Positive pressure ventilation as a support for firefighting in subways

 

Positive pressure ventilation (PPV) fans are widely used by the fire service during firefighting operations in buildings. Fans are positioned to create a flow through the enclosure. This flow can remove the smoke after the fire or affect the direction of the smoke to support firefighting operations. In subway stations, it is less common to use PPV fans.

New subway systems are designed with smoke- and heat-control systems. In the case of a fire, these systems should remove sufficient smoke so the fire service can attack the fire from a smoke-free environment. Working in good visibility enables the crews to extinguish the fire faster without being exposed to all the risks that working in a smoke-filled environment with zero visibility bring along: getting lost, potential for ignition of the smoke, running out of air, increased risk for cancer, tripping hazards and so on.

Old subway systems, however, are often not equipped with smoke-control measures. A big fire in such a station or a tunnel leading to such a station will create a huge challenge for the fire service. Is it possible to use a large number of PPV fans to compensate for the lack of a fixed smoke-control system?

Critical velocity

A subway tube or a subway station differs from an ordinary building. A tube is characterized by the fact that one dimension is way longer than the other two. To a lesser degree this is also valid for the subway station or at least the level where the platforms are. In such a volume, it is possible to clearly define the inlet(s) and the outlet(s).

Typically, the smoke from the burning carriage will spread in both directions. After a while, the smoke layer has descended to the floor level and the firefighters are confronted with zero visibility and heat when approaching the fire. A fixed smoke-control system is designed to create a longitudinal airflow through the tunnel or the station. This airflow will make it harder for the smoke to flow in the opposite direction of the airflow. At a certain distance from the fire, there will be an equilibrium. The smoke will stop spreading in the opposite direction. This distance is called the backlayering length.

The higher the velocity of the airflow, the shorter the backlayering length will be. At a certain velocity, all the smoke of the fire will be pushed downstream of the fire, creating a smoke-free environment at one side of the fire. This velocity is called the critical velocity.

Karel Lambert of the Brussels Fire Department performed research into using PPV fans in subway stations as a replacement for a fixed smoke-control system. The goal of the research was to find out whether it was possible to generate the critical velocity by using PPV fans.

Test site

The Feuerwehr & Rettungs Trainings Center (FRTC) in Frankfurt has a building that mimics a subway station. This mock-up station is 60m long and 4.53m high. It is equipped with one track, a subway car and one platform, which is 3.22m wide. A staircase connects the platform to the floor above which mimics the ground level. The door at the ground level is 1.93 x 2.47m (W x H). This door was used as an outlet.

The subway cars enter the building through an opening that measures 3.17 x 4.15m (W x H). This opening served as an inlet throughout the experiments.

The building is not connected to a real tunnel. However, the results of the tests were used as input for CFD simulations where a simulated tunnel was attached to the training building. The tunnel had the same dimensions as the opening.

Tests & CFD simulations

One hundred and six full-scale tests with up to four fans have been performed in the subway station building in Frankfurt. For each test, the configuration of the fans was changed, and the resulting velocity was measured. These measurements were performed with nine bidirectional velocity probes. A mobile weather station was on site to be able to correct for changes in ambient wind conditions between the different tests.

All the fans used in the tests were high-flow electrically driven fans. The advantage of electric motors is that the fans will run flawlessly in an environment where there is smoke present. A combustion-engine fan only functions if there is sufficient oxygen present. This means that – up to a certain temperature – the fans can be positioned between the fire and the outlet.

In each of the tests, a fan was positioned at the top of the staircase. This fan was pointed towards the door. It worked as an extraction fan. This fan was fed air by one or multiple fans on the platform or the landing of the staircase. The distance to the stair was varied. In some cases, all the fans on the platform were positioned at equal distance from the staircase whilst in other tests the fans were at different distances from the staircase.

A fire with an airflow coming from the left. The smoke is moving predominantly to the right. The backlayering length is indicated in the picture. The picture shows what happens when the flowrate is increased and the critical velocity is attained: no more smoke upstream of the fire.

Results

The best result was achieved with three fans on the platform, each at a distance of 7m from the bottom of the staircase, combined with one fan at the top of the staircase. The fans on the platform were inclined 10° vertically. This angle generates a flow that is directed to the middle of the height of the opening from the platform towards the staircase.

At a distance larger than 7m, the cone of air that is generated by the fan has become so big that part of it is no longer entering the staircase. When this happens, the effectiveness of the set-up reduces.

The top result was 47.8m³/s or 172,000 m³/h. This flow would exceed the critical velocity in a tunnel, if one were attached to the opening. Firefighters could then use the station upstream to enter the tunnel and attack the fire from a smoke-free environment.

In the station itself, the velocity would drop since the cross section of the station is larger than the cross section of the tunnel. However, the velocity in the station would still be sufficiently high to limit the backlayering length to 15m. This means that firefighters have a smoke-free environment up to 15m from the seat of the fire. This would still be a large advantage compared to the situation where they had to find their way to the fire over a very large distance.

The subway building at FRTC in Frankfurt.

Conclusion

The conclusion of the research is that using PPV to support firefighting in a subway system without a fixed smoke-control system is a viable option. The number of fans needed and their optimum location is dependent on the local geometry.

Bigger stations or larger tubes (e.g. for two tracks) will need a higher flowrate to achieve the critical velocity or limit the backlayering length. On the other hand, stations with two tracks often have two platforms which allow for the number of fans to be doubled. If the platforms are wider than in the mock-up building in Frankfurt, this allows for more space in between the fans. In figure 2, one can see that the space between two fans is limited due to the limited width of the platform. Increasing the space between fans will increase the performance of the fans due to the better entrainment. Lastly, a bigger subway station will usually have more than one staircase. This will lead to a larger (combined) outlet opening, lower velocities in the staircases leading to these openings and therefore lower friction losses. This will help as well to increase the performance.

The platform and the track. The staircase can be seen in the back.

To use this tactic successfully, some preparation is needed before a fire breaks out in a subway station or tunnel. A fire department can preplan for such incidents. The selection of the number of fans and their ideal positions can be determined for every station. The number of generators and the lengths of cables needed to connect the fans with the generator should also be noted in the preplan.

Without such preplans, only highly trained staff will be able to decide on the spot where to place the fans. Firefighters will have to figure out how to place the cables and the fans as they go. Even though this method can work, it is preferrable to prepare them for such an event and for the tasks that will await them.

The result of this study is the knowledge that PPV fans can make up for the absence of a fixed system. At least the fire service has one method to reduce the difficulties it is confronted with during such a complex fire as one in a subway system.

For more information, go to www.cfbt-be.com/en or email karel.lambert@skynet.be

References

Lambert K, Welch S, Merci B (2018) The use of positive pressure ventilation fans during firefighting operations in underground stations: an experimental study, Fire Technology, Vol 54, p 625-647

Lambert K (2014) Positive pressure ventilation in underground systems – an experimental and modelling study, thesis in the framework of the International Master in Fire Safety Engineering, The University of Edinburgh

Republication from https://iffmag.mdmpublishing.com

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