Failure investigation of a deaerating feed-water heater in a power plant

Abstract The aim of the current investigation is to understand the reason of failure of a deaerating feed-water heater in Bisotun power plant located in Kermanshah, Iran. The vertical deaerator failed after a service life of around 25 years. Optical Microscopy (OM), Scanning Electron Microscopy (SEM) equipped with the accessory Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) methods were used to characterize microstructural changes during in-service process. In addition, to understand integrity of microstructure, mechanical testing including universal tensile and hardness tests were conducted. The appearance of deaerator wall surface was both smooth and pitted, but a distinctive wave-like pattern with smooth scallops was generally prevalent in the areas of more severe damage, especially at the bottom position of the deaerator as a result of Flow Accelerated Corrosion (FAC). FAC resulted in thickness thinning of the deaerator, as in some location caused through-thickness loss (a leaky hole). Microstructural observation showed that leaky hole was formed at the locations in which the microstructure underwent recrystallization, during deaerator manufacturing process. This in turn led to low strength (hardness) of the deaerator steel against FAC.