Direct burial of district heating pipe bends – A limited casing pipe temperature ap-proach
Magazine article, 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.