Dams | capture device, the final product is a 2D plan view
Below: Interior of the Bonneville dam powerhouse
image of the test object. Discontinuities are identified by a lighter or darker image on the film. Because the radiation travels at a constant rate through a homogeneous material, any discontinuities located within the test object will have a lower density (cracks, porosity, slag, etc.), which will exhibit a darker colored image due to more radiation reaching the film or higher density (tungsten inclusions, excessive weld reinforcement, etc.). This results in a lighter image due to less radiation reaching the film. RT works well with simple geometry structures such as CJP welds in butt- joint configurations and is capable of locating various types of discontinuities, however because of the small change in density with tight linear discontinuities, such as hairline cracks, they are often not identifiable on radiographic images. RT, however, does excel at locating rounded indications such as porosity and slag inclusion. While it does require access to both sides of the test object, the source of radiation is on one side and the detector on the opposite side. Additionally, RT has significant safety issues due to the use of ionizing radiation and special licensing and training is required
for RT technicians. Also, because of the high safety issue, RT can only be performed during off hours when workers are not present. When comparing the two-test methods UT and RT, one would likely choose UT due to the higher sensitivity to linear indications, speed of testing, and having no safety issues.
Technicians and certification One element that is often overlooked is the skill of
the technician. In NDT, technicians are certified to specific methods as Level I, II or III. Level I technicians have limited experience, knowledge and training and are required to be observed by either Level II or III technicians during examination of test objects. Level II technicians have an increased skill and knowledge level and are capable of performing NDT tests without direct oversight. Level III technicians are at the supervisory level and are typically tasked with developing NDT procedures, training, and certification practices for their company. One unique element of NDT is that certification
to Level I, II, or III is typically the responsibility of the company in which they are employed. Central certification programs do exist but are rarely utilized with the exception of the ASNT Level III program. It is under this mentorship where the choice between NDT methods may be swayed. Instructors often understand that UT is a far more complex test method with an almost uncountable number of variables which can skew the test results. This requires a higher level of training, experience, and equipment familiarity that frankly most UT technicians do not receive when trained by their employers. RT on the other hand is relatively simplistic to perform, the image interpretation requires a higher skill level but because of its permanent nature, the image interpretation is often performed by individuals with a higher skill level. With this in mind, UT should be the preferred method, but RT is often more readily utilized. In addition to these factors, the certification document used in NDT is the ASNT Recommended Practice SNT-TC-1a: Personnel Qualification and Certification in Nondestructive Testing. Though this document recommends minimum training, experience, and examination requirements for employers to certify their technicians, it is not a requirement for the employers to follow these recommendations. This stems from a time when quality standards were unique to each manufacturer and the qualification/certification requirements of NDT technicians were modified to fit that manufacturer’s needs. Because this process continues today, the education and skill level of NDT technicians can swing wildly from employer to employer. As America strives to balance responsible water management and sustainable energy generation, the expertise of NDT professionals becomes ever more vital. The competence of NDT technicians serves as the foundation of accurate inspection and precise data collection. It is critical to elevate the expertise of NDT professionals through comprehensive and standardized training programs. These programs ensure that technicians are equipped with the necessary knowledge and skill to perform high-quality inspections, interpret results accurately, and make informed decisions based on their findings. These credentials not only validate the expertise of
36 | October 2023 |
www.waterpowermagazine.com
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