Failure Analysis of an Elbow Tube Break in a Pyrolysis Furnace

Abstract

The pyrolysis furnace, a critical component in a pyrolysis unit, inevitably faces operational challenges during its use. This study investigates a case of pyrolysis furnace failure, particularly focusing on an occurrence at the 90° lug elbow and furnace tube weld. The failure, characterized by a comprehensive fracture of the furnace tube in the circumferential direction along the weld vicinity, transpired within a timeframe significantly shorter than one-third of the design life. To unravel the root cause, a series of experiments was conducted on a sample extracted from the failed tube. These experiments, comprising visual inspection, chemical composition analysis, metallographic examination, microstructure analysis, fracture scanning electron microscopy, and energy spectrum analysis, collectively aimed at a comprehensive understanding of the failure mechanisms. The results disclosed that the fracture between the lug elbow and the inlet pipe stemmed from the presence of porosity and inclusions in the butt weld. The initiation of cracks was traced to the pores and inclusions in the fusion line of the inner wall of the pyrolysis tube, extending to connect with the pores in the heat-affected zone on the side of the pyrolysis tube parent material. Subsequently, under the influence of high temperature and stress, the cracks propagated, crept, and expanded along the circumference of the pyrolysis tube parent material until the final fracture occurred. In light of these findings, practical recommendations are proposed to mitigate the risk of similar failures in the future.

1. Introduction

A pyrolysis unit serves as a critical apparatus for pyrolyzing feedstock substances, with the primary purpose of converting complex organic molecules into simpler components [1]. This process, conducted under high-temperature and high-pressure conditions, induces fission of feedstock molecules, yielding more valuable products. These pyrolyzing units play a pivotal role in petroleum refining and chemical production.

The pyrolyzer, a key component within a pyrolysis unit, facilitates the pyrolysis process by breaking down large compounds into smaller molecules. This chemical reaction, occurring under high-temperature and high-pressure conditions, is commonly used in fields such as petroleum refining and chemical production [2]. The fundamental principle of a pyrolysis furnace is to produce simpler compounds by supplying sufficient thermal energy to pyrolyze raw materials at elevated temperatures [3]. In industrial production, pyrolysis furnaces are essential in petrochemical fields, providing crucial raw materials for diverse chemical production.

Pyrolysis furnace tubes, as integral piping components in the pyrolysis system, are responsible for transporting raw materials, media, and products. These tubes are characterized by high temperature and pressure resistance, meticulous material selection, dependable connection technology, optimized hydrodynamic design, and inclusion of a safety monitoring system [4,5]. The design and operation of these tubes are intricately tied to the stable execution of the pyrolysis process and the efficient transportation of products. Regular maintenance and overhauls are imperative for ensuring the prolonged and stable operation of the system.

In the context of piping systems, including those of pyrolysis furnace tubes, an elbow serves as a critical pipe fitting designed to facilitate a change in direction between two lengths of pipe or tubing. This directional shift often occurs at either a 90° or 45° angle, allowing for the efficient conveyance of fluids and gases within a wide range of applications [6,7]. Elbows play a pivotal role in various industries, most notably in the transportation of oil, gas, and even steam within geothermal systems, underlining their indispensable role in ensuring the smooth operation of these systems [8,9]. One of the distinguishing characteristics of a 90° elbow is the ever-changing flow pattern it introduces into the pipeline. This dynamic alteration in flow direction and flow velocity is responsible for inducing variations in the corrosion behavior exhibited at different locations along the surface of the elbow [10,11,12,13,14]. As the fluid or gas navigates through the elbow’s curve, it encounters shifts in momentum and velocity, thereby influencing the way it interacts with the inner surface of the pipe.

This study delves into the analysis of a malfunctioning elbow tube of a pyrolysis furnace, specifically addressing a critical issue involving the leakage of an inlet tube within a pyrolysis furnace, which occurred in proximity to the elbow section. This breach in the system led to a fire incident, necessitating the complete shutdown of the entire unit for safety reasons. Figure 1 presents the macroscopic image of the compromised tube segment and a comprehensive process schematic diagram titled ‘Pyrolysis furnace tube elbow fracture location’. The visual representation in the figure vividly demonstrates the extent of damage to the pyrolysis furnace tube. It also outlines the relevant components, with a particular emphasis on the section where the failure occurs.

