Eddy Current Testing of Copper Nickel Alloy Heat Exchanger Tubes

Eddy current inspection and eddy current thickness measurement of copper nickel alloy heat exchanger tubes using dual frequency eddy current and dual probe inspection techniques are introduced. For the eddy current testing of such nickel alloy materials with weak magnetism, a reasonable testing scheme is proposed and discussed. In order to reduce the interference of the weak magnetic materials on the testing signal and improve the signal-to-noise ratio, a permanent magnet saturation probe is proposed; In order to improve the detection efficiency, it is proposed to use the self comparison of dual probe signals to obtain differential signals for eddy current flaw detection, and use the other comparison of another group of probes to obtain comparative signals for eddy current thickness measurement, so that eddy current flaw detection and eddy current thickness measurement can be carried out synchronously.

1. Testing the basic situation

The test object is in-service equipment heat exchanger tube R51A21, heat exchanger tube specifications Ф54 × 2 × 4200, the material for copper-nickel alloy NO4400 (GB/T: MCu-28-1.5-1.8), the magnetic properties of weak magnetic properties, testing requirements for the heat exchanger tube damage and wall thickness thinning to make a judgment. Therefore, in order to reduce the weak magnetic properties of the material on the eddy current detection signal interference, improve the signal-to-noise ratio, the heat exchanger tube bundle needs to be locally magnetized. Instrument selection with multi-channel multi-frequency eddy current meter.

2. Detection method

According to the structure and material characteristics of the equipment: heat exchanger tube material for copper-nickel alloy UNS NO4400, its magnetic permeability μ>1, a weakly magnetic material. General conventional eddy current detection methods will be interfered by the weak magnetic properties of the tube itself, the signal-to-noise ratio is low, and the depth of eddy current penetration is affected. In order to detect local corrosion damage and uniform corrosion of the heat exchanger tube wall thickness changes after the weak magnetic properties, the detection method is determined in the conventional eddy current detection probe design by adding a permanent magnet ring to magnetize the tube, eliminating the influence of the weak magnetism of the tube on the detection signal. At the same time the use of dual differential measurement system to measure the wall thickness of the tube (i.e., the use of two differential probe detection with the same specifications to obtain the sample tube and the workpiece under inspection comparison signal, measurement of the wall thickness of the tube) to achieve eddy current flaw detection and eddy current thickness measurement synchronization, without step-by-step detection, thereby improving the detection efficiency.

3. The choice of detection system

3.1 The choice of testing instruments

The choice of domestic EEC-35RFT portable all-digital computer multi-frequency eddy current instrument. The detector has multiple detection channels, can simultaneously detect with multiple operating frequencies, with the detection of the signal phase, amplitude analysis function and mixing function. At the same time with storage, access to all testing parameters, real-time display and other functions. Operation once can read the wall thickness change value, the impedance map of independent defects and other information.

3.2 System tuning settings

Before detection mainly includes the detection system parameters settings, probe and converter connection settings, comparison and adjustment of the relationship between artificial defects and signal in the comparison sample tube, calibration of the flaw detection defect curve and thickness change curve.

• (1) Calibration of the defect curve for eddy current flaw detection: the artificial defect calibration of the inner and outer walls of the tube A is selected for flaw detection. The use of conventional eddy current in the depth direction of the phase lag effect, in the detection parameters are set appropriately, by establishing a comparison of the sample tube on the different depths of artificial simulation of the impedance diagram and the depth of defects – phase diagram to produce calibration curve, used to distinguish the nature and type of defects in the wall of the tube, see Figure 1, Figure 2.
• (2) Eddy current thickness measurement with the calibration curve: the thickness of the selected comparison sample tube B for thickness change curve calibration. Prepare the two groups of comparison sample tube, a group of probes placed in the flawless sample tube A, another group of probes placed in the thickness of comparison sample tube B, through the two groups of probes he than the comparative signal obtained to determine the thickness change caused by the amplitude change, the production of thickness – amplitude calibration curve.

4. Detection results

Figure 3 eddy current flaw detection bar graph, Figure 4 thickness-probe bar graph, Figure 5 eddy current thickness measurement bar graph is the actual test mapping. From the graphical signal to see the detection effect to meet the requirements. The signal rules near the refractory plate are clear and unambiguous, and the signal-to-noise ratio of the amplitude curve is good. In the thickness curve can be seen on the wall wall thickness curve without obvious offset, that is, the wall thickness without thinning. From Figure 6 near the folded plate signal map can be seen in which the folded plate near the left side of the thickness measurement bar graph offset and flaw detection bar graph also appeared at a smaller signal display, but the offset amplitude is small, combined with the signal impedance graph and in the defect depth – impedance graph and thickness – amplitude graph to determine the size of the signal, analysis and judgment for the local range of wall thickness slightly Thinning corrosion, the maximum wall thickness thinning in less than 15%. According to the test results, the set of testing system in line with the relevant requirements of the purpose of the test, the instrument is stable in the testing process, to achieve the desired effect.

5. Conclusion

(1) Verifies that eddy current inspection in the detection of tube bundles with weak magnetic properties, the addition of magnetic saturation auxiliary parts in the design of the probe can effectively circumvent the impact of weak magnetic materials on eddy current detection data. Magnetic saturation methods can be achieved by adding permanent magnets or DC magnetization methods within the probe. However, attention should be paid to the magnetic field strength, the magnetism is too weak, the magnetic saturation effect is poor; the magnetism is too strong, it will lead to the probe and the wall of the tube is too large attraction, can not be detected properly.

Figure.1 Contrast sample tube defect impedance map (through-hole 40°)

Figure.2 Defect depth – phase diagram

Figure.3 Eddy current flaw detection bar graph and Figure.4 Thickness measurement – flaw detection bar graph

Fig.5 Eddy current thickness measurement bar graph

Figure.6 Signal diagram near the refractory plate
(2) The eddy current flaw detection and eddy current thickness measurement of the same tube bundle are carried out simultaneously, and the flaw detection calibration curve and thickness measurement calibration curve can be used to analyze and judge the inspection data respectively.
(3) The use of self-ratio differential probe for eddy current flaw detection of the tube bundle, the detection of abrupt defects of high sensitivity, while the use of two sets of differential probe other than to obtain a comparison signal for the detection of slowly changing wall thickness changes of high sensitivity. The use of two detection signals and the establishment of separate inspection channels to obtain inspection data can be more efficient to complete the inspection task. For the use of heat exchangers in service tube to provide more intuitive data.
(4) Eddy current detection is an effective way to compare the detection of heat exchangers in service tube detection, so in order to obtain more accurate detection results, detection before the heat exchanger should try to obtain the tube specifications, process media, corrosion, tube dirt and other conditions, select the appropriate comparison sample tube, detection probe, detection parameters and detection methods.
Author: Yu Qiang

Source: China Copper Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)