Hose Liner Inserted into Pipe System
Rehabilitation of District Heating Pipes Simplified

District heating networks in Germany have been installed increasingly since the 1950s and are now reaching the end of their planned useful life. At the same time, replacing the pipelines is very costly: Masses of soil have to be excavated and the surfaces subsequently restored. Furthermore, road and delivery traffic is disrupted and residents are inconvenienced by construction noise and dust.

In the "FW-Liner" project funded by the German Federal Ministry for Economic Affairs and Climate Action, experts led by "AGFW - Der Effizienzverband für Wärme, Kälte und KWK" have currently developed a more efficient and environmentally friendly solution. In order to do this, they are using a process that is already being applied in pressureless sewer systems and pressurized potable water networks: resin-impregnated hose liners made of glass fiber-reinforced plastic (GRP) are inserted into the pipe system and harden on site. One manufacturer of these liners, SAERTEX multiCom GmbH, is part of the FW-Liner research consortium.

Special challenges in district heating pipelines

District heating pipes are subject to higher temperatures and different pressure conditions than sewer systems and potable water networks. "We have adapted the hose liners previously used there to the thermal, mechanical and chemical stresses in district heating pipelines," explains project manager Sebastian Grimm from AGFW. "In doing so, we focused on the hot water district heating systems which are the most commonly used ones in Germany."

In particular, changes in temperature load place a strain on district heating pipelines. As more volatile renewable energy is fed into the grids in the future – and sector coupling and the associated changes in the electricity and heating markets become more prominent –experts expect these stresses to increase, for which the older district heating systems, in particular, are not designed.

Of the typical damage to district heating pipes, only those where the internal service pipe itself is affected can be repaired with a liner. These include defective weld seams, which can lead to leaks or corrosion damage. "If a FW liner is used, maintenance measures can also be carried out selectively so that only individual network sections are affected. This reduces costs and means that the supply can be restored more quickly," says Grimm.

Materials suitable for district heating sought

Resin, glass fiber fabric, and protective film: These components of the hose liner, in particular, had to be optimised for the high loads in district heating networks. This means that the overall system must remain sufficiently leakproof and load-bearing at temperatures of up to 130 degrees Celsius. In addition, the experts had to ensure that medium- and long-term material changes would not have a detrimental effect on the heating medium.

"One of the biggest challenges at the beginning of the research project was to find a resin that could withstand the high material stresses in district heating networks," says Grimm. The glass fibers are processed in a "scrim" which is impregnated with the resin. The combination of the glass fiber and resin components forms the glass fiber reinforced plastic (GFRP). The inner film serves as a further sealing element before, during and, if required, after the resin has cured. Only when the resin cures has the flexible tube become a stable tube.