Special Hazards Fire Protection: Developments Since Halon 1301

Reprinted with permission from Fire Protection Engineering QT3 2016 issue – magazine.sfpe.org

By Jeff L. Harrington, P.E., FSFPE

It has been nearly 22 years since the production of halon 1301 ceased 
in the United States and many other developed and developing countries. This article will reflect on that time in history and examine many of the significant developments in fire and explosion suppression that were a direct result.

Halogenated hydrocarbons have been in commercial use as fire extinguishing agents since the early 1900s. Their use began to decline and had all but ended by the 1950s due to their relatively high toxicity coupled with the increasing popularity and availability of dry chemical agents. In the late 1940s, halon 1301 was verified to possess effective fire extinguishing capability while also having the lowest toxicity of all known halons. In the 1960s, halon 1301 received renewed interest as a means to extinguish fires in computer rooms without collateral damage from the agent itself with the added benefit of low toxicity.

Halon 1301 grew in popularity for the next 20 to 25 years becoming the extinguishing agent of choice in fixed fire extinguishing systems protecting a majority of electronic equipment facilities, including electronic data processing and communications equipment rooms. Halon 1301 also grew in popularity as an option for protecting a variety of other high-value and high-criticality assets benefiting significantly from the absence of direct damage by the agent, including vital records and priceless artifacts in museums. In 1989, it was reported that halon 1301 was in use according to estimates shown in Table 1.[1]

article1_table1

Halon 1301 Phases Out

The National Fire Protection Association (NFPA) formed a committee on Halogenated Fire Extinguishing Systems in late 1966 for the purpose of writing a new standard on halon 1301 extinguishing systems. In 1968, the first edition of the new standard was adopted in tentative form by the NFPA membership, and in 1970 it was officially adopted as NFPA 12A, Standard on Halon 1301 Fire Extinguishing Systems.[2] Just 27 years later, the continued viability of halon 1301 would be forever changed due to its particular effectiveness in depleting ozone in the earth’s stratosphere.

On September 16, 1987, the United States and 23 other nations signed the Montreal Protocol, which was adopted into US law through the Clean Air Act Amendments of 1990.[3] These new laws scheduled the phase-out of the production of ozone-depleting substances (ODS) that were causing the destruction of stratospheric ozone, including substances used as fire extinguishing agents. Stratospheric ozone is an essential minor atmospheric constituent responsible for the absorption of harmful UV-B radiation. The ODS chemicals are primarily chlorine and bromine-containing gasses, each of which has a characteristic potency for destroying ozone called the Ozone Depletion Potential (ODP). Halon 1301, as it turns out, has the highest ODP of any man-made ODS. Halon 1301 was targeted for the cessation of production in developed countries by ­January 1, 1994.

Since the cessation of production of halon 1301 in 1994, new fire extinguishing agents have been developed to serve as alternatives. Many of these new gaseous agents have attributes similar to those of halon 1301 including low toxicity to humans, nil electrical conductivity, and “clean” extinguishment (i.e., they cause no direct damage to the protected assets). These new “clean” agents are addressed by NFPA 2001, Clean Agent Fire Extinguishing Systems,[4] first published in 1994.

The phaseing out of halon 1301 not only prompted a search for alternative gaseous agents with characteristics similar to it, but also motivated a renewed interest in the further development of other fire extinguishing technologies, including water mist[5] and powdered aerosols.[6] Water mist fire protection systems are addressed by NFPA 750, Water Mist Fire Protection System,[7] first published in 1996. Powdered aerosol extinguishing systems are addressed in NFPA 2010, Standard for Fixed Aerosol Fire-Extinguishing Systems,[8] first published in 2006.

