An investigation was carried out into the failure of 30 of 33 HP superheater tubes that occurred during a single incident at a modern combined cycle gas turbine power plant.Metallurgical examination of a failed tube uncovered a multi-layered corrosion product that once cleaned revealed significant wall thinning of the sample tube.Analysis of the corrosion product revealed a grey inner layer containing the elements Na,Fe,P and O,possibly indicating the presence of maricite(NaFePO_4),covered by a dark outer layer probably of magnetite(Fe_3O_4).It is proposed that the tubes failed as a result of an acid phosphate corrosion mechanism caused by excessive liquid and contaminant carry-over from the HP steam drum.A liquid entrainment test was carried out during the course of the investigation that revealed a carry-over of approximately 2-3 %.Contributing factors to the failures included a minimal level of control instrumentation and substandard condition of the drum water/steam separation equipment.
展开▼
