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Fire safety's hidden agenda

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In the last of a series of fire engineering articles, Lawrence Webster Forrest advises on reducing unseen fire hazards Modern and complex building design, especially in large commercial or industrial projects, ultimately leads to an ever-increasing demand for a more extensive and varied form of building services installation. For aesthetic reasons, such services are often housed in purpose-built shafts, or ducts hidden from view.

However, due to their sheer size and nature, inherent fire risks associated with the function and form of shafts may pose a significant threat to both life and property. Therefore, it is essential that architect, and building services' engineers give careful thought to these potential risks at the initial design stage of the project.

For example, it is essential to consider in detail the nature of the operations likely to be carried out within ducts and shafts during the projected life of the building, to ensure that, internally, the services' configuration allow adequate space for the safe movement of maintenance personnel and fire fighters.

Also, regular maintenance of building services plays an essential role in preventing fires due to non-maintained or neglected services. So not only do the structural characteristics of service shafts or ducts have to be considered in detail (the dimensions, type of construction and means of ventilation), but designers should also consider what goes on inside them.

Lagging

Fires within service ducts can be caused by combustible materials in contact with, or close to, hot pipes and surfaces, faulty electrical cables or flammable gas/liquid leakage from piped services. Ensuring a correct choice of equipment and a reduction in the number of other services run in the same duct or in close proximity will help to reduce the inherent risks. For example, electrical services should not be installed within air ducts. Furthermore, pipes hotter than 100infinityC should be separated from combustible parts or contents of the building by an air space of at least 50mm non-combustible materials.

Pipes conveying flammable gases or liquids should not be allowed to run adjacent to hot plant and service pipes, although if this proves to be unavoidable, then good practice dictates the hot pipes be routed above the flammable services. Ventilation of such areas becomes all the more important when flammable gas and liquids are present.Without adequate ventilation, the potential for explosion becomes a real possibility, especially where services are old or prone to damage. For this reason it is imperative that services containing flammable liquids or gases be correctly jointed and strong enough to withstand accidental damage.

Materials which are easily melted or damaged by fire, such as plastics and aluminium, must never be used to carry flammable liquids, gases or compressed air. If the contents of these types of services do escape then it is inevitable that they will both assist and accelerate the combustion process in any fire situation. In the event of a fire breaking out it is of the utmost importance that fuel supplies within these areas are immediately restricted. Although building plant can be interfaced with modern fire alarm and detection systems to ensure all plant and operating equipment be shut down upon activation of the fire alarm, it is still a pre-requisite of services design that emergency cut-off valves be conveniently sited so that hazardous supplies can be cut off in an emergency. Such valves are generally provided at the bulk supply point; at the start of each branch line from a main supply line; at the entry point into each building; and in close proximity to the item of plant that is being supplied by the service.

In addition to the services content, associated features which have been known to spread fire and smoke include PVC, bitumen and rubber cable insulation; bitumen sealed lagging on hot pipes; plastic and fibreboard air conditioning; and warm air heating ducts. If fire breaks out within a service duct, the first line of defence against its spread to other areas of the building is the passive fire-resistant qualities incorporated within the building design and construction.

Compartmentation BS 8313 states: 'A fire-resistant duct should have a fire resistance at least equal to the highest standard required for any of the divisions it crosses.'

2In order that the levels of compartmentation are not compromised, BS 8313 also recommends that: 'Any openings provided within the enclosures of fire resisting ducts should be kept to a minimum and fitted with doors or panels having not less than half the fire resistance required for the duct enclosure, with a minimum of 30 min integrity.'

3However, an adequate number of openings and access points will have to be present to allow safe access and egress for maintenance personnel and fire fighters, although these will have to be carefully sited to ensure that other parts of the building are not severely compromised or put in unnecessary risk in the event of a fire.

Non-combustible infilling/firestopping is also a necessity where services penetrate a fire-resisting duct or shaft. Services with small diameters are recommended for this situation as this will ensure that only minimal gaps are left in the fire-resistant duct or shaft, thus restricting the passage of smoke, heat and flame to other areas of the building.

