Infrared Thermography Test for High Temperature Pressure Pipe

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.