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FIXED FIRE extinguishing systems can provide essential protection in commercial and industrial premises where fi re risks are high, suppressing a fi re in its incipient stages. Th ere are fi ve types of fi xed

fi re extinguishing systems – gaseous, foam, water mist, dry powder and wet chemical – which can protect enclosed areas of a site.

A knowledge and understanding of the applications and limitations of these systems is needed, in order to assess whether they provide the required protection and to make the appropriate selection. It is also important to check and ensure that a fi re extinguishing system, once installed, remains fully operational and continues to address the fi re hazard over time, including any changes to that hazard.

Th is article provides some general information on this subject. Th e newly revised standard, BS 5306-0: Fire protection installations and equipment on premises. Guide for selection of installed systems and other fi re equipment, will also assist.

Gaseous systems

Gaseous systems comprise gaseous extinguishing agent stored in cylinders under pressure. Once the cylinder valves are actuated, the agent is released into distribution pipework and on to discharge nozzles. Th e agents may be inert gases, such as carbon dioxide (CO2), nitrogen and argon, or combinations of these, which extinguish fi re by reducing the oxygen level. Alternatively, they may be chemical liquefi ed gases, which cooland inhibit combustion but can decompose in the process.

While CO2 can be used for ‘local application’ protection of objects, such as printing presses, all others are intended for

‘total fl ooding’ of enclosures. Upon discharge, the gaseous system builds up a concentration of agent in air greater than that needed to extinguish a fi re in the protected space.

As none of the agents provide any signifi cant object cooling, the fi re extinguishing concentration needs to be retained for long enough to prevent re-ignition (hold time) – usually 10 minutes for total fl ooding systems.

Th e discharge of gas into an enclosure creates a rise in pressure within the space (chemical agents also create an initial negative pressure). It is therefore necessary to establish during the design whether pressure vents are needed to limit pressure excursions to within the structural strength of the enclosure. On system completion, an integrity test is carried out using a portable, variable speed fan, to obtain data points to plot the leakage characteristic of the enclosure boundaries and to compute the hold time and area of natural leakage.

The effectiveness of a fire extinguishing system depends on the correct choice of system and riskassessors judging thatit is fit for purpose, says Bob WhiteleyKeeping fit

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Gaseous, total flooding fire extinguishing systems, both chemical and inert, are engineered to BS EN 15004: Fixed firefighting systems. Gas extinguishing systems. CO2 systems, for local application and total flooding, are engineered to BS 5306-4: Fire extinguishing installations and equipment on premises. Specification for carbon dioxide systems.

All gaseous systems are deemed ‘clean agent’ because they leave no residue and find applications in electrical risks, such as data centres, control and switchgear rooms, and telecoms. They can also be used for storage/archives, as well as flammable liquid hazards in turbine and generator enclosures. However, such systems are not suitable for hazards with poor integrity to retain the agent, or where the source of ignition remains.

To ensure an existing gaseous system is fit for purpose, it is necessary to check that:• there is the required quantity of agent• there has been no loss/leakage of agent from the cylinders• the enclosure integrity has not deteriorated• gas distribution has not been impaired by obstructions• the size and nature of the hazard has not changed• external ventilation is shut down upon actuation• the system is regularly maintained by competent personnel

Foam systems

Foam systems consist of supplies of water and foam concentrate, a foam/water metering (proportioning) system, and distribution pipework to nozzles where air is mixed into the foam/water solution. The discharged foam forms a blanket over the surface of flammable liquids to suppress the release of flammable vapours, separate the vapours from the air necessary for combustion, and cool the fuel and hot surfaces. This applies to low (typically 6:1) and medium (typically 50:1) expansion foams. High expansion foams (typically >500:1) are used to fill volumes, such as warehouses, to suffocate a fire in Class A materials.

Foams may be protein based or synthetic based. Where foams are to be used on fuels which absorb water (alcohols/polar solvents/water miscible fuels), polymers are added to form a chemical barrier around each foam bubble. Aqueous film forming foams (AFFFs) have a unique property whereby the AFFF drains quickly to become an aqueous film, which flows rapidly across the surface of hydrocarbon fuels ahead of the foam blanket to achieve rapid fire knockdown. This is achieved even when the AFFF/water solution is discharged through ordinary water nozzles (sprinklers and sprayers) with minimal expansion.

Foam systems are installed where flammable liquid hazards exist as either potential spill fires (drum/carton storage), or fuel in depth (storage tanks and bunds), where loss of containment could result in the release of flammable/toxic vapours and damage escalation. They should comply with BS EN 13565-2: Fixed firefighting systems. Foam systems. Design, construction and maintenance, although the US standards, NFPA 11, 16, and 409, are also widely used.

