RU2714868C1 - Method of detecting pitting corrosion - Google Patents

Abstract

FIELD: defectoscopy.

SUBSTANCE: use to detect pitting corrosion (pitting) in controlled articles by directed acoustic waves. Summary of invention consists in the fact that using ultrasonic piezoelectric transducers designed to conduct ultrasound thickness gauging of pipeline wall, starting pulse is generated and reflected echo signals are received through a constant distance-scan interval, a digitized oscillogram of the received echo signal is amplified, digitized and transmitted to the on-board computer on each piezoelectric transducer, digitized oscillograms are accumulated and averaged over two or more sounding cycles in each scan, measuring the time interval between the front of the emitted direct ultrasonic signal and the front of the received reflected ultrasonic signal from the inner surface of the pipeline wall, which forms the first echo signal, calculating the energy of the first echo signal, measuring time interval between front of first echo signal and front of received reflected echo signal from outer surface of pipeline wall, forming second echo signal, calculating energy of second echo signal, comparing the energy of the second A-scan second echo signal with the previous A-scans second echo energy values, detecting the third echo signal in the interval between the first and second echo signals, in the presence of the third echo signal, the energy of the third echo signal is calculated, the time interval between the maximum energies of the second and third echo signals is measured, and the depth of pitting is determined.

EFFECT: enabling detection of pitting corrosion by ultrasonic transducers designed to conduct ultrasound thickness gauging of a pipeline wall.

3 cl, 8 dwg

Description

The invention relates to the field of non-destructive testing of pipelines, other metal structures and can be used to detect pitting corrosion (pitting) in controlled products by the method of directed acoustic waves.

State of the art

The prior art method for ultrasonic inspection of pipelines using a high-resolution ultrasonic thickness gauge device disclosed in the patent for invention RU 2554323 C1 (IPC: G01N 29/04, G01B 17/02, published 07.03.2018). The high-resolution ultrasonic thickness gauge method is based on the immersion method, which consists in the propagation of an ultrasonic pulse in liquid and solid media, as well as its reflection from the interface. Radiation and reception of ultrasonic pulses is carried out by piezoelectric transducers (hereinafter referred to as PEP). In the immersion method, ultrasonic pulses from the probe to the object propagate through the liquid. The data obtained when calculating the boundary of the beginning of the change in wall thickness in the form of an electric signal is fed to an analog-to-digital converter, and then processed in a logic unit, which, by the presence of additional and exceeding the threshold of echo pulses in the received diagnostic information, includes an analysis of signal amplitudes and initiates correction values are calculated, then the diagnostic data are corrected in order to determine the clear boundaries of the defect, and the corrected updated data are sent to ok information storage device.

Along with determining the wall thickness of the pipeline, this method can be applied to the diagnosis of some corrosion defects. However, the possibility of detecting pitting in this way is characterized by a low probability

The prior art also knows the method of inspection of oil and gas pipelines during their operation, protected by patent RU 2153163 C1 (IPC: G01N 29/04, G01N 29/10, publ. 20.07.2000). The method includes continuous movement of a flaw detector with an electro-acoustic transducer along the pipeline wall, generation of ultrasonic vibrations pulses by the electro-acoustic transducer, non-contact transmission of ultrasonic vibrations pulses through the pumped medium to the pipeline wall, excitation of ultrasonic vibrations in the pipeline wall, reflection of ultrasonic vibrations from inhomogeneities of the pipeline wall material, transmission of reflected ultrasonic vibrations from the pipe wall to an electro-acoustic transducer, recording reflected ultrasonic vibrations and determining from the results of measurements the nature, size and location of defects in the pipeline wall. This flaw detection method, based on a comparison of the amplitudes of the forward and reverse pulses, determination of the absolute value of the reflection coefficient and comparison with the reference data of the defect sizes by the thickness of the pipeline, also does not allow to detect discontinuities (pitting), the opening plane of which is not oriented normally to the direction of the acoustic axis of the transducer. The closest analogue of the claimed invention is the method of ultrasonic testing by surface methods, disclosed in chapter 9 of the monograph (Diagnostics of technical devices / [G. A. Bigus, Yu.F. Daniev, N.A. Bystrova, D.I. Galkin]. - M. : Publishing House of MSTU named after NE Bauman, 2014-615 s), based on the registration of changes in the parameters of elastic waves excited in a controlled object. When implementing this method, the control object is sounded by short pulses of ultrasonic vibrations excited using a piezoelectric transducer. Oscillations reflected from discontinuities are recorded by the same transducer; as a result, an echo signal appears on the screen of the flaw detector. The larger the size of the discontinuities, the greater the energy of the reflected oscillations and the amplitude of the echo signal.

