A shell and tube heat exchanger is utilized to condense propylene gas. The temperature of the propylene gas entering the shell side cycles through a range of approximately 200ºC multiple times a day. This temperature cycling induces thermal (secondary) stresses within the condenser, which are exacerbated by the rapid increase in the temperature of the incoming gas. Additionally, the inlet and outlet nozzles experience thermal loading from the connected piping. This study aims to analyze the critical components of the condenser from a fatigue perspective.
Analysis
The critical regions of the condenser were evaluated using Finite Element models developed with the commercially available software packages FE/Pipe and FEMAP. These models employed 3-D brick elements to capture the temperature gradients through the wall. A transient simulation was conducted to determine the bending stresses induced by the varying rates at which different parts of the condenser warm up. The secondary stresses calculated were assessed for fatigue in accordance with the EN13445 pressure vessel design code.
Results
The analysis identified the nozzle through which the propylene enters the shell side of the condenser as a critical area. Significant bending stresses were observed here due to the differing rates at which the nozzle and the condenser shell warm up or cool down. To comply with the design code, DRG recommended rounding the intersection and improving the quality of the weld.
The tubesheet of the heat exchanger also experiences the cycling propylene temperature. However, the results indicated that its high thermal inertia allows it to warm up or cool down relatively uniformly, preventing high secondary bending stresses. Consequently, no modifications were necessary for the tubesheet to ensure compliance with the design code.
“A transient Finite Element simulation allowed DRG to identify the large bending stresses arising due to the cycling of the inlet temperature.”
