The SABS has approved and is legislating the adoption of ISO 14520 as the clean agent gas extinguishing standard for South Africa. In this issue we look at ISO 14520 Part 1, which includes use limitations, safety and system designs.
The demise of halon gas brought about a large variety of gas extinguishing systems being released onto the world markets. ISO 14520 lists 13 different gas fire extinguishants from various manufacturers but many other manufacturers have produced systems matching the physical properties of those listed.
If one considers the physical and chemical properties of these various agents and forgets the trade names the choice becomes a lot easier. Out of the 13 listed agents in ISO 14520 only five are actively promoted in South Africa.
The first two extinguishing and chemical agents are known as halocarbons, agents that extinguish the fire mainly by physical means. Containers are super-pressurised with nitrogen to achieve the pressure rating of either 25 or 42 bar. Their benefits include relatively low design concentrations typically from 8,5 to 12,8%, thereby providing smaller storage requirements and the ability to provide modular systems requiring no pipework and therefore low installation costs. Low hardware costs are balanced by high gas cost and therefore high re-filling costs.
HFC BLEND A
Chemical formula
Has been banned in many European countries for use in firefighting due to its decomposition products, health concerns and environmental impact problem. Its popularity is waning and use restrictions are being imposed around the world.
HCF 227 ea
Chemical formula: CF3CHFCF3
Chemical name: Heptafluoropropane
Is a popular product installed throughout the world by many of the larger contractors, manufactured by Great Lakes and distributed by many companies and available from several suppliers within South Africa.
IG products
The IG products all work by reducing the oxygen content within the room or enclosure, to below 15% and above 10%. The majority of fires cannot be sustained with less than 15% oxygen and humans can survive in rarefied oxygen atmospheres down to 10% oxygen availability.
Design concentrations for these agents vary from 36 to 48% requiring far more gas than chemical agents. Gas is stored in its natural form in either 150 or 200 bar containers. Cylinders are generally banked via a common manifold and piped into the protected space to discharge nozzles. High hardware costs are balanced with low gas cost and therefore more cost-effective refills.
Use and limitations
Throughout ISO 14520 the word 'shall' indicates a mandatory requirement, the word 'should' indicates a recommendation or that which is advised but not required.
The design, installation, service and maintenance of gaseous fire-extinguishing systems shall be performed by those competent in fire extinguishing system technology.
The hazards against which these systems offer protection, and any limitations on their use, shall be contained in the system supplier's design manual.
Total flooding fire-extinguishing systems are used primarily for protection against hazards that are in enclosures or equipment that, in itself, includes an enclosure to contain the extinguishant. The following are typical of such hazards:
(a) Electrical and electronic hazards.
(b) Telecommunications facilities.
(c) Flammable and combustible liquids and gases.
(d) Other high-value assets.
Extinguishants
The extinguishants referred to in ISO 14520 are electrically nonconductive media and shall not be used on fires involving the following unless relevant testing has been carried out to the satisfaction of the authority:
(a) Chemicals containing their own supply of oxygen, such as cellulose nitrate.
(b) Mixtures containing oxidising materials, such as sodium chlorate or sodium nitrate.
(c) Chemicals capable of undergoing autothermal decomposition, such as some organic peroxides.
(d) Reactive metals (such as sodium, potassium, magnesium, titanium and zirconium), reactive hydrides, or metal amides, some of which may react violently with some gaseous extinguishants.
(e) Environments where significant surface areas exist at temperatures greater than the breakdown temperature of the extinguishing agent and are heated by means other than the fire.
Electrostatic discharge
Care shall be taken when discharging extinguishant into potentially explosive atmospheres. Electrostatic charging of conductors not bonded to earth may occur during the discharge of extinguishant. These conductors may discharge to other objects with sufficient energy to initiate an explosion. Where the system is used for inerting, pipework shall be adequately bonded and earthed.
Compatibility with other extinguishants
Mixing of extinguishants in the same container shall be permitted only if the system is approved for use with such a mixture. Systems employing the simultaneous discharge of different extinguishants to protect the same enclosed space shall not be permitted.
Temperature limitations
All devices shall be designed for the service they will encounter and shall not readily be rendered inoperative or susceptible to accidental operation. Devices normally shall be designed to function properly from -200 to +500°C, or marked to indicate temperature limitations, or in accordance with manufacturers' specifications which shall be marked on the nameplate, or (where there is no nameplate) in the manufacturer's instruction manual.
Safety
Hazard to personnel
Any hazard to personnel created by the discharge of gaseous extinguishants shall be considered in the design of the system, in particular with reference to the hazards associated with particular extinguishants in the supplementary parts of ISO 14520. Unnecessary exposure to all gaseous extinguishants shall be avoided. Adherence to ISO 14520 does not remove the user's statutory responsibility to comply with the appropriate safety regulations.
The decomposition products generated by the clean agent breaking down in the presence of very high amounts of heat can be hazardous. All of the present halocarbon agents contain fluorine. In the presence of available hydrogen (from water vapour, or the combustion process itself), the main decomposition product is hydrogen fluoride (HF).
