Guidelines for Emergency Ventilation Smoke Control in Roadway Tunnels (2017)

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16 Road Tunnel Fires 2.1 Main Design Fire Parameters The design fire parameters used for the design of tunnel emergency ventilation and fire life and safety systems have a significant impact on the tunnel design and users’ safety. The key criteria are the fire size, heat release rate, fire growth/ decay rate, smoke production, resultant temperatures, and fire duration. Emergency ventilation system design and opera- tion depends on numerous factors of which the most impor- tant are tunnel geometry, types of vehicles and cargos, fire scenario considered, and the main design fire parameters. NFPA 502 [1] provides a list of factors to consider as part of an engineering analysis which is also updated and expanded with every NFPA 502 review cycle based on recent experience. The current list of factors includes: 1. Users of the facility 2. Restricted vehicle access and egress 3. Fire emergencies ranging from minor incidents to major catastrophes 4. Fire emergencies occurring at one or more locations inside or in close proximity to the facility 5. Fire emergencies occurring in remote locations at a long distance from emergency response facilities 6. Exposure of emergency systems and structures to elevated temperatures 7. Traffic congestion and control during emergencies 8. Built-in fire protection features, such as the following: a. Fire alarm and detection systems b. Standpipe systems c. Water-based firefighting systems d. Ventilation systems e. Emergency communication systems 9. Facility components, including emergency systems 10. Evacuation and rescue requirements 11. Emergency response time 12. Emergency vehicle access points 13. Emergency communication 14. Vehicles and property being transported 15. Facility location, such as urban or rural (risk level and response capacity) 16. Physical dimensions and configurations, including road- way profile 17. Natural factors including prevailing wind 18. Anticipated cargo 19. Impacts to buildings and landmarks near the facility 20. Impacts to facility from external operations and/or incidents 21. Traffic operating mode uni-directional, bi-directional, switchable, or reversible Each of the above parameters has an impact on anticipated fire parameters, fire severity, and emergency ventilation and other fire life safety systems needed for the tunnel. Table 2.1 was developed for NCHRP Synthesis 415: Design Fires in Road Tunnels [3] and summarizes the main design fire variables and provides the typical range for variables. The table illustrates that time dependent design fire variables depend on a number of factors to be studied. The magnitude and development of a fire depends on the: • Vehicle combustion load (often called the fuel load) • Source of ignition • Intensity of ignition source • Distribution of the fuel load in the vehicle • Fire propagation rate, which could be dependent on many factors including ventilation • Tunnel and its environment Specification of a design fire may include the following phases: • Incipient Phase—Characterized by the initiating source, such as smoldering or flaming fire • Growth Phase—Propagation spread potentially leading to flashover or full fuel involvement C h a p t e r 2

17 • Fully Developed Phase—Nominally steady ventilation or fuel-controlled burning • Decay Phase—Declining fire severity • Extinction Phase—Point at which no more heat energy is being released When there is a fire that involves new types of energy car- riers, it can lead to explosions (BLEVE) with catastrophic consequences. The field of new energy carriers is very diverse and constitutes many different fields of research. However, while they do not necessarily carry higher risks, they represent new scenarios and imply new risks. NFPA 502 provides reference to the fire size and time to reach the maximum fire heat release rate based on the type of vehicle. The maximum fire heat release rate and fire growth rate could be reduced if a fixed fire suppression system is used for tunnel protection and is activated in a timely man- ner. Factors that should be considered when designing the fire and life safety systems include the following: • The successful management of tunnel fires requires that fires are detected quickly and accurately while they are still at a controllable size (in the order of 1–5 MW [3–17 MBtu/hr]). Accurate fire detection is critical in preventing fire spread and fire growth. If a fire is detected early, the fire protec- tion system could suppress a small fire or take a larger fire under control, not allowing it to grow further or spread to other vehicles. Time Dependent Design Fire Variables Values Range Design Fire Variables Are a Function of: Fire Size - Maximum FHRR 1.5 – 300 MW [1] (5 – 1020 MBtu/hr) Type of vehicle (cars, buses, HGV, Tanker; Alternative fuel) Fire Growth Rate (slow, medium, fast, ultra fast) 0.002 – 0.178 kW/sec2 as high as 0.331 kW/sec2 measured at one test. 20 MW/min linear fire growth rate has been used for several tunnel projects where Flammable and Combustible Liquid Cargo were allowed to pass through the tunnel Type of cargo including bulk transport of fuel Fire Decay Rate 0.042 – 0.06 (min-1) (Note that this parameter is often ignored for conservative evaluations) Fire detection system and delay in activation of fire life safety (FLS) systems Perimeter of Fire Car – truck perimeter or pool of liquid fuel spill Ventilation profile Maximum Gas Temperature at Ceiling 110 °C – 1350 °C (212 °F – 2462°F) (higher with new energy carriers) Fire suppression system Fire Duration 10 min – 6+ days Tunnel geometry Smoke and Toxic Species Production Rate 20 – 300 m3/sec (42 – 640 kCFM) - tunnel width, height, cross section, length Radiation From 0.25 to 0.40 of total heat flux up to 5125 W/m2 (1625 Btu/hr/ft2) - tunnel volume (available oxygen) Flame Length Up to full tunnel height - shape of tunnel, grade - location of exits Tunnel drainage system Table 2.1. Design fire variables [3].

