Failure Analysis, NDE Limitations & Remedial Measures of Tube Failures in a DRU Kettle Boiler (Type BKU Exchanger)

Abstract This paper presents a comprehensive failure analysis of a premature tube bundle failure in a kettle boiler (Steam Generator) operating at a Diluent Recovery Unit (DRU). Originally designed as a reboiler in 2007 and only commissioned in 2022, the exchanger faced multiple tube leak events within its first two years of operation. The equipment was designed to process hot oil on the tube side and boiler feed water on the shell side to generate steam. Unexpectedly, multiple tube leaks occurred within six months of commissioning. Initial inspection involved a pressure test, after which the failed tubes were plugged. Non-destructive examinations using Remote Field Testing (RFT) indicated no significant anomalies in the remaining tubes, and the bundle was returned to service. However, within the following year, additional leaks developed, ultimately leading to complete replacement of the tube bundle. A detailed metallurgical failure analysis was subsequently conducted on the removed tubes. The investigation revealed that the primary failure mechanisms included localized cavitation, erosion of protective oxide layers due to mechanical wear at the baffle plate locations, and possible galvanic corrosion. The combination of these degradation modes was not anticipated during design and inspection phases, making this a significant case study in understanding complex in-service degradation under multiphase flow and thermal cycling conditions. The findings of this study provide critical insights into the limitations of conventional inspection techniques like RFET in detecting corrosion at baffle plate locations. This work significantly contributes to improving boiler reliability, offering practical recommendations for future design, inspection, and maintenance strategies in similar process environments.