Breakthrough In Ferromagnetic Tube Testing

Combining Near Field™ and Remote Field sensors in ferromagnetic tube testing

Detecting Outside Defects with Near Field™ Eddy Current in Heat Exchanger Tubes
Defect detection at Support Plates in Ferromagnetic Tubes

Near Field™ signals are easier to analyze than Remote Field signals and Near Field™ testing has much better sensitivity and ease of analysis for defect signals close to structures such as support plates and tube sheets.  Remote Field Eddy Current testing has better sensitivity to small defects away from structures, such as support plates and tube sheets than Near Field™ testing does.  So how do you decide to inspect a bundle with Near Field™ or Remote Field technology, or do you just inspect the bundle twice?

Now you can inspect a ferromagnetic tube bundle with Near Field™ and Remote Field simultaneously, using the new combination probes from Eddy Current Technology Incorporated.

In the screen clips used in this report, the four signals on the top are in differential mode and the four signals on the bottom are in absolute mode. The signal on the left, the red one, is Near Field™ low frequency. The signal to its right in green is Near Field™ high frequency. The next signal to the right is Near Field™ Very High Frequency (for eddy current testing, that is). The signal on the right in magenta is Remote Field. The probe used has dual send coils for the remote field function.

ferromagnetic tube testing 25%, 50% and 75% O.D. Grooves, 0.125 Inch(3 mm) Long Figure 1

25%, 50% and 75% O.D. Grooves, 0.125 Inch(3 mm) Long
Figure 1

Above appears a screen clip in which the signal is from a 25% I.D. groove and a 25% O.D. groove in the same calibration tube. The screen sensitivity has been increased by a factor of two from Figure 1.

On the left, the 25% O.D. groove is the nearly vertical signal, while the signal that is rotated clockwise slightly from horizontal is from the 25% I.D. groove. You can clearly make the determination as to whether the groove is on the inside or the outside, just as you would in a non-ferrous application. The very high frequency Near Field™ in blue shows a solid response to the 25 per cent I.D. groove and virtually no response to the O.D. groove due to the high frequency used. For the signals from the Remote Field probe, the 25% I.D. and O.D. grooves are almost superimposed on top of each other. As Remote Field is a through wall technique, it is not possible to distinguish between inside and outside defects.

Above appears a screen clip using the new combination Near Field™ Remote Field probes. The signals are from 25, 50, and 75% through the wall grooves. The Near Field™ low frequency and high frequency and the Remote Field signals both in differential and absolute show the grooves with a good phase spread for easy data analysis. The very high frequency Near Field™ has very little sensitivity to the outside grooves due the very high frequency that is used.

In Figure 1, take a close look at the RF differential signal (upper right). In addition to the three signals from the grooves, note a spur that goes up and to the left, rotated counterclockwise from the 75% groove signal. This occurs when one of the send coils passes the 75% groove. Actually, these spurious signals occur for each of the grooves and for each of the send coils, but in this figure, they are hidden behind the other signals. As Near Field™ does not use the remote send coils, Near Field™ is immune to these spurious signals.

Also look at the RF absolute in Figure 1 (lower right). The 75% groove is the larger signal. Rotated clockwise from that and smaller in amplitude are the 50 and 25% grooves. The signal that is about two and a half divisions long and almost at 45 degrees rotated counterclockwise from the 75% groove is also a spurious signal that occurs when one of the send coils passes under the 75% groove. Rotated clockwise to this and about 1 division long, you can make out the spurious signal from one of the send coils passing under a 50% groove.

ferromagnetic tube testing 25% I.D. and O.D. Grooves, 0.125 Inch (3 mm) Long Figure 2

25% I.D. and O.D. Grooves, 0.125 Inch (3 mm) Long
Figure 2

Above appears a screen clip in which the signal is from a 25% I.D. groove and a 25% O.D. groove in the same calibration tube. The screen sensitivity has been increased by a factor of two from Figure 1.

