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

Literature Overview

The paper by Guo Qingyun and Wang Qiang, published in Petrochemical Equipment Technology (Vol. 39, No. 6, 2018), presents a comprehensive failure analysis of tube elbow leakage in the low-temperature economizer of a catalytic waste heat boiler at a Sinopec Tianjin Branch facility. The authors conducted macroscopic examination, wall thickness measurement, chemical composition analysis, metallographic examination, and thermodynamic calculations to determine the root cause of failure. The analysis conclusively identifies sulfuric acid dew point corrosion as the primary failure mechanism and proposes engineering countermeasures including material upgrade, furnace wall sealing improvement, and feed water temperature elevation.

Core Technical Content

The catalytic waste heat boiler (referred to as "reboiler" or "catalytic reboiler") is a critical piece of equipment in fluid catalytic cracking (FCC) units. It recovers heat from the regenerator flue gas to generate steam for the power block and process heating. The low-temperature economizer is located in the low-temperature section of the boiler, where flue gas temperatures have dropped below the sulfuric acid dew point.

Failure Investigation Methodology

The investigation followed a systematic approach:

1. Macroscopic examination: Visual inspection of the failed elbow revealed localized thinning and perforation at specific locations along the tube wall. The affected areas showed characteristic corrosion product deposits and pitting patterns.
2. Wall thickness measurement: Ultrasonic thickness gauging was performed at multiple locations around the elbow circumference and along the tube length. The measurements revealed significant wall thinning in specific zones, with the minimum wall thickness reduced to less than 50% of the original design thickness.
3. Chemical composition analysis: Spectroscopic analysis confirmed the tube material composition and identified the presence of sulfur and other corrosive elements in the corrosion products.
4. Metallographic examination: Cross-sectional metallographic analysis revealed the extent of wall thinning, the presence of corrosion attack at the inner tube surface, and the condition of the heat-affected zone near any weld joints.
5. Thermodynamic calculations: The dew point temperature of the flue gas was calculated based on the sulfur content, water vapor content, and oxygen partial pressure in the flue gas.

Sulfuric Acid Dew Point Corrosion Mechanism

The sulfuric acid dew point corrosion mechanism proceeds as follows:

• Sulfur compounds in the FCC regenerator flue gas (primarily SO₂ and SO₃) react with water vapor to form sulfuric acid vapor (H₂SO₄).
• When the flue gas temperature drops below the sulfuric acid dew point temperature (typically 120°C to 180°C, depending on SO₃ concentration), sulfuric acid condenses on the tube surfaces.
• The condensed sulfuric acid, even at low concentrations (0.1% to 5%), is highly corrosive to carbon steel and low-alloy steel at temperatures above 100°C.
• The corrosion rate increases with sulfuric acid concentration, temperature, and the presence of particulate matter (fly ash) that can abrade the protective corrosion product layer.

Failure Root Cause Analysis

The investigation identified several contributing factors:

• Design temperature margin: The economizer outlet flue gas temperature was designed to be above the sulfuric acid dew point, but operational conditions (variations in feed composition, catalyst properties, and regenerator operating parameters) caused the actual temperature to drop below the dew point.
• Material limitation: The tube material (likely 20G or 12Cr1MoVG carbon or low-alloy steel) does not have adequate resistance to sulfuric acid corrosion at dew point conditions.
• Furnace wall air leakage: Air infiltration through furnace wall joints introduced additional oxygen and moisture into the flue gas path, altering the dew point temperature and increasing corrosion aggressivity.
• Low feed water temperature: Insufficient feed water temperature to the economizer resulted in lower tube wall temperatures, promoting acid condensation on the tube inner surface.

Engineering Practice Insights

The failure analysis provides several important lessons for engineers designing and operating catalytic waste heat boilers:

Material upgrade strategies: The most effective countermeasure is to upgrade the tube material to a sulfuric acid resistant grade. Suitable materials include:

The selection of the upgrade material should be based on a cost-benefit analysis that considers the expected service life, the cost of material, and the consequences of continued failure.

Furnace wall sealing improvement: Air leakage through furnace wall joints must be minimized through proper refractory installation, gasket selection, and joint design. The target is to reduce air infiltration to less than 2% of the flue gas flow rate.

Feed water temperature management: The economizer feed water temperature should be maintained above 120°C to prevent acid condensation on the tube inner surface. This can be achieved through feed water preheating using low-pressure steam or by utilizing the heat from the deaerator.

Monitoring and inspection program: A systematic inspection program should include:

• Ultrasonic thickness measurement at critical locations (elbow outer radius, tube joints, and areas near air leakages) at intervals of 3 to 6 months.
• Flue gas temperature and composition monitoring at multiple points along the economizer to detect dew point violations.
• Corrosion coupon monitoring using representative materials placed at critical locations.
• Visual inspection of furnace wall joints for air leakage indicators (soot deposits, temperature anomalies).

Failure prevention through design review: During the design phase of catalytic waste heat boilers, the following checks should be performed:

1. Calculate the sulfuric acid dew point temperature for the worst-case flue gas composition (maximum SO₃ and water vapor content).
2. Ensure that the minimum tube wall temperature throughout the economizer is at least 20°C above the calculated dew point temperature.
3. Select tube materials with adequate sulfuric acid resistance based on the expected operating conditions.
4. Design adequate feed water preheating capacity to maintain the required minimum tube wall temperature.
5. Provide inspection ports and access for thickness measurement at critical locations.

Study Reflections and Recommendations

This failure analysis exemplifies the value of systematic investigation in identifying root causes and implementing effective corrective measures. The combination of macroscopic examination, non-destructive testing, destructive metallurgical analysis, and thermodynamic calculation provides a comprehensive picture of the failure mechanism. The identification of sulfuric acid dew point corrosion as the root cause is well-supported by the evidence, and the proposed countermeasures are practical and cost-effective.

For engineers managing catalytic waste heat boilers, I emphasize that dew point corrosion is a preventable failure mode. The key is to maintain adequate temperature margins above the dew point, use appropriate materials, and implement a rigorous inspection and monitoring program. The cost of preventive measures is invariably lower than the cost of unplanned tube replacement and associated production losses.

The paper also underscores the importance of considering the entire flue gas path in the design of waste heat boilers. The interaction between flue gas composition, temperature profiles, and material properties must be carefully analyzed to ensure long-term reliability. Engineers should adopt a holistic approach that integrates thermodynamic analysis, materials selection, and operational monitoring to achieve optimal equipment performance and reliability.