Investigative studies on various metal loss flaws and their progression from a deep pinhole size to a lengthy size in pressurized tubes

Abstract

Localized metal loss (LML) in pressurized tubes is a primary contributor to tube failures and unplanned shutdowns in industrial boilers. Initially, these tubes may have inconspicuous anomalies introduced during manufacturing, testing, and inspection. Over time, as the vessels are deployed and exposed to harsh environmental, operating, or aging conditions, these anomalies evolve, eventually leading to their failure. In this paper, detailed investigative studies on various metal loss flaws were carried out to acquire insights on their failure behavior and mechanism, especially as they progress from very small size to lengthy size. Three investigations were carried out to identify which geometric parameters are most critical for predicting failure of the tubes and to what extent will they influence their failure. First, a range of real flaws that failed while operating under high internal pressure in a steam boiler were considered. The study compared two different modeling methods for evolving flaws to their failure geometry. The outcome shows that for most flaws, the failure evaluation is insensitive to the method of evolving the flaw to failure. Second, all the flaw geometries were modeled using the same material and operating conditions and their failure behavior studied. The findings show that failure pressure depends solely on the flaw thickness, and not on the other flaw dimensions, except for pinhole flaws. Finally, a range of flaw geometries, spanning the space from localized pinhole flaws to wide lengthy flaws, were investigated. Outcomes of these studies show that failure pressure for small flaws is sensitive to the flaw geometry. However, the failure pressure ceases to depend on the flaw length once the flaw length is greater than twice the tube diameter, and ceases to depend on the flaw width when the flaw length is greater than the tube diameter. These results suggest that gross plastic collapse is not the dominant failure mechanism for small flaws. However, for flaws with a length at least twice the tube diameter, the flaw dimension which predicts failure is the tube remaining thickness in the deepest part of the flaw. These studies establish the applicability limits of gross plastic collapse analysis for pressurized tubes experiencing metal loss and demonstrate opportunities for simplifying flaw analysis for sufficiently large flaws. Ultimately, the insights from this study will contribute toward appropriate preventive maintenance of these critical assets while in operation.