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Carbon Dioxide Co2
NFPA 12: Standard on Carbon Dioxide Extinguishing Systems

Carbon Dioxide System

Carbon dioxide is a clean gaseous agent, in plentiful supply and can be stored in either high pressure cylinders or low pressure tanks. It has been used effectively as a fire extinguishing gas since the early 1900’s.Carbon dioxide continues to be used in numerous applications around the world for the extinguishment of flammable liquid fires, gas fires, electrically energized fires and, to a lesser degree, fires involving ordinary cellulosic materials such as paper and cloth. Carbon dioxide can effectively suppress fires of most materials with the exception of active metals, metal hydrides, and materials containing their own oxygen source, such as cellulose nitrate. The use of carbon dioxide is limited primarily by the factors influencing its method of application and its intrinsic health hazards.

Carbon dioxide is used internationally in marine applications in engine rooms, paint lockers, vehicle transport areas on cargo vessels, and in flammable liquid storage areas. Large marine engine room systems may require as much as 20,000 lb of carbon dioxide per system. Carbon dioxide fire suppression systems are currently being used by the U.S. Navy and in commercial shipping applications.

The steel and aluminum industries also rely heavily on carbon dioxide fire protection. In the aluminum industry, for example, the rolling mill process requires the use of kerosene-like lubricants and coolants. Fires are prevalent in this application, occurring on the average of 1 per week in the typical aluminum plant. One particular aluminum processing company averages about 600 system discharges per year worldwide in all their fire protection applications using carbon dioxide, such as rolling mills, control rooms, and aluminum sheet printing. Many carbon dioxide systems in the metal processing industry are rapid discharge local application systems. In these applications, the carbon dioxide storage containers are located close to the outlet nozzles such that liquid carbon dioxide starts to discharge from the nozzle(s) in under 5 seconds. These local application carbon dioxide systems range in size from 800 to 10,000 lb of compressed carbon dioxide.

Carbon dioxide systems also are used in computer rooms (subfloor), wet chemistry benches, particle board chippers, equipment dust collectors, printing presses, cable trays, electrical rooms, motor control centers, switch gear locations, paint spray booths, hooded industrial fryers, high-voltage transformers, nuclear power facilities, waste storage facilities, aircraft cargo areas, and vehicle parking areas (Willms 1998, Wysocki 1998). Small carbon dioxide systems, such as those protecting paint lockers or fryers, use approximately 50 lb of carbon dioxide. Other systems use an average of about 300 to 500 lb of carbon dioxide, but can use as much as 2,500 lb.

Several properties of carbon dioxide make it an attractive fire suppressant. It is not combustible and thus does not produce its own products of decomposition. Carbon dioxide provides its own pressurization for discharge from a storage container, eliminating the need for superpressurization. It leaves no residue, and hence precludes the need for agent clean up. (Clean up of fire-released debris would, of course, still be necessary in the case of a fire event.) Carbon dioxide is relatively nonreactive with most other materials. It provides three-dimensional protection because it is a gas under ambient conditions. It is electrically nonconductive and can be used in the presence of energized electrical equipment.

Extinguishing Mechanism of Carbon Dioxide

Flame extinguishment by carbon dioxide is predominantly by a thermophysical mechanism in which reacting gases are prevented from achieving a temperature high enough to maintain the free radical population necessary for sustaining the flame chemistry. For inert gases presently used as fire suppression agents (argon, nitrogen, carbon dioxide, and mixtures of these), the extinguishing concentration (As measured by the cup burner method (NFPA 2001)) is observed to be linearly related to the heat capacity of the agent-air mixture.

Although of minor importance in accomplishing fire suppression, carbon dioxide also dilutes the concentration of the reacting species in the flame, thereby reducing collision frequency of the reacting molecular species and slowing the rate of heat release.

Extinguishing Effectiveness of Carbon Dioxide

Carbon dioxide is the most commonly used "inert" gas extinguishing agent, followed by nitrogen (Friedman 1992). On a volume basis, carbon dioxide is approximately twice as effective as nitrogen (e.g., for ethanol fires, the minimum required volume ratios of carbon dioxide and nitrogen to air are 0.48 and 0.86, respectively). However, because carbon dioxide is 1.57 times heavier than nitrogen [44 and 28 molecular weight (MW), respectively] for a given volume, the two gases have nearly equivalent effectiveness on a weight basis.

