June 2006

Engineering Case Histories

Case 33: Vessels in high-cycle service

Weld failures can be due to manufacturing defects

Sofronas, A., Consulting Engineer

Most vessels in processing plants pressure up during startup and depressurize for planned or unplanned down times. This is hardly a fatigue type of service and most country codes don't require fatigue-type calculations for such a low number of cycles. Several years ago, my colleagues and I wrote a technical paper1 on pressure vessels that had failed because of high-cyclic weld fatigue. The process required these type vessels to pressure up and blow down several times an hour. The failures were due to the poor manufacturing techniques used. Each pressure cycle introduced a bending stress in the weld that added to the operating pressure stress. Consider the three welds shown in Fig. 1. The

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