Failure Analysis of Boiler Tubes Used in a Thermal Power Plant

摘要

Failure of boiler water-wall tubes in a thermal power station is a very common phenomenon. 'High-temperature creep' is one of the main reasons behind these failures. A systematic study of the failures may lead to minimization of the failure occurrences preventing disruption of power generation and associated monetary losses. In this context, three failed boiler tube samples received from a 2x 210 MW coal fired power plant have been characterized in detail and a fresh tube sample was also studied for comparison. One of the tubes had failed due to localized overheating and change in microstructure in this tube from normal ferrite-pearlite to Widmanstatten ferrite could be observed near the failure zone confirming the same. No excessive tube wall thinning could be noticed in this sample. The manifestations of the failed part resembled those of a typical 'thick-lip' rupture tube. The other two tube samples showed typical 'thin-lip' ruptures. There was substantial tube wall thinning in these cases. The microstructures near the failed zone revealed dark globular particles distributed randomly in the decarburized ferrite matrix. Electron probe microanalysis confirmed incidence of elemental carbon in the dark globular particles indicating graphitization of the matrix owing to its long exposure at elevated temperature. Proper cooling of the tubes, periodic removal of inner wall scale and treatment of feed water can minimize these kinds of boiler tube failures.