JPH0628693Y2 - Eddy current flaw detector for fin tube with land - Google Patents

Description

[Detailed Description of the Invention] [Industrial field of application] The present invention relates to an eddy current flaw detector for a fin tube, which is widely used as a metal tube for a heat exchanger, etc.

It is difficult to apply the ultrasonic method to the flaw detection on the outer surface of such a fin tube because of the presence of fins, and the eddy current flaw detection method, particularly the through-coil method is usually applied. In the eddy current flaw detection method using this through-coil, two coils for applying high frequency are arranged concentrically at intervals slightly smaller than twice the fin pitch, fitted onto the fin tube, and moved relative to each other in the axial direction. The presence / absence of a defect is detected by detecting the impedance difference between the two coils that occurs in the portion in which there is.

[Problems to be solved by the device]

By the way, in the fin tube, there are no fins at predetermined intervals in the axial direction for fixing to the support during assembly,
A so-called land portion is formed. When such a fin tube with land is formed by using a rolling roll, the outer diameter of the pipe changes rapidly from the fin portion to the land portion and from the land portion to the fin portion. A tube diameter variation portion is formed. Therefore, when eddy current flaw detection is performed by the through coil method as described above, a large impedance difference is generated between the two coils in the portion where the pipe diameter changes abruptly, which causes a defect that a defect signal in this portion cannot be detected. there were.

As a countermeasure against this, it has been attempted to pass currents of different frequencies through the cross-type feedthrough coils to eliminate the outer diameter variation signal and extract the defect signal. However, this method conversely has a problem that the flaw detection sensitivity of the surface flaw on the land portion is lowered.

The present invention has been made in view of such circumstances, and its purpose is to eliminate a signal associated with a change in pipe diameter for a portion where the outer diameter changes abruptly, and to detect and detect only a defect signal. An object of the present invention is to provide an eddy current flaw detection device for a fin tube with a land, which is capable of performing flaw detection with high sensitivity even on a surface flaw in a land portion.

[Means for Solving the Problems]

A flaw detection apparatus according to the present invention comprises two coils arranged to cross each other, a cross-type coil for applying a voltage of a different frequency and a non-cross-type coil for applying a voltage of a single frequency. From the non-intersecting type coil by detecting the outer diameter variation portion between the land portion and the fin portion, the flaw detection head portion including the coil of Means for controlling on / off of the output of the detection signal.

[Action]

According to the present invention, therefore, the fin portion and the pipe diameter variation portion between the fin portion and the land portion are detected by the cross type coil, and the land portion is detected by the cross type coil and the non-cross type coil. It becomes possible to do.

〔Example〕

Hereinafter, the present invention will be specifically described with reference to the drawings showing an embodiment thereof.

FIG. 1 is a schematic diagram showing an eddy current flaw detector for a finned tube with a land according to the present invention (hereinafter referred to as “device of the present invention”).
In the figure, H indicates a flaw-finding portion, and P indicates a finned tube with lands, which has land portions P ₂ on the fin portion P ₁ at predetermined intervals.

The flaw detection head portion H is provided with two cross-type through coils 1 and 2 and a non-cross-type through coil 3, that is, a parallel type through coil 3 concentrically with each other at a required interval in this order. It is fitted and inserted into the outer circumference of the shaft, and relative to this, it can be relatively moved in the axial direction. The coils 1a, 1b, 2a, 2b of the double cross type through coils 1 and 2 are made to intersect each other at two points in the circumferential direction, and the crossing positions of the coils are arranged so as to be displaced by approximately 90 ° in the circumferential direction. This is because in the cross type through coils 1 and 2, the flaw detection sensitivity is lowered at the portions where the coils intersect, so that the low sensitivity portions do not overlap.

FIG. 2 is a diagram showing the sensitivity characteristic of the cross-type through coil, and the sensitivity characteristic of the portion (the upper and lower portions in FIG. 1) where the coils do not intersect with the center O of the cross-type through coil 1. Is 100%, the sensitivity characteristics of the parts where the coils cross each other (left and right parts) are about 50%, so to cover this, the cross-type feedthrough coils 1 and 2 have two sets of them. The coils are arranged with their crossing points displaced in the circumferential direction (usually 90 °).

