Causes for damaging of a gas pipeline pipe, which was accompanied by significant deformations and involves local loss of stability in a short pipe section, are considered. Certain characteristic indices of damage are established, namely damage is located in direct vicinity of the circumferential weld and develops on a lower strength pipe. A set of physico-mechanical studies did not reveal any lowering of metal performance that allows looking for the cause for pipe damage in the features of pipeline stress-strain state in service. It is shown that temperature deformations in the pipeline under the most unfavourable conditions induce insignificant stresses, which cannot lead to any local loss of pipe stability. Analysis of pipeline laying route showed that it passes through mining area and the pipeline stress-strain state is influenced by earth surface deformation. Magnitude of displacements and stress levels in the pipe, caused by earth surface deformation in the mining area, allow regarding them to be the cause for gas pipeline damage. 11 Ref., 2 Tables, 10 Figures.
Analysis of design and service documentation, as well as correspondence of actual loads and service modes of the new ventilation pipe (NVP) to this documentation requirements, has been performed. Results of non-destructive testing of base metal and welded connections of structural elements on the NVP at ChNPP have been analyzed. Cracks in tower nodes of ChNPP NVP are of systematic nature. They formed in the zones of node welded connections of tower three upper tiers, in which the lattice elements are made of pipes of 193.7 × 6 mm cross-section. At present cracks were detected in 17 nodes of slotted gusset of the post and in 17 nodes of brace attachment to straining sill. Templates were cut out, which are crack-containing fragments of gussets, i.e. structural elements of ChNPP NVP, samples were prepared and experimental investigations of the composition, microhardness, mechanical properties and structure of base metal, HAZ and weld metal have been performed. Causes for and mechanisms of cracking in structural elements of the NVP have been determined during performance of materials science investigations in two cutout templates. Fractographic studies revealed the fatigue nature of surface crack initiation and propagation in gussets. Aerodynamic and strength analyses of the construction and its individual elements have convincingly demonstrated that the cause of cracking are resonance vibrations of braces in the upper three sections of the tower, directed out of face plane and inducing alternating bending stresses in the nodes. Vibration frequencies and forces transferred to the nodes have been determined. Stress fields caused by periodic forces in the braces, confirm the existence of the zone of fracture initiation and development, and maximum stress levels greatly exceed welded connection fatigue limits. A conclusion was made that the tower was designed using outdated design concepts for nodes, characterized by numerous zones of high stress concentration, low fatigue and corrosion resistance. 12 Ref., 1 Table, 20 Figures.
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