TunnelTECH Sprinkler limitations for tunnel fire fighting Sep 2011
Haukur Ingason and Ying Zhen Li
Tests conducted by the SP Technical Research Institute of Sweden show that automatic sprinkler systems for fighting fires in tunnels are only suitable for tunnels with low air speed ventilation velocities. The results, reported here by Ying Zhen Li and Haukur Ingason of the SP, recommend that automatic sprinkler systems be used in tunnels with transverse ventilation or in bi-directional tunnels only, rather than in longitudinally ventilated tunnels in which air velocities are high.
Consequences of the Mont Blanc tunnel fire (1999)
- Interest in tunnel fire safety has increased dramatically in recent years owing to a number of catastrophic tunnel fires all of which have earned extensive media coverage. Accordingly, new technologies, such as water sprinkler systems, have been developed to improve fire safety in tunnels. At the same time it is generally accepted that automatic water sprinkler systems (AWS) can be adversely affected by ventilation during tunnel fires, but until now this viewpoint has not been fully investigated. This was why, before adopting a position on the matter, we decided to investigate how AWS systems function in a variety of longitudinal ventilation flow conditions.
- The system widely accepted today is the deluge system, where one or two zones of sprinklers are activated in the event of a fire. The heat from a fire plume rises vertically, but only if there is no wind in the tunnel. When there are longitudinal air flows, the heat and flames will be deflected and accumulate downstream of the fire, leading to a situation in which sprinklers are activated too far away from the actual fire to deliver water effectively.
- We also wanted to address the question of whether too many sprinklers might activate, thereby exceeding the water delivery capacity of the system. A deluge system activates one or two zones and is not sensitive to the longitudinal air flows. However, a full AWS system in a tunnel is assumed here to cover its entire length, without being split into different zones. When individual sprinkler bulbs activate over a large area, the AWS system will fail as it cannot deliver the amount of water needed to control the fire. This study aims to find out why such systems fail. A scale model was used, as it is cost-effective and can, if well designed, provide vital, reliable information.
In all, 28 tests were carried out in a 1:15 scale tunnel model, which was 10m long, 0.6m wide and 0.4m high (Fig. 1). The scale model corresponds to a tunnel that is 150m long, 9m wide and 6m high. The fire-spread between wood cribs spaced 1.05m apart (the equivalent of 15.75m in full scale) was tested, as were the effect of different ventilation velocities and water flow rates on the activation of nozzles. The heat release rate, fire growth rate and fire spread were measured. Further and more in depth information about the tests can be found in SP Report 2011:31.
Activating auto sprinklers
The tests showed that the heat release rate at activation of the first nozzle (sprinkler head) increases in line with ventilation velocity.
Fig 1. Scale model of a tunnel with an automatic sprinkler system
- The activation temperatures were about 206°C and 109°C for a link temperature of 141°C and 68°C, respectively.
- The tests also showed that the location of the first nozzle to activate depends mainly on the ventilation velocity. The other nozzles in the measured region activated shortly after the activation of the first nozzle.
During a tunnel fire, the suppression resulting from an AWS system can be divided into two modes: the fire is controlled by suppression through surface and gas cooling near the fire, while downstream it is controlled by gas cooling.
- At the preliminary stage, too few nozzles close to the fire source are generally activated shortly after ignition to sufficiently suppress the fire development and cool the hot gases above the fire. The nozzles activated at this stage play the most important role in the fire development. At the second stage, many nozzles downstream of the fire may be activated to cool down the hot gases.
Collapse of the system
The activation range is directly related to the longitudinal ventilation velocity and the water flow-rate of the automatic sprinklers. This is illustrated as a three dimensional plot of the activation range, as a function of the longitudinal ventilation velocity and the water flow rate (TL=141oC) (Fig 2). L'range is the dimensionless activation range of auto sprinklers, q*w is the dimensionless water flow rate of single nozzle, and V* is the dimensionless longitudinal velocity.
Fig 2. Activation range
- Failure of an AWS system during a fire is defined as it is having an activation range equal to or greater than 100m at full scale, i.e. a dimensionless activation range of about 15 (that is, 15 times the tunnel height). High ventilation and low water flow-rates can result in the failure of an AWS system during a tunnel fire. Note that the water flow-rates tested corresponds to 16.5mm/min, 20mm/min and 25mm/min at full scale, respectively. Note that nozzles having such flow-rates can suppress the fire efficiently as part of a deluge system.
- Under the specified water flow-rate tests, longitudinal ventilation plays the most important role in the failure of a system by stimulating development of the fire (by increasing the rates of maximum heat release and spread).
- It can be concluded that, on the basis of the water flow-rates tested, the most important parameter for an AWS system in a tunnel fire is ventilation velocity, rather than the water flow-rate; under such conditions, fire development is closely related to ventilation velocity, and almost independent of the water flow-rate.
To improve the performance of an AWS system in a tunnel fire, special strategies of variant ventilation and special control were also tested.
- The variant ventilation strategy changed the longitudinal velocity from 2m/s (equivalent to a full-scale 8m/s) to 0.5m/s (full-scale equivalent 2m/s), following an estimated fire detection time. Using the variant ventilation strategy effectively suppresses the fire development and reduces the maximum heat release rate. However, the maximum ceiling temperature is slightly higher.
- The special control strategy activated the sprinkler located 0.6m upstream. Using the special control strategy for nozzles can also efficiently suppress fire development, reduce the gas temperature and prevent the failure of an AWS. Although the tested ventilation velocity was 2m/s (equivalent to 8m/s in full scale), it should still work under low ventilation.
Conclusion: Use AWS systems under low ventilation
High ventilation promotes fire development, resulting in the failure of an AWS system. It should be used in tunnels with low ventilation velocities or natural ventilation. Therefore, it is recommended that AWS systems should be used in tunnels having transverse ventilation or in bi-directional tunnels, since in these tunnels longitudinal ventilation velocities will be relatively low. AWS systems are not recommended for use in longitudinally-ventilated tunnels with high ventilation velocities, except when variant ventilation strategies or special control strategies are applied.
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