The Clean Air Act Amendments provided the authority to the EPA (Environmental Protection Agency) to develop and enforce rules and create a program that would facilitate the replacement of ODS including halon 1301 with alternative chemicals that reduce the overall risk to human health and the environment. In 1994, the EPA developed the Significant New Alternatives Policy (SNAP) program as a result. Under the Clean Air Act, Title VI regulations on ozone layer protection and Section 612 that is the basis for the SNAP program, all new alternative substances developed to replace Halon 1301 must be submitted to EPA for review and determination of acceptability. The submitted information must include the submitter’s health and safety studies for the alternative being proposed. EPA categorizes all listed substitutes for ozone-depleting substances by use sector. Substitutes for halon 1301 are listed in the Fire Suppression and Explosion Protection use sector.

article1_table2

Agency review of SNAP program submissions includes the following criteria:[9]

  1. Atmospheric effects and related health and environmental impacts.
  2. General population risks from ambient exposure to compounds with direct toxicity and to increased ground-level ozone.
  3. Ecosystem risks.
  4. Occupational risks.
  5. Consumer risks.
  6. Flammability.
  7. Cost and availability of the substitute.

The Clean Air Act also required the EPA (Agency) to publish and maintain a list of substitutes that are unacceptable for specific uses and a corresponding list of substitutes that are acceptable for specific uses. This is commonly referred to as the SNAP List. Upon review, the submitted substitute will be placed on the SNAP List in one of five categories as shown in Table 2.

The EPA completes a risk screen on all substitutes submitted for inclusion on the SNAP List using the submitter’s health and safety studies. The information required to be submitted for halon 1301 substitutes is detailed in the EPA’s risk screen guide and is categorized as follows:[10]

  1. Physical-Chemical Properties.
  2. Conditions of Manufacture, Installation, Maintenance, and Use.
  3. Toxicological Effects.
  4. Additional Considerations for Powdered Aerosols Used in Occupied Spaces.

The purpose of the risk screen is to evaluate the human health and environmental risks of the proposed substitute and analyze its acceptability. The EPA further asks for information specific to the type of substitute being proposed. The EPA defines two substitute types in the Fire Suppression and Explosion Protection sector:

  • In-kind Halon Alternatives:
    • Halocarbons
    • Inert Gas
    • Carbon Dioxide
  • Not-in-kind Halon Alternatives:
    • Powdered Aerosols
    • Foam
    • Water Mist

Substitutes are further distinguished by their proposed use as follows:

  1. Flooding Agent
  2. Streaming Agent
  3. For Occupied Spaces
  4. For Unoccupied Spaces

For total flooding substitutes, a SNAP listing of “Acceptable” means that the substitute can be used in occupied areas and also unoccupied areas. EPA approval of a substitute for use in occupied areas clears the way for its use as a replacement for halon 1301 in the predominant share of the market. This approval offers the greatest potential return on investment for a new substitute, which often provides the necessary justification to develop the substitute fully and bring it to the marketplace.

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Gaseous Agent Alternatives to Halon 1301

Numerous collaborative efforts to find suitable alternatives to halon 1301 were undertaken in the years immediately following the signing of the Montreal Protocol in 1987. These efforts shared funding, expertise, and other resources from various government and private organizations. One such effort led to the publication of a report by NIST (National Institute of Standards and Technology) that presented an exploratory list of 103 chemicals in 9 chemical families for the purpose of helping to facilitate the search for halon alternatives.[11]

Candidate chemicals would need to possess performance characteristics in three major categories to be considered a viable replacement for halon 1301, including:

  1. Environmental impact,
  2. Agent Performance, and
  3. Toxicity.

Environmental impact considerations for a gaseous agent alternative include an assessment of its ozone depletion potential (ODP) and global warming potential (GWP), which are both related to the agent’s atmospheric lifetime (ALT). A candidate agent’s ODP value must be zero or near zero and the GWP must be very low to have commercial viability.

Agent performance is determined primarily by its effectiveness in extinguishing fires. Secondary factors include an agent’s form factor (weight and volume), total cost (installation, maintenance, and refill), and discharge effectiveness (evaporation and mixing).

Toxicity considerations are dependent on the agent’s chemical family. Halocarbon agents must be tested to determine their cardiotoxicity. When a person inhales halocarbons, it can cause an increase in the heart’s sensitivity to elevated levels of adrenaline, which can lead to cardiac arrhythmia and possibly a heart attack. Inert gasses have no toxicity effect; however, their use in total flooding applications creates the potential that a person could be exposed to low oxygen concentrations at high agent concentrations with the risk of asphyxiation.

