Eddy Current (ET Inspection)
Eddy current inspection is often used to detect corrosion, erosion, cracking and other changes in tubing. Heat exchangers and steam generators, which are used in power plants, have thousands of tubes that must be prevented from leaking. This is especially important in nuclear power plants where reused, contaminated water must be prevented from mixing with fresh water that will be returned to the environment. The contaminated water flows on one side of the tube (inside or outside) and the fresh water flows on the other side. The heat is transferred from the contaminated water to the fresh water and the fresh water is then returned back to is source, which is usually a lake or river. It is very important to keep the two water sources from mixing, so power plants are periodically shutdown so the tubes and other equipment can be inspected and repaired. The eddy current test method and the related remote field testing method provide high-speed inspection techniques for these applications.
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 signal 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.
Internal Rotating Inspection System (IRIS Inspection)
IRIS is a technique that can be applied on both ferrous and non-ferrous materials and even on non-conductive materials like plastics. With IRIS the remaining wall thickness of tubes can be accurately measured. IRIS inspection is more accurate than other tube inspection techniques and has the advantages of presenting information about the geometry of defects. Local defects and wall-loss on both sides of the tube can be accurately measured. Defects under support plates can be measured without any limitations.
The probe used in IRIS examination is made up of a centering device, an ultrasound transducer and a rotating mirror. An ultrasound pulse will be generated in the transducer that is mounted in axial direction, then a 45 degree rotating mirror in the probe will guide the sound bundle towards the tube wall. Next an ultrasound reflection (echo) will take place at the inner and at the outer wall of the tube. These echoes are reflected back and processed by the equipment. The time between these two echoes represents the wall thickness of the tube. Knowing the sound velocity in the material under test enables accompanying wall thickness to be calculated. Water is then used to rotate the probe mirror and is also needed as a couplant between the transducer and the tube wall.
A calibration standard of the same material and dimensions as the tubes to be examined is used to check the IRIS system response in preparation for the inspection and the tubes should be cleaned to an acceptable standard.
After an IRIS inspection an “on-site” report detailing the condition of each tube will be presented to the client and any critical issues can be identified immediately.
Remote Field Testing (RFT Inspection)
As EC cannot be used in ferrous tubes, another technique should be applied. One of the commonly used techniques is the Remote Field Technique. This technique is very suitable for detection and quantification of overall wall-loss. Local defects can be detected and quantified provided that they have some volume (diameter pit >5 mm). Remote Field Technique can detect both internal and external defects but it is not possible to distinguish between them. Defects under or close to the tube sheet are hard or not possible to detect. Only a basic cleaning of the tubes will be sufficient. Remote Field Technique is slightly slower than EC and approximately 300 tubes, with a standard 6 meter length, can be done on a first inspection day. Speed can go up to as many as 450 tubes on additional inspection days.
The probe used in Remote Field Technique examination contains a send and a receiver coil. In the bigger send coil an alternating magnetic field is generated. This field is indirectly coupled to the receiver coil as a direct coupling between the two coils is shielded by the strong magnetic fields originating from the eddy currents that are being generated in the tube. At a low enough frequency the shielding will lose some of its strength allowing the exciter field to penetrate the tube wall in axial direction. Once the magnetic field reaches the exterior of the tube it will spread rapidly along the tube with little further attenuation. Research found that a portion of the magnetic field re-diffuses back through the pipe wall to the interior of the tube at a certain location. At this position the smaller receiver coil is placed to detect the remaining field. Now the indirect coupling path between send and receiver coil is complete. The magnitude and the phase of the received magnetic field depend on the amount of material that was crossed in the indirect coupling path. If wall-loss occurs in a tube there will be less attenuation and delay of the exciter field before it reaches the receiver coil. The signal on the computer screen represents the change in the received magnetic field, and thus the condition of the tube.
During signal analysis, the signals acquired during a Remote Field inspection will be compared to the signals from reference defects. Reference defects are defects with known depth and shape and are machined into a calibration standard. The calibration standard needs to be of the same material and dimensions as the tubes to be examined.