Thinning Failure Analysis of Tee Fitting at Hydrogenation Unit Stripping Tower Outlet

Incident Description and Operational Context

The case study by Yang Wenxue from CNOOC Zhongjie Petrochemical Co., Ltd., published in Petroleum and Chemical Equipment in 2024, documents a failure analysis of a tee fitting that experienced significant wall thinning at the outlet of a stripping tower in a hydrogenation unit. During scheduled maintenance, operators discovered obvious wall thinning on the tee fitting connecting to the top outlet of the desulfurization hydrogen stripping tower. This type of failure is particularly concerning because hydrogenation units operate under high temperature and pressure conditions with hydrogen-rich environments containing hydrogen sulfide, creating aggressive conditions for material degradation.

Failure Analysis Methodology and Root Cause Identification

The investigation employed a systematic failure analysis approach combining macroscopic examination, metallographic analysis, hardness testing, and fluid velocity distribution mapping. The researchers obtained fluid velocity distribution diagrams for the tee fitting geometry to correlate the observed thinning patterns with local flow conditions. This integrated analytical approach is essential for distinguishing between different degradation mechanisms that may contribute to wall thinning.

The investigation revealed that the wall thinning resulted from the combined action of two degradation mechanisms: corrosion by the H2S-HCl-H2O system and erosion by the gas-oil medium. The H2S-HCl-H2O corrosion represents a well-documented sulfide stress cracking and general corrosion mechanism in refinery environments, where dissolved hydrogen sulfide and hydrochloric acid create an acidic, reducing environment that attacks the base metal. Simultaneously, the high-velocity gas-oil flow through the tee fitting causes mechanical erosion of the pipe wall, particularly at locations where flow direction changes or where flow accelerates.

Fluid Velocity Distribution and Differential Thinning Patterns

A key finding of this study is that erosion is the primary factor causing different corrosion damage patterns between the tee main body and the straight pipe sections. The fluid velocity distribution in a tee fitting is inherently non-uniform, with significant acceleration at the throat region where the cross-sectional area decreases, and flow separation occurring at the inner bend where the flow direction changes. These flow features create localized high-shear regions that intensify both mechanical erosion and corrosion rates.

The differential thinning between the tee body and the straight pipe sections provides important diagnostic information. In straight pipe sections, corrosion typically proceeds uniformly across the circumference, while in tee fittings, the thinning pattern follows the fluid velocity distribution with maximum thinning occurring at the outer wall of the bend and at the throat. This pattern recognition capability is valuable for predicting remaining life of tee fittings in similar service conditions and for identifying which fittings require priority inspection during maintenance campaigns.

Engineering Recommendations and Preventive Measures

Based on the failure analysis findings, several engineering countermeasures can be recommended for similar hydrogenation unit stripping tower outlet tee fittings. Material selection should consider alloys with enhanced resistance to H2S-HCl-H2O corrosion, such as duplex stainless steels or nickel-based alloys, depending on the economic and operational constraints. Surface protection measures including erosion-resistant coatings or overlay welding with hardfacing materials can provide additional protection at high-velocity regions.

Inspection procedures should be enhanced to include ultrasonic thickness mapping at critical locations identified by fluid velocity analysis, with particular attention to the throat region and outer bend wall. The inspection frequency should be increased based on the demonstrated degradation rate, and inspection results should be tracked over time to establish degradation rate trends for remaining life prediction. For new installations, tee fitting geometry optimization to reduce flow separation and velocity peaks can be considered, although this may require trade-offs with pressure drop and mechanical design constraints. The case study serves as a valuable reference for reliability engineering programs in hydrogenation units where aggressive service conditions demand rigorous material selection and inspection practices.