Performance-based approach to protecting a membrane roof aircraft hangar

AEI was retained to provide mechanical, electrical, piping/plumbing (MEP) and fire protection design services for two hangars at Chicago Rockford International Airport in Rockford, Ill. Three design challenges are highlighted.

By Affiliated Engineers February 27, 2020

AEI was retained to provide mechanical, electrical, piping/plumbing (MEP) and fire protection design services for two, 10-story, 90,000 square foot hangars at Chicago Rockford International Airport (North and South), and 24,977 sq ft of adjoining maintenance and office space.

The 100-foot high hangars are classified as Group IV Hangars under NFPA 409, meaning that the structures are constructed of a membrane-covered (fabric) steel frame, not a standard design for the majority of large hangars. Both hangars accommodate the Boeing 747 and/or Airbus A380 wide-body aircraft. Key facility features include a high-expansion foam system for fire protection and an extensive electrical system supporting in-floor hatches, grounding, and hangar lighting.

High-expansion foam system

This system serves each hangar and features:

  • Two diesel engine-driven fire pumps rated at 3,000 GPM each.
  • Two deluge systems consisting of 15 foam generators per system. Each generator weighs 720 pounds and requires 203 GPM at 50 PSI. Fan motors pull air through the generators to transform the foam concentrate and water solution by passing through the attached, five-foot diameter foam dispenser. Each fan is powered by the flow of water eliminating the need for any electrical connection to power the generator.
  • Dedicated foam supply for each hangar versus a primary and secondary supply due to hangar proximity to foam concentrate vendor.
  • Two 1,000-gallon foam concentrate bladder tanks.
  • 12-minute system discharge duration with a discharge rate of three feet per minute across the entire hangar floor resulting in a total of approximately 36 vertical feet of foam per discharge.
  • Infrared flame detectors are located within each hangar along the perimeter walls. Operation of the foam system occurs only after a detector and one of its adjacent detectors both move into alarm mode, preventing a false discharge from the failure of any single detector. The detector design also includes adjustable sensitivity. This measure excludes extraneous arcs from welding or other flashes resembling flame from activating the detector.
  • Total water output of 3,278 GPM at 115.5 PSI.
Trench drains are located in the hangar floors to route any spilled fuel and foam solution to a 50,000-gallon underground tank via an automatic diverter valve. As phosphorous levels are too high in the fuel/foam per the Illinois EPA regulations, this measure prevents the solution from entering the city sewage. Following a discharge event, the tank contents are pumped into a truck and treated offsite.

Prior to acceptance testing of the high-expansion foam system, AEI witnessed two tests and provided pressure setting adjustments to ensure only one fire pump started while the other pump was maintained as a backup. Each pump has its own controller and pressure settings are manipulated there. A water-only test was also performed to ensure all generators turned on and had the appropriate water supply. Acceptance testing involved a two-minute test of the high-expansion foam system in each hangar.

Three design challenges

  • Rockford Fire Department hangar adjacency concerns. AEI demonstrated a fire in one hangar is not intended to start a fire in the other hangar. The design implemented automatic safety locks within the programming of the system releasing panel. This prevents simultaneous operation of both foam systems, preventing manual operation of the second system after the first system has been deployed. Abort switches are also in place to stop a system discharge.
  • Aggressive schedule. The layout of the pump room was time-consuming as space was tight and equipment had to be configured for easy access for service and future equipment replacement. In addition, the pump room size was contingent on the final size of the hangars as this informed all equipment sizing and calculations.
  • Generator air source. The generators were designed to draw in air from within the hangar instead of outside air as required by code, which necessitated approval from the City of Rockford. AEI added a safety factor to the system calculation which compensates for reduced foam production based on contaminants in the hangar air such as smoke and particles of combustion.

This article originally appeared on Affiliated Engineers’ website. Affiliated Engineers is a CFE Media content partner.

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