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Contents
  1. 1Agents and how they work
  2. 2System types and components
  3. 3Design concentration and agent quantity
    1. 3.1Worked example
  4. 4Enclosure integrity and hold time
  5. 5Safety of people
  6. 6Reliability and performance
  7. 7See also
  8. 8Notes
  9. 9References

A gaseous extinguishing system is a fixed fire suppression system that puts out a fire by discharging an extinguishing agent in gaseous form, usually into a closed room. A gaseous agent stays a gas at normal room temperature and pressure, spreads easily and distributes itself evenly through an enclosure.[1] The agents fall into two classes: halocarbons, such as Halon 1301 and its replacements, and inert gases, such as nitrogen and mixtures containing argon. Carbon dioxide is a third, long-established agent.[1][2]

Carbon dioxide systems are useful where an inert, electrically non-conductive, three-dimensional gas is needed and where clean-up must be minimal. Hazards protected include flammable liquids, electrical equipment such as transformers and switchgear, engines and computer rooms.[3][4] An important advantage of gaseous agents is that no cleaning is needed after a release in the absence of a fire; a few minutes of venting is enough.[1] The protected room must be tight enough to hold the agent, and a halogenated system in most cases has only one chance to put the fire out. Several agents are also dangerous to people at the concentrations needed.[5][6]

Halon may contribute to the depletion of the Earth's ozone layer and is therefore considered harmful to the environment.[7] Production of halons ended in developed countries on 31 December 1993 under the Montreal Protocol. Since then, several clean halocarbon and inert gas alternatives to Halon 1301 have been commercialised.[2]

Agents and how they work

Gaseous agents extinguish fires by different mechanisms. Inert gases act mainly by oxygen depletion, while halocarbon agents act largely by absorbing heat, with some chemical effect on the flame reactions.[1] Inert gases lower the flame temperature below the level needed to sustain the combustion reactions. They do so by reducing the oxygen concentration and by raising the heat capacity of the atmosphere.[8] Adding enough inert gas to bring the oxygen concentration in air below 12 % leads to flame extinction in flaming fires.[8]

Carbon dioxide displaces the normal atmosphere and reduces the oxygen content below the 15 % that a report of the US Nuclear Regulatory Commission (NRC) gives as necessary for a diffusion flame.[5] Its action is mainly thermophysical: the reacting gases are prevented from reaching a temperature high enough to keep up the free radicals that sustain the flame.[5] It is colourless, odourless, electrically non-conductive and stored as a liquid. After release it becomes a gas, with some small solid particles, about 50 % heavier than air.[5]

Halon 1301 works chemically. It inhibits the chain reaction of combustion and therefore suppresses a fire very quickly, unlike agents that remove heat or oxygen.[9] On a weight basis it is about 2.5 times as effective as carbon dioxide.[9] Its replacements, the halocarbon clean agents, act by a mix of chemical and physical mechanisms. Compounds containing bromine or iodine scavenge flame radicals like Halon 1301, while most others work mainly by extracting heat from the flame zone.[8] Clean agents are defined as extinguishants that vaporise readily and leave no residue.[2]

Inert gas agents can also be blends. In World Trade Center Building 7 (WTC 7), a room containing a 6,000 gal fuel oil tank was protected by Inergen, a proprietary mixture of 52 % nitrogen, 40 % argon and 8 % carbon dioxide by volume.[10][11] A small amount of carbon dioxide in an inert gas mixture appears to reduce the effects of low oxygen on people substantially.[12]

System types and components

A gaseous system can protect a room or an object. In total flooding, the whole enclosure is filled with the agent to a prescribed concentration, through nozzles placed to give a uniform concentration everywhere.[3] Local application directs the agent at a localised hazard without filling the enclosure. It suits surface fires in flammable liquids, gases and very shallow solids, for example spray booths, printing presses and rolling mills.[3] Total flooding is the most common method for halogenated agents.[13]

Cross-section of a room with agent cylinders, pipework to ceiling nozzles, detectors, control panel, warning device and closed door.
Main parts of a total flooding gaseous extinguishing system. Based on Iqbal and Salley (2004), pp. B-47 to B-48, Hopkins et al. (2005), p. 58 and DiNenno (2002), p. 4-187.

