The two major applications of eddy current testing are surface inspection and tubing inspections. Surface inspection is used extensively in the aerospace industry, but also in the petrochemical industry. The technique is very sensitive and can detect tight cracks,
corrosion and wall loss.
Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubing in nuclear plants and heat exchangers tubing in power, oil and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate.
A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat exchanger. With the differential probe, no
dsignal will be seen on the eddy current instrument as long as no metal thinning is present. When metal thinning is present, a loop will be seen on the impedance plane as one coil of the differential probe passes over the flawed area and a second loop will be produced when the second coil passes over the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag. The size of the indication provides an indication of the total extent of the corrosion damage
LONG RANGE GUIDED WAVE ULTRASONIC TESTING (GWUT)
Guided Wave testing (GWT) is one of latest methods in the field of non-destructive
evaluation. The method employs mechanical stress waves that propagate along an
elongated structure while guided by its boundaries. This allows the waves to travel a long
distance with little loss in energy, guided wave technology is now commonly used as a
complementary screening tool to improve corrosion detection in various pipeline, the
technology may be applied to thin- and thick-walled pipe, with diameters ranging from 4” to
60”, rapidly and economically. Use reliable engineering data to assess exactly where your
pipeline needs follow-up nondestructive or visual inspection.
With guided wave, low-frequency waves are sent along pipe; these waves propagate over
long distances, covering 100% of the pipe wall thickness from a single inspection position.
Guided wave testing is used to identify areas of concern, which are then locally scanned
with conventional UT or phased array to size the indication, ensuring that the rest of the line
is free of corrosion.