NEAR FIELDTM EDDY CURRENT INSPECTION OF CARBON STEEL FIN FAN TUBES

Monty O’Connor, President
Eddy Current Technology Incorporated
201A Horace Avenue
Virginia Beach, VA 23462
USA

Telephone: 1 (757) 490-1814
Fax: 1 (757) 490-2778
Web Site: http://www.eddy-current.com
E-Mail Address: montyect@gmail.com

The tube used in this study is a 1.000 inch (25.4 mm) O.D. x 0.120 inch (3 mm) wall Carbon Steel tube wrapped with aluminum fins. The outside diameter of the aluminum fins is about 2.200 inches (55.8 mm). The purpose of the fins is to help dissipate heat. These tubes are commonly used in heat exchangers in Petro Chemical Plants. These tubes are referred to as Fin Fan tubes.

Remote Field Eddy Current Testing (RFT) cannot be used to inspect these tubes because the magnetic field must travel outside of the tube from the send coil to the receive coil. The external aluminum fins block the magnetic field. The aluminum fins block all signal response, just as support plates block signal response in Remote Field Testing when the support plate is between the send and the receive coil.

In Near FieldTM Eddy Current Testing (NFT), the electro magnetic field does not travel outside of the tube wall but remains in the tube wall; therefore, it is not blocked by external support structure like support plates nor by aluminum fins in Fin Fan tubes.

The defects in the calibration tube used for this study are as follows:

1. 1.000 inch (25.4 mm) x 20% O.D. groove
2. 0.500 inch (12.7 mm) x 25% O.D. groove
3. 0.500 inch (12.7 mm) x 50% O.D. groove
4. 0.500 inch (12.7 mm) x 75% O.D. groove
5. 0.125 inch (3 mm) through wall hole
6. 0.500 inch (12.7 mm) x 10% I.D. groove

In the attached diagram, three of the eddy current channels that can be used with this probe are not important for the detection of the defects in this calibration tube; therefore, these channels have been deliberately turned off in order to simplify the understanding of the signals that are presented here.

The Channels that have been turned off are Channels 1, 5 and 8, the NFTTM Low Frequency (LF) Differential Channel, the NFTTM High Frequency (HF) Channel and the NFTTM Very High Frequency (VHF) Absolute Channel.

The O.D. defects show up best in the NFTTM HF Absolute Channel, Channel 6. They also show up in the NFTTM HF Differential Channel, Channel 2, but they are easier to understand in the NFTTM HF Absolute Channel.

In addition to these being visible in Screens 2 and 6, they also show up in the left most strip chart. In that strip chart from the top down, you first see the 20%, followed by the 25, 50 and 75% O.D. grooves. The 10% I.D. groove is also visible in Screen 6 as a horizontal signal moving to the right from center. It does not show up well in the Strip Chart because it is a Vertical Strip Chart only.

The 10% I.D. groove is more visible in Screen 6, the NFTTM HF Absolute Channel. It is the horizontal signal from the center moving to the right about 1 division. The O.D. defects are rotated clockwise from that horizontal signal with the largest being the 75% groove followed by the 50% and 25%. The 20% is not visible in Screen 6 because it is overwritten by the 25% signal. All of these signals are also visible in the NFTTM HF Differential Channel, Screen 2, although the signals tend to overwrite each other and are more difficult to distinguish individually.

The through wall hole and 10% I.D. groove show up best in the NFTTM I.D. Differential Channel. The10% I.D. groove is the very large horizontal signal that goes off the screen to the right and into the RFT Differential Signal to the left.

The defect detection of the Data Analysis feature has detected the through wall hole and turned the signal white to make it easily visible to the Inspector. This white signal also shows up clearly in the right strip chart for Channel 4 vertical. The NFTTM I.D. Channels have been optimized for detecting inside defects; therefore, I.D. pits of a similar diameter to the through wall hole will have equal sensitivity to the signals seen here.

The 10% I.D. groove also shows up very clearly in the same strip chart. It is the very large signal that is in line with the bottom of the menus.

None of these defects are detected by the RFT Differential or RFT Absolute Channels because the aluminum fins completely block the magnetic field that must pass from the send coil to the receive coil outside of the tube. The small signal that is visible for RFT Channel 3 and RFT Channel 7 is when the send coil comes out of the end of the tube.

cs-id-pit

The best NDT Method for inspecting Fin Fan tubes is Near FieldTM Eddy Current.