The disadvantage of this method is the inability to identify those discontinuities, the opening planes of which are not oriented normally to the direction of control (the acoustic axis of the transducer), and are not able to form a reflected echo signal. For pitting, it is this arrangement of the disclosure plane that is characteristic.

In the given analogues, the amplitude criterion for defect detection is implemented, which assumes the presence of a signal detection threshold set with an excess of noise level of at least 6 dB or more. On the strongly corroded surface of metal structures, the reflected echo signal is scattered, which leads to a decrease in the amplitude of the echo signal. When the amplitude of the echo signal decreases below the threshold, the signal loss is fixed and, accordingly, the information on the state of metal structures (in particular, the pipeline) is lost in the controlled areas. For these reasons, in the selected analogues it is impossible to measure the pitting and determine its depth.

Pitting (pitting corrosion) is a typical stress concentrator, in the area of which there is a sharp increase in mechanical stresses, increasing the risk of pipeline failure, even in the absence of through damage. Pitting corrosion is assessed not by the loss of metal mass, but by the number of pitting per unit area, their diameter and depth. Therefore, the identification of pitting corrosion, given the small size of the pits and the complexity of their detection by known methods of ultrasonic thickness gauging, including the methods stated in the selected analogues, is a difficult task when conducting in-line pipe diagnostics.

Disclosure of the invention

The technical problem to which the invention is directed is the development of a method for detecting pitting corrosion by ultrasonic transducers designed to conduct ultrasonic thickness measurement of a pipe wall.

The technical result of the claimed invention is the provision of detection of pitting corrosion in the pipe wall by applying the energy criterion for detecting defects based on a comparison of the energy of the reflected signals from the pipe wall without a defect and with the presence of a defect, and determining the depth of the pitting.

The set of essential features sufficient to achieve the indicated technical result and defining the scope of legal protection of the present invention includes a method for detecting pitting corrosion using ultrasonic piezoelectric transducers designed to conduct ultrasonic thickness measurement of the pipe wall, while generating a triggering pulse and receiving reflected echo signals through constant distance interval - scan, amplify, digitize and transmit to the onboard The calculator for each piezoelectric transducer digitized waveform of the received echo signal, accumulate and average the digitized waveforms for two or more scoring cycles in each scan, measure the time interval between the front of the emitted direct ultrasonic signal and the front of the received reflected ultrasonic signal from the inner surface of the pipeline wall, forming the first echo, calculate the energy of the first echo; the time interval between the front of the first echo signal and the front of the received reflected echo signal from the outer surface of the pipe wall forming the second echo signal is measured, the energy of the second echo signal is calculated, the energy value of the second echo of the current A-scan is compared with the energies of the second echoes of previous A-scans, detect the third echo, in the interval between the first and second echoes, in the presence of the third echo, calculate the energy of the third echo, measure the time interval between the maxim s energies of the second and third echoes determine the depth of pitting.

In addition, the specified technical result is achieved in special cases of the invention due to the fact that:

- apply coherent accumulation of digitized waveforms of echo signals in one A-scan with subsequent averaging;

- form on the A-scan a sliding analysis window T _a , the width of which is determined based on the frequency of the ultrasonic signal according to the ratio:

T _a = p × 1 / F _s ,

where p is a coefficient that determines the width of the analysis window, equal to 0.5 ... 2.0;

F _s - the frequency of the ultrasonic sound signal, sec ^-1 ;

- calculate the energy of the echo signals with the subsequent determination of the maximum energy.

The implementation of the method

The claimed method is not based on the amplitude criterion for detecting defects in the pipeline, implemented in analogous methods, but the energy criterion, based on comparing the energy of the reflected signals from the pipeline wall without a defect and with the presence of a defect (pitting).

