Assessing the impact fire heat release rate has on infrastructure design and constructability of rail and road tunnels ventilation systems.
Even though many tunnel designs have been completed over the years, there is still one question no one has answered: how does the fire heat release rate really impact the design and constructability of a tunnel ventilation system? Harvey (2009) tries to address the issue, but in the end, he does not answer the question. He just basically joins the thought of NFPA 502 (2008).
We seem to believe that bigger is better. But is it really? The first thing we need to understand is the cost of safety. After all, tunnel ventilation systems are capital projects very closely associated with taxpayer's money one way or another, and cost is a very important component of the project. Figure 1 depicts in simple terms that if we build a project without safety being taken into consideration, the increase in risk causes the cost accidents to rise to a high enough level where the project could be stopped in order to limit the liability. By the same token, if we go to the extreme and dream of very unlikely events, the project could become unbuildable. As engineers and designers, we must consider events that are probable, but likely to happen. We cannot concentrate on the events and design scenarios with very small likelihood to occur. Otherwise, the project could not be built. For instance, we should determine the best credible design Fire Heat Release Rate (FHRR), fan airflows, and fan pressures, and then add a safety factor. However, we should not estimate a large FHRR in order to determine a larger fan airflow and pressure. We must determine the level of risk and safety we can afford in a project. This will set the protocol and policies on how to handle the hazards we could not consider for the project, and how to mitigate hazards of those we considered in order to avoid loss of life and transportation infrastructure. We should build something practical, functional, with reasonable degree risk and safety. Just remember: zero risk does not exist.
[FIGURE 1 OMITTED]
FIRE HEAT RELEASE RATE
To this extent, we come to the issue of fires. Many think that the larger the design FHRR, the safer the infrastructure. However, too large of a FHRR would require strenuous fire safety measures. If the very large fire scenarios are very likely to occur, by default that makes the project risk high enough to create an unacceptable high liability level and it should be stopped. We cannot talk about fire hazards without talking about how we intend to handle the hazards so that they do not become an incident where there is significant loss of life and/or transportation infrastructure. For example, looking at the airline industry, an airplane is designed considering how much fuel it will carry, sufficient to allow a safe journey. The tanks are specified to safety standards that have been developed through lessons learned. In additions, if is it a cargo airplane, precautions are taken to ensure the materials do not create an accident (within the risk limits the air cargo has identified). If it is a passenger airplane, we have learned since 11 September 2001 events that passengers and luggage must be screened in order to control the hazards that could create an accident, based on the new risk levels the industry has embraced. Tunnel ventilation facilities, for road or rail tunnels, should not be different from the airline industry. Both face the challenge of determining the appropriate design requirements that will manage the levels of risk and limit the magnitude of the loss consequences.
NFPA 502 (2008) has some strong design fire scenarios, but there is no justification for their use. Past history of accidents alone is not enough. As stated above, we must specify the fire scenario by describing how the hazards are to be handled. In this case, we must consider the requirements that the hazards must comply with in order to satisfy the design requirements.
NFPA 130 (2010) does a better job in this regard. In the rolling stock section, the standard sets out fire hardening requirements for the materials that are to be used in the construction of the rolling stock. If these requirements are satisfied, there is a very likely possibility that the rolling stock will not blast in flames. Passengers may be protected by designing a ventilation system good enough to control the smoke in the tunnel in the event of a fire for which requirements have been specified for the hazards entering the system.
Ingason (2006) has evaluated design FHRR, but the author finds these FHRR very unlikely. The author has recently considered the fire hardening properties of the New York City Transit rolling stock and moved toward the opposite direction. If materials are fire hardened, it makes sense that design fire sizes would go down as well. To support this premise, train manufacturers agree claiming that their rolling stock does not burn. In the early 1990's, NYCT considered 14MW fire scenarios. But, NYCT has reconsidered the fire properties of the rolling stock materials and has changed the design train FHRR for stations and tunnels to 5MW. This was determined through Computational Fluid Dynamics (CFD) using the JGS fire model (Sanchez 2008) using thermophysical properties of the rolling stock materials.
It is imperative to mention that today's designs are carried out through the use of computer modeling. For the tunnel sections that are predominantly one dimensional, the Subway Environment Simulation (SES) (2002) computer program is used to determine the ventilation requirements. For areas where the airflow is not one dimensional, such as high ceiling stations, CFD is used. However, this paper only will present tunnel environment results. Thus the SES computer program was used. The principle used to determine the criteria used in the tunnel ventilation are based on the so called "critical velocity". Kennedy, et.al. (1996) presents how this is derived and applied.
