DWDI Elliptical Technique Proposed for 168 mm Pipe
Twelve butt welds on 168.3 mm OD × 7.11 mm wall pipe are to be radiographed in a rack. The pipe is accessible only from the outside; there is no internal access and no crawler. A 3.0 mm effective focal-spot Ir-192 source is available, and the company procedure requires geometric unsharpness ≤ 0.25 mm and film density 2.0–4.0. The Level II proposes double-wall single-image elliptical exposures at 600 mm source-to-object distance, three exposures per joint at 120° increments. The client asks for 100% coverage and a technique sheet. The radiographer has D7 film and lead screens. The weld cap is not ground flush. The Level II must decide whether the proposed technique meets the unsharpness requirement, how many exposures are needed, and whether an alternative should be proposed.
1. Calculate the geometric unsharpness for the proposed technique and the source-to-object distance needed to meet the 0.25 mm limit. What does this imply for the technique?
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For double-wall single-image elliptical work, the source-side weld is roughly one pipe diameter from the film, so t ≈ 168 mm. With F = 3.0 mm and a = 600 mm, Ug = F × t / a = 3.0 × 168 / 600 = 0.84 mm, which is more than three times the 0.25 mm limit. To meet the limit with the same source, a = F × t / Ug = 3.0 × 168 / 0.25 ≈ 2016 mm. That is not practical in a rack with a 3 mm Ir-192 source and would also produce low density unless exposure time is greatly increased. A 1.0 mm focal-spot X-ray unit would give Ug = 1.0 × 168 / 600 = 0.28 mm, still slightly over; increasing to 800 mm gives 0.21 mm and would comply. The proposed DWDI elliptical technique is therefore not acceptable under the procedure, regardless of the number of exposures.
2. The client insists on 100% coverage without internal access. Recommend a compliant approach, address coverage and image interpretation, and list what must appear on the technique sheet.
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The first option is single-wall single-image panoramic radiography with a gamma source inside the pipe, which gives one exposure for the full circumference and much better geometry, but it requires internal access; removing a spool or using a crawler may be justified. If internal access is impossible, use a small-focal-spot X-ray unit from outside, with enough SFD to meet Ug, and a marked exposure map with overlap. Three exposures at 120° can cover the circumference in DWDI elliptical mode, but the unfushed weld cap and the double-wall image make the film-side weld difficult to interpret and can hide lack of fusion; coverage at the overlaps must be demonstrated, not assumed. If neither approach is possible, do not quietly accept the noncompliant technique - raise a deviation for engineering approval. The technique sheet must state the source size and type, SFD, object-to-film distance, Ug, kV/mA or activity, film and screens, density range, exposure angle and number, coverage map, IQI placement, and the acceptance standard.