Diagnosis Of Defects In The Inner Wall Of Heat Equipment Based On Infrared Temperature Measurement

During the production process, many industrial equipment need to work continuously at bad conditions of high temperatures, high pressure and chemical corrosion for a long time, resulting in damage or loss in the inner wall of equipments, which make defects or malfunctions for the equipments and bring great dangers to the safety of production. In our country, there are a great number of accidents in the chemical industry, electricity and other vicious systems every year. A large part of them are because of the lack of quantitative analysis of the defects in the industrial equipment. It fully shows the importance of the detection of the defects in the industrial equipment's inner wall.Infrared thermography is a relatively new technique in the field of nondestructive detection. Infrared scanner can measure the surface temperature of the object rapidly, leaving the object untouched and undamaged. Through the surface temperature distribution, the working status of the equipment can be evaluated. Infrared technique is a new, sensitive detection method and an important online monitor method, and has been widely used in many fields, such as electricity, metallurgy, petrochemistry, construction, transportation and so on. At present, from quantitative analysis to qualitative detection is a new trend of infrared detection technology. According to the laws of heat transfer, the temperature of object surface entirely depends on the structure of the object, thermal properties of the material, the thermal diffusion rate and the heat exchange between surface and environment. The defects in the inner wall of industrial heat equipment would be represented by abnormal temperature distribution of related part of the equipment. The information of defects can be obtained by using infrared scanner to get the temperature data at surface. Then the inverse heat conduction problem can be solved by appropriate physical and mathematical method with corresponding model to estimate the geometry boundary of the industrial equipment inner wall including character, location, size and geometry shape of the defects.The main tasks of this paper are as follow.1) A two-dimensional steady heat transfer model of hollow cylindrical equipment with unknown irregular geometry boundary of the inner wall is built. 2) The finite element method (FEM) is used to solve the direct heat conduction problem, and the temperature result replaces measurement data of infrared scanner for the input parameter to the direct heat conduction problem.3) The inverse heat conduction problem is transformed into a direct problem and a optimization problem and solved by minimizing the differences between actual temperature from direct problem and computed temperature value on the outer surface based on the conjugate gradient method (CGM) to estimate the defect configuration of the equipment inner wall.4) The rule of temperature distribution on the surface of the equipment with defects in the inner wall is studied and the effectiveness of the algorithm is certified well by numerical experiments. Furthermore, the effects of the measurement errors, choice of the initial value, boundary conditions and number of discrete temperature points are also discussed. The results show that this algorithm can hold back the magnifying ability for measurement error effect of the inverse problem and can be used in various heat transfer conditions, the initial value has little effect on the identification results. The method in this paper needs very short computer time and is easy to implement, providing a theoretical basis for infrared diagnosis of industrial equipment defect.