The aim of this paper is to demonstrate how the correct application of FEM analysis can be used to find effective solutions for the design of mechanical structures. The design of the inspection openings on the tanks is being considered. There are several existing tanks of the same dimensions (20 m high and diameter 10,2 m), but they have different wall thicknesses (9,6; 15; 20 and 25 mm). For inspection purposes, assembly of manhole hatch on all tanks is required. The manhole hatch is designed applying standard API 650. All tanks are filled with the water to the top 20 m high. Several different analyses have been carried out in order to ensure that there are not too high stresses in the materials of the existing tanks due to the insertion of the manhole hatch and finally qualify construction according to EN-13445 norm. The elastic analysis shows that stresses in the material are too high and the design hasn’t been approved. In order to avoid redesign procedures, which can be expensive and sometimes difficult to do in reality, plastic analysis has been done. After plastic analysis, the design could be approved with the restriction on the max. preload force in the bolts 40 kN/per bolt.
Abstract:Micro cracks have been found near some of the welds on the pipe unit. This paper investigates the root cause of this damage and suggests improvements to prevent such damage from occurring. The investigation includes several simulations using Pipestress and Ansys Mechanical, a welding and materials investigation, and a more theoretical creep investigation using the Larson-Miller parameter. A sensitivity study of the outlet pigtail installation and support configuration is performed as well as a cyclic plastic analysis. A complementary analysis of the catalyst tubes, inlet and outlet system shows that the only areas where the stress and strain indicate an elevated risk of damage are the areas around the ends of the outlet pigtails. Suggested improvements include reconfiguration and load reduction of constant hanger supports, improving welding procedures in order to lower residual stresses and undertake more material and on site investigations to verify causes of failure.
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