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

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49 Tunnel Fixed Firefighting Systems and Ventilation Impact Active tunnel fire protection systems can be classified as fixed systems and standpipe systems. Fixed fire protection systems used in road tunnels are fixed, water-based firefight- ing systems. Standpipe systems are used in the United States primarily by firefighters to suppress and extinguish fires. Fixed water-based firefighting systems (Figure 5.1) are categorized based on their performance objectives as: • Fire suppression systems with the goal to reduce the FHRR by sufficient application of water; Consideration should be given to the water flows from those systems—water density. • Fire control systems with the goal to stop or significantly slow the fire growth rate. • Volume cooling systems with the goal to provide substan- tial cooling of the products of combustion but not intended to reduce FHRR. • Surface cooling systems with the goal of protecting the main tunnel structural elements but not intended to reduce FHRR. Additional information on tunnel fixed firefighting systems can be found in Appendix B. Activation of the deluge system may lead to less tenable con- ditions in the immediate vicinity of the deluge due to some layer cooling and de-stratification. The undesirable consequences of water-based fixed firefighting system activation, such as smoke de-stratification, increased humidity, and decreased visibility are typically outweighed by their positive outcomes such as fire growth rate control, containment of fire spread, and reduced temperatures. Table 5.1 discusses the impact of the fixed fire- fighting system on tunnel fire safety and its interaction with different tunnel ventilation systems. Research projects are investigating to what extent an active fire protection system can limit the maximum heat release rate of a fire and whether an active fire protection system combined with ventilation offers equal or better life safety than ventilation alone. The projects are also investigating how to specify design or performance test criteria for active tunnel fire protection systems. 5.1 Interaction Between Water-Based Fixed Fire Suppression and Tunnel Ventilation Systems The tunnel ventilation system is still the main tunnel fire life safety system to control smoke and provide a tenable envi- ronment for evacuation. Table 5.2 illustrates that there are a number of common benefits from ventilation and fixed fire suppression system (FFSS) which can support each other in cooling down the tunnel environment and supporting fire- fighting procedures. However, there are several expected conflicts noted between the two systems, such as fire growth, fire spread, visibility, and tenability. 5.1.1 Longitudinal Ventilation The impact of the fixed fire suppression system largely depends on the type of tunnel ventilation and on the magni- tude of the longitudinal airflow along the tunnel. With lon- gitudinal ventilation, smoke is pushed through the tunnel towards the portal at air velocities not less than “critical veloc- ity” in order to prevent the smoke from backlayering. NFPA 502 and other standards allow for a maximum air velocity in a tunnel of 12 m/s (2200 fpm) [1]. Ventilation systems are designed for significantly smaller critical air velocities, but in combination with wind, other natural factors, and the traffic pattern, the resultant air velocities may reach those velocities. At high longitudinal air velocities, light water mist drop- lets will most likely be blown away, while heavy deluge water droplets will be displaced. As of today, water mist systems have been tested at significantly lower longitudinal air velocities than expected in an emergency scenario, not exceeding 5 m/s (1000 fpm). Field tests performed by water mist companies in C h a p t e r 5

