Applications
Currently, there are no generally accepted prescriptive design rules for water mist systems. This is exemplified by the fact that different water mist systems that have successfully passed exactly the same test protocol have varying system designs, such as nozzle types, pressures, flow rates and nozzle spacings. There are important corollaries to this. First, all current water mist technologies must prove their fire performance in a performance test. Second, once the systems are tested to a test protocol, their installation must be restricted to applications that are related closely to the fire scenarios included in the test protocol. Third, a clear distinction should be made on the type of testing: is it an ad-hoc test agreed, for example, between the system manufacturer and the end-user or is it a consensus test protocol accepted by a wide variety of parties, including a number of manufacturers, end- users and others (such as approval organisations, test laboratories and authorities having jurisdiction). Thus, a necessary prerequisite for installing water mist systems is proof, preferably through a consensus test protocol, that the most important property of the system, the fire performance, is adequate. Potential benefits of water mist systems alone are not sufficient to justify choosing water mist.
The first internationally recognised test protocols for water mist systems have been developed by the International Maritime Organization (IMO). Test protocol for water mist systems protecting shipboard category A machinery spaces and cargo pump rooms (IMO MSC/Circ.668, as amended by MSC/Circ.728) has become the benchmark for total flooding water mist systems protecting such spaces, and it has been adopted to land-based standards as well (UL2167, FM Global Class 5560).
Water mist systems are efficient against large fires in enclosed spaces, but may have difficulties extinguishing small concealed fires. When capable only of fire control, they are, however, very efficient thermal management systems, with an ability to keep the enclosure temperatures at around 100ºC or even less. With this in mind, the 49th session of the IMO Fire Protection Sub-Committee proposed to revise the requirements for total flooding systems to better reflect their true capabilities. If endorsed at the 80th session of the Maritime Safety Committee, the revised requirements may come into effect from the beginning of 2006, enabling the use of total flooding water mist systems in somewhat larger protected volumes than to date.
In addition to total flooding systems, water mist can be applied as a local application system for high-risk portions of machinery spaces. Marine local application systems are tested according to IMO MSC/Circ.913. These systems are intended to be activated very quickly, and they are designed to control the fire and cool the surroundings, and to provide time for preparing the activation of the total flooding system. Even if the local application systems are not designed to be extinguishing systems, properly engineered and operated local application systems may actually extinguish fires, as exemplified by a fire incident on-board M/S Explorer of the Seas in June 2002. An explosive fire in the engine room was extinguished in less than a minute using a high pressure water mist system. The total damages of this fire amounted to US$500. Water mist systems for accommodation areas, public spaces and service areas are tested according to IMO Res.A.800 (19). The test method builds on the concept of sprinkler equivalency: the fire performance required of the water mist sprinkler systems has been found by conducting reference fire tests using a conventional sprinkler system as specified in the Safety Of Life At Sea regulations. The efficiency of water mist as an extinguishing agent is shown by the fact that water mist systems in general achieve the fire performance of the reference system by using substantially less water.
Conclusion
Water mist technology is an emerging fire protection technology. It has properties that make it an attractive choice for selected applications, in particular where low water consumption by the fire protection system is beneficial. A responsible way to implement this new technology is to make sure that the technology is properly tested to meet the requirements set by the application. The true challenge with water mist systems is to understand the limits of installation parameters within which a tested system can be safely applied to real-life situations
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