CN107941905B - Low-frequency array vortex detection device and steel pipe inner wall corrosion defect detection method - Google Patents
Low-frequency array vortex detection device and steel pipe inner wall corrosion defect detection method Download PDFInfo
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- CN107941905B CN107941905B CN201810025874.0A CN201810025874A CN107941905B CN 107941905 B CN107941905 B CN 107941905B CN 201810025874 A CN201810025874 A CN 201810025874A CN 107941905 B CN107941905 B CN 107941905B
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Abstract
The invention relates to a low-frequency array eddy current detection device and a method for detecting corrosion defects on the inner wall of a steel pipe, and the technical scheme is that the device comprises an eddy current detection host, a connecting wire and a low-frequency array eddy current probe; the invention eliminates the interference of space scattering leakage magnetic flux through the magnetic shielding technology of a magnetic tank and a shielding coil, improves the defect resolution, and 8 detection coils can independently image in the detection process, and the detection sensitivity is always consistent, thus avoiding the defect missing detection, and the defects of the inner wall of the steel tube of the heating surface covered by the probe can be detected by one operation without reciprocating operation.
Description
Technical Field
The invention relates to the technical field of eddy current detection of a heating surface tube of a boiler of an electric power system, in particular to a low-frequency array eddy current detection device and a steel tube inner wall corrosion defect detection method.
Background
The heating surface pipe of the power station boiler is in a high-temperature and high-pressure working environment for a long time, and the inner and outer surfaces of the pipe are easy to generate defects such as corrosion abrasion, hydrogen damage, corrosion pits, oxidation pits, cracks, smoke corrosion, corrosion holes and the like, the continuous development of the defects leads to leakage, the safe operation of a unit is seriously threatened, and the unplanned shutdown of the unit is caused. Therefore, the periodic inspection of the inner wall corrosion condition of the heated surface steel pipe is an effective means for preventing accidents. Through detection, the service state of the heated surface steel pipe can be evaluated, the service life is estimated, the pipe row is reasonably replaced, the number of unplanned shutdown times is reduced, and the running cost of the unit is reduced.
At present, the corrosion detection of the inner wall of the steel pipe on the heating surface benefits from a low-frequency steel pipe corrosion scanning instrument, but the instrument probe mainly comprises a single coil, has a simple structure and limited effective coverage area of the coil, has higher defect resolution and detection precision of the pipe on the center part of the opposite fire side, and has the detection sensitivity of the edges on the two sides which are sequentially reduced, thus the detection is easy to be missed, and due to the structural problem, a plurality of inconveniences exist in the use process, so the improvement and innovation are necessary.
Disclosure of Invention
Aiming at the situation, the invention aims to overcome the defects of the prior art, and the invention aims to provide a low-frequency array vortex detection device which can effectively solve the problem of defect detection of a heating surface of a boiler.
The technical scheme of the invention is as follows:
the low-frequency array eddy current testing device comprises an eddy current testing host, a connecting wire and a low-frequency array eddy current probe; the eddy current detection host is connected with the low-frequency array eddy current probe through a connecting wire;
the eddy current detection host comprises a host shell, wherein a display and a probe connecting socket are respectively arranged on the host shell, a low-frequency oscillator, a lithium battery, a controller, an amplifier, a balance filter, a phase shifter, a gain adjustable amplifier and a digital-to-analog converter are respectively arranged on the shell, the output end of the lithium battery is connected with the input end of the low-frequency oscillator, the output end of the low-frequency oscillator is connected with the input end of the controller, the output end of the controller is connected with the probe connecting socket, the probe connecting socket is connected with the input end of a low-noise amplifier, the output end of the noise amplifier is connected with the input end of the controller through the balance filter, the phase shifter, the gain adjustable amplifier and the digital-to-analog converter which are connected in series, and the output end of the controller is connected with the display;
the probe connecting socket is connected with the probe socket of the low-frequency array eddy current probe through a connecting wire;
the connecting wire comprises a connecting wire body, the connecting wire body comprises a total shielding woven net and an eddy current acquisition signal transmission line, an excitation oscillation transmission line, a positioning coding signal transmission line and a power line which are wrapped in the total shielding woven net, the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line are formed by a single shielding woven net and core wires wrapped in the single shielding woven net, two ends of the connecting wire body are respectively provided with aviation plugs corresponding to probe sockets of low-frequency array eddy current probes and probe connection sockets of an eddy current detection host, and pins corresponding to the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line one to one are respectively arranged on the aviation plugs, and the core wire ends of the corresponding transmission lines are connected with the rear ends of the corresponding pins.
