CN107643321B - Multi-frequency alternating current field fingerprint method metal pipeline corrosion detection technology based on phase identification - Google Patents
Multi-frequency alternating current field fingerprint method metal pipeline corrosion detection technology based on phase identification Download PDFInfo
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Abstract
A metal pipeline defect detection technology based on a multi-frequency alternating current field fingerprint method is characterized in that alternating current excitation current with the frequency from high to low is applied to a detected pipeline, the current gradually permeates from the outer wall of the pipeline to the inner wall of the pipeline under the action of a skin effect, the amplitude and the phase of a detected signal can be changed due to the existence of defects, and the defect depth can be solved by selecting the maximum phase angle of a series of voltage signals. The method can measure the absolute defect depth without measuring reference voltage, original voltage and original wall thickness, has good drift resistance, and is suitable for long-term online monitoring.
Description
Technical Field
The invention relates to the field of oil-gas metal pipeline defect detection and online monitoring, in particular to a technology for detecting and online monitoring the defect depth of an oil-gas metal pipeline. The specific technical scheme is as follows: two current injection electrodes are fixedly installed at two ends of a region to be measured of the metal pipeline to be measured at a certain distance, so that the current distribution of the region to be measured of the pipeline is uniformly distributed as much as possible; then fixedly installing measuring electrodes in a to-be-measured area of the pipeline according to a certain rule, and forming a measuring electrode pair by two adjacent electrodes along the axial direction of the pipeline; the metal area of the pipeline between the electrode pairs can be equivalent to a resistor, so that under the action of excitation current, a voltage can be generated between the electrode pairs; injecting a group of alternating current excitation currents with high to low frequencies into the metal pipeline to be detected, and measuring and recording voltage values under the excitation of the currents with different frequencies; due to the skin effect, when high-frequency excitation current is injected, the current on the pipeline is intensively distributed in a shallow layer close to the outer wall of the pipeline, and the current on the pipeline gradually permeates from the outer wall of the pipeline to the inner wall of the pipeline along with the reduction of the frequency of the excitation current, so that the measurement voltages corresponding to the currents with different frequencies are different. Since the presence of defects changes the amplitude of the measured voltage and, at the same time, the phase of the measured voltage more significantly, the phase angle of each voltage value can be calculated and recorded asΨ f Then finding a maximum phase angleΨ fmax Then useΨ fmax The depth of the defect can be determined.
The conventional Direct Current Field Signature Method (DCFSM) is essentially a relative wall thickness detection technology, i.e., the original thickness of a measured pipeline must be accurately measured, then the change condition of the relative wall thickness is judged according to the change of a voltage signal, if a DCFSM circuit system is repaired and replaced, the actual wall thickness value of the measured pipeline must be measured again, and for a buried pipeline or a submarine pipeline, the detection is obviously difficult to realize. Secondly, the DCFSM technology must accurately measure and record an original voltage value at the beginning of system installation, and then during the whole service period of an instrument system, the measured voltage needs to be subjected to ratio operation with the original voltage; as long-term on-line monitoring equipment, aging of components, change of working environment and the like can cause the instrument system to obviously drift, so that the working principle of the DCFSM determines that the DCFSM cannot overcome measurement errors caused by the drift. In addition, the traditional DCFSM is also provided with a reference plate, a reference electrode is arranged on the reference plate, the reference plate is electrically connected with the pipeline to be detected only through an excitation current cable, and an insulating material is filled between the reference plate and the pipeline to be detected; researches show that the temperature difference between the reference plate and the measured pipeline can cause great measurement errors, the errors caused by the temperature difference are generally reduced by a temperature correction method in an industrial field, and if the temperature sensor is damaged, the whole DCFSM system can not work normally for the working environment in which the temperature sensor is difficult or impossible to repair such as buried pipelines, particularly submarine pipelines and the like.
Therefore, the invention has the following effective benefits: firstly, the invention can realize absolute measurement of the depth of the defect, namely, the original wall thickness value of the measured pipeline does not need to be measured; secondly, a reference plate is not needed to be arranged, so that adverse effects caused by temperature changes are avoided; in addition, the depth value of the detected defect can be solved only by measuring the phase information of the signal without measuring the original voltage.
