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- Bergström, Gunnar, et al.
(författare)
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Direct burial of district heating pipe bends – A limited casing pipe temperature ap-proach
- 2003
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Ingår i: Euroheat & Power - English Edition.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Bergström, Gunnar, et al.
(författare)
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Durability testing for 100 year lifetime for buried non-pressure plastic pipes
- 2006
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Ingår i: Plastics Pipes XIII.
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Konferensbidrag (refereegranskat)abstract
- For plastics pipes used for underground drainage and sewerage there is at present no internationally accepted method for the evaluation of durability. This makes it difficult to make an objective assessment for both new materials and new pipe designs entering the market. This paper presents a possible future structure of such a durability evaluation for nonpressure pipes. Based on an experimental study of pipes made from a filled and an unfilled PP material and one made from an HDPE material different damage mechanisms and changes in pipe characteristics were observed when the pipes were exposed to long-term deflection and long term (one-year) ageing at +95 °C. A CAED methodology is also described which was used to investigate the distribution and time dependence of pipe stresses.
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| 10. |
- Bergström, Gunnar, et al.
(författare)
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The influence from pipe surface, weld beads and protective skins on long term failure times for PE butt fusion joints
- 2004
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Ingår i: Plastics Pipes XII.
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Konferensbidrag (refereegranskat)abstract
- When making creep tensile tests on butt fusion joints in PE pipes almost all failures emerge from the outside of the pipe. Results indicate an influence from the outside surface on the long term behaviour of butt fusion joints. It is a well established practice to scrape the outside surface of PE pipes when making electro-fusion joints. This study stress the question if scraping or bead removal is a positive thing also when making butt fusion joints. Using PE100 pipes butt fusion joints were made using different levels of surface scraping before the butt fusion and with and without removing the weld beads before testing. PE-pipes with an outside PP or PE-skin were also included with and without the skin. Tensile test pieces were cut parallel to the pipe axis centered over the butt fusion joint. The long term strength evaluation were made using a standard accelerated test procedure with the test pieces exposed to a 5 MPa tensile loading immersed in 80 °C water with 2 % Arkopal N100 surfactant. With outside weld beads left intact failure times 200 h to 400 h are normal. Leaving the inner beads but removing the outer beads increase failure times to 1000 h to 2000 h. Failure times for test pieces without weld beads approach 10000 h. Scraping of the pipe surface to a depth of 0.2 mm did not substantially change the failure-time above that for an unscraped surface. Removing 0.7 mm from the pipe surfaceincrease failure times to 2000 h to 3000 h. Outside weld beads and outside pipe surface clearly interact to lower the time to failure in accelerated creep tensile tests. Weld bead removal and scraping of outside pipe surface significantly increase failure times. The exact mechanism is not clear and the implication on butt fusion joint performance in service is not demonstrated but the results indicate possible ways to enhance safety margins when making butt fusion joints. The topic will be given further attention in coming studies.
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