Radiography of a 22 mm shell declared 100 %: a Level I signature, a suspicious indication and missing films
A fabricator is completing a 22 mm Q345R vertical shell, inside diameter 2000 mm, design pressure 2.5 MPa, design temperature 120 °C. The A and B butt welds have been declared 100 % radiographed so that a joint efficiency of 1.0 can be used in the thickness calculation. The RT report cites an Ir-192 source, source-to-film distance 500 mm, 300 × 80 mm films, source-side wire IQI, weld density 2.7, and lists all 96 films as class II. On review you find that the report is signed only by a Level I radiographer; film 63 shows a 15 mm long, dark, slightly zigzag linear indication along the fusion line, recorded as 'linear slag, class II'; and the three films covering the last 1.8 m of the longitudinal seam are missing from the archive, explained as 're-shot, and the bad ones thrown away'.
1. Is the radiography report acceptable as a basis for accepting the welds? What do you check first?
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No. The report cannot be accepted as it stands, for reasons that are structural rather than a matter of opinion:
- Personnel. A Level I radiographer may perform the technique under supervision but cannot sign and interpret the report; interpretation and grading require Level II or III. The signature alone invalidates the report as an acceptance document.
- Technique. Check the source-to-film distance against the geometric unsharpness limit for 22 mm steel with Ir-192, the IQI sensitivity achieved, the film density range, the coverage and overlap, and the film identification and traceability. A stated density of 2.7 in the weld does not by itself prove the film meets the required sensitivity.
- Coverage. The 96 films must be traceable to weld lengths. Missing films mean 100 % coverage is not demonstrated, and 'bad ones thrown away' means the original record was destroyed, which is a documentation failure in its own right.
- Procedure. Confirm the RT procedure was qualified for this thickness and technique and that the acceptance criteria are the correct ones for the design.
Until these are resolved, the report is a draft at best. Do not accept, and do not reject the welds on the basis of it; require a compliant re-interpretation and re-shooting where coverage is missing.
2. How do you resolve film 63, and what does the 'class II' label actually tell you?
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The label tells you what someone wrote, not what the indication is. Grading by name is exactly where errors hide.
- A dark, slightly zigzag linear indication along the fusion line is crack-suspicious: lack-of-fusion or a crack, not slag. The shape, the position at the fusion line and the branch-like ends all point that way.
- A crack is unacceptable at any quality class; there is no class II crack. So if it is a crack, the weld fails regardless of length.
- Even if it were slag, a 15 mm linear indication in 22 mm material must be checked against the acceptance table for its type and length; the inspector should apply the table, not accept a colleague's classification.
- Characterise it: surface NDT (MT or PT) on the cap and, where accessible, the root, plus ultrasonic testing to establish through-thickness extent and whether it is planar. Radiography alone is weak for planar defects oriented along the beam.
- If it is planar lack-of-fusion or crack, remove it completely, repair with a qualified procedure and qualified welders, and re-radiograph the repair plus the required adjacent length.
The decision must be recorded as 'indication characterised as … by … and dispositioned …', with the film, the characterisation method and the repair record cross-referenced.
3. What are the consequences of the missing films for the joint efficiency of 1.0 and for the shell thickness?
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The joint efficiency of 1.0 is an assumption that rests entirely on demonstrated 100 % volumetric examination. With 1.8 m of seam unaccounted for, the assumption is unsupported.
Two legitimate routes:
- Re-establish the basis. Re-radiograph the missing length and review the archive to confirm that no other films were discarded; then the 100 % claim can be restored. Until that is done, the correct assumption is φ = 0.85.
- Recheck the design. Recalculate the required thickness with φ = 0.85 and the allowable stress for Q345R at 120 °C from the material table. Taking about 170 MPa for illustration: t = 2.5 × 2000 / (2 × 170 × 0.85 − 2.5) + C = 5000 / 286.5 + C ≈ 17.5 + C, so about 18.8 mm with 1.3 mm allowance. With φ = 1.0 the figure is about 5000 / 337.5 + C ≈ 14.8 + 1.3 = 16.1 mm. The 22 mm plate may still be adequate, but that must be shown by calculation, not assumed from a report that is not valid.
Either way the report must state which basis was used, and the fabricator must be told that discarding films is a records violation that undermines every '100 %' claim made on the project, not just this seam.