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This post provides a professional overview of the eight methods used to analyze the failure due to breakage of thermocouple sleeves. Based on field investigations, practical experience, and relevant data, Changhui Instruments has summarized the common causes of thermocouple sleeve breakage for reference by instrument technicians. Thermocouples are commonly used temperature-sensing elements in temperature measurement instruments, and they are widely applied in fields such as industrial production and scientific research. They directly measure the temperature of a medium, convert the temperature signal into a thermoelectromotive force signal in real time, and then use electrical instruments to convert this signal into the temperature of the medium being measured, thereby enabling the display, transmission, and control of temperature. Since they are often used in environments with high temperatures, high pressures, highly corrosive media, and high-velocity fluid impacts, thermocouple sleeves are generally used to protect thermocouples in order to ensure their proper, safe, and stable operation. Although the design of thermocouple sleeves is intended to provide protection for the proper functioning of thermocouples, and they do indeed serve a protective role in practical applications, analysis of the types of failures that occur in thermocouples shows that sleeve fracture remains the most common form of failure. Changhui Instruments classifies and explains the research methods for thermocouple sheath fractures, summarizes the general causes of such failures, and provides an overview of preventive measures against thermocouple sheath fractures. Reasons for the breakage of thermocouple sleeves: yunrun.com.cn/tech/2904.html 1. Appearance and selection of thermocouple sleeves. Figure 1: THERMOWELL, a protective tube for temperature sensors. Figure 2: Thermocouple sleeves. Figures 1 and 2 show images of common thermocouple sleeves; more details on the selection and technical parameters of thermocouple sleeves can be found in “THERMOWELL, a Protective Tube for Temperature Sensors”. 2. Failure analysis methods for cracked thermocouple sheaths: In this article, Changhui Instruments conducts an analysis and study of the thermocouple sheaths that have fractured. To determine the cause of the fracture and find appropriate countermeasures, methods such as macroscopic fracture analysis, chemical composition analysis, mechanical property analysis, metallographic analysis, and scanning electron microscopy analysis are generally used. Furthermore, corrosion evaluation analysis, modal analysis, and numerical simulation analysis are also commonly used methods in the analysis of casing failures caused by structural corrosion, severe vibration, and fluid impact. ①Macrofracture analysis is generally used, after either no treatment is applied to the fractured area of the thermocouple sleeve or after simple ultrasonic cleaning, to determine the location of the fracture zone as well as the overall condition of the components in that area. It helps to identify the characteristics of the fracture surface, its dimensions, the distinction between the instantaneous fracture zone and the propagated fracture zone, the distribution of fracture colors, the flatness and smoothness of the fractured area, the type of fracture, the presence of significant plastic deformation, any corrosion spots, and welding defects such as undercuts. This analysis provides an initial understanding of the sleeve’s fracture condition, laying a foundation for further investigation. Figure 3 shows a macro photograph of the fracture surface of a thermocouple sleeve; the fracture surface is smooth and polished, with the characteristic shell-like pattern typical of fatigue fracture visible. http://yunrun.com.cn/upload/202002/05/202002051714288764.png Figure 3: Macroscopic photo of the fracture surface of the thermocouple sleeve. ② Chemical composition analysis: A bulk sample is taken from the fractured thermocouple sleeve, and chemical analysis or energy dispersive spectroscopy is used to determine the chemical composition of the material forming the sleeve. This helps to verify whether the actual elemental contents in the material meet the requirements specified for the corresponding standard grade of material. For example, Xia Mingliu and others conducted a chemical composition analysis on a fractured stainless steel sleeve and found that its P content met the requirements for high-quality steel materials ； However, the sulfur content is on the high side, and an excessive sulfur level has a severe negative impact on the properties of this heat-resistant steel; it can cause thermal brittleness in the material and reduce its strength at high temperatures, thereby affecting the service life of the casing. ③Mechanical property analysis: The mechanical property analysis of thermocouple sleeves is primarily carried out through tensile tests to determine whether the actual mechanical properties of the material meet the specified standard requirements. Studies on thermocouple sleeves made of the material 20Cr25Ni20 have shown that the tensile strength of these sleeves meets the standard requirements for this material, whereas their plasticity is below the requirements for 20Cr25Ni20 steel; significant plastic deformation occurred in the specimens after the tensile tests. ④Metallographic analysis: For the metallographic analysis of broken thermocouple casings, metallographic samples from the fracture surface, welding points, and joints are taken. After being ground and polished, they are etched using an oxalic acid solution in an appropriate concentration, and then their metallographic structure is examined under a microscope to determine the type of matrix structure and the quality of fusion at the welding points. The metallographic micrograph of a certain thermocouple sleeve is shown in Figure 4. Analysis revealed a high number of carbide particles in the matrix structure, indicating that the carbon content of this sleeve material is very high. Further analysis showed that not only was the number of carbides high, but their distribution also appeared in distinct bands, which can directly affect the properties of the steel and lead to anisotropy in the material. Furthermore, hardness testing revealed that the precipitation of a large number of carbide particle phases caused significant hardening in the thermocouple sheath material. http://yunrun.com.cn/upload/202002/05/202002051711177149.png Figure 4: Metallographic micrograph of the thermocouple sleeve. ⑤ Scanning electron microscope analysis: This analysis is carried out based on metallographic analysis, using a scanning electron microscope to further examine the microscopic structure of the fracture surface. Depending on the