RT Coverage Dispute on a Circumferential Weld
A contractor is welding a 610 mm (24 in) diameter, 12.7 mm wall carbon steel pipe spool. The specification requires 100 percent radiography of circumferential butt welds. The RT contractor proposes a single-wall single-image technique with a source inside the pipe and film outside, using four exposures at 90-degree intervals. The welding coordinator notes that the weld was made with a GTAW root and SMAW fill, and that the RT film shows a dark linear indication near the root on one exposure. The contractor argues the indication is a film artifact from the source position and wants to accept the weld without further evaluation.
1. Evaluate whether the proposed RT technique provides the required 100 percent coverage and whether the dark linear indication can be dismissed as an artifact.
Read the worked answer
Coverage: For a 610 mm diameter pipe with a source inside and film outside, a single-wall single-image technique with four exposures at 90-degree intervals can provide full circumferential coverage if the effective source-to-film distance and beam spread are adequate. However, the coordinator should verify that each exposure covers the required arc without excessive geometric unsharpness at the edges. The technique must be documented and the coverage demonstrated - four exposures is a reasonable starting point but must be confirmed against the required sensitivity and density limits.
The indication: A dark linear indication near the root cannot be dismissed as an artifact without evidence. Possible causes include lack of fusion, incomplete penetration, or a true crack. Film artifacts (such as a source-side image or film handling damage) usually have characteristic shapes and positions, but the contractor's assertion is not sufficient.
Required action:
- Re-examine the film under proper viewing conditions.
- If the indication persists, perform additional NDE - UT or a second RT technique - to characterize it.
- If confirmed as a weld defect, evaluate against acceptance criteria and repair if required.
Do not accept the weld on the contractor's verbal assurance.
2. The contractor proposes increasing source-to-film distance to improve resolution. Explain the trade-off and whether this solves the coverage question.
Read the worked answer
Increasing source-to-film distance reduces geometric unsharpness, which improves image quality and the ability to resolve fine indications. The trade-off is that radiation intensity at the film decreases with the square of distance, so exposure time must increase substantially or a stronger source is needed. For a 610 mm pipe with an internal source, the maximum practical source-to-film distance is limited by the pipe radius and the need to keep the source centered.
This change improves image quality but does not by itself answer the coverage question. Coverage depends on the number and positioning of exposures and the beam spread at the film edges. The coordinator should:
- Confirm that each exposure's effective coverage overlaps adjacent exposures.
- Check that density and sensitivity meet the specification at the edges, not just the center.
- Require a technique sheet showing exposure positions and coverage.
Increasing source-to-film distance is a quality improvement, not a substitute for demonstrating 100 percent coverage. The linear indication still requires evaluation regardless of the technique change.