Both halocarbon and inert agents can be used to extinguish fires in total flooding applications under conditions considered safe for humans. Rules to ensure safe exposure limits are contained in NFPA 2001,[12] which includes design requirements that define safety limits including an agent’s NOAEL (no observed adverse effect level) and LOAEL (lowest observed adverse effect level).

Enormous progress in the search and validation for new gaseous alternatives to halon 1301 was made during the late 1980s and the first half of the 1990s. By 1995, five commercially available halocarbons and three commercially available inert gasses were determined to be acceptable for total flooding applications in occupied spaces.[13] Since that time, additional agents have been commercialized and added to the SNAP List as acceptable for total flooding applications in occupied spaces and to the list of clean agents in NFPA 2001. Also, several have been discontinued. Table 3 includes the gaseous agents concurrently listed on the SNAP List as acceptable for total flooding use in occupied spaces and the current edition of NFPA 2001. Carbon dioxide is also included because it is a gaseous agent on the SNAP List; however, it is addressed in NFPA 12.[14]

Listing and approval agencies have developed standards and guidelines for validating the design, performance, and reliability of the new alternative gaseous clean agent systems. Standards organizations in addition to NFPA have also developed clean agent system standards addressing design/performance, installation, testing and maintenance. These documents provide a means to achieve uniformity in design, quality of components and installation, and performance effectiveness and reliability. Key examples are shown in Table 4.

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Water Mist Alternatives to Halon 1301

Advances in water mist technology since the signing of the Montreal Protocol in 1987 have been significant for two primary reasons. The lack of a drop-in replacement for halon 1301 motivated renewed interest in water mist technology to fill the void and improved the economics resulting in more research funding. Also, in 1995, the International Maritime Organization implemented a new requirement mandating the installation of automatic marine sprinklers on all ships capable of carrying 35 or more passengers. This new requirement motivated interested parties globally to develop water mist technology further as a means to comply with the objectives of the sprinkler requirement with much less water and overall additional weight.[26] By 1996, nearly 50 agencies worldwide were engaged in some form of fundamentals research or applications development involving water mist technologies.[27]

Water, as an extinguishing agent, was approved by the EPA as an acceptable alternative to halon 1301 and placed on the SNAP List with its first publication in 1994. The general category of water mist systems was added to the SNAP List in 1995.

EPA did raise several concerns regarding the lack of evaluation of the health risks associated with exposure to water mist.[28] The primary risks of concern included respiration of water mist with an unknown droplet size distribution with and without water additives, and the physical properties of mist which may reduce visibility, interfering with safe evacuation from the protected space. In response to these concerns, the water mist industry facilitated the process of assembling an expert panel to evaluate the health risk concerns raised by the EPA. The panel report states that water mist systems using potable water do not present a toxicological or physiological hazard, and are safe for use in occupied spaces. EPA included the use of natural seawater as an acceptable alternative to potable water in the SNAP Listing for water mist systems. Furthermore, the panel report states that water mist with additives should be evaluated on a case-by-case basis, and the EPA has included this requirement in its process for SNAP program assessments of new water mist system proposals.

By the late 1990s, water mist system research and development efforts had made substantial progress in understanding and documenting the fundamental extinguishing mechanisms involved as well as the role of spray characteristics in suppression performance.[29] Three distinct types of water mist systems have evolved in the marketplace, defined by the operating pressure:[30]

  1. Low Pressure: operating pressure ≤12.1 bar (175 psi);
  2. Intermediate Pressure: operating pressure >12.1 bar (175 psi) but ≤34.5 bar (500 psi); and
  3. High Pressure: operating pressure >35.4 bar (500 psi).

Water mist systems have solidified market share in the protection of turbine and diesel powered machinery, protection of machinery spaces aboard ships, and the protection of passenger cabins aboard ships.[31] They continue to make inroads into other market areas protecting diverse hazards including industrial hot oil cookers, flammable liquid storage, and computer data/communication rooms.

