Radiography acceptance followed by planar indications in a thick nozzle weld
A 50 mm thick C-Mn steel nozzle-to-shell weld in a pressure vessel was radiographed with an Ir-192 source using a single-wall technique. The film showed no unacceptable indications and the weld was accepted. Six months later, a phased array UT inspection during a turnaround reports planar indications 8-15 mm below the surface, oriented parallel to the fusion face, up to 20 mm long. The client asks whether the original RT was invalid and wants all similar welds re-radiographed. The original RT density and IQI were acceptable and the RT procedure was qualified. The vessel operates at 40 bar and 250 C. The nozzle is set-on, with a 30 mm thick reinforcing pad. The UT operator is qualified, but the client's inspector doubts PAUT for this geometry.
1. Explain why the RT may have missed these planar indications even though the radiograph was technically acceptable.
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Radiography is a density-based method. It detects a change in thickness or density along the radiation path. A tight planar defect such as lack of fusion, oriented parallel to the radiation beam or parallel to the fusion face, may produce very little density change on the film. The image quality indicator and density can be acceptable while the defect remains invisible because the technique is not sensitive to that orientation and tightness.
At 50 mm thickness with an Ir-192 source, image unsharpness is significant. The source is not a point, and the geometry includes a set-on nozzle and a 30 mm reinforcing pad, which adds scatter and complicates the interpretation. The original RT may have been technically valid for the procedure but still poor at detecting planar lack of fusion.
So the original RT is not necessarily invalid; it was limited by method and geometry. Re-radiographing the same welds with the same technique is unlikely to resolve the question. The new indications must be evaluated on their own merits.
2. Advise the client on the most appropriate NDT and acceptance route for the affected weld and similar welds, including whether re-radiography is useful.
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Re-radiography is of limited use here. The defect type is planar and the geometry is thick and stiffened; RT has already demonstrated its limitation. A blanket re-radiograph of similar welds would add cost and delay without giving reliable detection of lack of fusion.
The better route is a qualified PAUT or TOFD procedure with a qualified operator, supplemented by conventional UT where needed. The procedure should be demonstrated on a reference block containing lack of fusion and sidewall defects at the same depth and orientation. Scanning should cover the shell side, nozzle side and the fusion faces. The reinforcing pad geometry must be addressed in the scan plan; it is a valid reason for extra qualification, not a reason to reject PAUT outright.
Acceptance should follow the applicable construction code and the client's specification. Planar indications that exceed the acceptance criteria require repair or fitness-for-service assessment. For similar welds, a risk-based sample is more sensible than 100% re-radiography. If the sample finds systematic lack of fusion, the sample size can be expanded.
3. What immediate fitness-for-service and repair decisions would you recommend?
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First, do not hydrotest or return the vessel to full service until the indications are evaluated. Record the indication positions, lengths, depths and orientation. Confirm the PAUT results with a second qualified operator or complementary UT technique if the client disputes them.
Carry out a fitness-for-service assessment using the applicable fracture mechanics procedure. The assessment should consider the operating pressure, the 250 C temperature, the weld residual stresses, the indication size and the material toughness. If the indications are acceptable under the assessment, define an inspection interval and operating limits. If they are not acceptable, repair is required.
For repair, excavate the indications to sound metal, verify removal with magnetic particle or penetrant testing, and re-weld with a qualified repair procedure. Re-inspect the repair with UT or PAUT after welding. Notify the client's inspector and the pressure vessel authority as required. The repair record and the FFS assessment should be retained with the vessel documentation.