The furnace tubes were installed in 2015, but after 3 years (only one-third of the designed life span (100,000 h)), the described tube failure occurred.

The whole pipe connection, including the broken section, contained three parts (see Figure 1): a horizontal inlet tube at the upper part, a 90° elbow with a lifting lug, and a vertical furnace tube at the lower part. The tube material was 25Cr-35Ni + Nb + MA with Φ63.5 × 6.4 mm dimension specification; the 90° elbow material was a CF8C casting type elbow. The vertical furnace tubes were supplied by Yantai Manior Heat Resistant Alloys Co., Ltd., Yantai, China. To enhance heat transfer, twisted slices (a patent developed by SINOPEC Beijing Research Institute of Chemical Industry) were placed outside the tubes.

The pyrolysis furnace was a SL-II type pyrolysis cracking furnace processing liquid feed. The radiation section of the pyrolysis furnace had six groups of coils, and each group of coils consisted of four groups of 5-1 two-pass furnace tubes. There were 120 pipes in total.

Failure analysis for radiant tubes of pyrolysis furnaces has been studied by many researchers and engineers; for example, tube failure caused by improper coking and decoking cycles [15], significant growth of carbides by inappropriate burning [16], bulk diffusion of carbon into the tube [17], and a decrease in creep resistance due to formation of coarse grains and coarse primary carbides caused by overheating [18]. A summary of typical failures in pyrolysis coils or tubes was reviewed in [19]. Wang [20] proposed a technology to calculate the failure probability of ethylene cracking furnace tubes. Cr-Ni alloy material failure analysis was studied in [17,21]. There has been no report about tube failure near the elbow section.

In this study, it was discovered that although the material of the pyrolysis furnace tube exhibits remarkable durability under normal operating conditions, even minor defects along the welding fusion line can lead to severe damage to the radiant tube. The significance of this research lies in elucidating the robustness of the pyrolysis furnace tube during regular operations while emphasizing the potential risks of significant damage due to minor imperfections along the welding fusion line. This not only contributes to the improvement of the design and manufacturing of pyrolysis furnace tubes, enhancing their overall performance and safety, but also provides crucial insights for the industrial processes involving pyrolysis. Through a thorough analysis of the impact of these minor defects on the radiant tube, the research findings serve as valuable guidance for related industries, aiding in the prevention of potential equipment failures and safety incidents, ultimately enhancing production efficiency and equipment reliability.

The rest of the paper is organized as follows. In Section 2, a macroscopic examination was conducted at the elbow of the fractured cracking furnace, and the failure site was sampled for comprehensive characterization through chemical composition analysis, metallographic analysis, fracture morphology observation, and energy spectrum analysis. These analyses were performed using established experimental methodologies. In Section 3, the acquired experimental results were synthesized to elucidate the underlying causes of the failure. The summary and conclusions drawn from this investigation are presented in Section 4.

2. Experimental Failure Analysis

2.1. Visual Inspection

From the images of the failed tube (see Figure 1) and its fractures show in Figure 2, we could observe that the crack sources originated from the circumference of the weld area, which are pointed out by red boxes in Figure 2a,b.

As shown in Figure 2a, the fracture morphology on the side of the 90° bend demonstrates that approximately two-thirds of the fracture surface displays melting characteristics due to the combustion of high-temperature flames. Conversely, Figure 2b exhibits the fracture morphology observed from the vertical plane of the furnace tube, which reveals no signs of melting. Moreover, the fracture surface does not exhibit apparent plastic deformation, and cleavage fracture characteristics—such as parallel and glossy striations—are present. Additionally, particle-like fracture features, i.e., small granular shapes, are also detected on the fracture surface. Collectively, these characteristics indicate that the furnace tube fracture is typical of brittle fracture.

To illustrate the relationship between the crack sources and the weld fusion line, the inner wall image of the tube is presented in Figure 2c. A significant fusion line area can be observed, which may be the terminal point of the arc welding. The distances between the crack sources and the weld fusion line vary from 2 to 7 mm. In the crack source zone on the elbow side, two large pits are located, with the larger one measuring approximately 3 mm in length and 0.5–1 mm in width, and the smaller one measuring approximately 1 mm in length and 0.5 mm in width. These pits might have provided stress concentration points for crack initiation.