The low-frequency array eddy current probe comprises a shell, the shell is of a hollow structure, the cross section of a bottom plate of the shell is in an upward concave cambered surface shape, guide rubber wheels are arranged on two sides of the bottom plate, a plurality of detection coils are circumferentially and uniformly distributed on the bottom plate along the cambered surface of the same radial cross section in the shell, a magnetic tank for eliminating space scattering leakage magnetic flux is sleeved outside the detection coils, exciting coils are arranged in the shell outside the magnetic tank, shielding coils for focusing magnetic circuits are arranged between the exciting coils and the detection coils, an amplifier is arranged in the shell, an encoder for synchronously recording detection data is arranged at one end of the shell, a probe socket connected with an aviation plug is arranged outside the shell, and the output end of each detection coil is connected with the input end of the amplifier, and the output end of the exciting coil and the output end of the encoder are connected with the probe socket.
A method for detecting corrosion defects of the inner wall of a ferromagnetic steel pipe by adopting a low-frequency array eddy current detection device comprises the following steps:
A. calibrating the detection waste judgment standard
Firstly, processing a sensitivity contrast test tube, taking a tube with a heating surface of the same batch, cutting along the center of the tube, sequentially processing 4 artificial defect blind holes with diameter phi 6mm and hole depth 20% and designed wall thickness at intervals in the circumferential direction of the inner wall of the tube at intervals of 20 degrees, scanning and detecting the sensitivity contrast test tube by a low-frequency array eddy current detection device to detect the depth value of the P-T (phase shift quantity-time) scanning curve amplitude calibration defect of the artificial defect on the sensitivity contrast test tube, setting the phase deviation value and the depth of the scanning signal and the percentage of the wall thickness occupied by the defect as detection waste judging standards, and judging the defect exceeding the amplitude in the detection process;
when scanning detection is carried out, two groups of guide rubber wheels are clung to the surface of a sensitivity comparison test tube, a probe is manually pushed to carry out detection, when an exciting coil carrying alternating current is close to the comparison test tube made of an electric conduction magnetic conduction steel tube, eddy currents are induced in the electric conduction magnetic conduction steel tube due to the effect of a coil magnetic field, the size, the phase and the flowing form of the eddy currents are influenced by the performance of an electric conduction material, the impedance of a detection coil is changed by a reaction magnetic field generated by the eddy currents, and the eddy currents are displayed in a display of an eddy current detection host after signal amplification treatment of an amplifier; the P-T scanning curve is a curve image of the eddy current phase shift quantity changing along with time;
B. defect localization
Two groups of guide rubber wheels are tightly attached to the surface of the steel pipe to be tested, and the probe is manually pushed to detect, so that the outer surface of the steel pipe to be tested is scanned at a constant speed at a scanning speed of less than 3-5 m/min; the current is processed by a low-frequency oscillator and then is sent into a probe excitation coil through a connecting wire by a controller, magnetic flux generated by the excitation coil covers 8 detection coils, signals collected by the detection coils are amplified by an 8-channel preamplifier and are transmitted to an eddy current detection host through the connecting wire, when a detected heating surface tube is defect-free, the space scattered leakage magnetic flux is balanced and symmetrical, the output of each detection coil is zero, when the detected heating surface tube is defect, the disturbance of the defect on the magnetic flux and the eddy current causes the leakage magnetic flux to be asymmetrically distributed, thus the detection coil on the defect can generate unbalanced signals, the unbalanced signals are amplified by a low-noise amplifier, then are filtered by a balance filter and automatically digital balanced, each line is placed at a reference zero level, the time constants of the filter and the automatic balance are set by the controller, and the filtered and balanced defect signals enter a phase shifter and a gain adjustable amplifier respectively for phase angle fixation and signal amplification, and are converted into digital signals after amplification, and are sent into the controller for display on a display, and the positioning of the defect position is realized:
1) Defect circumferential positioning
The method comprises the steps that inner wall defect detection is carried out on a boiler heating surface pipe through a low-frequency array eddy current probe, the probe can cover a 125-degree range of a detected pipe wall, a P-T scanning curve of each detection coil is independently displayed and corresponds to the position of a coil array, when the probe passes through the defective pipe wall, the corresponding detection coil cuts magnetic force lines leaking out of the pipe wall to induce a maximum voltage signal, impedance of the coil is changed and displayed on the P-T scanning curve, so that the circumferential position of the defect is determined, and the circumferential defect is recorded;
2) Defect axial positioning
If the defect is found to need to store detection data in the detection process, external triggering can be selected, namely pulse signal triggering of an encoder on the probe is performed, at the moment, a P-T scanning curve is synchronous with probe scanning detection, the probe scans forward and detects, the P-T scanning curve synchronously rolls forward and displays, the distance travelled by the probe scanning detection is displayed on a screen at the same time, when the defect is detected and needs to be rechecked, the P-T scanning curve distance which is equal to the distance travelled by the probe backwards is erased on the screen in real time, so that the axial position of the defect is determined, and the axial defect is recorded;
C. defect review
Cleaning the surface of the positioned defect, coating a coupling agent, and re-measuring by using an ultrasonic thickness meter to confirm the depth of the defect, wherein the geometric shape of the surface of the pipe fitting is irregular, butt welding seams, bent pipe positions, pipe ends and internal stress can possibly cause trouble to defect discrimination, so that the final detection result is checked by using the ultrasonic thickness meter;
D. checking with the sensitivity contrast test tube every 2 hours, and if the manual injury on the sensitivity contrast test tube is difficult to distinguish, the previous detection is invalid.
