Ratcheting Assessment of a Fixed Tube Sheet Heat Exchanger Subject to In Phase Pressure and Temperature Cycles

Abstract

An investigation of the cyclic elastic-plastic response of an Olefin plant heat exchanger subject to cyclic thermal and pressure loading is presented. Design by analysis procedures for assessment of shakedown and ratcheting are considered, based on elastic and inelastic analysis methods. The heat exchanger tube sheet thickness is nonstandard as it is considerably less than that required by conventional design by formula rules. Ratcheting assessment performed using elastic stress analysis and stress linearization indicates that shakedown occurs under the specified loading when the nonlinear component of the through thickness stress is categorized as peak stress. In practice, the presence of the peak stress will cause local reverse plasticity or plastic shakedown in the component. In nonlinear analysis with an elastic–perfectly plastic material model the vessel exhibits incremental plastic strain accumulation for 10 full load cycles, with no indication that the configuration will adapt to steady state elastic or plastic action, i.e., elastic shakedown or plastic shakedown. However, the strain increments are small and would not lead to the development of a global plastic collapse or gross plastic deformation during the specified life of the vessel. Cyclic analysis based on a strain hardening material model indicates that the vessel will adapt to plastic shakedown after 6 load cycles. This indicates that the stress categorization and linearization assumptions made in the elastic analysis are valid for this configuration.