The design of natural ventilation systems to control smoke movement in tunnels.
The control of smoke movement in tunnels is of critical importance to the safety of people in a tunnel. Two general approaches are available to designers of these systems. The first is through passive means and the second is through the use of mechanical equipment. The passive systems are often referred to as natural ventilation (NV) systems. The active systems are referred to as mechanical ventilation (MV) systems. NV systems are only used in existing systems and in short tunnels, while MV systems are implemented in the remaining tunnels. There are many practical advantages to using natural ventilation systems and this paper discusses them along with important design elements that help insure a successful design.
What Is a Natural Ventilation System
NV systems are designed to allow smoke to move naturally away from people who may be located within the tunnel. This movement is accomplished generally through the buoyancy forces that hot smoke generates. This force naturally moves the hot smoke upward toward the ceiling of the tunnel. Once the smoke reaches the ceiling the smoke spreads laterally along the ceiling. In a NV system the smoke is accumulated in the ceiling area or channeled away from any people in the tunnel. Another characteristic of NV systems is that there are no mechanical devices that need to activate for the system to operate (although certain systems utilize dampers that must open or close in a fire emergency).
Types of NV Systems. The first type is one in which smoke is carried away from the fire through ventilation openings in the ceilings that lead to the surface. By removing the smoke through these openings, a tenable environment can be sustained in the tunnel so that evacuation can occur. The second type of NV system utilizes a smoke capture system in the crown of the tunnel. Sufficient space is provided such that a layer of smoke forms high above any patrons and spreads laterally as the fire progresses.
What Is a Mechanical Ventilation System
MV systems are characterized by the use of fans to force smoke to move in the proper direction. Dampers are also commonly used. The fans generally overpower any buoyancy forces that may be present. In these types of systems more precise control over where smoke travels can be accomplished.
Why Choose a Natural Ventilation System?
The use of NV systems in tunnels has many advantages over their MV counterparts. The first being that the added costs of MV systems is often quite large compared to that of NV systems. Some of the costs are associated with the actual purchase of equipment such as fans, dampers, sensors, motor controllers, and wiring. Other costs come from the actual construction costs for the physical spaces necessary to house the equipment. Still other costs stem from providing adequate power for this equipment. Finally costs stem from the maintenance associated with the equipment. Natural ventilation systems have little to none of these costs, with perhaps an exception being in the physical space requirements.
NV systems are also generally less complex than MV systems. Because NV systems are generally passive systems, they must generally perform without any action by operators. These systems perform based upon mostly geometric conditions within the tunnel such as ceiling height and openings to the surface.
NFPA 130 and NFPA 502
The two main sources for fire life safety standards in tunnels are the National Fire Protection Association Standards 130 and 502. NFPA 130 covers rail tunnels while NFPA 502 covers road tunnels. Both standards have similar goals with respect to smoke control. Primarily, the goal is to provide a tenable path of evacuation from a fire incident. To accomplish this smoke must be kept away from evacuating passengers through either active or passive means.
Tunnel Length. Both of these standards use tunnel length as one means of determining whether MV or NV is applicable. NFPA 130 requires an MV system for rail tunnels greater than 300 m (984 ft). For rail tunnels between 60 m (197 ft) and 300 m (984 ft), this standard allows for engineering analysis to determine whether MV or NV systems are required. For rail tunnels less than 60 m (197 ft), MV systems are not required. For road tunnels, NFPA 502 does not set a definitive lower limit on tunnel length for which MV systems are required, but allows for NV systems where engineering analysis shows acceptable performance.
Tenable Environment. Both standards have slightly different definitions of what a tenable environment is. In either case, the standards cover exposure to high temperature, radiation, toxic gases, high noise levels, high air velocities and limited visibility.
Zone of Untenability
Only NFPA 130 mentions this, but because of impracticality of providing a tenable environment at points very close to a tunnel fire, there is a zone around a fire where the smoke control requirements are relaxed. According to NFPA 130, this 'zone of untenability' can include areas up to 30 m (98 ft) away from the fire location. The size of this zone is most directly related to the severity of the design fire incident. This zone allows designers to provide a reasonable amount of smoke control away from a fire without over-designing for those cases, close to the fire, which are difficult to impossible to control without excessive cost.
Path of Egress
The standards require a path of egress from the fire. The standards are vague on whether all patrons need to have a path of evacuation provided or only a single path is required for some of the patrons. Because of this vagueness, longitudinal systems, which push smoke and hot gasses over a portion of the tunnel which may contain patrons are allowable. In these cases only one path of egress is provided for a portion of the patrons.
Smoke Control in Chosen Evacuation Path. One of the characteristics of a controlled system is that it respond as the designer or operator intended under a variety of conditions. If the designer expected the ventilation direction to be in a certain direction then the system should operate in that direction when implemented. For NV systems this intended direction is always the same for any given fire location or size. In NV systems the smoke movement must be away from the path of egress for all conditions that may be reasonably encountered. If the results are different for different conditions the NV system does not really provide control of the path of egress.