2.2. Chemical Composition Analysis

The failed tubes were sampled for chemical composition analysis. The S_tube samples were collected from the vertical furnace tube; the S_weld samples were collected from the welding fusion line of the 90° elbow.

Table 1. Chemical composition of samples in comparison with ASME SA351/351M (wt. %).

Element	C	Si	Mn	P	S	Cr	Ni	Cu	W	Al	Nb	Mo	V
STube	0.5	1.68	1.32	0.0237	0.03	27.87	34.79	0.0168	0.110	0.0035	0.97	0.107	0.14
SWeld	0.0766	0.268	4.42	0.004	0.0170	24.52	53.92	0.184	0.113	0.0247	2.72	0.200	0.0428
Standard	0.37–0.5	1.5–2.0	≤1.5	≤0.03	≤0.03	24–27	34–37	≤0.0025	≤0.3	≤0.0005	0.8–1.2	≤0.5	-

shows the chemical composition of the samples in comparison with ASME SA351/SA351M. The analysis results indicate that the main chemical composition of the S_tube meets the standards [22], although the elements Al and Cu was slightly higher than the standards. However, the Mn content for S_weld is significantly higher than the standards and its Al and Cu contents are slightly higher.

2.3. Fracture Surface Characterization

An scanning electron microscope (ZEISS EVO 10 by Carl Zeiss Microscopy GmbH, Oberkochen, Germany) was employed to acquire detailed images and insights into the morphology of the crack source zone, enabling a comprehensive examination of the defect’s characteristics and origins.

2.3.1. Morphology of the Break Section from the Vertical Furnace Tube Side

In order to provide a comprehensive understanding of the crack source zone and to delve into the root causes of crack initiation, Figure 3 offers a detailed side view of the vertical furnace tube. The image reveals the presence of multiple crack sources, which are denoted as areas A, B, and C in Figure 3a. These crack sources were conspicuously located in close proximity to the fusion line of the tube.

To gain further insight into the composition of these critical areas, energy-dispersive spectroscopy (EDS) analysis was conducted specifically on areas 1 and 2, as highlighted in Figure 3b. The results of this analysis are meticulously depicted in Figure 3e,f. Notably, the EDS analysis unveiled a significant discrepancy in the manganese (Mn) content within these areas. Area 1 was found to contain a Mn content of 5.76%, whereas in Area 2, the Mn content was markedly higher at 8.07%. These elevated levels of Mn inclusions in the crack source zone were found to be in stark contrast to the composition of the base material.

Moreover, an additional critical observation was made regarding the fracture surface on the side of the vertical furnace tube. This surface exhibited noticeable contamination, primarily from molten aluminum slag, which resulted from the melting of the insulation layer. This contamination added an additional layer of complexity to the circumstances surrounding the crack initiation. In summary, the comprehensive analysis of Figure 3 not only highlights the presence of Mn inclusions in the vicinity of the cracks but also underscores the adverse influence of molten aluminum slag on the fracture surfaces.

2.3.2. Morphology of the Fracture from the Elbow Side

Figure 4 provides a revealing glimpse into the fractural morphology of the crack source area, specifically focusing on the side of the 90° elbow. The images clearly depict the profound impact of the high-temperature flame, which has partially melted the surface of the fracture. However, the significance lies in the observations made regarding the presence of pores, notably discernible in Area B of Figure 4b and Area 1 in Figure 4c. These pores or blowholes serve as the initiation sites for the cracks and exhibit characteristics typical of intergranular fractures. To gain further insights into the composition of these areas, an energy-dispersive spectroscopy (EDS) analysis was carried out on samples obtained from Areas 1 and 2, as delineated in Figure 4c. The EDS results, thoughtfully presented in Figure 4d,e, unveil the primary elements comprising the fractural surface of the 90° elbow side. Notably, the main constituents identified are chromium (Cr), nickel (Ni), and iron (Fe). This elemental composition is in harmony with that of the vertical furnace tube, further substantiating the interconnection between the crack source and the base material. An additional noteworthy discovery is that the areas under investigation, namely Areas 1 and 2, do not exhibit signs of contamination by the melting of the insulation layer. This finding underscores that the crack initiation is primarily linked to the presence of pores and blowholes located at the fusion line of the elbow. The confirmation of these compositional and morphological features plays a pivotal role in unraveling the complex dynamics behind the crack initiation on the 90° elbow side.

