CN108680607A - Pipeline crack corrosion monitoring process based on multi-communication potential drop - Google Patents

Abstract

The present invention provides the pipeline crack corrosion detecting method based on multi-communication potential drop, present invention firstly provides multidirectional current methods, it is passed through exciting current from four different directions, obtain four groups of voltage ratios, according to the relationship of electrode direction of check and voltage ratio, maximum voltage ratio is selected to substitute into depth solving formula, solution obtains the depth of crackle.The multidirectional current method of the present invention makes crack position and angle (complementary angle) range of electric current pole line become 67.5 90 degree from 0 90 degree, significantly improves the precision of crack depth solution.

Description

Pipeline Crack Corrosion Monitoring Method Based on Multidirectional AC Potential Drop

technical field

The invention belongs to the technical field of pipeline detection, and in particular relates to a pipeline crack corrosion monitoring method based on multidirectional alternating current potential drop.

Background technique

Pipeline corrosion is the most dangerous of many risks faced by oil and gas pipelines, and 70%-90% of pipeline safety accidents are caused by pipeline corrosion.

At present, commonly used pipeline corrosion detection technologies, including the relatively advanced Field Signature Method (FSM), only have high detection accuracy for localized corrosion and general corrosion; the latest AC field fingerprint technology Alternating Current Field Signature Method (ACFSM), using the measurement data of multiple frequency points to evaluate the corrosion depth, can improve the detection accuracy of crack corrosion, but this method requires that the angle between the crack direction and the current electrode connection direction is greater than 45 degrees, and cannot To solve cracks with an included angle of less than 45 degrees, the connection direction of current excitation electrodes is parallel to the pipeline axis, which will limit the detection range of crack defects.

Therefore, in order to meet the actual detection requirements, it is extremely necessary to study the random crack corrosion detection technology in depth.

Contents of the invention

The object of the present invention is to solve the above-mentioned defects in the prior art, and provide a pipeline crack corrosion detection method based on multi-directional AC potential drop. The included angle (complimentary angle) ranges from 0-90 degrees to 67.5-90 degrees, which can detect the corrosion depth of random cracks more accurately.

The present invention adopts following technical scheme:

A pipeline crack corrosion detection method based on multi-directional AC potential drop, the steps include:

Step 1. Weld and arrange several groups of test electrodes on the circumference of the test pipeline.

Step 2. The signal generator generates an excitation signal. The excitation current is generated through the power amplifier, and the excitation current is sequentially passed to the input electrode from four different directions. The frequency of the power amplifier can be adjusted, and the amplitude of the excitation current can be adjusted.

Step 3. Use a high-precision lock-in amplifier to measure the voltage of a normal pipeline without cracks, obtain the test voltage value of the normal pipeline and input it into the computer, then use a high-precision lock-in amplifier to measure the voltage to be tested in the area to be tested in the test pipeline, and input the measurement results Computer, get four groups of different voltage ratios;

Step 4. According to the relationship between the crack direction and the voltage ratio, select the largest voltage ratio and substitute it into the following depth solution formula, and use a computer to solve it:

Step 5. Obtain the depth of the pipeline crack according to the results displayed by the computer.

A further technical solution is that in step 2, the four groups of test electrodes arranged in the circumferential direction refer to the horizontal direction, the vertical direction, and the diagonal direction, wherein the input electrodes and the output electrodes are arranged oppositely, and their connections all pass through the parts to be tested .

A further technical solution is that in step 3, when using a high-precision lock-in amplifier to test the voltage of the area to be tested, the connection between the input electrode for inputting the excitation current and the output electrode for outputting the excitation current is consistent with the test direction of the measurement probe connection.

The preferred technical solution is that the frequency of the power amplifier is selected to be 59-500 Hz, and the amplitude of the excitation current is selected to be 2A.

The preferred technical solution is that in step 2, the frequency of the power amplifier is selected to be 100 Hz.

Beneficial effects of the present invention:

1. The present invention proposes the multi-directional current method for the first time, and the excitation current is fed from four different directions to obtain four sets of voltage ratios. According to the relationship between the direction of the electrode crack and the voltage ratio, the maximum voltage ratio is selected and substituted into the depth solution formula; the present invention The multi-directional current method makes the angle (complimentary angle) between the crack position and the current electrode line changed from 0-90 degrees to 67.5-90 degrees, which significantly improves the crack depth and solution accuracy.

2. Since the present invention adopts the principle of AC potential drop technology, it has the advantages of small excitation current, high measurement sensitivity, and strong anti-interference ability.