Listing and approval agencies have developed standards and guidelines for validating the design, performance, and reliability of water mist systems. Standards organizations in addition to NFPA have also developed water mist system standards addressing design/performance, installation, testing, and maintenance. These documents provide a means to achieve uniformity in design, quality of components and installation, and performance effectiveness and reliability. Key examples are shown in Table 5.

article1_table5

Hybrid Inert Gas-Water Mist Alternatives to Halon 1301

During the last decade, a distinct variant of water mist and inert gas fire extinguishing systems has emerged and is actively being further defined and developed. These systems are being called Hybrid (Water and Inert Gas) Fire Extinguishing Systems, from now on called hybrid systems. Hybrid systems employ a larger quantity of inert gas in combination with water than traditional twin-fluid type water mist systems.

The EPA, as of this writing, has added one manufacturer’s hybrid system to the SNAP List, effective January 2, 2009, as acceptable for use in occupied spaces.[38] EPA recommends that the use of this manufacturer’s system conform to the safe exposure guidelines for inert gas systems in the latest edition of NFPA 2001 (clean agent systems), specifically the requirements for residual oxygen levels, and should conform to the relevant operational requirements in NFPA 750 (water mist systems).

Research by FM Global determined that one hybrid system tested extinguished the test fires at a dry-basis oxygen concentration ≥12.5 percent and ≤16.0 percent, providing a strong indication that fire extinction was accomplished by contributions from both the inert gas and water components.[39] This research further suggests that a twin-fluid water mist system should be treated as a gaseous system if the dry-basis oxygen concentration at fire extinction is <12.5 percent, and as a water mist system if the dry-basis oxygen concentration at fire extinction is >16.0 percent.

FM Global has published an approval standard for hybrid systems (Class No. 5580[40]), which borrows heavily with from their water mist system approval standard (Class No. 5560). It is likely that other listing and approval agencies will follow suit, but this may take some considerable time. Two manufacturers have received FM approval of their twin-fluid water-inert gas extinguishing systems by FM Global’s Hybrid Systems approval standard Class No. 5580.

NFPA, in the latter part of 2014, initiated the development of a new document on hybrid systems, and approved the roster for the new Technical Committee (TC) in April 2015. The TC has held two pre-first draft development meetings as of this writing. It is estimated that the first edition of this document may be ready for approval approximately in 2019, preliminarily named NFPA 770, Hybrid (Water and Inert Gas) Fire Extinguishing systems.

Powdered Aerosol Alternatives to Halon 1301

Research and development efforts related to powdered aerosol fire extinguishing systems increased dramatically, as with most other alternative fire suppression technologies, subsequent after the signing of the Montreal Protocol in 1987.[41] Powdered Aerosol agents are typically one or a mixture of several solid salts and oxides of alkaline metals (dry chemicals) that are dispersed rapidly into a protected space by one of the several mechanisms. The dry chemical particles that are dispersed are typically <10 µm in diameter. Condensed aerosols exist as a solid, which is a mixture of the dry chemicals in powder form, an oxidizer, a reducer, and a binding resin. This solid mass is thermally ignited, and the ensuing combustion reaction ejects the combustion reaction products as a dispersion aerosol. The dry chemical particles produced in this manner are in a size range that provides maximum fire extinguishing effectiveness, and would be very challenging to create and deliver by other means.

Some testing has shown that condensed aerosol systems may not be effective extinguishing deep-seated Class A type fires.[42]

The EPA, as of this writing, has added several powdered aerosol extinguishing systems to the SNAP List as acceptable for total flooding of occupied spaces. The EPA recommends that the use of these systems conforms to the safe exposure guidelines for inert gas systems in the latest edition of NFPA 2001 (clean agent systems), specifically the requirements for residual oxygen levels, and should conform to the relevant operational requirements in NFPA 2010 (aerosol agent systems). The EPA has added several other powdered aerosol agent systems to the SNAP List as acceptable for total flooding of unoccupied spaces.

NFPA publishes a design/performance standard addressing powdered aerosol extinguishing systems.[43] NFPA 2010 was first published in 2006. The standard defines two types of aerosol extinguishing systems: Condensed Aerosol and Dispersed Aerosol. Condensed aerosols are finely divided dry chemical powders in the form of a solid aerosol forming compound that requires a combustion process to generate and disperse the aerosol. Dispersed Aerosols are finely divided dry chemical powders that are resident in a pressurized agent storage container, suspended in a halocarbon or an inert gas.

