Evaluation of Near-Drum Thinning Data in Recovery Boiler Generating Bank Tubes, 2015 PEERS Conference

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Near-drum thinning affects recovery boiler generating bank tubes near the surface of the mud drum. Although sophisticated thickness scanning equipment has been developed to make tens of thousands of thickness measurements in the portion of a tube vulnerable to near-drum thinning, methods for using these data to evaluate fitness-for-service have not shown similar advances. Non-destructive testing companies typically use a technician's subjective judgment, to identify the "thinnest reliable" thickness measurement on each tube. Some mills decide whether tubes can continue in operation or should be plugged or replaced based on this single thickness measurement. However, rigorous Finite Element Analysis of the remaining strength of individual tubes thinned in the near-drum area suggests that it is essentially impossible to identify the weakest tubes from simple empirical rules. In the absence of an industry standard for evaluating these data, different mills could reach different conclusions about the fitness-for-service of a tube from the same data set. This paper will review the technology for scanning the thickness of generating bank tubes and discuss approaches that have been used to identify the tubes most weakened by near-drum thinning and evaluate the fitness-for service of individual tubes. We hope that this may form the basis for discussions to develop industry guidelines or a new Technical Information Paper to provide guidance to boiler operators, testing companies and insurance companies about how to interpret near-drum thinning data.  

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