The low-frequency array eddy current detection device has novel and unique structure, is simple and reasonable, is easy to produce and operate, has low cost, adopts 8 detection probes which are arrayed along the circumferential direction, can effectively cover the 125-degree circumferential area of a detected heating surface tube, eliminates the interference of space scattering leakage magnetic flux through the magnetic shielding technology of a magnetic tank and a shielding coil, improves the defect resolution, can independently image the 8 detection coils in the detection process, keeps the detection sensitivity consistent all the time, avoids the defect leakage detection, can detect the defects of the inner wall of the heating surface tube covered by the probes by one-time operation, does not need to carry out reciprocating operation, synchronously records detection data through a data encoder, is beneficial to the calling analysis and archiving of the detection data, improves the detection efficiency, amplifies signals firstly to offset the attenuation of the signals in the transmission process, prepositioned a freezing zero-setting switch, eliminates the trouble of time difference and can adjust zero in time when a base line deviates from zero point; the method can effectively solve the problem of positioning the corrosion defect of the inner wall of the heating surface pipe of the power station boiler, improves the detection precision and the on-site working efficiency, ensures the reliable and stable operation of the heating surface pipe of the power station boiler, is convenient to use, has good effect and has good social and economic benefits.
Drawings
Fig. 1 is a perspective view of a housing of an eddy current testing host according to the invention.
Fig. 2 is a left side view of the eddy current inspection host housing of the present invention.
Fig. 3 is a partial front view of the connection line of the present invention.
Fig. 4 is a front view, partially in section, of the connecting wire body of the present invention.
Fig. 5 is a cross-sectional side view of the connector body of the present invention.
Fig. 6 is a front view in cross section of the use of the low frequency array eddy current probe of the invention.
Fig. 7 is a right side view showing the use state of the low frequency array eddy current probe of the present invention.
FIG. 8 is a cross-sectional side view of a low frequency array eddy current probe according to the invention in use
Fig. 9 is a schematic block diagram of the circuit principle of the present invention.
Detailed Description
The following describes the embodiments of the present invention in further detail with reference to the drawings.
1-9, a low frequency array eddy current testing apparatus comprising an eddy current testing host, connecting wires, and a low frequency array eddy current probe; the eddy current detection host is connected with the low-frequency array eddy current probe through a connecting wire;
the eddy current testing host comprises a host shell 19, wherein a display 20 and a probe connecting socket 24 are respectively arranged on the host shell, a low-frequency oscillator, a lithium battery, a controller, an amplifier, a balance filter, a phase shifter, a gain adjustable amplifier and a digital-to-analog converter are respectively arranged on the shell 19, the output end of the lithium battery is connected with the input end of the low-frequency oscillator, the output end of the low-frequency oscillator is connected with the input end of the controller, the output end of the controller is connected with the probe connecting socket 24, the probe connecting socket 24 is connected with the input end of a low-noise amplifier, the output end of the noise amplifier is connected with the input end of the controller through the balance filter, the phase shifter, the gain adjustable amplifier and the digital-to-analog converter which are connected in series, and the output end of the controller is connected with the display 20;
the probe connecting socket 24 is connected with the probe socket 2 of the low-frequency array eddy current probe through a connecting wire;
the connecting wire comprises a connecting wire body 13, the connecting wire body 13 comprises a total shielding woven net 131 and an eddy current acquisition signal transmission line, an excitation oscillation transmission line, a positioning coding signal transmission line and a power line which are wrapped in the total shielding woven net 131, the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line are formed by a single shielding woven net 132 and a core wire 133 wrapped in the single shielding woven net 132, two ends of the connecting wire body 13 are respectively provided with an aviation plug 14 corresponding to a probe socket of a low-frequency array eddy current probe and a probe connection socket of an eddy current detection host, the aviation plug 14 is respectively provided with pins 18 corresponding to the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line one by one, and the core wire end of the corresponding transmission line is connected with the rear end of the corresponding pin.
