With up to 2,300 m of rock on top of it, the Gotthard Base Tunnel, which is scheduled to go into regular operation in December 2016, is not only the world’s longest, but also the deepest railway tunnel ever built. Accordingly, the temperatures in the tunnel are very high, with the surrounding rock expected to reach 45 C (113 F) once the tunnel is operational. Since unrestricted rail operations are only permitted in temperatures of 40 C (104 F) or less, the heat has to be controlled with suitable equipment. Large fans developed and built especially for this application by TLT-Turbo GmbH provide fresh air for regular operations as well as during maintenance and repair work.
Separate fans ensure that the emergency exits are properly ventilated in case of fire. In addition to coming up with a technical solution that could handle the special challenges of this project, the engineers and planners had to develop detailed preliminary plans as well as a sophisticated logistics and installation concept.
The Gotthard Base Tunnel consists of two 57-km long tubes, one in each direction. Including all connecting and access tunnels, the entire underground system measures 152 km. At Faido and Sedrun, at approximately 16 and 36 km respectively, two multifunction stations were built that cut each of the two tubes into three sections of roughly equal length. Here, trains can make emergency stops and change tracks, if necessary. This is also where eight large axial fans made by TLT were installed that exchange the air, i.e. they extract a certain amount of heated air and inject the same amount of cool fresh air a few hundred meters farther down the train tracks. In addition to the four exhaust and four supply fans, 24 so-called jet fans were installed along with associated components like silencers, shut-off dampers, drives and lubrication systems from TLT were installed.
In addition to exchanging the air, these fans are used to enable maintenance and repair operations in the tunnel. Plans call for blocking each tunnel tube for one night per week in order to perform preventive maintenance work, thus enhancing the tunnel’s overall functionality and availability.
During these hours, many people will be in the tunnel who require fresh air.
“One of the major criteria for winning this bid was the fans’ ability to quickly switch between a wide range of modes. TLT fans are among the best in the world in this regard,” says Andreas Kuhn, TLT’s project manager for the Gotthard Base Tunnel.
In the third potential operating mode, the event case (i.e., a fire), four exhaust fans with a power rating of 2.4 MW each extract smoke from the emergency stop. The fans were designed to handle gases with temperatures of up to 400 C (752 F) for two hours. To put this in perspective, Kuhn explains that 2.4 MW correspond to 3,263 hp, which means that each exhaust fan has the power of four Formula 1 race cars. If a train reports a fire via sensors that are mounted on the train or in the tunnel, it is routed to the nearest multifunction station and instructed to make an emergency stop.
Following a sophisticated evacuation system, passengers can exit the train at the emergency stop and escape to secure areas. To keep the emergency stations from filling with smoke and enable passengers to take refuge in separately ventilated, secure tunnel areas, the four supply fans with ratings of 1.5 MW each are used, each of which can inject 275 m3 of fresh air per second into the protected areas.
During normal operations, the system controls only the temperature in the tunnel. During maintenance operations, the system controls what is called the dry temperature, i.e. the combination of temperature and relative humidity. The exhaust fans must run at top speed if a fire breaks out, so that they can extract as much smoke as possible.
The supply fans, on the other hand, must run at top speed in a specific maintenance scenario, when people are present in the tunnel. In this mode, the supply fans are operated in parallel to inject 420 m3 of air per second. The amount of air being moved by the fans is adjusted in two ways: via a speed controller with frequency converters, and via the blade controller, which uses hydraulic pressure to adjust the fans’ blade angles while they are rotating.
The special aerodynamic conditions of the 57-km tunnel deep under the mountain posed complex technical challenges. Since the trains can run at speeds of up to 250 km per hour, they generate a pressure shock in front of them and a vacuum behind them, which can cause serious problems for the giant fans. To come up with a solution, the R&D department of TLT conducted special tests early on.
As a result, each fan is now being permanently monitored by a so-called stall warning unit. Stalls must be prevented at all cost. Equally dangerous is excessive impeller acceleration, the so-called windmill effect, because it may cause mechanical damage. The stall warning unit, an innovative development of the TLT engineers, measures any changes in the tunnel’s air pressure 10 times per second and uses the fan controller to prevent stalls by hydraulically adjusting the fan blade angles.
The logistics of moving large and heavy components onto and into the mountain and assembling them in tight spaces posed major challenges. In addition, schedule changes made it necessary to deliver four of the large fans to the Sedrun ventilation station in the High Alps in deepest winter. The problem was that the roads in this region are designed for passenger cars, small vans and trucks with weights of no more than 28 tons. The heavy trucks needed to ship the fans, however, weighed roughly 70 tons. After making sure that the bridges could accommodate them, Swiss police escorted the fans to the mountain without any problems.
Since the tunnel shell was completed earlier than planned, its opening was pushed up by one year. As a result, all the shell equipment contractors, which included the consortium of TLT and Swiss company ABB Schweiz AG, had to perform much of their work side-by-side instead of sequentially. After the components had been successfully installed and tested, the actual commissioning could commence. To start the process, the engineers checked and documented the interaction of the system’s components and their safety compliance by conducting extensive tests.
The special challenge facing the consortium was the fact that the fan controls are the lead controls for all components. Accordingly, each scenario has to be initiated and controlled via the TLT master computer. To explain: The Sedrun multifunction station is linked to the ventilation center via two 800-m tall air shafts. One of these includes a large freight elevator. While this elevator moves, no fan may be in operation. This means that the controller must check whether the elevator is in its final position – top or bottom – when a “Start” command is issued for a fan. If it is not, the controller must move the elevator into this position before the fan can actually commence operation.
TLT initially became involved in the project as early as 2007, when a feasibility study was conducted to explore whether it is even possible to install fans with this level of performance into such small spaces. The pressure and vacuum problem caused by fast-moving trains was discussed as well. The actual bid was developed between late 2009 and August 2010. TLT won the contract in early 2011.
Unlike in other tunnel projects, no standard fans could be used, but the engineers had to develop, test, install and commission prototypes. “This was a very exciting time for us in terms of the contract size as well as the related delivery and performance parameters,” says Kuhn.