Corroded Shell Patch Near a Nozzle Weld
A vertical process vessel is opened for turnaround inspection. The shell is 2.4 m inside diameter, 19 mm nominal wall, SA-516-70, with a 3 mm corrosion allowance. Design pressure is 1.75 MPa, and the operating temperature is 120 degrees C. The required minimum wall thickness, after allowance, is calculated as 14.5 mm. Ultrasonic thickness mapping finds a localized external corrosion patch 250 mm by 180 mm with a minimum reading of 13.8 mm. The patch is on the shell, 35 mm from the toe of an existing nozzle-to-shell weld. The rest of the shell reads 17.9 to 18.4 mm. The vessel is out of service and empty. The maintenance planner proposes to grind the patch smooth and build it up with weld metal, then grind flush and return to service without post-weld heat treatment because it is only 0.7 mm low. The welder has a current SMAW procedure for this material, but the procedure was qualified at 20 mm thickness and 150 degrees C preheat. The site has local PWHT equipment.
1. Evaluate the planner's weld-build-up proposal. Consider the thickness deficiency, the proximity to the nozzle weld, and the missing PWHT.
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The patch is below the required minimum thickness, so the vessel cannot simply be ground smooth and returned to service. Weld buildup can restore thickness, but it is a permanent pressure-boundary repair, not a cosmetic fix. The corroded metal must be excavated to sound metal and the repair welded with a qualified procedure. The location 35 mm from the nozzle-to-shell weld is a concern because that region is restrained and already carries nozzle loads and residual stress. Welding there can alter the existing weld and increase cracking risk. The existing procedure qualified at 20 mm and 150 degrees C preheat may be close, but it must be reviewed for the actual repair geometry and heat input. Omitting PWHT because the deficiency is only 0.7 mm is not sufficient reasoning; PWHT may be required by the repair procedure or engineering assessment. An insert plate or shell section replacement should also be considered if the repair cannot be properly heat treated.
2. Outline the repair and inspection plan if weld buildup is approved by engineering.
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Prepare an engineering repair procedure that covers excavation depth, preheat, welding process, bead sequence, interpass temperature, and PWHT. Grind the corroded area to sound metal and confirm with PT or MT that no cracks remain. Preheat per the qualified procedure, weld with a qualified welder, and control heat input to limit distortion near the nozzle. After welding, grind flush and perform UT thickness mapping over the repaired area. Perform PT or MT on the build-up and the adjacent nozzle weld. If PWHT is required, apply it with thermocouples and a chart record, then check hardness. Repair documentation should include the thickness map before and after, the WPS/PQR used, preheat and PWHT records, and NDT reports.
3. What hydrotest and return-to-service checks are required after the repair?
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The hydrotest pressure should be 1.3 times the maximum allowable working pressure unless the applicable code or engineering calculation specifies otherwise: 1.3 x 1.75 = 2.275 MPa. Use water, vent all air, use calibrated gauges, isolate the vessel, and pressurize gradually. Inspect the repaired patch, the nozzle weld, and all temporary closures for leakage or distortion. Because the repair is near a nozzle, check nozzle alignment and flange face condition after the test. After depressurizing, dry the vessel, restore any internal linings or coatings, and complete the inspection record before returning to service. If the test reveals leakage, repair and retest rather than tightening under pressure.