The low-frequency array eddy current probe comprises a shell 1, the shell 1 is of a hollow structure, the section of a bottom plate 1a of the shell 1 is in an upwards concave cambered surface shape, guide rubber wheels 4 are arranged on two sides of the bottom plate 1a, a plurality of detection coils 11 are uniformly distributed on the bottom plate 1a along the cambered surface shape of the same radial section in the shell 1 in the circumferential direction, a magnetic tank 10 for eliminating space scattering leakage magnetic flux is sleeved outside the detection coils 11, an excitation coil 7 is arranged in the shell 1 outside the magnetic tank 10, a shielding coil 9 for focusing a magnetic circuit is arranged between the excitation coil 7 and the detection coils 11, an amplifier 12 is arranged in the shell 1, an encoder 8 for synchronously recording detection data is arranged at one end of the shell 1, a probe socket 2 connected with an aviation plug is arranged outside the shell 1, the output end of each detection coil 11 is respectively connected with the input end of the amplifier 12, and the output end of the amplifier 12, the input end of the excitation coil 7 and the output end of the encoder 8 are all connected with the probe socket 2.
In order to ensure the use effect, the radian of the bottom plate 1a is 2 pi/3, the detection coils 11 are uniformly distributed with 8, and the eddy current acquisition signal transmission lines are in one-to-one correspondence with the detection coils with 8.
The shell is respectively provided with a digital knob 21, a power switch 22, a charging socket 23, a USB interface 25, a grounding socket 26 and an RS-232 printer interface 27 which are connected with the controller, the display is a touch screen, and the digital knob 21 is used for adjusting the gain value of the gain adjustable amplifier, namely the sensitivity of the instrument can be manually adjusted within the range of 0.01-15.00 degrees/div. When a wide range of sensitivity changes is required, the adjustment is made by the touch screen.
The handle 3 is arranged at the top of the shell 1, so that the handheld operation is convenient; the number of the guide rubber wheels 4 is 4, the guide rubber wheels are respectively arranged on two sides of the bottom plate of the shell in pairs, the bottoms of the guide rubber wheels extend out of the lower end face of the shell and are obliquely arranged downwards and inwards, so that the guide rubber wheels can be completely attached to the pipe wall of the steel pipe 5 to be tested, the function of telescopic adjustment can be added, and the stability of a system is improved conveniently;
the encoder 8 is arranged at the tail part of the shell, can be provided with a torsion spring, is tightly pressed by the elasticity of the torsion spring, is always in contact with the wall of the steel pipe 5 to be tested in the walking process of the shell, is used for collecting the axial scanning distance of the array scanning probe in real time, synchronously records detection data, and is convenient for post-processing and archiving of the data.
The shell 1 is provided with a freezing zero-setting switch 6 connected with the probe socket 2, and when an abnormal signal appears on the instrument, the freezing zero-setting switch can lock a screen image so as to further observe and judge. In the prior art, an operator of the instrument usually needs to put the probe aside and then press the freezing switch of the instrument, and as the phase shift scanning curve of the instrument rolls along with time, time difference is generated, detection signals are difficult to capture in time, the freezing zero setting switch of the instrument detection signals is arranged in front, and the low-frequency array scanning probe shell is provided with a device capable of freezing suspected signals in time, so that the trouble of the time difference is eliminated. In addition, various interferences can occur in the scanning detection process according to the field conditions, so that the base line deviates from the zero point, and the zero needs to be adjusted in time. Obviously, it is not feasible to rest the probe to zero. It is difficult to perform the zeroing operation while maintaining the probe state. The instrument zeroing switch is arranged in front, and the zeroing switch is arranged on the shell of the low-frequency array scanning probe, so that zeroing operation is convenient.
The magnetic jar 10 adopts ferrite to make, and every magnetic jar all covers rather than the detection coil of one-to-one, carries out magnetism shielding through the magnetic jar that covers detection coil, and ferrite has high permeability, utilizes the magnetic circuit reposition of redundant personnel principle to shield the low frequency electromagnetic field, eliminates the interference of the leakage magnetic flux of space scattering, has improved the defect resolution, and 8 detection coils all can independently image in the testing process, and the detection sensitivity remains unanimous throughout, has avoided the defect to leak to examine.
The controller comprises a singlechip with the model of stm32f103 (mpu) and an ep4ce10e22 (fpga) programmable logic controller.
