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November 2006:Water Mist Technology

Background

Steam flooding as a fire extinguishing medium is a predecessor of water mist that was introduced in the early 1900's. These early applications included oil refining and wood drying kilns. Water mist appeared in the 1940's as a method to extinguish fires on ships and ferries. Interest in water mist fire extinguishing applications faded in the 1960's with the arrival of gaseous suppression agents such as Halon. Along with technological advances, renewed interest in water mist technology found legs following the 1987 international accord of the Montreal Protocol (the Montreal Protocol was signed in September 1987 and became effective in 1989). This protocol was an effort to reduce potentially damaging greenhouse gases. This included the phase out of halogenated hydrocarbon extinguishing agents Halon 1301 and 1211.

How It Works

Water mist is defined as airborne water droplets which 99% by volume are less than 1,000 microns. Producing water mist requires forcing water through extremely fine orifices at elevated pressures from that typically range from 100 psi and 1,000 psi. Unlike large water drops associated with standard spray sprinklers, water mist has a much higher capacity for heat absorption. The result is a water fire extinguishing agent that requires less than 20% the amount of water needed for suppression than with traditional water extinguishing methods.

In addition to heat absorption, water mist also has the ability of providing a radiant heat barrier. This property can help minimize heat damage to surrounding objects and contents. Along with its cooling and heat barrier properties, water mist also had the capacity to deprive fire from oxygen by displacement. These combined characteristics can prevent reflash in Class B fires involving flammable liquids, combustible liquids, petroleum greases, tars, oils, oil-based paints, solvents lacquers, alcohols, and flammable gases; and Class K fires in cooking appliances that involve combustible cooking media (e.g., vegetable or animal oils and/or fats). Water mist is also regarded as environmentally friendly.

In designing water mist systems there is a wide range of factors involved that require rigorous calculations, modeling and actual performance testing. From an engineering calculation standpoint, important parameters include:

- Room size and shape

- Location of objects and equipment

- Drop size distribution

- Mass flow rate

- Velocity

- Direction

From an equipment standpoint, either high pressure positive displacement pumps or compressed gas can be used to develop to necessary high pressure required to achieve an effective water mist spray. Because the nozzle mist orifices are so small, a high degree of water purity is needed. Using a prescriptive approach, such as those applied for standard spray sprinklers, is very problematic and does not guarantee success. There are a few pre-engineered systems available that are known to work in small areas. For instance, FM Global restricted total flooding applications to rooms with a maximum area of 400 ft2. The design and arrangement of systems for water mist equipment is simultaneously varied and rigid.

System strategies for water mist application include:

- Local-application systems

- Total compartment application systems

- Zoned application systems

Applications

There are a variety of water mist applications. These include:

- Flammable and combustible liquids storage in cut-off rooms

- Machinery spaces

- Clean room wet benches

- Ships

- Steam Turbine Generator Bearings

- Compartmentalized Gas Turbines

- Diesel Generators

- Historical Structures

Pros / Cons

While water mist as a fire extinguishing agent is promising, the reliability and predictability of this technology is problematic. Some recognized pros and cons from a variety of reputable sources is summarized below:

Pros

- Fast Extinguishment

- Reduced Water Damage

- Non-toxic agent

- Smaller volumes of water required

Cons

- Controversy over reliability of testing vs. modeling

- Challenge of replicating test results

- Expense and complexity of equipment

- Expense of testing

- Expense of maintenance

- Not all situations can be predicted by test results

References

FM Global Data Sheet 4-2, Water Mist Systems

NFPA 750, Standard on Water Mist Fire Protection Systems 2006 Edition

NFPA 301, Code for Safety to Life from Fire on Merchant Vessels 2001 Edition

NFPA 850, Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations 2005 Edition

NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection 2007 Edition

NFPA 914, Code for Fire Protection of Historic Structures 2007 Edition

Related Publications

IMO Publications. International Maritime Organization, 4 Albert Embankment, London, SEI 7SR, United Kingdom.

IMO Code for Application of Fire Test Procedures, 1998.

IMO MSC/Circ. 668, Alternative Arrangements for Halon Fire-Extinguishing Systems in Machinery Spaces and Pump-Rooms.

IMO MSC/Circ. 728, Amendments to the Test Method for Equivalent Water-Based Fire Extinguishing Systems for Machinery Spaces of Category A and Cargo Pump-Rooms Contained in MSC/Circ. 668.

IMO MSC/Circ. 913, Guidelines for the Approval of Fixed Water-Based Local Application Fire-Fighting Systems for Use in Category A Machinery Spaces.

IMO Resolution A.800(19), Revised Guidelines for Approval of Sprinkler Systems.

E.1.2.9 UL Publication. Underwriters Laboratories Inc., 333 Pfingsten Road, Northbrook, IL 60062-2096.

ANSI/UL 2167, Standard for Water Mist Nozzles for Fire Protection Service, 2002 with revisions through February 2004.

Factory Mutual Research Corporation, Norwood, MA, USA

FMRC Draft Performance Requirements for Fine Water Spray Systems for the Protection of Combustion Turbine Enclosures, Machinery Spaces, and Special Hazard Machinery Spaces with Volumes Not Exceeding 2825 ft3 (80 m3).

FMRC Draft Performance Requirements for Fine Water Spray Systems for the Protection of Combustion Turbine Enclosures, Machinery Spaces, and Special Hazard Machinery Spaces with Volumes Not Exceeding 9175 ft3 (260 m3).

FMRC Draft Performance Requirements for Fine Water Spray Systems for the Protection of Combustion Turbine Enclosures, Machinery Spaces, and Special Hazard Machinery Spaces with Volumes Exceeding 9175 ft3 (260 m3).

FMRC Draft Performance Requirements for Fine Water Spray Systems for the Protection of Light Hazard Occupancies.

FMRC Draft Performance Requirements for Fine Water Spray Systems for the Protection of Wet Benches and Other Processing Equipment.

FMRC Draft Performance Requirements for Water Mist Systems for Local Application System Protection.