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

Specifically, Annex E now suggests the considerationof fixed fire suppression systems when it canbe shown by engineering analysis that the level ofsafety can be equal or exceeded by their use as partof an integrated approach to the management of lifesafety. Annex E also addresses, as follows, the previouslylisted concerns which had been expressed bytunnel designers, engineers and worldwide (authorities)regarding the application of fixed fire fightingsystem in road tunnels:Concern (1) Typically fires in road tunnelsusually occur inside vehicles or inside passengeror engine compartments designed tobe waterproof from above; therefore, fixedfire fighting systems might not have an extinguishingeffect.It is now recognized that the purpose of afixed fire fighting fire suppression system is toprevent fire spread to other vehicles so thatthe fire does not grow to a size that cannot beattacked by the fire service.Concern (2) If any delay occurs betweenignition and fixed fire fighting system activation,a thin water spray on a very hot firecould produce large quantities of superheatedsteam without material suppressingthe fire.Fire tests have now shown this concern notto be valid. A properly design fixed fire fightingsystem suppresses the fire and cools thetunnel environment. Since a heavy goodsvehicle fire only needs 10 minutes to exceed100 MW and 1200°C, which are fatal conditions,it is important to operate the fire suppressionsystem as soon as possible.Concern (3) Tunnels are very long and narrow,often sloped laterally and longitudinally,vigorously ventilated, and neversubdivided, so heat normally will not belocalized over a fire.Nevertheless advances in fire detection technologyhave now made it possible to pinpointthe location of a fire in a tunnel with sufficientaccuracy to operate a zoned fixed firefighting system.Concern (4) Because of stratification of thehot gas plume along the tunnel ceiling, anumber of the activated sprinklers would not,in all probability, be located over the fire. Alarge number of the activated sprinklerswould be located away from the fire scene,producing a cooling effect that would tend todraw this stratified layer of smoke downtoward the roadway level, thus impeding therescue and fire fighting effort.Independent laboratories have commentedthat they do not observe smoke stratification.Any activated fixed fire fighting system, notover the fire, would cool the tunnel to helprescue services to intervene. Zoned systemsare released by a detection system that isaccurate even with forced ventilation.Concern (5) Water spraying from the ceilingof a subaqueous tunnel could suggest tunnelfailure and induce panic in motorists.This was a theoretical concern not borne outin practice. In the event of fire motorists arelikely to recognize water spraying from nozzlesas a fire safety measure. Behavioralstudies have shown that people do not panicin a fire, even when they are unable to see.Concern (6) The use of sprinklers couldcause the delamination of the smoke layerand induce turbulence and mixing of the airand smoke, thus further threatening thesafety of persons in the tunnel.This has been shown not to be a valid concern.Fire tests have demonstrated thatsmoke does not usually form a layer at thetop of the tunnel but quickly fills the crosssection.Normal air movement in the tunnelaccelerates this process. A fixed fire fightingfire suppression system reduces temperaturesand the risk of fire spread to other vehicles.Concern (7) Testing of a fixed fire fightingsystem on a periodic basis to determine itsstate of readiness could be impractical andcostly. Inspection can be performed whenother facilities are inspected.A full discharge test is normally only performedat system commissioning. Duringroutine testing the system can be configuredto discharge flow to the drainage system.PROTECTION OF STRUCTURE ANDSTRUCTURAL ELEMENTSIn the last edition (2004) NFPA 502 introduced newrequirements for the protection of structural elementswithin road tunnels. The 2008 edition has190