The total shielding woven mesh 131 and the single-strand shielding woven mesh 132 are both made of nylon woven mesh.
The method for detecting the corrosion defect of the inner wall of the ferromagnetic steel pipe by adopting the low-frequency array vortex detection device comprises the following steps of:
A. calibrating the detection waste judgment standard
Firstly, processing a sensitivity contrast test tube, taking a tube with a heating surface of the same batch, cutting along the center of the tube, sequentially processing 4 artificial defect blind holes with diameter phi 6mm and hole depth 20% and designed wall thickness at intervals in the circumferential direction of the inner wall of the tube at intervals of 20 degrees, scanning and detecting the sensitivity contrast test tube by a low-frequency array eddy current detection device to detect the depth value of the P-T (phase shift quantity-time) scanning curve amplitude calibration defect of the artificial defect on the sensitivity contrast test tube, setting the phase deviation value and the depth of the scanning signal and the percentage of the wall thickness occupied by the defect as detection waste judging standards, and judging the defect exceeding the amplitude in the detection process;
when scanning detection is carried out, two groups of guide rubber wheels are clung to the surface of a sensitivity comparison test tube, a probe is manually pushed to carry out detection, when an exciting coil carrying alternating current is close to the comparison test tube made of an electric conduction magnetic conduction steel tube, eddy currents are induced in the electric conduction magnetic conduction steel tube due to the effect of a coil magnetic field, the size, the phase and the flowing form of the eddy currents are influenced by the performance of an electric conduction material, the impedance of a detection coil is changed by a reaction magnetic field generated by the eddy currents, and the eddy currents are displayed in a display of an eddy current detection host after signal amplification treatment of an amplifier; the P-T scanning curve is a curve image of the eddy current phase shift quantity changing along with time;
B. defect localization
Two groups of guide rubber wheels are tightly attached to the surface of the steel pipe 5 to be detected, and the probe is manually pushed to detect, so that the outer surface of the steel pipe to be detected is scanned at a constant speed at a scanning speed of less than 3-5 m/min; the current is processed by a low-frequency oscillator and then is sent into a probe excitation coil through a connecting wire by a controller, magnetic flux generated by the excitation coil covers 8 detection coils, signals collected by the detection coils are amplified by an 8-channel preamplifier and are transmitted to an eddy current detection host through the connecting wire, when a detected heating surface tube is defect-free, the space scattered leakage magnetic flux is balanced and symmetrical, the output of each detection coil is zero, when the detected heating surface tube is defect, the disturbance of the defect on the magnetic flux and the eddy current causes the leakage magnetic flux to be asymmetrically distributed, thus the detection coil on the defect can generate unbalanced signals, the unbalanced signals are amplified by a low-noise amplifier, then are filtered by a balance filter and automatically digital balanced, each line is placed at a reference zero level, the time constants of the filter and the automatic balance are set by the controller, and the filtered and balanced defect signals enter a phase shifter and a gain adjustable amplifier respectively for phase angle fixation and signal amplification, and are converted into digital signals after amplification, and are sent into the controller for display on a display, and the positioning of the defect position is realized:
1) Defect circumferential positioning
The method comprises the steps that inner wall defect detection is carried out on a boiler heating surface pipe through a low-frequency array eddy current probe, the probe can cover the range of 120 degrees of a detected pipe wall, a P-T scanning curve of each detection coil is independently displayed and corresponds to the position of a coil array, when the probe passes through the defective pipe wall, the corresponding detection coil cuts magnetic force lines leaking out of the pipe wall to induce a maximum voltage signal, the impedance of the coil is changed and displayed on the P-T scanning curve, so that the circumferential position of the defect is determined, and the circumferential defect is recorded;
2) Defect axial positioning
If the defect is found to need to store detection data in the detection process, external triggering can be selected, namely pulse signal triggering of an encoder on the probe is performed, at the moment, a P-T scanning curve is synchronous with probe scanning detection, the probe scans forward and detects, the P-T scanning curve synchronously rolls forward and displays, the distance travelled by the probe scanning detection is displayed on a screen at the same time, when the defect is detected and needs to be rechecked, the P-T scanning curve distance which is equal to the distance travelled by the probe backwards is erased on the screen in real time, so that the axial position of the defect is determined, and the axial defect is recorded;
C. defect review
Cleaning the surface of the positioned defect, coating a coupling agent, and re-measuring by using an ultrasonic thickness meter to confirm the depth of the defect, wherein the geometric shape of the surface of the pipe fitting is irregular, butt welding seams, bent pipe positions, pipe ends and internal stress can possibly cause trouble to defect discrimination, so that the final detection result is checked by using the ultrasonic thickness meter;
D. checking with the sensitivity contrast test tube every 2 hours, and if the manual injury on the sensitivity contrast test tube is difficult to distinguish, the previous detection is invalid.
