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Another inconvenience that arises during the eddy current testing of heat exchanger tubes is that although phase analysis (measuring defect depth based on the angle on the screen) is usually the best measure of a defect’s depth, it is not perfect.
The two eddy current screens shown in the diagram are a bit cluttered because you can see the dent and five defects of an ASME calibration tube that we’ve used for examples in previous articles. The green horizontal signal is, of course, the dent signal. Then, rotated clockwise from this at about 45 degrees, are the through wall hole, followed by the 80, 60, 40, and 20 per cent O.D. pits, all of similar amplitude. Then, superimposed on these is an additional defect larger in amplitude. Just what might this be?
In the ect MAD 8D Eddy Current Software, a new feature has been turned on. This is the ect AutoMAD Analysis Mode, in which the software measures the defect depth of all the defects it encounters in a tube, and then displays a list of up to the 10 deepest defects. This list appears where the Mixer Menu used to be; therefore, appears under the word “Mixer”. The deepest defect encountered is listed as 99 per cent on the I.D. This is, of course, the through hole. (The one per cent error is actually excellent and not to be expected in the field environment. This example was created in a carefully controlled laboratory environment.) Listed below this through wall hole you can pick out the other five outside defects; specifically, the 80, 60, 40, and 20 per cent O.D. defects, all ranging in amplitude from 1.0 volts to 1.8 volts. (Note that below the 20 per cent O.D. defect is listed a nine per cent I.D. defect. This is, in fact, a 10 per cent I.D. groove, which is not shown on the screen.)
Now back to this new, larger defect that appears on the screen, which is marked by the cursors. (Square at the top and a cross at the bottom.) The signal is larger in amplitude; therefore, it is logical to assume that the defect is of greater volume. The angle of this defect is rotated counter clockwise from the 20 per cent O.D. pit; therefore, it would be logical to assume that this is an O.D. pit deeper than the 20 per cent in the calibration tube, but shallower than the 40 per cent, but larger in diameter in order to make up a larger volume. In fact, the data analysis shows this defect as being 36 per cent through the wall on the outside with a magnitude of 2.9 volts. Unfortunately, this is not what the defect is.
This signal actually originates from a concentric O.D. groove 3 mm long but only 10 per cent through the wall from the outside!
Apparently and unfortunately if a defect were to grow larger in volume while retaining the same defect depth, its resulting eddy current signal would rotate counterclockwise and the signal would grow in amplitude. In this case, by directly applying phase analysis using a calibration table based on the ASME O.D. pits, this 10 per cent O.D. defect is incorrectly measured as 36 per cent O.D.
The same is true if the defect was a 100 per cent through wall hole. If you drill holes of different diameters in a calibration tube and measure the eddy current response, you will notice that the larger the diameter of the hole, the more the hole rotates counter clockwise.
This problem becomes even worse for defects on the inside of the tube wall. One reason that this problem is greater on the inside (assuming that these tubes are being inspected from the inside) is that the most important part of the defect is the part in the deepest part of the flaw. This part is farthest from the probe; therefore, the probe has the least sensitivity to it. The eddy current signal is more due to the shallower portion of the defect closer to the probe. The result is that very large diameter pits on the I.D. can easily be interpreted as being much less deep than they actually are.
So, what can be done about it? The most common advice you will hear in eddy current testing is to make the defects in your calibration tube as close to the size and shape of the defects you are trying to measure as possible. As a result, you may need more than one calibration table to correlate phase to defect depth if you have more than one defect type in a tube. Actually, the ect MAD 8D Software has multiple tables, which can be selected by the user depending upon the type of damage that is being measured.
An example of how this may occur could be a water cooled condenser. This type of tube may have multiple, small diameter pits caused by corrosion, but the occasional very large diameter pit caused by erosion where an object had been stuck in the tube. Based on noting the amplitude difference, the Inspector analyzes with the appropriate calibration table and accurately measures the defect depth.