Selecting the correct clean agent suppression system

Clean agent fire suppression systems are used to protect facilities with high value per volume. They work on a total flooding concept, causing quick extinguishment of a detected fire, limiting damage to the protected space and its contents.


This article has been peer-reviewed.Learning objectives:

  • Learn to apply, design, and install clean agent systems while considering agent selection.

  • Understand the importance and effects of enclosure integrity/leakage, pressure-relief venting, and design concentration on the performance of a clean agent system.

  • Discuss the advantages and disadvantages of the commercialized agents.

Clean agent fire suppression systems are primarily used to protect facilities with high value per volume. They work on a total flooding concept (filling the entire protected space) and, consequently, their cost is directly proportional to the volume of the space to be protected. Typical spaces protected with clean agent systems include data processing facilities, telecommunications facilities, art-storage facilities, and other high-value buildings.

The NFPA 2001: Standard on Clean Agent Fire Extinguishing Systems covers the application of the clean agent systems. This standard addresses the design, installation, maintenance, inspection, and testing of clean agent systems. Note that clean agent systems employ a supplemental fire-detection system—usually smoke detection—to cause activation that is integral to the overall system. These smoke-detection systems are covered by NFPA 72: Fire Alarm and Signaling Code. Features of smoke-detection systems require separate discussion and are beyond the scope of this article.

Clean agents

Table 1: Shown are the selected properties of the most commonly used clean agents in fire-extinguishing systems. Courtesy: JENSEN HUGHESThere are seven primary clean agents that consist of either halocarbons or inert gases and are used worldwide. These include HFC-227ea (FM-200), HFC-125 (FE-25), FK-5-1-12 (Novec 1230), IG-100 (nitrogen), IG-01 (argon), IG-55 (Argonite), and IG-541 (Inergen). Another, HFC-23 (FE-13), has been used in a few applications, mostly involving low temperatures. Selected properties of these agents are shown in Table 1. These agents can cause fire extinguishment through a combination of three primary mechanisms:

  • Increasing the heat loss from the fire by increasing the heat capacity of the environment within the protected enclosure.

  • Displacing the oxygen in the environment within the protected enclosure.

  • For the halocarbon agents, absorbing energy from the fire to cause decomposition of the agent.

HFC-227ea, HFC-125, FK-5-1-12, and HFC-23 are grouped together as halocarbon agents. These agents use the agent-decomposition mechanism to cause extinguishment. This mechanism results in lower agent design concentrations for this group of agents. As the products formed by decomposition of the agent are toxic (primarily HF), caution must be used and appropriate protective equipment should be worn when re-entering a protected space after a fire event.

The amount of decomposition products formed is directly related to the size of the fire at the time of system activation. Early detection and prompt system activation are key for limiting the amount of decomposition products formed. The decomposition products can have a negative effect on the property being protected; however, the primary concern is the noted toxicity.

IG-100, IG-01, IG-55, and IG-541 are grouped together as inert gas agents. They do not undergo the agent-decomposition reaction, thus they have higher design concentrations than the halocarbon agents. While they are less expensive on a mass basis, larger quantities of inert gases would be needed.

Figure 1: Agent storage area requirements are given for clean agents based on minimum Class A design concentrations and typical agent cylinder capacities and dimensions. All graphics courtesy: JENSEN HUGHESThe halocarbon agents are stored as liquids, having the advantage of a reduced storage-space requirement as compared with the inert gas agents. Figure 1 compares the storage area needed for these agents based on minimum Class A design concentrations and typical agent cylinder storage capacities and dimensions.

There currently are no U.S. Environmental Protection Agency (EPA) regulations that restrict the use of fire suppression agents based on their global warming potential (GWP). However, regulations and other restrictions based on the use of agents with higher GWP, such as HFC-227ea, HFC-125, or HFC-23, may be imposed in the future due to concerns over greenhouse gas emissions.

FK-5-1-12 has a low GWP due to its short atmospheric lifespan. This halocarbon agent, along with the four inert gases noted above, avoids the concerns regarding potential future regulations with GWP.

