TECHNICAL | SAFETY/FIRE
For the system to operate as intended, every step must be done correctly. An availability assessment for the smoke control
Right, figure 4: A later, larger smoke damper in another tunnel
function must consider fire detection, control system, power supply, smoke dampers, exhaust fans, jet fans and airflow monitors. Automatic fallback solutions for failing equipment may improve the availability of the system. Indeed, maintenance requires a significant effort. I consider the airflow monitor to be the weakest spot. A typical and probably worst-case scenario is a
faulty airflow monitor feeding a wrong flow velocity to the controller. This is why sufficient redundancy must be installed. With several airflow monitors, automatic plausibility checks are possible. Faulty measurements can be identified and disregarded. Some countries define a minimum requirement of two triplets of flow monitors for any tunnel section with controlled airflow. If powerful jet fans are installed, dynamic control
With distributed smoke extraction, ventilation operation requires only three steps: detect the fire;
assign the fire to a ventilation section (~1km in length); and, start the smoke extraction. With only three steps in the control routine, there is only a small risk that the system does not operate as intended, such as the fire might not be detected, the system could assign the wrong ventilation section to the task, or the fans might not start. While more effective, local smoke extraction is also
more complex in that the system requires more steps, as follows: ● Detect the fire ● Assign the fire to a fire section (100m length, approximately)
● Open the smoke dampers ● Start the extraction fans ● Select the airflow monitor as input for the controller ● Measure and process the airflow velocity in the tunnel ● Select and start/stop jet fans to control the airflow, and
● Repeat the last two steps, measuring and processing airflow, and starting/stopping jet fans
requires the fans to be equipped with variable speed drives (VSD). In some countries, it has been found sufficient to have a small number of jet fans controlled by VSD. Variable speed drives usually have half the design life of a tunnel jet fan. Again, availability is a main driver for the design. High availability is achieved by sufficient redundancy and by thorough maintenance.
CONCLUSIONS Today, distributed smoke extraction has disappeared from ventilation design considerations for new tunnel projects. Instead, smoke control by local smoke extraction is a ventilation concept proven in multiple applications, in smoke tests and in real fires. Design requirements for the local smoke extraction
concept, and the necessary technical requirements for equipment and system control are available in the technical literature, and in various national standards. Some national codes include detailed recipes for the selection of parameters for jet fan control. And, of course, any complex safety system requires
rigorous testing during commissioning and as part of regular maintenance. Current developments aim at reducing costs,
improving availability and for the specific operation of the tunnel ventilation system in combination with other safety elements, such as emergency exits and/or fixed firefighting systems (FFFS).
REFERENCES 1 PIARC (1987): Road Tunnels, XVIII World Road Congress, Brussels, Sept. 1987
2 PIARC (2007): Systems and Equipment for Fire and Smoke Control in Road Tunnels 05.16.B. 3 PIARC (2011): Road Tunnels: Operational Strategies for Emergency Ventilation, 2011R02 4 Riess, I. (2022): On smoke stratification in a 1-D tunnel ventilation model, 11th “Tunnel Safety and Ventilation”, Graz, Austria
Int Conference
14 | August 2023
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