Evaluation on Thermal Spraying Applications of Boiler Tubes
During the annual inspection in April 2007, one coal-fired power plant in Taiwan discovered that the thickness of the broiler’s tube wall was under the borderline. Because it was not the time yet for an overall inspection, this problem was handled by welding. Later on in July 2008, this welded waterwall tube ruptured. The section of the broken tube was examined by observing the cross-section of the broken tube, testing the rigidity, and performing a microscopic metallographic analysis. The results suggested that this matter may be stress and corrosion induced rupture, and the stress here may be the residual stress from wielding. It was suggested that some cracks were formed at the internal wall of the starting or stopping end of welding. Once these cracks received water pressure inside the tube, the residual stress from welding and corrosion factors acted together and grew bigger. At the end, cracks were formed throughout the tube wall, causing the high pressure water inside the tube to spout and create more rupture. The objective of this study is to clarify causes for tube rupture using relevant tests and analyses. High velocity oxy fuel will be used to coat the surface of the wall of broiler tubes with WC/Co. This treatment will be followed by onsite testing. Three years later, the thickness of the sprayed surface will be compared for assessing the feasibility of giving broiler tubes comprehensive welding in the future.