High temperature boiler tube failures: case studies

Abstract

Numerous investigation on high temperature failures of boiler tubes received from different thermal power plants had been carried out This paper presents three case studies of such failures. These include failures of (i) reheater tube of 1.25Cr-0.5Mo steel, (ii) carbon steel tube and (iii) final superheater tubes of 2.25Cr-IMo steel. The reheater tube had extensive damage in the form of pitting on outersurface. X-ray microanalysis using SEM revealed the presence of corrosive elements viz. K, Ca etc. on the pitted surface. Sticking of fly ash particles containing such elements causing reduction in effective tube wall thickness is responsible for the failure of reheater tube. The carbon steel tube showed a brittle window fracture. Decarburized metal containing intergranular cracks at the inner surface is indicative of hydrogen embrittlement responsible for this failure. In contrast, an extensive study on final super heater tube conclusively proves that the failure took place due to short term overheating. The extent of overheating had been estimated from the kinetic data on oxide scale growth and was found to be about 830'C. Surprisingly the tube had experienced a circumferential expansion as high as 19%. The wall thinning due to fireside corrosion, embrittlement and oxidation are primarily responsible for the failure of these tubes. It had been estab¬lished considering the influence of wall thinning that irrespective of operating temperature, pressure and damage development, modified 9Cr 1 Mo steel exhibits longest life among the various grades of Cr-Mo steel.