* Eddy Current Technology, Inc., 201-A Horace Avenue, Virginia Beach, VA 23462; (757) 490-1814; fax (757) 490-2778; email: firstname.lastname@example.org.
In the previous article entitled Eddy Current Testing of Heat Exchanger Tubes Is Easy, I explained how it was easy to measure the severity of a defect in a heat exchanger tube during eddy current inspections by measuring the angle of a signal. Unfortunately, it is not always that easy.
The tubes in heat exchangers are supported by support plates. Although the purpose of the support plate is to protect the tube, the majority of outside defects occur adjacent to support plates. The presence of a support plate near a defect alters the defect signal, making it difficult to analyze.
Figure 1 shows only the 60 and 80 per cent O.D. pits of the ASME calibration tube defects that were shown in the earlier article. The 60 per cent O.D. pit is the signal which is rotated 99 degrees clockwise of horizontal and is analyzed as 60 per cent through wall from the O.D. in the sixth line in the menu. The 80 per cent O.D. pit is rotated 82 degrees clockwise from horizontal.
Figure 2 shows the signal from the support plate on its own.
Figure 3 shows the two O.D. pits but with the support plate located close to the 60 per cent O.D. pit as shown in Figure 4.
Note that the signal from the 60 per cent O.D. pit has been rotated to an angle similar to the 80 per cent O.D. pit due to the proximity of the support plate and incorrectly indicates 78 per cent through wall. If the relative position of the 60 per cent O.D. pit with relation to the support plate were changed, it could alter the slope of the signal approximately the same number of degrees in the opposite direction. This adds significantly to the error in the measurement of these defects; therefore, this problem must be addressed.
The solution to this problem is to add an additional channel to the system, operate the channel at a lower frequency, usually half the primary inspection frequency, in order to get a good signal from the support plate. The signals from these two channels which operate simultaneously are combined in a mixer which subtracts the two signals in a manner to eliminate as much as possible the support plate signal.
Figure 5 shows the signals from the 60 per cent O.D. pit and the 80 per cent O.D. pit with the support plate adjacent to it in the upper left corner of the screen. In the lower left corner of the screen, the angle of the same two signals appear, but in this case, this is after processing the signals through the mixer. The mixer signal from the 60 per cent O.D. pit is no longer distorted by the presence of the support plate in the channel. There is a distinct angle difference between the 60 and 80 per cent O.D. pits and the 60 per cent O.D. pit is analyzed correctly as 60 per cent O.D. Again, we are able to accurately determine the defect depth by measuring the angle of the signals.
Once again, the analysis from the signals in eddy current inspections of heat exchanger tubes is easy,although we have had to add the complexity of a second frequency channel and a mixer. The mixer must be adjusted very accurately, but fortunately in today’s modern eddy current instruments, the eddy current system automatically adjusts the mixer for elimination of the support plate signal.
Watch for the next in these series of articles, in which an additional layer of complexity will be added.