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The primary Waste Heat Boiler (WHB) in an ammonia plant experienced cap leaking and the outer tube rupture after ten months since the last repair and replacement (total retubing). The leaking cap and the outer tube materials are low carbon steel SA-204 Gr. B and SA-209 Gr. T1a. The inappropriate vertical part of the leaked cap, which is 2.4 mm shorter than the design, might trigger turbulence flow inside the cap and lead to flow-accelerated corrosion (FAC), as suggested by the appearance of wall thinning and horseshoe pattern in the inner surface. This condition is severed by improper cap material selection with low chromium content (0.01%), which is more susceptible to FAC. The local turbulence flow might erode the oxide layer at the cap bottom and accumulate the oxide deposit around the circumference weld joint and the nearest nail spacer in the tube, represented by a thick Fe3O4 deposit. The primary WHB outer tube failure might occur due to the lack of cooling from boiler water because of cap leakage combined with a thick Fe3O4 scale deposit on the nail spacer that causes significant local temperature increases on the failed tube, which resulted in yielding and thin-lip rupture., Failure analysis, Flow accelerated corrosion, Short-term overheating, Thin-lip rupture, Waste heat boiler tube.