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OCR for page 114
115 of the car, or can increase if the fire is ignited in the back most interest. Later, elements of the tunnel, such as linings, of the car. might become involved. The influence of increased ventilation on the observed fire behavior depends on the ignition location. Materials exposed to higher levels of heat will ignite Under the influence of a high-ventilation velocity, the more readily, release more heat, and potentially lead to fire development accelerates for a covered load at a rate more flame spread. 2 to 3 times faster. The fire size is expected to be 20% The best way to prevent strength loss of the concrete is to 50% higher owing to a high-ventilation speed. by reducing the heat penetration. If the wood cribs are densely packed, the increase in Spalling is a highly relevant failure mechanism in con- peak HRR by ventilation can be up to a factor of 1.5. crete tunnel structures, which is driven by the tempera- No significant HRR changes owing to ventilation [up to ture increase rate and thermal gradient over the structure air velocities of 2 to 2.5 m/s (394 to 492 fpm)]. than by temperature alone. In most cases, mechanical ventilation will lead the fire It is very important to understand the post-cooling to burn fully. Thus, the total duration of the fire will be spalling mechanism because it leads to a better under- limited and the structure will not be subjected to a high standing of a fixed fire suppression system application thermal load concentration. for structural fire protection. There could be a negative effect of ventilation because A fixed fire suppression system application on a very forced ventilation may cause significant flame deflec- early stage of a fire development can actually help to tion, which leads to the chance that the fire might spread cool down the fire and surface and protect the structure, to other vehicles and threaten the integrity of the tunnel whereas delay with its activation can initiate a post- structure on a larger surface, assuming the ventilation cooling spalling. cooling effect and reduction in radiation at the source are insignificant. EXAMPLE OF DESIGN FIRE SIZE ESTIMATE The fastest fire growth occurs at about 3 m/s airflow velocities. Both higher and lower ventilation rates may Every tunnel is unique and this example is for illustration result in slower growth fires. purposes only. Each project has to establish the design fire size accepted by the stakeholders and the Authority Having Tunnel geometry may have a significant impact on fire Jurisdiction. HRR: Consider that a tunnel is twice the width of the Runehamar For a given combustible load, the FHRR of a fire will Tunnel and the designer is using the most conservative test vary depending primarily on the relative width of the result--the maximum FHRR from the HGV fire of 200 MW. tunnel and the fire source: This example illustrates the impact of tunnel geometry, tun- Fires that are small relative to the size of a tunnel nel exit design, reliable rapid fire detection, and benefits of will not be significantly influenced by the tunnel fast activation of the fire suppression system capable of con- geometry; trolling the fire (Table 35). Fires up to about one-half of the width of a tunnel will be enhanced by the tunnel geometry; and Table 36 represents the resultant fire curve modified by the Fires with dimensions close to the width of the tun- fixed fire suppression system rapid activation. Rapid fire detec- nel will be reduced. tion, early start of self-rescue, and fast application of a suffi- When compared, fires within narrow tunnels will gen- cient fixed fire suppression system could reduce the design fire erate larger HRR for the same fuel load than within size 10 times or more. An insufficient fixed fire suppression wider tunnels, assuming sufficient air is available for system design will not control fire, which will keep growing. burning in both cases. Proper fixed fire suppression system design will either keep the The slope of the tunnel has an important influence on fire at the starting rate (10 MW in this example) or reduce the the dispersion of the flue gases. In general it can be fire up to extinguishing. (In reality the process is more compli- said that owing to the chimney effect, the dispersion cated and fire may keep growing for a short period of time velocity of the flue gases increases with the increase after fixed fire suppression system activation, and then get in tunnel slope. reduced.) Early deactivation of the fixed fire suppression sys- tem may lead to explosion and unmanaged fire. This exam- It is unlikely that the fire will immediately involve all of ple is not applicable to the liquid fuel fires or alternative fuel the available fuel. In the growth stages, road vehicles are of vehicles fires.

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116 TABLE 35 EXAMPLE OF DESIGN FIRE Design Fire Scenario for Design Fire Scenario Design Fire Scenario Central Mechanical for Self-rescue for Structural Protection Equipment Fire Rating Maximum fire HRR for 200 MW 200 MW; RWS curve 200 MW HGV with cargo similar (see "Full-Scale (see "Time-temperature to the Runehamar tests Tests" in chapter six) . . ." in chapter eleven) (1350C; 2462F) With geometry 100 MW (see example 100 MW RWS curve 100 MW correction (no tunnel in "Influence of (see "TimeTemperature grade correction made) Tunnel Geometry . . ." . . ." in chapter eleven) in chapter thirteen) (1350C; 2462F) Consider self-evacuation 80 MW using ultra- No correction No correction to the nearest exit is 10 fast fire growth curve min (see "Combined Curve for Evacuation . . ." in chapter eleven) Consider fast fire 10 MW (see revised 10 MW, but with a 10 MW, but design detection and sufficient fire curve due to rapid faster temperature temperature not less than wet FFSS system FFSS activation growth rate (additional 250C (482F, see chapter activation within 4 min illustrated in Table structural protection nine) before ventilation in full 36) may not be required. effect Computational analysis needed) Correction for N/A as FFSS is activated before ventilation. ventilation Correction for tunnel N/A as HGV was used in the example assuming no liquid fuel spillage. drainage For illustration purposes only. N/A = not available. TABLE 36 EXAMPLE OF DESIGN FIRE CURVE Self-Rescue FLS Systems Activation A. Make a decision to evacuate 1. Detection time B. Disembark the bus 2. Operator reaction time (alarm) C. Walk away from the fire-effected zone 3. FFSS activation D. Reach cross passage 4. All fans activated 5. Ventilation mode in full operation For illustration purposes only.