Diagnosing Boiler Tube Failures Related to Overheating

Abstract

Unexpected tube failure is the major factor causing unreliability in utility boilers. The first step in analyzing tube failures is to identify the active damage mechanisms. Three tube damage mechanisms related to overheating are presented and possible root causes are discussed to resolve these tube failures. Damage mechanisms can be recognized by metallurgical evaluation, comprising a combination of both visual and microexamination, complemented by chemical analyses of tube or fireside deposits, as appropriate. Characterizing the degree of microstructural degradation can also help to confirm and separate various potential high temperature tube damage modes, such as long or short term overheating, as well fuel ash attack. Carbon steel is the standard tube material for high pressure boilers (typically up to 625 psig steam) and has a normal design temperature limit of about 440 °C. However, microstructural changes occurring as a result of higher temperature exposures in service can include carbide spheroidization, graphitization and other transformations. Metallography is powerful tool for evaluating overheated failures and also for fire damage assessment. Evaluating overheated steel microstructures utilizes the principles of steel heat treatment and application of the iron-iron carbide equilibrium diagram.