Innovative ventilation solutions for Gotthard 25 Aug 2016

The Gotthard Base Tunnel consists of two 57km-long tubes, one in each direction. When all connecting and access tunnels are added, the entire underground system measures 152km. At Faido and Sedrun, located at 16km and 36km respectively, two multifunction stations have been constructed to effectively split the alignment into three about equal sections. Here, trains can make emergency stops and switch tracks, if necessary. This is also where eight large axial heat exchange fans, manufactured by TLT, are installed, with the purpose of extracting heated air and injecting the same amount of cool fresh air a few hundred metres further down the tracks. In addition to the four exhaust and four supply fans, 24 LRT-manufactured jet fans are also installed, along with associated components including silencers, shut-off dampers, drives and lubrication systems.

As well as exchanging the air, these fans are used to facilitate maintenance and repair operations in the tunnel. Current plans call for the blocking off of each tunnel tube for one night per week in order to perform preventative maintenance work, thus enhancing the tunnel’s overall functionality and availability. But during these hours, many personnel will be inside the tunnel and will require fresh air. One of the major criteria for award of this ventilation contract was the necessity for a solution that incorporated the ability of the installed fans to quickly switch between a wide range of modes.

Power of four Formula 1 cars

In the third of these operating modes – the so-called event case, or fire mode – four exhaust fans, each with a power rating of 2.4MW, extract smoke from the emergency stop location. These are designed to handle gases at temperatures of up to 400°C for two hours; to put this in perspective, 2.4MW corresponds to 3,263HP, which means that each exhaust fan has the power of four Formula 1 racing cars. If a train reports a fire, for example via sensors that are mounted on the train or in the tunnel itself, 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 therefore enable passengers make their way to separately ventilated refuge areas, four supply fans with ratings of 1.5MW each are used, each of which is capable of injecting 275m³ of fresh air per second into the protected areas.

During normal operations, the system controls only the temperature in the tunnel. During maintenance operations, however, the system can control 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, however, must run at top speed in a specific maintenance scenario, when many people are present in the tunnel. In this mode, the supply fans are operated in parallel to inject 420m³ 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.

Innovative stall warning system

The special aerodynamic conditions of a deep 57km tunnel posed complex technical challenges. Since trains using the tunnel will be capable of running at speeds of up to 250km/hr, they will generate pressure-shock in front of them and a vacuum behind them, a scenario that has the potential to cause major problems for a fan-based system.

To find a solution, the R&D department of TLT conducted a special testing program, the outcome of which is that each fan is permanently monitored by an innovative 'stall warning unit'.

Stalling of the fans must be prevented at all costs; equally dangerous is excessive impeller acceleration, the so-called 'windmill effect', because this may result in mechanical damage. The stall warning unit, an innovative development by TLT engineers, measures changes in the air pressure inside the tunnel 10 times every second and uses the fan controller to prevent stalls by hydraulically adjusting the angles of the fan blades as necessary.

Logistical challenges

The logistics of moving very large and heavy components onto and into the mountain, and assembling them in extremely tight spaces, also posed major challenges. Schedule changes also made it necessary to deliver four of the large fans to the Sedrun ventilation station in the High Alps during the depths of winter. The problem for LRT was that the roads in this region are designed for passenger cars, small vans and trucks with weights of no more than 28 tonne. The heavy trucks needed to ship the fans, however, weighed 70 tonne. After carrying out safety checks to ensure the bridges along the route were capable of bearing the extra load, Swiss police escorts were used to help with transportation.

Scheduling challenges

With the tunnel excavation completed ahead of schedule, the opening date was pushed forward a year. As a result, all the fit-out 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, as had been originally planned. The corresponding coordination and schedule management turned out to be a real challenge. Because of the complexity of the project TLT-ABB Schweiz had to coordinate more than 1,000 technical interfaces in order to ensure a smooth execution.

After the components had been successfully installed and tested, commissioning could commence. To start the process, engineers checked and documented the interaction of the system’s components and their safety compliance by conducting extensive tests, including live scenario testing using a rented train capable of operating at speeds of up to 275km/hr. Every eventuality was tested for, including failure scenarios – both automated ones and ones where the operator must step in.

A special challenge facing the consortium was the fact that the fan controls lead the control of all components. Accordingly, each scenario had to be initiated and controlled via the TLT master computer. The Sedrun multifunction station is linked to the ventilation centre via two 800m-deep air shafts, one of which includes a large freight elevator that precludes the operation of any fan when it is in operation. This means that the ventilation 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 operations.

TLT initially became involved in the project in 2007, when a feasibility study was conducted to explore whether it would even be possible to install fans with this level of performance into such small spaces. The pressure and vacuum problem caused by fast-moving trains formed an integral part of early discussions. The bid itself was developed between late 2009 and August 2010, with TLT winning the contract in early 2011. Unlike other tunnel projects, standard-design fans could not be used and engineers had to develop, test, install and commission prototypes.

For TLT, the project will not be over when the tunnel opens to commercial operations in December 92016) – it has already received an order for spare parts and expects to sign a five-year maintenance contract very shortly.