3. Compared with the traditional potential drop technology ACFSM using a single excitation current parallel to the pipeline axis, the method of the present invention quantitatively solves the depth of random crack defects for the first time. According to the results of simulation and experimental data, the current flows through the crack defects When , the direction of the crack will affect the distribution of the current field, thereby changing the voltage of the measuring electrode.

Description of drawings

Figure 1 is a schematic diagram of skin current distribution;

Fig. 2 is the voltage distribution diagram at different thicknesses tested in Table 1;

Figure 3(a), Figure 3(b), Figure 3(c), Figure 3(d), and Figure 3(e) are schematic diagrams of five crack directions and electrode connections;

Figure 4 is the ratio depth map of cracks at different angles;

Fig. 5 is the schematic diagram of multidirectional current measurement of the present invention;

Fig. 6 is the maximum ratio depth diagram of two kinds of crack defects in the simulation test.

Fig. 7 is the experimental setup figure of the inventive method;

Figure 8 is a schematic diagram of the layout of the metal flat probe;

Figure 9 is a schematic diagram of the back of the experimental plate.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention clearer, the following technical solutions in the present invention are clearly and completely described. Obviously, the described embodiments are some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

Explanation of terms:

1.1, skin effect

The alternating current (AC) of different frequencies is passed through the pipeline, and the current distribution follows the skin effect. Calculation formula of skin depth δ:

In the above formula, μ _r is the relative magnetic permeability of the material; μ ₀ is the vacuum magnetic permeability; σ is the electrical conductivity of the material; f is the frequency of the excitation current. The schematic diagram of current distribution is shown in Fig. 1.

It can be seen from formula (1) that when a high-frequency excitation current is applied, the current concentrates on the outer wall of the pipe, and as the frequency of the excitation current decreases, the current will gradually penetrate into the inner wall, and the current density J(r) in the pipe wall satisfies the formula:

in,

In the formula, I is the current amplitude with angular frequency ω; R is the outer radius of the pipeline; r is the radial distance; J ₀ (kr) is the zero-order Bessel function of the first kind, and J ₁ (kR) is the first A class of first-order Bessel functions.

According to the relationship between current density and voltage, the expression of voltage value U(r) is:

where l is the distance between the measuring electrodes.

The Bessel function in formula (3) is approximately replaced by an exponential function, then the voltage amplitude can be expressed as:

The contact depth between the measuring electrode and the outer wall of the pipeline is d ₀ =Rr≈0.5mm. At this time, after the excitation current is applied, the voltage amplitude U measured by the measuring electrode can be expressed by formula (5):

The wall thickness of the pipeline is T, and the original voltage value measured before corrosion is U _(d=0) . After the pipeline is put into production, the actual measured voltage value is U _(d) . When the corrosion defect on the inner wall of the pipeline is extremely shallow (d≈0), as the frequency decreases, it always satisfies:

U _(d) /U _(d=0) ≈1;

If there is a uniform corrosion defect with a depth of d at the bottom, then when δ=Td, the measured voltage value U _(d) remains unchanged, U _(d) /U _(d=0) ≈m/U _(d=0) (m is a constant); and when the defect is a crack defect, when δ=Td, as the frequency decreases, the current around the defect "layer" will penetrate downward, U _(d) <m, U _(d) / U _(d=0) <m/U _(d=0) .

Therefore, the U _(d) /U _(d=0) value of crack defects can be approximated by the linear superposition of U _(d) /U _(d=0) under the two limit cases (no corrosion defects, uniform corrosion defects):

In the formula, a ₁ , a ₂ and a ₃ are constants related to the properties of the measured material.

1.2, multi-directional current

A finite element analysis is performed on a pipe with a length of 400mm, an inner diameter of 140mm, and an outer diameter of 160mm. The finite element software is COMSOL Multiphysics 5.0. The measurement area is located in the middle of the pipeline, the probe spacing is 20mm, the injection current amplitude is 2A, and the defect depth is 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, 5.5mm, 6mm.

The material parameters are shown in Table 1:

It can be known from formula (3) that when the excitation current frequency is different, the voltage distribution at different "thickness layers" is different. In order to ensure that the permeation current touches the "defect layer", it is necessary to set a suitable excitation current. The voltage (voltage equivalent distribution) is used as the reference, and the voltage distribution is shown in Figure 2 when the input frequency is 2.5KHz and 500Hz.

It can be seen from the figure that if the excitation current frequency is too high, such as _2.5KHZ , the current cannot completely penetrate the pipeline wall. In this case, the penetration current cannot touch the shallow "defect layer", and the measured signal cannot reflect Defect information: reduce the excitation current frequency, increase the skin depth, and the penetration current can reach the "defect layer" at various depths, such as when the frequency is 500Hz.

