Overview of NFPA 502 Standard for Road Tunnels ... - SCATnow

MAINTAINING A TENABLEENVIRONMENTThe 2008 Edition includes a complete revision to theannex material relating to tenable environment. Thisis intended to correlate with similar guidance materialin NFPA 130, Standard on Fixed GuidewayTransit and Passenger Rail Systems. The purpose ofthe annex is to provide guidelines for the evaluationof tenability within the tunnel evacuation paths. Currenttechnology is capable of analyzing and evaluatingall unique conditions of each path to provideproper ventilation for pre-identified emergency conditions.The same ventilating devices might or mightnot serve both normal operating conditions and preidentifiedemergency requirements. The goals of theventilation system, in addition to addressing fire andsmoke emergencies, are to assist in the containmentand purging of hazardous gases and aerosols such asthose that could result from a chemical/biologicalrelease.Environmental conditions, geometric considerations,and time considerations should be taken intoaccount. Following are some factors that should be consideredin maintaining a tenable environment for periodsof short duration.Heat EffectsExposure to heat can lead to life threat three basic ways:• Hyperthermia• Body surface burns• Respiratory tract burnsFor use in the modeling of life threat due to heat exposurein fires, it is necessary to consider only two criteria—thethreshold of burning of the skin and theexposure at which hyperthermia is sufficient to causemental deterioration and thereby threaten survival.Air Carbon Monoxide ContentAir carbon monoxide (CO) content should be as follows:• Maximum of 2000 ppm for a few seconds• Averaging 1150 ppm or less for the first 6 minutesof the exposure• Averaging 450 ppm or less for the first 15 minutesof the exposure• Averaging 225 ppm or less for the first 30 minutesof the exposure• Averaging 50 ppm or less for the remainder of theexposureThese values should be adjusted for altitudes above984 m (3000 ft).Smoke Obscuration LevelsSmoke obscuration levels should be continuouslymaintained below the point at which a sign internallyilluminated at 80 lx (7.5 ft-candles) is discernible at30 m (100 ft) and doors and walls are discernible at10 m (33 ft).Air VelocitiesAir velocities in the enclosed tramway should begreater than or equal to 0.76 m/s (150 fpm) and lessthan or equal to 11.0 m/s (2200 fpm).Noise LevelsNoise levels should be a maximum of 115 dBA for afew seconds and a maximum of 92 dBA for the remainderof the exposure.TRANSPORT OF REGULATED ANDUNREGULATED CARGOESRecent road tunnel fires suggest that goods traditionallynot characterized as ‘hazardous’; i.e. flour andmargarine (1999 Mont Blanc Tunnel), paint (1999Gothard Tunnel), and tires (2005 Frejus Tunnel),may constitute a greater risk to tunnel users and tunnelstructures than expected. As a result Chapter 13Control of Hazardous Materials has been re-titledRegulated and Unregulated Cargoes to provide guidanceto tunnel operators, first responders andenforcement officials on developing rules regardingany and all cargoes allowed passage through a particularroad tunnel facility.The authority having jurisdiction must adoptrules and regulations that apply to the transportationof regulated and unregulated cargoes. Design andplanning of the facility must address the potentialrisk presented by regulated and unregulated cargoes.When developing vehicle cargo regulations fora particular road tunnel facility, fire, accident, andresearch experience of the vehicles and cargo of thetype expected within the tunnel—and particularly ofgoods and vehicles not normally characterized ashazardous or otherwise regulated—should be considered.Some types of cargoes not normally consideredhazardous may in certain circumstances inconfined spaces within tunnels behave as, or equivalentto, hazardous materials in terms of the rate offire growth, the intensity of the fire, discharge ofnoxious materials, destruction to infrastructure, andthreat to users’ safety.In developing regulations, the following mustbe addressed:1. Population density2. Type of highway3. Types and quantities of hazardous materials192