УДК 620.179.141.1
OPTIMIZATION OF THE MAGNETIC CIRCUIT IN THE MFL INSPECTION SYSTEM FOR STORAGE TANK FLOORS1
Song Xiao-chun'2, Huang Song-ling1, Zhao Wei'
'State Key Lab of Power Systems, Dept. of Electrical Engineering, Tsinghua University, Beijing 100084, China;
2College of Mechanical Engineering, Hubei University of Technology, Wuhan 430068, China
ОПТИМИЗАЦИЯ МАГНИТНОЙ ЦЕПИ В СИСТЕМЕ КОНТРОЛЯ
ДНИЩ РЕЗЕРВУАРОВ ПО РАССЕИВАНИЮ МАГНИТНОГО
ПОТОКА
Сонг Ксяо-чун12, Хуанг Сон-линь', Жао Вей'
'Государственная лаборатория энергетических систем, Факультет электроэнергетики, Циньхуаньский университет, Пекин, 100084, Китай 2Механический факультет Хубейского технологического университета, Вухань, 430068, Китай
Намагничивание — это ключ к проверке днищ резервуаров по рассеянию магнитного потока (MFL). Для оптимизации магнитной цепи MFL-детектора и получения наилучшей чувствительности контроля методом конечных элементов (FEM) изучено влияние размеров постоянного магнита на намагничивание днища, плотность магнитного потока рассеяния и магнитодвижущую силу, а также проанализировано влияние некоторых других параметров, таких как межполюсное расстояние и толщина полюсного наконечника, на соотношение сигнал/шум. Результаты расчетов показывают, что ширина магнита влияет на намагничивание много больше, чем его толщина и для магнита толщиной 30 мм и шириной 40 мм можно достичь оптимального соотношения между намагничиванием и магнитодвижущей силой. При условии намагничивания днища до насыщения увеличение межполюсного расстояния и толщины полюсных наконечников может повысить чувствительность контроля и улучшить соотношение сигнал/шум.
Abstract
Magnetization is the key to inspect the tank floor using magnetic flux leakage (MFL) technique. In order to optimize the magnetic circuit of the MFL detector and obtain the best detection effects, the influences of the magnet sizes on the floor magnetization condition, the gap magnetic flux density and the magnetic force were studied with the help of the finite element method (FEM), and the effects of some other parameters like the magnet pole spacing and pole piece thickness on the signal-to-noise ratio were also analyzed. The simulation results indicate that the variation of the magnet width affects the magnetization much more than that of the thickness, and the detector can get a trade-off between the magnetization effects and the driving force while the magnet is about 30 mm thickness and 40 mm width. And under the condition that the floor has reached its magnetizing saturation, increasing the magnet pole spacing and the pole piece thickness can improve the testing sensitivity and better the signal-to-noise ratio.
1. INTRODUCTION
Storage tanks are the important equipments to store oil and other petrochemical products, and the tank floors run in a very hazardous environment,
'Foundation item: Project (50305017) supported by National Natural Science Foundation of China and Project (2005038317) supported by Chinese Postdoctoral Science Foundation.
Correspondence: Song Xiao-chun, PhD; Tel: +86-10-62773070; E-mail; songxc@ mail.tsinghua.edu.ch
the hydrous medium on the top surface and the groundwork underside of the floor often attack the plates, the corrosion and erosion cause the steel plate thickness decrease and being failure, even lead to leakage and cause the fire, explosion and pollution of the environment, which the safety and cost-effec-tive of the oil plant is influenced seriously. Therefore, the periodical corrosion inspection for the floor is one of the most important tasks for tanks. To realize the full inspection quickly and size the defects accurately, several non-destructive testing (NDT) methods that could potentially be applied to the tank floor were studied [1,2]. Comparing with other methods, the magnetic flux leakage (MFL) NDT technique has such advantages as low-cost, efficient-inspection and easy-implementation, and tank floors are often produced by ferromagnetic materials with good permeability, so it is very suitable for using the MFL technique to assess the condition of the floor and locate the flaws.
Magnetization is the base of the MFL testing, if the floor has not been magnetized to a suitable extend, the defects could not leakage enough magnetic flux to be captured or the thin flux could be flooded by the noises, that is to say, the magnetization capability is the key to influence the MFL inspection system reliability [3]. In general, for the same defect, the stronger the applied magnetic field is, the larger the generated magnetic flux leakage is. However, to provide the strong magnetic strength, it must increase the sizes of the mag-netizer, which can cause the detector weight and the magnetic force between the magnet and the floor increase directly. As a result, it can also lead the driving power increase in the automatic NDT apparatus. Hence, it is very important to design an optimal magnetic circuit and select an appropriate external magnetic field strength in the development of the tank floor MFL inspection system.