Background
The corrosion is the most main factor causing the leakage of the oil gas metal pipeline, and the oil gas pipeline leakage accidents in China occur frequently, so that the development of a safe, efficient and reliable metal pipeline corrosion detection technology is very important. Furthermore, as the concept of integrity management goes deep, offline detection technology is inevitably replaced by online real-time monitoring technology. A corrosion monitoring technology widely adopted in the petrochemical industry at present, namely a field fingerprint method based on a direct current potential drop method, has the working principle as follows:
1. two excitation current electrodes are fixedly arranged at intervals of a certain distance at two ends of a region to be measured of the measured metal pipeline, and direct current excitation current with a certain amplitude is injected into the measured pipeline through the two electrodes; fixedly mounting a measuring electrode matrix in a to-be-measured area of a measured pipeline, wherein two adjacent electrodes in the axial direction form a pair of measuring electrode pairs; the device is characterized in that a homogeneous reference plate which is made of the same material as the tested pipeline is arranged, a reference electrode is arranged on the reference plate, the reference plate is electrically connected with the tested pipeline through a current cable, and an insulating material is filled between the reference plate and the tested pipeline to prevent the reference plate and the tested pipeline from being electrically connected except a cable connection point.
2. The pipeline metal body between a pair of measuring electrode pairs can be regarded as an equivalent resistor body, once the measuring electrode matrix of the measuring area is determined, the length and the width of the equivalent resistor body are kept unchanged, and the resistance value of the equivalent resistor body can be changed only by the thickness reduction of the equivalent resistor body caused by defects such as corrosion and the like, so that under the action of the inrush current with constant amplitude, the voltage signal change condition of the measuring electrode pairs represents the change condition of the wall thickness of the measuring area.
3. At the beginning of installation of the DCFSM system, the original wall thickness value of the measured area needs to be measured and recorded and is recorded asWT(t 0 ). At the same time, the initial reference voltage of the measurement reference electrode is recordedV ref （t 0 ）And measuring the initial measurement voltage of the electrode pair, notedV i （t 0 ）,. During the service period of the instrument system, the measured pipeline is corroded, and the sum of the measured voltage and the reference voltage on the pipeline measured at the moment is recorded as the sum of the measured voltage and the reference voltage respectivelyV i （t x ）AndV ref （t x ）and then calculating the corrosion depth by using the following formula:
FC i (t x ）=((V i (t x )/V i (t 0 ))/Vref(t x )/V ref (t 0 )-1)×1000 （1）
in the formula:
V i （t 0 ），V i （t x ）-measured voltages at the initial and corrosion moments;
V ref （t 0 ），V ref （t x ）-reference voltages at the initial and etching moments;
D(t x )=WT(t 0 )-WT(t 0 )×1000/(FC i (t x )+1000) （2）
in the formula:D(t x ) -depth of defect;WT(t 0 ) -the original wall thickness of the pipe under test;
as can be seen from the formula (1), the DCFSM needs to introduce a reference voltage for corresponding calculation and needs to perform ratio operation with the original voltage; in practical engineering application, along with the aging of an instrument system, the change of the working environment of equipment and the like, various drifting problems of the instrument system are necessarily caused, including baseline drifting, zero drift and the like, namely, the measured voltage and the reference voltage are increased or reduced by the same amplitude value at the same time. As shown in the formula (1), when the reference voltage and the measured voltage are increased or decreased by the same value, the final ratio will change, and therefore the drift will cause a large error in the detection result of the field fingerprinting method. As can be seen from the formula (2), when the defect depth is calculated by the direct current field fingerprint method, the original wall thickness of the measured pipeline also needs to be accurately measured, the measurement error of the original wall thickness also causes adverse effect on the calculation of the defect depth, and once a circuit system of the circuit system is damaged and replaced, the actual wall thickness value of the measured pipeline needs to be measured again to serve as the original wall thickness value, which is very difficult to realize for buried pipelines or submarine pipelines;
this patent aims at providing one kind and based on frequency conversion alternating current field fingerprint method metal pipeline defect detection technology, and this technique no longer need measure reference voltage, need not measure original voltage, need not measure the original wall thickness of being surveyed the pipeline, only calculates the aassessment to the defect degree of depth through voltage signal's phase place, can eliminate the influence to measuring such as system drift, temperature variation, original wall thickness measurement error very effectively.
