About 45 kilometers of Helsinki's district heating network have been built in tunnels under the city, says Timo Nevalainen, Helsinki Energy's project manager. Construction work on district heating trunk lines would cause considerable inconvenience on the streets of the city center. In addition, tunnels allow energy production to be optimized in a network of large transmission lines. In addition to district heating, a 110 kV electric cable, a Helsinki Water cold water trunk pipe and a district cold trunk line will be installed in the tunnels.

The fifth and final pipeline contract for the three-kilometer Kamppi-Erottaja-Kruununhaka tunnel, which is being contracted by Destia, is now under construction. In the autumn, the Plumbing work on the six-kilometer-long Meilahti-Pasila-Käpylä tunnel will begin. Destia's contract also includes plumbing in the downtown service tunnel and the Viikinmäki tunnel.

Assurance for users

By connecting all our plants to the same network, we can increase the security of district heating and district cooling distribution and optimize the operation of different production plants. The work has more precise quality and safety requirements than usual, mainly due to the important large trunk lines running in the tunnels, Nevalainen says.

The elements are installed in the tunnel on brackets at 18 meter intervals. This brings its own additional requirements to the work and its design. The thermal movements of free-moving elements are different from those of friction-fixed elements in the trench. For this reason, network stress tests are performed at the site. - It is important that the stresses do not exceed the permissible design values ​​of the materials. The design pressure used in the tunnel is 25 bar, while the reading on the ground is 16 bar. An ST 52.0 flow pipe is installed in the tunnel, which is about 50 percent stronger than the pipe material of the surface network.

An electrical sleeve was required for the insulation work
As such, there is not much difference between installing in a trench or a tunnel. However, due to the importance of the network, among other things, welds and branches are inspected even more carefully in tunnels than in the surface network.

The work has certain quality requirements. For example, the insulation work of the element joints has the required electrical sleeve. The contractor has been able to choose how he does the work, Nevalainen states. Destia's contract includes the transport of district heating and cooling pipes from the warehouse to the tunnel, as well as their installation and insulation. The transport of 16-meter pipes in the center of Helsinki and into the tunnel has brought challenges to the work.

- We chose a TSC welding sleeve for insulation work. The split sleeve can be installed in a finished line. We rented the equipment from a supplier and received guidance on its use, says Erkki Virtanen, Destia's work manager.

The TSC method is suitable for special conditions

-The system is well suited for tunnel joints. It is a good solution, especially in special conditions, for example when the pipe is exposed to seawater, Virtanen thanks. The sequel has traditionally been the weakest point of the district heating line. When using a TSC welding sleeve, the joint of the steel flow pipe is protected by a similar insulation and surface material as the pipe element itself.

The split polyethylene sleeve is as well suited for a trench as a pipe bridge. The sleeve weighs 2 to 13 pounds and requires approximately the same amount of space for welding as for welding a flow pipe by hand. The size of the welding machine is only 35 x 35 x 20 centimeters.

- The installation of the sleeve therefore does not require large machines on the construction site, sums up project manager Jan-Erik Svarvén. Once the flow pipes have been welded and the leak detection wire connections have been made, the TSC sleeve is tightened in place at the pipe joints. A stainless steel mesh is placed between the sleeve and the shell. Power is supplied from the tool to the steel mesh. When the required connection temperature is reached, the welding is OK. The longitudinal seam is finally passed through a hand extruder. The joint is usually 300 to 340 mils, the welding head 150 to 170 mils and the sleeve 500 mils long.

- After installation, the joint is test-tensioned from the drilled hole and foamed with the same insulation as the element itself. Finally, the borehole is welded with a 25 mm polyethylene rod. The end result is a line where the insulation and surface material are uniform. One of the advantages of the method is that it also allows angular deviations, Svarvén states.

Six different sizes of TSC couplings have been delivered to the connections of the service tunnels of Helsingin Energia's Johanneksenpuisto, Viikinmäki and the city center.