Underwriters Laboratories (UL) has published a listing standard for validating the design, performance, and reliability of fixed condensed aerosol extinguishing system units. The scope of this standard evaluates performance on surface fires involving Class A, Class B, and Class C hazards. Four manufacturers have fixed condensed aerosol extinguishing system units listed by UL by UL 2775 as of this writing. Several standards organizations have developed powdered aerosol system standards addressing design/performance, installation, testing, and maintenance. These documents provide a means to achieve uniformity in design, quality of components and installation, and performance effectiveness and reliability. Key examples of the listing and design/performance standards for powdered aerosol systems are shown in Table 6.

article1_table6

article1_table7

Oxygen Reduction Fire Prevention Systems

Oxygen reduction fire prevention systems are also commonly referred to as hypoxic air fire prevention systems. These systems were developed in the 1990s and have gained market share in many countries as alternatives to conventional fire suppression systems of all types. There are several manufacturers of these systems and their reach in the global marketplace continues to expand.

Oxygen reduction systems work by ventilating a protected volume continuously with a supply of air that has been modified such that the oxygen volume percent has been reduced[48]. The manufacturers commonly refer to this as the hypoxic air supply to the protected space. Hypoxic air is supplied and oxygen sensors in the protected space provide information to the control system to enable a specified design level of oxygen to be achieved and maintained. Typically, this design level will be in the range between 14 percent and 17 percent, and will be based in part on an evaluation by the designer of the combustible fuels present. The manufacturers claim that such a system will prevent fire ignition in the protected space. Many also claim that the hypoxic atmosphere maintained by these systems is safe for human exposure. This claim has been challenged and debated in various countries by the authorities having jurisdiction and addressed by the organizations that have written design/performance standards. This concern must be addressed on a case-by-case basis since each country will have some health standards that address human exposure to hypoxic environments in some manner, and there will be differences that will impact on how these systems will be allowed to be configured.

In the U.S., for example, The U.S. Occupational Safety & Health Administration (OSHA) Respiratory Protection Standard[49] defines as oxygen deficient any atmosphere that contains less than 19.5% oxygen and requires, generally, that all oxygen-deficient atmospheres be considered immediately dangerous to life or health (oxygen-deficient IDLH). Certainly, any country or jurisdiction with similar health and safety laws will have concerns about workplace environments that are maintained at oxygen volume percent levels between 14 percent and 17 percent for indefinite periods of time. Safety procedures would have to be developed and implemented that meet the approval of the health and safety enforcement authorities.

The EPA, as of this writing, has received no applications to review oxygen reduction fire prevention systems for inclusion on the SNAP List as an acceptable halon 1301 replacement. There is currently no NFPA design/performance standard for oxygen reduction fire prevention systems, nor is there a standard under development. Likewise, FM Global has no current plans to develop an approval standard for oxygen reduction fire prevention systems.

UL has published a listing standard for validating the design, performance, and reliability of oxygen reduction fire protection system units. The scope of this standard covers oxygen reduction fire protection system units, intended to separate oxygen and nitrogen from the ordinary air to produce a flux of reduced-oxygen (elevated nitrogen) air for limiting the potential for ignition and fire spread of combustibles in a protected volume. The standard applies specifically to oxygen reduction fire protection system units intended for design, installation, operation, and maintenance under prEN 16750. One manufacturer has oxygen reduction fire protection system units listed by UL in accordance with UL 67377 as of this writing. Several standards organizations have developed oxygen reduction system standards addressing design/performance, installation, testing, and maintenance. These documents provide a means to achieve uniformity in design, quality of components and installation, and performance effectiveness and reliability. See Table 7.

Jeff L. Harrington is with Harrington Group, Inc.

