Aircraft hangar fire protection (a la USAF)
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
The per copy cost of the Air Force’s advanced fighter jet, the Lockheed Martin F-22 Raptor, is approximately $140 million. Given the cost and the critical 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 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.
ETL 02-15 addresses protection criteria for Air Force hangars in which fueled aircraft are located. The suppression systems components for the hangar consist of the following: 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 protection, 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 requirements, 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 protection. 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 aircraft in the event of a fuel spill and fire.
The automatic sprinkler protection is generally understood to provide protection for the hangar structure only and consists of wet pipe sprinklers, unless freezing temperatures 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 temperature rating. A 500 gpm hose stream allowance must also be included. Draft curtains are required to be provided 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 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 criteria 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.
The high expansion foam concentrate is typically available 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 proportioning. The foam concentrate can be stored in an atmospheric tank and delivered to an inline balance-pressure proportioning 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 generators 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 02-15 permits use of air within the hazard. This allows the designer to avoid designing costly outside air supplies to the generators. Foam generators may be either electrically powered or water powered.
The high expansion foam system will have its own fire suppression control panel. The activation of foam system discharge is initiated by a sprinkler system water flow alarm switch, manual release stations, and, where preaction sprinkler systems are used, the heat detection system. Separate notification appliances are required to activate upon the foam system discharge and must be blue visual alarm devices, either stroboscopic or rotating beacons. ETL 02-15 has a fire suppression system matrix of operation that captures all the functions of the suppression system controls.
Normally, municipal or military base water supply systems are not capable of meeting the total demand of the hangar fire suppression system and an onsite storage/fire pump system is required. Two equal size water tanks are required, sized to support 120 percent of the maximum demand, for a period of 30 minutes. The purpose of two tanks is to permit the fire protection system to be operational with one tank out of service for maintenance. Fire pumps are limited to a maximum individual size of 2,500 gpm at 125 psi. Similar to NFPA 409, one redundant fire pump must be provided, i.e., the system must be capable of meeting the largest water demand with the largest fire pump out of service. Fire pumps are to be electrically driven when supplied by a reliable electrical power supply source; otherwise, diesel engine driven pumps are to be used. It is noted that pumps must be automatically started from a pressure drop in the system. NFPA 409 also recommends that fire pumps be arranged to start via an electrical start signal from the fire suppression control panel.
Surge suppressor (expansion) tanks are also required for each sprinkler riser, for the high-expansion foam system riser and at the discharge of the fire pump system.
In my next article I will discuss the specific fire suppression system design features of a recent project using high-expansion foam.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.