The agent is stored in containers and released through pipes and nozzles. Carbon dioxide is held either in high-pressure steel cylinders at room temperature, at about 850 psi (5.9 MPa) at 21 °C, or in refrigerated low-pressure vessels at about −18 °C and 300 psi (2.1 MPa).[14] Cylinders hold 50 to 100 lb (23 to 45 kg) of agent each, so most high-pressure systems use several cylinders manifolded together.[14] Halon 1301 is stored as a liquid under pressure and released as a vapour.[9]

Detection starts the system. In the World Trade Center towers, smoke and heat detectors actuated the release of carbon dioxide into the protected computer rooms.[4] The Inergen system in WTC 7 was to be actuated by two heat detectors at the ceiling above the fuel tank.[11] In the United States, NFPA 12A (1997) of the National Fire Protection Association (NFPA) required automatic actuation of Halon 1301 total flooding systems for ordinary combustible materials, unless the authority having jurisdiction accepted manual actuation.[13]

The discharge must be fast. A Halon 1301 discharge must be substantially complete in a nominal 10 seconds, to prevent the fire from becoming deep-seated, to limit decomposition products and to disperse the agent throughout the enclosure.[13] For halocarbon clean agents, the maximum discharge time is also 10 seconds. For inert gases, the requirement is currently 60 seconds, and longer times are typically used in Europe.[15] For carbon dioxide on deep-seated fires, NFPA 12 requires the design concentration within 7 minutes and 30 % within 2 minutes. For surface fires, it requires the design concentration within 1 minute.[6]

Design concentration and agent quantity

The design starts from the concentration needed to extinguish the fuel. For halocarbon agents, the minimum extinguishing concentration is measured with a laboratory cup burner and then checked in full-scale approval tests.[16] Halon 1301 design concentrations were historically set at the cup burner value plus a 20 % safety factor, with a minimum of 5 %.[17] Full-scale tests later suggested that 20 % might be insufficient. The first edition of ISO 14520-1, a standard of the International Organization for Standardization (ISO), therefore required a safety factor of at least 30 %. NFPA 2001 requires 30 % for Class B hazards and for systems actuated only by hand.[16]

Carbon dioxide needs much higher concentrations. NFPA 12 sets a minimum of 34 % by volume for surface fires such as liquid fuels and 50 % for deep-seated hazards such as electrical equipment. Bulk paper requires 65 % and some special hazards 75 %.[3] Most surface burning stops at about 20 % carbon dioxide or less, so these values include a large margin of safety.[3] For inert gases, IG-541 is used at a minimum design concentration of 37.5 % where ordinary combustible materials are involved.[15]

Bar chart of design concentrations: Halon 1301 5 %, HFC-227ea 8.6 %, CO2 surface fires 34 %, IG-541 37.5 %, CO2 deep-seated 50 %; line at 9 %.
Minimum design concentrations of gaseous agents, with the carbon dioxide level above which most people lose consciousness. Based on DiNenno (2002), pp. 4-176 to 4-189 and Iqbal and Salley (2004), pp. B-47 and B-49.

Once the design concentration is known, the quantity of agent follows.[15] The protected volume is the net volume of the space.[15] For a halocarbon agent, the mass needed to reach the design concentration in a closed room depends on the room volume and the specific volume of the agent vapour:[15]

w=VS⋅C100−C

Symbols
Symbol Description Unit
w mass of agent required kg
V net volume of the protected space m³
S specific volume of the agent vapour at the minimum expected room temperature, S=k1+k2T m³/kg
C design concentration % by volume

The factor C/(100 − C) assumes that the agent and air mixture that escapes during discharge is well mixed at concentration C. The formula makes no allowance for leakage after discharge.[15] For inert gases, a logarithmic version is used, which is identical to the expression for carbon dioxide and assumes that the displaced atmosphere is freely vented.[15] The minimum room temperature is used because a colder room needs more agent.[15]

Worked example

HFC-227ea is one of the halocarbon agents acceptable for normally occupied areas.[12] Its cup burner extinguishing concentration for n-heptane is 5.8 to 6.6 %.[18] With the upper value and the 30 % safety factor for Class B hazards, the design concentration is 1.3 × 6.6 ≈ 8.6 %.[16] This lies below the no observed adverse effect level (NOAEL) of HFC-227ea, which is 9.0 %.[19]

For a minimum room temperature of 20 °C, the constants for HFC-227ea give S = 0.1269 + 0.0005 × 20 = 0.137 m³/kg.[15] Assume a closed room with a net volume of 100 m³. The agent mass is then:

w=1000.137⋅8.6100−8.6≈69 kg

If the room can cool to 0 °C, S falls to 0.127 m³/kg and the required mass rises to about 75 kg. The calculation gives only the agent for the design concentration. Leakage, unclosable openings and the required hold time must be assessed separately.[17][15]

