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co2-2Carbon dioxide is a most effective clean agent. It can be used to protect a wider range of risks than any other clean agent and at lower cost! The first use for fire protection was in 1902, when a fire chief was faced with a mountain of coal dust, burning in the centre. The Lux factory nearby was producing large volumes of CO2 as a by-product. The fire chief organised the piping of CO2 to the deep-seated fire, which all other means had failed to extinguish, and within a few hours it was under control. The value of CO2 as a fire extinguishant was recognised in the USA, the concept developed, and the first six page standard for the systems was published by the NFPA in 1928. Before halon was even conceived, carbon dioxide systems had already extinguished thousands of fires.

Given the development of new gaseous halon alternatives, such as FM 200 and Inert Gas, and fine water spray systems, which are each, excellent in their own right, why are carbon dioxide systems finding increasing use in both land based and marine applications?

Carbon dioxide as a fire extinguishant

Carbon dioxide extinguishes fire mainly by displacing oxygen in the flame zone with an inert atmosphere. There are also degrees of physical and chemical action. It is therefore a little more efficient than the inert gases, requiring lower concentrations for a given application. At the other end of the spectrum of uses, since it is in a liquid state until it reaches the nozzle, it has a reasonable ‘throw’ and thus can be readily used to smother fires on the surface of flammable liquids in local application mode.

Carbon dioxide is about 1.5 times as heavy as air, but readily mixes in an enclosure to form a uniform concentration. It is a colourless, odourless, electrically non-conductive gas. On discharge it leaves no residue. It is almost completely inert, not breaking down in a fire to form corrosive by-products. However, although it is non-toxic at low concentration levels, it becomes hazardous at concentration levels below those necessary to extinguish fires, and therefore it can only be discharged with safety once areas are unoccupied. With the appropriate safety measures in place it is ideally suited to:

  • Permanently unoccupied areas
  • Areas where evacuation of personnel can be assured before discharge of agent
  • Local application to plant or flammable liquid hazards in larger enclosures
Environmental impact

Carbon dioxide naturally exists in small quantities in the lower atmosphere. It is a by-product of many industrial processes, available worldwide at low cost to a consistent specification for many uses, such as in the soft drinks industry. Although carbon dioxide is known as a ‘green house’ gas, its use in fire protection is environmentally neutral.

Carbon dioxide for fire protection systems is either drawn from atmosphere, or is taken as a waste by-product from other processes. Since it is stored in cylinders until a fire occurs, only a small proportion ever reaches the atmosphere. Carbon dioxide does not contribute to ozone breakdown. Table 1 shows the relative environmental impact of carbon dioxide in comparison with halon and other gases, and potential agent breakdown in fire.

Table 1 Comparison of agents for environmental impact and agent breakdown
Ozone depleting potential Global warming potential vs CO2 (100 yrs) Agent breakdown to corrosive by-product
Carbon Dioxide zero 1 none
Halon 1301 x 16 3,500 HF
FM 200 zero 2,900 HF x 7
Argon zero zero none
Typical applications

Unlike the other halon alternatives, CO2 systems can be designed for use in a variety of ways. The wide range of techniques used means that skilled fire protection engineering is vital. Four main techniques for its use are described in the Standards:

  • total flooding for surface fires and deep seated fires
  • local application for surface application,
  • local application for volume or object protection,
  • extended discharge for rotating machinery and turbines
  • snuffing out of vent tip fires in petrochemical risks.

A very wide range of hazards can be protected, such as:

  • switch rooms, control cabinets, floor voids, electrical and electronic equipment rooms, engine test bays, ships engine and cargo holds
  • drying ovens, large electrical and electronic cabinets
  • paint spray booths, dip varnishing units, vapour exhaust systems
  • transformers, generators, turbines
  • flammable liquids and chemical products, foam materials
Deep-seated burning risks

Carbon dioxide offers an unequalled capability in the control of ‘deep-seated’ fires. Whether or not a system needs to be designed to meet the challenge of a deep-seated fire depends upon skilled risk assessment. Most fires in electrical/electronic equipment, ducts and voids can be treated as surface fires if an appropriate fire detection system is employed. Fires in paper stores, archives and bundled cable runs are more likely to become deep seated, and require greater concentrations and longer hold times, which are given with other requirements in the Standards.

Safety systems

For all gaseous fire extinguishing systems, it is now normally required that the room or enclosure be evacuated before the gas is discharged. Usually systems are on automatic control, and discharge takes place after a time delay sufficient to allow evacuation. With carbon dioxide much stricter controls are necessary. If it is physically possible for persons to be in the protected area a any time, then a ‘lock-off’ system must be installed to ensure that there can be no discharge whilst the area is occupied.

In recent years, very secure and reliable ‘lock off’ procedures and equipment have been developed, which we incorporates in system design to ensure safety.

Design concentrations

For total flooding applications, the basic design concentration is 34% by volume, except where fires might become deep seated, in which case concentrations of up to 75% can be required, with hold times of 20 minutes or more. The extinction of fires in ships’ cargo holds often requires far longer hold times. Where flammable liquids may be involved, higher concentrations may be needed, as indicated in the Standards.co2-1

Quality and reliability of installed systems.

As has already been indicated, although carbon dioxide itself is one of the simplest extinguishants available, system design requirements can vary greatly. The design of safety equipment and procedures is critical, and the absolute reliability of systems and interfaces is vital for effective system operation in the event of fire. Accordingly, the use of very well proven equipment such as valves, nozzles, detection and electronic controls is imperative.

Even the best equipment can fail to operate satisfactorily if it is not installed correctly, and companies such as Universal Fire put a great deal of effort in ensuring correct installation.

Conclusions

More than 70 years of use in fire protection systems, plus the very wide range of risks that carbon dioxide systems can protect, establishes the platform for the optimum application of carbon dioxide today. Despite the good track record of performance in a multiplicity of applications, there is still a need to ensure that systems are designed, installed and maintained by skilled experts, from approved companies with long standing expertise in this field of fire protection.