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

Localized metal loss (LML) in pressurized tubes is a primary contributor to tube failures and unplanned shutdowns and in industrial boilers. In this paper, detailed investigative studies on various metal loss flaws were carried out to acquire insights on their failure behaviour 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. Outcome of the investigations show that for most flaws, the failure evaluation is insensitive to the method of evolving the flaw to failure. Furthermore, that failure pressure depends solely on the flaw thickness, and not on the other flaw dimensions, except for pinhole flaws. The results additionally 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. 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 towards appropriate preventive maintenance of these critical assets while in operation.