Enclosure integrity and hold time

The enclosure is a critical part of the system. It must be tight enough to prevent preferential loss of agent during discharge and excessive loss of the agent and air mixture afterwards, so that the concentration is held long enough.[17] Doors, ventilation fans and other openings must be secured before discharge. Systems should not be installed in rooms with unclosable openings unless test data show that adequate concentrations can be reached.[17]

The density of the agent and air mixture is the driving force for leakage in still air.[20] Halon 1301 is about five times as heavy as air, so the agent and air mixture flows out through low openings while fresh air enters at the top.[21] An interface forms and descends; the space above it is unprotected, while the space below keeps the original concentration.[5] The concentration at the height of the highest combustible therefore matters most.[21]

Two room sections: just after discharge the room is uniformly filled; later, agent leaks out at the bottom, air enters at the top and an interface descends.
Leakage of a heavy agent and air mixture from an enclosure: the protected zone shrinks as the interface descends. Based on Iqbal and Salley (2004), pp. B-45 to B-46 and DiNenno (2002), p. 4-198.

If the extinguishing atmosphere is lost too quickly, glowing embers may reignite when air returns. Openings must then be closed, or extra agent must be discharged.[14] For rotating electrical equipment such as generators, an extended discharge at a reduced rate maintains the concentration while leakage continues.[14]

Deep-seated fires need long soaking times. If a 5 % concentration of Halon 1301 does not extinguish a fire within 10 minutes, NFPA classes it as deep-seated.[13] Visible flames on solid materials go out at once, but glowing combustion can continue inside the material.[13] In full-scale tests on cable trays, a 6 % Halon 1301 concentration held for 10 minutes extinguished fires in unqualified cables, while qualified cables needed 15 minutes.[22] For carbon dioxide on deep-seated fires, the design concentration must be held for at least 20 minutes.[6]

Safety of people

Several gaseous agents are dangerous at the concentrations needed to put out a fire. Carbon dioxide replaces the oxygen in the air; harmful effects become noticeable at 6 to 7 %, and most people lose consciousness quickly above 9 %.[6] Since the minimum extinguishing concentrations exceed 9 %, every carbon dioxide system needs adequate safety precautions.[6] According to the NRC report, carbon dioxide should not be used in occupied areas unless the documented fire risk is greater than the risk to personnel and no viable alternative exists.[23] In the United States, the Occupational Safety and Health Administration (OSHA) requires a predischarge alarm with enough time delay for people to leave when the design concentration exceeds 4 %.[23]

Accidents show why these rules exist. A US Environmental Protection Agency review found 51 incidents from 1975 onwards, with 72 deaths and 145 injuries from discharges of carbon dioxide systems. All deaths attributed to carbon dioxide were caused by asphyxiation.[23] In 1998, a total flooding system discharged without warning during maintenance at a US national laboratory, killing one worker.[24] In 1992, a test discharge at the Duane Arnold nuclear plant let carbon dioxide leak into the control room. The oxygen level fell to 17 % at chest height and 15 % at floor level, below the plant's criterion of 19.5 %.[25]

Clean agents are limited by toxicity data. For halocarbons, the NOAEL is the highest concentration at which no adverse effect on the heart was observed in animal tests. The lowest observed adverse effect level (LOAEL) is the lowest concentration at which an effect was measured.[19] Where no further modelling data exist, the design concentration in occupied areas must be below the NOAEL. On this basis, HFC-227ea, HFC-23 and FC-3-1-10 are acceptable for normally occupied areas.[12]

Inert gases are physiologically inert; their hazard is the low oxygen concentration that they create. Exposure is limited to 5 minutes up to 43 % agent (12 % residual oxygen), to 3 minutes between 43 and 52 %, and to 30 seconds above 52 %.[12] Keeping a room permanently at a reduced oxygen level is a different approach, described under oxygen reduction system.

Step chart of allowed exposure time against inert gas concentration: 5 min up to 43 %, 3 min from 43 to 52 %, 30 s above 52 %.
Exposure time limits for inert gas agents, with the corresponding residual oxygen concentration. Based on DiNenno (2002), p. 4-184.

