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Infrared Thermography Test for High Temperature Pressure Pipe Gongtian Shen, Gang Chen, National Center of Boiler and Pressure Vessel Inspection and Research Beijing 100013, China Tao Li, Chunshu Li Machinery Research Institute of Tianjin Petro-chemical Corporation Tianjin 300271, China. Contact

Abstract:

High temperature pressure pipelines are extensively used in petrochemical plant and power station. The pits or wall loss defects can be severely internally produced after a certain period of time due to media corrosion and cavitation erosion. These defects finally lead to leakage of the pipes. In order to search for a good non-destructive method for these defects, a big pipe testing installation was constructed. A serious of infrared thermography testing experiments were performed for 4 kinds of stainless steel and carbon steel pipes which are drilled different size holes on inner-surface. The testing results show that infrared thermography test is a very suitable method for non-destructive test of corrosion pits and wall loss defects of high temperature pressure pipes. The corrosion pits or wall loss defects that size are bigger than F10mmX40% wall thickness can be detected by infrared thermography test. The testing sensitivity satisfies the requirements of pipe safety operation.

Keywords: High temperature; Pressure pipe; Pipe, Infrared thermography test; Infrared test.

1. INTRODUCTION

High temperature pressure pipelines are extensively used in petrochemical plant and power station. The leakage and explosion accidents often take placed after a certain period of time because of media corrosion, cavitation erosion, welding defects cracking, stress corrosion cracking and materials deterioration. According to the statistic of large number of pipe leakage accidents, media corrosion and cavitation erosion is the main reasons lead to leakage and explosion accidents. The ratio is larger than 50% of the total accidents. In order to insure the safety operation of the pipes, it is necessary to search for some good NDT methods to assess wall thickness and then replace only critically damaged pipes. X-rays (or g-rays), ultrasonics, and infrared thermography are useful for such purposes. Due to infrared thermography test possess the advantages of being noncontact, fast, harmless, useful for either the reflective or transmissive method, and easy to deloy, it has the potential to test the pits and wall loss defects on-line for high temperature pressure pipes.

The discontinuities in solid materials can change the heat flow condition. The change of the heat flow condition can result in the fluctuation of the temperature on the surface of the materials. Both infrared testing and thermal image testing use this principle to measure the change of the surface temperature and then to deduce the discontinuity condition in the materials.

In the aspect of non-destructive test for industrial equipment, infrared thermography test is applied to operation condition monitoring of electrical equipment, power plant machinery and high temperature equipment [1-3]. In the aspect of non-destructive test for pressure pipe, just a few papers were found by use of international literature retrieve from INSPEC, EI, ISMEC, METADEX and etc. Almost of these papers investigated the insulation condition and heat loss for high temperature pressure pipes [4], heat transfer for heat exchangers [5-7], defects detect for composite pipes [4] and leakage test for underground concrete pipes [8]. Due to metals have very high thermal conductivity, few people try to test the wall loss defects for steel pipes by use of infrared thermography. It was just found that Maldague performed the infrared thermography test for a small pipe in laboratory [1, 9]. Two wall thinning defects on an elbow were detected. It is not found that systemic investigation was performed for different type of pipes with different wall thickness. We also can not find the sensitivity of the infrared thermography test for steel pipes with different wall thickness.

In order to get solution for such problems, a big pipe testing installation was constructed. A serious of infrared thermography testing experiments during heating and cooling were performed for 4 kinds of stainless steel and carbon steel pipes which are drilled different size holes on inner-surface.

2. TESTING EQUIPMENT AND INSTALLATION

2.1 TVS-2100 Thermal Video System
All the experiments were performed by the TVS-2100 Thermal Video System. The infrared camera head of this system is optically mechanical scanning type. The detector is InSb with 10X10 cell arrays. The detecting wavelength is 3~5.4mm. The operating ranges of temperature is -40^°C~950^°C. The minimum detecting temperature difference is 0.1^°C. at 30^°C and the sensitivity is 0.01^°C. The field of view is 10^°(V)´15^°(H). The field resolution is 2.2mrad. Table 1 lists the minimum resolving size for different view distance. The detection distances are from 20cm to infinity. The scanning speed is 30 frames per second. One floppy disc drive can save one frame per 10 seconds. There is an analogue video output, all the testing image can be saved in the tape recorder.

