THE EFFECTS OF WATER MIST ON INTERIOR FIREFIGHTING
BY CHRISTIAN DUBAY
While water mist fire protection systems have gained some successful experience in Europe, they have failed to achieve much attention in the United States until recent years. This initial lack of interest stems from the reliance on a combination of separate systems including halon; carbon dioxide (CO2); and, to a lesser extent, an array of traditional sprinkler systems and other water spray protection schemes. However, in view of the recent findings concerning halon`s negative impact on the environment in conjunction with the life-safety risks posed by the actuation of CO2 systems, steps have been taken to restrict the use of CO2 systems while phasing out halon systems entirely. As a result, research efforts began in the late 1980s to find a suitable replacement system to deal with the hazards most typically protected by CO2 or halon.
Since the onset of these developments, water mist systems have been tested in numerous applications with varying degrees of success. Tests have included residential, computer, shipboard machinery space, combustion turbine, and library/records storage applications. While no system has received blanket approval, many manufacturers have developed systems actuated by various means including smoke detection, annual operation, and thermal activation of individual nozzles. This in turn activates pumps, traditional water supplies, or pressurized storage tanks, usually ranging in pressures from 100 to 2,000 psi. While some of these systems operate at high pressures, the injury potential for occupants and firefighters is minimal due to the low system flows and small water droplet size. Water mist systems can be expected to flow on the order of one-tenth the water used by automatic sprinklers, decreasing the time needed for water removal and overhaul.
The new technology provides a different challenge for firefighters entering spaces protected by water mist. Applying water mist to a fire provides additional hazards for the interior structural firefighter. How these hazards will affect operations and the safety of firefighters is discussed below.
The National Fire Protection Association (NFPA) Draft Standard 750, Standard for the Installation of Water Mist Fire Protection Systems, defines water mist as a water spray for which the Dv0.99 is less than 1,000 microns. This means that 99 percent of the generated droplets have a diameter of 1,000 microns or less. To put this in perspective, 1,000 microns is equal to one millimeter. The extremely small water mist particles allow water mist to attack the fire with the benefits of a water-based extinguishing agent (i.e., automatic sprinklers) capable of being entrained by the fire. This interaction allows the water mist protection system to extinguish or control the fire in several ways.
Flame cooling. On activation of the system, the space or area to be protected is flooded with water mist. Due to the small size of the water mist particles, they rapidly absorb heat from the flames, making it difficult for the flaming combustion to continue. This rapid heat absorption causes the flames to be extinguished in the area affected by the water mist application.
Oxygen displacement. The displacement of the oxygen is caused primarily by the production of steam. When the fine water mist particles interact with the flames or superheated gases, they are rapidly turned into steam. The steam in turn takes up much more volume than the water mist particle and, in the process of expansion, displaces the oxygen in the area of the steam. This is most pronounced in the flaming region of the fire, since it is in this region that the oxygen is consumed.
Obstructed/shielded fire penetration. The fine spray characteristics of water mist allow the small water particles to penetrate into many of the obstructed or shielded fires that traditional sprinklers could not. Additionally, these qualities allow the water mist to be drawn into spaces open to the water mist nozzles (i.e., computer cabinets).
Radiant heat exposure protection. When total compartment application is desired, system activation in the space results in the enclosure`s being flooded with water mist. The water mist not converted to steam settles on the surfaces of surrounding fuels, providing a form of prewetting that protects the other fuels from the fire`s radiant exposure. This protection helps to prevent adjacent fuels from igniting. By cooling the ceiling, walls, and contents, the water mist also helps to delay or prevent flashover.
EFFECTS ON INTERIOR OPERATIONS
These extinguishing qualities of water mist have several effects on the environment of the enclosures. These effects will in turn have some impact on the interior firefighting operations.
The overall visibility of the space is drastically reduced. The large quantity of small water droplets mixes with large volumes of steam, and smoke may reduce visibility to near zero throughout the space. The visibility is further reduced when light is present in the space. The reflectivity of the water mist particles and the steam is very similar to that of fog. During some of the residential test scenarios, visibility often was greater with no additional light source.
The expected thermal layer separation present in most fires will not be present once the system is activated. The water mist acts much in the same way as an automatic sprinkler system in that it provides mixing throughout the space so that the firefighter will encounter a relatively uniform elevated temperature throughout the space. This mixing allows the smoke, steam, and hot gases to be present at all levels within the space. However, a general temperature trend of higher temperatures at the ceiling and lower temperatures at the floor still will be pres-ent.
Again, as with automatic sprinklers, steam is produced during the extinguishment process. This not only reduces visibility but may provide a serious burn potential. The mixing characteristics of water mist may enhance this hazard in large fire environments by allowing the steam to be mixed throughout the space.
Due to the small water mist particles and large volumes of steam, many of the products of combustion become attached to the water particles, allowing them to be more readily inhaled even after fire extinguishment.
SAFETY CONSIDERATIONS
Several considerations should be ad-dressed when attacking a fire in a building or space protected by a water mist suppression system.
Protective clothing is of the utmost importance when entering an area where a water mist system has been activated. The mixing of the thermal layers will have caused the floor temperatures to be increased above what would normally be expected, and the large volumes of steam will be spread throughout the space. As with the steam produced from fog nozzle water application, it will rapidly penetrate any openings in the firefighter`s protective clothing. As always, extra care should be taken to ensure that the protective clothing has been donned completely and correctly.
While the activation of the water mist system has caused mixing throughout the space, the lower temperature will still remain at the floor. Therefore, it should be a priority to remain as low as possible. While the temperature will be elevated at the floor, the steam and hot gases will continuously try to rise and collect at the ceiling. This will allow some relief from exposure to the steam and hot gases at the floor level.
As always, during overhaul it is important to continue to use self-contained breathing apparatus (SCBA). While the space may appear to be clear or to have only water mist present, the products of combustion experienced in normal firefighting operations still are present and have become attached to the small water particles. These small water particles are more readily absorbed into the body.
Ventilation should be handled in the traditional ways. Note that it may take longer to remove all the smoke and water particles. The water mist particles are heavier than smoke or steam. They will move more slowly than smoke and may collect at lower levels of the building or space. In many of the residential tests, positive pressure proved to be the most effective method for removing smoke and mist.
Salvage covers should still be used in the early stages of the operation, as the water mist will tend to settle on flat surfaces. The water mist should be expected to travel into adjacent spaces through doors, windows, cracks, and other openings. Due to the nature of the water mist particles, they will tend to travel with the smoke. Salvage covers will help protect the contents from water damage and staining.
By understanding the processes of water mist and the new hazards they present, the firefighter can operate in these situations with little modification to existing standard operating procedures. Preplanning and building surveys along with an on-site walk-through can provide the knowledge officers and firefighters need. n
CHRISTIAN DUBAY is associate fire protection engineer with the National Fire Protection Association and has been a volunteer firefighter for 10 years. He has a bachelor`s degree in fire protection engineering from the University of Maryland and is a certified fire instructor, inspector, and fire officer.
