Failure Analysis of Hydrogenation Exchanger Shell-Side Outlet Elbow Cracking

Literature Overview

This 2003 paper by Chai Keliang, Peng Qiong, Guo Zhijun, and Zhong Yanping, published in "Petrochemical Equipment" (Volume 32, Issue 2, pages 15–17), presents a detailed failure analysis of a hydrogenation exchanger shell-side outlet elbow that experienced cracking. The investigation was conducted by CNPC Lanzhou Petrochemical Refinery and the Lanzhou Petroleum Machinery Research Institute.

Core Technical Content

The failure occurred at the weld junction between the shell-side outlet elbow and the first reducer-to-flange connection of exchanger E301B. A longitudinal through-thickness crack measuring 26 mm in length was discovered at this location. Upon dissection of the elbow, a large number of transverse cracks were found in the near-weld region on the inner wall surface. The root cause was identified as wet hydrogen sulfide stress corrosion cracking (SSC) initiated from welding delayed cracks.

The failure mechanism involved a sequence of events:

Welding delayed cracks: During or shortly after welding, hydrogen atoms trapped in the weld and heat-affected zone (HAZ) caused delayed cracking. These cracks formed in the high-stress region near the weld.
Wet H₂S exposure: The service environment contained hydrogen sulfide in a wet (aqueous) environment, which is the classic condition for wet H₂S stress corrosion cracking as defined by NACE MR0175/ISO 15156.
SSC propagation: The welding delayed cracks served as initiation sites for SSC. The combined effect of tensile stress (from residual welding stress and operational loading) and the corrosive H₂S environment caused the cracks to propagate through the wall thickness.

Technical Analysis and Defect Characterization

Parameter	Observation	Significance
Crack location	Weld junction between elbow and reducer flange	High stress concentration region
Crack orientation	Transverse (perpendicular to weld)	Characteristic of SSC initiation from HAZ
Crack length	26 mm (surface); extensive internal cracking	Through-thickness propagation
Initiation mechanism	Welding delayed cracks	Hydrogen-induced cracking from welding
Propagation mechanism	Wet H₂S stress corrosion cracking	Environmental cracking in sour service
Material condition	Near-weld region on inner surface	HAZ susceptible to SSC

The combination of welding delayed cracks and wet H₂S SSC represents a particularly dangerous failure mode because the welding cracks provide pre-existing initiation sites that dramatically reduce the time to failure under SSC conditions. In a properly controlled welding process, the HAZ would not contain cracks, and the SSC initiation would be much slower.

Engineering Practice and Countermeasures

This failure case highlights several critical engineering practices that must be followed in sour service applications:

Welding procedure control: Welding procedures for sour service components must be qualified in accordance with NACE MR0175/ISO 15156. Preheat, interpass temperature, and post-weld heat treatment (PWHT) parameters must be carefully controlled to prevent hydrogen-induced cracking.
PWHT implementation: Post-weld heat treatment is essential for reducing residual welding stresses and relieving hydrogen in the weld and HAZ. The PWHT parameters should be verified by hardness testing, which should not exceed the limits specified in NACE MR0175 (typically 22 HRC for carbon steel).
Post-weld NDT: 100% radiographic testing (RT) and magnetic particle testing (MT) should be performed after welding and after PWHT to detect any delayed cracks that may have formed.
Material selection: For sour service, materials should comply with NACE MR0175/ISO 15156. Low-carbon steels with hardness below 22 HRC are generally acceptable, but higher-strength materials require careful qualification.
FMEA integration: The failure mode of welding delayed cracks leading to SSC should be included in the FMEA of sour service components, with appropriate preventive and detective controls.

Study Insights

This failure analysis demonstrates the critical importance of welding quality control in sour service applications. The welding delayed cracks, which are a welding metallurgy issue, created the conditions for environmental cracking. This case underscores that material selection alone is insufficient; the manufacturing process, particularly welding, must be controlled to the same level of rigor. The lesson for engineers is that every component in sour service must be treated as critical, from material procurement through welding, PWHT, and NDT, to final installation and in-service monitoring. A single welding defect can compromise the entire integrity of a pressure boundary in a corrosive environment.