CO2

Carbon Dioxide

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 that necessary to extinguish fires, therefore it can only be discharged with safety once areas are unoccupied.

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 unequaled 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.

System engineering.

For so called 'high pressure' systems, the carbon dioxide is stored as a liquid in cylinders at a pressure of about 60 bar at 20^oC. Cylinders can be racked together in cylinder banks, from 1 or 2 cylinders to as many as 160 cylinders for a risk. For larger volume protection, the carbon dioxide can be stored in refrigerated tanks at a temperature of -20^oC, in quantities of from 2 to 100 tonnes. Several areas can be protected from the same central bank of cylinders or low pressure storage tank. Systems can be designed with long pipe runs, so that multiple risks at some distance from each other can be protected by the same supply system.

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.

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.