1、 Maevski I. is a Jacobs Engineering fellow , Principal Member of NFPA 502 and Secretary of ASHRAE TC5.9 New York, NY. Klein R. is a Sr. Mechanical Engineer with Jacobs Engineering, New York, NY. Impact of Tunnel Ventilation on Tunnel Fixed Fire Suppression System Igor Maevski, PhD, PE Raymond C. Kle
2、in, PE ASHRAE Member ASHRAE Member ABSTRACT Recently there have been major fires in a number of European tunnels that required reevaluation of road tunnel fire loads. This showed that tunnel safety requires applying a fixed fire suppression system in addition to a tunnel ventilation system. This int
3、roduces a new challenge when designing tunnel safety systems. The type of ventilation system influences the type of sprinkler system and the sprinkler system design impacts the ventilation system performance. The design is also affected by the sequence of system activation. For example, sprinkler ac
4、tivation during calm airflow will differ from activation during fully developed turbulent air flow, which may carry away sprinkler droplets. On the other hand, the ventilation system may experience significant resistance due to water curtains created by the sprinklers. This paper addresses the types
5、 of tunnel ventilation systems, types of sprinkler systems, their influences and new design challenges to achieve a final goal, which is a safe tenable environment for evacuation during a fire emergency. INTRODUCTION PIARC, NFPA and several European countries are rethinking fixed fire suppression ap
6、plication for tunnels. Before the Alpine tunnel fire disasters, Japan and Australia were the only two countries to require and use sprinkler systems in road tunnels. It is noted that sprinklers were installed in several other tunnels in other places in the world, including the USA. However, those in
7、stallations were driven by specific requirements and jurisdictions. Fixed Fire Suppression Systems have been successfully used for more than 40 years in Japans congested urban road tunnels and, more recently, in all of Australias congested urban tunnels. Today, over 100 tunnels are equipped with an
8、active fire protection system. This paper uses the terms “sprinkler system”, “active fire protection” and “fixed fire suppression system” to mean Water-based Fixed Fire Suppression System (FFSS). NFPA 502 recognizes the benefits of the FFSS for road tunnels, but was concerned with the reliability of
9、 fire detection technology, further visibility reduction and the impact of the FFSS on the effectiveness of tunnel ventilation. For an active fire protection system to be effective, it is essential that fires be quickly and accurately detected. The latest developments in tunnel FFSS applications wer
10、e possible due to achievements in fire detection technology. It is recognized that active fire protection systems can limit the size and growth of a fire and prevent the fire from spreading. It could also protect tunnel lining, possibly reducing the amount of passive structural fire protection and m
11、aking significant construction and operation savings. A fire involving several heavy goods vehicles would not close the tunnel protected with a FFSS for a lengthy period. Tunnel ventilation systems are still the main tunnel fire life safety systems to control smoke and provide a tenable environment
12、for evacuation. Table 1 illustrates that there is a number of common benefits from ventilation and FFSS, which can support each other in cooling down the tunnel environment and supporting fire fighting procedures. However, there are LV-11-C076 2011 ASHRAE 6292011. American Society of Heating, Refrig
13、erating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.several expecte
14、d conflicts noted between the two systems, such as fire growth, fire spread, visibility and tenability. An in-depth analysis of the impact of tunnel ventilation on the tunnels water based fixed fire suppression systems, as well as the analysis of the impact of fire suppression on tunnel ventilation,
15、 is required. Table 1. Expectations from Tunnel Ventilation and Fixed Fire Suppression Systems Tunnel Ventilation Fixed Fire Suppression Expected benefits Expected Concerns Expected benefits Expected Concerns Controls smoke and other gases Increases the fire growth Slow down the fire growth, reduces
16、 fire size and overall smoke production Reduce visibility and thus negatively impact the tenable environment Provides tenable environment for evacuation, including visibility Supports spreading fire further impacting other vehicles Prevent spreading of fire further impacting other vehicles Destroy s
17、tratification of hot air and smoke and may disrupt ventilation system operation Cools down the tunnel environment Cool down the tunnel environment Increased humidity and its impact on tenability and fans Supports fire fighting procedures Support fire fighting procedures Produce toxic smoke due to in
