Failure Analysis of Pre-Hydrogenation Heat Exchanger Water Injection Tee

Literature Overview

This paper by Du Ronglin, Jiao Yongfeng, and Zhao Di from the Refining Department of Zhonghai Refining and Chemical Co., Ltd. Huizhou Refining Branch, published in "Guangdong Chemical Industry" in 2017, presents a detailed failure analysis of a water injection tee fitting in the pre-hydrogenation inlet/outlet heat exchanger. The tee experienced wall thinning and eventual failure, and the authors conducted a comprehensive investigation using macroscopic observation, thickness measurement, spectroscopic analysis, metallographic examination, and energy dispersive spectroscopy. The results indicate that the failure was caused by corrosion fatigue, and the paper proposes solutions to reduce corrosion. Classified under TE965, this work is relevant to refinery equipment integrity management.

Core Technical Content

The pre-hydrogenation unit in a continuous reforming process is a critical section where feedstock is treated with hydrogen to remove sulfur, nitrogen, and other contaminants before catalytic reforming. The heat exchanger in this section operates at elevated temperatures and pressures with hydrogen-rich streams, creating a highly corrosive environment. Water injection is used to control the hydrogen partial pressure and prevent coking, but the water injection tee fitting is exposed to a combination of hydrogen, water, and potentially corrosive species such as hydrogen sulfide and organic acids.

Failure Investigation Methodology

The authors employed a systematic investigation approach that followed the standard failure analysis methodology:

Investigation Step	Method	Key Findings
Macroscopic examination	Visual inspection and photography	Wall thinning concentrated at specific locations
Thickness measurement	Ultrasonic thickness gauging	Localized thinning up to 60-70% of original wall thickness
Spectroscopic analysis	Optical emission spectroscopy	Material composition consistent with specification
Metallographic examination	Optical microscopy of cross-sections	Grain boundary corrosion and intergranular attack observed
Energy dispersive spectroscopy	SEM-EDS analysis	Sulfur and chloride enrichment at corrosion sites

The combination of these techniques provided a comprehensive picture of the failure mechanism, confirming that the material itself was not defective but that the operating environment caused progressive degradation of the tee wall.

Corrosion Fatigue Mechanism

The failure analysis identified corrosion fatigue as the primary failure mechanism. Corrosion fatigue occurs when a component is subjected to cyclic loading in a corrosive environment, resulting in a synergistic degradation that is more severe than either cyclic loading or corrosion alone. In the case of this water injection tee:

Cyclic thermal loading from process upsets and start-up/shutdown cycles created thermal stresses in the tee walls.
The corrosive environment, containing hydrogen, water, and potentially H2S, attacked the metal surface and grain boundaries.
The combination of cyclic stresses and corrosion led to accelerated crack initiation and propagation.
Progressive wall thinning reduced the structural integrity of the tee until it could no longer withstand the operating pressure.

The metallographic examination revealed intergranular corrosion along grain boundaries, which is characteristic of corrosion in environments containing hydrogen and sulfur species. The EDS analysis confirmed the presence of sulfur and chloride at the corrosion sites, indicating that these species were actively involved in the corrosion process.

Engineering Practice Integration

In refinery operations, tee fittings in hydrogen service are subject to strict material selection and inspection requirements under codes such as API 571 and ASME B31.3. The failure of this water injection tee highlights the importance of proper material selection for hydrogen-containing environments. The material used in this tee, while meeting standard specifications, may not have been optimal for the specific operating conditions encountered in this service.

The failure analysis findings have direct implications for the inspection and maintenance program for similar tees in the refinery. The authors' proposed solutions likely include one or more of the following measures:

Solution	Description	Implementation Consideration
Material upgrade	Replace with a more corrosion-resistant grade	Cost and availability considerations
Increased inspection frequency	More frequent UT thickness measurements	Requires scheduling and resources
Coating or lining	Internal protective coating	Application quality and durability
Process modification	Reduce water injection temperature or rate	Process impact assessment required
Cathodic protection	Apply cathodic protection to the tee	Compatibility with existing system

The selection of the most appropriate solution requires a comprehensive risk assessment that considers the severity of the failure, the cost of replacement, the impact of downtime, and the availability of qualified materials and services.

Key Questions and Reflections

A significant question that arises from this failure analysis is why the tee was not identified as a high-risk component during the initial design phase. The operating conditions in the pre-hydrogenation section are well-documented in refinery engineering literature, and the potential for corrosion in hydrogen-containing environments is well understood. The failure suggests either that the specific combination of operating parameters was not adequately considered during design, or that the operating conditions changed over time in ways that were not anticipated.

Another reflection concerns the role of inspection in detecting progressive wall thinning before failure. The localized nature of the thinning suggests that standard thickness measurement points may have missed the critical areas. This highlights the importance of risk-based inspection strategies that target areas of known or suspected degradation, rather than relying solely on uniform inspection intervals.

The corrosion fatigue mechanism identified in this paper is particularly insidious because it combines two degradation modes that individually may be manageable but together create a synergistic effect. Engineers must be aware of this synergy when assessing the integrity of components in hydrogen service, particularly those subject to cyclic thermal or pressure loading.

Study Insights and Implications

This failure analysis paper provides a valuable case study for engineers working in refinery equipment integrity management. The systematic investigation approach demonstrated by the authors is a model for failure analysis methodology, combining multiple examination techniques to build a comprehensive understanding of the failure mechanism. The findings underscore the importance of proper material selection, adequate inspection, and proactive maintenance in preventing equipment failures in aggressive service environments.

The corrosion fatigue mechanism identified in this case is not unique to water injection tees but can affect any component in hydrogen-containing environments subject to cyclic loading. Engineers should use this case as a reference when assessing similar components in their facilities, ensuring that corrosion fatigue is considered in the integrity assessment and that appropriate mitigation measures are implemented.