Steady-state creep peak rupture stresses in 90° power plant pipe bends with manufacture induced cross-section dimension variations

Pipe bends represent geometric discontinuities in the steam pipe systems of power plants, therefore understanding the behaviour of these potential locations of weakness is of great industrial importance for component inspection, design and analysis. Due to the high operating temperatures encountered, the failure mechanism of creep is a justified concern. Furthermore, while the geometry of pipe bends appears to be simplistic, the manufacturing process employed results in variations to the critical dimensions of the pipe bends. It is these variations in geometry that can cause potentially significant differences in peak steady-state rupture stress magnitude (approximately 48% in some of the cases considered in the present work). Through analysis of industrial data, several novel non-dimensional parameters have been established, allowing for (with suitable constraint equations depending on the type of bends analysed) the approximation of the complexity of pipe bend geometry in only a few dimension factors. Using these factors, systematic finite element analysis (FEA) studies may be completed with these non-dimensional parameters taking account of a range of geometry variation. Using this philosophy, the stress states and failure lives of pipe bends of the same type (i.e. Hot Reheat or Main Steam) with similar, but not identical, dimensions may be estimated and compared using approximations of the peak rupture stress function. By way of example, this procedure is applied to Main Steam and Hot Reheat type 90° pipe bend geometries. The accuracy of interpolation for the stress function is also analysed, along with comments on failure locations and possible future improvements.

► Pipe bends are critical components in power plant piping systems.
► Manufacturing may induce variations in cross-section dimensions.
►  Analysis of industrial data has highlighted several non-dimensional parameters.
►  These parameters can be used to describe the complex bend geometry.
►  Systematic studies have been completed producing peak stress surfaces.