Leakage Analysis of Atmospheric Overhead Heat Exchanger Outlet Elbow Under Dew Point Corrosion Conditions

Literature Overview

Authored by Hou Yanhong and Li Qiang from CNOOC Huizhou Petrochemical Co., Ltd., this paper appeared in Petrochemical Corrosion and Protection (2020, Vol. 37, No. 3, pp. 53-57). The study investigates the failure of an outlet elbow on atmospheric overhead heat exchanger E-301A in a refinery. The analysis identified the HCl-H2O-H2S corrosion environment as the primary driver, with contributing factors including sub-dew-point outlet temperature, piping elevation deviations causing uneven vapor distribution, and insufficient water injection. The paper proposes corrective measures including increased overhead water injection, elevation correction, and amine injection upstream of the branch point.

Core Technical Content

The failure scenario involves a heat exchanger in the atmospheric distillation overhead system where the outlet elbow experienced a leak. The analytical approach followed a systematic root cause investigation methodology, and the key findings are organized as follows:

The paper's central argument is that the failure resulted from a confluence of factors rather than a single cause. The HCl-H2O-H2S environment is inherently aggressive, but the specific conditions at E-301A amplified the corrosion rate beyond acceptable limits. The piping elevation deviation is a particularly instructive finding, as it demonstrates how a seemingly minor installation error can have disproportionate consequences in a corrosion-prone system.

Corrosion Mechanism Analysis

The HCl-H2O-H2S corrosion system is well-documented in refinery overhead systems, but the specific interaction at sub-dew-point temperatures warrants detailed examination. When the overhead vapor cools below the acid dew point, the following sequence occurs:

1. HCl gas dissolves in the condensate water film to form dilute hydrochloric acid, which is highly corrosive to carbon steel.
2. H2S simultaneously dissolves in the condensate, creating a mildly acidic environment that can contribute to sulfide stress cracking in susceptible materials.
3. The combined acid condensate forms a thin, highly conductive liquid film on the pipe wall, establishing an electrochemical corrosion cell.
4. At the elbow, the flow pattern change creates shear stress on the condensate film, leading to erosion-corrosion synergy where the mechanical removal of the acid film exposes fresh metal surface to attack.

The table below summarizes the typical corrosion rates under different conditions:

Piping Elevation Deviation as a Contributing Factor

A particularly valuable insight from this paper is the role of piping elevation deviation in exacerbating corrosion. In a typical atmospheric distillation overhead system, multiple heat exchangers are connected in parallel branches from a common header. The vapor distribution among these branches depends on the relative elevations of the branch connections. When a branch is slightly lower than designed, it receives a disproportionate share of the vapor flow, resulting in higher HCl concentration and lower residence time for water injection to take effect.

The paper found that the E-301A branch had a negative elevation deviation relative to design, causing it to receive more overhead vapor than intended. This led to two compounding problems: the increased vapor flow rate reduced the contact time between injected water and the vapor stream, and the higher HCl concentration in the E-301A outlet stream lowered the acid dew point further below the actual operating temperature. The result was a more aggressive condensate environment at the E-301A outlet elbow.

Corrective Measures and Engineering Recommendations

The paper proposes three corrective actions, which can be evaluated using the PDCA framework:

The recommendation to inject amine upstream of the branch point is particularly astute, as it ensures that all branches receive the benefit of neutralization regardless of flow distribution. Amine injection (typically using morpholine or diethanolamine) reacts with HCl to form non-corrosive amine hydrochloride salts that remain in the gas phase at operating temperatures.

Study Insights and Practice Integration

This case study reinforces the principle that corrosion management in refinery overhead systems requires a holistic approach that integrates process design, piping installation quality, and chemical injection strategy. The piping elevation deviation finding is especially relevant for construction and mechanical integrity teams, as it demonstrates that installation tolerances must be strictly controlled in corrosion-prone systems. A deviation of even a few millimeters in branch elevation can have cascading effects on corrosion performance.

For integrity management programs, the paper suggests incorporating piping elevation verification as part of commissioning and periodic inspection activities. Additionally, the use of online corrosion monitoring probes at critical elbow locations would provide early warning of deteriorating corrosion conditions. The combination of increased water injection, elevation correction, and amine injection represents a layered defense strategy that addresses the problem at multiple points in the corrosion pathway, consistent with the philosophy of barrier-based risk management.