Issue 5: Clean Agent Fire Suppression Update
By Tom Wysocki
For over half a century from 1910 to the late 1960s, when a clean, dry, gaseous fire-extinguishing agent was required, carbon dioxide was the choice, usually the only available choice. In the late 1960s, the "halons" became commercially viable alternatives to carbon dioxide for applications where the life safety risks posed by carbon dioxide were considered unacceptable. In particular, halon 1301 became popular as a fire protection agent for total flooding spaces where personnel might be present.
For nearly two decades, halon 1301 was the gaseous agent of choice for protection of areas containing high-value electronic equipment. Halon 1301 also made inroads to the marine fire protection market for total flooding machinery spaces where flammable liquids presented the primary risk. Halon 1301 was also widely used in aerospace and military applications.
By the mid-1980s, a growing body of scientific data indicated that halons were contributing to depletion of the stratospheric ozone layer. In 1987, the Montreal Protocol was adopted requiring a phase-out of halon 1301 production in developed countries. The U.S. Clean Air Act Amendments of 1990 banned new production and import of halons into the United Stated as of 1994. These regulatory actions triggered an accelerated search for gaseous fire extinguishing agents to replace the popular halon 1301. In the U.S., existing halon systems are still permitted and the use of recycled halon is allowed. However, in recent years, new installations have become very rare.
Next year marks the 20th anniversary of the Montreal Protocol. During the last two decades, a number of gases have gained acceptance as halon replacements --- "life-safe" alternatives to traditional carbon dioxide fire protection for total flooding applications. NFPA 20011 covers the use of "halon alternatives." The current Standard 2001 lists over a dozen "halon alternatives."
Some of the halocarbon agents currently included in NFPA 2001 are HFC227ea (FM-200™, FE-227™), HFC-125 (FE-25™), HFC-23 (FE-13™), and FK-5-1-12 (Novec 1230™) as well as a number of "blends," that is, combinations of various chemical agents. The inert gases identified in NFPA 2001 are nitrogen (IG-100) and argon (IG-01) and mixtures of 50 percent nitrogen and 50 percent argon (IG-55). There is also a mixture of nitrogen and argon with a small amount of carbon dioxide (IG-541, trade name "Inergen™").
Choosing a Gaseous Agent
If the fire protection engineer determines that a space would best be protected by total flooding with a gaseous agent, the choice of the many available agents should be given careful consideration. The engineer may consider whether traditional carbon dioxide protection is a desirable or viable option. If people will normally be present in a space, carbon dioxide should generally be eliminated as an option. (There may exist specialized applications where people are normally present that are best protected by total flooding carbon dioxide. NFPA 122 covers the considerations for such hazards.)
When a clean agent is required, halocarbon and inert gas clean agents each have advantages and disadvantages.
"Clean Agent" Considerations
When floor space for the agent storage is limited, halocarbon clean agents often have an advantage. If the clean agent storage must be located some distance from a protected space, inert gases may sometimes have an advantage. When flammable liquid hazards are to be protected with a clean agent, availability of data for inerting concentrations using the agent/fuel combination must be considered. Availability of listings or approvals for specific hazards, such as Marine hazards, can affect the choice of agent. Other considerations may include cost, local availability of the agent for recharge, and operating temperature range.
Jurisdictions within the United States generally accept agents that are included in NFPA 2001. If the system is to be installed outside the United States, the engineer should check local regulations for possible restrictions on the use of the agent of choice.
"Class A" and "Class C"
If the fuel is electrical equipment, such as computer or telecommunications equipment, virtually all of the clean agents in NFPA 2001 are listed and approved for such hazards when electric power to the equipment is shut down upon discharge of the clean agent. All clean agents listed by UL or approved by FM per NFPA 2001 guidelines have been tested to determine their "Class A" fire-extinguishing concentrations. The "Class A" fire tests include not only the traditional "wood crib" fire but also arrays of plastic materials simulating the fuels in computer and telecommunications equipment.
Although the "Class A" fire tests use fuels simulating those found in electrical equipment, they are not representative of fire scenarios where electrical energy is continuously supplied to the fuel array. Limited testing with clean agents on fuel arrays where electrical power is maintained after application of the clean agent indicates that higher concentrations of clean agent than the "Class A" concentrations may be needed to extinguish the fire. As of this writing, the NFPA 2001 technical committee continues to study this issue. Electric power to the protected equipment should be shut down upon discharge of a clean agent unless issues related to possible reflash and continued agent decomposition (in the case of halocarbons) are adequately addressed. Presently, the designer should seek advice from the system manufacturer for such hazards where electrical power shutdown has untenable repercussions.
"Class B"
If the space to be protected contains quantities of flammable liquids and gases, the engineer must select an agent for which the flame-extinguishing and possibly the inerting concentrations for the specific flammable fuels have been determined. Alternatively, the engineer could arrange for determination of the required concentration by laboratory testing.
A notable difference between the fire protection using carbon dioxide and the "clean agent" alternatives to CO2 is the consideration of inerting versus flame-extinguishing. All total-flood carbon dioxide systems designed with CO2 concentrations per NFPA 12 produce an "inert" atmosphere --- precluding fire or explosion of even stochiometric mixtures of fuel and air with a persistent ignition source for as long as the specified CO2 concentration remains.
NFPA 2001 recognizes two levels of protection for flammable liquids and gases --- flame-extinguishment and inerting. NFPA 2001 requires inerting concentrations to be used for hazards involving Class B fuels if "conditions for subsequent reflash or explosion could exist." 1 Annex A of NFPA 2001 describes such "conditions."
Practically speaking, nearly all hazards where flammable liquids are the primary fuel load would best be protected by systems employing an inerting concentration.
Going Forward
Although many alternatives have been developed, no single alternative has emerged with all of the desirable characteristics of halon 1301. There are, however, some fine alternatives that provide excellent fire protection. "Clean agent" development is now focusing on developing design criteria for hazards involving electrical equipment where equipment cannot be de-energized, standardizing the cup burner test used to determine flame-extinguishing concentrations for Class B fuels, refining requirements for room pressure venting during clean agent discharges, and expanding the database of inerting concentrations.
Tom Wysocki is with Guardian Services, Inc.
1 NFPA 2001, Clean Agent Fire Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2004.
2 NFPA 12, Standard on Carbon Dioxide Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2005.
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