How warm does it get in a tunnel smoke extraction duct when a water-mist FFFS is activated?
Results from the SOLIT² research project
Within the SOLIT² research project, numerous full scale fire tests were carried out in a test tunnel. For the first time, fire tests with a water-mist FFFS and a semi-transversal ventilation, including installation of a false ceiling in the test tunnel, were implemented. To cover several fire scenarios, both solid fires (Class A) and liquid fires (Class B) with different fire sizes were tested. To determine the temperatures in the smoke extraction duct, thermocouples were installed in the false ceiling at a height of 0.5m and 1.5m.
Figure: Temperatures in the exhaust duct with a 150 MW solid fuel fire and activated water mist FFFS
Although the ventilation in the test tunnel is only dimensioned for a fire size of 30 MW HRR, in fire tests with a significantly higher fire load, the amount of smoke gas could be discharged through the exhaust air duct after activation of the FFFS. The amount of smoke gas was also greatly reduced after activation. An exemplary evaluation of the duct temperatures with a 150 MW solid fuel fire (truck mock-up) and a semi-transverse ventilation with 120 m³/s at 3 m/s wind speed looks as follows:
Figure: Temperatures in the exhaust air duct with a 150 MW solid fuel fire and activated water-mist FFFS
The red bar marks the activation of the FFFS. The FFFS was activated very late (about 8 minutes after ignition) to represent a worst-case scenario. It is recognisable that shortly after activation of the FOGTEC FFFS, temperatures did not exceed 50°C and settled at less than 40°C, thus once again demonstrating the enormous cooling effect of water mist.
Increasingly water-mist FFFS are used for compensatory purposes in tunnels. One possibility is the compensation of structural fire protection to protect the structure. Fire tests have shown that temperatures can also be considerably reduced in the exhaust air duct, which means that structural measures such as fire protection panels and PP fibres can be dispensed with. It was also shown that ventilation systems that are designed too weak – as it is the case with many older tunnels - can be compensated by using water-mist FFFS.
If the safety systems are coordinated, such compensatory measures can be very cost-effective. The simple implementation of a water-mist FFFS can thus preserve against expensive construction measures. Furthermore, the overall safety level in the tunnel will be increased.
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