18、complete combustion Protect assets Become slippery Hazard for evacuation TYPES OF TUNNEL VENTILATION AND THEIR IMPACT ON FIXED FIRE SUPPRESSION SYSTEMS There are two major types of ventilation for tunnel applications: longitudinal and transverse. Longitudinal system is defined by the longitudinal ai
19、rflow movement along the tunnel initiated either by natural factors (wind, stack effect, piston effect of vehicles) or by fans (portal fans, shaft fans or jet fans along the tunnel). Transverse ventilation is defined by the transverse airflow movement in the tunnel. Transverse ventilation systems fe
20、ature the uniform collection and/or distribution of air throughout the length of the tunnel roadway and can be of the full transverse or semi-transverse type. In addition, semi-transverse systems can be of the supply or exhaust type. Single Point Extraction (SPE) systems are conceptually similar to
21、transverse exhaust ventilation systems. However, SPE systems utilize a limited number of large extraction openings that provide localized exhaust during a fire emergency, which supplements the performance of the transverse exhaust ventilation system at the fire site. Single Point Extraction systems
22、can be supported by longitudinal ventilation systems, such as jet fan systems, to prevent the spread of smoke and hot gases along the tunnel and directing them to a SPE opening. Longitudinal Ventilation Impact on fixed fire suppression system largely depends on the type of tunnel ventilation and on
23、longitudinal airflow along the tunnel. The only feasible way to evacuate smoke with longitudinal ventilation is by pushing it through the tunnel toward the portal at air velocities not less than “critical velocity” in order to prevent it from backlayering. NFPA 502 and other standards allow for a ma
24、ximum air velocity in a tunnel of 12 m/s (2200 fpm). Ventilation systems are designed for significantly smaller critical air velocities, but in combination with wind, other natural factors, and traffic pattern, the resultant air velocities may be that high. What will high air velocities do to the pe
25、rformance of the fixed fire suppression system? At high longitudinal air velocities light water mist 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 not exceeding
26、5 m/s. The deluge system, using much heavier (large) water droplets, may be considered with high longitudinal air velocities once it is proven that it provides similar or a better level of safety. 630 ASHRAE TransactionsDoes the fixed fire suppression system add any benefits to the longitudinal vent
27、ilation system, especially in the evacuation phase? While it takes about 30 seconds to activate a wet sprinkler system, it may take 3 to 4 minutes to fully implement the ventilation mode due to inertia and electrical loads. It means that the sprinkler system can be activated in a relatively calm env
28、ironment before ventilation is at full speed. This allows for a faster wet FFSS system to be activated over the incidence zone. However, once ventilation mode is in full effect, the sprinkler activation zones should be switched to account for a blow-away effect from the longitudinal ventilation. Thi
29、s could require deactivation of one downstream zone and activation of an upstream zone under full system pressure. The middle zone would stay activated throughout the process. System activation, by opening the emergency control valve and deactivation by closing the valve, is a critical component of
30、controlling the deluge system for optimum effectiveness. Recent technological advances have made it possible to activate and deactivate the deluge system with one simple ball valve and/or a remotely controlled solenoid valve. Valve operation utilizes standpipe water pressure fed to the back side of
31、the valve. Under no circumstances should there be complete deactivation of the deluge sprinkler system until either the fire is extinguished or managed by the fire department. Figure 1 Longitudinal Ventilation with Fixed Fire Suppression System Fast, reliable fire detection and prompt activation of
32、fixed fire suppression and ventilation systems are of vital importance in order to obtain benefits from the combined operation of FFSS and longitudinal ventilation systems. With the fixed fire suppression system, longitudinal airflow may need to be selected to ensure an appropriate droplet spread an
33、d mass flow performance for given water pressures. Otherwise, appropriate fixed fire suppression zones need to be activated depending on longitudinal airflow. The ventilation system is also affected by sprinkler operation. Water curtains create a significant resistance to longitudinal ventilation ne