analysis requirements, multiple different magnification levels are typically used for observation; it is a crucial step for determining the origin of cracks, their direction of propagation, the size of the propagation areas, the morphology of the sudden fracture zone, and the characteristics of the fracture surface, thereby enabling an assessment of the causes of sleeve failure. ⑥Corrosion evaluation analysis: Corrosion evaluation analysis is not a mandatory option; it is only necessary for casings that exhibit obvious corrosion spots, and whose failure may be caused by corrosion. Furthermore, corrosion analysis may also be conducted on thermocouple casings in which some fractures have remained untreated for an extended period, resulting in obvious oxidation at the fracture surfaces. ⑦Modal analysis: In many cases, the failure of thermocouple sleeves is caused by intense vibrations or even resonance; therefore, modal analysis is often employed. This analysis can involve either on-site modal testing or simulation studies using finite element methods. The purpose of such analysis is usually to determine the natural frequencies of the structure and the fluid at various orders. In particular, it is important to avoid situations where the natural frequencies of the structure and the fluid coincide or are similar, as this can have a significant impact on vibrations. ⑧Numerical simulation analysis: The use of finite element analysis software (such as ANSYS) for modal analysis, as mentioned in modal analysis, also falls under the category of numerical simulation analysis. However, it is also common to study the fracture of thermocouple sleeves using computational fluid dynamics methods (i.e., CFD). For example, some studies have employed large eddy simulation to numerically model the impact of pipe flow on thermocouple sleeves; the forces acting on these sleeves were calculated, and further analysis of the results led to the identification of effective measures to reduce the forces exerted on them. 3. Causes of thermocouple sleeve fracture: There are various reasons for the fracture of thermocouple sleeves, and the actual situations may differ. Based on field investigations, practical experience, and relevant data, Changhui Instruments has summarized the common causes of such fractures: ① Welding defects. Statistical analysis of the locations where thermocouple sleeves fracture shows that the welding areas and their vicinity are regions where fractures occur frequently, and this is often related to welding defects. The literature, in analyzing the causes of fracture in the thermocouple sleeves of a pressure-reduction oil transfer line, mentions that poor welding quality, uneven microstructure in the heat-affected zone, and welding defects such as undercutting lead to the segregation of impurities in the root area of the outer wall. The microstructure also exhibits defects such as porosity and voids; under intense impact loads, cracks are generated and spread rapidly, ultimately resulting in the fracture and failure of the thermocouple sleeve. ②Casing vibration: The main pipeline connecting the thermocouple casing is usually filled with fluid. If the fluid has a high density, high flow velocity, and is in a turbulent state, vortices will form near the casing. As these vortices periodically detach from both sides of the casing, they generate periodic lift and drag forces on the casing, resulting in constant changes in the pressure distribution within the pipeline. The fluid pressure acting on the casing also changes continuously in terms of direction, which ultimately leads to vibration of the pipeline. Resonance occurs when the fluid excitation frequency is close to or even identical to the natural frequency of the casing. Once strong resonance occurs, periodic alternating stresses are generated within the casing. Casing that is subjected to high alternating stresses over a long period of time will experience fatigue fracture in areas where the stress levels are highest. ③Corrosive degradation: Thermocouple sheaths are often exposed to extreme conditions such as high temperatures, high pressures, and high humidity, as well as highly corrosive substances. It is common for these sheaths to break or fail due to corrosion. Some forms of corrosion can be observed with the naked eye, while others require metallographic analysis to be detected; intergranular corrosion is one such example. Areas affected by intergranular corrosion tend to become sources of cracks that lead to fatigue failure. ④Improper selection of the sleeve material: Thermocouple sleeves require high strength and resistance to extreme environments. There are three main situations in which sleeve failure occurs as a result of improper material selection; one is that the material does not meet the relevant standard requirements at all ； Second, the actual materials used do not match those specified in the design (this is a problem that is often identified through chemical composition analysis, and it is also a key reason why such analysis is necessary) ； Thirdly, the selection of materials focuses on strength considerations, with little regard for factors such as resistance to high temperatures and pressures as well as corrosion resistance, or the long-term effects of these factors are not taken into account. ⑤Improper installation of the thermocouple: If the break in the thermocouple sheath occurs near components such as elbows, orifice plates, tees, and valves, it is very likely due to improper installation of the thermocouple. These areas are regions where excitation forces and energy are concentrated; therefore, unless it is necessary for temperature measurement and control, the thermocouple should be installed away from such areas as much as possible. ⑥Improper installation of the thermocouple: If the thermocouple sleeve is installed vertically on the pipeline, its stress condition can be simplified to that of a cantilever beam. Fractures usually occur at the junction between the sleeve and the main pipeline; this area corresponds to the root of the cantilever beam, which is the point under the greatest stress and where stress concentration is most likely to occur. Therefore, for thermocouples for which an inclined installation also meets the functional requirements, it is possible to install them at an appropriate angle. Additionally, the length to which the thermocouple sleeve extends into the main pipeline is also an important factor affecting its likelihood of fracturing, and this length should be chosen carefully. This article mainly introduces the methods for analyzing the failure of thermocouple sleeves due to fracture, as well as the common causes of such fractures. Please stay tuned for the article titled \"What Are the Measures to Prevent Thermocouple Sleeve Fractures?\" Authors: Yuan Wei, Yang Zhirong