Because of having the strong nonlinear behaviors, it is hard to get an ideal result by solving the Maxwell equations directly in the magnetic circuit design. And the magnetizer sizes are traditionally specified by one's experiences and experiments firstly, and then the magnetic circuit could be computed and analyzed using such techniques as the permeance method [4], the difference method [5], the magnetic charge integral calculus (MCIC) [6], the finite element method (FEM) [7-11], the boundary element method (BEM), and so on. Especially with the high-efficiency numerical method being used widely, the finite element method has being become a very useful technique to solve the electromagnetic fields problem, and the different defects, magnetic circuit structure and various inspection factors can all be simulated easily using the FEM software.
2. MAGNETIC CIRCUIT STRUCTURE AND ITS FINITE ELEMENT MODEL FOR TANK FLOOR MFL INSPECTION
Since the area to be inspected is much bigger than that of the magnetizer, the tank floor can only be magnetized locally, and the magnetic circuit to inspect the floor could be much more complicated than that of the pipelines and the steel bars. In addition, the entrance of the tank is limited (with diameter about 500 mm), which requests the magnetizer to be compact, whereby there will be a tradeoff between the magnetizer sizes and magnetization capabilities. Therefore, under the condition that the floor has reached its magnetizing saturation, the sizes of the magnetizer should be compacted and its weight should be small as far as possible.
According to the tank floor structure and its flaws characteristics, a 3D finite elements model, shown in Fig. 1, was designed to study the influence of the magnetizer sizes on the floor magnetization condition and the signal-to-noise ratio, where the W and H are the magnet width and thickness respectively. The magnetic circuit is constituted by yoke, permanent magnets, pole pieces and the tank
64
CoHr KcHO-nyH, XyaHr Coh-jihhl, 5Kao Ben
floor. The Nd-Fe-B N 48 permanent material is selected as magnets and its capability are shown as Table, the industrial pure iron and the material Q235 are cho-
Fig. 1. MFL finite element model to inspect the tank floor.
sen as the magnetic yoke and the tank floor respectively, which their magnetic properties parameters are specified by the Institute of Physics, Chinese Academy of Sciences.
The capability of permanent magnet Nd-Fe-B
No Residual magnetism (Br) Coercive force (He) Maximum energy Product (BHmax)
N48 Max 14.3 kGs Min 13.7 kGs kOe >10.5 kA/m >836 Max 49 MGOe Min 45 MGOe
3. OPTIMIZATION OF THE MAGNETIC CIRCUIT SIZES USING 3D FINITE
ELEMENT MODEL
In the MFL testing, the external magnetic field is generally selected in accordance with the magnetic properties of the material to be inspected. And the magnetic strength, which represents the share of the saturation magnetic strength being more than 80 %, is commonly considered as a proper one. Some factors like the magnet pole area, the thickness and the magnetic gap, have direct effects on the floor magnetization. In the model shown in Fig. 1, the gap between the magnet pole and the floor is 5 mm height and the sensor lift-off value is 2.5 mm. In order to enter into the tank through the entrance easily, the length of the magnet poles is only specified as 300 mm. Using the OPERA-3D finite element analysis software, the magnetic flux density in the steel floor and the gap magnetic flux density are calculated, and the emphasis is to analyse the influences of the magnet sizes on the floor magnetization, which the results can be used to design the magnetic circuit and optimize the sizes of the magnetization system. Taking the symmetry of the model into account, only a quarter of the magnetic circuit is to be calculated.
3.1 Relationships between the magnet sizes and the floor magnetization
Fig. 2(a) and 2(b) show the influences of the magnet width and thickness on the floor magnetization if other parameters keep constant. The parallel component (Bx), which mainly indicate the magnetization condition of the floor, is the magnetic flux density in the floor (Z = -5 mm). From Fig. 2(a), it is found that
the Bx increases sharply with the magnet width increasing at first, but when the width is over a certain point and the floor is being magnetized saturation gradually, the increment of Bx would become slow. And Fig. 2(b) demonstrate that the Bx value only has a slightly increases with the magnet thickness increasing continuously. For example, the increase extent of Bx is less than 10 % when the magnet thickness increases from 15 to 50 mm, but the increment of Bx is about more than 100 % if the magnet width transforms from 15 to 50 mm. Apparently, the magnet width variation has greater influence on the magnetization than that of the thickness.
Magnet width W/mm Magnet t
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