Lifetime of HDPE under stress relaxation at large strains

Slow Crack Growth (SCG) in HDPE under stress relaxation was studied by measuring the time to fracture of notched pipe samples subjected to constant hoop strains. Tests were made in a 2 % Arkopal N-100 tenside solution at various temperatures. The constant deformation was accomplished by pressing the pipe samples onto oversized steel cylinders with diameters giving strain levels ranging from 5 to 50 %. In addition, efforts were made to measure the stress relaxation by applying strain gauges to the steel cylinder wall. Two grades of HDPE were studied—one PE63 grade and one PE100 grade. The two materials were found to behave very differently. Somewhat surprisingly, the PE100 grade generally exhibited shorter fracture times. Furthermore, the time to fracture showed significant temperature dependence but was more or less independent of strain level above the yield point. For the PE63 material, on the other hand, the temperature effect was much less prominent, but a slight strain level dependence was seen. It is likely that the reason for this is differences in the stress relaxation behaviour. As the stress level decreases over time, so does the crack driving potential, and an elevated temperature may not necessarily lead to an overall acceleration of the crack propagation. A frequently used empirical model for the calculation of fracture times was expanded to account for a time dependent stress level. The results indicate that SCG under stress relaxation may be analytically treated in analogy with the more common creep case

Durability of District Heating Pipes

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

Durability of District Heating Pipes

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

Skador på skyddsmantlade plaströr vid grävningsfri förläggning

Damage to coated plastics pipes from trenchless laying techniquesThis project aims at evaluating an external coating on plastics pipes, and its protective function against scratches and scoring damage from trenchless laying techniques.PE100 pressure pipes with four different coatings were tested in horizontal drilling and pipe bursting field-trials. During the tests, the lengthwise and cross-sectional deformations of the pipe were measured along with the traction force. The pipes’ surfaces wereexamined with respect to scratches, and the depth of the most prominent ones were determined.Of the two laying techniques, pipe bursting was the one causing the deepest scratches.Horizontal drilling was not significantly worse than ordinary handling during transport and preparation with respect to scratch depth. Dragging of the pipe above ground on asphalt surfaces caused severe damage to the pipe wall.The deepest scratches were seen on two pipe alternatives with soft coatings. Due to thic kness, however, these coatings were not penetrated and thus successfully protected the main pipe wall. The hardest coating tested, a polypropylene alternative, were also thethinnest one, and was penetrated at one location during the pipe bursting test.The traction force on the pipe was significantly higher during horizontal drilling than during pipe bursting. On the other hand, ovalisation of the pipe’s cross-section was greater during pipe bursting.Keywords: Horizontal drilling, pipe bursting, polyethylene, coated pipes, scratche

Damage to coated plastics pipes from trenchless laying techniques

This project aims at evaluating an external coating on plastics pipes; and its protective function against scratches and scoring damage from trenchless laying techniques.PE100 pressure pipes with four different coatings were tested in horizontal drilling and pipe bursting field trials. During the tests, the lengthwise and cross-sectional deformations of the pipe were measured along with the traction force. The pipes’ surfaces were examined with respect to scratches, and the depth of the most prominent ones weredetermined. Of the two laying techniques, pipe bursting was the one that caused the deepest scratches.Horizontal drilling was not significantly worse than ordinary handling during transport and preparation with respect to scratch depth. Dragging of the pipe above ground on asphalt surfaces caused severe damage to the pipe wall.The deepest scratches were seen on two pipe selections with soft coatings. Due to their thickness, however, these coatings were not penetrated and thus successfully protected the main pipe wall. The hardest coating tested, made of polypropylene, was also thethinnest one, and it was penetrated at one location during the pipe bursting test. The traction force on the pipe was significantly higher during horizontal drilling than during pipe bursting. On the other hand, ovalisation of the pipe’s cross-section was greater during pipe bursting