The implementation of the claimed method for detecting pitting corrosion is illustrated in FIG. 1-8 where

in FIG. 1 shows the design of a probe carrier with trajectories of ultrasonic signals;

in FIG. 2 shows a pitting shading zone of a portion of the area of a pipe wall sound zone;

in FIG. Figure 3 shows a graph of the calculated dependence of the relative value of shading on the pitting diameter for sound zones with a diameter of 6 mm and 8 mm;

in FIG. 4 shows a waveform of an echo signal obtained by ultrasonic scoring of a metal plate 12 mm thick without pit simulators;

in FIG. Figures 5-8 show oscillograms of echo signals obtained by ultrasonic scoring of a metal plate 12 mm thick with pit simulators with a diameter of 1.3 mm, 2.0 mm, 3.2 mm, 4.0 mm, respectively. The depth of pit simulators is 2.0 ... 2.5 mm.

In FIG. 1-8 the following notation is accepted:

1 - piezoelectric transducer (PEP);

2 - skid carrier PEP;

3 - wall of the pipeline;

4 - pitting;

5 - sound zone on the outer surface of the pipeline wall;

6 - sound zone on the inner surface of the pipeline wall;

7 - immersion medium;

8 - direct ultrasonic signal;

9 - reflected ultrasonic signal;

10 is a radiation pattern of the probe;

11 - the inner surface of the pipe wall;

12 - the outer surface of the pipe wall;

13 - shading zone pitting;

14 - pitting diameter;

15 - diameter of the sound zone of the pipeline wall;

16 - coordinate K (coefficient of relative shading of the sound zone);

17 - coordinate D _p (D _p - pitting diameter, in mm);

18 is a graph of the calculated dependence of the relative shading of the scoring zone on the pitting diameter for the scoring zone with a diameter of 6 mm;

19 is a graph of the calculated dependence of the relative shading of the scoring zone on the pitting diameter for the scoring zone with a diameter of 8 mm;

20 - echo signal from the internal (relative to the direction of direct ultrasonic signals) surface of the plate (first echo signal);

21 is the magnitude of the energy;

22 - echo signal from the outer surface of the pipe wall (second echo signal);

23 - an echo signal from the outer surface of the plate with the presence of a pitting simulator in the scoring zone (third echo signal);

24 - echo from the flat top of the pit simulator.

The technical result is realized by a method that is based on the immersion method of ultrasonic thickness measurement, which consists in the propagation of an ultrasonic pulse in liquid and solid media, as well as its reflection from the interface. In the immersion method, ultrasonic pulses are emitted and received by piezoelectric transducers 1 (Fig. 1), mounted on a skid of the probe 2 carrier, and propagate to the sounding object through the immersion medium 7, which is a liquid pumped through the pipeline. To determine the wall thickness of the pipeline 3, the probe 1 is located around the entire circumference of the wall of the pipe 3 on the skid of the carrier of the probe 2, while the acoustic axis of the probe 1 are located normal to the axis of the pipeline. Ultrasonic pulses, forming direct ultrasonic signals 8 and reflected ultrasonic signals 9, in addition to solving thickness gauging problems, are used to detect pitting 4 on the inner surface 11 or outer surface 12 of the pipe wall 3.

The design of the carrier of the probe 1 (Fig. 1) provides the location of the emitting and receiving surface of the probe at a certain distance from the inner surface of the wall of the pipe 11. The radiation pattern 10 of the probe 1 provides the formation of sound zones 5, 6 on the walls of the pipeline.

The cavity between the probe 1 and the inner surface of the pipe wall 11 is filled with an immersion medium 7. The ultrasonic pulse emitted by the probe 1 reaches the surfaces of the pipe 3 wall. After partial reflection from the defect on the inner surface 11 or the outer surface 12 of the pipe wall, the reflected ultrasonic signals 9 enter PEP 1, where they are converted into an electrical signal.

In this case, the process of generating a triggering pulse and receiving reflected echo signals is carried out at a certain constant distance interval — a scan, which is generated by electronic equipment located in the electronics section of the ultrasonic thickness gauge device (not shown in Fig. 1-8). Using electronic equipment, amplify, digitize and transmit to the on-board computer (not shown in Fig. 1-8) for each probe 1 digitized waveform (Fig. 4-8) of the received echo signal, where A-scan is generated by accumulation and averaging digitized waveforms obtained for N scoring cycles in this scan (N is an integer equal to or more than two).