Advantages of FFSS Challenges of FFSS General FFFS is designed to react on an early stage of the fire. Controls fire by not allowing it to further grow, or grow slowly, or extinguishing a small fire. Reduced visibility especially on an early stage when people evacuate. When the sprinkler system is activated on an already large fire, smoke stratification will be destroyed and a large amount of water will be evaporated and, thus, the visibility will be further diminished. Protection of tunnel users and structure. Duration of the fire can be limited and the structure of the tunnel will be subjected to less harsh conditions. Incomplete combustion creates smoke, gases and steam. Studies needed on critical time to activate the FFFS system to protect tunnel structures. Support rescue and firefighting: help rescue and firefighters to reach the fire source. Creates slippery environment when water applies. Malfunctioning of the system with accidental water release may create panic which may lead to an accident. When FFFS applies with transverse ventilation based on smoke extraction (including single point extraction) Reduced fire size; also see general. Destroys stratification of hot air which makes ceiling extraction inefficient and makes evacuation more difficult especially in a fire suppression zone. Reduced fire duration. Increases in mass of air/water mixture and mass of smoke due to incomplete combustion which results in increased ventilation rate needed for extraction system. When FFFS applies with longitudinal ventilation– uni-directional tunnel with manageable traffic Reduced fire size may result in reduced ventilation rate. Increases in mass of air/water mixture and mass of smoke due to incomplete combustion –which might increase required ventilation rate. Cools environment and protects fan units from high temperature. Creates water curtains which impact ventilation design by increasing pressure (thrust) needed for ventilation to overcome. See general. Ventilation blows the FFFS substances away from fire which may increase number of FFFS zones for activation depending on weight of water particles and pressure. Longitudinal ventilation– uni-directional tunnel with unmanageable traffic or bi-directional tunnel Protects tunnel structure and reduces fire size. Destroys stratification making evacuation difficult to both sides of the fire especially in a fire suppression zone. Traffic control for low traffic tunnels is imperative. Table 5.1. Impact of FFSS on tunnel fire safety. Figure 5.1. Effects of water-based fixed firefighting systems. W at er -B as ed F ix ed F ire - Fi gh ti ng S ys te m Fire Suppression Example: Deluge system with or w/o additives such as AFFF Fire Control Example: Deluge system Volume Cooling Example: Water Mistsystem Surface Cooling ample: Deluge or Water Ex Mist system

51 San Pedro de Anes (Spain) [26] indicated that a low longitudi- nal air velocity will have a minor impact on FFFS performance. Experimental model scale tests performed in Sweden con- cluded that fully automatic FFFS are only suitable for tunnels with low velocities and recommended that automatic FFFS be used in tunnels with transverse ventilation or in bi-directional tunnels rather than in longitudinally ventilated tunnels with high velocities [27]. The deluge system, using much heavier (larger) water droplets, may be considered for operation in high longitudinal air velocities once it is proven that it provides a similar or a higher level of safety. The deluge system with and without 3% AFFF additives was tested in different rela- tively low longitudinal air velocities as shown in Figure 5.2, and while some displacement of heavy water droplets was Tunnel Ventilation Fixed Fire Suppression Expected Benefits Expected Concerns Expected Benefits Expected Concerns Controls smoke and other gases Supplies oxygen to fire and increases the fire growth Slows down the fire growth, reduces fire size and overall smoke production. Smoke production could be significantly reduced by starting FFFS at incipient fire development stage. Reduces visibility and thus negatively impacts the ability to evacuate Provides tenable environment for evacuation, including visibility Supports spreading fire further impacting other vehicles Prevents spreading of fire further impacting other vehicles Destroys stratification of hot air and smoke and may disrupt ventilation system operation Cools down the tunnel environment Cools down the tunnel environment. Cooling effect of FFSS on critical velocity and ventilation requirements Increased humidity and its impact on tenability and on fan design Supports firefighting procedures Supports firefighting procedures Produces toxic smoke due to incomplete combustion Protects assets and possibly structural protection Develops slippery hazard for evacuation Table 5.2. Expectations from tunnel ventilation and FFSS. Figure 5.2. Displacement of FFFS water particles as the result of longitudinal air velocity [50].

