Overall fire hazard is directly related to the type and quantity of the fuel present in a space. The ease of ignition/reignition of the fuel, the fire growth rate, and the difficulty of achieving control, suppression, extinguishment, or any combination thereof, shall be considered when selecting or designing a water mist system.
Water mist systems offer potential benefits for many specialized applications, particularly when available water supplies are limited or when the application of water must be restricted. Potential benefits may also exist for applications previously protected by gaseous and other specialized fire suppressant system or extinguishing agents.
A water mist system is a fire protection system using very fine water sprays (that is water mist). The very small water droplets allow the water mist to suppress or extinguish fires by cooling the flame and fire plume, oxygen displacement by water vapour and radiant heat attenuation.
Due to the presence of larger drops, higher mass flow rates are earlier to achieve with Class 2 sprays than with Class 1 sprays. The larger drops are not so large as to be ineffective on liquid fuel fires. However, considerable surface wetting occurs with sprays in this range; therefore a Class 2 mist is also likely to be effective on fires involving ordinary combustibles. Class 3 water mists are typically generated by intermediate pressure, small orifice sprinklers, impingement nozzles of various sorts and fire hose fognozzles.
A water mist distribution system is a system that is connected to a water supply and equipped with one or more nozzles capable of delivering water mist. A water mist system shall meet the performance requirements of its intended application.
A water mist system having predetermined flow rates, nozzle pressures and volumes and spray densities of water mist. These systems have the specific pipe sizes, maximum and minimum pipe lengths, flexible hose, number of fittings and number and types of nozzles (see Fig. 2).
A water mist system where, as required by the accepting authority, individual calculation and design shall be done to determine the system flow rates, pipe sizes, area or volume protected by the system, discharge density of water mist, number and types of nozzles and nozzle placement.
A water mist system using sealed nozzles attached to a piping system containing air, nitrogen or inert gas under pressure, the release of which (as from an opening of sealed nozzle) allows the water supply pressure to open a dry-pipe valve. The water then flows into the piping system and out through any nozzles that have opened.
A water mist system using sealed nozzles attached to a piping system containing air that might or might not be under pressure, with a supplemental detection system installed in the same areas as the mist nozzles. The actuation of the detection system opens a valve and the water then flows into the piping system and out through any nozzles that have opened.
A water mist system using sealed nozzles attached to a piping system containing water and connected to a water supply so that water discharges immediately from nozzles that are operated by the heat from a fire.
The clearance is the air distance between water mist system equipment, including piping and nozzles, and unenclosed or uninsulated live electrical components at other than ground potential. The minimum clearances provided are for the purpose of electrical clearance under normal conditions; they are not intended for use as safe distances during water mist system operation.
Water mist system with additives protecting normally occupied areas shall incorporate a manual lock off valve unless the additive in the concentration used has been evaluated as safe for human exposure by an appropriate authority. The valve, where fitted, shall be locked whenever the protected area is occupied.
Floor area, enclosure volume, ceiling height, ventilation conditions and the like, shall be considered when designing a system in regard to nozzle location, system flow rate and the total water usage of the system.
The number, size and location of the openings in the space (for example, the door, window) shall be addressed in the design and installation of the system. In some cases, special precautions are necessary to minimize the effects of these openings. These precautions include, but are not limited to, automatic door closures and water mist curtains.
A fire hazard analysis shall be conducted to determine both the design parameters of the water mist system and the type of detection/activation scheme employed by the system. The system shall be based on the fuel type, combustible loading and anticipated fire growth rate as well as the desired fire fighting performance objectives.
Overall fire hazard is directly related to the type and quantity of the fuel present in a space. The ease of ignition/reignition of the fuel, the fire growth rate and the difficulty of achieving control, suppression, extinguishment or any combination thereof, shall be considered when selecting or designing a water mist system.
A reserve supply of water and atomizing or pressurizing gas shall be provided where the extinguishing agent can not otherwise be replaced within 24 h following system operation or if a single water supply is used for multiple risks. If a manual changeover is necessary, the mechanism shall be readily accessible from outside of the risk. Means shall be provided to prevent discharge of reserve supplies from open manifold connections when supplies are removed for servicing. The reserve quantities shall be identical to the initial supply.
Where practicable, full flow tests of the system piping, using water, should be made as a means of checking the nozzle layout, discharge patterns and any obstructions, determining the relationship between design criteria and actual performance and ensuring against the clogging of the smaller piping and nozzles by foreign matter carried by the water.
The heart of every FOGTEC system are special water mist nozzles. Due to a modular nozzle concept, nozzles can be provided covering standard applications or which can also be used in raised floors, suspended ceilings, areas with particularly high temperatures or that are exposed to wind conditions. Their reliability has been intensively tested such as at Factory Mutual.
The entire pipework is made of stainless steel, being in line with international requirements for water mist firefighting systems. The pipe cross-sections range between 10 and 50 mm and can be installed and be designed according to recognised standards.
Cylinder systems comprise separate cylinders for water and nitrogen, which are interconnected via a nitrogen manifold. When triggered, all water cylinders are pressurised with nitrogen. They are activated via an external fire detection system or manually.
With FOGTEC high pressure wall cabinets, the fire can be manually fought immediately upon detection without time losses resulting from the build-up of an extinguishing water supply. The water mist is generated by the connected FOGGUN which creates a considerably greater cooling effect compared to conventional low pressure wall cabinets. Unlike with hand-held fire extinguishers or small extinguishing trolleys, there is no limitation to the extinguishing time. The continuous availability of water makes firefighting with mist even more effective and safer than with conventional systems. The hoses have lengths of up to 60 m. The flow rate of the FOGGUN is 25 l/min at a pressure of 100 bar.
The misunderstandings of a Watermist system during the initial stages of the Crossrail project and the subsequent designs produced which failed to meet the fundamental request to provide an engineered solution to the traditional sprinkler system leads me to explain the Watermist system in more depth.
This Code of Practice gave recommendations for the design, installation, commissioning and maintenance of Watermist systems, and gives performance criteria for fixed Watermist systems for specific commercial and industrial hazards.
(2) The parts to which this paragraph applies must be provided with a means for fighting fire comprising a system which is activated automatically in that part of the premises in case of fire in that part.Reference to the Crossrail Programme Functional Requirements (CPFR) document was also key in the initial design of the Watermist system as Table 1 outlines:
While traditional water sprinkler systems remove only the heat element of the triangle, Watermist systems remove both the heat and oxygen elements by dispersing water through specially-designed nozzles.
First to the market were high pressure water mist solutions, but low-pressure systems are increasingly preferred by owners and authorities. It must be noted that the first nozzle appeared as early as 1870 by FogtecÂ.
However, there remains a misperception that low pressure water mist systems use more water than high pressure systems. But approval data published in DNV EXCHANGE demonstrates that there is very small difference between the systems and that low-pressure systems can even use less water on occasions.
Another misperception is that low pressure systems can cause more damage to electric and electronic equipment than high pressure systems. In fact, cabinets and cables may be better cooled by the larger water mist droplets.
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