FPE Corner Water mist fire protection systems, Part 1
Samuel S. Dannaway, PE, FSFPE President, S.S. Dannaway Associates, Inc., Honolulu
W
ater mist fire protection systems have been in lim- ited 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) require-
ment for installation of fire sprinklers on all existing and new passenger ships capable of carrying more than 35 pas- sengers. Most of the development of water mist technology came
from the search for Halon alternatives and the IMO require- ments, 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
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 dis- tribution. 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.
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
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 mecha-
nisms 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 poten- tial for increasing fire intensity by flare-up or extinguish- ment by “blowing out” the fire.
Page 24/Plumbing Engineer 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 sys- tems, but they are helpful in understanding the extinguishing mechanisms prevalent in particular systems. The finer droplet sizes of Class 1 mists and their high sur- face 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 extin- guishment effectiveness. Other factors include fuel proper- ties, 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 sys- tems are in the range of 175 to 500 psi and high pressure sys- tems 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 pres- surized 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 con- stantly attended. Water mist system piping is either copper tubing or stain-
less steel piping. Fittings must be appropriate to the system pressure rating. Consideration must be given to the need for
Continued on page 26 October 2011
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