PSI - Issue 68

Reza Khadem Hosseini et al. / Procedia Structural Integrity 68 (2025) 409–414 R. K. Hosseini / Structural Integrity Procedia 00 (2025) 000–000

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In CCR units, tube materials such as 2¼Cr1Mo are typically used in the fired heaters; however, ferritic tubes with higher chromium contents, such as 5Cr1Mo and 9Cr1Mo, are also used, especially the latter. As a general rule, tube dimensions and corrosion allowances are in accordance with API RP-530 (Brear and Williamson, 2007 ) . As creep is the predominant mode of failure for these tube grades at elevated temperatures, this standard has been traditionally used to determine their remaining life for safe operation (Khadem Hosseini, 2022). Meanwhile, modern methodologies, including machine learning techniques, have recently gained attention for improving the accuracy of tube lifespan estimations (Zare and Hosseini, 2024). Creep is not the only factor contributing to furnace tube and reformer degradation . In spite of the fact that these tubes are designed to function at high temperatures, overheating can significantly compromise their integrity (Hosseini and Yareiee, 2019). A catastrophic phenomenon commonly observed within the refining industry is metal dusting at high temperatures, particularly in tubes, thermowells, and other components of catalytic reforming furnaces. The presence of CO and H ₂ gases within the stream, combined with temperatures ranging from 482 °C to 816 °C (900 °F– 1500 °F), creates conditions conducive to metal dusting (Orlikowski, 2018). As elevated temperatures cause changes in microstructure, microstructural examinations serve as an effective method for identifying failure mechanisms (Hosseini, 2018; Franks et al., 2017). Moreover, assessing microhardness variations can provide valuable insights into degradation processes (Hucińska and Gajowiec, 2010). This study investigates the root cause of premature failure in two furnace tubes from the CCR unit of an oil refinery after seven years of service. The design temperature for the furnace was set at 625 °C. However, according to the technical inspection report from the plant, there is a possibility that actual operating temperatures exceeded the design specifications due to thermocouple malfunctions. 2. Experimental Procedures 2.1. Visual observations Two tube samples were analyzed: Tube A exhibited signs of overheating characterized by a hot spot, while Tube B displayed severe damage resulting in a significant hole located just below the junction of the thermocouple shield and the tube. These samples were sent from the refinery for failure investigation, as illustrated in Fig. 1.

Fig. 1. Tube samples A, and, B, were sent from the refinery for failure investigation.

Figure 2a presents images of the internal and external surfaces of the damaged section of Tube B, revealing a hole that extends close to the surface of the tube. In Figure 2b, the inner surface of the damaged portion of Tube A is depicted, indicating numerous shallow small holes. Stereographic microscopic imaging has provided a clearer view of these cavities, which are fully rounded with a hemispherical morphology. 2.2. Tube material One of the tube samples underwent chemical analysis using spark emission spectrophotometry with worldwide Analytical Systems AG (model PMI–Master). The chemical composition of the tube material is presented in Table 1, conforming to the ASTM A213 standard. The material composition aligns with the specifications for 9Cr-1Mo steel, commonly referred to as grade T9.