18 • Additional considerations are to be given to the impact of the FFSS on smoke stratification and visibility. The design process may expect the ventilation and fire safety design engineer to consider the following questions to come to a decision on the fire scenario and design fire parameters for ventilation and other fire life safety system’s design: • The chart in Figure 2.1 presents a simplified process and is used for considerations of the design process only. Num- bers presented in the diagram are for illustrative purposes only. The final decision process for design fire parameters should include other factors and follow NFPA 502 require- ments previously identified in the document. 2.2 Impact of Fire Size on Smoke Management Requirements An objective of the tunnel ventilation system is to con- trol and/or extract smoke and heated gases and provide a non-contaminated environment for egress of tunnel users. Another objective is to support firefighting and rescue opera- tions. In both cases, the primary goal is to control smoke and hot gases as the result of fire. The amount of smoke and heat generated depends on the fire size. It is often assumed that the smoke generation and heat generation rates are proportional to the fire size (note that this may not always be the case as smoke is a product of incomplete combustion and depends upon the combustion mechanism of burning materials). In Figure 2.1. Sample decision making chart on design fire parameters. 1) Are flammable vehicles and hazardous materials allowed to travel through the tunnel? 2) Are a fixed fire suppression and reliable rapid fire detection system considered for tunnel protection? 3) Are heavy goods vehicles and combustible materials allowed to travel through the tunnel? Yes Example: Consider a linear fire growth rate of 20 MW/min or faster for flammable or HAZMAT with the possible reduction of FHRR based on fire detection, sprinkler activation and sprinkler system design Example: Consider a linear fire growth rate of 20 MW/min or faster for flammable or HAZMAT with the maximum FHRR specified in NFPA 502 4) Are a fixed fire suppression and reliable rapid fire detection system considered for tunnel protection? 5) Is this a bus tunnel without HGV and FLC? 6) Are a fixed fire suppression and reliable rapid fire detection system considered for tunnel protection? 7) Is this a special tunnel? Is this tunnel for alternative fuel vehicles? Example: Consider the quadratic ultra-fast fire growth rate of 187.6 W/s2 to maximum FHRR specified in NFPA 502 with the possible reduction of FHRR based on fire detection, sprinkler activation and sprinkler system Example: Consider the quadratic ultra-fast fire growth rate of 187.6 W/s2 with the maximum FHRR specified in NFPA 502 Example: Consider a quadratic fire growth rate of 100 W/s2 to maximum FHRR specified in NFPA 502 with the possible reduction of FHRR based on fire detection, sprinkler activation and sprinkler system design Example: Consider a quadratic fire growth rate of 100 W/s2 with the maximum FHRR specified in NFPA 502 Identification of alternative-fuel vehicles is critical and the tunnel should be evaluated on a case-by-case basis, which might be handled by risk analysis, computer modeling, experimental testing, or all of the above. Unique type tunnel could be evaluated on a case-by-case basis No Yes No Yes No Yes No Yes No Yes No Yes No Note that t2 fire growth rate is typical while linear fire growth rate in the above example is used for illustration of verity of fire growth to be considered.

