P04.01: Failures of stiffened ducts stressed by temperature

Abstract

ABSTRACTSteel pipelines are an important part of industrial technology equipment in many sectors. Ducts with a ratio of pipe diameter to wall thickness of up to 150 are generally considered as simple beams, ducts with a higher ratio have to be considered as shell elements. Technical literature provides a number of methods for designing such structures stressed by various kinds of loads. Failures that have occurred in recent past suggest that the existing common design method for industrial piping shows some insufficiencies. This paper is focused on the study of stiffened steel shells stressed by temperature changes. Due to a high temperature load, there is a big temperature deformation of the whole system. This deformation caused by temperature differs within particular parts of the system. As the high temperature source is inside the pipe, the highest temperature is on the inner surface of the shell. The variation of the temperature in the shell and the partial warming of the ring stiffener is caused by heat conduction. Due to the different temperatures of the surrounding environment the stiffener is always colder compared to the shell. The research is focused on the shell behaviour close to the ring stiffener, which represents the area most tending to cracks initiation and their expansion due to the low‐cycle fatigue phenomenon. The research procedure consists of several sub‐parts. In the first part, the authors focus on determining the thermal load based on the theory of heat transfer. The resulting heat transfer coefficients were verified by measurements performed on the real structure. These findings served as boundary conditions for numerical modelling in Ansys. The resulting values of strain and stress were one of the inputs for the subsequent analysis of lifetime prediction from the viewpoint of low cycle fatigue. The number of cycles to failure (creation of macro‐cracks) for various types of structural solutions has been determined in the final part of research.