Water mist fire protection systems, Part 1
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
Water mist fire protection systems have been in limited use since the 1940s, and their introduction into our fire protection engineering toolbox has been a story of very slow progress. According to the SFPE Handbook of Fire Protection Engineering, three events in the 1980s spurred renewed interest in the development of water mist technology:
• The aviation industry response to the Manchester air crash in 1984 with efforts to develop an efficient method of onboard fire suppression in aircraft cabins;
• The 1987 signing of the Montreal Protocol, to phase out the use and manufacture of ozone depleting substances (Halons); and
• An International Maritime Organization (IMO) requirement for installation of fire sprinklers on all existing and new passenger ships capable of carrying more than 35 passengers.
Most of the development of water mist technology came from the search for Halon alternatives and the IMO requirements, as most research has been directed into these two areas. In 1993, the first meeting of the NFPA Technical Committee on Water Mist Fire Protection Systems was held and, in 1996, the first edition of NFPA 750 Standard for Water Mist Systems was published.
What is water mist? The 2010 edition of NFPA 750 defines water mist: a water spray for which the Dv0.99, for the flow-weighted cumulative volumetric distribution of water droplets, is less than 1,000 microns at the minimum design operating pressure of the water mist nozzle.
OK, so what is a Dv0.99? Again, NFPA 750 provides us with a definition: a drop diameter such that the cumulative volume, from zero diameter to this respective diameter, is the fraction 99/100 of the corresponding sum of the total distribution. Now, if I understand it correctly this means that, out of every 100 water mist droplets, 99 of them will have a droplet diameter of less than 1,000 microns.
Now, do not ask me what a micron is. OK, I’ll look it up. A micron is one-millionth of a meter, a thousand microns is one millimeter.
The next question you ask is, “How does that compare to the droplet sizes from a sprinkler?” Research on sprinkler droplet size indicates a wide range of sizes from about 100 to 5,000 microns. Typically, sprinkler droplet sizes are in the range of 400 to 2,000 microns.
Extinguishing mechanisms. The extinguishing mechanisms for water mist systems are very complex and depend on several factors. The SFPE FPE Handbook describes three primary mechanisms: gas phase cooling, oxygen depletion and flammable vapor dilution, and wetting and cooling of the fuel surface. The secondary mechanisms are radiation attenuation and kinetic effects. Kinetic effects include potential for increasing fire intensity by flare-up or extinguishment by “blowing out” the fire.
The 1996 edition of NFPA 750 identified three classes of water mists based on droplet size. Class 1 mist has 90% of the volume of the spray (Dv0.9) within drop sizes of 200 microns or less; Class 2 mist has a Dv0.9 of 400 microns or less and Class 3 mist has a Dv0.9 value larger than 400 microns. These designations no longer appear in NFPA 750 or other standards related to water mist fire protection systems, but they are helpful in understanding the extinguishing mechanisms prevalent in particular systems.
The finer droplet sizes of Class 1 mists and their high surface area-to-volume ratio exhibit better gas phase cooling and O2 depletion through enhanced heat absorption and water vapor creation. Those of us who have stood in line with the munchkins at a Disney park on a hot summer day may have been able to appreciate the cooling effect from misting systems. The larger droplet sizes of Class 3 mists are better at wetting and cooling of the fuel surface. Class 2 mists tend to apply all three primary mechanisms more equally.
As noted, droplet size is not the sole determiner of extinguishment effectiveness. Other factors include fuel properties, enclosure configuration, ventilation, spray flux density and spray momentum.
The system. Water mist fire protection systems can either be the closed-head type or the open-nozzle (deluge) type. NFPA 750 categorizes water mist systems by maximum pressures developed in system piping. Low pressure systems have pressures of 175 psi or less, intermediate pressure systems are in the range of 175 to 500 psi and high pressure systems operate at pressures greater than 500 psi.
Water can be stored as part of the system or supplied by a water main, similar to automatic sprinklers. Stored water is typically held at atmospheric pressure in a tank and is pressurized at system actuation by compressed nitrogen or air from separate compressed gas cylinders in what is called a single fluid system. Some systems deliver water and gas (atomizing medium) via separate piping to each nozzle. They are called twin-fluid systems. Pressure can also be developed by a pump or multiple pumps in series.
Both NFPA 750 and Factory Mutual (FM Data Sheet 4-2 Water Mist Systems, July 2011) require a reserve supply for systems where replenishment is not available within 24 hours. This may be encountered with systems using an atomizing medium. This reserve supply is not required to be connected, but a connected supply is recommended. FM Data Sheet 4-2 requires a connected reserve if 1) the system protects multiple hazards or 2) if protection cannot be restored in 24 hours and the protected hazard is not constantly attended.
Water mist system piping is either copper tubing or stainless steel piping. Fittings must be appropriate to the system pressure rating. Consideration must be given to the need for strainers. Note that strainers are required on all Factory Mutual systems. Pumps used in water mist systems must comply with NFPA 20.
Nozzles that are available as open head or closed head type must be listed, either individually as a component or as part of a system. Nozzles are listed by one of the following standards:
• FM Approval Standard 5560, Water Mist Systems
• UL2167, Water Mist Nozzles for Fire-Protection Service
• IMO 800 (19), Revised Guidelines for Approval of Sprinkler Systems Equivalent to that Referred to in SOLAS Regulation II-2/12
NFPA 750 requires the nozzle listing to include the following information:
• Specific hazards and protection objectives
• Volumetric flow rate characteristics of water discharge for each nozzle
• Maximum height of protected space
• Minimum distance between nozzle tip or diffuser, as applicable, and plane of protection
• Maximum and minimum spacing between nozzles
• Maximum coverage area per nozzle
• Maximum height between ceiling and nozzle diffuser or tip, as applicable
• Nozzle obstruction spacing criteria
• Maximum spacing of nozzles from walls
• Minimum- and maximum-rated operating pressures of nozzles
• Allowable range of nozzle orientation angle from vertically down
• Classification of automatic nozzle thermal response characteristics as fast, special or standard response
• Maximum compartment volume, if applicable
• Maximum time delay for water mist delivery to the most remote nozzle
The Design. The current state-of-the-art in water mist fire protection system design is that there is no comprehensive set of guidelines that can be used to engineer a system, such as we are able to do for sprinkler systems using NFPA 13. Therefore, unless you are bold and like to go where no person has gone before, you are constrained with protecting hazards that fit into the limited number of approved pre-engineered system designs. More on that next month.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.