Point of Safety. A point of safety is a location that affords adequate protection to patrons during a fire emergency. By adequate, it is meant that the area will remain tenable for the duration of the incident. In deep tunnels, far from the surface, points of safety are often employed so that patrons can evacuate there instead of traveling long distances to the surface.
Control of Smoke Movement
In order to successfully control smoke movement the system must definitively move smoke in the intended direction. The intended direction is that which the designer chose as a response to a particular fire incident. In the MV case this generally means using fans to move smoke away from the path of egress. In the NV case, control is based upon the geometric configuration of the tunnel and how buoyant hot smoke moves up and away from a fire incident. The geometric configuration is designed to facilitate this smoke movement in an intended direction, therefore maintaining control of the situation.
The designer of NV systems for tunnels has to balance many items; those both under his/her control and those that are not.
The impact of those items beyond the designers control needs to be mitigated such that they do not have great influence on the control of smoke movement. There are several items that the designer has no control over in designing a natural ventilation system for a tunnel. These items must be taken into account and measures must be taken to mitigate against their effects.
Wind. Wind is perhaps the single most difficult influence on a natural ventilation system. When present, wind forces can be larger than the buoyancy forces generated by a fire. Since wind direction and magnitude are variable, even for locations with a prevailing wind, the wind force can not be relied upon to provide natural smoke control or neglected in its adverse impact on a NV system.
The only design choice for NV systems is to shield the exposed external connections to a tunnel (portals and vent shafts) from the wind. This generally involves shielding these exposed elements from the wind. When the elements are not exposed directly to oncoming winds, the impact of the wind can often be minimized.
Ambient Temperature. The ambient temperature both external to and inside the tunnel can have an impact on the buoyant forces generated by a tunnel fire. In warm conditions, the temperature difference between the smoke and ambient air is less than in colder conditions. The size of these temperature differences are directly related to the size of the buoyant forces generated by a tunnel fire.
Generally, NV systems perform better when buoyant forces are stronger, therefore to be conservative, the design should definitely examine situations where ambient temperatures are high and therefore buoyant forces are low. This should not preclude also looking at how the NV system performs when low ambient temperatures are encountered.
An important ancillary item related to ambient temperature is the wall temperatures that exist in the tunnel. The wall temperature impacts the amount of fire heat absorbed by the walls and also impacts the buoyancy forces generated.
Heat Release Rate and Smoke Release Rate. Although trains and road vehicles can be designed to be fire hardened, generally the tunnel designer has little control over the vehicles traversing the tunnels they are designing. Therefore the fires that can be expected within a tunnel are beyond the designer's control. Most projects have a set fire heat release rate (FHRR) and fire smoke release rate (FSRR) profile based upon a reasonable worst expected case scenario.
The FHRR and FSRR have two effects on the design of NV systems. The first is the impact that the FHRR has on the strength of the buoyant forces. The larger the FHRR the larger the buoyant forces due to a greater mass of hot fire gases. This is generally a beneficial effect. The second is that larger the FHRR the larger the resulting hot smoke cloud will be. This is generally a detrimental effect as the size of the resulting untenable zone is larger. Generally the larger untenable region has a greater detrimental effect than the beneficial effect of the added buoyant forces for larger fires.
The choice of tunnel geometry is the most important consideration when developing a working NV system. Design decisions are made to facilitate one of the two types of NV systems described earlier. These geometric choices are often costly to implement and care needs to be taken to minimize their costs while still providing a working NV system.
Length. There are no particular reasons that NV systems can not work in longer tunnels. The reason they are not often employed is that the costs of ventilation openings to the surface and the increased underground space requirements of ceiling capture type systems is prohibitive. NFPA 130 has an upper limit of 300 m for rail tunnels before MV systems are required, while NFPA 502 has no upper limit at which MV systems are required.
Cross Sectional Area. Increasing the tunnels cross sectional area can be beneficial to a NV system in that a larger cross section can hold more smoke and hot gasses near the ceiling. This makes the smoke layer thickness shallower and thus generally higher away from patrons. Unfortunately, tunnels are often built with the smallest cross section possible to reduce costs.
Ceiling Height. The ceiling height has a profound impact on the performance of a NV system. The higher the ceiling height the higher any smoke layer will be with respect to patrons. Unfortunately, tunnels are often built with the lowest possible ceiling. In certain cut-and-cover tunnels a higher ceiling height can be achieved at relatively low cost if less overfill is placed on top of the tunnel.
Grade. The vertical curvature or grade of a tunnel is another determining factor in the design of NV systems. The grade of the ceiling gives a buoyant plume of smoke a natural direction to move laterally. In combination with a tailored ceiling height and ventilation openings the clever use of the tunnel grade can make a NV system work by funneling smoke and hot gases to the high point of the tunnel.