Reliability and performance

Gaseous systems are credited with a high, but not perfect, reliability. For fire risk studies of United States nuclear power plants, recommended unreliability values are 0.04 for carbon dioxide systems and 0.05 for halon systems.[26] For a Halon 1301 system in a computer facility, Steciak and Zalosh estimated the probability that the system fails to protect against fire damage. They found 0.05 for an electrical cable fire, 0.13 for a waste paper fire and 0.08 for a fire outside the protected room.[27]

A halogenated agent system in most cases has only a single chance to extinguish a fire. The NRC report therefore recommends testing systems and measuring concentrations at several heights, at least at the highest combustible.[5] Field experience includes both successes and failures. At Rancho Seco in 1984, a hydrogen fire at the main generator was extinguished by the fixed automatic carbon dioxide system within 14 minutes.[28] At Millstone Unit 3 in 1999, an inadvertent carbon dioxide discharge in a cable spreading room migrated to other rooms. The control room reached more than 17,000 ppm, and operators wore breathing apparatus for about six hours.[24]

Engineering analysis of these systems relies on standards rather than general methods. The International Fire Engineering Guidelines state that calculations for inert and chemical gaseous systems should be based on their design specifications and on standards such as AS 4214 and NFPA 2001. These calculations are not covered by the guidelines themselves.[29]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Iqbal and Salley (2004), p. B-41.
  2. ↑ 2.0 2.1 2.2 DiNenno (2002), p. 4-173.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Iqbal and Salley (2004), p. B-47.
  4. ↑ 4.0 4.1 Hopkins et al. (2005), p. 58.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Iqbal and Salley (2004), p. B-46.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Iqbal and Salley (2004), p. B-49.
  7. ↑ Nolan (2000), as cited in Iqbal and Salley (2004), p. F-14.
  8. ↑ 8.0 8.1 8.2 DiNenno (2002), p. 4-174.
  9. ↑ 9.0 9.1 9.2 Iqbal and Salley (2004), p. B-42.
  10. ↑ McAllister et al. (2008), p. 73.
  11. ↑ 11.0 11.1 Hopkins et al. (2005), p. 71.
  12. ↑ 12.0 12.1 12.2 12.3 DiNenno (2002), p. 4-184.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 Iqbal and Salley (2004), p. B-43.
  14. ↑ 14.0 14.1 14.2 14.3 Iqbal and Salley (2004), p. B-48.
  15. ↑ 15.00 15.01 15.02 15.03 15.04 15.05 15.06 15.07 15.08 15.09 DiNenno (2002), p. 4-189.
  16. ↑ 16.0 16.1 16.2 DiNenno (2002), p. 4-188.
  17. ↑ 17.0 17.1 17.2 17.3 DiNenno (2002), p. 4-187.
  18. ↑ DiNenno (2002), p. 4-176.
  19. ↑ 19.0 19.1 DiNenno (2002), p. 4-183.
  20. ↑ DiNenno (2002), p. 4-198.
  21. ↑ 21.0 21.1 Iqbal and Salley (2004), p. B-45.
  22. ↑ Iqbal and Salley (2004), p. B-44.
  23. ↑ 23.0 23.1 23.2 Iqbal and Salley (2004), p. B-50.
  24. ↑ 24.0 24.1 Iqbal and Salley (2004), p. B-51.
  25. ↑ Iqbal and Salley (2004), p. B-52.
  26. ↑ Parkinson et al. (1994), as cited in Nowlen et al. (2005b), p. P-6.
  27. ↑ Steciak and Zalosh (1992), as cited in National Research Council of Canada et al. (2005), p. 3.7-3.
  28. ↑ Iqbal and Salley (2004), p. H-12.
  29. ↑ National Research Council of Canada et al. (2005), p. 2.7-8.

References

  • DiNenno, P. J. (2002). Halon replacement clean agent total flooding systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-173–4-200). National Fire Protection Association; Society of Fire Protection Engineers.
  • Hopkins, M., Schoenrock, J., & Budnick, E. (2005). Fire suppression systems (NIST NCSTAR 1-4B). National Institute of Standards and Technology.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • McAllister, T. P., Gann, R. G., Averill, J. D., Gross, J. L., Grosshandler, W. L., Lawson, J. R., McGrattan, K. B., Pitts, W. M., Prasad, K. R., Sadek, F. H., & Nelson, H. E. (2008). Structural fire response and probable collapse sequence of World Trade Center Building 7 (NIST NCSTAR 1-9, Vols. 1–2). National Institute of Standards and Technology.
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
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