Frequency compensation (1/4)Frequency compensation (1/4)

After the calibration, display the absolute signal. Scan a few tubes in the bundle and determine the average support- plate signal orientation. If the orientation of the support plate signal in the bundle is similar to the orientation of the support plate signal in the calibration tube, there is no need for compensation. If the difference in orientation is greater than 45°, refer to the following slides for the procedure to compensate the frequency.

Frequency compensation (2/4)Frequency compensation (2/4)

Change the operating frequency to orient the support plate signal horizontally: - To produce a counterclockwise rotation on the support plate signal move the operating point up, reduce the frequency (F↓). - To produce a clockwise rotation on the support plate signal and move the operating point down, increase the frequency (F↑).

Frequency compensation (3/4)Frequency compensation (3/4)

Rescan the tube in the bundle and look at the support plate signal. The support plate signal should now be rotated toward the horizontal axis. Repeat the frequency correction, if required until the support plate signal is horizontal.

Frequency compensation (4/4)Frequency compensation (4/4)

Save your setup under a new name and use it for your inspection.

Basic defect signal shapeBasic defect signal shape

The analysis of remote field signals is a challenging task. It is very important to keep in mind the theory behind remote field testing as well as the signal patterns for a few simple defects. The pattern of the support plate signal is particularly important to remember because it can reveal some hidden defects. The next few slides show the responses of typical defects: short, long, taper, and pit-type defects. Although these defects are artificial, they offer a good starting point for analysis.

Short defect signalsShort defect signals

Short defects create two signals on the strip chart (the first one, when the receiver moves past the defect; and the second one, when the driver moves past the defect). The signal angle in the voltage plane indicates the depth of the defect ( here the blue-circled defect is deeper than the red-circled defect)

Long defect signalsLong defect signals

Long defects create a combined signal known as a “head & shoulder” signal in the strip chart and a “dog leg” signal in the voltage plane.

Taper defect signalsTaper defect signals

In the voltage plane, a taper defect creates a signal that moves gradually along the logarithmic spiral. In the strip chart, the signal starts gradually and finishes with two edges coming, respectively, from the receiver and the exciter.

Pit defect signalsPit defect signals

Pit defects create a sinusoidal signal on the vertical strip chart. The data on the strip chart is read from the top to the bottom, with the defect signal beginning on the left and ending on the right, when measured with ASME code.

Tube support platesTube support plates

The analysis of remote field signals is usually more difficult than the analysis of eddy current signals because: - The behavior of carbon steel tubes is not constant, even within the same bundle. - RFT probes have a large sensitivity area due to the double diffusion of the electromagnetic field inside the tube wall. During analysis, the orientation of the support plate is important because it indicates variation in the tube (the material properties or thickness) Before measuring any defects in the tube, the operator should verify that the support plate signal is correctly oriented. If not, a correction should be made before sizing the defect.

Normal support plate signalsNormal support plate signals

It is very important to remember the normal tube support plate signal shape. For the DIF channel: - The user should note the angle of the support plate signal in the calibration tube. This becomes the reference angle for the differential analysis. - The reference angle is normally between 200° and 230°. The exact value depends on the selected frequency.

Support plate variationsSupport plate variations Bundle effect - The orientation of the support plate signal is affected by the electromagnetic environment which is different in the middle of the bundle than it is on the edge. Variation in the nominal wall thickness - In tube manufacturing the mechanical tolerance for wall thickness is ± 10%. Such variation can cause the support plate signal orientation to be changed by up to 70°. Variation in material properties - The tubes in the bundle may not all have the same grade ( for example, due to retubing).If this is the case, different magnetic properties will give different support plate signal orientations. Defect at the support plate - A defect at the support plate distorts the signal.Before sizing a defect, the operator should look at the other support plate signals of the tube to verify that the general orientation is correct.

Support plate correction (ABS)Support plate correction (ABS)

To correct a support plate signal: - In the voltage plane, zoom on the support signal. - Activate manual measurement by right-clicking in the voltage plane or by using the SHIFT key. - Measure the support plate signal, from the operating point to the end of the signal. - Click the Remote field calibration icon

Support plate correction (ABS)Support plate correction (ABS)

To correct a support plate signal: - In the voltage plane, zoom on the support signal. - Activate manual measurement by right-clicking in the voltage plane or by using the SHIFT key. - Measure the support plate signal, from the operating point to the end of the signal. - Click the Remote field calibration icon( ) After this correction, you can size the defect.

Support plate correction (DIF)Support plate correction (DIF)

To correct the support plate signal: - In the impedance plane, zoom on the support plate signal. - Click the Automatic measurement icon . - Hold the CTRL key and right-click the mouse to rotate the signal. - Drag the mouse around the center of the impedance plane until the signal reaches the reference angle previously noted for the calibration tube (in this example, the angle was 207°) After this correction, you can size the defect.

Defect measurementDefect measurement

Click the Automatic measurement icon Click the Operation menu, and the click Defect sizing.

Gradual corrosion defectsGradual corrosion defects

If there is gradual corrosion along the tube the support plate signal correction should be made based on the thickest part of the tube. The thickest part of the tube corresponds to the support plate signal that has rotated the most (clockwise). For the ABS channel use manual measurement to measure the support plate signal, and then click the Remote field calibration tab. For the DIF channel, hold the CTRL key and manually rotate the support plate signal in order to reach the reference angle.

Measuring defects in a corroded areaMeasuring defects in a corroded area

If there is a general corrosion problem and also localized defects: - Measure the general corrosion by using the ABS channel and comparing the corrosion signal with the thickest area of the tube. - Measure the localized defect: >> Select the support plate signal closest to the localized defect. >> Adjust the support plate signal orientation. => For the ABS channel use the Remote field calibration tab . => For the DIF channel, manually rotate the signal. >> Select and measure the localized defect. - The total wall loss is a combination of the general corrosion and the localized defect.