As is clear from the above, the low-frequency array eddy current detection device has novel and unique structure, simple and reasonable structure, easy production, easy operation and low cost, the current is processed by the low-frequency oscillator and then is sent into the probe excitation coil through the connecting wire by the controller, the frequency range is kept between 1Hz and 2000Hz due to the addition of the low-frequency oscillator, the minimum stepping value is 0.1Hz, the lower the frequency is, the deeper the detected wall thickness is, the magnetic flux generated by the excitation coil covers 8 detection coils, the signals are collected by the detection coils through the 8-channel preamplifier and are transmitted to the eddy current detection host through the connecting wire, when the detected heating surface tube has no defect, the space scattering leakage magnetic flux is balanced and symmetrical, when the detected heating surface tube has defects, the defects cause the leakage magnetic flux to be asymmetrically distributed, so that the detection coils on the defects generate unbalanced signals, the unbalanced signals are amplified by a low-noise amplifier and then are filtered by a balance filter and automatically balanced digitally, the wires are placed at a reference zero level, the time constants of the filter and the automatic balance are set by a controller, and the filtered and balanced defect signals respectively enter a phase shifter and a gain adjustable amplifier to be subjected to phase angle fixation and signal amplification, and are converted into digital signals by an analog-to-digital converter to be sent to the controller, and the three-dimensional display and data analysis functions are realized; the defect-free material can be determined by measuring the impedance change rule of the detection coil; the encoder synchronously records detection data, the quality of the pipe wall under the coverage area of the probe can be detected by one scanning, and as the sensitivity of each detection coil under the effective coverage area is equal, the defect omission factor is effectively controlled, the detection efficiency is improved, the magnetic tank is made of ferrite materials with high magnetic permeability, and the leakage magnetic flux scattered in space can be effectively eliminated; the shielding coil is arranged outside the magnetic tank and used for limiting and focusing the magnetic circuit and leading the magnetic circuit to the detection coil; two mutually perpendicular electromagnetic field transmission modes are formed between the exciting coil and the detecting coil, so that linear defects with different orientations can be found; the signals of the coils can be separately transmitted into an instrument, mutual inductance among different coils is effectively avoided, 8 detection probes which are arrayed along the circumferential direction can be used for effectively covering the 125-degree circumferential area of a detected heating surface tube, the interference of space scattering leakage magnetic flux is eliminated through the magnetic shielding technology of a magnetic tank and a shielding coil, the defect resolution is improved, the 8 detection coils can be independently imaged in the detection process, the detection sensitivity is always consistent, the defect missing detection is avoided, the defects of the inner wall of the heating surface tube covered by the detection probes can be detected through one-time operation, the reciprocating operation is not needed, the detection data is synchronously recorded through a data encoder, the detection data calling analysis and archiving are facilitated, the detection efficiency is improved, a front amplifier is arranged in the probe, the signals are amplified firstly, the attenuation of the signals in the transmission process is placed, a front freezing zero setting switch is eliminated, the trouble of time difference is eliminated, and the baseline is adjusted to zero in time when the baseline deviates from zero point; the device comprises an eddy current acquisition signal transmission line, an excitation oscillation transmission line, a positioning coding signal transmission line and a power line which are arranged in 8 channels, wherein a 24-pin aviation plug is arranged at the end part of the device, the device is convenient to connect, the shielding effect is good, the impedance matching is matched, the signal transmission is stable and lossless, the frequency range is kept between 1Hz and 2000Hz by a low-frequency oscillator, the minimum stepping value is 0.1Hz, the lower the frequency is, the deeper the detected wall thickness is, and therefore the device can be used for pipeline detection with different wall thicknesses; the 8 detection coils can independently image, the detection sensitivity is always consistent, the defect missing detection is avoided, the defect of the inner wall of the heating surface steel tube covered by the probe can be detected by one-time operation, the reciprocating operation is not needed, and the detection efficiency is improved; the advantages of the low-frequency array eddy current probe scanning image imaging display are utilized, the problem of positioning of corrosion defects on the inner wall of the heating surface pipe of the power station boiler can be effectively solved, the detection precision and the on-site working efficiency are improved, the reliable and stable operation of the heating surface pipe of the power station boiler is ensured, the use is convenient, the effect is good, and good social and economic benefits are achieved.