Design concentrations

The minimum design concentration for a clean agent system is based on the minimum extinguishing concentration with a safety factor between 20% and 30% applied. The safety factor is applied to ensure that the system will perform as intended in actual installations where conditions are not as controlled as they are during the laboratory tests.

The minimum extinguishing concentrations are based on three series of tests based on the fuel. The first of these tests is the cup-burner test. During this test, a 30-mm (1.18-in.)-diameter cup is located inside an 85-mm (3.35-in.)-diameter chimney. The fuel is fed from the bottom of the cup and the fuel level is maintained level with the lip of the cup. The agent is introduced into the air flowing through the chimney. The agent concentration in the airflow is gradually increased until extinguishment occurs. Details on this test are given in Annex B of NFPA 2001.

The second series of tests is a pan-fire test, in which a 0.23-m2 (2.5-sq-ft) pan fueled with n-heptane to a depth of 5 cm (2 in.) with a 5-cm (2-in.) freeboard and is generally conducted as part of UL 2166: Standard for Halocarbon Clean Agent Extinguishing System Units, UL 2127: Standard for Inert Gas Clean Agent Extinguishing System Units, or FM Global approval (Approval Standard for Clean Agent Extinguishing Systems, Class Number 5600). The pan is elevated in the center of a 100-m3 (3,531-ft3) enclosure. The agent is discharged into the enclosure using the clean agent system manufacturer’s hardware. This test effectively confirms the scaling for the cup-burner test results.

The third series of tests involves Class A materials and is generally performed as part of a UL listing or FM Global approval. These tests involve a wood-crib fire, and polymeric-material arrays of polymethyl methacrylate, acrylonitrile-butadiene-styrene copolymer, and polypropylene. The wood crib or the polymeric material array is located in the center of a 100-m3 (3,531-ft3) enclosure. The agent is discharged into the enclosure using the clean agent system manufacturer’s hardware. ISO-14520: Gaseous fire-extinguishing systems—Physical properties and system design is a similar test method that uses a variation on the apparatus used for the polymeric array fire tests, which consists of a larger ignition pan. This variation in the ISO text generally causes a small increase in the agent concentration required to successfully result in extinguishment as compared with the UL or FM Global test method.

The design concentrations for a given agent are determined based on the hazard class (A, B, or C) of the fuel and the actual fuel present. For a hazard involving a Class B (flammable liquid), the design concentration includes a 30% safety factor applied to the concentration required to cause extinguishment in the cup-burner test for the fuel involved. If this concentration is less than that used for the pan-fire test with a 30% safety factor applied, then the higher concentration is used.

For a hazard involving Class A (solid fuel) hazards, the design concentration includes a 20% safety factor applied to the agent concentration used to extinguish the wood crib and polymeric material arrays during the test. The concentration required to extinguish n-heptane in the cup burner is used as a minimum value for the design concentration.

For hazards involving Class C (energized electrical equipment) hazards, the design concentration includes a 35% safety factor applied to the concentration required to cause extinguishment in the cup burner for the fuel involved. If the energized equipment involves voltages greater than 480 V, a higher design concentration may be necessary and should be determined by tests. It is always recommended to shut down power to electrical equipment to prevent reignition/reoccurrence of the fire event once the clean agent has dissipated.

For hazards involving multiple hazard classes, the highest agent design concentration should be used. In most cases, the determined minimum concentrations are appropriate. For Class B fuels, methanol and other alcohols require a higher concentration than the n-heptane baseline. Cup-burner tests with the specific fuel are required to determine the required concentration.

For some Class A materials that have a tendency to smolder or become “deep-seated,” a much higher concentration with a long soak time may be required to cause complete extinguishment. There is not a standardized test for determining the agent requirements for smoldering or deep-seated fires. These are handled in a case-by-case manner, as the specific configuration can have a significant effect on the performance of the system. Examples of smoldering fires that may become deep-seated include excelsior, electrical cable bundles, shredded paper, bales of cotton jute, and saw dust or mulch.

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