Water Mist Fire Suppression as an Alternative to Military Specification Foam in Aircraft Hangars

Bridgett Ashley | Air Force Civil Engineer Center



The goal of a water mist system in an aircraft hangar is to suppress and mitigate fire events and prevent damage to aircraft surfaces that are directly above and/or adjacent to the fuel spill fire. Many current airframes can suffer severe structural damage from a fire in less than one minute, and existing Aqueous Film Forming Foam (AFFF) and high expansion foam agent application systems are designed to minimize damage to the incident aircraft. Inadvertent system activation, however, can damage assets and harm personnel. The objective of this study is demonstration and evaluation of water mist systems, in combination with an Ignitable Liquid Drainage System (ILDS) as an alternative to synthetic foams for suppression and mitigation of fuel spill and running fuel fires in hangars. The demonstrations will include evaluation of the technologies in a small research aircraft hangar, in a relevant operating environment, and at an operational scale. Preliminary assessment will include anticipated implementation costs, lifecycle metrics, environmental impact, and safety considerations compared to AFFF for various firefighting scenarios. The study leverages a parallel project at Air Force Civil Engineer Center/Airbase Technologies Branch to minimize fuel spill threats and heat damage to aircraft in hangars using ILDS as a fuel spill drainage and removal system.

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Technical Approach

The combination of active spill drainage and water mist provides a potentially innovative solution to fire suppression in aircraft hangars, while simultaneously eliminating the use of surfactants. The National Fire Protection Association standard 750 has approved the use of water mist for office, residential, and data centers, but water mist application technologies are not proven for aircraft hangar protection in the Air Force. Water mist, with its associated fine droplet size, presents a unique opportunity to limit damage to aircraft from fire and to limit environmental impact from AFFF. Upon detection of a fire and activation of the suppression system, fine water mist systems are designed to fill a given volume with a fog of small water particles on the order of 25-200 micron. Water mist systems suppress and reduce fire size by providing increased cooling to the fire due to the increased surface area of the smaller droplets. Water mist provides displacement of oxygen to the fire as steam is produced, and acts as a shield against heat radiated from the fire back to the fuel (and assets/equipment/occupants) within the protected space. Water mist systems are typically found in areas where nozzles can be located close to the space being protected, and as a result have traditionally not been installed in spaces with high ceilings like hangars. As distance from the nozzle increase, water droplets tend to coalesce, increase in diameter, and lose surface area and cooling capability. Advancements in water mist systems over the last several years, including floor-mounted nozzles and nozzle design to optimize droplet size, show promise for use in high ceiling facilities and aircraft hangars. The water mist system will be evaluated in conjunction with ILDS that works by allowing ignitable liquid to drain through small holes in the floor, down a slope through channels to an exterior tank. The system is actively flushed with water to both cool and separate the fuel from atmospheric oxygen to minimize fire risk.

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The National Defense Authorization Act for 2020 directs the Department of Defense (DoD) to phase out military use of per- and polyfluoroalkyl substances in firefighting foams by 2024. Adoption of water mist will remove the environmental toxicity, persistence, and bioaccumulation issues associated with legacy fluorosurfactants. This will significantly reduce the potential for subsequent remediation at DoD aircraft hangar facilities. Implementation will comply with recent Secretary of the Air Force/Installation Energy guidance (August 2019) to protect human health and the environment by mitigating perfluorooctanesulfonic acid/perfluorooctanoic acid and reduce future environmental liability to the Air Force, while complying with federal, state and DOD policy. Successful demonstration of an installed water mist suppression system will validate the technology for protecting aircraft from fuel spill fires and compatibility with existing facility operations across the DoD. An optimal water mist system will be cost effective, have lower toxicity, and support operations at both DoD and civilian airports. The combined factors provide broad commercial appeal and favor regulatory acceptance.

Distribution Statement A: Approved for public release; distribution is unlimited. AFEC-202105, 11 March 2021.

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