Critical Solids Contamination
The mountain water, groundwater and drilling water produced as tunneling progresses is highly contaminated with solids and creates special challenges for the pumps. The proportion of solids in the water can be as high as 10 percent. Making the situation even more difficult, the geology of the region is characterized by granite rock, which results in highly abrasive particles. Sprayed concrete residuals are also part of the mix. It is essential that this sludge mixture is not only continuously removed but also processed so that the remaining wastewater can be discharged into the city‘s sewer network.
Multiple Pumping Stations
Immersible motor pumps working directly at the tunnel face collect the sludge mixture, which is loaded with drilling chips and sprayed concrete, and move it to collection troughs at the nearest pumping station. Each of these pumping stations has one or two high-performance Sulzer EMW sludge pumps that are the core components of the drainage system. Each pump is powered by a 22-kW drive and has a flow rate of 150 m³/h with a pump head of 25 m. The EMW series pumps are capable of maximum performance of 4000 m³/h and 95 m.
Pumping stations are placed every 500 m in the tunnel. As the tunnel grows longer, the number of stations increases. Wastewater is pumped through 6-inch pipes from station to station until it reaches the tunnel portal where it is treated with neutralizer. 13 pumping stations will be put into operation one by one as the tunnel advances.
The pumps are regulated with frequency transformers and are monitored remotely. Each pump is complemented by an XRW-series mixer with a double mechanical seal. Stations integrated into the cascade drainage system usually have two pumps and two mixers. Due to the size of the troughs at the double pumping stations, both mixers remain in continuous operation. The second pump ensures that operations continue even in the event of disturbance. The collection troughs of less critical stations are smaller and are equipped with just one pump and one mixer.
The mixers keep the sludge in a pumpable condition by preventing solids from settling on the bottom of the collection trough. If solids were allowed to settle on the bottom, it would be necessary to completely empty the trough with a vacuum truck every 48 hours. But by using very durable and highly efficient pumps in combination with mixers, the construction site‘s operating and energy costs can be kept to a minimum. The savings on energy costs and the vacuum truck alone exceed the total procurement costs of the pumping systems.
Durable Pumps with up to 80 % Efficiency
The design of the EMW pumps gives the operator additional benefits. Many years of practical experience, together with advanced liquid-flow modeling techniques and Finite Element Analysis have resulted in optimized hydraulics. Their efficiency is as high as 80 percent, which is significantly better than comparable pump units. Pump design and careful selection of materials reduce wear considerably, especially with problematic solids contamination. High-performance bearings can stay in service for more than 50 000 hours. The pumps’ Mean Time Between Repair (MTBR) is as long as one year. This keeps lifecycle costs very low. “From the very beginning, Sulzer won us over with very durable pumps. They have really met our expectations since commissioning of the first pump station,” says Bessi Sveinsson, Marti Tunnelbau AG‘s responsible for all activities in Norway.
Sulzer promises its customers intensive, comprehensive and continuous support at the construction site. On the Bergen project, this began during organization of the construction site and individualized planning of the system and pump configuration. This is complemented by ongoing support regarding supply of spare parts, training, and commissioning of new pump stations. Sulzer provides these services not just for individual construction sites. Experts assigned to the “Site drainage center” in Isernhagen, Germany, are active at locations throughout Europe.
Service Water Supplies
Sulzer expects to start supplying the site with service water depending on progress of the tunnel. This water will come from the city‘s existing network. Booster sets of model VMS125/3 are used to compensate for pressure losses in the lines, which will be up to 3.8 km long. Thus, the system can reliably supply water needed for drilling rigs, concrete mixing stations, and potential firefighting operations.