ENGINEERING THERMOPLASTICS | MATERIALS
A joint study has identified the feasibility of using coiled PVDF for slipline rehabilitation of district network heating pipes
Putting PVDF to work in district heating pipes
In 2016, there were 4,500 district heating networks present in a total of 32 countries. In France alone, these networks represent around 5,000km of piping. For the most part, district heating networks comprise pairs of parallel steel pipes: one is for the distribution of hot water, overheated water or steam; the other, which is generally smaller in diameter, is for the return of condensates. The heat-transporting pipe is insulated, usually with glass wool, in order to limit energy loss, while the condensate return pipe is coated with a protective substance (such as polypropylene) to prevent corrosion. The overall system is installed within a concrete duct which is usually buried 1-1.5m deep. Given the operating conditions of these net-
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works, corrosion problems are unavoidable – and leaks are inevitable. In the urban environment, repairing leaks and renewing these networks is costly and disruptive. To limit recurrent leaks, particularly in the pipes carrying condensate, it is worth considering non-metallic pipes that can withstand the required temperature and pressure – and are not prone to corrosion.
Technical requirements The search for polymers capable of meeting the requirements of district heating networks has focused on those whose thermal and mechanical performances surpass those of conventional polyolefins. The need to resist permanent high temperatures and pressure for long durations requires:
Main image: PVDF copoly- mer grades had the correct balance of stiffness and coilability to be used for pipes in district heating systems
� January/February 2019 | PIPE & PROFILE EXTRUSION 13
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