19 general, the larger the fire size the more combustion products the fire produces and hence, more air is required to control smoke and hot gases. However, this ratio is not linear. There are two commonly used concepts for smoke manage- ment in road tunnels to achieve a smoke free environment for egress. They are: 1. Longitudinal ventilation concept—directing smoke along the tunnel in the opposite direction of egress. The longi- tudinal ventilation concept is achieved by producing air velocity that meets or exceeds the critical velocity along the tunnel which prevents smoke backlayering. The critical velocity depends on the fire size as described below. 2. Extraction ventilation concept—extracting smoke at the fire location and relying on smoke stratification (see Fig- ure 2.2a) to allow for egress under the smoke layer. The extraction concept is achieved by maximizing the exhaust rate in the ventilation zone that contains the fire and by avoiding disruption of the smoke layer by longitudinal air velocities. This concept depends on the smoke production rate which is a function of the fire size. Note that smoke stratification may not occur for fires with relatively small heat production rates (low buoyancy) (see Figure 2.2b) and especially when the flame is not visible (for example, rubber tire fires). Also, stratification can be destroyed by air- flow passing by the fire site or by a fixed firefighting system. Extraction ventilation systems designed for large size fires should be designed and analyzed for both stratified smoke and non-stratified smoke. 2.2.1 Critical Velocity There are two main types of models to estimate critical velocity, the critical Froude model and the non-dimensional model. NFPA 502 currently uses the critical Froude number model [1]. The equation for critical velocity can be found in Annex D of NFPA 502. It shows a direct relationship between the critical velocity and fire size. The non-dimensional model [30] concludes that the critical velocity tends to be independent of the fire size (fire heat release rate HRR) for large fires (over 100 MW [341 MBtu/hr]). For the small fires, such as from 5 to 30 MW (17 to 100 MBtu/hr), the non-dimensional model critical velocity results are much higher values. 2.2.2 Smoke Stratification A smoke layer may be created in tunnels at the early stage of a fire with essentially no longitudinal ventilation. However, if a smoke layer is formed, it will descend further away from the fire as the hot smoke and gases near the ceiling cool. If the tunnel is sufficiently long, the smoke layer may descend to the tunnel sur- face at a specific distance from the fire depending on the fire size (temperature/buoyancy effect), the tunnel shape, air velocity, ambient conditions and the perimeter and height of the tunnel cross section. The smoke layer can also descend if the smoke generation rate is higher than smoke movement (transporta- tion) along the tunnel flow rate which causes smoke expansion towards the road surface. Typically, as the longitudinal ventila- tion is gradually increased, the stratified layer will gradually cool and spread throughout the tunnel. The particular dimension- less group, which determines whether a gas will stratify above another, is the Froude number (Fr) or the Richardson number (Ri) which defines mass transfer between layers [30]. The de-stratification downstream from the fire is a result of the mixing process between the cold air stream and the hot plume flow created by the fire. The gravitational forces tend to suppress the turbulent mixing between the two different (a) (b) Figure 2.2. Diesel fuel pan fire of 5 MW in I-90 tunnel in Seattle (left); Winnipeg Transit bus on fire (right).

20 density flows. There is a correlation between the local tem- perature; the gaseous composition (CO, CO2, O2, etc.); and smoke (soot, ash, gases and other solid particles) stratification in tunnels. The temperature stratification is not only related to the air velocity but also to the HRR and the height of the tun- nel. These parameters can be related through the local Fr or Ri. Small fires, with relatively low temperatures generated, create less buoyancy in the combustion products, and thus decrease the likelihood of smoke stratification under the tunnel roof than with hotter fires (see Figure 2.2b). Three distinct regions of temperature and thus smoke stratification are defined in several of H. Ingason’s publications with citation to J. Newman by the Fr or Ri and are shown in Figure 2.3 [30]. • The first region (region I), when Fr < 0.9, results in severe stratification, in which hot combustion products travel along the ceiling. For region I, the gas temperature near the floor is essentially ambient. This region consists of buoy- ancy dominated temperature stratification. This region is next to the fire location and allows for motorist evacuation. • The second region (region II), when 0.9 < Fr < 10, is domi- nated by strong interaction between imposed horizontal flow and buoyancy forces. Although not severely stratified or layered, it involves vertical temperature gradients and is mixture-controlled. In other words, there is significant interaction between the ventilation velocity and the fire- induced buoyancy. • The third region (region III), when Fr > 10, has insignificant vertical temperature gradients and consequently, insignifi- cant stratification. 2.2.3 Extraction Ventilation Concept The main design parameter for smoke extraction is the smoke flow rate produced by the fire. The smoke flow rate depends on the combustion, but in general can be considered to vary nearly linearly with the heat release rate—from about 50 m3/s (1765.7 ft3/s) at approximately 10 MW (34 MBtu/hr), to about 250 m3/s (8828.7 ft3/s) at approx. 150 MW (512 MBtu/ hr), as shown in Figure 2.4 [14]. Table 2.2 presents smoke production rates, CO and CO2 published in different literature sources (summarized exper- imental results and values from standards) [14] [17] [18] [31]. In order to convert the smoke masses produced to smoke Figure 2.3. Sketch with three stratification regions. Figure 2.4. Smoke flow rate versus fire heat release rate [18] [31].