In larger buildings or building complexes, service ducts sometimes start in subterranean levels, rising vertically through the various levels of the building.Although the interior of such ducts may include horizontal firebreaks at differing levels, it is not uncommon to find vertical risers with horizontal firebreaks with large distances between them. Further problems include adverse environmental conditions (hot or cold temperatures); irregular air currents; and dirt and dust, all of which may cause false alarms with conventional smoke or heat detection.

Hot-headed

Where the vertical rise within a shaft is open, a phenomenon known as stratification has to be considered. It is known that under certain conditions a fire plume may stop short of reaching a ceiling (or in this case the head of the shaft). Initially, a fire plume will be hotter than the surrounding air. However, subsequent mixing between the plume and air will eventually reduce the temperature of the diluted fire plume towards that of the surrounding air.

If the plume cools so much that the air above it at the head of the duct or shaft is warmer (and hence lighter) than the plume, the plume will stop rising and spread laterally and radially as though it had hit a ceiling. Therefore it is possible that the plume may not have the energy to reach a conventional detector mounted at the head of the shaft or duct and activate the fire alarm.

This phenomenon is similar to the problems that may be encountered under pitched or north-light roofs, which are heated by the sun.

Here the volume of air in contact with the underside of the roof is heated and rises to eventually fill the roof down to the eaves level, with a colder layer of air beneath.

Factors that influence the passage and route of a fire may require alternative forms of fire detection. New linear heat detection capabilities provide a means where the temperature of hot gases and radiated heat can be recorded at any point along the whole length of a cable, either detecting a localised 'hot spot' or a lower level of temperature increase over its total length.

Modern cables can also be manufactured to virtually any length, and thus can be easily installed for the full height of a shaft rise. Due to their flexibility and ease of installation the cables also possess the ability to provide constant monitoring at the precise point of risk. Therefore they can be installed directly adjacent to services within any shaft which possess an inherent fire risk, to pinpoint the location of a fire occurring along a section of the cable.

All other fire and safety precautions have to be considered in the same context. Precautions (such as ventilation and the incorporation of automatic fire extinguishing systems) should be installed as appropriate.

However, where fixed extinguishing or suppressant systems are a proposed component of the shaft or duct design (in accordance with BS 5306: Part 0 and BS 7273: Part 2), further design considerations may have to be considered, given the impact these may have on the safety of personnel operating within the shaft or duct. These range from inert gases being discharged while operatives are present, or the need for drainage considerations for total flooding systems, especially where the duct or shaft is below ground level.

Travel distances in ducts Although the application of fire engineering methods may provide solutions to difficult problems, the designer must also assess the confined nature of service areas, which will impact on an operative's safety.

Travel distances apply and should not be exceeded 4, bearing in mind it is likely that escape may be via ascent or descent using fixed ladders. It may be necessary to incorporate additional means of escape to ensure operatives can move rapidly in an emergency.

Adequate and safe design of any duct or shaft containing building services is essential from the outset of any project. There are numerous safety considerations that must be borne in mind and these become far more complex when it is likely that persons are likely to operate within them. Due regard should be paid to their safety in addition to the ultimate safety of the building.

Suitable design precautions are of the utmost importance as service shafts and ducts are areas where fires can start and may grow unseen, and where persons may become trapped undetected. Service shafts and ducts also provide an ideal route for smoke and fire to spread to other parts of the building and, due to their configuration and confined nature, it is unlikely that any fire will be easily extinguished.

LWF Fire & Engineering Consultants, London, tel 0208 655 1605.

REFERENCES 1Fire Safety Data Sheets, Buildings and Fire, FPDG9, Fire Protection Association, London, December 1990 2 British Standard 8313:1997 - Code of practice for accommodation of building services in ducts 3 ibid 4 Building Regulations 2000: Approved Documents: Fire Safety

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