52 observed, displacement of the zone covered by the FFSS was insignificant [50]. However, it is expected to be more sig- nificant at high air velocities and with lighter water particles. The design of the fixed firefighting systems should be fully coordinated with the tunnel ventilation system design as ven- tilation equipment, such as the jet fans or the fan/supply open- ings, may have a significant impact on the performance of the water droplets of the sprinkler heads. Jet fans, lights, signage, and other possible obstructions should be taken into account when installing nozzles. Nozzles are normally installed under the tunnel ceiling and/or at the upper part of the walls, pointing downwards or at the center of the tunnel and disperse into the protected area. Nozzles should be arranged such that vehicles will not reduce their efficiency by blocking the spray to a critical extent. This applies especially to systems having nozzles installed on the walls or with just one row of nozzles along the tunnel ceiling. This may lead to local variations of nozzle spacing or lowering the installation posi- tion of the nozzles below obstructions, such as jet fans. Varia- tions in tunnel geometry, e.g., emergency stop lanes or other areas where vehicles potentially can enter, should also be taken into account. It is advisable to locate sprinkler heads above and under the jet fans and locate the nearest sprinkler head further away from the jet fan discharge. While it may take about 30 seconds to activate a wet sprin- kler system, it may take 3 to 4 minutes to fully implement the ventilation mode due to inertia and electrical loads. This means that the sprinkler system can be activated in a relatively calm environment before ventilation is at full speed. This allows for a faster wet FFSS system to be activated over the incident zone. However, once the ventilation mode is in full effect, the sprin- kler activation zones may need to be switched to account for a blow-away effect from the longitudinal ventilation. This could require deactivation of one downstream zone and activation of an upstream zone under full system pressure. The fire zone would likely stay activated throughout the process. System activation, by opening the emergency control valve and deactivation by closing the valve, is a critical component of controlling the deluge system for optimum effectiveness. Recent technological advances have made it possible to acti- vate and deactivate the deluge system with one simple ball valve and/or a remotely controlled solenoid valve. Valve oper- ation utilizes standpipe water pressure fed to the back side of the valve. Under no circumstance should there be a complete deactivation of the deluge sprinkler system until either the fire is extinguished or managed by the fire department. Tests in I-90 tunnels in Seattle demonstrated that with a lon- gitudinal air velocity, the water/foam droplets enter a shielded volume and can reduce the fire size inside the enclosure [49]. While the FFSS was unable to extinguish a 2 MW (7 MBtu/hr) shielded fire inside the van during the tests, the tunnel air temperature around the burning van was reduced prevent- ing the fire from spreading (Figure 5.3). It was an important observation that the temperatures inside the van were also reduced as water droplets blown by the tunnel airflow enter the van through the open windows and evaporate, lowering the energy inside of the van. The suppression system cools the outside of the van and the surrounding air. This leads to convection of the hot air (inside the van) to the cooler envi- ronment outside of the van. Ventilation controlled smoke movement kept one side of the fire completely tenable when stratification was destroyed by the FFSS [49]. In the absence of any ventilation, the activation of a FFSS may reduce visibility as shown in Figure 5.4. In these tests, stratified smoke was allowed to build up along the ceiling and therefore, if a fixed fire suppression system is activated quickly, the loss in visibility may not be as drastic. The ven- tilation should be activated as soon as possible to control the destratified smoke caused by the activation of the fixed fire suppression system. Fast, reliable fire detection and prompt activation of the fixed fire suppression and ventilation systems are of vital importance in order to obtain benefits from the combined operation of the FFSS and the longitudinal ventilation system. With the FFSS, longitudinal airflow may need to be selected to ensure an appropriate droplet spread and mass flow per- formance for given water pressures. Otherwise, appropriate fixed fire suppression zones need to be activated depending on the longitudinal airflow. The ventilation system is also affected by the sprinkler operation. Water curtains create a significant resistance to longitudinal ventilation that is necessary in preventing smoke backlayering. However, the critical velocity requirement may also be reduced due to the reduced fire heat release rate and Perimeter of fireUntenable zone Cross passage Figure 5.3. Longitudinal ventilation with fixed fire suppression system.

