Enhanced design of safety systems

Fire detection

Conditions in the tunnels in the Stockholm region are quite aggressive on equipment such as fire detection systems due to contamination and regular tunnel washes.

Air velocities are usually high, approximately 5-8 m/s, due to either environmental ventilation or the piston effect created by the traffic flow.

Even though congestion should normally be avoided, with the exception of accidents and incidents, the safety concept must have an inherent robustness to handle congested conditions both upstream and downstream of a fire. Part of this work is to further enhance the possibilities to detect fires in the tunnel system, and this at the high air velocities created by the environmental ventilation that is the case if the traffic is congested. Early fire detection is important for the traffic control centre in order to quickly start the appropriate action plan. Thus, a research study lead by the Swedish Transport Administration has been undertaken.

Historically, linear heat detection has been used as a robust fire detection system, and the research study has investigated the possibilities to complement this with some kind of smoke detection. The tests were undertaken in the tunnel Södra länken, and the test equipment, consisting of systems from various suppliers, has been exposed to actual traffic conditions, including contamination and tunnel washes, for approximately a year.

One of the goals of the study was to investigate if there are systems able to detect a 1 MW fire within 90 seconds, at high air velocities. The results look promising, and several of the tested smoke detection systems were able to detect a fire as small as 0.5 MW at an air velocity of approximately 5.5 m/s.

One challenge is to find a balance between the risk of false alarms and the benefit of very early fire detection. One way to handle this is to use detection of a 0.5 MW fire as a “pre-alarm” level, which gives the traffic control centre an early notification of the accident.

Fixed fire fighting system

The decision to install a fixed fire fighting system (FFFS) was made due to the desire to increase the robustness of the personal safety aspects of the safety concept. Under ordinary conditions, the tunnel can be evacuated without a FFFS should a fire occur. Experience from past fires however, highlights the difficulty of predicting road users’ behaviour during an incident. A FFFS can slow down and reduce the development of a fire, and as a result, prevent a large fire from developing into a catastrophe, even if the road users’ evacuate the tunnel at a slower pace than expected. The primary purpose of the system is therefore to slow down the fire growth and assist evacuation. The purpose is not to extinguish vehicle fires.

Naturally, fire fighting operations will be assisted by installation of the FFFS.