Figure 5 provides a comprehensive view of the fractographic images taken from the inner wall near the crack source on the elbow side. These images reveal the presence of multiple fracture pits, with the largest pit measuring approximately 3 mm in length and 0.5–1 mm in width. These distinctive features are indicative of critical points in understanding the nature of the crack initiation. Further insights into the composition of these pits were obtained through energy-dispersive spectroscopy (EDS) analysis, as illustrated in Figure 6. The EDS results indicate that the primary components found within both the interior and exterior of these pits are chromium (Cr), nickel (Ni), and iron (Fe). Importantly, this elemental composition aligns with the material composition of the vertical furnace tube, affirming the connection between these pits and the base material.

A noteworthy finding from the EDS analysis is the presence of manganese (Mn) within these fracture areas. This additional element suggests a more complex origin for the observed pits. Based on the combined evidence from the scanning electron microscope (SEM) images and the EDS analysis, it is reasonable to deduce that these pits originate from inclusions or blowholes within the material. The presence of inclusions or blowholes in these critical areas not only provides valuable insights into the crack initiation process but also underscores the importance of the quality and manufacturing processes of materials in ensuring the integrity of industrial components. In summary, Figure 5 and Figure 6 offer compelling evidence of the pit formation near the crack source on the elbow side, with EDS analysis confirming the key elements present within these pits. The presence of manganese adds an intriguing layer to the analysis, suggesting that the role of inclusions or blowholes in the crack initiation process is more complex than initially presumed.

2.4. Analysis of Microstructure

2.4.1. Microstructure of the Vertical Furnace Tube

Microstructure analysis was meticulously carried out employing the ZEISS Axio Observer-A1m optical microscope, capable of both low- and high-magnification resolutions. Figure 7 is dedicated to displaying the optical metallography of the fracture section located on the side of the vertical furnace tube. In adherence to ASTM standards [22], the sample preparation process involved cutting, mounting, grinding, polishing, and etching.

The microstructural examination of the furnace tube’s base material, as depicted in Figure 7a,b, reveals the presence of a relatively uniform columnar crystalline structure. However, an intriguing observation comes in the form of secondary cracks that are distinctly visible. These secondary cracks exhibit a pattern of propagation along the crystal boundaries, a phenomenon underscored in Figure 7c,d. This profound insight into the microstructure of the base material, combined with the identification of secondary cracks and their propagation behavior, constitutes a critical aspect of the comprehensive analysis.

2.4.2. Break Section on the Elbow Side

Figure 8 presents a detailed view of the microstructure on the elbow side, where a conspicuous feature emerges in the form of pronounced and extensive pores and blowholes concentrated in the fusion line section. This observation provides a key insight into the structural integrity of this critical area.

Further insights into the microstructure are revealed in Figure 9, incorporating both optical and electronic metallographic images obtained from the sample. The examination of the break section, the heat-affected zone, and the base material discloses the presence of various shapes of pores and blowholes, including round and stripe formations, distributed throughout these regions. This widespread distribution strongly suggests that the presence of pores and blowholes plays a significant role in the initiation and subsequent propagation of cracks within the vertical furnace tube.

To gain a deeper understanding of the composition of the inclusions within these pores and blowholes, an energy-dispersive spectroscopy (EDS) analysis was thoughtfully conducted on selected areas of the break section. The analysis results unequivocally demonstrate that these inclusions are primarily composed of aluminum (Al), silicon (Si), and iron (Fe) elements. Remarkably, these findings align closely with the composition of the insulation material used within the vertical furnace tube.