Ventilation Openings. Sufficient openings need to be provided and located throughout the tunnel such that during a fire emergency the majority of the smoke will exit through these openings. These openings should be located in or near the tunnel ceiling. This is an important consideration because sometimes these openings are located slightly below the ceiling level where their effect is reduced. In this situation, smoke must fill the area above the opening soffit before a significant amount of smoke exits the openings. The design of these openings should be such that there are no long horizontal runs which tend to hamper the buoyant flow of smoke.
Special care should be taken at the surface where ventilation openings are exposed to winds. In certain situations where the openings are flush with the surface the buoyant flow can be hampered by the flow of wind. Care should be taken to shield these opening from the wind. Raising the openings above the surface, when feasible, is one way to avoid this phenomena.
Portal and Approach Design. Tunnel portals are very important elements of NV system design. Tunnel portals provide the primary connection between the tunnel and the surface. They also supply a means of egress from the tunnel and a source of fresh air to the tunnel. The main item to be addressed with portal design is the wind. Wind directly impinging upon an exposed portal will wreck havoc with the intended operation of a NV system during a fire emergency.
For a NV system to work portals must be shielded from direct exposure to the wind. This is often accomplished by placing curtain walls near the portals to deflect the wind away from the openings. The best configuration with respect to wind would be to have the portal at one end of a u-shaped open cut approach section. This give the best shielding from the wind as wind from any direction never directly impinges upon the portal.
Along with the geometry a NV design should take into consideration the egress elements present. The whole point of a NV system for smoke control is to provide a tenable path of egress. By adding additional egress elements to the design it may be possible to reduce the amount of time necessary to completely evacuate the fire zone. This decrease in evacuation time may make it easier for a NV system to meet its goals as passengers may be fully evacuated before tenability is lost in the fire zone.
Cross-Passages. One way a tunnel designer can improve evacuation is to add additional cross passages to an adjacent tunnel. An adjacent tunnel may provide a tenable environment for a longer period than the incident tunnel. If it can be shown that the adjacent tunnel remains tenable for the duration of the incident, it may be possible to classify it as a point of safety. Adding sliding doors that can be closed after the evacuation can also help keep the adjacent tunnel tenable for a longer period of time.
Walkways. In rail tunnels, walkways are often provided next to the trainway to assist evacuation from rail vehicles with elevated exits. Generally these have been designed to accommodate a single lane for evacuation, because of the extra costs associated with wider walkways. By making these walkway wider to accommodate multiple evacuation lanes, evacuation times can be reduced and perhaps enable a working NV system where one did not exist before.
Portals. Adding extra stairways near the portals can improve evacuation and therefore evacuation times can be reduced and perhaps enable a working NV system where one did not exist before.
Car-to-Car. In short rail tunnels it is often the case where the train is longer than the tunnel itself. In this case, during any fire scenario at least part of the train is outside the tunnel. The possibility of evacuating through the train should be explored since the environment is generally more tenable inside rather then outside the vehicle (except perhaps the incident car).
Stairways. Similar to cross-passages, additional emergency exits to the surface can improve evacuation and therefore evacuation times can be reduced and perhaps enable a working NV system where one did not exist before. These can be costly compared to other options
Points of Safety. Specially designed points of safety can often be built that provide a tenable environment during a fire emergency in a tunnel. These can be built adjacent to but physically separated from the mainline tunnel except for closable doors or supplied positive mechanical ventilation. Although this design option is expensive compared with others, in certain situation building these refuge areas may be feasible.
The following is a description of a NV system that is being deployed for a 171 m (561 ft) long rail tunnel. The description includes an analysis of the system that was performed using computational fluid dynamics (CFD). Several of the design elements described above are also addressed and their impact quantified through the CFD analysis.
The tunnel includes provisions for ventilation opening for almost its entire length. The openings all connect from the side; some connect to actual shafts, while others are merely open windows to the outside. The portals are protected from winds due to u-shaped approach sections. The modeled fire was a large rapidly growing 37 MW fire (126.2 MBtu/h). A 3.78 m/s (744 ft/min) traverse wind was also modeled based upon available wind data.
In this case study two different geometric configurations were analyzed using CFD. The first was of the initial conceptual design for the NV system. It was found that this configuration failed to meet NFPA 130 criteria for smoke control. The second was for a modified design that addressed the shortcomings of the conceptual design by adjusting several of the design elements discussed earlier. Figure 1 shows the CFD Model used to analyze the NV system for use in this rail tunnel. It shows the modified design after it was found that the initial conceptual failed to meet NFPA 130 criteria.
