Canadian Consulting Engineer

Feature

Water Mist

One of the most common natural resources we know is also an environmentally friendly and economic substance in sophisticated fire suppression systems.The production of Halon-1301, one of the most effe...



One of the most common natural resources we know is also an environmentally friendly and economic substance in sophisticated fire suppression systems.

The production of Halon-1301, one of the most effective chemical fire suppressants ever developed, was banned in early 1994 by the Montreal Protocol because of its effects on the earth’s ozone layer. As a replacement, water mist has received considerable attention because it is environmentally friendly, non-toxic, economical and effective.

Though the concept of using water mist to extinguish liquid and solid fuel fires is not new, the technology was not considered practical for fire suppression until the phase-out of halon renewed interest in its potential. Over the last few years there have been a number of studies of the capabilities of water mist systems for various applications, including using water mist systems for kitchen hood fire protection. Research has also been done on increasing the fire suppression performance of water mist by combining additives and using cycling discharges.

Technology review

The early studies identified flame cooling and oxygen displacement as the dominant mechanisms in water mist fire suppression. More recent investigations, however, suggested that other mechanisms also play a role. One such mechanism is the radiation attenuation provided by water mist which can stop a fire from spreading to an unignited fuel surface and reduce the vaporization or pyrolysis rate at the fuel surface. Tests conducted at National Research Council of Canada (NRC) showed that the radiant heat flux to the walls of a test compartment was reduced by more than 70 per cent by water mist. Water mist’s other extinguishing mechanisms, considered as secondary, include the dilution of flammable vapours, and the direct wetting and cooling of combustibles.

Water mist characteristics, such as drop-size distribution, flux density and spray momentum, have a direct relation to its fire suppression effectiveness. To work effectively, a water mist system must generate and deliver to the fire optimum-sized droplets with an adequate concentration. The selection of the optimum size of droplets is dependent on the potential size of the fire, the properties of the combustibles, and the degree of obstruction and ventilation in the compartment. There is no one drop-size distribution to fit all fire scenarios.

The momentum of the spray, as determined by the spray mass and velocity and its direction relative to the fire plume, is also an important parameter in determining the difference between success and failure of these fire suppression systems. The efficacy of the water mist systems will be highest when most of the spray goes in a desired direction relative to the fire plume.

Shipboard machinery spaces

The use of water mist as a fire suppression system has been considered in a wide range of applications such as for shipboard machinery and in turbine engine rooms, aircraft cabins, engine and dry bays. Its application in shipboard machinery spaces showed considerable potential. One of the concerns raised though was about how effective such a system might be in a large space with a high ceiling and a high degree of obstruction. Full-scale tests conducted by NRC and other research agencies, however, showed that water mist systems can effectively extinguish large fires in unventilated machinery spaces using a small amount of water. The performance was comparable to that of gaseous halon alternatives.

Water mist systems were able to extinguish a wide variety of fires when natural ventilation such as open doors and hatches were allowed, while gaseous agents were not effective under such conditions. Water mist also rapidly reduced the compartment temperature and significantly improved visibility. Thus it would allow access to a compartment while fire is being suppressed. However, the tests showed that the effectiveness of a water mist system was affected by obstructions, fire size and ceiling heights.

Based on the results of various studies, the U.S. Navy and Canadian Navy identified water mist systems as a candidate to replace Halon-1301 in shipboard machinery spaces.

Electrical and electronic equipment

Water mist also has potential as an effective alternative to halon in the protection of electrical and electronic equipment. However, the telecommunications and utilities industries have traditionally been reluctant to use water as a fire suppressant because of potential water damage.

Recently, NRC conducted studies for the “IntelMist” project to assess the feasibility of using water mist to protect facilities with substantial electronic equipment. The basic principle of IntelMist is to use state-of-the-art fire-detection technology to control a zoned water-mist fire suppression system so that water can be applied only to the smallest possible area directly associated with the fire.

As a part of the IntelMist project, NRC conducted a series of full-scale fire suppression tests using water mist in electronic cabinets, underfloor cable plenums and overhead cable trays. The investigations showed that the traditional total-flooding approach (as used for Halon-1301), whether in a single cabinet, an underfloor plenum or a large space with many cable trays, was unreliable with water mist. On the other hand, reliable fire suppression was achieved with water mist by rigorously controlling the direction of the spray relative to the hazard. This control was done by laying out the nozzles to suit the physical arrangement of the obstructions or structural elements. The investigations showed that a water mist system can be used to suppress fires in electrical and electronic equipment without causing short circuits or damage to electrical and electronic components. Evidently, with suitable spray characteristics and an intelligent detection system which has an appropriate signal processing logic, water mist can replace halon in electronic and telecommunication equipment rooms.

Computer modelling

We do not understand the relationship between a fire scenario and the characteristics of the water mist system well enough yet to apply a “first principles” approach to the design of water mist systems. Until now, the evaluation of water mist systems has been based on costly full-scale tests, with the result that there has been a slow development of the technology.

A computational fluid dynamics (CFD) model can enable us to study fluid dynamics and heat transfer as well as the interaction between water mist and the flame. To study liquid pool fire extinguishment by water mist, NRC developed a computational fluid dynamics model. The model showed that the fire created a strong upward plume directly over the fire, and did not allow a low-momentum water spray to penetrate the fire plume. However, water mist cooled the fire plume rapidly and introduced a flow of cool gases over pan fires.

NRC plans to develop computer models further in order to increase our understanding of the extinguishment processes and the design of water mist systems. CCE

Andrew K. Kim, Ph.D., P.Eng. is a fire researcher at the NRC’s Institute for Research in Construction, Ottawa. For more details call (613) 993-9555.