Each coil 1a, 1b, 2a, in the cross type through coil 1, 2
The dimension between the coils 1a and 1b in the area where 2b does not intersect, the dimension between 2a and 2b, and the dimension between the coils 3a and 3b of the parallel type through coil 3 are both more than twice the fin pitch of the fin portion P _1. Is also set to a slightly smaller value.

FIG. 3 is a block diagram showing flaw detection circuits A, B, and C of the cross-type through coils 1 and 2, and the parallel-type feed-through coil 3, respectively. , B are substantially the same. 11a and 11 in flaw detection circuits A and B
b, 21a and 21b respectively indicate oscillators, and oscillators 11a and 21a
Of the frequency ₁ and the oscillators 11b and 21b output high frequency signals of the frequency _{2 to} the cross-type through coils 1 and 2. The high frequency signals are combined and amplified by the amplifiers 12 and 22, and are input to the cross-type through coils 1 and 2 via the bridge circuits 13 and 23, whereby eddy currents are formed on the surface of the fin tube with land 1. It The pair of coils forming each of the cross-type through coils 1 and 2 are connected so as to form the opposite sides of the bridge circuits 13 and 23, respectively, and are maintained in a balanced state at the fin portion P ₁ , and the fin tube with land P If there is a defect or dimensional change in the coil, the balanced state is disturbed by the accompanying impedance change of each coil, and the unbalanced signal at that time is amplified by the amplifiers 14 and 24 and the band pass filter 15a, 1
Input to 5b, 25a, 25b.

Bandpass filter 15a, 25a is the frequency ₁ and the band-pass filter 15b, 25b is configured to pass only frequencies _2, frequency ₁ which has passed through this, ₂ signals synchronous detector 16a, 16b, 26a , 26b.

Each synchronous detector 16a, 16b, 26a, 26b is an oscillator 11a, 11b, 2 respectively.
The signals from the bandpass filters 15a, 15b, 25a, 25b are detected in synchronization with the signals input from 1a, 21b, and the frequency ₂ signals of the synchronous detectors 16b, 26b are directly subtracted by the subtractors 19, 29, respectively.
To one input terminal of each, and the frequency of each synchronous detector 16a, 26a
The signal _{No. 1} is output to the other input terminals of the arithmetic units 19, 29 via the phase converters 17, 27 and the amplifiers 18, 28, respectively.

The phase converters 17, 27 and the amplifiers 18, 28 are the synchronous detector 16a,
As shown in FIG. 4 (c), the outer diameter variation signal in the Y-axis direction for the finned tube with land P related to frequency ₁ as shown in FIG. Then, the outer diameter variation signal in the Y-axis direction related to frequency ₂ shown in FIG. 4D is adjusted approximately and output to the subtracters 19 and 29.

In the subtracters 19 and 29, the outer diameter variation signals obtained by the pair of coils 1a, 1b, 2a and 2b forming the paired cross-type through coils 1 and 2 are subtracted from each other, and as shown in FIG. As shown, the outer diameter variation signal is output in a deleted state.

On the other hand, the defect signal has different magnitude and phase when the flaw detection frequency is different, so it is not erased even if the subtraction process is performed, and only the unnecessary signal is erased and a signal with high sensitivity S / N is obtained. It will be.

On the other hand, also for the parallel type through coil 3, the signal of the frequency ₃ is amplified from the oscillator 31 by the amplifier 32 and input through the bridge circuit 33. The parallel type through coil 3 is composed of a pair of coils arranged on the same axial center line with a required interval, and when energized, an eddy current is formed on the surface of the finned tube with land P. The balanced state is maintained in the land portion P ₂ , and if there is a defect or the like, the impedance of the coil changes and the bridge circuit 33 enters the unbalanced state, and the unbalanced signal is input to the synchronous detector 36 via the amplifier 34. To be done. The synchronous detector 36 synchronously detects the signal from the bridge circuit 33 in synchronization with the signal from the oscillator 31, and the gate circuit
It is designed to output to 37.

