Weld Inspection Method Selection for Thick Section
A fabrication shop is welding a 50 mm thick carbon steel pressure vessel. The weld is a double-V butt joint with a single-sided access for most of the circumference. The customer specification requires volumetric examination of the full weld volume. The shop has available: radiographic testing (RT) with a 192Ir source, ultrasonic testing (UT) with phased array, and magnetic particle testing (MPT). The project schedule is tight, and the weld is in a remote location with limited access. The RT source has a 10 Ci activity, and the required film density is 2.0. The UT technique can cover the full volume from one side. The MPT is only for surface. The shop must choose a method that meets the specification and is practical.
1. Which NDT method should be selected for volumetric examination of the full weld volume, and why?
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The selection should be based on the requirement for volumetric examination of the full weld volume and the access constraints. RT with 192Ir can provide volumetric examination, but the 50 mm thickness and single-sided access may require a source inside the vessel or double-wall technique, which is challenging. The 10 Ci source may not provide sufficient penetration for 50 mm steel with acceptable exposure time. UT phased array can cover the full volume from one side, which is suitable for single-sided access. MPT is only surface. Therefore, UT phased array is the most practical method. However, the specification may require RT; if so, a higher activity source or alternative technique would be needed. The answer should weigh code requirements, access, and capability.
2. What are the limitations of RT in this scenario?
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Limitations include: the 10 Ci 192Ir source may not provide adequate penetration for 50 mm steel; the single-sided access may require double-wall single-image technique, which reduces sensitivity and coverage; the remote location may pose logistical challenges for source handling and safety; and the required film density of 2.0 may be difficult to achieve with the given source and thickness. Also, RT is less sensitive to planar defects oriented parallel to the radiation beam.
3. If the specification mandates RT, what alternatives could be considered?
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Alternatives include: using a higher activity source (e.g., 100 Ci or more) to reduce exposure time; using a linear accelerator for higher energy; employing double-wall double-image technique if access allows; or using a different volumetric method such as UT with code approval. The answer should note that code requirements may allow UT as an alternative to RT with proper qualification.