 






References

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  2. NFPA 12A, Standard on Halon 1301 Fire Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2015.
  3. 
The Clean Air Act Amendments of 1990, Public Law 101-549, 42 U.S. Code 7401, et seq.
  4. 
NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2015.
  5. Mawhinney, JR and Back III, GG, Water Mist Fire Suppression Systems, Chapter 46, p. 1588. In: “SFPE Handbook of Fire Protection Engineering, 5th edition,” Society of Fire Protection Engineers, 2016.
  6. Kibert, CJ and Douglas Dierdorf, “Solid Particulate Aerosol Fire Suppressants,” Fire Technology, November 1994; vol. 30, issue 4: pp. 387-399.
  7. 
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NFPA 2010, Standard on Fixed Aerosol Fire-Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2015.
  9. Agency Review of SNAP Submissions, Code of Federal Regulations, title 40, vol. 14, sec. 82.180(a)(7).
  10. U.S. EPA, 1994. A Guide to Completing a Risk Screen: Collection and Use of Risk Screen Data: Fire Suppression Sector, Stratospheric Protection Division, April 1994.
  11. Pitts, William M., Marc R. Nyden and Richard G. Gann, NIST Technical Note 1279: Construction of an Exploratory List of Chemicals to Initiate the Search for Halon Alternatives, National Institute of Standards and Technology, August 1999.
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  13. Su, Joseph Z., Andrew K. Kim, and Jack R. Mawhinney, Review of Total Flooding Gaseous Agents as Halon 1301 Substitutes, “Journal of Fire Protection Engineering,” May 1996; vol. 8, no. 2: pp. 45-63.
  14. NFPA 12, Standard on Carbon Dioxide Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2015.
  15. DiNenno, PJ and Forssell, EW, Clean Agent Total Flooding Fire Extinguishing Systems, Chapter 44, p. 1484. In: “SFPE Handbook of Fire Protection Engineering, 5th edition,” Society of Fire Protection Engineers, 2016.
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  17. U.S. EPA, Significant New Alternatives Policy (SNAP) Program, Acceptable and Unacceptable Substitutes, Fire Suppression and Explosion Protection, Total Flooding Agents, http://www.epa.gov/snap/substitutes-total-flooding-agents (accessed March 2, 2016).
  18. UL 2166, Standard for Halocarbon Clean Agent Extinguishing System Units, Underwriters Laboratories, Northbrook, IL, 2012.
  19. UL 2127, Standard for Inert Gas Clean Agent Extinguishing System Units, Underwriters Laboratories, Northbrook, IL, 2012.
  20. ISO 14520, Gaseous Fire Extinguishing Systems: Physical Properties and System Design, International Organization of Standardization, 2015.
  21. Approval Standard for Clean Agent Extinguishing Systems, Class No. 5600, FM Approvals, LLC, Norwood, MA, April 2013.
  22. VdS 2381 (2009-06), Fire Extinguishing Systems using Halocarbon Gases, Planning and Installation, Verband der Sachversicherer e.V., 2009.
  23. VdS 2381-S1 (2014-06), Guidelines for Fire Extinguishing Systems using Halocarbon Gases, Amendment 1, VdS, 2014.
  24. VdS 2380 (2014-06), Fire Extinguishing Systems using non-liquefied Inert Gases, Planning and Installation, Verband der Sachversicherer e.V., 2009.
  25. VdS 2381-S1 (2011-07), Fire Extinguishing Systems using non-liquefied Inert Gases, Amendment 1, VdS, 2011.
  26. Mawhinney, JR and GG Back, III, Water Mist Fire Suppression Systems, Chapter 46, p. 1589. In: “SFPE Handbook of Fire Protection Engineering, 5th edition,” Society of Fire Protection Engineers, 2016.
  27. Mawhinney, JR and JK Richardson, A Review of Water Mist Fire Suppression Research and Development, Fire Technology, January 1997; vol. 33, issue no. 1: pp. 54-90.
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  29. Su, Joseph Z. and Andrew K. Kim, “A Review of Water Mist Fire Suppression Systems: Fundamental Studies,” Journal of Fire Protection Engineering, August 1999; vol. 10, no. 3: pp. 32-50.
