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

• The direct burial of district heating pipe bends, i.e., without the use of expansionabsorbing elements, has reportedly frequently been done withoutany observed pipe-line damage. This in spite of current design principles,predicting so large stresses on a directly buried L bend that the techniqueshould be avoided. A reason for the lack of correspondence between practicalexperience and design theory is the difficulties in formulating relevantcriteria for tolerable stresses and deformations in the pipe. Conservative designlimits are traditionally used, particularly regarding allowable stresses inthe PUR foam.An alternative method for design with respect to the PUR foam may bedeveloped based on the idea that the deformation of the PUR foam, and thusalso the displacement of the steel pipe relative to the casing pipe, must belimited only to avoid overheating of the polyethylene. The factors influencingthe deformation of the PUR foam is mainly a question of the temperatureinduced steel pipe displacement versus the stiffness and strength of thebackfill material. When the loading properties of the PUR foam and thebackfill material are known, the relationship between the steel pipe displacementand the PUR foam deformation is defined. Thus also a relation betweena specified thermal motion of the steel pipe and the resulting maximumtemperature on the casing pipe is established. This makes it possible touse the maximum casing pipe temperature as a design value.The conclusion using this approach is that the laying of bends directly inthe ground should be possible with normal backfill materials and compactionconditions. Certain care must be taken, though, when bends are laid invery stiff and strong backfill, where high reaction pressures may develop. Ingeneral, the risk for pipeline damage is minimised if directly buried bendsare laid in only moderately compacted backfill.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Samhällsbyggnadsteknik -- Infrastrukturteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Civil Engineering -- Infrastructure Engineering (hsv//eng)

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