Fitness for purpose of existing foam systems requires that:• the foam type is suitable for the fuels• foam storage tanks are fully charged• the hazard and its configuration have not changed • water supplies are operational• foam concentrate and proportioners are performance-

checked annually• control and isolating valves are operational and in their

required status• nozzles and piping are undamaged• the system is regularly maintained by competent personnel

Water-mist systems

Developed from the 1990s, water-mist systems deliver water in very small droplets, thereby creating a relatively high surface area of water that is able to rapidly absorb large amounts of heat. As the droplets have low mass, they are readily able to be vapourised into steam, absorbing large amounts of heat in the process and expanding 1620:1. The droplets are created by discharge through small orifices, via swirl chambers or deflector plates.

A feature of water-mist systems is the relatively small quantities of water they discharge (compared with sprinkler and water-spray systems), which enables small systems to be fed from banks of nitrogen-purged water cylinders, although larger systems are fed from dedicated pumps and water

Gaseous systems are often used as total flooding solutions, to protect enclosures within IT data centres or storage archives

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storage tanks. Extinguishing systems may be local application for object protection or total flooding for protection of enclosures involving flammable liquids or gases.

Water-mist systems usually operate with nozzle pressures of either 100 bar (high pressure) or 12 bar (low pressure). Due to the small nozzle orifices, used to produce the low flows, system pipework needs to be corrosion-resistant and the water clean to prevent nozzle blockage.

Water-mist using nozzles which activate individually due to the heat of a fire (with frangible bulb release elements) are used to protect low fire loads, such as accommodation, data centres and offices. Open nozzles can provide local application or volume protection of machinery, including engines, turbines and pumps. Other applications include industrial oil cookers. Any and all mist systems can only be used for applications for which full-scale fire testing has been carried out to determine their design basis.

When it comes to standards for land-based water-mist systems, BS DD 8489 and 8458 have been recently published and CEN TS 14972 is available.

Water-mist systems that are fit for purpose are those where:• the design is supported by a fire test report• the hazard and its configuration have not changed• nozzles are not obstructed, damaged or clogged• water supplies are operational• control and isolating valves are operational and in their

required status• the system is regularly maintained by competent personnel

Dry powder systems

Dry powder systems (sometimes called dry chemical systems) are essentially large fire extinguishers piped to discharge nozzles. They can provide local application or total flooding fire extinguishing systems for flammable liquids, gases or combustible metals. Due to the corrosion and contamination created by the distribution of the fine particulate powder, these systems are not in common usage, although they are highly effective.

Systems are designed to either BS EN 12416-2: Fixed firefighting systems. Powder systems or NFPA 17. Such systems are fit for purpose when:• the powder type is suitable for the fuels• the powder tanks and nitrogen cylinders are fully charged• the hazard and its configuration have not changed• nozzles and piping are undamaged• valves are free-moving and in their operational positions• the powder is dry and not compacted• the system is regularly maintained by competent personnel

Wet chemical systems

Wet chemical systems are used to protect commercial cooking equipment, such as deep-fat fryers, griddles, woks, grease filters and extract exhaust plenum. The systems comprise cylinders containing a foam-like chemical connected to distribution pipework and nozzles. The discharged wet chemical forms froth on or over hot oil and grease, to blanket flammable liquids, suppress the release of

flammable vapours, displace air from the fuel surface, and cool hot surfaces. A dedicated heat detection system triggers the release of nitrogen to purge the wet chemical from the cylinders.

The systems are designed to NFPA 18 and carry Under-writers Laboratories and/or Loss Prevention Council type approvals. Such systems are fit for purpose when:• the nozzles are clean and/or capped• nozzles are aimed at the fire hazards• the wet chemical cylinders are fully charged• the nitrogen purge cylinders are full • the detection and actuation system is operational• piping is undamaged• the system is regularly maintained by competent personnel

Effective protection

Fixed fire extinguishing systems have various applications and limitations, so a knowledge of what they do and how they work is important when selecting suitable systems to protect life and property. The use of suitable and correct codes and standards will ensure best engineering practice is adopted and a validated approach used.

Where systems have been installed to protect an asset, the risk assessor needs to be able to judge the suitability/fitness for purpose of the chosen system for the hazard. The assessor must be able to assess whether it will achieve the expected outcome, should a fire occur.

The effectiveness of the system depends upon the correct system being chosen for the application, and it must be correctly engineered and adequately maintained. Checks must also be made to ensure that changes in the nature and geography of the hazard have not jeopardised the fire protection outcome

Bob Whiteley is chairman of the Fire Industry Association’s Foam and Water Mist Groups

Water-mist systems create a very high surface area of water that can rapidly absorb large amounts of heat