Crack Analysis of Reducer Tee Body in Hydrotreating Unit

Literature Overview

This technical note by Zhang Jianmin, published in Chemical Machinery (2002, Vol. 29, No. 5), reports on the analysis of a crack in the body of a reducer tee (eccentric reducer) installed in a hydrotreating unit at a Sinopec refinery in Cangzhou. The paper, while brief, addresses a critical failure mode in high-pressure process piping: the initiation and propagation of cracks in the body of a reducer tee at the transition zone between the larger and smaller diameter sections. The analysis focuses on the relationship between crack location, hardness distribution, and wall thickness variation, providing valuable insights into the metallurgical and mechanical factors that contribute to this type of failure.

Technical Background

Hydrotreating units operate under severe conditions: high pressure (typically 30-200 bar), elevated temperature (250-400°C), and the presence of hydrogen, sulfur compounds, and other corrosive species. The process piping in these units is typically constructed from low-alloy steels such as ASTM A335 P11 or P22, or from carbon steel with heat-resistant alloy cladding. Reducer tees, which combine the functions of a reducer and a tee in a single fitting, are commonly used to connect pipes of different diameters at branch points.

The transition zone between the larger and smaller diameter sections of a reducer tee is a critical area because it experiences:

• Stress concentration due to the geometric discontinuity at the transition.
• Thick-thin wall interaction where the thicker section constrains the deformation of the thinner section.
• Residual stress gradients from the manufacturing process, particularly if the fitting is forged or formed.

Crack Analysis and Root Cause Identification

The paper's analysis focuses on three key parameters: crack location, hardness distribution, and wall thickness. Based on these parameters, the following root causes are identified:

The elevated hardness in the transition zone suggests that the material in this area has undergone work hardening during the forming process, or that the post-weld heat treatment was inadequate to fully relieve the residual stresses and temper the microstructure. In low-alloy steels, hardness values above 22 HRC are generally considered unacceptable for high-pressure service because they increase the susceptibility to hydrogen-induced cracking and stress corrosion cracking.

Metallurgical Considerations

The hydrotreating environment is particularly aggressive due to the presence of atomic hydrogen, which can diffuse into the steel and cause hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC). The following metallurgical factors are critical in preventing cracking in reducer tees:

1. Microstructure control: The microstructure should be a tempered martensite or a fine pearlite-ferrite structure, with a hardness below 22 HRC.
2. Inclusion control: Sulfide and oxide inclusions should be minimized and elongated in the rolling direction to prevent crack initiation.
3. Residual stress relief: Complete stress relief through PWHT is essential to prevent residual stress from contributing to crack propagation.
4. Hydrogen resistance: The material should have adequate resistance to hydrogen-induced cracking, which can be enhanced by adding microalloying elements such as vanadium or niobium.

Engineering Practice and Preventive Measures

Based on the analysis presented in the paper, the following preventive measures should be implemented for reducer tees in hydrotreating service:

• Material specification: Use of a low-alloy steel with proven resistance to HIC and SSC, such as ASTM A335 P11 with a hardness below 22 HRC.
• Manufacturing control: Strict control of the forming and welding procedures to minimize work hardening and residual stress.
• PWHT verification: Mandatory verification of PWHT effectiveness through hardness testing and residual stress measurement.
• In-service inspection: Regular UT or MT inspection of the transition zone to detect early-stage cracking before it becomes a safety hazard.
• Design modification: Consideration of using a separate reducer and tee instead of a combined reducer tee, to eliminate the stress concentration at the transition zone.

Key Questions and Reflections

This paper raises an important question about the adequacy of current design and inspection practices for reducer tees in high-pressure, hydrogen-containing service. The fact that a crack was detected in the body of a reducer tee suggests that either the design was inadequate, the manufacturing quality was substandard, or the in-service inspection was insufficient. In any case, the failure represents a significant safety risk, as a crack in a high-pressure pipe fitting can lead to a catastrophic leak of hydrogen-containing hydrocarbons.

The analysis also highlights the importance of hardness control in high-pressure piping. Hardness is a simple but effective indicator of the material's susceptibility to cracking, and it should be routinely monitored during manufacturing and in-service inspection. The paper's emphasis on the relationship between hardness, wall thickness, and crack location provides a useful framework for diagnosing and preventing similar failures in the future.

Study Insights and Implications

The crack analysis of the reducer tee in the hydrotreating unit serves as a reminder of the complex interplay between material properties, manufacturing processes, and operating conditions in determining the integrity of high-pressure pipe fittings. The transition zone of a reducer tee is a particularly vulnerable area because it combines geometric stress concentration, metallurgical non-uniformity, and exposure to an aggressive chemical environment. Engineers should adopt a holistic approach to the design, fabrication, inspection, and maintenance of reducer tees in such service, recognizing that no single factor alone is sufficient to ensure long-term reliability.