Under the original design FHRR (14MW), a double-track tunnel section would require 121kcfm past the train to control the smoke and the tunnel ventilation fan plant would need to draw 400kcfm (Figure 2). If a fan plant strong enough to ventilate a 14MW fire scenario were installed, and a 5MW fire were the scenario to be ventilated, the required airflow past the train would be 84kcfm, while the predicted airflow past the train is 150kcfm (Figure 3). This could be considered an over design. However, if the fan plant were to be designed for a 5MW fire scenario, the fan plant would need to draw only 250kcfm (Figure 4). It should be understood that this example has taken only the airflow performance. There may be other design considerations that may alter the airflow requirements and design of the facility.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
IMPACT ON THE DESIGN AND CONSTRUCTABILITY
The FHRR considered for a design has some impact. However, it is not directly proportional. In this particular case it was shown that for a 64% FHRR reduction, the ventilation requirement only dropped 37%. However, the impact is not just the ventilation requirement, but all that comes with it. For example, a 400kcfm fan plant may be a two level fan plant requiring 4 fans of 100kcfm each, with 121 hp motors (assuming 5 in wg total fan pressure, and a 65% efficiency); while a 250kcfm fan plant may be a single level fan plant requiring only 2 fans of 125kcfm each, with 90 hp motors (assuming 3 in wg total fan pressure due to reduction of airflow, and a 65% efficiency). But the impact goes further. The foot print of the fan plant has been reduced significantly. In addition, the impact also reaches to the number of silencers (and its dimensions), dampers, controls, etc. This could very well make a very costly project into a more feasible one. The schedule, testing, and commissioning would be shorter. All this has a direct impact on cost, the ultimate ruler of any rail and road tunnel project.
Although only a rail case was presented, the impact on road tunnels would follow the same logic. If the tunnel FHRR to be considered is unjustifiably very large, it could lead to an over design. Care must be taken into consideration when selecting the FHRR for road tunnel designs.
This paper has demonstrated the importance of establishing the appropriate FHRR has on the design and constructability of a rail and road tunnel project. First and foremost, every project must start by establishing the risk it will consider acceptable and the policies to be enforced to meet the risk set for the project. Then, a proper and credible fire scenario must be established. This could be the deciding factor between go or no go for a project.
From a ventilation point of view, it was shown that there is no direct numerical correlation between the FHRR and required ventilation airflows, but we must rely on computer simulations to establish credible FHRR scenarios and assess the tunnel configurations throughout the system. The case shown was for a single-tunnel, but for a multi-junction track system, computer modeling would be required and the outcome may be similar or totally different. However, it must be recognized that there will be an impact, not just in the ventilation airflow requirements, but in the equipment. The number of equipments, and sequence of operations; the testing and commissioning; the size of the construction site; the time table. Every case must be assessed individually. There is no rule of thumb.
FTA, 2002, Subway Environmental Simulation Computer Program, SES Version 4.1. Volpe national Transportation Systems Center, Cambridge, MA, USA
Harvey, N., and Fuster, T. 2009. Design fire heat release rate selection - Impacts for road tunnels. 13th International Symposium on aerodynamics and Ventilation of Vehicle Tunnels. BHR Group. New Brunswick, NJ, USA
Ingason, H. 2006. Design Fires in Tunnels. Safe & Reliable Tunnels. Innovative European Achievements Second International Symposium, Lausanne, Switzerland.
Kennedy, W.D., Gonzalez, J.A., and Sanchez, J.G. 1996. Derivation and Application of the SES Critical Velocity Equations. ASHRAE Transactions: Research, paper 3983, pp. 40-44.
NFPA. 2008. NFPA 502 Standard for Road tunnels, Bridges, and Other Limited Access Highways. Quincy, MA, USA
NFPA. 2010. NFPA 130 Standard for Fixed Guideway Transit and Passenger Rail Sytems. Quincy, MA, USA
Sanchez, J.G. 2008. Predicting Fire Growth and Smoke Conditions in Tunnels and Metyros-An Advanced Fire Model. 4th International Conference in Tunnel Safety and Ventilation. Graz, Austria.
FTA, 2002, Subway Environmental Simulation Computer Program, SES Version 4.1. Vope national Transportation Systems Center, Cambridge, MA, USA
J. Greg Sanchez, P.E.
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|Author:||Sanchez, J. Greg|
|Date:||Jan 1, 2011|
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