Carbon dioxide is an effective clean agent. It can be used to protect a wider range of risks than any other clean agent ... and at lower cost.
It was first used for fire protection 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.
What is carbon dioxide?
Carbon dioxide is a colourless, odourless and electrically non-conducting inert gas which is almost 11/2 times as dense as air: about 0,3% of the air we breathe is carbon dioxide. It is readily available throughout the industrialised world (one of the main reasons for its historical popularity).
How does it extinguish fire?
Carbon dioxide is used to reduce the oxygen content of the atmosphere from its normal level of approximately 21% to a level that will no longer support combustion. Most fires are extinguished once the oxygen level is reduced to 15%, but some situations require a greater reduction.
The cooling effect of carbon dioxide when discharged also contributes to fire fighting. The gas is stored under pressure as a liquid and considerable cooling results from its rapid vaporisation at discharge, characterised by 'fogging' in the discharge area.
Carbon dioxide and the environment
Mindful of the withdrawal of halon on environmental grounds, it is worth pointing out that carbon dioxide has no effect on the ozone layer. When used to extinguish fire, it does not produce any environmentally harmful decomposition products.
Its global warming potential (GWP) when used as an extinguishant is generally disregarded on the basis that the gas used is an industrial by-product that would otherwise be discharged to atmosphere. There are no environmental restrictions on discharge testing carbon dioxide systems.
Suitable applications
Carbon dioxide is suitable for fighting class A, class B and live electrical apparatus fires. It is also suitable for deep-seated fires such as document and tape stores. Standards exist for 'local application' as well as 'total flooding' systems.
It is not effective on fires involving reactive metals or chemicals, which generate their own oxygen. Upon discharge, electrostatic discharges may be produced which make carbon dioxide unsuitable for inerting explosive atmospheres.
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.
Types of CO2 systems
Carbon dioxide can be used as an extinguishant in the following fixed installations:
* Total flooding system. This description is applied when the risk is contained in a room or other enclosure, which will retain the extinguishing atmosphere. Small uncloseable openings can be compensated for by allowing additional carbon dioxide.
* Local application system: where the risk to be protected is not enclosed, or can only be partially enclosed, different design criteria are needed to engineer a 'local application' system.
* In addition, protection can be provided via a manual hose reel connected to a supply of carbon dioxide.
High pressure or low-pressure storage?
The majority of systems in existence are 'high pressure' systems. In modern versions the extinguishant is stored at ambient temperature in cylinders containing 45 kg of carbon dioxide. The storage pressure at room temperature is approximately 59 bar.
A characteristic of stored carbon dioxide is that the pressure of vapour above the liquid carbon dioxide remains unchanged as the level of carbon dioxide reduces. Consequently, pressure gauges cannot be used for contents monitoring. Where increased system integrity is required, cylinders are supplied with weight monitoring devices, which raise the alarm when leakage occurs.
Alternatively, carbon dioxide can be stored in refrigerated bulk storage vessels at -180°C, in these 'low pressure' systems; the corresponding storage pressure is approximately 21 bar. Low-pressure storage is only cost- effective on large installations, but can offer benefits in the form of reduced storage space and weight, reserve supply from the same vessel and pipework, and inclusion of a liquid level monitoring facility.
Safety considerations
At the concentrations employed in total flooding fire-fighting installations, carbon dioxide is both toxic and an asphyxiant.
Vigilance is required with respect to the following safety requirements:
* All exit doors from the protected space should open outwards. (Self-closing mechanisms are generally fitted to aid retention of extinguishant).
* If doors can be locked, then exit from the protected space should still be possible by means of escape deadlocks or similar mechanisms.
* Local application systems are generally installed because the risk is not enclosed and it follows that escape is generally easily achieved. However, consideration must always be given to the concentration of carbon dioxide, which results in the room or factory as a whole.
* If there is a possibility that the extinguishant could drift and accumulate at low level, for example in pits or basement areas, consideration should be given to adding an odoriser to the carbon dioxide to aid detection.
* Indication of each entrance to the protected area.
* Audible and visual warning of imminent discharge should be provided within the protected space.
* Care should be taken to include the minimum safety precautions outlined in BS5306: Part 4: 1986 for areas 'normally occupied' or 'not normally occupied' as appropriate.
In addition the specifier should give full consideration to the following aspects:
* The method of providing switching from 'automatic/manual' to 'manual only' mode upon entry; this may be achieved by switching a microswitch in the door lock or through an interlocking arrangement.
* Where fast response detectors are employed to activate the extinguishant system, these are generally configured to operate on a coincidence circuit ('double knock') principle and the system will incorporate a time delay prior to discharge.
* Consideration should be given to whether the protected area can be naturally ventilated following discharge, or low level extraction needs to be installed for the purpose.
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 time of 20 minutes or more. The extinction of fires in ships, cargo spaces often requires far longer hold times. Where flammable liquids may be involved, higher concentrations may be needed, as indicated in the standards.
Conclusion
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.
For details contact the FDIA on tel: (011) 397 1618, or e-mail: [email protected]
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