Failure Analysis of Low-Temperature Economizer Tube Elbow Leakage in a Catalytic Waste Heat Boiler

Literature Overview and Equipment Description

The paper by Guo Qingyun and Wang Qiang, published in Petrochemical Equipment Technology (2018, Vol. 39, No. 6), presents a detailed failure analysis of tube elbow leakage in the low-temperature economizer section of a catalytic waste heat boiler (catalytic cracker waste heat boiler, often abbreviated as "WHEB" or "余锅" in Chinese). The waste heat boiler is an integral part of the fluid catalytic cracking (FCC) unit, where it recovers heat from the hot flue gas exiting the regenerator and converts it into steam for the power generation system and process heating.

The low-temperature economizer is located in the low-temperature section of the boiler, where the flue gas temperature is below the dew point of sulfuric acid formed from sulfur-containing compounds in the fuel. This makes the low-temperature economizer particularly vulnerable to acid dew point corrosion, which is the primary failure mechanism identified in this study.

Failure Investigation Methodology and Findings

The investigation followed a systematic failure analysis approach, combining macroscopic examination, thickness measurement, chemical composition analysis, metallographic examination, and thermodynamic calculations. The following methodology was applied:

Macroscopic Examination and Thickness Measurement

The failed elbow was recovered from service and subjected to a thorough macroscopic examination. The leakage site was located on the outer surface of the elbow, where a thin, crystalline deposit of sulfuric acid corrosion products was observed. The surrounding area showed signs of general thinning, with the wall thickness reduced from the original 6.35 mm to approximately 1.2 mm at the thinnest point. The thickness measurement was performed using ultrasonic thickness gauging at multiple locations around the elbow circumference and along the axial direction.

Chemical Composition Analysis

The chemical composition of the tube material was analyzed to verify conformance with the specified grade. The results confirmed that the material met the requirements of the specified grade (typically ASTM A210 A or similar carbon steel grade for economizer tubes). The sulfur content was within acceptable limits, but the presence of residual sulfur in the steel contributed to the formation of iron sulfide inclusions that served as initiation sites for corrosion.

Metallographic Examination

Metallographic examination of cross-sections taken from the failed elbow revealed the following features:

• A distinct corrosion attack zone on the outer surface, extending approximately 0.8 mm into the wall thickness.
• Sulfidation attack characterized by the presence of iron sulfide (FeS) phases along the grain boundaries and in the matrix.
• No evidence of high-temperature oxidation or thermal fatigue cracking.
• The microstructure in the unaffected region showed a normal ferrite-pearlite structure consistent with the original material condition.

Thermodynamic Analysis of Dew Point Corrosion

The authors calculated the sulfuric acid dew point temperature of the flue gas based on the sulfur content of the fuel and the water vapor content of the flue gas. The sulfuric acid dew point was determined to be approximately 165°C. The operating temperature of the low-temperature economizer inlet was measured at 155°C, which is below the dew point temperature. This temperature differential of 10°C is sufficient to cause condensation of sulfuric acid on the tube surfaces, initiating the corrosion process.

The following table summarizes the key parameters of the failure analysis:

Root Cause Determination and Remediation Recommendations

The root cause of the elbow failure was conclusively determined to be sulfuric acid dew point corrosion. The failure sequence can be described as follows:

1. The flue gas, containing sulfur dioxide (SO2) and water vapor (H2O), enters the low-temperature economizer at a temperature below the sulfuric acid dew point.
2. Sulfuric acid (H2SO4) condenses on the cold tube surfaces.
3. The condensed acid attacks the carbon steel surface, dissolving the metal and forming iron sulfide corrosion products.
4. The corrosion products form a porous, non-protective layer that does not prevent further acid attack.
5. Progressive wall thinning occurs until the remaining wall thickness can no longer withstand the internal pressure, leading to rupture and leakage.

The authors recommend the following remediation measures, which are presented in order of priority:

1. Material upgrade: Replace the carbon steel tubes with corrosion-resistant materials such as 304L stainless steel, 316L stainless steel, or high-silicon cast iron (e.g., HS-6 or HS-8 grades). The material upgrade provides a direct defense against sulfuric acid attack and is the most effective long-term solution.
2. Improvement of boiler wall sealing: Enhance the sealing of the boiler wall to reduce the infiltration of cold air into the flue gas stream. Cold air infiltration lowers the flue gas temperature and increases the water vapor content, both of which exacerbate dew point corrosion. The target is to reduce the oxygen content in the flue gas to below 3% (dry basis).
3. Increase of feedwater temperature: Raise the feedwater temperature to increase the tube wall temperature above the sulfuric acid dew point. This can be achieved by installing a feedwater preheater or by optimizing the steam extraction system to provide higher-temperature extraction steam for feedwater heating.
4. Chemical treatment of flue gas: Install a flue gas desulfurization (FGD) system or inject a neutralizing agent (such as magnesium oxide or calcium oxide) into the flue gas to reduce the sulfur content and raise the dew point above the operating temperature.
5. Implementation of a corrosion monitoring program: Install corrosion coupons and ultrasonic thickness gauges at critical locations in the economizer to monitor the corrosion rate and predict the remaining service life of the tubes.

Engineering Practice Insights

The failure analysis presented in this paper illustrates several important principles of failure analysis methodology. First, the integration of multiple analytical techniques (macroscopic examination, chemical analysis, metallography, and thermodynamic calculations) is essential to reach a conclusive root cause determination. No single technique provides sufficient evidence on its own; it is the convergence of evidence from multiple sources that establishes the failure mechanism with confidence.

Second, the importance of thermodynamic analysis in corrosion failure investigations cannot be overstated. The calculation of the dew point temperature provided the critical link between the operating conditions and the corrosion mechanism. Without this calculation, it would have been difficult to explain why corrosion occurred at the specific temperature and location observed.

Third, the failure analysis highlights the importance of considering the entire system context, not just the failed component. The root cause of the failure was not a material defect or a manufacturing defect, but rather an operating condition (temperature below dew point) that was not adequately controlled. This underscores the importance of system-level analysis in failure investigations and the need to consider operating procedures, control systems, and maintenance practices as potential contributing factors.

In summary, this paper presents a well-structured failure analysis of a low-temperature economizer tube elbow failure caused by sulfuric acid dew point corrosion. The systematic investigation methodology, the clear identification of the root cause, and the practical remediation recommendations make this a valuable reference for engineers dealing with similar corrosion problems in waste heat recovery systems. The case study reinforces the importance of thermodynamic analysis, material selection, and operating condition control in preventing corrosion-related failures in high-temperature process equipment.