Disclosure of Invention
The invention provides a multi-frequency alternating current field fingerprint metal pipeline defect detection technology, which mainly aims at the defects that the absolute defect depth cannot be measured, reference voltage is required to be introduced, so that the absolute defect depth is easy to be subjected to drift interference, a great measurement error is introduced due to the temperature difference between a reference plate and a detected pipeline, and the like in the conventional direct current field fingerprint technology. Specifically, the technology comprises the steps that firstly, two current injection electrodes are fixedly installed at two ends of a to-be-measured area of a to-be-measured pipeline at a certain distance, a measuring electrode matrix is fixedly installed in the to-be-measured area of the pipeline according to a certain rule, and two adjacent electrodes in the axial direction form a pair of measuring electrode pairs; applying a series of alternating current excitation currents with high to low frequencies to a measured pipeline through a current injection electrode, enabling the currents to gradually permeate from the outer wall of the pipeline to the inner wall of the pipeline by utilizing a skin effect, measuring and recording voltages generated by the excitation currents at different frequencies, and calculating a phase angle of each voltage, and recording the phase angle asΨ f Using the maximum phase angleΨ fmax The defect depth of the measured area can be obtained.
The skin effect refers to that when an alternating current passes through a conductor, the distribution of the current on the cross section of the conductor is uneven, the current density on the surface of the conductor is the largest, and the trend is more obvious when the frequency of the alternating current is higher, which is called skin effect (skin effect), and the skin effect is also called skin effect. This current is called the skin current, the skin depth of whichδDetermined by equation (3):
δ=1/(πμ r μ 0 σf)½（3）
in formula (3)μ r Is the relative magnetic permeability of the measured pipeline material,μ 0 in order to have a magnetic permeability of air,σis the electrical conductivity of the material of the pipeline to be measured,fis the frequency of the excitation current. It is known from the formula (3) that the skin depth of the skin current decreases as the frequency of the excitation current decreasesδThe skin current will gradually permeate from the outer wall of the metal pipe to the inner wall of the metal pipe, which will cause the depth of the metal block in the current flowing area to increase, that is, the equivalent resistance of the current flowing area to decrease, and under the action of the excitation current with the same amplitude, the voltage between the measurement electrodes will gradually decrease, and at the same time, the phase angle of the corresponding voltage will also change. If the detected pipeline has defects, the defects can obviously change the phase angle of the voltage, so the invention utilizes the maximum value of the phase angle of the voltageΨ fmax Solving the depth of the detected region defectdThe specific scheme comprises the following steps:
(1) installing a pair of current injection electrodes at two ends of a region to be measured of the metal pipeline, arranging a measuring electrode matrix in the region to be measured of the pipeline, and forming a pair of measuring electrode pairs by two adjacent measuring electrodes along the axial direction of the pipeline; all the electrodes are fixedly arranged on the detected pipeline, so that the defect of the detected pipeline can be monitored on line according to the following steps:
(2) injecting alternating current excitation current with high-to-low frequency into the measured pipeline through a current injection electrode, enabling the current on the metal pipeline to gradually permeate into the inner wall of the pipeline from the shallow layer of the outer wall of the pipeline by utilizing the skin effect, measuring and recording the voltage of a pair of measuring electrode pairs under each excitation frequency, and recording the voltage asV f ；
(3) Calculate each voltageV f Is described asΨ f ；
(4) Finding the maximum phase angleΨ fmax The defects of the detected pipeline can be calculated by using the following formulad：
d=-2.004Ψ fmax +10.5。
The invention has the beneficial effects that: the method has the advantages that a series of alternating current excitation currents with high frequency to low frequency are applied to the measured pipeline, a group of voltages are obtained through measurement, the depth of the defect is solved by utilizing the maximum voltage phase angle, the original voltage does not need to be measured, a reference plate does not need to be arranged, absolute wall thickness loss measurement can be carried out, and the method is very suitable for long-term online monitoring of the defect of the metal pipeline.
Drawings
FIG. 1 is a schematic diagram of a DC field fingerprint technique
FIG. 2 is a schematic diagram of multi-frequency alternating field fingerprint technique
Figure 3 phase angles of different defects.
