Direct burial of district heating pipe bends – A limited casing pipe temperature ap-proach

Artikel i övrig tidskrift, 2003

The direct burial of district heating pipe bends, i.e., without the use of expansion absorbing elements, has reportedly frequently been done without any observed pipe-line damage. This in spite of current design principles, predicting so large stresses on a directly buried L bend that the technique should be avoided. A reason for the lack of correspondence between practical experience and design theory is the difficulties in formulating relevant criteria for tolerable stresses and deformations in the pipe. Conservative design limits are traditionally used, particularly regarding allowable stresses in the PUR foam. An alternative method for design with respect to the PUR foam may be developed based on the idea that the deformation of the PUR foam, and thus also the displacement of the steel pipe relative to the casing pipe, must be limited only to avoid overheating of the polyethylene. The factors influencing the deformation of the PUR foam is mainly a question of the temperature induced steel pipe displacement versus the stiffness and strength of the backfill material. When the loading properties of the PUR foam and the backfill material are known, the relationship between the steel pipe displacement and the PUR foam deformation is defined. Thus also a relation between a specified thermal motion of the steel pipe and the resulting maximum temperature on the casing pipe is established. This makes it possible to use the maximum casing pipe temperature as a design value. The conclusion using this approach is that the laying of bends directly in the ground should be possible with normal backfill materials and compaction conditions. Certain care must be taken, though, when bends are laid in very stiff and strong backfill, where high reaction pressures may develop. In general, the risk for pipeline damage is minimised if directly buried bends are laid in only moderately compacted backfill.