Failure Analysis of Once-Through Steam Generator (OTSG) Tube

The once-through steam generator (OTSG) produces superheated steam using purified feed water. The plant-specific water quality, steam quality, high temperature, and pressure operations lead to the leakage of the OTSG tubes with economic, safety, and environmental consequences. Tube leakage is one of the most frequent causes of OTSG tube failure. A leaking tube was discovered within the OTSG unit of the 110 MW cogeneration plant. The failed section of the tube was removed from the steam generator. Several metallurgical examinations of this tube segment were performed to identify the failure mode and cause. A portion of the tube was analyzed using optical emission spectroscopy (OES) to determine the alloy composition. The results confirmed that the tubing was fabricated from a material consistent with chemical specifications for ASME Specification SB 407 Inconel Alloy 800 (UNS N08800). Glass bead blasting was used to determine the deposit-weight-density (DWD). The DWD value was a maximum of 5.1 g/ft2. The maximum internal deposit thickness was 0.002 in. No evidence of overheating was observed. Scanning electron microscope-energy-dispersive x-ray analysis (SEM-EDXA) was used to determine the elemental composition of the internal deposits. The results indicated that the internal gray deposits primarily comprised iron, chromium, and nickel compounds. There were also fewer amounts of sodium, silicon, aluminum, potassium, and calcium species. The subject tube failure involved a through-wall crack that occurred as stress corrosion cracking (SCC). Additions of caustic solution used in OTSG water treatment practices potentially induced corrosive substances into the tube.