Aircraft hangar fire protection (a la USAF)
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
As discussed last month, the criteria used for the fire protection of new Air Force hangars is found in Air Force Engineering Technical Letter ETL 02-15: Fire Protection Engineering Criteria — New Aircraft Facilities. The other codes and standards addressing the fire protection systems used in hangars include NFPA 11 Standard for Low-, Medium-, and High-Expansion Foam, NFPA 409 Aircraft Hangars and NFPA 101 The Life Safety Code. I also described the general design criteria for a system complying with ETL 02-15.
Now I would like to share some design tips learned after having been through the design, construction and acceptance testing of several hangar fire protection systems in recent years. Our office also currently has four aircraft hi-ex foam system designs in progress at this time.
When hydraulically designing the overhead automatic sprinkler system, one should be careful to balance the sprinkler demand with the high-expansion foam system demand. The sprinkler demand will generally be the larger of the two. The designer should attempt to get the minimum residual pressure required for the sprinkler system to closely match the minimum pressure required for the foam. Otherwise, a large “overflowing” of the systems can occur. This can affect water supply flow rates and quantities as well as foam concentrate flow rates and quantities. When balancing the system, I do not recommend the use of orifice plates or pressure regulating valves. Instead, judicious use of pipe sizing and sprinkler layout should be used.
A supply hydraulic calculation must also be used for sizing foam concentrate storage and flow rates. A supply calculation is one in which the demand flow and pressure are matched to the available flow and pressure, i.e., the demand point rests on the supply curve. This is a requirement of NFPA 409.
The previous article noted that ETL 02-15 requires activation of foam system discharge be by a sprinkler system water flow alarm switch, manual release station, and, where preaction sprinkler systems are used, the heat detection system. Activation using a water flow switch has been a concern of mine. In my opinion, it will most certainly be the cause of an “accidental” system activation somewhere down the line. If anyone has any ideas how to minimize the chance of accidental operation via the flow switch without impeding system reaction time, please let me know. The best we have come up with is using a 30-second retard on the flow switch, which eats into the time limits to meet foam coverage performance.
The room containing foam storage, proportioning and the foam and sprinkler riser manifold should be amply sized so one can easily move around the equipment for maintenance. Also, this room should be located as central to the foam generator locations as is possible to minimize flowing time to the generator. Do not let the architect squeeze you down. Remember to provide adequate space for the required surge suppressors and any required strainers, as they can be huge. This room should be part of the hangar building. Avoid routing foam concentrate piping or foam solution piping underground at all costs.
There are two options for foam storage and proportioning; 1) inline balance-pressure proportioning system via a foam concentrate pump and atmospheric storage tank and 2) a single bladder tank and pressure proportioner. We have had success using bladder tanks.
A word to the wise on bladder tanks. Make sure that only experts attempt to fill the foam bladder. If it is not properly supported during this operation, it is possible to tear the bag; that occurrence would kill any acceptance test deadline you were required to meet.
ETL 02-15 now requires that all system control valves must be electrically supervised. In addition to control valves on sprinkler and foam system risers, the critical valves that are part of the foam concentrate system should also be supervised with tamper switches.
The most critical factor in the system design is selection and layout of high expansion foam generators. Also remember that, unlike NFPA 11, ETL 02-15 permits use of air within the hazard. This allows the designer to avoid including costly outside air supplies to the generators. Foam generators may be either electrically powered or water powered. We have used water-powered foam generators with much success. Foam generator layout must be arranged so that the foam is dropped as close as possible to the aircraft without dropping it on the aircraft. In this way, the chances of flowing foam beneath the aircraft can be optimized. Also try to lay out foam solution piping going to the generators in a balanced tree, so that each foam generator will receive foam at about the same time and that all generators will receive about the same flow rate. Also consider hangar floor slopes and the roughness of the floor when positioning the generators.
It is important to know where the aircraft will be parked. For small aircraft, it may be necessary to consider multiple layouts. We also try to keep the discharge pressure at the foam generator as low as possible. This helps in balancing the hi-ex system with the sprinkler system. We recommend finding the most efficient foam generator you can to get the most foam discharge volume with the fewest generators.
We also recommend that a substantial overdesign be used in the total number of generators provided. We did a B-2 hangar in which a minimum of seven generators were needed. We provided ten. The contractor complained about the cost but was happy when the system passed the first time. There is no wiggle room if you find out during the acceptance that you need more generators to meet the performance requirement. (Remember that ETL 02-15 calls for 90% floor coverage of the silhouettes of all aircraft in the hangar within 60 seconds from discharge initiation and a minimum foam depth of one meter over the entire hangar floor area within four minutes).
For system controls, make sure you fully understand the hazardous classification requirements for electrical within the hangar bay. “Explosion-proof” manual release stations and notification appliances may be required. Note that explosion proof notification appliances have a large current draw, so additional NAC panels will be needed.
In my next article, I will provide some tips on getting through acceptance testing. You know, fire protection engineers like me have been doing acceptance testing for decades. Imagine my surprise when I recently found out that all this time I was not doing acceptance testing. I was doing “system commissioning.” I think it is nice that other engineering disciplines have also recently discovered the concept. There’s an idea for another article.
Aloha, Sam
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.