Infrared Thermography Test for High Temperature Pressure Pipe cont.................

3.2 Outside Cooling After Steam Heating After pipes heated a short time (from 30 seconds to 5 minutes), the stationary heat flow condition will be constructed. Temperature differences between areas of different wall thickness within a pipe are nonexistent. In order to get significant thermal contrasts for the defects, outside cooling should be performed to break the stationary heat flow condition. The outside cooling are tried by use of ice and refrigeration gas. The minimum detectable defect is a inside hole with diameter 6mm and depth 1mm for F114´4mm stainless steel pipe and a inside hole with diameter 10mm and depth 2mm for F140´5mm carbon steel pipe. Figure 8 shows the instantaneous thermal images of the defects for these two pipes after ice cooling. Fig 8: Thermal images of defects after steam heating and ice cooling. 4. ANALYSIS FOR THE TESTING RESULTS Through viewing the whole infrared thermal imaging process and analyzing the testing data listed in table 3, a serious of phenomena and laws are found as follows: Comparing the minimum detectable defects of F114´4 stainless steel and F140´5 carbon steel, it can be found that the testing sensitivity of stainless steel is much higher than carbon steel. For a F10mm opening hole defect, the duration time is about 2 minutes from emerge to disappear of the defect on thermal image for stainless steel pipe, but it is just 30 seconds for carbon steel pipe. The reason is that the thermal conductivity of stainless steel is 15W/(m.°C) and carbon steel is 48W/(m.°C). That is, the thermal conductivity of materials is a key factor affected the sensitivity of infrared thermography test. The lower the thermal conductivity is, the higher the sensitivity is and the longer the duration of defects emerging is. For defects of different diameters in the same pipe, the detectable depths of the defects are different. This phenomena show that the size of defects is another key factor affected the sensitivity of infrared thermography test. The larger the area of defect is, the higher the sensitivity of wall loss is. Comparing the testing results of 3 type carbon steel pipes with different thickness, it was found that the thinner the wall is, the smaller the size of detectable defects is. But the thicker the wall is, the longer the duration of defects emerging is. For a F10mm opening hole defect, the duration time is about 30 seconds for F140´5 pipe, 120 seconds for F168´16 pipe, and more than 200 seconds for F180´36 pipe. Thus it can seen that the thickness is also a key factor affected the sensitivity of infrared thermography test. The thicker the material is, the lower the testing sensitivity is, but the longer the duration of defects emerging is. Comparing the testing results of inside heating and outside cooling, it was found that the detecting sensitivity of inside heating is higher than outside cooling. The detecting sensitivity of refrigeration gas cooling is higher than ice cooling. Assuming that the operation pressures of the 4 type pipes in table 2 all are 3 MPa, fracture mechanics calculation shows that a 10mm diameter and 80% wall loss defect is safe for all the 4 pipes. That is, the sensitivity of infrared thermography test is much more over the accepted defects for safe operation of the pipes. 5. CONCLUSION The testing results show that infrared thermography test is a reliable non-destructive method for detecting wall loss defects produced by corrosion and flow erosion of high temperature pipe. The sensitivity of infrared thermography test is much more over the accepted defects for safe operation of the pipes. The thermal conductivity of materials is a key factor affected the sensitivity of infrared thermography test. The lower the thermal conductivity is, the higher the sensitivity is and the longer the duration of defects emerging is. The shape and size of defects is another key factor affected the sensitivity of infrared thermography test. The larger the area of defect is, the higher the sensitivity of wall loss is. The thickness is also a key factor affected the sensitivity of infrared thermography test. The thicker the material is, the lower the testing sensitivity is, but the longer the duration of defects emerging is. The excitation method breaking temperature balance is one of factors affected the sensitivity of infrared thermography test. The detecting sensitivity of inside heating is higher than outside cooling. The detecting sensitivity of refrigeration gas cooling is higher than ice cooling. The outside cooling method is suitable for on-line infrared thermography test for high temperature has very large potential in future. REFERENCE Edited by Xavier Maldague, Nondestructive Testing Monographs and Tracts Volume 7, Infrared Methodology and Technology, Gordon and Breach Science Publishers, 1994. Roderic K. Stanley, Patric O. Moore and Paul Mclntire eds, Nondestructive Testing Handbook, Volume 9, Special Nondestructive Testing Methods, American Society for Nondestructive Testing, 1995. T.-M. Liou, C.-C. Chen, T.-W.Tsai, Heat transfer and fluid flow in a square duct with 12 different shaped vortex generators, Transactions of the ASME. Journal of Heat Transfer vol.122, no.2, p.327-35, May 2000. Richard N. Wurzbach, David A. Seith, Infrared Monitoring of Power Plant Effluents and Heat Sinks to Optimize Plant Efficiency, Proceedings of SPIE - The International Society for Optical Engineering Thermosense XXII April 24-27, 2000, Orlando, FL, USA. R. Rozenblit, M. 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