Claims (9)
1. The low-frequency array eddy current testing device is characterized by comprising an eddy current testing host, a connecting wire and a low-frequency array eddy current probe; the eddy current detection host is connected with the low-frequency array eddy current probe through a connecting wire;
the eddy current detection host comprises a host shell (19), wherein a display (20) and a probe connecting socket (24) are respectively arranged on the host shell, a low-frequency oscillator, a lithium battery, a controller, an amplifier, a balance filter, a phase shifter, a gain adjustable amplifier and a digital-to-analog converter are respectively arranged on the host shell (19), the output end of the lithium battery is connected with the input end of the low-frequency oscillator, the output end of the low-frequency oscillator is connected with the input end of the controller, the output end of the controller is connected with the probe connecting socket (24), the probe connecting socket (24) is connected with the input end of a low-noise amplifier, the output end of the low-noise amplifier is connected with the input end of the controller through the balance filter, the phase shifter, the gain adjustable amplifier and the digital-to-analog converter which are connected in series, and the output end of the controller is connected with the display (20);
the probe connecting socket (24) is connected with the probe socket (2) of the low-frequency array eddy current probe through a connecting wire;
the connecting wire comprises a connecting wire body (13), the connecting wire body (13) comprises a total shielding braided net (131) and an eddy current acquisition signal transmission line, an excitation oscillation transmission line, a positioning coding signal transmission line and a power line which are wrapped in the total shielding braided net (131), the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line are all formed by a single shielding braided net (132) and core wires (133) wrapped in the single shielding braided net (132), two ends of the connecting wire body (13) are respectively provided with aviation plugs (14) corresponding to probe sockets of low-frequency array eddy current probes and probe connection sockets of an eddy current detection host, pins (18) which are in one-to-one correspondence with the eddy current acquisition signal transmission line, the excitation oscillation transmission line, the positioning coding signal transmission line and the power line are respectively arranged on the aviation plugs (14), and the core wires of the corresponding transmission lines are connected with the rear ends of the corresponding pins;
the low-frequency array eddy current probe comprises a shell (1), the shell (1) is of a hollow structure, the section of a bottom plate (1 a) of the shell (1) is in an upwards concave arc shape, guide rubber wheels (4) are arranged on two sides of the bottom plate (1 a), a plurality of detection coils (11) are uniformly distributed on the bottom plate (1 a) along the circumferential direction of the arc shape of the same radial section, a magnetic tank (10) for eliminating space scattering leakage magnetic flux is sleeved outside the detection coils (11), exciting coils (7) are arranged in the shell (1) outside the magnetic tank (10), shielding coils (9) for focusing magnetic circuits are arranged between the exciting coils (7) and the detection coils (11), an amplifier (12) is arranged in the shell (1), an encoder (8) for synchronously recording detection data is arranged at one end of the shell (1), a probe socket (2) connected with an aviation plug is arranged outside the shell (1), and the output end of each detection coil (11) is respectively connected with the input end of the amplifier (12), and the output end of the amplifier (12) and the output end of the encoder (8) of the exciting coil (7) are connected with the output end of the encoder (8).
2. The low-frequency array eddy current testing device according to claim 1, wherein the radian of the bottom plate (1 a) is 2 pi/3, 8 testing coils (11) are uniformly distributed, and 8 eddy current collecting signal transmission lines are in one-to-one correspondence with the testing coils.
3. The low-frequency array eddy current testing device according to claim 1, wherein the host shell is respectively provided with a digital knob (21), a power switch (22), a charging socket (23), a USB interface (25), a grounding socket (26) and an RS-232 printer interface (27) which are connected with the controller.
4. The low frequency array eddy current testing arrangement according to claim 1, wherein the top of the housing (1) is provided with a handle (3).
5. The low-frequency array eddy current testing device according to claim 1, wherein the number of the guide rubber wheels (4) is 4, the guide rubber wheels are respectively arranged on two sides of the bottom plate of the shell, the bottoms of the guide rubber wheels extend out of the lower end face of the shell, and the guide rubber wheels are obliquely arranged downwards and inwards.
6. The low frequency array eddy current testing device according to claim 1, wherein a freeze zeroing switch (6) connected to the probe socket (2) is provided on the housing (1).
7. The low frequency array eddy current testing device as claimed in claim 1, wherein the magnetic tanks (10) are made of ferrite, each magnetic tank being covered with a detection coil corresponding to each other.
8. The low-frequency array eddy current testing device according to claim 1, wherein the total shielding woven mesh (131) and the single strand shielding woven mesh (132) are both made of nylon woven mesh.