[FIGURE 1 OMITTED]
The initial conceptual design featured relatively small ventilation openings (1.83 m [6 ft] x 0.91 m [3 ft]) with soffits 0.61 m (2 ft) from the tunnel ceiling. Window type openings on the northeast end of the tunnel were exposed to the oncoming wind. The other ventilation openings featured elbow shaped shafts with horizontally oriented base legs. The horizontal surface openings were flush with the surrounding terrain.
The modified design featured larger ventilation openings (2.13 m [7 ft] x 1.52 m [5 ft]) with their soffits 0.46 m (1.5 ft) from the tunnel ceiling. The window type openings were shielded from the oncoming wind by the placement of a curtain wall a short distance away from the openings. The other shafts were modified to include a chamfered shaft design eliminating the horizontal leg in the initial design. The horizontal surface openings were raised above the surrounding terrain help deflect the effect on the oncoming wind on the surface openings.
Light attenuation plots are used to present the simulation results. The light attenuation plots show the light attenuation coefficient at a height of 2.5 m (8.2 ft) above the walkway. The height of 2.5 m (8.2 ft) corresponds to the requirements of NFPA 130. The attenuation plots were used to evaluate the extent of smoke movement. The plots also include cross-sectional views taken 30 m (98.4 ft) from the edge of the incident car, on either side. This distance is from the NFPA 130 recommendation for definition of the zone of tenability 30 m (98.4 ft) away from the perimeter of the fire. On each of the sectional contour plots, a line that indicates the NFPA 130 criteria height of 2.5 m (8.2 ft) above the walkway has been placed. In the attenuation plots, the areas that are red exceed the maximum attenuation coefficient of 0.267 [m.sup.-1] (0.081 [ft.sup.-1]).
Initial Conceptual Design
Figure 2 shows the resulting light attenuation pattern in the tunnel Figure 3 shows the resulting smoke cloud for the initial conceptual design CFD simulation. The results for this simulation indicate that smoke travels throughout the whole tunnel from portal to portal resulting in untenable conditions well beyond the zone on untenability 30 m (98.4 ft) from the fire car. The results here indicate an unacceptable NV system design. It was felt that the exposed north vents and opening flush with the surface were causing the unacceptable conditions.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Figure 4 shows the resulting light attenuation pattern in the tunnel Figure 5 shows the resulting smoke cloud for the modified design CFD simulation. The results for this simulation indicate that smoke is extracted efficiently near the fire incident and does not spread much into the area beyond the zone of untenability. The results here indicate an acceptable NV system design.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
NV systems can provide a cost effective solution to smoke control in relatively short tunnels that generally require little maintenance or operator interaction. NV system design for tunnels involves balancing the many elements that impact their performance. Often times, the designer must carefully address the geometric and egress element to find a working NV system solution that meets the requirements set forth in NFPA 130 and NFPA 502. Most critical of these element being the impact of winds, tunnel ceiling height, available egress elements, and the number of ventilation openings provided.
Kang, Mia: Mechanical Engineer, Parsons Brinckerhoff, Inc.
Mitchell Stephen: Structural Engineer, STV, Inc.
Ohlin, Erik: Structural Engineer, STV, Inc.
NFPA 130. 2007. Standard for fixed guideway transit and passenger rail system. 2007 Edition, published by the National Fire Protection Association, Quincy, MA.
NFPA 502. 2007. Standard for road tunnels, bridges, and other limited access highways. 2004 Edition, published by the National Fire Protection Association, Quincy, MA, USA.
Igor Maevski, Senior Mechanical Engineer, Jacobs Engineering, New York, NY: Changes are being made to NFPA 502, which will require engineering analysis to justify natural ventilation for short tunnels.
Thomas P. O'Dwyer: This is a good development. Hopefully, the standard is clear and concise about what is required for a natural vent system to be acceptable.
Kevin Gallen, Senior HVAC Engineer, Dewberry, New York, NY: You (Tom O'Dwyer) claim that natural ventilation strategies can be cost effective for small tunnels. Is this assuming a fixed wind direction and velocity?
Thomas P. O'Dwyer: No, wind direction can never be assumed to be from a fixed direction or velocity. There are measures that can reduce the impact of these items, such as shielding portals and openings at the surface from the wind.
J. Greg Sanchez, Principal Mechanical Engineer, MTA-New York City Transit, New York, NY: How does the size of the street vent openings compare with a mechanical ventilation system?
Thomas P. O'Dwyer: In my opinion, a natural vent system requires substantially more street openings compared to a mechanical system. This is a large generalization, and it may be possible to make the tunnel ceiling tall enough to reduce this requirement and funnel smoke through smaller openings.
Thomas O'Dwyer is a supervising mechanical engineer in Mechanical and Electrical Technical Excellence Center of Parsons Brinckerhoff Americas Inc. New York.
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|Author:||O'Dwyer, Thomas P.|
|Date:||Jan 1, 2010|
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