In the gate circuit 37, the output signal of the synchronous detector 16b in the flaw detection circuit A of the cross type through coil 1 is supplied to the comparator 3
8. The ON / OFF signal of the gate circuit 37 is input through the timing circuit 39. That is, the gate circuit 37 is normally set to the off state, and when the cross-type through coil 1 balanced in the fin portion P ₁ reaches the outer diameter variation portion and a large outer diameter variation signal is output, The comparator 38 compares it with a predetermined reference value (see FIG. 4 (d)), and if it exceeds this, it is judged as an outer diameter fluctuation signal at the time of transition from the fin portion P ₁ to the land portion P ₂ , and the parallel type The gate circuit 37 is turned on at the timing when the through coil 3 reaches the outer diameter variation portion, and the flaw detection signal by the parallel through coil 3 is output to start the flaw detection of the land portion P ₂ and the outer diameter variation signal is output. Becomes smaller than the reference value, the land portion P ₂ to the fin portion P ₁
It is judged that it is the outer diameter fluctuation signal at the time of the transition to, and the gate circuit 37 is turned off at the same timing as the parallel type penetration coil 3 reaches the outer diameter fluctuation part, and the flaw detection signal from the parallel type penetration coil 3 is again detected. The output is stopped and the flaw detection of the land portion P ₂ by the parallel type through coil 3 is completed.
As a result, flaw detection is performed by the cross-type through coils 1 and 2 over the entire length of the finned tube P with land, and flaw detection is also performed by the parallel through coil 3 for the land portion P ₂ .

Thus, in such a device of the present invention, eddy current flaw detection over the entire length of the finned tube P with land is caused by the two cross-type through coils 1 and 2, and by the non-cross type, that is, the parallel type through coil 3. , The land portion P ₂ is subjected to eddy current flaw detection.

As a result, if a non-intersecting through coil 3 is used for the outer diameter variation portion, an excessive signal due to the outer diameter variation is generated and it is difficult to detect flaws. On the other hand, since the output of the non-crossing type through coil 3 is stopped and the flaw detection is performed by the two crossing type through coils 1 and 2, the outer diameter variation signal is erased and only the defect signal is extracted. It is possible to perform flaw detection without any trouble, and when the crossed type through coils 1 and 2 are used in the land portion P ₂ , the outer diameter signal is subtracted and erased. Although a part of the data is erased and the sensitivity is lowered, in the device of the present invention, the land portion P _{2 is also} subjected to flaw detection using the non-intersecting type through coil 3, and therefore by using this signal. obtained perform flaw detection of high sensitivity to the land portion P ₂ And thus.

In the above-described embodiment, the structure is described in which the flaw detection hend portion H is fitted to the land-attached fin tube P and the flaw detection is performed while moving the flaw detection portion H relatively. Needless to say,

〔effect〕

As described above, according to the present invention, it is possible to detect flaws in the outer diameter varying portion between the land portion and the fin portion while the outer diameter varying signal is erased by using the cross type coil, and to the land portion. The present invention has excellent effects such as highly sensitive flaw detection using a non-intersecting type coil, and highly accurate flaw detection efficiently over the entire length of the finned tube with land.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a coil arrangement mode of the device devised by the book,
FIG. 2 is a distribution diagram of flaw detection sensitivities of the cross-type through coil,
FIG. 4 is a signal processing block diagram of the device of the present invention, and FIG. 4 is an explanatory view showing signal processing contents. 1, 2 ... Cross-type through coil, 3 ... Parallel through coil, 11a, 11b ... Oscillator, 33 ... Bridge circuit

Claims (1)

[Scope of utility model registration request] 1. A device for eddy-current flaw detection of a finned tube with a land, wherein a fin portion having a fin formed on the outer periphery and a land portion having no fin formed thereon are alternately arranged. Flaw detection head unit including a plurality of coils, each of which applies a voltage of a different frequency to each other, and a non-crossing coil that applies a voltage of a single frequency, and detection signals of the respective coils of a crossing type And a means for detecting the outer diameter varying portion between the land portion and the fin portion and controlling ON / OFF of the output of the detection signal from the non-intersecting type coil. Eddy current flaw detector for fin tubes with land.