9. A method for detecting corrosion defects on the inner wall of a ferromagnetic steel pipe by using the low-frequency array eddy current testing apparatus as claimed in claim 1, comprising the steps of:
A. determining detection waste judgment standard and calibrating
Firstly, processing a sensitivity comparison test tube, taking a tube with a heating surface of the same batch, cutting along the center of the tube, sequentially processing 4 artificial defect blind holes with diameter phi 6mm and hole depth 20% and designed wall thickness at intervals in the circumferential direction of the inner wall of the tube at intervals of 20 degrees, scanning and detecting the sensitivity comparison test tube by a low-frequency array eddy current detection device to detect the depth value of the P-T scanning curve amplitude calibration defect of the artificial defect on the sensitivity comparison test tube, setting the phase deviation value and depth of a scanning signal and the percentage of the wall thickness occupied by the defect as detection waste judgment standards, and judging the defect exceeding the amplitude in the detection process;
when scanning detection is carried out, two groups of guide rubber wheels are clung to the surface of a sensitivity comparison test tube, a probe is manually pushed to carry out detection, when an exciting coil carrying alternating current is close to the comparison test tube made of an electric conduction magnetic conduction steel tube, eddy currents are induced in the electric conduction magnetic conduction steel tube due to the effect of a coil magnetic field, the size, the phase and the flowing form of the eddy currents are influenced by the performance of an electric conduction material, the impedance of a detection coil is changed by a reaction magnetic field generated by the eddy currents, and the eddy currents are displayed in a display of an eddy current detection host after signal amplification treatment of an amplifier; the P-T scanning curve is a curve image of the eddy current phase shift quantity changing along with time;
B. defect localization
Two groups of guide rubber wheels are tightly attached to the surface of a steel pipe (5) to be detected, a probe is manually pushed to detect, and the outer surface of the detected pipe is scanned at a constant speed at a scanning speed of less than 3-5 meters/min; the current is processed by a low-frequency oscillator and then is sent into a probe excitation coil through a connecting wire by a controller, magnetic flux generated by the excitation coil covers 8 detection coils, signals collected by the detection coils are amplified by an 8-channel preamplifier and are transmitted to an eddy current detection host through the connecting wire, when a detected heating surface tube is defect-free, the space scattered leakage magnetic flux is balanced and symmetrical, the output of each detection coil is zero, when the detected heating surface tube is defect, the disturbance of the defect on the magnetic flux and the eddy current causes the leakage magnetic flux to be asymmetrically distributed, thus the detection coil on the defect can generate unbalanced signals, the unbalanced signals are amplified by a low-noise amplifier, then are filtered by a balance filter and automatically digital balanced, each line is placed at a reference zero level, the time constants of the filter and the automatic balance are set by the controller, and the filtered and balanced defect signals enter a phase shifter and a gain adjustable amplifier respectively for phase angle fixation and signal amplification, and are converted into digital signals after amplification, and are sent into the controller for display on a display, and the positioning of the defect position is realized:
1) Defect circumferential positioning
The method comprises the steps that inner wall defect detection is carried out on a boiler heating surface pipe through a low-frequency array eddy current probe, the probe can cover the range of 120 degrees of a detected pipe wall, a P-T scanning curve of each detection coil is independently displayed and corresponds to the position of a coil array, when the probe passes through the defective pipe wall, the corresponding detection coil cuts magnetic force lines leaking out of the pipe wall to induce a maximum voltage signal, the impedance of the coil is changed and displayed on the P-T scanning curve, so that the circumferential position of the defect is determined, and the circumferential defect is recorded;
2) Defect axial positioning
If the defect is found to need to store detection data in the detection process, external triggering is selected, namely pulse signal triggering of an encoder on the probe is performed, at the moment, a P-T scanning curve is synchronous with probe scanning detection, the probe scans forward and detects, the P-T scanning curve synchronously rolls forward and displays, the distance travelled by the probe scanning detection is displayed on a screen at the same time, when the defect is found to need to be rechecked, the probe scans backward and detects in a retrace mode, the P-T scanning curve distance which is equal to the distance travelled by the probe backward is erased on the screen in real time, so that the axial position of the defect is determined, and the axial defect is recorded;
C. defect review
Cleaning the surface of the positioned defect, coating a coupling agent, and re-measuring by using an ultrasonic thickness meter to confirm the depth of the defect, wherein the geometric shape of the surface of the pipe fitting is irregular, butt welding seams, bent pipe positions, pipe ends and internal stress can possibly cause trouble to defect discrimination, so that the final detection result is checked by using the ultrasonic thickness meter;
D. checking with the sensitivity contrast test tube every 2 hours, and if the manual injury on the sensitivity contrast test tube is difficult to distinguish, the previous detection is invalid.
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