Oxygen free copper tubes, copper tubes and copper alloy tubes, which contain a large proportion of copper, are collectively referred to as copper tubes. They have excellent electrical conductivity and thermal conductivity. Therefore, this type of copper pipe is often used in various parts and components of power plants, heat exchanger tubes in chemical equipment, and cooling pipes in air conditioning units.
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(1) method and device of flaw detection

The eddy current testing methods for copper and copper alloy tubes are classified according to the types of probes used. There are three kinds of through coil method, rotary point probe method and inserted coil method.

Because the copper and copper alloy tubes have higher production speed (100 meters / fraction), they are especially suitable for the eddy current inspection of the through method. In addition to the finished stage of flaw detection, because the production process of the pipe size changes, material temperature changes and so on the impact of flaw detection is very small, so it is easy to carry out flaw detection in the intermediate stage.  Detection of intermediate stage is generally performed before cutting after heat treatment. The self testing coil is usually used in the through flaw detection of copper and copper alloys. The coil has low sensitivity to detect the long strip defects along the pipe line.

The through flaw detector includes: eddy current probe, eddy current flaw detector, marking device, alarm, good or bad pipe separating mechanism. If the inspection leave production line, should also include the transmission device and the loading mechanism etc..

(2) contrast sample

Artificial defects in standard sample tubes used for eddy current testing of copper and copper alloy tubes are usually subject to national standards.  Through the use of artificial defect detection is mostly through hole or semi through hole.

For general penetration eddy current testing, it is necessary to determine the detection parameters, such as detection speed, filling factor, detection frequency, amplifier gain, detection phase, filtering frequency band, alarm power equality. For the eddy current testing of copper and copper alloy tubes, due to their material properties and higher detection speed, it is particularly important to select the frequency and filling factor in the above conditions.

Selection of flaw detection frequency

Inspection frequency and defect detection sensitivity of the relationship between the larger, in the choice of testing frequency, should be considered in addition to detection of defect location (inner or outer wall), shape and size, but also consider the factors of detection coil length, detection speed etc..

Experience tells us, for copper and copper alloy pipe inspection, in the use of through the coil, the detection frequency is generally selected in the range of 1~100kHz.

The inspection frequency of copper and copper alloy tubes with different diameters and thicknesses is also different when the cross method is used for flaw detection. In general, the larger the diameter, the wall is thicker, the frequency of use is low, whereas the more high. Table 1 recommended some testing frequency of different diameter and wall thickness of copper tube used for reference.

Table 1 Relationship between diameter, wall thickness and frequency

The diameter (mm) wall thickness (mm) and frequency (kHz)
Less than 20 is less than 1.516
Less than 20 is greater than 1.58
More than 1.58 20 - 28
More than 1.54 2850

Note: the diameter, wall thickness and frequency described above are suitable for on-line flaw detection, while the detection frequency of in-service flaw detection is several times higher than that of good phase separation angle.



Selection of filling coefficient

In general, the greater the filling coefficient, the detection sensitivity is high, but the possibility of abrasion surface of the pipe is also larger. So the best choice to fill coefficient, diameter and pipe flaw detection sensitivity, detection speed, pipe bending and other factors. For copper and copper alloy tube, as the testing speed is higher, the general standard is enough to reach 60%. Table 2 gives the copper and copper alloy of different diameter pipe selection fill rate reference value.

Table 2 coil diameter selection

Do (mm) D-do (mm)
< 101.5 below
10-15 2 below
15-25 2.5 below
25-35 3 below
35-50 4 below

D: coil inner diameter do: outer diameter of pipe

(4) influencing factors

Generally, there are many factors affecting the eddy current testing results. The changes of the material, the size of the workpiece and the detection coil, the shape and location of the defect, the flaw detection condition and so on, all affect the correct evaluation of the results of the flaw detection. In the eddy current testing of copper and copper alloy tubes, most of them are the through coil method, and the various influencing factors are briefly described as follows:

Defects: including the depth, length and width of the defect, the location of the defect (the inner surface, the outer surface), the types of defects (holes, slots), etc..

Material: copper and copper alloy pipe material for eddy current testing is mainly reflected in the electrical conductivity, the same alloy components in the material, segregation, residual stress, etc., will lead to differences in conductivity.

The tube size and filling coefficient: diameter change has a direct impact on the size of the filling rate.

The wall thickness of noise signal caused by the change of wall thickness of copper tube.

The relative position between the tube and the detecting coil: when the copper tube passes through the inside of the through coil, it is inevitable that the vibration occurs. This vibration will change the relative position between the tube and coil.

The speed fluctuation: eddy current testing of copper and copper alloy tubes to 100 meters per minute or higher speed. When the velocity changes also affect the defect indication. For example, the speed of transmission in the case of 120m/min, the velocity fluctuation should be no greater than 10%.