Aircraft Hangars Can Now Turn to Other Means

Prior to last year, NFPA 409, Standard on Aircraft Hangars, gave hangar operators only one option for fire protection in Group 1: overhead foam-deluge systems. While effective, these systems have always been costly to operate and often damage aircraft when used, because the water supply and low expansion foam requirement is based on the height of the ceiling.

07/01/2002


Prior to last year, NFPA 409, Standard on Aircraft Hangars , gave hangar operators only one option for fire protection in Group 1: overhead foam-deluge systems. While effective, these systems have always been costly to operate and often damage aircraft when used, because the water supply and low expansion foam requirement is based on the height of the ceiling. In other words, as the ceiling height increased, so did the number of systems that were expected to operate in a fire situation. And with a design density at the roof of 0.16 gallons per minute per square foot, these systems proved expensive due to the large water and foam concentrate requirements.

In 2001, NFPA revised its code, and in doing so, the fire prevention criteria for Group 1 hangars also changed, providing hangar operators with more fire-prevention options. One new approach allowed under the revision is to use under-wing foam systems in conjunction with standard closed-head, water-based sprinkler systems on the ceiling. This new system not only offers the potential to save money, but also provides a more effective system.

Anchors Away

A leading proponent of this methodology is the U.S. Navy, who fit out its new F-18 Aircraft Maintenance Hangar at the Naval Air Station Oceana in Virginia Beach, Va., with such a system.

The concept, specifically a system of nozzles mounted in drainage trenches, was first proposed by engineers from the Naval Facilities Engineering Command (NAVFAC) as an outcome of their efforts to overhaul fire-protection requirements for Navy and Marine Corps hangars. At the time, the Navy was concerned with the high costs associated with overhead-foam deluge systems, and began testing to determine the best method of applying foam solution to the floor.

Based on its initial findings, NAVFAC investigated many schemes and discharge devices, eventually settling on the system that applied foam directly at the floor, where a fire would start from the ignition of liquid fuel igniting.

Prototype nozzles were developed and tested in consultation with Baltimore-based fire protection engineer Hughes Associates, Inc. Design, development and commercial production of the grate-nozzle system was handled by the Viking Corporation of Hastings, Mich.

The finished fire-protection system has nozzles located at 25-ft. centers in trenches 50 ft. apart—or 1,250 sq. ft. per nozzle. Each discharges foam in a radial pattern at a height of approximately 12 to 18 in. above the floor—a significant factor as it is advantageous to release foam from many points over the hangar floor. This helps reduce the impact of a blocked nozzle as in-fill will occur from surrounding nozzles. Furthermore, the grate system allows even blocked nozzles to discharge foam in the area because they don't suffer from the limitations of oscillating monitor nozzles.

Another benefit of the grate system is that the low-discharging foam minimizes the risk of foam or water entering the aircraft and fouling any sensitive avionics that may be exposed for maintenance.

The grate nozzle itself was developed to overcome the problems observed with the foam cannons that were part of the traditional systems. In such designs, cannons sweep in an arc pattern across a hangar floor. The cannons are placed around the perimeter of a hangar bay, with each device covering anywhere from 4,000 to 15,000 sq. ft. The problem with covering such a large area means that any misalignment or cannon obstruction results in parts of the hangar floor receiving little or no foam at all.

By comparison, the grate nozzle is installed flush with the trench grate, has no moving parts and each nozzle is identical, with no varying individual settings such as the arc radius and direction requirement for foam cannons.

Also, the nozzle is designed to take the load of any aircraft or support vehicle. Under torsion, the top plate will rotate about the center of the nozzle, allowing the nozzle to absorb turning and off-center wheel loads. The top plate of the nozzle overhangs the openings through which the foam/water solution is discharged, making it unlikely that any debris will enter the system piping through the nozzle.

A win-win system

As noted earlier, hangars using the grate nozzle system will also require overhead wet-pipe or pre-action sprinklers, as well as a rapid-acting fire-detection system, such as triple infrared optical detectors. The overhead closed-head sprinkler system, however, only need discharge in conjunction with the grate nozzles. Previous code requirements mandated for a deluge foam/water sprinkler system and an under-wing foam system. With foam removed from the overhead sprinkler system, and provided only by the low-level system, the amount of foam required in the hangar is significantly decreased. Not only is this a money saver in equipment installation, it also reduces the size of retention and containment systems. Because discharge of foam into the environment—or into wastewater treatment facilities—is often restricted, it is mandatory to construct containment systems.

Thus, the new low-level fire-fighting foam delivery system for aircraft hangars promises to provide enhanced fire protection, with its rapid activation by optical detectors and placement of foam directly onto any fuel spills. At the same time the design changes aim to improve system reliability by reducing false activations, which, in turn, should help protect avionics in averting unnecessary contact with foam.

In nearby Norfolk, at Chambers Field Naval Station, construction has already begun for two new hangars, with designs for other hangars in progress.





No comments