Detailed Description
An embodiment of the present invention is described in detail as follows:
the invention relates to a metal pipeline defect detection technology by a multi-frequency alternating current field fingerprint method, which comprises the following specific implementation steps:
(1) selecting a common carbon steel pipeline with the wall thickness of 10mm, the pipe diameter of 219mm and the length of 1000mm as a measured object, welding a pair of current excitation electrodes at two ends of the pipeline, welding and installing a measuring electrode matrix on the outer wall of the center of the pipeline, and measuring the electrode distance of 35mm in the axial direction;
(2) respectively processing the defects with the depths of 1mm, 2mm, 3mm, 4mm, 5mm and 6mm on the inner wall of the pipeline corresponding to the measuring electrode, wherein the sizes of the defects are 35mm multiplied by 35 mm;
(3) adjusting the frequency of the excitation current according to the material characteristics of the tested pipeline, and sequentially increasing the skin depth of the skin current from 0.1mm to 10mm in equal increments of 0.1mm according to a calculation formula of the skin current, namely setting 100 frequency points in total;
(4) sequentially measuring voltage values of 6 pairs of measuring electrodes under excitation currents with different frequencies;
(5) respectively calculating the phase angle of each group of voltageΨ f However using the maximum phase angleΨ fmax The defect depth calculated according to the formula for finally calculating the defect depth is shown in the following table:
Claims (3)
1. a frequency conversion alternating current potential drop metal pipeline defect detection method based on phase recognition is characterized in that a pair of current injection electrodes are installed at two ends of a metal pipeline region to be measured at a certain interval, and a measuring electrode matrix is arranged in the pipeline region to be measured; along the axial direction of the pipeline, two adjacent measuring electrodes form a pair of measuring electrode pairs; all the electrodes are fixedly arranged on the detected pipeline, so that the defect of the detected pipeline can be detected according to the following steps:
(1) injecting alternating current excitation current with high-to-low frequency into the measured pipeline through a current injection electrode, enabling the current distribution on the metal pipeline to gradually permeate into the inner wall of the pipeline from the shallow layer of the outer wall of the pipeline by utilizing the skin effect, measuring and recording the voltage between a certain pair of measurement electrode pairs under each excitation frequency, and recording the voltage as Vf；
(2) Calculate each voltage VfIs denoted by ψf；
(3) Finding the maximum phase angle psifmaxThe defect d of the measured pipeline can be calculated by using the following formula:
d＝-2.004ψfmax+10.5。
2. the phase identification-based variable frequency alternating current potential drop metal pipeline defect detection method according to claim 1, wherein the detection method can be used for off-line detection and on-line monitoring, and the frequency of the excitation current can be adjusted accordingly according to the requirement of detection precision.
3. The method for detecting the defects of the variable frequency alternating current potential drop metal pipeline based on the phase identification as claimed in claim 1, wherein the excitation current source is composed of a high-precision alternating current source, and the voltage between the measuring electrode pair is measured by a high-precision alternating current voltage tester.
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| CN108845000A (en) * | 2018-08-20 | 2018-11-20 | 四川大学 | A kind of method of pulsed field fingerprint technique measurement defect of pipeline |
| CN108680607B (en) * | 2018-08-30 | 2021-02-26 | 四川大学 | Pipeline crack corrosion monitoring method based on multidirectional alternating current potential drop |
| CN110133053B (en) * | 2019-06-14 | 2024-02-02 | 中国特种设备检测研究院 | Metal pipeline corrosion monitoring method and system |
| CN110220947B (en) * | 2019-06-14 | 2021-11-30 | 中国特种设备检测研究院 | Method for determining corrosion degree of pipeline |
| CN110133054B (en) * | 2019-06-14 | 2024-02-02 | 中国特种设备检测研究院 | Metal pipeline characteristic analysis system |
| CN110470726B (en) * | 2019-08-14 | 2023-03-07 | 中国特种设备检测研究院 | Steel pipeline defect detection method and device based on multi-frequency electromagnetism |
| CN114090942B (en) * | 2021-11-11 | 2023-04-21 | 四川大学 | High-temperature smelting pipeline online monitoring method based on multi-sensor fusion |
| CN114624313A (en) * | 2022-03-14 | 2022-06-14 | 中国特种设备检测研究院 | Corrosion monitoring system |
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| CN102954998A (en) * | 2011-08-26 | 2013-03-06 | 中国石油天然气股份有限公司 | Non-contact detection method for abnormal wall thickness change of steel pipeline |
| CN102759567B (en) * | 2012-07-18 | 2015-10-28 | 南昌航空大学 | The EDDY CURRENT identification of steel pipe inside and outside wall defect and evaluation method under DC magnetization |
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