Research On Magnetic Flux Leakage Testing Methods For Storage Tank Floors

It is very important to inspect tank floors regularly to ensure the safety of storage tanks. With the development of economy and establishment of the national strategic crude oil margin system, the capacitances and amounts of storage tanks in petrochemical industry increase a lot, and testing works become burdensome. At present, there is no effective inspecting tool and evaluation method for tank floor testing. In this paper, a magnetic testing method is studied and a testing system is designed. It can inspect almost all the tank floor quickly and reliably. The consent of this paper is composed of four parts:First, the characteristic of magnetization for tank floor testing is studied. The concept of local magnetization mode is presented. Among those methods applied to study the magnetic fields distribution in the steel plate, numerical computing technique is the only effective one. The problems by using 2D finite element computing modeling to study the local magnetization for tank floor are analyzed, and 3D finite element modeling is set up. The results by computing different plate width models show that magnetic field concentrate around the magnetizing system, and local region of a steel plate even with infinite large area can also been magnetized to saturation. When plate area is large enough, the area change doesn't affect the magnetization status of the local region any more. That means the same inspection sensibility can be got even if the floor areas of different tank are different. Then, the effects on local magnetic field by the specimen material and specimen thickness are also studied. At last, the relationship between the magnetic flux leakage field and the local magnetization status is given. The effects of defect parameters on the peak value of magnetic flux leakage field are also presented. The results show that the MFL testing method is the powerful tool for tank floor testing.Second, a testing transducer composed of magnetizing system and detecting system is studied. The effects of magnetizing system parameters on local magnetization are studied first. The relationships between the local magnetization of steel plate and permanent magnet thickness, magnet pole area and the air gap between magnetic pole and the surface of steel plate are given. Results show that magnetic pole area is one of the key factor affected local magnetization. According to the experiments and optimization results, a magnetizing system with optimized parameters is given. The testing transducer, which combines magnetizing system and detecting system, is then designed. Experiments show that the transducer has good sensitivity.Third, inversion methods of signal processing are studied to identify defects from magnetic flux leakage testing data. A combined finite element – nonlinear conjugate gradient method for the reconstruction of defects profiles is studied first. The optimization results from the conjugate gradient method are used as the parameters of finite element models. Difference method is applied smartly to fulfill the joint of the two techniques. The inversion results from ideal data and data overlapped noise are shown respectively. Another method is based on artificial neural network, which overcomes the shortcomings of finite element –conjugate gradient method. The method not only has higher inversion precision, but also can do real-time work.Based on the research ahead, the magnetic flux leakage testing system is completed. The testing system consists of transducer subsystem, signal preprocessing and data acquisition subsystem, signal processing subsystem and power subsystem. Compared with other instruments applied to inspect tank floor, the system has high sensitivity, reliability and inspecting efficiency. It has enormous economic benefits and extensive application foreground.