Common root causes of recent failures of flanges in pressure vessels subjected to dynamic loads

J.L. OTEGUI; P.G. FAZZINI; A. MARQUEZ

Sitges, España

Conferencia; ICEFA 3; 2008

Present rules in fabrication codes are aimed to reduce the probability of fatigue cracks in flanges welded to pressure vessels subjected to cyclic pressure or vibrations. Yet, several leaks and ruptures have recently occurred at flanges in pressure vessels and pipes. A review of three cases is presented, and their common root causes highlighted and discussed. Common root causes for the failures are investigated, and the influences of manufacture and operation conditions on crack initiation and propagation mechanisms are highlighted. Common aspects of these failures are crack initiation in heat affected zone of welds, all related to an inadequate design of the reinforcement and of the welding procedure. Weld inadequacy is sometimes a result of large differences in thickness. Increase in thickness as an attempt to increase safety is found to be detrimental, since larger than expected cyclic stresses are sometimes due to excessive weight of the vessel. Improper foundation due to design errors or maintenance deficiencies leads to higher than expected vibration amplitudes. The use of fillet welded reinforcements induces rigidity inconsistencies within the flange joint, and concentrates deformations in the failed regions. In one case, it is believed that preoperational hydrotest could have been detrimental, by coalescing small weld defect into a small but larger than critical crack. Most cracks initiate from the outer surface, so an adequate non-destructive inspection program during service helps prevent failures. But other cracks initiate in the outer half of the thickness of the reinforcement, from very difficult to avoid weld defects. Changes in the design of the reinforcements is required. Ultrasonic testing, with an adequate procedure, can be reliably used to detect these defects.