Crack in Firetube Boiler Tubesheet Ligament
During the internal inspection of a 1996 packaged firetube boiler, 150 psig steam, 3/8 in. shell, the AI finds a 4 in. longitudinal crack in the rear tubesheet ligament between two 2 in. tubes. The crack is tight but magnetic particle testing shows it extends from the tube hole toward the shell knuckle and stops about 1/8 in. short of the knuckle. The boiler was washed out and the AI entered through the front handhole. The owner wants to return the boiler to service for the weekend because the plant has no backup steam. The boiler has a current certificate and no prior repair history. The AI has authority to recommend shutdown but the owner is pressing to 'monitor it' until the next outage. The AI must decide what to require and what to report.
1. Should the AI agree to return the boiler to service and monitor the crack until the next outage?
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No. A crack in a tubesheet ligament is a pressure-boundary defect in a region that carries tube loads and shell stresses. The 1/8 in. remaining ligament to the knuckle does not make the crack safe; it shows the crack has propagated through most of the ligament and is close to a higher-stress transition. Magnetic particle testing has already confirmed a sharp crack, and the boiler is in steam service with no backup. The AI should not recommend return to service on a 'monitor' basis. The AI should document the finding in the inspection report, notify the owner in writing that the boiler should be removed from service until repaired, and explain the basis: risk of crack extension, leakage, or ligament failure. The owner makes the operating decision, but the AI's report should be clear that the boiler is not acceptable for continued service in its present condition. The AI should not issue a favorable inspection report or certificate endorsement based on monitoring alone.
2. If the owner agrees to repair, is this a repair or an alteration, and what must the R-stamp holder demonstrate?
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This is a repair, not an alteration, because the work restores the tubesheet to a sound condition without changing the design or pressure rating. The repair must be performed by an R-symbol holder with an AI. The R-stamp holder must demonstrate that the crack is completely removed, typically by grinding to sound metal with MT or PT confirmation, and that the excavation does not unacceptable reduce the ligament or tubehole support. The welding procedure and welder must be qualified for the material and joint, and the repair must comply with the original code of construction and NBIC Part 3. The AI should require a repair procedure that addresses preheat, welding sequence, peening if permitted, and post-repair examination. Because the crack stopped near the knuckle, the AI should also require examination of the knuckle and adjacent ligaments for additional cracking. The completed repair must be documented on an NBIC repair report and the boiler must be hydrostatically tested as required before return to service.
3. The owner proposes to drill a stop hole at each end of the crack and operate until the next outage. How should the AI respond?
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The AI should reject that proposal as a permanent repair. A stop hole may be used as a temporary measure in some situations to arrest crack growth, but it does not restore the pressure boundary, does not remove the cracked ligament, and does not satisfy NBIC repair requirements. The crack could still extend from the stop hole, and the remaining ligament is already thin. The AI should explain that the boiler would remain in a known defective condition and that continued operation is not justified on the basis of a stop hole alone. If the owner insists on a temporary operating decision, the AI should document the AI's recommendation against it and the owner's acceptance of the risk, but the AI should not represent the boiler as repaired or acceptable. The proper path is a welded repair by an R-stamp holder after crack removal, followed by required examination and hydrostatic testing.