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Geoff Wilkinson's Regs: A mist opportunity

New water-mist systems use less water, and less space, than sprinklers 

There are significant developments occurring in the sprinkler industry following the publication in October 2010 of a new British Standard. BS DD 8458 Pt 1 covers the design and installation of residential and domestic water-mist systems, and a second part is proposed for commercial applications.

The theory behind mist systems is that they produce smaller water droplets than traditional sprinklers (typically 70-100 microns) which are better able to absorb the heat produced from a fire, as there is more surface area between the droplet and the heat source. Contrary to popular assumption, it is not the wetting effect of a sprinkler that causes a fire to be controlled or extinguished. In fact it is the conversion of the water from liquid to vapour that takes the heat energy out of a fire, causing the droplet to expand to over 1,600 times its original volume, thereby displacing oxygen and disrupting the combustion process.

The main advantage of water-mist systems over sprinkler systems is that they use a lot less water – they don’t have the same need for water tanks – and hence potentially offer lower post-fire water-damage costs too. Also, because they require smaller pipes and tanks, they can be fitted into places that are simply too small for traditional sprinklers, such as domestic buildings, retrofit properties and heritage sites.

A mist system is made up of the same three main parts as a sprinkler system: a water supply, an alarm system and a distribution system. These systems fall into two main categories: low pressure and high pressure. A low-pressure system uses less water than a sprinkler but more than the high-pressure option, whereas the high-pressure option involves a lot less water storage and still smaller pipework.

Water-mist systems were first developed by the marine industry in reaction to the problem of capsizing due to large volumes of water when sprinklers operated on a boat, and they have a proven 20-year track record in such applications. Recently the manufacturers and installers of such systems have put them forward as an alternative to conventional sprinklers, in providing local or complete building protection, particularly for vulnerable occupancies such as schools, hospitals and nursing homes.

However, there have been problems getting such systems accepted by building control as there is no current British Standard for the systems to be tested to. It should be noted that the National Fire Protection Association (the body which oversees the design and installation of sprinkler systems in the United States) has had a water-mist standard (NFPA 750) for several years.

Last week, I attended a day of live demonstration of water-mist technology at the BRE, and the first-ever full tests of equipment against the new BS DD 8458. The tests were on Ultra’s new low-pressure water-mist nozzle ‘Ultramist PV1’ and opened the way to wider acceptance and specification of such systems.

Although there are currently no mandatory requirements for sprinklers in dwelling houses, the Welsh Assembly has voted in favour of introducing them when they take devolved powers to set their own building regulations later this year. Until there are mandatory requirements, sprinklers still open up design opportunities and by ‘trading off’ sprinklers you can enjoy design flexibility, reduce costs and comply with building regulations. 

Increasingly, building-control officers are seeing architects proposing their use in both existing properties and new developments. Typical recently-presented trade-offs include the following:

1. Secondary means of escape: could be relaxed where sprinklers are used, for example where the provision of external staircases is not possible or desirable. 

2. Passive requirements: the 30-minute fire resistance for walls, doors and floors could be relaxed. 

3. Open stairways: stairways open to ground-floor lounge areas from new second-floor or loft accommodation could be acceptable with a sprinkler system. 

4. Inner rooms: to overcome the problem of inner rooms opening into an open-plan space. 

5. Internal protected corridors or hallways: currently restricted to 9m long, but this could be extended to 18m with a sprinkler system. 

6. Travel distances: as a sprinkler system will significantly reduce the risk of a fire developing and spreading beyond the room of origin, this allows the potential for greater evacuation times. Therefore, there is scope to introduce increased flexibility in travel distances. 

The use of sprinklers should still not been seen as a panacea for all design situations and any application to use them in lieu of code-compliant solutions should be assessed and supported by a fire engineer.

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