21 volumes, it is necessary to know the smoke temperatures. The theoretical stoichiometric combustion temperature of regular gasoline is about 2000°C (3632°F). The real fire temperatures are usually much lower, mainly because the combustion is not stoichiometric or because the smoke mixes with air. Along with smoke production rates, buoyancy and other factors described herein, additional consideration should be given to the impact of FFFS on smoke stratification. Smoke management is usually achieved using tunnel ventilation equipment and is impacted by the fire suppression system con- sidering that FFFS can reduce the fire size and fire growth rate. Reduction of the fire size should reduce overall smoke produc- tion and propagation; however FFFS may cause incomplete combustion, which may increase smoke production. Once the fire size and possible fire scenarios are determined, the requirements of ventilation and fire suppression systems can be determined based on the length of the tunnel and traf- fic mode along with other tunnel operational characteristics. Table 2.3 represents a simplified example of considering the fire safety risks for tunnels and the required fire life safety sys- tems to mitigate those risks, such as tunnel ventilation and FFSS. This table is for illustrative purposes only and cannot replace engineering analysis which may include risk analysis based on traffic volumes and other factors discussed previously (e.g., long mountain tunnel with low traffic volumes bears less risk than a shorter urban tunnel with high traffic volumes). Another impact of fire size on tunnel ventilation require- ments is related to the ventilation equipment design tempera- ture. This heavily depends on the type of ventilation system and its configuration. Jet fans and tunnel dampers could be directly exposed to the fire temperature and need to be designed accordingly. This impact is further discussed in Chapter 3 (see Table 3.4). Burning vehicle Smoke flow CO2 production CO production [m3/s (ft³/s)] [kg/s (lb/s)] PIARC 1987 [32] RABT 1994 [33] EUREKA tests [34] CETU (1996) [31] EUREKA tests [34] Passenger car 20 (706) 20 – 40 (706 -1412) - 20 (706) - - Passenger van (plastic) - - 30 (1059.4) 30 (1060) 0.4-0.9 (0.88–2) 0.020-0.046 (0.04–0.1) 2 – 3 passenger cars - - - 30 (1060) - - 1 van - - - 50 (1765) - - Bus/truck without dangerous goods 60 (2120) 60 – 90 (2120 – 3180) 50 - 60 (1765 – 2120) 80 (2825) 1.5-2.5 (3.3–5.5) 0.077-0.128 (0.17–0.28) Heavy goods vehicle - - - 50 – 80 (1765 – 2825) 6.0-14.0 (13.2–30.9) 0.306-0.714 (0.67–1.57) Gasoline tanker 100 -200 (3531 – 7063) 150 –300 (5300–10600) - 300 (10600) - - Table 2.2. Smoke, CO2 and CO production [17].

22 Tunnel fires significantly increase the air temperature in the tunnel roadway and in the exhaust air duct (if utilized in the design). Therefore, both the tunnel structure, air ducts (where applicable), and ventilation equipment will be exposed to high smoke/gas temperatures. Protection of structural elements from heat is required by NFPA 502 [1]. Loss of structural ele- ments could be life threatening for evacuees; it may also have an impact on the tunnel ventilation system. For example, the jet fan support system should be designed for high temperature exposure. Ductwork should maintain structural integrity. Ven- tilation equipment, including tunnel ventilation fans, should be designed for high temperature exposure (see Section 3.3). Ca te go ry Tu nn el Le ng th [ft ] Uni-directional traffic Bi-directional traffic w/HGV; w/FLC w/HGV; no FLC no HGV; no FLC w/HGV; w/FLC w/HGV; no FLC no HGV; no FLC X < 30 0 No Ventilation; No Fire Suppression No Ventilation; No Fire Suppression A 30 0 - 8 00 Ventilation (suppression likely to protect structure) No Fire Suppression (Ventilation questionable) No Ventilation; No Fire Suppression Ventilation; (suppression likely to protect structure) No Fire Suppression (Ventilation questionable) No Fire Suppression (Ventilation unlikely) B 80 0 - 1 00 0 Ventilation (suppression likely to protect structure) No Fire Suppression (Ventilation likely) No Fire Suppression (Ventilation questionable) Ventilation, Fire Suppression Ventilation; (suppression likely to protect structure) No Fire Suppression; Ventilation required C 10 00 - 32 80 Ventilation, Fire Suppression Ventilation; (suppression likely to protect structure) No Fire Suppression (Ventilation likely) Ventilation, Fire Suppression Ventilation; (suppression likely to protect structure) No Fire Suppression; Ventilation required D > 32 80 Ventilation, Fire Suppression Ventilation, Fire Suppression No Fire Suppression; Ventilation required Ventilation, Fire Suppression Ventilation, Fire Suppression No Fire Suppression; Ventilation required Notes: Tunnel categories X; A; B; C; D are defined by NFPA 502 [1] and are illustrated in Figure 3.11 discussed in Section 3.2.2 of these guidelines. FLC - Flammable liquid cargo (e.g., fuel tankers) HGV - Heavy goods vehicles (trucks) Colors represent the level of risk and hence relative cost of fire life safety systems, with Green being the least risky and least expensive and Red being the most risky and most expensive (color PDF available online at www.trb.org). Table 2.3. Simplified example of tunnel fire safety risk and fire life safety system needs based on the tunnel length and traffic conditions.