Analysis of a steam generator using the fmea and fta techniques

Abstract

The structural health of a steam generator can be affected by several factors, such as the type of material of the structure, the quality of weld joints made, the boiler manufacturing parameters and the operating procedures performed. In this context, corrosion stands out as one of the major sources of degradation and can lead a surface under pressure to complete collapse. In this study, a failure analysis was made to investigate the process that lead to rupture and corrosion in a steam generator, using FMEA (Failure Mode and Effects Analysis) and FTA (Fault Tree Analysis) techniques. Metallographic tests of structure and a physicochemical study of the system feed water were also analysed. The metallographic tests showed small cracks in the base metal, in addition to lack of penetration in the welded joint and a misalignment of the joint. Water analysis showed a high concentration of chloride ions and a low pH of the feedwater, factors that cause the rupture of the passive layer and result in corrosion. The FMEA and FTA techniques allowed the understanding of the factors that act as stress concentrators, associated with lower quality of the steam generator feed water and errors during the welding process, which resulted in localized corrosion followed by rupture and leakage of liquid and steam water.