On the left, the 25% O.D. groove is the nearly vertical signal, while the signal that is rotated clockwise slightly from horizontal is from the 25% I.D. groove. You can clearly make the determination as to whether the groove is on the inside or the outside, just as you would in a non-ferrous application. The very high frequency Near Field™ in blue shows a solid response to the 25 per cent I.D. groove and virtually no response to the O.D. groove due to the high frequency used. For the signals from the Remote Field probe, the 25% I.D. and O.D. grooves are almost superimposed on top of each other. As Remote Field is a through wall technique, it is not possible to distinguish between inside and outside defects.

ferromagnetic tube testing 25% O.D. Thinning, 7 Inches (180 mm) Long Figure 3

25% O.D. Thinning, 7 Inches (180 mm) Long
Figure 3

Above appears a screen clip in which the signal is from a 25% O.D. thinning 7″ (180 mm) long. The screen sensitivities are the same as in Figure 1. For the Near Field™ 400 Hz and 800 Hz signal, the amplitude is a little bit larger and rotated counterclockwise slightly from the 25% groove signal in Figure 1. This is because when one of the coils is centered under the 3 mm groove, it still has some sensitivity to the original wall thickness portions of the tube on either side of the groove; therefore, this slight counterclockwise rotation from the 3 mm groove signal and increase in amplitude is logical and similar to what is seen in non-ferromagnetic heat exchanger tube inspection. The effect is more pronounced in the lower frequency because it penetrates farther along the axis of the tube away from the groove, seeing more of the original tube wall thickness.

In Figure 3, the signal from the 25% 7 inch long thinning varies dramatically from the signal for the 25% 3 mm long O.D. groove. This is because this thinning is of such a great length that both send coils are in the thinned area at the same time that the receive coils are; therefore, the signal from the send coils passes through the thinned portion of the tube twice before reaching the receive coils. At first thought, it might seem that this long thinning defect would have signals more like the 50% groove than the 25% groove; whereas, in fact, the signal is larger in amplitude than for the 50% 3 mm groove, and the angle has rotated between the 50 and 75% 3 mm long O.D. grooves.

Closely examining the RF absolute signal, you will note that the signal has six distinct zones. The signal starts by moving down about four divisions. This is when the first send coil enters the thinned area. The next downward section is when the receive coils enter the thinned signal and the final portion of the downward signal is when the second send coil enters the thinned region. The remaining three sections are when the first send coil exits the thinned area, the receive coils exit the thinned area, and then finally the second send coil exits the thinned area. This phenomena substantially complicates data analysis for remote field testing.

To successfully analyze thinning such as this, the Operator must first examine the strip chart and determine the length of the thinned area, and then use different amplitude or angle tables to determine the defect’s depth. Determining a long defect’s depth will be further complicated by the fact that natural thinning will be tapered, as opposed to be a machined long groove with sharp edges, as this calibration defect is. When trying to determine the deepest portion of the defect, one cannot be certain of the defect depth at the send coils, so there will always be significant margin of error. Determining defect depth on a long defect will be much easier with Near Field™.

INSPECTING FIN FAN TUBES

ferromagnetic tube testing 25%, 50% and 75% O.D. Grooves, 1/2 Inch (13 mm) Long Figure 4

25%, 50% and 75% O.D. Grooves, 1/2 Inch (13 mm) Long
Figure 4

Fin fan tubes are ferromagnetic carbon steel tubes with aluminum fins wrapped on the outside to improve heat transfer. Figure 4 above shows the result of inspecting a fin fan calibration tube with 25%, 50% and 75% O.D. grooves. In the high frequency Near Field™ on the left, a very strong response is shown to the 75 and 50% grooves, and a small response, almost horizontally to the left, is the response to the 25% groove.

The very high frequency Near Field™ has no response because these are all O.D. defects.

Also, the Remote Field on the right has no response to these flaws. In Remote Field testing, the signal from the send coil passes through the tube wall and then travels outside the tube wall towards the receive coils, where it again passes through the tube wall. The aluminum fins completely block this external signal path; therefore, there is no response to any of these outside defects.

Fin fan tubes often suffer from external corrosion close to a tube sheet. The corrosion can also extend under the tube sheet. Wall loss in both of these areas is detectable with Near Field™ because the signal does not have to travel outside the tube wall and therefore is not effected by aluminum fins or the tube sheet.

CONCLUSION

By combining a Near Field™ sensor with a Remote Field sensor, you add the following capabilities to the capabilities of the Remote Field probe alone:

  1. An ability to accurately measure defects close to support plates and tube sheets.
  2. An ability to detect a hole under a support plate.
  3. The ability to distinguish between inside and outside defects.
  4. An ability to inspect fin fan tubes (ferrous tubes wrapped with external aluminum fins).

In addition to the above new capabilities, the following improvements are also added:

  1. Measure general thinning without the effect of the remote send coils being under the receive coils because Near Field™ does not use the remote send coils.
  2. No spurious signals when send coils pass under defects because Near Field™ does not use the remote send coils.

Watch this site for more information coming soon.