Leakage Analysis of Atmospheric Distillation Overhead Heat Exchanger Outlet Elbows

Literature Overview

This paper by Hou Yanhong and Li Qiang from CNOOC Huizhou Petrochemical Co., Ltd., published in "Petroleum and Chemical Corrosion and Protection" (2020, Vol. 37, No. 3), presents a detailed failure analysis of elbow leakage at the outlet of atmospheric distillation overhead heat exchanger E-301A. The study identifies dew point corrosion exacerbated by insufficient overhead water injection as the primary mechanism, with pipeline elevation deviations playing a critical role in the non-uniform distribution of overhead vapor and water injection. The analysis is particularly instructive for engineers managing overhead systems in atmospheric distillation units where acid dew point corrosion remains a persistent challenge.

Root Cause Analysis

The investigation reveals a multi-factor failure mechanism centered on the HCl-H2O-H2S corrosion environment in the atmospheric distillation overhead system. The outlet temperature of heat exchanger E-301A was below the acid dew point temperature, creating conditions for liquid-phase acid corrosion. However, the critical aggravating factor was the non-uniform distribution of overhead vapor across parallel branches due to pipeline elevation deviations. The branch connected to E-301A received a disproportionately higher share of overhead vapor, which in turn resulted in a severely insufficient water injection rate relative to the actual acid loading.

The combination of elevated acid concentration and inadequate dilution created an aggressive localized corrosion environment. The elbow at the heat exchanger outlet experienced both dew point corrosion from the acid condensate and erosion-corrosion from the high-velocity acid-laden vapor flow. This dual mechanism accelerated wall thinning at the elbow's outer arc, ultimately leading to perforation and leakage.

Technical Parameters and Corrosion Mechanism

The acid dew point corrosion mechanism operates through the condensation of HCl and H2S on the metal surface when the temperature drops below the respective dew points. In the presence of water injection, the acid is partially neutralized, but when the injection rate is insufficient, the residual acid concentration exceeds the threshold for aggressive attack. The erosion-corrosion component arises from the interaction between the high-velocity vapor flow and the thin acid film on the metal surface, which removes the protective corrosion product layer and exposes fresh metal to continued attack.

Mitigation Measures and Engineering Recommendations

The authors propose three complementary mitigation strategies. First, increasing the overhead water injection rate to ensure adequate dilution of the acid condensate across all branches. Second, correcting the pipeline elevation deviations to achieve balanced vapor distribution among parallel branches. Third, installing neutralizer injection points upstream of branch takeoffs to provide localized acid neutralization where the acid loading is highest.

A particularly valuable engineering insight is the recognition that overhead water injection systems must be designed and operated with consideration for the actual vapor distribution pattern across branches, not just the total overhead vapor rate. Pipeline elevation deviations that seem minor during construction can have outsized effects on corrosion performance over time. This underscores the importance of precise installation tolerances and periodic verification of flow distribution in overhead systems.

Study Insights

This case study highlights the often-overlooked interaction between hydraulic design and corrosion management in distillation overhead systems. The failure was not caused by a single deficiency but by the compounding effects of temperature control, flow distribution, and chemical injection. For engineering practice, the key lesson is that overhead system integrity requires a holistic approach that integrates process control, mechanical design, and corrosion management. Regular verification of water injection distribution and pipeline elevation profiles should be incorporated into the integrity management program for atmospheric distillation units.