With a rock overburden of up to 2,300 metres, Gotthard Base Tunnel, which has been opened for scheduled train services in December 2016, is not only the world’s longest, but also the world’s deepest railway tunnel ever constructed.
Hence, temperatures within the tunnel rise quite high: During operation the rock temperature is expected to be 45 °C and the air will heat up accordingly. Since 40 °C is the permitted maximum for trouble-free rail traffic, temperature must be regulated by an appropriate ventilation system. TLT-Turbo GmbH’s heavy-duty fans, developed specifically for this purpose, do not only provide the tunnel with fresh air during normal operation, maintenance, service and repair work, but also ensure smoke extraction in case of fire and a separate ventilation of escape ways. The project’s high level of complexity, in addition to a technical solution mastering those special demands, also required extensive preliminary planning and a well-designed concept for logistics and assembly.
Gotthard Base Tunnel consists of two 57 km long single-track tunnels. Including all cross-passages, access tunnels and shafts, the underground system runs over 152 km. Two multifunction stations, at Faido and Sedrun, at km 18 and 36, divide both tunnel tubes into three sections of approximately equal length. The stations contain emergency-stop stations and allow for track crossover. Here, TLT also installed eight large axial flow fans exchanging the air in these stations, i.e. the fans extract a certain amount of heated air and a few hundred meters further into driving direction, they blow in the same amount of cool fresh air. In addition to the four air supply fans and four extractor fans, TLT installed 24 jet fans with accessories such as silencers, shut-off dampers, drives and oil supply units.
These fans are used not only for air exchange, but also for ventilation during maintenance, service and repair work in the tunnel. The current plan is to block one of the tunnel tubes for one night per week to carry out maintenance work at the system components of that tube. Such preventive action is to ensure high availability of the plant. During maintenance there are a lot of people in the tunnel who need fresh air supply. “The ability of the fans to approach a very large number of different operating points in no time at all, was an essential award criterion. Within this category TLT’s fans are among the best in the world”, Andreas Kuhn, TLT’s project manager of the Gotthard Base Tunnel project, explains the decision for the manufacturer.
Power of Four Formula 1 Racing Cars
During the third possible operation mode called emergency mode – in other words in case of fire – four exhaust fans, each with a capacity of 2.4 MW being designed to pump hot gases of up to 400 °C over two hours, will extract exhaust gas from the emergency-stop station. “As a comparison, 2.4 MW are equivalent to 3,263 PS, this means an exhaust fan has the power of four Formula 1 racing cars” Kuhn outlines the dimension of the facility. If a train signals fire in a tunnel, for instance by the sensors installed at the train or in the tunnel, it will be lead to the nearest multifunction station to stop in an emergency-stop station. Due to a smart evacuation system, passengers in the emergency-stop station can get off the train and evacuate to safe areas. The four supply air fans, each with a power of 1.5 MW and each capable of blowing 275 m3 of fresh air per second into the escape areas, will be used to prevent the emergency-stop stations from filling with smoke and to allow for passengers to seek shelter in separately ventilated safe tunnel areas.
Under normal operating conditions, the temperature in the tunnel is the control value, during maintenance the dry temperature is the control value, this means temperature combined with humidity. The highest power requirement of exhaust fans is needed during an incident, i. e. to extract exhaust gas. The highest power requirement of air supply fans arises during a very specific maintenance scenario if much maintenance personnel is working deep down in the tunnel. The air supply fans are then run at maximum power in parallel mode to blow in a total of 420 m3/s. The air volume pumped by the fans is adjusted by two factors: via speed control by aid of frequency converters and via blade control by aid of a hydraulic unit that can synchronously adjust the pitch angle of all blades during operation by different oil pressure settings.
