Weld Failure on a Pressure Vessel Repair
A 12 mm thick mild steel air receiver (AS 1210 territory, design pressure 1.0 MPa) developed a pinhole leak along a longitudinal seam weld. A maintenance fitter proposes to grind out the leak and run a single-pass 6 mm fillet weld over the ground area using a general-purpose electrode, then hydrotest to 1.0 MPa and return to service. The vessel is 900 mm diameter, 2.4 m long, and sits in a compressor room where it cycles from 0.3 to 1.0 MPa about 40 times per day. The seam is a double-V butt weld, full penetration. The fitter argues the repair is 'just a patch' and that the hydrotest will prove it.
1. Identify the technical problems with the proposed repair and explain why a hydrotest would not validate it.
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Several faults compound. First, a 6 mm fillet weld on a 12 mm full-penetration butt seam does not restore section thickness - it leaves at most half the wall carrying the hoop stress, creating a stress concentration at the toe. Second, a fillet profile on a butt seam introduces a sharp geometric discontinuity exactly where the cyclic pressure load peaks, so fatigue cracking is likely given 40 cycles/day (~14,600/year). Third, a general-purpose electrode may not match the parent metal strength or toughness, and without preheat or a qualified procedure on 12 mm steel, hydrogen-assisted cold cracking is a real risk. Fourth, grinding a leak out of a seam may remove the very material that was containing the defect.
A hydrotest only proves the vessel holds pressure for the test duration. It does not detect lack of fusion, slag, or undercut, and a fatigue crack can initiate after the test under cycling. It is a proof test, not a fitness-for-service assessment. The repair needs a qualified welding procedure, full-penetration restoration, NDT (e.g. UT or MPI) of the completed weld, and a fatigue assessment.
2. Outline the correct course of action and the engineering justification for each step.
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Step 1: take the vessel out of service and depressurise. Step 2: engage a qualified welding procedure specification and procedure qualification record for the repair - the repairer must demonstrate the joint achieves parent-metal strength and toughness. Step 3: excavate the defect fully (not just grind the surface) to sound metal, back-gouge and re-weld as a full-penetration butt repair with preheat appropriate to the thickness and controlled interpass temperature. Step 4: perform volumetric NDT (UT or radiography) on the repaired region plus MPI at the toes. Step 5: hydrotest at the specified test pressure with the correct hold time and inspect for weeping. Step 6: assess fatigue life for the 0.3-1.0 MPa cycle and record the repair in the vessel's history file.
Justification: pressure equipment repairs are safety-critical, so the governing principle is that the repaired joint must be at least equivalent to the original design, verified by procedure qualification and NDT rather than by a single proof pressure. The fatigue assessment is essential because the cycling duty is the actual failure mechanism driving the pinhole.