To ensure safe and reliable operation, steam turbine casings must have acceptable stresses and maintain sealing when subjected to internal pressures and temperatures.&nbsp; To show turbine casings acceptable, analysts conduct structural evaluations using finite element analysis (FEA) techniques.&nbsp; This paper outlines the analytical methods used to perform these types of analyses, provides analysis examples, and outlines the process to create pressure and temperature limit maps.&nbsp;&nbsp;The type of steam turbine evaluated has a main casing with a welded-on steam chest and two crossover pipes from the steam chest to feed steam to the bottom half of the main casing.&nbsp; The set of models evaluated included an overall model of the entire system, separate models of the main casing and steam chest, and a detailed model of the crossover pipe flange.&nbsp;&nbsp;The design conditions considered are normal operation, hydro test, heat-up, cool-down, and the cold-pipe condition.&nbsp; The cold-pipe condition arises when the valve feeding steam to the crossover pipe is closed during operation.&nbsp;&nbsp;The overall model is used to determine the forces and moments acting on the pipes for the various design conditions.&nbsp; These forces and moments are then used to assess the acceptability of the pipe for allowable stress and for the loadings to be applied to the detailed flange model to determine if pipe flange leakage will occur.&nbsp;&nbsp;The main casing and steam chest models are used to determine stresses and sealing of the casing horizontal split line and the steam chest cover.&nbsp; The sealing evaluations consider the sealing capabilities of the bolted joints when the casing is subjected to internal steam pressure and consider the effects of bolt stress relaxation at elevated temperatures, joint contact surface separation, and penetration of the internal pressure into the sealing surface.&nbsp; The acceptance criteria for the bolted joint sealing is based on the minimum width of the contacting surface and the minimum joint contact pressure.&nbsp;&nbsp;A series of analyses were conducted on the various models to create pressure and temperature limit maps, so that the design can be applied for the appropriate conditions.&nbsp; These maps plot maximum allowable working pressure (MAWP) versus maximum allowable working temperature (MAWT), and allow an application engineer to easily determine the acceptability of the casing for a particular application.&nbsp; An explanation of the process used to create the limit maps is presented.

Steam Turbine Casing Analyses to Determine Temperature and Pressure Limits

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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