At sea aboard USS John C. Stennis (CVN 74) on January 28, 2003. Damage Controlman 3rd Class Nicholas Lewis inspects aqueous film forming foam overhead sprinkler systems in the ship’s hangar bay. (U.S. Navy photo by Photographer’s Mate 2nd Class Jayme Pastoric.)
By Jacob McAfee
Water-based fire protection systems have been around since 1860, and water mist systems are making a strong argument that they will be around for years to come in the form of water mist systems.
Can water mist systems take over the fire protection industry? With the growing safety concerns and environmental impact of other suppression agents such as halogenated hydrocarbons and carbon dioxide as well as dry and wet chemical, water mist systems could be the answer.
The most common fire protection system used in the United States today is wet pipe automatic fire sprinklers. So, why not just improve what already works: water. In 1996, the National Fire Protection Association (NFPA) created NFPA 750, Standard on Water Mist Fire Protection Systems, which is now the standard on the installation of water mist fire protection systems. A water mist system is a distribution system connected solely to a water supply or alternatively to a water supply and an atomizing media (air or nitrogen) that is equipped with one or more nozzles capable of delivering water mist intended to control, suppress, or extinguish fires; this is defined by NFPA 750.
The performance objectives set fourth by NFPA 750 are to control the fire through a reduction in thermal exposure, to suppress the fire by drastically reducing the heat release rate and prevent restart, and to ensure complete extinguishment. Further studies have shown that water mist systems can also be effective at controlling a room’s temperature, which allows for a safe egress and reduced damage as well as exposure protection through prewetting of combustibles ahead of the advancing fire.
The key to a water mist system working more proficiently than traditional fire sprinklers is the water droplet size. The water droplet size on water mist systems at the minimum operating pressure is no less than 1,000 microns. With the smaller droplet size, this creates more droplets. With a larger water surface area exposed to heat, more drops will evaporate and turn to steam. The steam then absorbs more heat per unit time from the flame, reducing the flame temperature. Not only will water mist systems control room temperature and convert the droplets into steam, suffocating the fire, but water damage from water mist systems as compared to ordinary water sprinklers is lowered considerably. The water damage from using these systems will been reduced to the point that they are proving to be effective on electrical fires as well as occupancies with high value items that require as little water damage as possible.
Water mist systems could possibly be the answer to effectively combating all classes of fire (with the exception of Class D combustible metal fires). Class A fires are traditionally controlled by water sprinklers, and Class B fires are controlled with foam and dry chemicals. Carbon dioxide and halogenated agents are used for Class C electrical fires, while Class K fires are currently extinguished by wet chemical.
For years, dry chemical and low-expansion foam have been the staples of fighting Class B flammable liquid fires. Low expansion Class B foam such as aqueous film forming foam (AFFF) are the most common along with high-expansion systems and, in some instances, alcohol-resistant foam. Since it’s the vapor and not the liquid that burns during flammable liquid fires, AFFF is used to create a blanket over the liquid that suppresses the vapors, cools, and excludes oxygen . High-expansion foam is a volumizer and relies on its ability to fill an area with a large volume of super-aerated foam. These foam systems are common in aircraft hangars and fuel farms where flammable liquid fires are commonplace.
Although water mist systems cannot create a blanket over the surface of a flammable liquid, they can be just as effective. Water mist systems can displace oxygen from the steam conversion and extinguish flammable liquid fires. In these instances, water mist systems would have to stay on for a period of time after extinguishment to prevent reflash. Water mist systems have to be used in a confined environment—not out in the open—for them to work to their full potential and for the steam conversion to be effective.
Dry chemical is a powder that consists of very small particles that are suspended in a gaseous medium, which permits distribution of the powder to the hazard. Dry chemicals’ ability to adhere to the surface and prevent reignition as well as the fact that it produces a rapid knockdown of flame by breaking the chain reaction can be both good and bad. Dry chemical can be hard to clean up and, if mixed with water, may solidify and be hard to get off. Maintenance can also be expensive; if dry chemical sits without being agitated, it can solidify in the storage container and be unable to discharge with the introduction of nitrogen. With dry chemicals, you also need to fulfill other requirements such as signs and markings to identify that there is a system that requires people to exit; it can also present a respiratory hazard. When used during Class C fires, dry chemical can damage other electrical equipment in the room from the powder spread from discharge, increasing the monetary loss from the fire.
Over the past decade, there has been substantial research done regarding water mist technology in fire suppression applications when extinguishing Class C fires in electronic equipment and computer rooms. This research shows that water mist systems can be just as effective at slowing the spread of fire during Class C fires without the cleanup that comes with dry chemical systems. Testing the system also becomes significantly easier than dry chemical systems because of the discharging of dry chemicals vs. water. Cost becomes a factor after use when you consider purchasing more dry chemical or refilling water.
Clean agents are called as such because of their ability to extinguish electrical fires without leaving a residue or creating damage to exposures, as would dry chemical or traditional water sprinklers. These agents displace the oxygen to extinguish fire. With these systems come the potential for significant safety considerations that may require additional costs such as marking and signage requirements, audible and visual notification systems, and employee training. Because of the droplet size inherent in water mist systems, very little damage is created from the water used in other areas, and the costs can be significantly less.
Two designs are used when installing or designing water mist systems: local application or total flooding. Total flood design uses nozzles placement to cover the whole room regardless of the location of the fire. The local application method places nozzles over a specific hazard area. Both design methods have their place, but a new design that is emerging—zoned application—has several distinct suppression zones. With special hazards or in occupancies with other high-value items, you may want to use a zoned or local application method to reduce water contact with other items.
Water mist systems appear to offer the best value among all fire suppression systems. Although it may not be practical for water mist systems to take over as an all-class fire suppression system for every scenario, it may be the case for fixed fire suppression systems. When considering the overall use and diversity of water mist systems such as effectiveness, cost, training, testing, and application, it has a chance to be the future of fire protection.
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Jacob McAfee is a 15-year fire and emergency services veteran, having served in a variety of positions across the military and Department of Defense (DoD). He is the assistant chief of operations at the United States Military Academy in West Point, New York. McAfee began his career with the United States Marine Corps as an aircraft rescue firefighter; after eight years of service and multiple deployments, he left active service honorably in 2007. He’s lead and mentored personnel in Iraq as a captain and division chief. After Iraq, he has served as an assistant chief of operations, a fire marshal, a fire prevention chief, and a health and fitness coordinator as a civilian with the DoD.
McAfee is a chief fire officer designee and a chief training officer designee through the Center for Public Safety Excellence. He is a member of the Institute of Fire Engineers and sits on the professional development and education committee. McAfee earned a Master’s Degree in occupational safety and health and emergency management. Currently, McAfee is working on his PhD in emergency management with Capella University and is attending the National Fire Academy’s Executive Fire Officer Program. He is an instructor for the California Office of Emergency services—hazardous materials section, the California State Fire Marshal, the National Safety Council, and the American Heart Association. McAfee also instructs hazardous materials, urban search and rescue, and incident command courses throughout California. He is IFSAC/Pro Board certified Fire Officer IV, Inspector III, Instructor III, hazmat technician, hazmat officer, confined space technician, swift water rescue technician, trench rescue technician, advanced rope, and collapse structure technician.