CREEP-RUPTURE ASSESSMENT OF SUPERHEATER TUBES USING NONDESTRUCTIVE OXIDE THICKNESS MEASUREMENTS

摘要

Steam-carrying superheater and reheater tubes in fossil-fired boilers operating at temperatures above 900F (482C) are subject to failure by creep-rupture. These tubes form an internal oxide layer that inhibits heat transfer through the wall and causes the tube metal temperature to increase over time. The thickness of this oxide can be used with unit operating data and wall thickness measurements to estimate the remaining creep-rupture life of a tube. In 1986 The Babcock & Wilcox Company designed and built the portable, ultrasonic Nondestructive Oxide Thickness Inspection System (NOTIS~R) for measuring this oxide. Prior to the development of this system, the only method to measure this internal oxide was through the destructive removal of tube samples. The application of NOTIS~R makes it possible to nondestructively assess a large number of tubes in a relatively short time. By assessing a large number of tubes within a superheater, rather than assessing only a few tube samples, better decisions can be made regarding component replacements and the identification of unusual unit operating conditions.