Durability of District Heating Pipes

Doktorsavhandling, 2002

This thesis is focused on mechanical loads and moisture related problems with a potential to critically harm the functionality of pre-insulated district heating pipes and pipe joints. Directly buried pipeline bends are investigated with respect to limiting deformations from a thermally induced lateral displacement. A new approach is introduced; stating that very large deformations of the insulation foam are tolerable as long as overheating of the casing pipe is avoided. A local increase in heat losses and loss of axial shear stiffness is not critical, but a too high temperature will accelerate the rate of deterioration of the polyethylene material. Measurements indicate that the earth pressures required to reach critical deformations are not likely to develop in regularly compacted backfill materials. Problems related to backfilling with reused excavated soil containing coarse-grained materials are of particular interest. The lifetime implications from large stones causing deep indentations in the casing pipe wall are studied. A conceptual framework for the force interaction between a laterally displaced pipe and a stone adjacent to it is presented, along with tests on the long-term strength of severely strained polyethylene. The risk for obtaining deep indentations depends on the compaction of the backfill, and is greatest in materials compacted to "ordinary" levels. There is an obvious temperature dependence in the expected lifetime. To avoid the combination of indentations and an elevated casing pipe temperature, compaction of backfill material containing large stones should be avoided around pipeline bends. The strength of pipe joints to axial movements through coarse-grained backfill material is studied experimentally. It is concluded that electro-fusion welded sleeves are superior to other types. But with a simple external protection, consisting of a polyethylene net, the much cheaper single-sealed non-shrinking sleeve can be used in very tough backfill materials. The consequences of a leaking joint seal are investigated experimentally and analytically. Homogeneous and defect-free PUR foam will act as a moisture barrier, protecting the medium pipe from water. Air-gaps or voids in the foam may promote an inflow of groundwater by a "breathing effect" caused by temperature variations. Hence in order to improve the performance of the joints, efforts should be made to develop the methods for foam injection to ensure a complete filling of the joint space. Vapour diffusion in plastics pipes heating systems and in cooling systems is studied analytically. In heating systems with polyethylene medium pipes, vapour will diffuse outward and possibly condensate and accumulate in the colder parts of the insulation foam. Calculations indicate that the increase in heat losses during the first 50 years of service is about 25 - 30 %. In cooling pipes, vapour passing in from saturated soil will condense in the colder insulation. Simplified calculations show that condensation of water starts almost immediately.