Circumferential cracking on the waterwalls of supercritical boilers: Volume 1. Final report

The aim of this project was to determine the root cause(s) of the circumferential cracking experienced on the fireside of waterwall tubes of supercritical steam boilers in the US. The approach taken was to correlate the thermal excursions experienced by affected tubes with local and global parameters through in situ measurements in an operating boiler. Instrumented tube panels were used, and the parameters monitored were: tube wall temperatures; water temperature and chemistry; composition of the local flue gas adjacent to the walls, as well as changes in boiler operating parameters such as load and sootblowing frequency. In addition, attempts were made to measure directly the strains experienced by the waterwall tubes. Data were collected over a sufficient period to allow observations of trends with boiler load, and with transient operation associated with start-up and shut-down. Tubes removed from the waterwall prior to installation of the instrumented panels, and the tubes used in the panels were examined metallographically to generate information on the circumferential cracks, and to measure the thicknesses of the internal scales and deposits. A heat transfer model was used to estimate the average tube metal temperatures experienced during service. Also, a finite element stress analysis was made ofmore » the waterwall tube configuration, using measured heat fluxes to calculate the magnitude and location of the stresses developed in the tubes. In this volume, the background to the circumferential cracking problem, the various scenarios postulated to account for the cracking, and the approaches taken for making the in situ measurements are described.« less