53 air temperature. The temperature tenability requirements can be met with less ventilation; however, smoke toxicity and air humidity are a concern. The performance of the tun- nel ventilation fans should be evaluated considering lower temperatures, water curtains, and the additional mass of moisture. With the longitudinal ventilation system, it seems reason- able to activate both systems simultaneously. As a result, the wet FFSS will start before the longitudinal ventilation reaches full speed. This allows the sprinkler system to discharge water in a low air velocity environment, thus protecting people and structures by taking control of a fire at an early stage of its growth. Once the ventilation reaches full speed, the sprin- kler zones may need to be revisited and either switched or additional activation zones will be required to account for ventilation. The question is: if the sprinkler is activated early enough, can ventilation be reduced or eliminated and what will be the impact on smoke production? 5.1.2 Transverse Ventilation Transverse ventilation uses both supply and exhaust ducts served by a series of fans usually housed in a ventilation building or structure. Exhaust openings are often located at the ceiling level relying on stratification to extract smoke and hot gases, while stratification could be destroyed by the fixed firefighting system. However, there are systems with sidewall exhaust air openings which depend less on stratification and more on pressure created by the fans. Recent scale model testing of an automatic sprinkler sys- tem showed that it could be used safely in tunnels with trans- verse ventilation or where longitudinal ventilation is used at low speeds. Concerns were raised regarding sprinkler system failure as a result of too low water pressure or too high ven- tilation air velocities. Longitudinal air velocities for full transverse, semi-transverse exhaust, and single point extraction systems (Figure 5.5) are relatively small, and the displacement of water droplets due to ventilation is diminished. However, water droplets will most likely destroy stratification and lower the efficiency of these systems. Since longitudinal air velocities are relatively low with trans- verse and single point extraction systems, any type of tunnel FFSS can be considered. The water mist system may be benefi- cial since it produces less stratification disturbance, provides a better cooling effect, and is somewhat safer for the mechani- cal and electrical equipment. However, reduction of visibil- ity in the path of evacuation due to the loss of smoke and hot gas stratification has to be considered. Does the FFSS bring additional benefits? Which fire suppression zones should be activated first? Would the delay in sprinkler activation be ben- eficial? Are there other means of fixed fire protection system activation to mitigate its possible negative impact on ventila- tion, such as a water shield mitigation system? Figure 5.4. Photographs of a fire test. (a) Tunnel with smoke stratification at ceiling right before FFSS activation. (b) 12 seconds later [full discharge] and beginning of smoke de-stratification. (c) Additional 8 seconds later, showing smoke descending to lower elevations. (d) Additional 8 seconds later, showing smoke at ground level [50]. (a) (b) (c) (d)

54 Table 5.3 provides a sample analysis of additional benefits and considerations to be addressed when analyzing a FFSS in addition to a single point extraction ventilation system. Similar analysis can be provided for other types of ventila- tion systems. The main benefit of controlling the fire growth and reducing the heat release rate (and thus a reduction in the overall smoke production rate) could be achieved with a reliable and rapidly activated fire suppression system before the fire gets too large. With a transverse ventilation system using ceiling exhaust, the sequence of activation may differ from a longitudinal ven- tilation system sequence. Destruction of the smoke layer, wors- ening of visibility, and potential generation of hot steam must all be considered when developing the system controls. The Japanese approach for the transverse ventilation system allows for a minimum of a 3-minute delay before sprinkler activa- tion so that people can leave the sprinkler zones. However, sprinkler activation delay may be dangerous for the tunnel structure and can lead to fire spread and growth. It may also require a greater water supply, due to a larger fire size at the time of activation. This needs to be evaluated in the design and additional research is required. The activation time of the FFSS may differ depending on the type of ventilation. There is a need for an integrated approach to all fire life safety system designs to coordinate each element and obtain the desired level of tunnel fire life safety. 5.2 Interaction Between Firefighting Operation and Tunnel Ventilation Systems Mechanical ventilation systems are a major factor in ensur- ing safety, both for tunnel users and emergency response teams. The best chance of successful firefighting is in the ini- Figure 5.5. Single point extractions with FFSS. Table 5.3. Analysis of FFSS impacts on performance of single point extraction ventilation. Acvaon to support Evacuaon (early acvaon) Acvaon to support First Responders (acvaon aer evacuaon) Acvaon to support Property Protecon Advantages Consideraons Advantages Consideraons Advantages Consideraons Temperature reducon Loss of straficaon, loss of visibility and toxicity Temperature reducon Venlaon mode may need to be changed to longitudinal Beneficial in early stage Possible post- cooling spalling needs to be considered Radiaon reducon Impact on fan design and required airflow rates Radiaon reducon