Then in the on-board computer produce:

- measuring the time interval between the front of the emitted direct ultrasonic signal 8 and the front of the received reflected ultrasonic signal 9 from the inner surface of the pipe wall 11 (first echo signal);

- calculation of the energy of the first echo signal;

- measuring the time interval between the front of the first echo and the front of the received reflected echo from the outer surface of the pipe wall 12 (second echo);

- calculation of the energy value of the second echo signal;

- comparison of the energy values of the second echo signal, the current A-scan with the energy values of the second echo signals of the previous A-scans;

- detection of a third echo, in the interval between the first and second echoes;

- in the presence of a third echo signal, the calculation of the energy value of the third echo signal;

- measurement of the time interval between the maximum energies of the second and third echo signals.

The practical implementation of this method is carried out by finalizing the signal processing paths of existing in-line flaw detectors operating in the mode of measuring wall thickness. Refinement can be carried out by adding to the traditional signal processing path a parallel channel that implements the proposed energy method for detecting pitting.

The claimed method differs from the prototype method in that when pitting 4 gets into the sounding zones 5,6, a part of the sounding zone is obscured by the pitting on the outer surface of the wall 12 or on the inner surface of the pipe wall 11, reducing the energy of the reflected echo signal.

In FIG. Figure 2 shows the shading zone by pitting 13 of the area of the sounding zone on the outer surface of the pipe wall 5, the diameter of the pitting 14 being less than the diameter of the sounding zone of the pipe wall 15, which is typical for pitting corrosion.

The third echo signal may be formed in the interval between the first and second echo signals when the energy of the ultrasonic pulse is reflected from a flat top or bottom of the pitting 4. The decrease in the energy of the second echo is proportional to the pitting area with the pitting diameter 14 (Fig. 2), which is located in the voicing zone with a diameter of 15 (Fig. 2) on the outer surface of the wall of the pipeline 12 or on the inner surface of the wall of the pipeline 11.

In FIG. Figure 3 shows the graphs of the calculated dependences 18, 19, plotted in the coordinates "relative shading coefficient 16 - pitting diameter 17" for two diameters of sound zones 6 mm and 8 mm.

The relative shading coefficient is determined by the formula:

where K is the relative shading coefficient;

- площадь зоны озвучивания, мм²;

- the area of the scoring zone, mm ² ;

- площадь зоны затенения питтингом, мм².

- the area of the shading zone by pitting, mm ² .

Considering that pits can have various forms, the level of reflected signals can be commensurate with the noise of the receiving path; therefore, to detect small signals, it is necessary to use coherent accumulation of N digitized waveforms of echo signals in one A-scan followed by averaging. This will increase the signal-to-noise ratio and, with a high probability, detect echoes from pits. In the presence of a third echo, the pitting depth is determined by the formula:

where H _pt - pitting depth, mm;

V is the speed of ultrasound in the metal, mm / s;

t is the time interval between the maxima of the energy of the echo pulses of the second 22 and third echo signals 23.

The energy of the echo pulses of the received echo signals is calculated by the formulas:

where A _i is the amplitude of the echo signal in the i-th sampling step, V;

T _a - width of the analysis window of the echo signal (analysis time, during which the calculation of the energy of the echo signal is performed), s;

F _s is the frequency of the ultrasonic sound signal, s ^-1 ;

i = 1 ... m is the number of samples in the analysis window;

δ is the sampling step of the echo signal, s;

p - coefficient determining the width of the analysis window.

To reduce errors from echo sampling, the sampling frequency should be 8 - 10 times higher than the probe working frequency. The coefficient p is changed when tuning the system to select the optimal width of the analysis window.

In the process of receiving and sampling echo signals in one scan, N arrays (oscillograms) of echo signals are generated, which are stored in the on-board computer. Next, coherent accumulation and averaging of N arrays and the formation of one A-scan are performed. In the obtained A-scan, a sliding window (analysis window) is formed, including m - discrete values, in which the energy E is calculated by the formula (3). The resulting value is recorded in the memory of the calculator with an arrival time equal to the middle of the analysis window. Then the window is shifted by one step of quantization δ of the echo signal and the process of calculating energy with writing to the on-board computer is repeated. After completing the process of calculating energy over the entire time interval of scoring, a search is made of the energy maxima in the obtained “energy” A-scan, which are the energy centers of the echo signals from the inner surface of the pipe wall 11 and from the outer surface of the pipe wall 12, and in the presence of pitting 4 - echo from its flat top. All received data is recorded in the on-board drive for further analysis.

