Failure Analysis of Steam Sparger Tubes

摘要

A number of steam sparger tubes made of 316L austenitic stainless steel showed premature failure after a short period of service in a polyethylene plant. The tubes were mainly used during emergencies in order to keep the flowing water within the piping system at a certain operating temperature so it could be used in the polyethylene reactor. The last premature accident occurred after only two months of continuous operation. The failed tubes contained numerous cracks that initiated from manually drilled holes and propagated until a critical size was reached and complete "fish mouth shape" rupture of the tube occurred in a ductile manner. Microscopic investigation using low- and high-magnification scanning electron microscopy (SEM) showed that the cracks propagated by fatigue after initiating from mechanically damaged holes with poor surface finish and high residual stress. Fatigue cracks are propagated by the alternating loading that occurs within the sparger casing during operation, particularly during startup and shutdown periods. Linear elastic and plastic fracture mechanics approaches (LEFM and LPFM) were used to consider two different scenarios: the monotonic or critical load that can lead to failure and the cyclic load leading to fatigue crack rate propagation.