Considering that the pipe wall thickness is 10mm, the maximum skin depth is 10mm. According to the formula (1) and the parameters in Table 1, the corresponding current frequency at this time can be calculated as 59Hz.

Taking the four positional defects in Figure 3(b) and Figure 3(e) as examples ( is the angle between the crack direction and the electrode connection direction).

Figure 3(a) is a non-defective pipeline, and the measured voltage value at this time is U(d=0,f=100Hz), which is used as the original voltage:

Comparing the measured voltage values U _(d) and U _(d=0) at different depths of the defects in the four positions in Figure 3(b)-Figure 3(e), the results are shown in Figure 4.

It can be seen from Figure 4 that when the angle between the crack direction and the electrode connection direction When it is less than 45°, the relationship between the value of U _(d) /U _(d=0) and the crack depth does not conform to the formula (6), and at this time U _(d) /U _(d=0) cannot be used to solve the defect depth;

when When greater than 45°, The closer to 90°, the more the relationship between the value of U _(d) /U _(d=0) and the crack depth follows the exponential distribution form of formula (6). Substituting the value of U _(d) /U _(d=0) into the formula ( 6) The closer the calculated crack depth is to the actual depth, When it is 90°, the solution accuracy can reach 97.16%; at the same time, the simulation data is also displayed: The closer to 90 degrees, the larger the value of U _(d) /U _(d=0) .

Therefore, for the depth solution problem of random cracks, using The closer to 90 degrees, the higher the measurement accuracy, innovatively proposed the multi-directional current method. By increasing three groups of excitation currents, making The range changes from 0-90° to 67.5-90°. Figure 5 is a schematic diagram of the measurement.

A set of characteristic signals can be obtained for the same defect at the same frequency:

Take the maximum U _(d) /U _(d=0) value and substitute it into formula (6) to solve the crack depth.

2. A pipeline crack corrosion detection method based on multi-directional AC potential drop, the steps include:

Step 1. Weld and arrange several groups of test electrodes on the circumference of the test pipeline.

Step 2. The signal generator generates an excitation signal. The excitation current is generated through the power amplifier, and the excitation current is sequentially passed to the input electrode from four different directions. The frequency of the power amplifier can be adjusted, and the amplitude of the excitation current can be adjusted.

Step 3. Use a high-precision lock-in amplifier to measure the voltage of a normal pipeline without cracks, obtain the test voltage value of the normal pipeline and input it into the computer, then use a high-precision lock-in amplifier to measure the voltage to be tested in the area to be tested in the test pipeline, and input the measurement results Computer, get four groups of different voltage ratios;

Step 4. According to the relationship between the crack direction and the voltage ratio, select the largest voltage ratio and substitute it into the following depth solution formula, and use a computer to solve it:

Step 5. Obtain the depth of the pipeline crack according to the results displayed by the computer.

A further technical solution is that in step 2, the four groups of test electrodes arranged in the circumferential direction refer to the horizontal direction, the vertical direction, and the diagonal direction, wherein the input electrodes and the output electrodes are arranged oppositely, and their connections all pass through the parts to be tested .

A further technical solution is that in step 3, when using a high-precision lock-in amplifier to test the voltage of the area to be tested, the connection between the input electrode for inputting the excitation current and the output electrode for outputting the excitation current is consistent with the test direction of the measurement probe connection.

The preferred technical solution is that the frequency of the power amplifier is selected to be 59-500 Hz, and the amplitude of the excitation current is selected to be 2A.

The preferred technical solution is that in step 2, the frequency of the power amplifier is selected to be 100 Hz.

3. Embodiment 1. Utilize a plate model to carry out simulation test

The flat plate model is firstly solved accurately.

The length of the plate is 220mm, the width is 220mm, the thickness is 10mm, the distance between the measuring probes is 20mm, the amplitude of the injected current is 2A, and the frequency is 100HZ, as shown in Figure 6 for details.

The defect depth is 2mm, 2.5mm, 3mm, 4mm, 4.5mm, 5mm, 5.5mm, 6mm, and the material parameters are as follows in Table 2:

Taking the 0-degree crack defect and the 45-degree crack defect as examples, the measurement results obtained by the two under the above simulation conditions are:

Take the value of 0 degree defect for depth fitting, the formula is:

The results calculated by formula (7) for 45-degree defects are shown in Table 2 above.

Embodiment 2 utilizes 5 aluminum plates to do the test

Experimental materials: five aluminum plates of 220mm×220mm×10mm. The overall experimental device is shown in Figure 7-8.

