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Case Study - Suspected SAC In Recovery Boiler Screen Tubes Revealed A Manufacturing Defect

Picture for Case Study - Suspected SAC In Recovery Boiler Screen Tubes Revealed A Manufacturing Defect
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Product Number: 51321-16587-SG
Author: Catherine A. Noble, P.E.
Publication Date: 2021
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$20.00
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A longitudinal crack in the swaged section of a screen tube was discovered in the black liquor recovery boiler at a paper mill. An adjacent tube to the failure contained an “indication” of a crack when X-rayed. The mill originally suspected stress assisted corrosion (SAC). These tubes were examined in the lab, and neither appeared to be damaged due to operation. Further investigation determined that the failure originated from a manufacturing flaw at the external tube surface in the swaged section. The examination indicated that the condition was likely isolated and not indicative of the general condition of other tubes with similar service. The mill planned to remove all of the screen elevation swaged tubes from the boiler during the next outage. These tubes were examined using phased array ultrasonic testing (PAUT) for SAC and no indications were detected. As a follow up, two additional tubes from the inspection were also metallurgically examined and there was no evidence of the manufacturing defect. This was confirmed to be an isolated incident.

Key words: black liquor recovery boiler, swage, screen tube, phased array ultrasonic inspection, manufacturing defect, stress assisted corrosion, corrosion fatigue.

A longitudinal crack in the swaged section of a screen tube was discovered in the black liquor recovery boiler at a paper mill. An adjacent tube to the failure contained an “indication” of a crack when X-rayed. The mill originally suspected stress assisted corrosion (SAC). These tubes were examined in the lab, and neither appeared to be damaged due to operation. Further investigation determined that the failure originated from a manufacturing flaw at the external tube surface in the swaged section. The examination indicated that the condition was likely isolated and not indicative of the general condition of other tubes with similar service. The mill planned to remove all of the screen elevation swaged tubes from the boiler during the next outage. These tubes were examined using phased array ultrasonic testing (PAUT) for SAC and no indications were detected. As a follow up, two additional tubes from the inspection were also metallurgically examined and there was no evidence of the manufacturing defect. This was confirmed to be an isolated incident.

Key words: black liquor recovery boiler, swage, screen tube, phased array ultrasonic inspection, manufacturing defect, stress assisted corrosion, corrosion fatigue.

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