Premature failure of superheater tubes in a fertilizer plant

Abstract

This work presents an unique premature failure of steam superheater tubes in a fertilizer plant. The heat exchanger tube bundle made of SA 213 grade T11 steel suffered from wall thinning in the vicinity of baffles causing reduction in wall thickness of the tubes leading to perforation and leakage of steam from the tube-side into the shell-side during service. The work presents details of metallurgical failure analysis involving: site inspection, visual inspection, chemical analysis, X-Ray diffraction, microstructural analysis, and computational fluid dynamics analysis. The qualitative and quantitative chemical analyses and X-Ray diffraction carried out on superheater residue followed by microstructural analysis of failed tubes confirmed that the tubes suffered from aggressive corrosion attack at certain locations due to hot flue gas carrying alkaline salts and refractory fines. To understand the mechanism of thinning and the reasons for the tube bundle failure only at certain locations, a detailed CFD analysis was carried out on model of the heat exchanger which simulated the flow pattern of the hot gas. The analysis comprehensively demonstrates that the hot gas carrying both corrosive and erosive species caused erosion-corrosion of the tubes in the vicinity of tube-baffle junction that lead to wall thinning and subsequent failure of the tube bundle. Suitable remedial measures are suggested to minimize such failures in future. © 2020 Elsevier Ltd