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A few years ago, I served as project manager of the mechanical, electrical and plumbing (MEP) design team for the new 185,200-square-foot Army National Guard Readiness Center and Army Aviation Support Facility (AASF) in Kankakee, Illinois. Located 60 miles south of Chicago, the facility is now home to two aviation companies, a medivac unit and approximately 10 UH-60 Black Hawk helicopters.
As the project began, I remember thinking how cool it would be to finagle a ride on one of the Black Hawks. Before that had any possibility of happening, of course, the Readiness Center and AASF had to be designed and constructed.
This included the Black Hawks’ maintenance and storage hangars, which utilized one of the unique features of the project: a complex fire protection and high-expansion foam (HEF) fire suppression system. (The HEF used on this project does not introduce any hazardous per- and poly-fluoroalkyl substances into the environment.)
At the time of this project, the systems were primarily designed based on the Air Force Engineering Technical Letter (ETL) 02-15: Fire Protection Criteria - New Aircraft Facilities, along with portions of NFPA 409, Standard on Aircraft Hangars. The following is a high-level summary of the system and its integration with the hangar infrastructure.
Discharging the Foam
In the event a fire is detected by multispectrum infrared flame detectors, overhead foam generators blanket the hangar floor with foam to extinguish the fire and protect the aircraft. The hangars also are protected with a double-interlock, pre-action sprinkler system to prevent sprinkler piping from freezing when hangar doors are open during winter.
The foam system is intended to protect the Black Hawks (the cost of which can be well upward of $10 million each); the water system is intended to protect the structure; and both systems serve to protect building occupants.
The flame detectors crisscross the aircraft bays approximately 8 feet above the floor and are aimed at the center of the bay underneath the aircraft. They can sense a flame as small as 1 cubic foot and are connected to the fire alarm system.
The HEF system can be activated in several ways: the flame detectors, manual foam activation pull stations located at the main exits (including between hangar doors), water flow signal from the overhead sprinkler systems, or the ceiling-level heat detection system associated with the double-interlock, pre-action system.
The hangars contain several color-coded alarm lights; when the HEF system is triggered, amber lights flash, indicating the HEF release countdown has begun. Up to that point, an abort button at the pull station can be used to stop the release. It requires one person to hold the button to prevent the release until another person can disable the system at the main panel.
Once the HEF system has been released, red alarm lights flash, indicating foam is on the way. The temperature sensors above the hangar doors trigger blue flashing alarm lights if the temperature approaches freezing. Green alarm lights flash at each safety shower and eyewash station located at intervals along the back wall of the hangar. Each of these stations is also connected to an annunciator panel located at the occupied front desk.
In the event of a fire, the discharged HEF exits the hangars via sloped floors leading to continuous trench drains just inside the hangar doors. The primary purpose of the trench drains, however, is to prevent a liquid spill within any aircraft servicing area from flowing into an adjacent aircraft bay. The trench drains are connected to oil separators located outside both the maintenance and storage hangars.
A bypass for the drainage piping circumvents each oil separator in the event of an HEF discharge; an automatic control valve located in a manhole upstream of each oil separator closes when the foam system is activated.
The control valve shuts off the pipe going to the inlet of the oil separator and allows the liquid level in the manhole to rise and exit via a gravity bypass pipe that has a slightly higher invert than the inlet to the oil separator. The bypass discharge reconnects to the drainage piping downstream of the oil separator.
Fire Flow Requirements
Two 75,000-gallon water storage tanks were installed on the backside of the hangars to meet the fire flow requirements of the building sprinkler and HEF system. The tanks were sized to provide storage capacity equal to 120% of the maximum demand for 30 minutes, with each tank storing half of the required volume.
The storage tanks are insulated; each tank includes a
7.5 kW heater to keep the water from freezing. A 6-inch water line with an automatic control valve located in a manhole provides makeup water for each storage tank. An ultrasonic sensor in each tank controls the water level, and a visible manual gauge is mounted on the exterior of each tank.
A 12-inch diameter outlet pipe extends from each storage tank to two 2,500 gallons/minute fire pumps — one of them standby — serving the readiness center and hangar HEF and double-interlock systems. The tanks are slightly elevated to ensure flooding the pump suction.
As this is a critical facility used in the case of civil emergencies, the Readiness Center and AASF are designed to meet seismic criteria. All sprinkler and HEF piping is designed accordingly, including bracing for the foam generators located high within the hangars.
Generators at Truss Level
The foam generators are centrally located within the hangars at roof truss level, a height avoiding the large bridge cranes that traverse the entire maintenance hangar. The foam generators are spaced along the entire length of the hangars so that foam discharge falls close to, but not directly on, the aircraft fuselage or wings. The intent is for the initial discharge of foam to protect the floor area underneath the aircraft and then spread to the remaining hangar floor area.
ETL 02-15 requires the foam to cover 90% of the aircraft silhouette area and be projected onto the floor in one minute or less. Foam must also be projected onto the aircraft servicing area and adjacent accessible areas to a depth of one meter (3.2 feet) in four minutes or less.
ETL 02-15 also requires a system of temperature sensors for hangars in geographic areas having a 99.6% dry bulb temperature of less than 30 F when wet-pipe sprinklers are present. Although wet-pipe sprinkler systems are not uncommon in hangars in the Midwest, a double-interlock, pre-action sprinkler system was recommended for the AASF due to the large hangar doors.
These large hangar door openings led to concerns that a wet-pipe sprinkler system might freeze, especially near the doors, even though the hangars are heated. All hangar doors face south — a basic but important design consideration to reduce the potential for the doors icing closed.
In addition, due to the critical nature of the facility, a glycol heating loop was installed in the concrete floor underneath all the doors to prevent them from freezing closed. A glycol snowmelt heating system was also installed in the concrete apron to 20 feet outside of the doors for the entire length of the hangars.
The chosen double-interlock pre-action system reduces the potential for frozen piping, but temperature sensors were included at 200-foot intervals above the hangar doors to monitor the temperature at the sprinkler piping level.
At the completion of construction, the HEF system was tested, which involved initiating and releasing foam. Plastic, 1-meter-tall (3.2 feet) barrels were placed at intervals down the center of the hangar to provide a quick reference as to the depth of the foam when timing it to reach that level within four minutes or less.
The maintenance hangar was tested before approval was given to construct the storage hangar; upon completion, it was also tested. Both foam tests went off without a hitch and easily filled the hangars to the 1-meter (3.2 foot) depth within the four-minute requirement.
Since foam suppression systems are rare, the tests were somewhat of a spectator event, with local fire department and building officials invited along with military personnel, contractors and others. A viewing stand made of scaffolding was erected for spectators to peer over an 8-foot-tall temporary plastic liner installed around the inside perimeter of the hangars to protect wall devices and minimize the cleanup afterward.
With the project successfully completed, my team and I were then able to score a ride in one of the Black Hawks. It was an experience I’ll never forget, and I thank the flight crew for the once-in-a-lifetime opportunity.
Kevin Magnuson, LEED AP, is a project manager and lead mechanical engineer at IMEG. He has a broad range of expertise in mechanical systems, including HVAC, chilled and hot water distribution, system controls, piping, and fire protection.