C O N T E X T 1 1 3 : M A R C H 2 0 1 0 31 Stewart Kidd works with the Loss Prevention Consultancy. A sprinkler system consists of a number of components, all of which require careful consideration when the system is being designed and installed in a traditional building. Most sprinkler systems in non-domestic buildings will comprise a water supply, a pump or pumps, an alarm valve, a mechanical local alarm, stop valves, a test and drain valve, a gridded array of water supply pipes, and sprinkler heads. Sprinkler systems operate successfully in a very high proportion of fires (more than 97 per cent) when the level of risk identified in the fire risk assessment can be counteracted by the delivery of a matching level of fire suppressant (in the case of sprinkler systems, water). Other fire protection measures introduced in the building will contribute to this equation. A number of factors affect the quantity of water delivered.These include the volume of water delivered to each head by the sprinkler distribution pipework, the number of heads and the floor coverage pattern of the head type chosen. The only other water-based system which would be appropriate for a traditional building would be water mist. Benefiting from recent research and proprietary development, water mist systems, employing heads discharging aerated water in a mist or fine spray, superficially appear very similar to conventional sprinkler systems. Indeed, these systems offer many of the benefits of conventional sprinkler protection. Several proprietary systems are available, ranging from very high pressure (up to 110 bars) systems producing a fine water particle mist, to low pressure systems providing a fine water spray extinguishing medium akin to a conventional sprinkler system. The water is propelled either by pumps or by an inert gas, and dispensed from nozzles that are designed to deliver water in fine droplets to the area of fire. The suppression mechanism relies on a combination of cooling by the water, the production of steam that displaces oxygen from the area of the fire to a level that can not sustain combustion, and inhibition of the chemical processes of combustion. In comparison with sprinklers, water mist systems use comparatively small amounts of water to fight a fire.This means that less water has to be stored, which can result in substantial cost savings. Mist systems also offer potential for installation in locations that are too small to accommodate the pumps of a conventional sprinkler system. A further benefit is the minimisation of water damage in the event of activation. Pioneering applications of water mist protection in heritage buildings include a gallery of the National Portrait Gallery in London and a number of very old Norwegian churches which were at particular risk from arson. The largest heritage-related mist installation to date is believed to be in the National Gallery of Art in Washington, DC. There are a significant number of mist systems in large palazzos in Venice, although these systems reportedly only protect the roof spaces. Many hotels are now protected by mist systems. At the time of writing, there are no published British or European standards for water mist systems, although a draft CEN (European) standard has been produced (pr EN 14972: 2004). This was rejected by the British Standards Institution (BSi) as being inherently flawed in respect to the lack of independent test data on water application rates. BSi has set up two working groups to produce draft BS codes of practice for mist systems in residential and domestic premises (BS DD 8458), and commercial and industrial premises (BS DD 8459). It is expected that these documents will be published and available for use in summer 2010. Much research has been undertaken into the appropriate use of these systems in confined spaces and large volume areas. High-ceilinged rooms with large floor areas may prove a significant problem in designing nozzle layout to ensure that potential fire locations fall within the effective range of the nozzle distribution (although six-metre-high rooms in Stirling Castle are currently being protected with a mist system). Tests have also indicated that the design of water mist systems needs careful consideration where the protected location is prone to significant air movement, as this may impact on the effectiveness of the mist. Mist systems are also less effective than traditional sprinkler systems at extinguishing slow, deep-seated fires in ‘normal combustibles’. In one test, trialling water mist protection in cellular archive storage areas, the results were disappointing. This and other limiting factors, such as personnel presence and detection parameters, require a careful technical assessment to be made before any conclusion is reached as to the type of protection needed. This assessment needs to take into account the fact that (unlike sprinkler systems) each mist system has to be specifically designed for the space it is to protect. In the present absence of any independent standards for the design of water mist systems, care needs to be taken when deciding on the validity of manufacturers’ claims. Many claims are based on system technology and components developed for use in marine applications, and so may not be directly relevant for protecting buildings. Although these issues will be resolved in time, caution should be exercised in specifying water mist systems for the protection of large areas or complete buildings until design and standards issues are resolved and the British Standards documents referred to above are published. In many cases it is possible that significant benefits can accrue where a fire suppression system is installed in a historic or heritage building which is being refurbished or modified for a change of use. Apart from providing compliance with current building standards, and making it easy for the building to meet fire safety standards, a fire suppression system may do less damage and be less intrusive than the conventional approach to escape routes and compartmentation.
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