FPE Corner Aircraft hangar fire protection (a la USAF)
Samuel S. Dannaway, PE, FSFPE President, S.S. Dannaway Associates, Inc., Honolulu
T
he per copy cost of the Air Force’s advanced fight- er jet, the Lockheed Martin F-22 Raptor, is approx- imately $140 million. Given the cost and the criti-
cal importance of this aircraft to our national defense, it is essential to provide fire protection for the hangars used for maintenance, service or storage of this and other high-tech and high-priced aircraft. 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. Other codes and stan- dards 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. ETL 02-15 addresses protection criteria for Air Force
hangars in which fueled aircraft are located. The suppres- sion systems components for the hangar consist of the fol- lowing: wet pipe sprinklers, hi-ex foam system, detection and control, foam concentrate, foam concentrate storage, foam concentrate proportioning, foam generation and water supply. Aircraft hangars have several other features of fire pro-
tection, including minimum construction requirements, internal fire separations, allowable floor area, building separations, draft curtains, floor and ramp drainage, fire hydrants, foam-water retention systems, egress require- ments, accessibility for firefighting, fire alarm system and hazardous location electrical. This article will focus on the fire suppression systems. The fire suppression system requirements of ETL 02-15
are relatively consistent with NFPA 409 requirements for Group I Aircraft Hangars. One option permitted by NFPA 409 is to provide automatic sprinkler protection with high- expansion foam for supplementary or low level protec- tion. That option is the basic requirement of ETL 02-15. The term low level protection is for a system that provides protection for under-fuselage and under-wing areas of air- craft in the event of a fuel spill and fire. The automatic sprinkler protection is generally under-
stood to provide protection for the hangar structure only and consists of wet pipe sprinklers, unless freezing tem- peratures are an issue, in which case, a preaction sprinkler system activated by a heat detection system is provided. The sprinkler system must be hydraulically designed to deliver a minimum design density of 0.20 gpm/square foot over a 5,000-square-foot remote area. Sprinklers must be the upright, quick-response type with an intermediate tem- perature rating. A 500 gpm hose stream allowance must also be included. Draft curtains are required to be provid- ed to surround each individual sprinkler system or to cover an area of 15,000 square feet, whichever is less. By definition, high expansion foam systems produce
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foam with a solution-to-foam ratio in the range of 1:200 to 1:1000. The foam is discharged using foam generators located to deliver a blanket of foam over the entire aircraft hangar floor area. The ETL 02-15 system performance cri- teria requires 90% floor coverage of the silhouettes of all aircraft in the hangar within 60 seconds from discharge initiation and to provide a minimum foam depth of one meter (3.2 feet) over the entire hangar floor area within four minutes.
The foam discharge rate is calculated using the formula
from NFPA 11 that considers the hangar floor area, the required submergence depth, a factor for foam leakage from unclosable openings, a factor for normal foam shrinkage and a factor to account for foam breakdown from sprinklers.
Foam bladder tank, pressure proportioner. The high expansion foam concentrate is typically avail-
able for proportion in the range of 2.00% to 2.75%. The amount of foam concentrate required must be enough to provide for a 15-minute discharge or to provide four times the submergence volume, whichever is greater. Unlike NFPA 409, ETL 02-15 does not require a reserve supply of foam concentrate. Also note that NFPA 409 requires that a hydraulic calculation using the supply calculation method be performed for the purposes of determining the quantity of foam concentrate required. There are two options for foam storage and proportion-
ing. The foam concentrate can be stored in an atmospher- ic tank and delivered to an inline balance-pressure propor- tioning system via a foam concentrate pump. The other option is to use a bladder tank and pressure proportioner. One of the most critical factors in the design of such
systems is the selection of foam generators. Foam genera- tors are typically listed or approved at a range of discharge pressure and volumetric foam flow rates (CFM). Though NFPA 11 has a general requirement that the air supply to foam generators come from outside the hazard area, ETL
Continued on page 24 May 2011
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