Evaluation of Near-Drum Thinning Data in Recovery Boiler Generating Bank Tubes, TAPPI JOURNAL July 2016

ABSTRACT: Near-drum thinning affects the fireside surface of recovery boiler generating bank tubes near the surface of the mud drum. Although sophisticated thickness scanning equipment has been developed to rapidly make tens of thousands of thickness measurements in the portion of a tube that is 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, 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 reviews the technology for scanning the thickness of generating bank tubes and discusses approaches that have been used to identify the tubes most weakened by near-drum thinning and to evaluate the fitness-for-service of individual tubes.

Application: The results of this analysis might help readers understand the difficulties of using thickness scanning equipment to identify severe thinning in generating bank tubes in a recovery boiler. They could also form the basis for discussions to develop industry guidelines or a new TAPPI Technical Information Paper (TIP) to provide guidance to boiler operators, testing companies, and insurance companies about how to interpret near-drum thinning data.

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