Hydrotreating Unit Reducing Tee Body Crack Analysis

Literature Overview

The paper by Zhang Jianmin of Sinopec Cangzhou Branch (published in "Chemical Engineering & Machinery," 2002, Vol. 29, No. 5, pp. 296–297) presents a failure analysis of cracks found in the body of a reducing tee (eccentric or concentric reducer tee) in a hydrotreating unit. The analysis focuses on the transition zone (the diameter change area) of the tee, examining hardness and wall thickness variations as key factors in the crack formation.

Technical Context

Hydrotreating units operate under severe conditions: high pressure (typically 15–30 MPa), elevated temperature (300–400°C), and hydrogen-containing environments. The reducing tee, which connects pipes of different diameters, is a critical component in the hydrotreating process where flow rate and pressure changes occur. The body of the tee, particularly at the diameter transition zone, is subject to complex stress states and environmental degradation.

Crack Analysis

The authors identified the crack formation in the reducing tee body as a multifactorial failure involving:

Stress Factors

1. Geometric stress concentration — The diameter transition zone of the reducing tee creates a geometric discontinuity that concentrates stress. The stress concentration factor at the transition can be 1.5–2.0 times the nominal stress, depending on the taper angle and diameter ratio.
2. Thermal stress — The temperature gradient between the hot process fluid and the cooler ambient environment creates thermal stresses in the tee body. The transition zone, with its varying wall thickness, experiences differential thermal expansion.
3. Mechanical stress — The weight of the tee and connected piping, combined with the internal pressure, creates a complex stress state at the transition zone. The stress is further amplified by any misalignment or external loading.

Material Degradation Factors

1. Hardness variation — The authors specifically examined hardness variations across the tee body. The transition zone may exhibit higher hardness due to the manufacturing process (forging, rolling, or welding), creating a gradient that can act as a crack initiation site. Hardness values exceeding 200 HB in carbon steel are generally considered susceptible to hydrogen embrittlement.
2. Wall thickness variation — Non-uniform wall thickness at the transition zone creates stress concentrations and can lead to localized thinning due to corrosion or erosion. The authors noted thickness variations as a contributing factor to the crack formation.
3. Hydrogen attack — In hydrotreating service, atomic hydrogen diffuses into the steel matrix, combining with carbon to form methane, which creates internal voids and cracks. This is known as high-temperature hydrogen attack (HTHA) and is particularly severe in the temperature range of 200–450°C.
4. Sulfide stress cracking — If the medium contains H2S, sulfide stress cracking (SSC) can initiate in the HAZ or in areas of high hardness. SSC is a form of stress-corrosion cracking that is particularly aggressive in carbon and low-alloy steels.

Failure Mechanism Synthesis

The crack formation in the reducing tee body can be understood as a synergistic interaction of stress and material degradation:

• Crack initiation — Occurs at the transition zone where stress concentration is highest and where material properties (hardness, wall thickness) are most variable.
• Crack propagation — Driven by the cyclic loading from pressure pulsation and thermal cycling, accelerated by hydrogen embrittlement and SSC.
• Final failure — The crack grows to a critical size where the remaining cross-section can no longer withstand the applied stress, leading to sudden fracture.

Engineering Recommendations

Material Selection

• Specify appropriate material grade — For hydrotreating service, use API 5L X65 or higher for the tee body, or consider low-alloy steel (e.g., 1.25Cr-0.5Mo) for improved HTHA resistance.
• Control hardness — Limit the hardness of the tee body and HAZ to ≤200 HB (or ≤22 HRC) to minimize hydrogen embrittlement susceptibility.
• Consider overlay welding — Apply a corrosion-resistant overlay (e.g., 309L or 316L stainless steel) to the inner surface to protect against HTHA and SSC.

Manufacturing Controls

• Uniform wall thickness — Ensure the wall thickness at the transition zone is uniform within ±5% of the nominal value.
• PWHT — Apply post-weld heat treatment to relieve residual stresses and soften the HAZ.
• NDT inspection — Conduct UT and MT inspection of the transition zone before and after PWHT to detect any pre-existing defects.

Operational Monitoring

• Regular UT thickness measurement — Monitor the wall thickness at the transition zone every 6 months to detect thinning due to corrosion or erosion.
• Hydrogen monitoring — Install hydrogen detectors in the unit to detect any hydrogen leakage that could contribute to HTHA.
• Pressure pulsation monitoring — Monitor the pressure fluctuations in the hydrotreating unit to identify any abnormal pulsation that could accelerate fatigue cracking.

Key Reflections

This failure analysis case study emphasizes the importance of understanding the interaction between stress and material degradation in high-pressure, high-temperature hydrogen service. The reducing tee, while a standard component, is a critical location for failure due to the geometric stress concentration at the transition zone. The authors' focus on hardness and wall thickness as key parameters in the crack analysis reflects the metallurgical understanding that material properties are not uniform across a component and that local variations can be the controlling factor in failure. This case also highlights the need for comprehensive material selection and manufacturing controls in hydrotreating applications, where the environmental conditions are particularly aggressive.