas chosen as well as the fuel class being protected. Specifics of this can be found in the standard related the appropriate type of agent.

An important concept to understand when it comes to total flooding clean agent systems is that these extinguishing agents needs to reduce the oxygen available for combustion to below the threshold where it would occur and hold it there until the items involved cool below their auto ignition temperature. If the concentration were to disperse prior to the items cooling enough the fire could reignite.

Since the agent needs to maintain a certain concentration for a period of time to suppress a fire it is important that the room air-tight enough to maintain concentrations for the minimum hold times. We have a great blog that dives further into this concept here.

Local application

As the name implies, local application systems discharge extinguishing agent, so the burning object is surrounded locally by a high concentration of agent to extinguish the fire. A local application system is often required because the enclosure itself is not suitable for a total flooding system. This means that when the protected object is not enclosed the discharge nozzles and rate of application must be capable of enveloping the object, which requires more agent to be discharged. The agent supply needs to be sufficient to maintain flow for the required time of protection, which is typically several minutes. Nozzle design is also critical, and the application design parameters must be determined by testing.

Types of clean agents

There are several distinct types of clean agents available, each with their own advantages, disadvantages, price points and design restrictions. The following are the main categories of clean agent types:

Carbon Dioxide

Even though NFPA does not classify it as one, Carbon Dioxide (CO2) can be considered the original clean agent. It works by both removing oxygen from the equation while simultaneously providing cooling to the fire. The biggest limitation when using this fire suppressant is that for it to be effective in extinguishing a fire it needs to displace oxygen at a level that is fatal for humans. For this reason, new CO2 systems are limited in their application and typically not permitted to be installed in normally occupied enclosures. More information on the specific requirements for the installation of CO2 systems can be found in the latest edition of NFPA 12, Standard on Carbon Dioxide Extinguishing Systems.

Halocarbon agent

Halocarbon agents are agents that contain as primary components one or more organic compounds containing one or more of the elements fluorine, chlorine, bromine, or iodine. Examples are hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs or FCs), fluoroiodocarbons (FICs), and fluoroketones (FKs).

Halocarbons extinguish fires through a combination of chemical and physical mechanisms. Primarily they work by interrupting the chemical chain reaction of fire. Halocarbons also extract heat from the fire, reducing the flame temperature until it is below what is needed to maintain combustion. Oxygen depletion also plays a vital role in reducing flame temperature.

Halocarbon Agents have been historically referred to as “Halon Replacement Agents” since they were developed to provide a more environmentally friendly alternative to Halon, which was an effective fire suppressant that is no longer produced. Halons have been identified as stratospheric ozone-depleting substances. In fact, halons have been identified as the most potent of all ozone-depleting substances. The Montreal Protocol on Substances That Deplete Stratospheric Ozone is an international agreement to control the production and trade of ozone-depleting substances. The agreement has been signed by over 140 countries and is administered by the United Nations Environment Program.

Specific requirements for halocarbon agents can be found in NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems

Inert gas

An inert gas agent contains one or more of the following gases as components: helium, neon, argon, or nitrogen, and that can also contain carbon dioxide as a minor component. Unlike CO­2 inert gases are non-lethal to humans at low concentrations (although there is still always a concern when oxygen levels are low). Inert gases suppress fires primarily by reducing the oxygen concentration and reducing the flame temperature below what is required for combustion. While inert gases are an effective means of fire suppression, they are not as effective as halocarbon agents and require more agent to be dispersed to extinguish a fire.

Like halocarbon agents, specific requirements for inert gas systems the can be found in NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems.

Initiation & activation

In the event of a fire clean agent systems are activated by a suppression releasing panel which detects the fire using automatic detection. Once a fire has been detected a releasing sequence starts often with a delay to allow occupants to evacuate. Notification appliances in the protected area sound for a pre-determined time before the system is activated. The gas is released from the cylinders by the releasing panel via an electronic signal to a solenoid valve on the agent tanks. The gas then flows through the piping and out the open nozzles to either protect a local area or flood the protected enclosure.

A manual option of activation is also often required where the releasing panel receives the signal from a manual station.

Should the activation be a false alarm, abort switches should be provided, which can stop the agent release during the pre-discharge phase.

These clean agent systems are typically installed in addition to the sprinkler system but occasionally it might be able to replace a sprinkler system completely. For more details on when this might be possible check out this blog.

Clean agent fire protection systems are a fantastic way to protect high value or sensitive electronic equipment. There are many options available between the inert gases, halocarbons and CO2 that vary in price, effectiveness, and design options. All these systems can be installed in either a total flooding or local application approach and have an involved process for activation and discharge. While these systems can have a high price point when compared to sprinklers, there will always be applications where these systems are needed.

Sign up for the NFPA Network Newsletter
Brian O'Connor
Technical Services Engineer

Related Articles