The LWF Blog
Facilities Management & Fire Safety – Water Mist Systems – Part 17July 20, 2017 8:44 am
In this blog series for people who work in Facilities Management and who have an interest in or responsibility for Fire Safety, we are going to be covering Water Mist Systems. In part 16, we looked at high and medium velocity Water Spray Systems and their use in cooling structural steel work and in flammable liquid plants. In Part 17, we will discuss the use of Water Mist Systems and how they have come to be in widespread use.
As mentioned in the previous blog in this series, water spray systems come in medium and high velocity head types and this refers to the momentum with which water droplets will be ejected from the heads. Momentum of water droplets is also required for sprinkler heads, to allow the water to penetrate the plumes of hot gases rising from the fire.
In the case of water mist systems, however, this momentum is not required. In fact, the application of much smaller water droplets than in the previously mentioned methods can douse a fire more efficiently in some cases. They work because the heat from the fire is removed and oxygen is displaced from the area of the fire.
While water mist systems are still not used generally in standard purpose buildings, like office blocks, there has been an increase in uptake in recent years, partially due to the banning of halon gaseous extinguishing agents because of their effect on the environment.
Effectively, a water mist system is a water spray system where the droplets are of a very small diameter, typically 10-500 microns. They have become an effective alternative to gaseous systems in some circumstances as the water acts in the same way a gas extinguishant would. While such a system will use a very small amount of water when compared to a hose, for instance, they have shown themselves capable of extinguishing large flammable liquid fires.
Initially, the development of these systems was intended for use in the protection of machinery spaces on ships, and this is where the value was proven. Since that time, the uses have expanded to be used on land, including the protection of gas turbines, for example.
It has been adopted for use in compartment enclosures, such as a specific room in a building or vessel which a slowly developing or smouldering fire is a possibility, rather than a high heat blaze. Although the performance in such scenarios is less well-proven, it should be noted that such systems generally work best in a smaller compartment or area, with less obstacles to block the sensors from a potential fire ignition.
In Part 18 of this series, we will continue to look at Water Mist Systems and how they are being used in residential premises too. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact Peter Gyere in the first instance on 0208 668 8663.
Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.