Gas Volume Equivalent (GVEq) = vol. ratio for N2 / vol. ratio for CO2 =1.8
Weight Equivalent = GVEq x MWN 2 / MWCO2 = 1.1

The amount of carbon dioxide needed to reduce the oxygen level to a point at which various fuels are prevented from burning is relatively high and is also at a level where humans will suffer undesirable healTheffects. Table 1 presents the minimum required ratios of carbon dioxide to air (v/v), the corresponding oxygen concentration that will prevent burning of various vapor fuels at 25 degrees C, the theoretical minimum carbon dioxide concentration, and the minimum design concentration of carbon dioxide for various fuels.

Use of Carbon Dioxide Extinguishing Systems

Carbon dioxide fire extinguishing systems are useful in protecting against fire hazards when an inert, electrically nonconductive, three-dimensional gas is essential or desirable and where clean up from the agent must be minimal. According to the NFPA, some of the types of hazards and equipment that carbon dioxide systems protect are "flammable liquid materials; electrical hazards, such as transformers, switches, circuit breakers, rotating equipment, and electronic equipment; engines utilizing gasoline and other flammable liquid fuels; ordinary combustibles such as paper, wood, and textiles; and hazardous solids" (NFPA 12).

Safety Measures

As with other fire protection systems, a number of regulatory agencies or authorities having jurisdiction (AHJ) administer the design, installation, testing, maintenance, and use of carbon dioxide systems. The authority that regulates the system depends on where the system is located, the intended scenario, and the type of system. Many AHJs that regulate industrial, commercial, and nonmarine applications utilize the NFPA consensus standard covering carbon dioxide extinguishing systems (NFPA 12). Although the standard itself does not hold the force of law, governments and local authorities adopt the standard as their governing fire code. Marine applications are regulated depending on whether the vessels navigate domestic or international waters. U.S. Coast Guard (USCG) regulations pertain to ships in domestic waters and are published in the Code of Federal Regulations (46 CFR Part 76.15). Internationally registered vessels are covered under the International Maritime Organization's Safety of Life at Sea (SOLAS) (IMO 1992). In workplaces that are land-based, the Occupational Safety and Health Administration (OSHA) regulates the exposure to carbon dioxide in order to ensure worker safety.

Design, Specification, and Component Approval

Generally, the process of acquiring fire suppression system approval starts with the manufacturer "listing" its components through organizations such as Underwriters Laboratory or Factory Mutual in the United States. Part of the listing process is the development of an instruction and maintenance manual that includes a description of the full operation of the system along with system drawings. Specifications or plans for the carbon dioxide system are prepared under the supervision of an experienced and qualified person knowledgeable in the design of carbon dioxide systems and with the advice of the AHJ. The designs are then submitted to the AHJ before installation begins.

Installation and Testing

Installation of the carbon dioxide system is usually performed by manufacturers' representatives or distributors. Although the installers are not given a formal accreditation or certification, they are trained by the manufacturer regarding proper installation of system components. The completed system is inspected and tested by appropriate personnel to meet the approval requirements of the AHJ. Often these requirements include:

(A) Performance of a full discharge test of the entire design quantity through the piping and into the intended hazard area, for each hazard area, if the system protects more than one. A check to verify that the design concentration is achieved and maintained for the specified hold time applies to total flooding type systems only.
(B) Operational checks of all devices necessary for proper functioning of the system, including detection, alarm, and actuation.
(C) Checks for proper labeling of devices and protected areas warning occupants of the possible discharge of carbon dioxide. In addition, signage must be present to warn personnel to vacate the area when the alarm sounds. (No foreign language requirements (e.g., Spanish) for signage are specified by U.S. AHJs. Ideally all labels and warning signs should be printed both in English and in the predominant language of non-English-reading workers (NIOSH 1976))
(D) Complete inspections of the system and the hazard area to ensure that the system meets the specifications and that it is appropriate for the type of fire hazard.