The energy criterion works effectively even when the amplitude signal-to-noise ratio is 1, since the spectral density of noise in the analysis window T _a will be less than the spectral density if there is a signal with noise in the analysis window. This is because the width of the echo analysis window T _{a is} optimally selected for receiving the echo signal. For example, when the frequency of the probing direct ultrasonic signal is 4 MHz (T _s = 0.25 μs), the maximum width of the analysis window T _a will be 0.5 μs. In addition, in the energy method, the use of coherent accumulation of N arrays of echo signals further increases the signal-to-noise ratio.

In the general case, the width of the analysis window T _a is determined by relation (4) based on the frequency of the ultrasonic signal and the coefficient p, the value of which is determined by the degree of damping of the probe crystal.

Currently, probes for thickness gauging in in-line diagnostics operate at frequencies from 2 MHz to 10 MHz.

A method for detecting pitting is to compare the energy values of the echo signals in previous A-scans with the corresponding energy values obtained in the last scan. In the presence of pitting 4, for example, on the outer surface of the wall of the pipeline 12, the energy of the echo signal from the inner surface of the wall of the pipeline 11 will not change, and the energy of the echo from the outer surface of the wall of the pipe 12 as a result of shading by pitting 4 will decrease in proportion to the increase in the diameter of the pitting 14. When the presence of a third echo, you can determine the depth of the pitting by the formula (2).

Oscillograms of echo signals (Figs. 4–8) illustrate the implementation of the proposed method by the example of an experiment to detect pitting in a metal plate 12 mm thick without pitting and with pit simulators of different diameters applied to it. (1.3 mm, 2.0 mm, 3.2 mm, 4.0 with a constant depth of pit simulators equal to 2.0 ... 2.5 mm).

In the absence of a pit simulator, the values of the energy of the echo signal from the inner surface of the plate 20 and the echo signal from the outer surface of the plate 22 practically do not differ from each other (Fig. 4), since there is no shading zone with pitting 13.

On the oscillograms of the echo signals (Fig. 5-8), the echo signal from the flat top of the pit simulator 24 is clearly fixed, which allows determining the location of the defect. The energy value 21 of the echo signal from the outer surface of the plate with the presence of a pitting simulator in the scoring zone 23 proportionally decreases with an increase in the diameter of the pitting simulator, which makes it possible to determine the size of the defect.

An analysis of the experimental results shows that the proposed method for detecting pitting corrosion, based on the energy analysis of echo signals, allows to detect pits that are not determined by known methods of ultrasonic thickness gauging, and to determine their depth.

Claims (7)

1. A method for detecting pitting corrosion using ultrasonic piezoelectric transducers designed for ultrasonic thickness measurement of a pipe wall, characterized in that they generate a triggering pulse and receive reflected echo signals at a constant distance interval - scan, amplify, digitize and transmit to the on-board computer by each piezoelectric transducer digitized waveform of the received echo signal, the accumulation and averaging of digitized waveforms for two or more scoring cycles in each scan, measure the time interval between the front of the emitted direct ultrasonic signal and the front of the received reflected ultrasonic signal from the inner surface of the pipeline wall, forming the first echo signal, calculate the energy of the first echo signal, measure the time interval between the front the first echo and the front of the received reflected echo from the outer surface of the pipe wall, forming the second echo, calculate the energy of the second echo, cf they take into account the energy of the second echo of the current A-scan with the energies of the second echoes of the previous A-scans, detect the third echo, in the interval between the first and second echoes, in the presence of the third echo, calculate the energy of the third echo signal, measure the time interval between the maximum energies of the second and third echo signals, determine the depth of the pitting. 2. The method according to p. 1, characterized in that the coherent accumulation of digitized waveforms of echo signals in one A-scan is used, followed by averaging. . 3. A method according to claim 1, characterized in that formed on the A-scan moving analysis window T _and whose width is determined based on the frequency of the ultrasonic signal by the relation: T _a = p × 1 / F _S , where p is a coefficient that determines the width of the analysis window, equal to 0.5 ... 2.0; F _s is the frequency of the ultrasonic sound signal, s ^-1 . 4. The method according to p. 1, characterized in that they calculate the energy of the echo signals with the subsequent determination of the maximum energy.

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