Visual Examination can be either direct or indirect.
Therefore, remote or indirect visual examination can be substituted for direct examination.
VT is performed with or without optical aids (mirror, magnifier, borescope) or measuring tools (ruler, micrometer, welding gauge, vernier caliper).
When performing VT, the observer’s eye shall be between 6” – 24” (150 mm through 600 mm) range of the surface to be examined and at an angle not less than 30 degrees to the surface.
According to Table T-921, ASME V, the essential variables for a visual examination procedure include any changes from direct to indirect examination; using visual aids and light intensity.
This means if any of these essential variables changed, then, the procedure shall be re qualified, that is, the examiner shall be able to see a 1/32” (0.8 mm) wide imperfection under new parameters.
A change of a requirement identified as a nonessential variable does not require requalification of the written procedure.
Reminder: The VT examination shall be carried out as per ASME V, but the indications shall be evaluated and interpreted in terms of the acceptance criteria of the referencing code or specification.
Visual testing is done to verify the object conformance to specification and find out anomalies if any.
In order to know what indication or defect we are looking for; it is important that one is familiar with the component configuration and manufacturing process, its service history and potential failure modes as well as having related industry experience for visually inspecting such items.
Visual testing is widely used on a variety of objects to detect surface discontinuities associated with various structural failure mechanisms.
Even when other NDEs are performed, VT is often provide a useful supplement ( to verify and more closely examine the surface condition).
Discontinuities that can be detected by a simple visual examinations are:
- Surface discontinuities,
- physical damage,
- Loose or missing parts.