An Introduction to Watermist: the Fire Sprinkler Alternative
Watermist systems are becoming widely recognised as a viable alternative to traditional fire sprinklers. This guide describes what they can (and can't) do and also provides an overview of the technology. We created this website, so all those involved in the building process, such as property developers, authorities having jurisdiction, architects, designers and end-users could learn more about the benefits of using this unique type of fire protection.
Watermist systems share many similarities with traditional fire sprinklers. However, as they are application and manufacturer-specific, they have some distinct differences. Each hazard or occupancy requires its own individual design, which is detailed in the manufacturer’s DIOM (Design Installation Operation and Maintenance). It is not possible to design a mist system simply by following one of the various Standards. This would be possible with traditional sprinkler systems, where you could follow BS EN 12845 or BS 9251 for example. With watermist, each manufacturer may vary the characteristics of the nozzle, appearance, spacing, orientation, activation methodology and even location. In this way, the DIOM is the 'application and manufacturer’s specific design standard', whilst the standard per se, lists the minimum performance requirements across applications.
1.1 Definition of Watermist
A water spray for which the cumulative volumetric distribution of water droplets is less than 1000 μm (1mm) within the nozzle operating pressure range.
1.2 Definition of a Watermist System
As per prEN 14972 (and very similar in other standards): distribution system of a firefighting system connected to a water supply equipped with one or more water mist nozzles capable of delivering water mist intended to control, suppress or extinguish the fire.
The key to the success of watermist is the ability of small water droplets to efficiently extinguish, suppress or control a fire.
The ‘intelligent use of water’ in the form of very small water droplets was first successfully employed by Grinnell in his ‘pepper pot’ nozzle in the 1890s. Such systems proved to be a fast and efficient method of extinguishing fires by rapid cooling and oxygen displacement.
The land-based breakthrough for watermist technology came at the beginning of the 1990s and was linked to the rapid expansion of food processing production plants that were constructed to cope with the growing demand for ready-prepared meals and other convenience foods. The food industry was quick to recognise the immense benefits of watermist in rapidly suppressing or extinguishing the potentially disastrous fires which could result from the range of oil-based cooking processes carried out on such premises.
Another watermist application developed following the fire on the passenger ferry Scandinavian Star in April 1990, which killed 158 people. This tragedy galvanised the watermist industry and in 1993 it conducted a large series of cabin and corridor fire tests. These fire tests were independently witnessed and led to improved International Maritime Organisation fire safety requirements on passenger ships and the development of installation guidelines and fire test procedures for alternative sprinkler systems.
These systems were originally limited to very specific applications requiring full-scale tests before deciding whether mist was a suitable agent. Over the last 20 years, however, much research has been carried out in the use of watermist in land-based applications and, as experience grows and fire test standards are developed, watermist systems are an option for a wider range of hazards including maritime engine rooms, gas turbines, data rooms, homes, heritage buildings and even the international space station (as a zero-G fire extinguisher).
Advances in watermist technology have led to the production of British, European and International Standards and Guidelines for the design, installation, commissioning and maintenance of watermist systems to combat Class A, B, C and F type fires in a wide variety of domestic, residential, commercial and industrial applications.
Watermist is still a developing technology as the number of applications and methods of deployment continue to diversify as more investment into research is carried out, both by industry and by research laboratories. Many of the existing applications will be added to the ever-increasing scope of watermist standards as these become common practice.
1.4 How Watermist Works
For a fire to survive, it relies on the presence of the three elements of the fire triangle: oxygen, heat and combustible material. Removal of any one of these elements can suppress or extinguish a fire.
Watermist removes both the heat and oxygen. It achieves this by dispersing water through specially designed nozzles at low, medium or high pressure. Generally, as system pressure increases, the water droplet size decreases. This, in turn, significantly increases the total surface area of the unit and so leads to the production of a greater volume of steam, removing more energy from the fire which generates the steam.
The smaller a water droplet size, the larger the surface area and the more effective the suppressant becomes in rapidly reducing the temperature and oxygen on the flame front. This is because the heat absorption capacity of watermist is greater than any other water-based suppression system. To put it another way, when water is converted to steam – a lot of energy is used, energy which is taken from the fire.
It takes 335kJ to heat 1 litre of water from 20°C to 100°C and 2257kJ to convert 1 litre of water to steam. Water expands 1700 times upon vaporisation so the high energy-absorption capability of small water droplets produces the rapid cooling and oxygen depletion characteristics that are unique to watermist.
Graphs 1 - Reduction of oxygen concentration (left) and 2 -Reduction of temperature (right) illustrate actual test results for rates of oxygen depletion and of cooling by a watermist system which actuates about 5.5min after the start of a fire.
Watermist requires the small droplets to reach the base of the fire so that it can be effective at suppression by cooling and suffocating and not by wetting, which is the case with large droplet systems (such as sprinklers which rely on gravity). Watermist, therefore, relies heavily on the Archimedes Principle where the hot, less dense air is rising and the cool oxygen-rich air is feeding the fire at its base. For watermist to be effective it needs to either be entrained in the denser cooler air at the base of the fire or it needs momentum to overcome the flow of hot air from the fire.
Watermist systems are designed so that this can take place in several ways, non-exclusive examples as follows:
Directional nozzles at the fire load from the top or sides to address the risk locally.
Nozzles placed closer to the base of the fire so that these are dragged to the flame front by the incoming oxygen consumption.
Deploying the mist in an enclosed compartment so that mist does entrain the oxygen draught, expands and suffocates the fire.
Using a high-speed outlet to generate enough momentum to overcome the upward draught.
This also means that watermist nozzles are not necessarily fitted on the ceiling of compartments, it depends on the application and the intended use. The International Watermist Association (IWMA) has an informal club called the “Eureka Club” of watermist applications that deliver mist most effectively to the base of the fire.
It also means that watermist nozzles rarely look alike because they are application and operation-specific. Some are designed to be directional in order to deliver a specific spray pattern at a protected risk whilst others are required to provide a diffusing mist for volume protection. In addition, some are designed for specific functions such as sidewall nozzles for the protection of rooms or to provide water curtains in atrium buildings for life safety applications. There are even nozzles specifically designed for floor void protection within data centres.
Nozzle design is also governed by the type of fire risk they are installed to protect and this varies greatly on whether the nozzle is designed to control, suppress or extinguish Class A, B, C or F fires.
1.5 Fire Classifications
Various watermist systems have been developed either to suppress or extinguish fires involving fuels of classes A, B, C and F.
Class A fires involve ordinary combustibles, solid materials, mainly of organic origin, such as wood, paper, textiles and straw etc.
Class B fires involve flammable liquids such as oils, spirits, fats and certain plastics.
Class C fires involve flammable gases such as methane, propane, hydrogen and natural gas.
Class F fires involve the flammable cooking oils used within the food industry.
1.6 Fire Protection Objectives
Fire suppression, control and extinguishment objectives:
1.6.1 Suppression: The sharp reduction of the rate of heat release of a fire and the prevention of re-growth.
1.6.2 Control: The limitation of the growth of fire by pre-wetting adjacent combustibles and controlling ceiling gas temperatures to prevent structural damage.
1.6.3 Extinguishment: The complete suppression of fire until there are no burning combustibles.