3. Failure Analysis

Macroscopic examination has yielded vital insights into the nature of the furnace tube failure. Remarkably, no substantial thinning or deformation of the compromised furnace tube was observed during the macroscopic inspection. This intriguing observation aligns with the fracture’s appearance, which exhibits distinctive characteristics of cleavage, thus classifying it as a classic example of brittle fracture. Furthermore, the metallographic analysis conducted on the base material near the fracture site of the vertical furnace tube revealed no apparent signs of deterioration. This encompasses the absence of creep voids, creep cracks, or carburization, which collectively indicate that the failure did not originate from the vertical furnace tube itself. Chemical composition analysis has provided essential information regarding the chemical makeup of the furnace tube and the weld. These analyses indicate compliance with the relevant standards. However, it is noteworthy that the aluminum (Al) and copper (Cu) contents in the vertical furnace tube surpass the defined technical standards. Additionally, the manganese (Mn) content in the weld significantly exceeds the standard limits. These deviations from the standard compositions create the necessary conditions for the formation of inclusions and pores or blowholes in the weld fusion line.

Drawing on fractographic analysis and scanning electron microscope (SEM) images, the results strongly suggest that the cracks have their origins in the pores and blowholes located within the wider fusion line area on the inner wall of the vertical furnace tube. These cracks propagate through the pores and blowholes within the heat-affected zone, ultimately evolving into larger cracks. Under the simultaneous influence of high temperatures and mechanical stress, these cracks progress towards the base material of the vertical furnace tube. Subsequently, they extend along the tube’s circumferential direction, persisting until the final failure occurs. This comprehensive analysis underscores the critical role played by the composition of the weld fusion line and its susceptibility to inclusions and pores.

After a comprehensive investigation involving macroscopic examination, metallographic analysis, and chemical composition evaluation, the insights obtained play a crucial role in advancing our understanding of furnace tube failure mechanisms. The absence of substantial thinning or deformation during macroscopic inspection, coupled with the identification of cleavage characteristics in the fracture, signifies a classic case of brittle fracture. Chemical composition analysis revealed deviations in aluminum (Al), copper (Cu), and manganese (Mn) contents from the defined technical standards, creating conditions conducive to the formation of inclusions and pores or blowholes in the weld fusion line.

The significance of this study for pyrolysis units and the chemical industry is paramount. By pinpointing specific factors contributing to furnace tube failure, such as composition irregularities and susceptibility to inclusions, the research provides valuable guidance for the design, manufacturing, and maintenance of pyrolysis furnace tubes. Implementation of these findings can lead to improved industry practices, minimizing the risk of similar failures, enhancing equipment reliability, and ensuring overall safety in pyrolysis processes.

4. Conclusions

In this investigation, the pyrolysis furnace tube, designed for a 100,000 h lifespan, experienced fracture and failure between the lug elbow and the inlet tube after 28,000 h (over 3 years) of operation. Employing a comprehensive array of methodologies including macroscopic examination, chemical composition analysis, microstructure characterization, and fracture morphology, we elucidated the underlying cause of the failure. The fracture between the lug elbow and the inlet pipe was ascribed to the presence of porosity and inclusions in the butt weld connecting the elbow and the inlet pipe; specifically, cracks initiated from the pores and inclusions within the fusion line on the inner wall of the furnace pipe. These fissures propagated towards the pores in the heat-affected zone on the side of the furnace pipe parent material. Under the combined influence of elevated temperature and mechanical stress, these fissures underwent creep and expansion along the circumferential direction of the furnace pipe parent material, culminating in eventual rupture.

It is imperative to underscore that the consequences of such weld defects can have far-reaching implications, potentially leading to catastrophic failures in industrial settings.

Furthermore, the propagation of cracking through the pores and blowholes in the heat-affected zone on the side of the vertical furnace tube’s base material further accentuates the importance of this weld fusion line. This underscores the need for vigilant monitoring and inspection of this critical component.

As a proactive measure to prevent similar failures in the future, it is strongly recommended that the weld fusion line undergoes thorough examination using non-destructive testing techniques. These techniques, such as ultrasonic testing and X-ray inspection, are instrumental in detecting defects such as inclusions and pores or blowholes, thereby ensuring the quality and structural integrity of the weld fusion line. This preventive approach is pivotal in averting potentially catastrophic consequences and maintaining the safe and efficient operation of petrochemical ethylene furnaces.