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Research on intelligent pipeline magnetic flux leakage detectorYang-Lijian, Fong-Haiying and Wong-Yumei
School of Information Science and Engineering, Shenyang University of Technology,Shenyang, China.
A new apparatus for non-destructive test (NDT) of underground pipeline-- magnetic flux leakage (MFL) is provided in this paper, as well as the present condition of overseas and domestic developments in this field. The basic principle of this method is described in this paper. An inspection apparatus, designed in Shenyang University of Technology for underground pipeline, is explained. The main performance and feature are also described.
IndexTerms: Pipeline, Non-destructive test (NDT),Magnetic flux leakage (MFL), Discrimination of defects, Detector
The basic requirement of transportation oil and gas pipeline is safety and high efficienty. However 50% of the world pipe network has been used for several decades. The pipeline may occur leakage and result in danger because of erosion, abrasion, unexpected damage and so on. Meanwhile, the long transferring pipelines are buried underground or ocean floor on the whole, which make pipeline detection become more difficult. As a result, many countries have paid more attention to the detection of the long transferring pipelines. The high efficiency and safety of long pipeline running has been an important problem with relation to the national economy and the people's life hood.
On the basis of plenty of studying correlative technology, we have studied and developed high precision pipelines magnetic flux leakage on-line detection system. Magnetic flux leakage pipeline installation, for short MFL-PIG, is presently the most advanced implement of oil and gas pipeline erosion in the international detection. Detecting state parameters of erosion in pipeline wall are acquainted by detecting the MFL generated by flaws, which can offer reliable firsthand data for quantitatively evaluating the residual intensity and the span of pipeline .The results detected by MFL method (assume excellent) are quantificational, objective and recordable. MFL-PIG is a kind of on-line detecting robot of high and novel technology, and is also a kind of intellectual pipeline detector based on MFL non-detective test principle. It utilizes excitation setting to magnetize the pipe wall and explores the location of flaws by sensor array at the same time. Erosion degree, flaw size and the area of defects can be located by means of computer analysis. Detecting apparatus is involved in many advanced technology fields such as machinery, electron, sensor, flaw detection, computer. When utilized, the setting is sent into the on-line pipeline by launching cylinder and goes forward at the speed of 0.7~4 m/s on the push of transportation medium's trust in the pipeline, meanwhile detection is completed, as well as data procession and data storag. The process can last umpty hours and test at least 100 kilometers long. When finished, the setting was pushed by reception cylinder. Then, by analyzing the recorded data, the erosion condition can be judged, and the service life can be ascertained, so as to prevent the possible malignant accident .
MFL test device obtains the magnitude of the defects by measuring the magnetic density leaked from the surface of the ferromagnetic material, shown in Figure 1. If the components to be measured are glossy and defectiveless and inclusive, all magnetic flux will pass through the components, seen from figure 1(a). But, as can be seen in figure 1(b), there are flaws in the parts, the relativity near the flaw increases. And this makes the magnetic flux aberrance. So the magnetic flux can be divided into three parts. First, most of the magnetic flux would pass through the detected components, making a detour from the defects. Second, a little part of the flux would transit the lacunas. And then, some flux would puncture surface of the components, detouring the lacunas. Hall sensor or mobile induced coils could detect the third part of the flux. Power or weakness of the flux signal has an intensive relation to the condition of the pipe surveyed .
|Fig 1: principle figure of MFL.|
The MFL method has been widely used on corrosion inspection of oil & gas pipeline and tank bottom, and also be applied in magnetic material inspecting, such as steel line, steel plate and so on.
Figure 2 shows the construction of a MFL apparatus for pipeline developed by Shenyang University Of Technology. It is composed of 4 parts, namely: impetuses section, measure section, computer section and battery section. Rubber bowls are mounted on the front and back part of each section, with gimbals between each section to permit the device pass through the bent belt. The pressure of oil or gas in pipe promotes the whole system. Each section is sealed tightly, and the maximum pressure it can bear is 10 Mpa.
|Fig 2: structure of MFL-PIG.|
Rubber bowl fixed on impetus section is used to generate pressure to push the device forward and clear the pipe. With magnetizing equipment, Hall sensor, and low frequency transmitting set and accepting device, measure section can gain MFL signals about the pipeline, and the tracks of the underground apparatus are followed. This can improve the locating precision. Computer section is the corn part of the system. Measure controlling, data processing and storage are disposed here. Battery section supplies the large amount of power for long-time work of the apparatus in pipeline.
Impelled by liquid pressure in pipe, and moving with the pipe liquid, the MFL pipeline inspection apparatus magnetizes the pipe with a constant magnetic intensity. At the same time, Hall sensors will sensor the magnitude of flux leakage inside the pipe. Data measured by sensors are stored in the memory after compressed. By disposing and analyzing the detected data, we can get the condition information of the pipe, including flows and corrosions. Based on the mileage of the odometer, the location of the flows and corrosions can be acquired. This can be used as the foundation for pipeline maintenance and life-assess .
1. The acquirement of MFL signal:
On the basis of analyzing and comparing of many kinds of sensors, the ones with such high performances as high sensitiveness, good-linear and high anti-interference can be selected. Enough information can also be obtained by increasing the number of sensors and using multi-dimensional composite probe, so as to improve the precision of the system.
2. Tracking and locating of the setting:
The setting specified the position of defect through the mileage value. In order to decrease accumulated error, such three methods as detecting equipment mileage record, signing pipeline and labeling mileage per 2 kilometer using specific equipment were adopted to follow up location. On the ground portable tracking device was developed to find detecting set in abnormal conditions
3. Data storage:
Because of the bulky recorded data and the fast record speed, big capacitance commercial grade IDE ,which can resist magnetic interference, high temperature and shock motion, is adopted as storage equipment. Advanced technologies of data compression and data transmission are used to solve the problem of data storage.
4. Mechanical passing performance and sealing performance:
The passing performance is the foundation of the setting. A set of reasonable mechanical size was obtained by elaborate design and repeated tests. Good mechanical sealing performance was insured through pouch and squash experiment .In order to ensure the safety of the setting when running, such processes as clearing pipelines, measuring variable diameters, passing experiments of the model with the same weight and size were done before field test.
5. The compensation of MFL and the discrimination of defects:
At first, using finite element method in computer, simulate the influence imposed on MFL signal by various interference, and take hold of the law. Then finde corresponding data processing method, and compensate the influence. At last, gain the pure signals only in correspondence with the shape and size of defents, and discriminate the flaw. On the basis of plenty of field measurements, field test and computer simulation, parameter database of defect characteristic was set up. Expert system of pipeline defect discrimination was also built.
6. Visualized figure of pipeline defect shape:
Blowback software which can read and judge data and support pipeline data base system was built to offer objective, exact and visual references to pipeline maintainers.
The setting creatively solved such pivotal technologies as measuring, high-speed data acquisition, bulky storage, magnetic circuit design, sashay location, computer procession, integral design. Exploration data indicate that the setting has reached the international nineties' level. This pipeline MFL detection system researched assumed such these characteristics as high veracity and high reliability, which offeres wider foreground for development of MFL detect technology.
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