55 tial phase of a fire. It is obvious that FFFS systems are benefi- cial for emergency and rescue services and could be effective in protecting the tunnel infrastructure and delivering human safety. See Section 5.1 on interaction between water-based FFSSs and tunnel ventilation systems. NFPA 502 [1] states that the “Smoke Control design goal shall be to provide an evacuation path for motorists who are exiting from the tunnels and to facilitate fire fighting operations.” As far as fire and rescue services are concerned, the most important conditions that can reduce the severity of acci- dents are: • Short distances to, and simple means of reaching, escape routes for those escaping from a fire and rescue work; • Firefighters can approach the fire as safely as possible and safely escape as needed; and • Fire cannot grow excessively before firefighting work can start. These various conditions can be achieved in different ways, but there must be an overall safety program that identifies all the parameters involved, ensures that they work together, and creates the best conditions for a high level of safety. When the evacuation phase is concluded, firefighting must be facilitated by proper smoke management. A basic require- ment is to provide maximum opportunity for firefighting access in minimum smoke conditions. During evacuation, the direction of smoke flow must not change. Upon rescue team and firefighter arrival, it can be decided on-site which mode of ventilation is the most effective for their mission. The ventilation mode should be provided to force heat and smoke or other noxious gases in the direction away from the first responders (Figures 5.6 and 5.7). The requirements of the emergency services should be taken into account when designing the ventilation system and response procedures for assisted rescue and firefighting phases. Heavy items, such as fans, subject to temperatures of 450°C (842°F), are to be designed not to fall during the firefighting phase [43]. In some cases, dedicated firefighting ventilation modes are designed to reverse longitudinal ven- tilation and to operate at velocities other than ones required for the evacuation phase. Considerations should be given to the time factor for achieving full reverse of smoke manage- ment during the fire event and that such operations can take a longer time, depending on the ventilation system, the tunnel geometry, the fans, electrical control system used, and other conditions. Smoke clearance could also be achieved by the use of portable smoke control equipment deployed by the fire services, such as movable ventilation units (MVU). When the fire department arrives at a fire scene, they may not have the means to reach a fire inside the tunnel because reaching the scene in the conditions of extreme heat and smoke may be beyond the endurance of human beings, even if they have appropriate equipment, such as long duration breathing apparatuses and firefighting protective clothing. Reversal of jet fans is generally not recommended dur- ing the evacuation phase, even if the fire is located near the entrance portal. In the period between the ignition of the fire and the reversal of the jet fans, smoke could have traveled sev- eral hundred meters. When the smoke layer flow is reversed, it will spread over the entire cross section during the phases of evacuation. It is important to maintain good visibility condi- tions. Therefore, only after evacuation is complete can reversal Figure 5.6. Fire and rescue operation dealing with a car fire in a twin-bore tunnel with queuing traffic [18]. Fan Figure 5.7. Fire and rescue operation dealing with a car fire in a single-bore tunnel [18]. Fan

56 of the air flow direction take place. The emergency response time is to be based on NFPA 1710. Figure 5.8 demonstrates a tunnel firefighting timeline. According to NFPA 1710, the fire department’s objective is 80 seconds for turnout time for fire response, from 240 to 480 seconds for the arrival of the first engine and from 480 to 610 seconds for the deployment of an initial full alarm assignment. Firefighting time (time of intru- sion) will depend on the nature of the incident. Often, the firefighters have to get very close to the fire site in order to fight the fire due to low tunnel ceilings. The water flow rate is sometimes maintained for up to 30 minutes or more in order to extinguish a HGV fire. Ventilation could manage hot gases, but thermal radiation from the fire and from any residual backlayering will be difficult to deal with. Development of some form of protection against thermal radiation such as water mist or a water curtain could assist in tackling fires of this type. Por- table radiant barriers and other vehicles already in the tunnel could be used to protect firefighters from thermal radiation. The design features must be understood by all agencies involved in tunnel safety, the operators, and the various emergency services. If such understanding does not exist, then beneficial, expensive, and complicated design features may not be used in the event of an emergency. An exam- ple of this would be the provision of a complex ventilation system for use by the fire department in case of fire. If fire department personnel are unaware of how the system oper- ates, it is unlikely to be used effectively. The tunnel operator must provide expert knowledge and advice on tunnel facili- ties and the ventilation system to the emergency services incident commander. An educational plan comprising both education of new employees and repeated education of current employees should be implemented covering all staff dealing with safety, includ- ing maintenance contractors. The plan should be a planning and follow-up instrument for individual staff, and acceptable time intervals for different education topics should be incor- porated. As part of the education plan, the traffic operators should, on a regular basis, carry out exercises which focus on prevention and handling of incidents/accidents and seldom used procedures. Or igin of fir e Fir e d ete ctio n Ac tiva tio n o f fi xed fir e figh ting re sou rce s De fin itio n o f m iss ion Arr iva l Im pa ct o f m iss ionAla rm Figure 5.8. Firefighting timeline [51] [52].