Session: 1.1.2 - Fuels, Combustion & Material Handling

Paper Number: 110584

110584 - Failure Analysis of OTSG Tube in a Cogeneration Power Plant 

Tube leakage is one of the most frequent causes of Once Through Steam Generator (OTSG) tube failure. Detection of OTSG tube leakage is vital for power plant functioning, as a significant number of generator outages are due to tube leaks. A leaking tube was found within the OTSG unit at the 110 MW Cogeneration Plant. The failed section of the tube was removed from a lower pass at the bottom of the generator. Several metallurgical examinations - visual examination, Deposit Weight Density (DWD), Energy Dispersive Spectroscopy (EDS) in conjunction with the Scanning Electron microscope (SEM), Atom Emissions Spectroscopy (AES), and Optical microscopy of this tube segment were performed to identify the failure mode and cause. The results of the metallurgical evaluation revealed that the tubing was furnished per the chemical requirement of ASME Specification SB 407 and UNS N08800 alloy. A tightly adhered scale (oxide) is evident on the ID surfaces of the tube segment.The through-wall cracking, longitudinal in nature, is evident. Deposits were observed on the ID surfaces on the opposite side of the cracking. The DWD of the scale removed from the ID surface of the tube sample measured 0.26 gm/ft2, which is considered very light. EDS analyses of deposits from the ID surface of the tube segment revealed elements associated with iron (Fe), oxygen (O), manganese (Mn), chromium (Cr), nickel (Ni), silicon (Si), aluminum (Al), magnesium (Mg), titanium (Ti), zinc (Zn), and carbon (C). The oxides formed from tube metal decomposition. The zinc (Zn) is from the wastage of heat exchanger tubing in the feedwater system. Smaller cracks were observed along the ID surface of the crack. A larger crack near the through-wall failure site measured 0.031" in depth. The cracks observed at the through-wall leak site and more minor cracks are primarily transgranular. The initiation mode of the cracking from the ID surface is intergranular. Some intergranular cracking was also observed away from the ID in the tube segment. The cracks were filled with corrosion products. The subject tube failure occurred as stress corrosion cracking. Residual stresses in the tube, created from forming of the tube at the time of manufacturing, combined with flow-induced stresses experienced in service as the steam generator operated, resulted in the formation of longitudinal cracking. The cracking formed and extended gradually into and through the tube wall, influenced by corrosion products in the cracks until the failure occurred. Additions of caustic solutions used in boiler water treatment practices potentially induced corrosive substances into the tube.

Presenting Author: Wahida Tina Lansing Board of Water and Light

Presenting Author Biography: Wahida Tina is a Mechanical Engineer at Lansing Board of Water and Light. She has 15 years of work experience in utility (power generation, transmission and distribution of electricity, Steam, drinking water, chilled water), oil and gas industry. She has experience designing power plants, gas processing plants, and Steam assisted gravity drainage (SAGD). Currently, she supports the asset management department to obtain and maintain ISO 55000 certificate for the cogeneration power plant. She developed Failure Modes and Effects Analysis (FMEA) and Root Cause Analysis (RCA) for a 110 MW gas-fired cogeneration power plant and a 40 MGD water plant. She has led the construction and commissioning of capital projects at cogeneration and water plants. She has conducted vibration analysis for compressor off skid piping and designed low-pressure gas stations for cogeneration power plants. She has managed a carbon reduction project for a 165 MW coal-fired power plant per EPA.

Wahida Tina received a bachelor's degree in mechanical engineering from Bangladesh University of Engineering & Technology, Bangladesh, and a master's degree focusing on machine design and production from Concordia University, Montreal, Quebec, Canada.

She is a licensed Professional Engineer from Michigan's Michigan Department of Licensing & Regulatory Affairs. She also holds a Reliability Engineering Certificate from the University of Kansas.

Ms. Tina is an active member of the American Society of Mechanical Engineers, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), and the Society for Maintenance & Reliability Professionals (SMRP).

Ms. Tina's recent publication is "Failure Analysis of Once-Through Steam Generator (OTSG) Tube," Wahida Tina, Elizabeth Donaldson, Thomas E. Dickinson, William Hal Schmidt, in the ASME Open J. Engineering. Jan 2023.