Title : Incident investigation of frequent failure of bank tubes above mud-drum of fuel oil fired water tube bi-drum boiler
Abstract:
Flue gas deposits in fired water tube boilers, particularly when furnace oil is used as fuel, are a common byproduct of combustion. These deposits predominantly accumulate in hard-to-reach areas like the bottom portion of the boiler bank tube, which complicates periodic maintenance and inspection (M&I) activity. While external corrosion of boiler tubes is rare, it can become a significant risk if water comes in to contact with the fireside surface, particularly during M&I activities or due to leaks. When water interacts with residual flue gas deposits in bottom bank tube area especially above mud drum, it forms an acidic environment that accelerates corrosion and leads to tube failures. The paper presents a failure analysis of a Utility Fired Boiler with 13 years of operation, which experienced frequent bank tube leaks and subsequent boiler outage for 3 months. Despite satisfactory feedwater chemistry and fuel composition, the failure was traced to under-deposit corrosion (UDC), triggered by acidic conditions in the bank tube area. Laboratory tests revealed a pH of 3.7 in the flue gas deposits after contact with leaked water, confirming the corrosive environment. The study emphasizes the critical need for advanced Non-Destructive Examination (NDE) of boiler tubes during every comprehensive boiler inspection. Techniques such as Remote Field Electromagnetic Testing (RFET), Remote Visual Inspection (RVI), and Internal Rotary Inspection System (IRIS) are essential for detecting any corrosion damage. Additionally, the study highlights the importance of designing boilers that are maintenance-friendly, facilitating effective cleaning strategies to prevent under-deposit corrosion. These measures play a vital role in maintaining boiler integrity and ensuring long-term reliability.
