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Inspecting the Inner Corrosion Defects by Magnetic Flux Leakage Testing MethodLI LUMING, WANG LAIFU, HUANG SONGLING, LIU SHIFENG
NDT CENTER, DEPARTMENT OF MECHANICAL ENGINEERING
TSINGHUA UNIVERSITY, BEIJING 100084 CHINA.
The inner surface of heat exchanging pipe of the boiler will be eroded by the electrochemical reaction when water flows inside the pipe during service. The corrosion of the pipe wall finally will result in problem. Typically, the wall thickness is measured periodically to make sure the boiler will work safely. The method is ultrasonic thickness measurement generally. The limitation of point-measuring and its random feature will cause reliability problem.
Magnetic flux leakage method (MFL), which is used to inspect the pipe inner condition, is discussed in this paper. The inspecting system established in our NDT center is introduced. Magnetic strength, inner defects characteristics and magnetic flux leakage field are discussed and show their relationship. The influence of out surface condition is also described. The testing results show the system will inspect the pipe quickly and reliably. The inner defects of the pipe can be easily measured if MFL combined with ultrasonic thickness measuring is used.
Key words: magnetic flux leakage testing, erosion testing, heat exchanging pipe ,sensor scanning, portable testing device
The inner surface of heat exchanging pipe of the boiler will be eroded by the electrochemical reaction when water flows inside the pipe. The other factors include the difference of temperature and stress condition in and out of the pipe. Generally, the corrosions consist of oxygenic and hydrogenous corrosions, corrosive slot and flaw. The occurrence and growth of the defects will result in the failure of the boiler, even the human loss. Therefore, the pipes of boilers are inspected periodically.
Ultrasonic thickness measurement is the common method for this inspection. It is determined by the point measuring that it can only do stochastic spot-check on the status of the corrosions of the heat exchanging pipes. It couldn't reflect reliably the inner corrosions and the status of the destroy of pipe, and leave over the hidden trouble. The testing technique of the heat exchanging pipe based on the theory of magnetic flux leakage method (MFL) is brought forward according to the need of the current safety detect.
MFL, which is used to inspect the pipe inner condition, is discussed in this paper. The potable inspecting system established in our NDT center is introduced.
|Fig 1: the configuration of potable system.|
The portable inspecting system on heat exchanging pipe is composed of sensor scanning device fitted for straight or flexed tubes, magnetization device, sensor array and host machine. The constitute frame is shown as Figure-1:
The scanning sensor device includes transducer carrier, slide machine and the framework. The magnetizing device is used to magnetize the pipe inspected. The sensor array consists of 8-9 hall sensors and their signal pre-processing circuits. The host part integrates signal sampling, signal processing, signal wave display, data storage, alarming and communicating with PC.
2.1 sensor scanning device
The sensor scanning system is mainly composed with the modules of transducer carrier, magnetization device and mechanics framework. The transducer carrier module is applied to carry the hall sensors array, hall sensors can quiver up and down in the runner, and they can be ensured to attach closely to the surface of pipe to increase the lift-off value furthest, it will improve the measurement precision.
|Fig 2: sensor scanning device.|
The magnetization device has 3 main components: permanent magnet, magic yoke and magic shoe. It magnetizes the pipes axially. It can cut or close the magnetic route by the mechanical method to implement detecting pipe during service easily.
The mechanics framework is to brace the whole machine, the framework is made into a dolly, it will slide axially through the surface of the pipes, the sensor array and the magnetization device are fixed on the framework, move followed with the framework.
2.2 magnetization device
Magnetization plays an important role in MFL Testing, It wouldn't generate magnetic flux leakage field if the material be inspected is not magnetized to a suitable level. It is considered that magnetization is suitable when defects could be detected in its magnetic flux leakage field according to the rules of economic and applicability.
The permanent magnet, which magnetism is mighty enough, was used as the magnetic pole replacing of electrical magnet order to decrease the power loss and make the structure compactly. The thulium magnetism material, NdFeB has high Hc, big (BH)max, which is suitable to used as the magnetic pole of the inspecting system which is applied to detect the pipes whose diameter and the width of pipe wall vary little. The NdFeB was chosen as the magnetic pole of the device.
Hall probes suitable for the measurement of magnetic strength inside a crack of 0.2 mm wide and lift off distances of 0.2 mm have been developed by Forster.[2-3] We adopt the hall sensor as the magnetic test element mainly for it can meet the need of the detecting precision, for its little bulk, light weight, for it possess good temperature adaptability and wide response frequency band. The most important reason is that the amplitude of the signal generated by the hall sensor is independent with the velocity of the sensor scanning.
|Fig 3: Pipe magnetization.|
We magnetize the pipe locally in axial direction considering the own character of the heat exchanging pipe, the transducer array is placed around the surface of the pipe in the middle of two magic shoes. The sensor array moves on the surface axially to detect the pipes. The sensor array covers 1/3 perimeter of the pipe, so it can do a full-scale detection for a pipe when it scans on the pipe axially for 3 times.
2.3 host machine
The part of host machine is composed with signal processing circuit and data sampling MCU system. The magnetic flux leakage signal is very tiny, so a high SNR, low yawp analog circuit is the important part of the system. The signal processing circuit includes the main amplify circuit, filter circuit and the impedance matching circuit.
The MCU data sampling system adopt AT89C51 as its CPU, the peripheral equipment includes AD convert circuit, LCD display module, keyboard, data storage circuit, alarming circuit and the interface circuit of RS232. It takes charge of the high precision data sampling, signal wave display, signal process, alarm function of the system.
The portable inspecting system used common battery as power supply, so the power consuming of the system is a very important parameter, the low power loss technique concerned is the kernel to resolve the problem.
The chief factor when we select an IC is to consider its power loss in order to decrease the power loss of the inspecting system. We also followed the rule of least power loss to design software, when the IC is not at the work station, we convert them to the waiting mode, low power cost mode or sleeping mode in order to decrease the power loss furthest.
Experimental research was done at heat exchanging pipe using the system. The results below was on a heat exchanging pipe of outer diameter 48mm, thick 3mm, there are some artificial defects on it, which are 0.5mm * 0.5mm, 0.5mm*1.0mm rectangular radical slots in the inner and outer of pipe wall, F0.5mm, F1.0mm holes on the wall. Parts of the signals and their peak-peak values were shown as below.
The results analyzing from the signals were accordant to the theory of magnetic flux leakage. According the same size, same shape defects in inner and outer pipe wall, the amplitude of the magnetic leakage field generated by the inner defects is smaller, the pace increasing or decreasing is slower. These characters are embodied in the amplitude and width of the detected signals which can used to separate inner or out defects..
For hole defect, the leakage flux generated is smaller, and the leakage flux signal reflects the shaper of the defects, so it can be identified expediently from its signal generated.
For practical application, the flux leakage signals generated by the cankerous pits on the outer pipe wall and signal generated by the narrow and deep flaw in the inner pipe wall are very similar in peak-peak value and width. Therefore we can judge the disputed area integrating the ultrasonic thickness measuring method.
The portable multi-channels inspecting system on the heat exchanging pipe of boiler developed meets the need of the inspecting inner defects of the heat exchanging pipe, and it can detect the pipes efficiently and fleetly.
The experiment result indicated that: the signal amplitude generated by the defects is in direct ratio with the deep of the defects mainly. This provides experimental proof for the quantification of the defects detected for heat exchanging pipe or other similar industry task based on the system, and the implement of non-destructive evaluation finally.
This work has been sponsored by the National Natural Science Foundation of China (Grant No.50001006).
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