Gulf Construction Online - Burning issues on passive protection

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Fire Protection's Flamebar BW-11 fire-resistant ductwork

Burning issues on passive protection

When, why and how should fire-resistant ductwork be used? And, are the right standards being enforced in the Gulf? Brian James, managing director of Firespray International and Fire Protection, UK, and Jonathan Ombler of Mosaco, UAE, which represents Fire Protection in the Gulf, delve into these issues.

01 August 2002

Most modern buildings are designed to provide facilities for the latest technology with a congenial indoor environment for those that work in them. The controlled comfort levels needed to achieve the right environment are usually provided by an air-conditioning or ventilation system which, in the event of a fire, must be able to help maintain compartmentation in order to limit the spread of fire.

Effective compartmentation must therefore be taken into consideration when designing and constructing a building. Under most circumstances, compartmentation is maintained by precisely-positioned fire dampers in the HVAC (heating, ventilation and air-conditioning) ductwork system. However, there are certain conditions under which fire dampers should not be used, when the ductwork itself must provide the same levels of fire resistance as that afforded by the compartment wall or floor. In general terms, any ductwork system that is intended to operate or has special use under a fire condition and cannot therefore utilise fire dampers should be fire rated.

The ventilation systems and applications which have this special task in the event of a fire are smoke extraction, dual ventilation/smoke extraction, car-park extraction, pressurisation and kitchen extract systems.

Smoke extraction systems handle the evacuation of those products of combustion, which could otherwise reduce visibility and impair human functions, thereby helping the buildings occupants to escape and assist fire-fighters to locate the seat of the fire.

In areas such as large or deep basements, or in high-rise buildings without opening windows where smoke clearance by natural ventilation may be virtually impossible, ductwork is needed to convey smoke to a suitable outlet. Fan-assisted systems are used in instances where the natural buoyancy of the combustion products is not adequate to ensure the required smoke extraction rate through the ductwork. Ducted air inlets may also be installed in order to provide replacement air.

When completely contained within the fire compartment, ductwork must be capable of resisting the anticipated temperatures generated during the development of a fire. BS 476 Part 24 (1987) and ISO 6944 (1985) also requires the duct which is intended for smoke extract to retain at least 75 per cent of its cross-sectional area within the fire compartment. If the ductwork penetrates a fire-resisting barrier, it must also be capable of providing the same period of fire resistance.

Dual ventilation/smoke extraction systems, which are growing in popularity, provide an economical solution by serving as a conventional ventilation system under normal conditions but converting to a smoke extraction in the event of a fire.

Car-park extraction systems, which must be both separate and independent, are obligatory in car-parks which require mechanical ventilation to expel polluted air. Due to the fire risk associated with car-parks, these systems should be treated as smoke extract systems and therefore maintain a 75 per cent cross-sectional area under fire conditions in accordance with BS 476 Part 24 (1987) and ISO 6944 (1985). Under no circumstances should fire dampers be installed in extract ductwork serving car-parks.

Restricting smoke penetration

Pressurisation systems are used to restrict the penetration of smoke into certain critical areas of a building by maintaining the air at higher pressures than those in adjacent areas. These systems are found in areas where smoke would inhibit escape, such as protected stairways, lobbies, corridors and fire fighting shafts serving deep basements. A pressurisation system is a special form of mechanical ventilation system in that the air supply creating the pressurisation must be maintained for the duration of the fire. Fire dampers cannot, therefore, be used and any ductwork penetrating fire-resistant barriers should be fire-resistant.

Kitchen extract systems in non-domestic situations are required, because of the polluted nature of the air, to have separate and independent extraction systems. Fire dampers should not be installed in kitchen extract ductwork and any ductwork penetrating fire-resisting barrier should itself be fire-resistant.

A particular hazard is presented by combustible deposits such as grease, which is likely to accumulate on internal surfaces. A fire in an adjacent compartment through which the ductwork passes could therefore lead to ignition of these deposits so fire would spread through the ductwork system, possibly putting kitchen staff at risk.

