Eddy Current Testing of Heat Exchanger Tubes Is Easy

by Monty O’Connor*

* Eddy Current Technology, Inc., 201-A Horace Avenue, Virginia Beach, VA 23462; (757) 490-1814; fax (757) 490-2778; email: montyect@gmail.com.

All you do is pull a probe through a tube and you get a unique signal from every defect.

The accompanying figure shows the signals from five defects in an ASME Calibration Tube: a through wall hole, 80 per cent, 60 per cent, 40 per cent, and 20 per cent O.D. pits, and a dent.

ASME Calibration Tube Signals

In the impedance plane portion on the left of this diagram, the dent is the signal that is almost totally horizontal.

In the strip chart representation to the right, the strip chart on the left is the horizontal and on the right is the vertical.

As the probe is pulled through the tube, it first passes the dent. The resulting signal is seen primarily in the horizontal channel. The next defects that are passed in sequence are the hole, at about 45 degrees in the impedance plane, and then increasing in the clock wise direction, the 80, 60, 40, and 20 per cent pits.

The depth of the defect can be determined by the angle of the signal. Also, the total volume of the defect can be determined approximately by the amplitude of the signal.

It’s easy!

A few controls must be set correctly in order to result in the signals looking like this.

The phase control is used to rotate the entire group of signals. The phase control is used to set the dent signal to be horizontal. (It is the author’s preference and recommendation to set the dent signal horizontal, but other schemes will work.)

The gain control is used to adjust the amplitude of the signals. A good starting point is to adjust the through wall hole to be approximately half the size of the screen.

Actually, the most critical setting is the selection of an appropriate inspection frequency, and conveniently, this information is available here.

If your specific tube alloy or wall thickness is not in the table, then a formula to calculate the frequency is available here and a list of electrical conductivities is available here.

Look for the next in this series of articles entitled “Eddy Current Testing of Heat Exchanger Tubes is Not Quite That Easy”.