Innovative New Developments Turn Fans Into Prototypes
The technical challenges arose from the particular aerodynamic conditions in the 57 km long railway tunnel deep under the Gotthard massif. Since trains passing the tunnel will be quite fast with up to 250 km/h, the high speed will create a pressure shock ahead of and a suction behind the train that may cause enormous problems to the fans. TLT”s R&D department faced the issue at an early stage by carrying out preliminary tests to create a solution. Now each fan is continuously monitored by a stall warning unit (SWU) to avoid stall during any operating mode and likewise to avoid inadmissibly high acceleration of the impeller called windmilling since it may cause mechanical damage. The SWU, an innovative new development of TLT’s engineers, registers a change in pressure in the tunnel at a clock frequency of 10 Hz, i.e. 10 times per second. The pitch angle is then adjusted via fan control by aid of a hydraulic blade adjustment to prevent stall.
The logistical situation, too, was a major challenge, because very large and heavy components had to be transported on or rather into the mountain and assembled under extremely limited space conditions. In addition, postponements delayed the delivery of four of the large fans to the ventilation center of Sedrun in the Grisons’ High Alps into the deepest winter. “Throughout the envisaged period there was always the risk of snow in the High Alps. However, we were lucky since the winter was mild and the period was nearly snow-free”, Kuhn remembers. Another problem were the streets in this region being built only for passenger cars, small transporters and trucks up to a maximum of 28 t. The heavy trucks used to deliver the fans, however, weighed about 70 t. Having confirmed that all bridges can withstand the heavy trucks, the Swiss police escorted the fans uphill without any problems.
Coordinating Over 1,000 Technical Interfaces
Having completed the shell structure of the tunnel ahead of schedule, the opening of the tunnel was brought forward by one year. As a consequence for the following work of all shell structure suppliers, which include the open consortium composed of TLT and the Swiss company ABB Schweiz AG, many operations once planned to be executed consecutively had to be carried out in parallel then. “It was quite a challenge to master the coordination and monitoring of deadlines. Given the complexity of the facility, we finally had to coordinate over 1,000 technical interfaces to ensure that everything would run smoothly”, TLT’s project manager says.
Upon completion of the installation and successful sub-testing of components and installations, the actual commissioning could be started. In the first instance, the test operation phase from October 2015 to May 2016 proved the functionality and fulfilment of the safety requirements. The interplay of all tunnel components was thoroughly tested by train runs – even by a special train run at 275 km/h with an ICE train rented from Germany. “Comprehensive scripts describe nearly any actions and their related, desired response. Failure scenarios are tested as well – automated ones and those requiring manual action by the operator”, Kuhn explains.
The consortium’s major challenge was the fact that the control system governs all components of the facility to the effect that any scenario is initiated and controlled by TLT’s head computers. To be more precise: Two 800 m high shafts connect the multifunction station of Sedrun to the ventilation center. One of the two shafts has been equipped with a lifting device – a large freight elevator. When the elevator is running, no fan must be started. This means if a fan receives the command “start”, the control unit first has to check if the elevator is in its end position – at the top or at the bottom. If not, the control unit has to get it to one of those positions.
Regular Railway Operation With Revised Timetable in Operation Since December 2016
In 2007, TLT got in touch with this project for the first time. At that time, a study should identify the feasibility of installing fans of such power into extremely narrow buildings. The issue of suction and pressure shocks had also been discussed on a fundamental level in those days. The actual quotation processing of the public invitation to tender took place between the end of 2009 and August 2010. At the beginning of 2011, TLT finally won the contract. “Unlike usual tunnel orders requiring standardized fans being catalogue products, this time we developed, tested, installed and commissioned prototypes. Due to this project, especially the size of its order and its scope of supply and services we witnessed a fascinating period”, Kuhn concludes.
The opening was on June 1, 2016. SBB [Swiss Federal Railways] as future operator of the Gotthard Base Tunnel then assumed responsibility for the subsequent trial operation aimed at proving that the passage of passenger and goods trains, personnel deployment, and incident management function smoothly. Only then the route started to be included in the regular timetable in December 2016. For TLT, however, the project has not ended yet: The manufacturer has been awarded another order on the delivery of spare parts and is also expecting a five-year maintenance agreement.