Failure Analysis of a J-nozzle on a Waste-Heat Boiler Tube

A waste heat boiler experienced a series of tube bundle failures due to severe material loss localized to a J-nozzle on a boiler tube. The J-nozzle is a baffle welded on to the tube to promote the flow of boiler feed water around the OD of the tube. The temperature of the feed water is approximately 550 °F at 850 PSIG. During service, the ID of the tube is exposed to a process gas environment consisting of a mixture of carbon monoxide, carbon dioxide, and hydrogen at temperatures ranging from 2200-2400 °F. Two nozzles were submitted for a metallurgical failure analysis- one exhibiting a leak site and one having a reduction in thickness at the apex of the J-nozzle. The J-nozzles exhibited deposits from the water feed on the waterside and iron oxide on the process-gas side. Numerous environmental contaminants, including the corrosive species of Cl and S were identified by performing chemical analysis via energy dispersive x-ray spectroscopy. Resulting concentrations for Cl and S were present in concentrations greater than trace quantities. Metallographic analysis of the leak site revealed extensive intergranular attack beneath the waterside deposits, and pitting from the process-gas side. Significant material loss of the J-nozzles was attributed to corrosion pitting on the tube surface exposed to the process-gas, and under-deposit corrosion observed on the steam-side surface. Recommendations to mitigate future failures of a corrosive nature included a reduction in operating temperature, reduction of the corrosive Cl and S species, and a materials substitution for a more intrinsically corrosion resistant alloy that also retains microstructural stability at higher temperatures.