34、eded for preventing smoke backlayering. However, the critical velocity requirement may also be reduced due to the reduced fire heat release rate. The temperature tenability requirements can be met with less ventilation; however, smoke toxicity and air humidity are a concern. The performance of tunne
35、l ventilation fans should be evaluated considering lower temperatures, water curtains and additional mass of moisture. 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 a
36、re often located at the ceiling level relying on stratification to extract smoke and hot gases. However, there are systems with sidewall exhaust air openings, which depend less on stratification, but more on pressure created by the fans. A semi-transverse supply system will most likely not be used f
37、or removing smoke to avoid its spread along the tunnel. The only exception could be if the fire were located close to the exit portal. In this case, the semi-transverse system will operate effectively, just as with longitudinal blowing smoke away through the exit portal. The traditional way to extra
38、ct smoke with semi-transverse exhaust systems is to use small ceiling openings distributed Perimeter of fire Untenable zone Cross passage 2011 ASHRAE 631at short intervals throughout the tunnel. The balance of airflow is made up via the tunnel portals. Single Point Extraction (SPE) uses large openin
39、gs with remotely controlled dampers directly above the incidence with suitable extraction ports. This system works best in conjunction with jet fans (see Figure 2) or portal (Saccardo) nozzles to localize smoke around openings and to prevent smoke driven by natural factors (such as wind and tunnel g
40、rade) from spreading along the tunnel. To maintain smoke stratification, a low speed longitudinal air velocity is usually required to push smoke to one side of the fire. Longitudinal air velocities for full transverse, semi-transverse exhaust, and SPE systems are relatively small and the displacemen
41、t of water droplets due to ventilation is diminished. However, water droplets will most likely destroy stratification and lower the efficiency of these systems. Figure 2 Single Point Extractions with Fixed Fire Suppression System Since longitudinal air velocities are relatively low with transverse a
42、nd single point extraction systems, any type of tunnel fixed fire suppression system can be considered. The water mist system may be beneficial since it produces less stratification disturbance, provides a better cooling effect and is somewhat safer for the mechanical and electrical equipment. Howev
43、er, reduction of visibility in the path of evacuation due to the loss of smoke and hot gases stratification has to be considered. Does the fixed fire suppression system bring additional benefits? Which fire suppression zones shall be activated first? Would the delay in sprinkler activation be benefi
44、cial? Are there other means of fixed fire protection system activation to mitigate its possible negative impact on ventilation, such as water shield mitigation system (WSMS)? While an engineer has to find answers to these questions, it seems obvious that FFSS systems are beneficial for emergency and
45、 rescue services. Table 2. Analysis of Fixed Fire Suppression System benefits to SPE Activation to support Evacuation (early activation) Activation to support First Responders (activation after evacuation) Activation to support Property Protection Advantages Consideration Advantages Consideration Ad
46、vantages Consideration Temperature reduction Loss of stratification, loss of visibility and toxicity Temperature reduction Ventilation mode may need to be changed to longitudinal Beneficial on early stage Possible post-cooling spalling needs to be considered Radiation reduction Impact on fans and ne
47、eded flow rate Radiation reduction Table 2 provides a sample analysis of additional benefits and consideration to be addressed when analyzing a fixed fire suppression system in addition to a single point extraction ventilation system. Similar analysis can be provided for any type 632 ASHRAE Transact
48、ionsof ventilation system. The main benefit of controlling the fire growth and reducing the heat release rate (and thus the overall smoke production rate reduction) can be achieved with a well designed, reliable and quick fire detection system with rapid activation before the fire gets too large. TY
49、PES OF WATER BASED FIRE SUPPRESSION SYSTEMS There are many types of water based fire suppression systems but only a few that are found to be applicable to the tunnel environment. Restrictions, such as open portals, natural ventilation and huge tunnel volumes, prevent the practical application of most suppression systems. The two types of water based fire suppression systems found to present the most benefits in the tunnel environment are intelligent water mist and deluge sprinklers. While a few automatic fire sprinkler systems have been installed in tunnels, most of them are del