Before the defects are not processed, 2A, 100Hz alternating current is injected into the plate from the I _in1 , I _in2 , I in3, and I _in4 electrodes _in sequence, as shown in Figure 5 and Figure 8, and the probes are measured with a high-precision digital lock-in amplifier SR850 The original voltage U ₁ ( _{d = 0 )} and U _{2 ( d = 0 )} , U _3(d=0) , U _4(d=0) .

Then, 5 cracks were drawn on the bottom of the 5 plates respectively, among which No. 1-3 plates are 0 degree crack defects, the depths are 2mm, 4mm, 6mm respectively, and the width is 2mm; No. 4-5 plates are 45 degree crack defects , and the depth distribution is 3mm and 5mm, as shown in Figure 9.

Then, from the I _in1 , I _in2 , I _in3 , and I _in4 electrodes, as shown in Figure 8, 2A current, 59-500Hz alternating current can be injected into the plate in turn, and 100Hz alternating current is preferably used in this experiment to measure U _{1(d )} , U _2(d) , U _3(d) , U _4(d) .

The measured voltage is compared with the original voltage, and the results obtained are shown in Table 3 below:

Note: U ₁₀ ＝U _1(d=0) , U ₂₀ ＝U _2(d=0) , U ₃₀ ＝U _3(d=0) , U ₄₀ ＝U _4(d=0) .

Substituting the maximum value of U _(d) /U _(d=0) corresponding to each depth into formula (7), the results are shown in Table 4 below.

Therefore, compared with the previous situation where random position defects cannot be detected, the multi-directional current method can solve the random crack depth.

Using the voltage ratio in formula (7) to solve the depth method can eliminate environmental interference and improve the anti-interference ability of the measurement system.

Conclusion analysis:

The traditional potential drop technology uses a single excitation current parallel to the pipeline axis, and the present invention utilizes the angle between the current and the crack When the temperature is 90 degrees, the ratio of the measured voltage to the original voltage is the largest, and the ratio and the defect depth function characteristics are the best at the same time. The AC excitation current is fed from four directions, so that The range is changed from 0-90 degrees to 67.5-90 degrees, and the maximum voltage ratio is selected to calculate the defect depth.

The experimental data of the simulation test shows that the new calculation method improves the detection accuracy of random cracks and can detect corrosion more effectively.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it can still be Modifications are made to the technical solutions described in the foregoing embodiments, or equivalent replacements are made to some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims (5)

1. The pipeline crack corrosion detection method based on the multidirectional alternating current potential drop is characterized by comprising the following steps of:

step 1, welding and arranging a plurality of groups of test electrodes in the circumferential direction of a test pipeline;

step 2, the signal generator generates an excitation signal and generates an excitation current through the power amplifier, the excitation current is sequentially introduced to the input electrode from four different directions, the frequency of the power amplifier can be adjusted, and the amplitude of the excitation current can be adjusted;

step 3, measuring the voltage of the crack-free normal pipeline by using a high-precision phase-locked amplifier to obtain a normal pipeline test voltage value, inputting the normal pipeline test voltage value into a computer, measuring the voltage to be measured in the area to be measured of the test pipeline by using the high-precision phase-locked amplifier, and inputting the measurement result into the computer to obtain four groups of different voltage ratios;

and 4, selecting the maximum voltage ratio to substitute the following depth solving formula according to the relation between the crack direction and the voltage ratio, and solving by using a computer:

and 5, obtaining the depth of the pipeline crack according to the result displayed by the computer.

2. The method for detecting the pipeline crack corrosion based on the multi-directional alternating current potential drop is characterized in that in the step 2, the frequency of a power amplifier is selected to be 59-500Hz, and the amplitude of an excitation current is selected to be 2A.

3. The method for detecting pipeline crack corrosion based on multidirectional alternating current potential drop as claimed in claim 2, wherein the frequency of the power amplifier is selected to be 100 Hz.

4. The method for detecting pipeline crack corrosion based on multi-directional alternating current potential drop as claimed in claim 1, wherein in step 2, 4 groups of test electrodes arranged in the circumferential direction are in the horizontal direction, the vertical direction and the diagonal direction, wherein the input electrode and the output electrode are arranged oppositely, and the connecting lines of the input electrode and the output electrode pass through the part to be tested.

5. The method for detecting pipeline crack corrosion based on multi-directional alternating current potential drop as claimed in claim 1, wherein in step 3, when the high-precision phase-locked amplifier is used for testing the voltage of the area to be tested, the connection line of the input electrode for inputting the excitation current and the output electrode for outputting the excitation current is consistent with the connection line test direction of the measurement probe.