Corrosion Repair and R-Stamp Question on a 250 psig Watertube Boiler
A 50,000 lb/hr, 250 psig watertube boiler is opened for its 10-year internal inspection. The lower mud drum shows a 4 in. by 6 in. area of external corrosion beneath deteriorated insulation; ultrasonic thickness readings are 0.44-0.46 in. against 0.50 in. nominal, and the calculated minimum allowable thickness is 0.42 in. A rolled tube adjacent to the area seeps at 200 psig during hydro. The contractor proposes to grind the pitted area, weld a 1/4 in. patch over it without postweld heat treatment, reroll the seeping tube, and hydrotest at 1.25 times MAWP. The owner wants the boiler back in service in five days and asks whether the repair can be done under the plant's own maintenance group without an R-stamp holder. The jurisdiction requires NBIC compliance.
1. Evaluate the proposed patch weld and the use of the plant maintenance group without an R-stamp holder.
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The patch is a weld repair to a pressure part of a code boiler. Under NBIC and the jurisdiction's requirements, such a repair must be performed by an organization holding the appropriate National Board repair stamp, normally the R stamp, using a qualified repair procedure, qualified welders, and the required inspection. The plant maintenance group cannot simply weld a patch because the measured thickness is above the minimum allowable; thickness acceptability does not make an unqualified weld repair acceptable. A 1/4 in. patch welded over external corrosion also raises fit-up, weld quality, and stress-concentration concerns. Postweld heat treatment may or may not be required depending on material, thickness, and procedure, but it cannot be omitted by the contractor's assertion alone. The repair procedure must be reviewed and accepted, and the Authorized Inspector must be involved before the work begins.
2. Evaluate the proposed reroll and the 1.25 times MAWP hydrotest. What must be verified before either is accepted?
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Rerolling a seeping tube may be acceptable if the tube end is not cracked, the tube sheet hole is not damaged, and the tube has enough remaining wall and expansion. The leak at 200 psig is a warning that the rolled joint has failed; a simple reroll may only hide a cracked or thinned tube end. The tube should be examined, and if there is any doubt, it should be replaced rather than rerolled. The proposed 1.25 times MAWP hydrotest must not be accepted just because it is convenient. The hydrostatic test pressure for a repair must be the value required by the applicable code and accepted by the jurisdiction; the contractor's proposed factor must be verified against that requirement. The test must be witnessed as required, with proper water temperature, venting, and inspection access, and all records retained.
3. What inspection and documentation plan would you require before returning the boiler to service?
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First, map the corroded area with a grid of ultrasonic readings and remove insulation farther out to find the true extent. Verify the minimum allowable thickness calculation and the remaining ligament around the tube holes. The R-stamp holder should submit a repair procedure, welding procedure specification, procedure qualification record, welder qualifications, and NDE plan. The Authorized Inspector should review and hold witness points before welding, during NDE, and at hydro. After the repair, perform the required NDE and hydrostatic test, then recheck the tube joint and the patch area. Documentation should include the repair scope, material certifications, welding and NDE records, hydro test chart, AI sign-off, and a revised inspection history. The boiler should not be returned to service until the jurisdiction's requirements and the AI's acceptance are complete; a five-day schedule is not a reason to skip them.