These decomposition products have a sharp, acrid odour, even in minute concentrations of only a few parts per million. This characteristic provides a built-in warning system for the agent, but at the same time creates a noxious, irritating atmosphere for those who must enter the hazard following a fire.
The amount of agent that can be expected to decompose in extinguishing a fire depends to a large extent on the size of the fire, the particular clean agent, the concentration of the agent, and the length of time the agent is in contact with the flame or heated surface. If there is a very rapid build-up of concentration to the critical value, then the fire will be extinguished quickly and the decomposition will be limited to the minimum possible with that agent. Should that agent's specific composition be such that it could generate large quantities of decomposition products, and the time to achieve the critical value is lengthy, then the quantity of decomposition products can be quite great. The actual concentration of the decomposition products then depends on the volume of the room in which the fire was burning and on the degree of mixing and ventilation.
Clearly, longer exposure of the agent to high temperatures would produce greater concentrations of these gases. The type and sensitivity of detection, coupled with the rate of discharge, should be selected to minimise the exposure time of the agent to the elevated temperature if the concentration of the breakdown products is to be minimised. Nonliquefied agents do not decompose measurably in extinguishing a fire. As such, toxic or corrosive decomposition products are not found. However, breakdown products of the fire itself can still be substantial and could make the area untenable for human occupancy.
Safety precautions
For normally occupied areas
The minimum safety precautions taken shall be in accordance with the table.
The maximum concentration shall not exceed the lowest observable adverse effect level (LOAEL) for the extinguishant used unless a lock-off valve is fitted. It is recommended that systems where the no observable adverse effect level (NOAEL) is expected to be exceeded be placed in nonautomatic mode whilst the room is occupied. Any change to the enclosure volume, or addition or removal of contents that was not covered in the original design will affect the concentration of extinguishant. In such instances the system shall be recalculated to ensure that the required design concentration is achieved and the maximum concentration is consistent with the table.
For unoccupiable areas the maximum concentration may exceed the LOAEL for the extinguishant used, without the need for a lock-off valve to be fitted.
In areas, which are protected by total flooding systems, and which are capable of being occupied, the following shall be provided.
(a) Time delay devices:
1. For applications where a discharge delay does not significantly increase the threat from fire to life or property, extinguishing systems shall incorporate a pre-discharge alarm with a time delay sufficient to allow personnel evacuation prior to discharge.
2. Time delay devices shall be used only for personnel evacuation or to prepare the hazard area for discharge.
(b) Automatic/manual switch and lock-off devices.
(c) Exit routes, which shall be kept clear at all times, and emergency lighting and adequate direction signs to minimise travel distances.
(d) Outward-swinging self-closing doors which can be opened from the inside, including when locked from the outside.
(e) Continuous visual and audible alarms at entrances and designated exits inside the protected area and continuous visual alarms outside the protected area, which operate until the protected area, had been made safe.
(f) Appropriate warning and instructions signs.
(g) Where required, pre-discharge alarms within such areas that are distinctive from all other signals that will operate immediately on commencement of time delay upon detection of the fire.
(h) Means for prompt natural or forced-draft ventilation of such areas after any discharge of extinguishant. Forced-draft ventilation will often be necessary. Care shall be taken to completely dissipate hazardous atmospheres and not just move them to other locations, as most extinguishants are heavier than air.
(i) Instructions and drills of all personnel within or in the vicinity of protected areas, including maintenance or construction personnel who may be brought into the area, to ensure their correct actions when the system operates.
In addition to the above requirements, the following are recommended:
* Self-contained breathing apparatus should be supplied and personnel trained in its use.
* Personnel should not enter the enclosure until it has been verified as being safe to do so.
Electrical hazards
Where exposed electrical conductors are present, clearances shall be provided, where practicable, between the electrical conductors and all parts of the system that may be approached during maintenance. Where these clearance distances cannot be achieved, warning notices shall be provided and a safe system of maintenance work shall be adopted. The system should be so arranged that all normal operations can be carried out with safety to the operator.
Electrical earthing and electrostatic discharge
Systems within electrical substations or switchrooms shall be efficiently bonded and earthed to prevent the metalwork becoming electrically charged or at risk from electrostatic discharge.
System design
All ancillary systems and components shall comply with the relevant national or international standards.
Extinguishant supply
Quantity
The amount of extinguishant in the system shall be at least sufficient for the largest single hazard or group of hazards that are to be protected against simultaneously. Where required, the reserve quantity shall be as many multiples of the main supply as the authority considers necessary. Where uninterrupted protection is required, both the main and reserve supply shall be permanently connected to the distribution piping and arranged for easy changeover.
Container arrangement
Arrangement shall be made for container and valve assemblies and accessories to be accessible for inspection, testing and other maintenance when required. Containers shall be adequately mounted and suitably supported according to the systems installation manual so as to provide for convenient individual servicing of the container and its contents.
Storage containers shall not be located where they will be subjected to severe weather conditions or to potential damage due to mechanical, chemical or other causes. Where potentially damaging exposure or unauthorised interference are likely, suitable enclosures or guards shall be provided. Direct sunlight has the potential to increase the container temperature above that of the surrounding atmospheric temperature.
For further details contact the FDIA on tel: (011) 397 1618, [email protected]
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