Consideration should, therefore, be given to the insulation performance of kitchen extract ducts. Also, access doors at intervals not exceeding 3 m are recommended where possible.

Given the fact that the systems are being used to protect buildings and ultimately life, the installation of fire-resistant ductwork systems is subject to specific regulations and codes in whatever country the systems are specified. These are applicable to new buildings and extensions or alterations to existing buildings.

However, the enforcement is not helped by the fact that the regulations and British Standards relating to passive fire-resistant ductwork are not absolutely definitive, in particular in the case of fire insulation; and, with the building regulations having to be interpreted alongside BS 5588 Part 9 and BS 476 Part 24 as well as taking into account any trade off between passive and active systems, the mechanical designer has a difficult part to tread in order to satisfy the relevant standards, the consultants interpretation of the standards, as well as remaining within the given budget.

Concern over inexperience

More worrying for the bonafide passive fire protection industry is that there are too many inexperienced companies in the industry doing work they don't understand, with some making misleading statements about the performance of products. The industry, fire test laboratories and enforcement authorities are very concerned that this has created conditions where some fire-resistant ductwork installations will not perform to specification under fire conditions, therefore putting buildings and possibly people, at risk.

Consultants and designers are therefore being urged to check very carefully that the products they are specifying are right for the job. Careful checking is essential to ensure that the products have a full-scale test report and that assessments, appraisals and certification are in order.

Also of concern is that, although an increasing percentage of ducting installed throughout the world is fire-resistant, by no means all of this is being manufactured and installed by specialist passive fire protection experts with a detailed understanding of the products and how they should perform.

In practice, when ventilation and air-conditioning systems are designed, the mechanical services contractors send out enquiries to ductwork contractors for the manufacture and installation of the ductwork in line with HVAC DW142 or SMACNA specifications. In the case of fire-resistant ductwork, the contractor either sends the fire duct package to a specialist fire duct contractor or asks the insulation contractor to fire-rate the ductwork.

The real danger in this is that the ductwork contractor is unaware that some of the ducting for which he is quoting is required to be fire-resistant and therefore he does not manufacture the ductwork to the correct standard - using the incorrect gauge of galvanised sheet steel, aluminium rivets, flammable sealants in longitudinal seams, flammable gaskets and not using correct cross joints, stiffeners, penetration seals and supports.

Also, where the duct passes through a fire compartment wall, the duct should be additionally stiffened and fire stopped to prevent the duct warping in the event of a fire. Failure to do this could result in fire escaping between the wall and the duct, thereby negating fire compartmentation.

In addition, when the duct is not constructed to the correct standard, it may not maintain the 75 per cent free area necessary under a fire condition to satisfy the requirements of smoke extract.

Finally, and possibly most importantly, the hangers for the ductwork may not be sized to take into account the fact that the tensile strength of steel in a fire situation would be reduced to approximately 2.5 per cent of its normal strength. Ultimately, any of the above problems could result in premature collapse and failure of the whole ductwork system.

An effective way to overcome conformity problems, and one advocated by this company is to have a single source responsibility for design, manufacture and installation. Under these circumstances, a certificate of conformity can be issued which confirms the period of fire resistance installed and confirms compliance with the relevant criteria of BS 476 Part 24 and ISO 6944.

This document also helps to maintain quality control through the design, manufacture and erection of the ductwork system and gives the client the added advantage of being able to check that the system installed is to the correct specification. It can also, of course, be used to show the enforcing authorities that the installation complies with all necessary criteria.

Unfortunately in the Gulf, there is little regulation concerning fire-resistant ductwork and many times, often unbeknown to the consultant or client, the system employed is often untested by any recognised laboratory or where testing has occurred, it is to the bare minimum and not supported by full test reports.

As in the UK, it should be required of the HVAC industry to work towards a position where the installation of products, which might not perform in the event of a fire, is a thing of the past.

It can only be in the interest of the designers, specifiers, regulatory bodies and enforcing authorities, as well as manufacturers and installers, that the passive fire protection industry sets and observes the highest standards. After all, these are critical products designed to protect buildings and ultimately life itself.