View distance (m)	0.2	0.4	0.6	0.8	1.0	1.2	1.4	1.6	1.8	2.0	3.0	4.0	5.0	10
Minimum size (mm)	0.44	0.88	1.32	1.76	2.20	2.64	3.08	3.52	3.96	4.4	6.6	8.8	11.0	22.0
Table 1: Minimum resolving size for different view distance of TVS-2100 thermal video system.

2.2 Pressure Pipe Testing Installation
Figure 1 is the pressure pipe testing installation constructed by 5 different type one meter length steel pipes and linking with a 150^°C steam pipeline. The 150^°C steam and normal temperature water can go into the pipe testing installation according to the testing requirement.

Fig 1: Pressure Pipe Testing Installation for Infrared Thermography Test.

2.3 The Preparation of Inside Defects of Pipes
First, 1 meters length one stainless steel pipe and three carbon steel pipes were vertically cut into 3 parts. Then, a serious of different diameter and different depth holes were drilled. Finally, the three parts were welded into a pipe again. Table 2 lists the size of all defects in the 4 type of steel pipes.

Type and Specification (mm)	Inside defects quantity and size (mm)
F114×4 stainless steel	24 defects: Diameter: (F2, F4, F6, F10)X Depth: (15%, 25%, 35%, 50%, 65%, 75%)
F140×5 20 Carbon Steel	24 defects: Diameter: (F2, F4, F6, F10)X Depth: (20%, 40%, 50%, 60%, 70%, 80%)
F168×16 20 Carbon Steel	12 defects: Diameter: (F4, F8, F12)X Depth: (10%, 20%, 40%, 60%) 12 defects: Diameter: (F5, F10, F15)X Depth: (20%, 40%, 60%, 80%)
F180×36 20 Carbon Steel	12 defects: Diameter: ( F4, F8, F12)X Depth: (10%, 20%, 40%, 60%) 12 defects: Diameter: (F5, F10, F15)X Depth: (20%, 40%, 60, 80%)
Table 2: The size of defects in 4 type of steel pipes.

3. INFRARED THERMOGRAPHY TEST FOR INSIDE DEFECTS OF PIPES

3.1 Inside Steam Heating
The pipe was passed through 150°C steam to heat from normal temperature. At the same time, the thermal image of the defects was looked and recorded in the floppy disc and tape recorder. The typical thermal images of inside defects for 4 type of pipes are shown from figure 4 to figure 7 respectively. Table 3 lists the minimum defects tested by infrared thermography for 4 type of pipes.

Type and Specification (mm)	The size of minimum defects tested (mm)
F114´4 stainless steel	F10´15%, F6´25%, F4´50%, F2´65%
F140´5 20 Carbon Steel	F10´20%, F6´40%
F168´16 20 Carbon Steel	F10´20%, F8´20%, F5´40%
F180´36 20 Carbon Steel	F12´20%, F10´40%, F8´60%
Table 3: The minimum defects tested by infrared thermography for 4 type of pipes.

Fig 4: F114×1000×4 stainless steel Pipe.

Fig 5: F140×1000×520 Carbon Steel Pipe.

Fig 6: F168×16 20 Carbon Steel Pipe.

Fig 7: F180×36 2020 Carbon Steel Pipe.

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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

1. 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.
2. 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.
3. 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.
4. 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.
5. 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

1. Edited by Xavier Maldague, Nondestructive Testing Monographs and Tracts Volume 7, Infrared Methodology and Technology, Gordon and Breach Science Publishers, 1994.
2. 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.
3. 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.
4. 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.
5. R. Rozenblit, M. Simkhis, et al, Heat Transfer in Horizontal Solid-liquid Pipe Flow, International Journal of Multiphase Flow, Volume 26, No.8, 2000.
6. Shigemichi Yamawaki, Toyoaki Yoshida, et al, Fundmental Heat Transfer Experiments of Heat Pipes for Turbine Cooling, American Society of Mechanical Engineers (Paper) Proceedings of the International Gas Turbine & Aeroengine Congress & Exposition June 2-5, 1997, Orlando, FL, USA.
7. Yuwen Qin, Naikeng Bao, Thermographic Nondestructive Testing Technique for Delaminated Defects in Composite Structure, Proceedings of SPIE - The International Society for Optical Engineering Thermosense XVII, 1995, Orlando, FL, USA.
8. Kenneth R. Maser, Mehdi S. Zarghamee, Proceedings of the Speciality Conference on Infrastructure Condition Assessment, August 25-27, 1997, Boston, MA, USA.
9. Xavier Maldague, Pipe Inspection by Infrared Thermography, Materials Evaluation, Volume 57, No.9, 1999.

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