Failure investigation of water wall tubes in a drum boiler of a thermal power plant

Abstract

In this study, failure of C-Mn steel water wall tubes in a radiative natural circulation drum boiler with capacity of 550 t/h is investigated. In addition to macroscopic observations, the failed tubes and deposits were characterized by optical microscope (OM), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Microscopically, pearlite decomposition, carbide spheroidization and void formation were the main features of the investigated tubes. Deposit analysis showed high thickness of oxide layers, mainly consisting of magnetite, corrosion product transferred from other part of the system, and some of the chemicals used in the boiler. Due to the phosphate-based regime of investigated boiler, this study revealed the phosphate hide-out phenomenon in the boiler. Although the macroscopic similarities between hide out phosphate corrosion and caustic gouging, this research shows that under-deposit caustic gouging is the predominant corrosion mechanism. Under-deposit corrosion accelerated creep was identified as the main mechanism of tubes failure. Mitigative actions were suggested based on the identified failure mechanism.