  30. NFPA 750, Standard on Water Mist Fire Protection Systems, National Fire Protection Association, Quincy, MA, 2015.
  31. Wickham, RT, Status of Industry Efforts to Replace Halon Fire Extinguishing Agents, Wickham Associates, March 16, 2002.
  32. UL 2167, Standard for Water Mist Nozzles for Fire Protection Service, Underwriters Laboratories, Northbrook, IL, 2002.
  33. Approval Standard for Water Mist Sprinkler Systems, Class No. 5560, FM Approvals, LLC, Norwood, MA, November 2012.
  34. VdS 3188 (2015-05), Guidelines for Water Mist Sprinkler Systems and Water Mist Extinguishing Systems (High Pressure Systems), Planning and Installation, VdS, 2015.
  35. ISO 6182-9, Fire Protection – Automatic Sprinkler System – Part 9 – Requirements and Test Methods for Water Mist Nozzles, International Organization of Standardization, 2005.
  36. MSC/Circ. 668, Alternative Arrangements for Halon Fire Extinguishing Systems in Machinery Spaces and Pump Rooms, International Maritime Organization, London, England: 1994.
  37. MSC/Circ. 728, Revised Test Method for Equivalent Water-Based Fire Extinguishing Systems for Machinery Spaces of Category A and Cargo Pump Rooms, International Maritime Organization, London, England: 1996.
  38. Determination of Acceptability, Protection of Stratospheric Ozone: Notice 23 for Significant New Alternatives Policy Program, Federal Register, vol. 74, no. 1 (January 2, 2009): 21-29.
  39. Yu, Hong-Zeng, Robert Kasiski and Matthew Daelhousen, Characterization of Twin-Fluid (Water Mist and Inert Gas) Fire Extinguishing Systems by Testing and Modeling, Fire Technology, August 2014; vol. 51, issue 4: pp. 923-950.
  40. Approval Standard for Hybrid (Water and Inert Gas) Fire Extinguishing Systems, Class No. 5580, FM Approvals, LLC, Norwood, MA, November 2012
  41. Kibert, Charles J. and Douglas Dierdorf, Solid Particulate Aerosol Fire Suppressants, Fire Technology, November 1994; vol. 30, issue 4: pp. 387-399.
  42. Back, Gerard, Michael Boosinger, Eric Forssell, David Beene, Elizabeth Weaver, and Lou Nash, An Evaluation of Aerosol Extinguishing Systems for Machinery Space Applications, Fire Technology, March 2009; vol. 45, issue 1: pp. 43-69.
  43. NFPA 2010, Standard on Fixed Aerosol Fire-Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2015.
  44. UL 2775, Standard for Fixed Condensed Aerosol Extinguishing System Units, Underwriters Laboratories, Northbrook, IL, 2014.
  45. ISO 15779, Condensed Aerosol Fire-Extinguishing Systems: Requirements and Test Methods for Components and System Design, Installation and Maintenance-General Requirements, International Organization of Standardization, 2011.
  46. MSC/Circ. 1270/Corr. 1, Revised Guidelines for the Approval of Fixed Aerosol Fire-Extinguishing Systems Equivalent to Fixed Gas Fire-Extinguishing Systems, as Referred to in SOLAS 74, for Machinery Spaces, International Maritime Organization, London, England: 2008.
  47. AS 4487, Condensed Aerosol Fire Extinguishing Systems: Requirements for System Design, Installation and Commissioning and Test Methods for Components, Standards Australia, July 17, 2013.
  48. Oxygen Reduction versus Inerting or Gas Extinguishing Systems, Tech Talk, vol. 11, pp. 1-4, Allianz Risk Consulting
  49. OSHA Respiratory Protection Standard, Code of Federal Regulations, title 29, std. 1910, sub. I, sec. 134.
  50. UL 67377, Standard for Oxygen Reduction Fire Protection System Units, Underwriters Laboratories, Northbrook, IL, 2016
  51. prEN 16750 Draft, Fixed Firefighting Systems: Oxygen Reduction Systems-Design, Installation, Planning, and Maintenance, European Committee For Standardization (CEN), May 2014.
  52. VdS 3527 (2015-05), Oxygen Reduction Systems: Planning and Installation, Verband der Sachversicherer e.V., 2015.
  53. PAS 95:2011, Hypoxic [air] Fire Prevention Systems for Occupiable Spaces: Specification, British Standards Institute, 2011.
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