Play misty for me

Sprinkler systems are very effective in reducing damage arising from a fire, but their drawback is amount of space required to accommodate the tank and pumps.

This is where water mist systems have the advantage and is one reason why they are increasingly being used in buildings. Like sprinklers, they consist of a water supply and an array of fixed pipework and discharge heads. Unlike sprinklers, water mist systems do not need a hundred tonnes of water, and can be designed to control or extinguish a fire using as little as 10% of the water required by a conventional sprinkler system.

Mist heads produce water droplets many times smaller than those of sprinkler heads, and this is the key to their effectiveness. Smaller droplets result in a larger surface-area-to-mass ratio, and this leads to greater absorption of heat. As the water heats up and turns to steam, more energy is absorbed and the resulting steam acts as an inert blanket that displaces oxygen and effectively smothers the fire. The small, extremely mobile droplets are entrained into the seat of the fire by the fire’s own convection currents, hitting directly at burning fuel. As a bonus, the mist particles suspended in air provide a surprisingly effective barrier to radiation and so reduce the potential for fire spread to adjacent fuels.

Water mist systems have been used extensively in the marine and offshore industries for the past two decades. In these environments, where space is at a premium, and where the movement of uncontained water in a tank can prove catastrophic, water mist systems won their spurs. It was only with the phasing out of ozone-depleting halons in the early 1990s, however, that the use of these systems in buildings was seriously considered.

In a 1993 report by the United Nations Environment Programme, water mist systems were rated more highly than conventional sprinklers, and equal to fast response sprinklers, for use in situations where halons would otherwise have been provided. Although not a direct replacement for gaseous systems in all environments, water mist does have several advantages: it has no environmental drawbacks; it doesn’t deplete the ozone layer and has no global warming potential; and water is still (relatively) plentiful. The mobility of the small droplets, especially in high pressure systems, is similar to that of the various gases used in extinguishing systems, and this makes these systems suitable for areas where gases would normally be provided, such as data halls, large server rooms and cable tunnels.

Systems generally fall into three categories: low pressure (up to 12.5 bar); medium pressure (12.5 to 35 bar); and high pressure (35 bar and higher). Each has its own benefits and drawbacks. The main disadvantage is with high pressure systems which require most of their components to be of stainless steel, with obvious cost implications. However they are probably the most effective in protecting data halls or similar areas. Prices for a typical water mist system are likely to be higher than for conventional sprinklers, but the much smaller space requirements will free up a significant amount of lettable space.

The pressure required to produce the water mist droplets can be provided by one or more pumps, either centrifugal or positive displacement, or by a number of cylinders of pressurised air or nitrogen.

One of the main advantages of water mist systems is also one of the greatest drawbacks. There is no unified standard, so each system must be designed to deal with the specific risk, with its effectiveness based on the results from a number of full-scale test fires. This adds to the cost, but at least the system has been designed to do exactly what we expect it to, whether this is to control, suppress or extinguish the fire. The effectiveness of these systems depends largely on the design of the discharge head, as this is where the stream is broken up to form the individual mist particles.

Each manufacturer designs and engineers their own heads, and again this raises the cost, although each head will probably protect a greater area than a conventional sprinkler.

In a study carried out for the government in 2005, BRE estimated there were 1000 water mist systems installed in buildings in the UK, ranging from houses and flats to factories, hotels, offices and residential care homes.

BSI has been working with CEN to produce a technical standard and a draft, known as prCEN/TS 14972:2006, is out for public consultation. At first sight, the influence of the sprinkler industry can be seen from the fact that this is intended as a specification rather than an approved code of practice, and is therefore presented as a set of prescriptive clauses that must be observed if one is to claim that a system has been designed to that standard.

It is interesting to note that the British Standard for automatic fire detection and alarm systems is written as a code of practice, and not as a specification, thereby giving the designer of the system more flexibility and scope for innovation, although both systems are probably of equal importance. Still, this is for another day.

In conclusion, water mist systems have been proven to be equally, if not more, effective than conventional sprinkler systems, but with considerably less water. These systems are suitable for most occasions when a sprinkler system is being considered, but may not be appropriate for dealing with deep-seated or low-energy fires. Each system must designed specifically for the particular risk, with this design being based on a number of full-scale fire tests, so all stakeholders can be sure the system will do exactly what it is supposed to do in the most cost-effective manner. This is not a claim that can be made by other systems.