Water sprays are recognised as a viable alternative for fire and explosion protection in offshore applications. Water mist systems aim at maximising the extinguishing and cooling effectiveness of water sprays by using relatively small amounts of water dispersed in fine droplets. Water mist fire protection technology has been developed and commercialised to a large degree in the shipping environment, but similar applications for water mist systems can be found in offshore environment, including machinery spaces and accommodation areas.
Water Mist
Water mist is ordinary water atomised to very small droplets. The US National Fire Protection Agency (NFPA) 750 standard (year 2000 edition) defines water mist as a water spray in which 99% of the water is in droplets whose diameter is less than 1,000mm. However, many commercial water mist firefighting systems are capable of producing mean droplet sizes down to 100–400mm, and some even below that. The word ‘mist’ relates to the fact that a water spray consisting of such small droplets has a ‘misty’ visual appearance. For reference, clouds, both in the upper atmosphere and close to ground (commonly termed fog or mist), consist of droplets with a typical diameter of 1mm, while a typical raindrop has a diameter of 1,000–2,000mm. These figures illustrate the effect of droplet size on the capability of a droplet to be suspended in gas. Thus, water mist droplets, as defined for firefighting purposes, are not entirely aerosol-like particles, nor entirely raindrops, but something in-between.
Water Mist Versus Other Systems
Small water droplets are highly effective in fighting fires. Laboratory-scale studies have shown that, on a mass basis, the extinguishing effectiveness of water equals that of Halon 1301, provided that the droplet size is optimum (experiments suggest about 10mm). It may be attractive to think that water mist systems generally exhibit superior fire performance in comparison to other firefighting systems. However, throughout the brief history of fixed water mist systems, a keyword in the design of these systems has always been ‘equivalency’ with respect to fire performance. In the marine environment, water mist systems were applied for the protection of machinery spaces (volume protection, a gas-type application), as well as accommodation, public space and service areas (area protection, sprinkler-type applications). The idea was to enable the introduction of alternative technologies with a similar fire performance to a reference system, which by a political decision was a conventional sprinkler or water spray system. Therefore, the choice of a particular system type for a vessel depended primarily on properties other than fire performance, such as price, weight, space requirements, reliability, ease of installation and maintenance, etc.
Properties of Water Mist Systems
Perhaps the most significant and general property of water mist systems is the low water consumption when compared with conventional water-based systems. Full-scale testing has shown that, for a typical sprinkler-like application, such as an accommodation area, water consumption by water mist systems may be 5–10 times lower than by conventional sprinklers designed for the same application. A low water consumption is possible by making use of the extinguishing effectiveness of small droplets: a similar fire performance to sprinklers is achieved by using water more efficiently. Practical benefits of a low water consumption are that the water supply, the pump size and the diameter of the pipework can be substantially smaller for water mist systems than for sprinkler systems. These are immediately reflected in the weight and space requirements of water mist systems. In some applications, an adequate water supply may be small enough to enable a water mist system to be a self- contained stand-alone unit that is not dependent on a connection to an external water supply.
An interesting and important property of water mist systems is their ability to successfully fight fires whenever the fires are enclosed in a room. For room protection, gas systems would also be possible. However, with water mist systems, the protected room does not need to be as tightly sealed as withgas systems. This is simply because the discharge of water mist is usually continuous, and loss of extinguishing agent via leaks is not crucial to performance. Another advantage for water mist in rooms or enclosures is the superior ability of water mist to control the gas temperature. This is because the large number of small water droplets evaporates effectively, and because evaporation of water absorbs large amounts of energy. This is also the explanation for the somewhat counter-intuitive behaviour of water mist systems for volume protection: the ability of water mist to extinguish fires increases with the fire size. Large fires release large amounts of heat, which is readily used by water mist systems to convert water droplets to water vapour. Water vapour in turn displaces oxygen and acts like an inert extinguishing gas. Small water droplets are effective in scattering heat radiation. Wide fog patterns are commonly used by firefighters as radiation shields when approaching an intense fire. The same principle applies to the water sprays delivered by water mist systems. The mist sprays are capable of offering protection against heat radiation and thereby provide protection against heat exposure for both evacuating people and advancing firefighters. The ability to scatter heat radiation is also considered as one of the extinguishing mechanisms of water mist. While protecting people from heat radiation, water mist also reduces the radiative heat flux to surfaces of combustible materials in the vicinity of the flame, which increases the ignition time of these materials and therefore slows down the spreading of the fire.
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