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Research on Measuring Abrasion of Pumping Oil PipeLI LUMING, HUANG SONGLING, WANG LAIFU, LIU SHIFENG
NDT CENTER, DEPARTMENT OF MECHANICAL ENGINEERING,
TSINGHUA UNIVERSITY, BEIJING 100084 CHINA.
Pumping oil pipe is widely used in oil well of petroleum industry. Occasionally, pumping shank will wear the pumping pipe during operation, and damage the pipe. To avoid the wearing off of over-tearing by the pumping shank, it is necessary to inspect the abrasion of pumping pipes. The magnetic testing principle for wearing is introduced in this paper, which is achieved by combining the techniques of computer, electro-mechanical control, signal processing and magnetic flux leakage testing. The probe used for magnetic testing rotates and scans inside of the pipe. The influence of geomagnetic filed and the other noise caused by the uneven magnetic fields of the pipe inner surface condition during magnetization are eliminated by applying differential sensors. Experimental data show that this method can detect the wearing of pumping pipes and also can evaluate the wear degree, which is beneficial to select the severely damaged ones from the pipes for application.
Keywords: magnetic testing, abrasion testing, pumping pipe
Pumping oil pipe is widely used in oil well of petroleum industry. Abrasion along the axis occasionally occurs due to the severe operation condition . To avoid accidents,pumping oil pipes are replaced regularly, which results in the waste of many pumping oil pipes with no dangerous abrasion defects. Therefore, it is essential to test and diagnose the amount of abrasion of pumping pipes. An effective method to inspect the wearing of pumping pipes is introduced in the paper, which is achieved by combining the techniques of computer, electro-mechanical control, signal processing and magnetic flux testing. Experimental data show that this method can detect the wearing of pumping pipes and the rotating probe can reduce the possibility of missing any wearing flaws.
The main principle of magnetic leakage testing is based on the high permeability of ferromagnetic materials. The permeability of wear pipes is very small than that of normal ones. Magnetized steel pipes will act differently as they have wear flaws or not. When there is no wear flaw, the main magnetic flux circuit is dominating. Otherwise,the amount of magnetic flux of secondary circuit will turn to be larger when there have wear flaws on the wall of pipes. Therefore,the wear flaws can be detected by testing the flux change of the main magnetic circuit. Figure 1 shows the distribution of magnetic field when there is a wear flaw.
|Fig 1: The distribution of magnetic circuit of wear pipe.|
To detect the full signal of wear flaw, the magnetic probe should scan along the inner surface of the pumping pipe. The geometrical form of the flaw can be gotten from the change of magnetic flux leakage. Different testing system suitable for different workpiece needs various scanning devices. In the auto-testing of pipes, the technique that magnetic sensor and magnetic field move together along the surface of the workpiece is usually adopted [2-4].
The wear inspection instrument of oil pumping pipe contains two large parts. First is hardware, the other is software. Hardware (Figure 2) contains probe scanning device, magnetizing device, sensors, analog signal processing circuit and computer.
|Fig 2: The components of wear inspection instrument of oil pumping pipe.|
Probe spiral scanning along the inner surface of pipes was adopted to inspect pipe wall thoroughly. This scanning mode can keep relatively high inspect efficiency under lower equipment cost, alleviate the working intensity of operator and quantify defects easier. Figure 3 is the sketch map of probe scanning. This scanning mode can reduce the possibility of missing any wearing flaws because most of wearing defects is relative longer along the axis of the pipe.
|Fig 3: The sketch map of probe scanning.|
Magnetizing plays an important role in magnetic flux leakage testing which influences testing sensitivity. Permanent magnet with good performances was adopted to get a more compact construct and have lower power consuming. Permanent magnet locates in the center of the pipe and the magnetic circuit consists of central permanent magnet, magnetic pole shoes and the wall of pumping pipe. It improved the effect of magnetizing largely [5,6].
The select of magnetic sensor is very important to designing the probe. There are Hall element, testing coil, etc as magnetic sensors. The large dimension of testing coil lower the testing sensitivity and the defect resolving power. Whereas, Hall element can be made very small(smaller than 0.2mm´0.2mm), so it can get higher defect resolving power, most of all, it has higher testing sensitivity, broad response frequency band, good input-output linearity and better adaptability to temperature. So Hall element was introduced as magnetic sensors in the wear testing system of oil pumping pipe.
The signal, which is detected by the Hall element, is affected largely by the interference of the electromagnetic source around. Differential method, which is achieved by encapsulating two Hall elements with similar parameters in one probe, is introduced to weaken this interference. Differential signal is collected using the computer after amplified and band-pass filtered. Computer is an important component of the testing system. It controls sampling and plays a roll as signal processing, showing defect signal wave form and judging defects.
The system software is programmed by language C. Simple signal processing methods such as median filter, relative threshold are adopted. Experimental data show that relative threshold method is very effective to judging defects. Figure 4 shows the system program flowchart.
|Fig 4: Program flowchart.|
An oil pumping pipe which has been used in an oil field for some time was taken to test. The diameter of the pumping pipe is 80mm, the thickness of pipe wall is 5mm. There is a wear defect along the axis of the pipe in the inner wall of it. The width of the wear zone is 10mm and the deepest area of it is 2mm. Signals detected by the probe are magnified and band-filtered, then collected and processed by the computer. Figure 5 shows the wear defect signal after the series processes.
|Fig 5: Wear defect signal in the inner wall of pumping pipe .|
This work has been sponsored by the National Natural Science Foundation of China (Grant No.50001006).
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