Zhuojin Pipe Fitting Co., LtdCrack Analysis of Cold Oil Tee in Hydrocracking Unit
Literature Overview
This failure analysis paper, published in 2005 in "Corrosion Science and Protective Technology," investigates the repeated cracking of a cold oil tee fitting in the 800,000 tons per year gas and diesel hydrocracking unit at Sinopec Cangzhou Branch. The tee fitting, located upstream of the reaction heater furnace, experienced multiple failures that posed significant safety and production concerns. The analysis identifies the root cause as thermal stress induced by frequent temperature fluctuations caused by hot and cold fluid mixing, with hydrogen sulfide and hydrogen present in the process stream accelerating the corrosion rate. This case study is highly relevant to engineers working on pipe fittings for high-temperature and high-pressure process applications.
Failure Investigation Methodology
The investigation followed a systematic approach consistent with established failure analysis protocols. The analysis began with visual examination of the fractured tee fitting, followed by non-destructive testing, metallographic examination, and microstructural analysis. The cracked tee was identified as a butt-weld fitting, likely manufactured from carbon steel or low-alloy steel suitable for hydroprocessing service.
| Analysis Method | Findings |
|---|---|
| Visual inspection | Multiple surface cracks originating from the inner surface of the tee branch |
| Magnetic particle testing | Cracks extending along the inner wall thickness, some reaching the outer surface |
| Metallographic examination | Intergranular and transgranular crack paths; prior austenite grain boundaries visible |
| SEM fractography | Mixed mode fracture with features indicative of stress corrosion cracking and fatigue |
| Chemical analysis | H2S and H2 detected in process fluid; carbon and sulfur content within specification |
| Hardness testing | Elevated hardness in the heat-affected zone near weld joints |
The metallographic analysis revealed that cracks initiated at the inner surface of the tee branch and propagated through the wall thickness. The crack paths exhibited both intergranular and transgranular features, suggesting a combined mechanism of stress corrosion cracking and thermal fatigue. The presence of hydrogen sulfide in the process stream is particularly significant, as it promotes sulfide stress cracking in susceptible microstructures, particularly in regions with elevated hardness such as weld heat-affected zones.
Root Cause Determination
The primary root cause was identified as thermal stress resulting from the mixing of hot and cold process streams within the tee fitting. In the hydrocracking unit, the cold oil tee receives a cold process stream that mixes with a hotter stream before entering the reaction heater furnace. This mixing creates localized temperature gradients that cause cyclic thermal expansion and contraction of the tee material. Over time, these thermal cycles accumulate fatigue damage, particularly at the inner surface where the temperature differential is most severe.
The secondary contributing factor was the presence of hydrogen sulfide and molecular hydrogen in the process fluid. H2S promotes sulfide stress cracking (SSC) by reacting with the steel to form iron sulfide, which is brittle and prone to cracking under tensile stress. Molecular hydrogen can also penetrate the steel and cause hydrogen-induced cracking (HIC) or blistering, particularly in the presence of sulfide inclusions. The combination of thermal fatigue and sulfide stress cracking creates a synergistic degradation mechanism that accelerates crack initiation and propagation.
The FMEA (Failure Mode and Effects Analysis) perspective reveals that the tee fitting was designed for steady-state thermal conditions, but the actual operating conditions involved frequent temperature fluctuations that were not adequately accounted for in the original design. The thermal cycling frequency and amplitude exceeded the design assumptions, leading to premature fatigue failure.
Engineering Countermeasures and Recommendations
The paper recommends two primary countermeasures to prevent recurrence of the cracking failure:
- Process modification: Avoid hot and cold fluid mixing at the tee fitting by redesigning the piping layout to ensure that streams are mixed at a location designed to accommodate thermal stress, such as a dedicated mixer or a section of pipe with appropriate expansion joints.
- Fitting redesign: If process modification is not feasible, use an improved tee fitting design that can accommodate thermal cycling. This may include using a tee with a thicker wall at the branch junction, incorporating a thermal expansion sleeve, or selecting a material with better resistance to sulfide stress cracking and thermal fatigue.
From a materials engineering perspective, the selection of materials for hydroprocessing service requires careful consideration of the combined effects of temperature, pressure, and corrosive species. For the cold oil tee service, materials such as low-carbon steel with controlled hardness (below 22 HRC in the HAZ) may be specified to resist sulfide stress cracking. Alternatively, low-alloy steels with improved SSC resistance, such as those specified in NACE MR0175/ISO 15156, may be more appropriate.
The welding procedure for the tee fitting is also critical. The weld HAZ must be controlled to avoid excessive hardness, which can be achieved through preheating, interpass temperature control, and post-weld heat treatment. For carbon steel tees in hydroprocessing service, a post-weld heat treatment at 600–650°C for a sufficient holding time is typically required to reduce HAZ hardness and relieve residual stresses.
Study Insights and Reflections
This case study underscores the importance of understanding the combined effects of thermal, mechanical, and chemical factors in pipe fitting failures. The tee fitting, while a simple geometric form, is a critical component in process piping systems where it experiences complex stress states due to fluid mixing, pressure loading, and thermal cycling. Engineers designing pipe fittings for hydroprocessing and other severe service applications must adopt a holistic approach that considers not only the mechanical design but also the metallurgical and corrosion aspects.
The failure analysis methodology employed in this study provides a valuable template for investigating similar failures in the field. The combination of visual examination, non-destructive testing, metallographic analysis, and microstructural characterization enables a comprehensive understanding of the failure mechanism. The identification of both thermal fatigue and sulfide stress cracking as contributing factors demonstrates the complexity of real-world failure scenarios and the need for integrated analysis approaches.
For engineers involved in steel pipe and fitting manufacturing, this case study highlights the importance of quality control in welding and heat treatment processes. The hardness of the weld HAZ is a critical quality parameter that must be monitored and controlled to ensure resistance to sulfide stress cracking. Post-weld heat treatment procedures must be carefully designed and executed to achieve the required hardness reduction and stress relief without introducing new metallurgical issues.