Second Catalytic Waste Heat Boiler Economizer Serpentine Tube Elbow Leakage Analysis

Literature Overview

This paper by Song Zhaohua from Luoyang Petrochemical Company and Liu Yingfan and Liu Chaofeng from the Key Laboratory of Interfacial Science, Zhengzhou University of Light Industry, published in Foundry Technology (2012, Vol. 33, No. 11, pp. 1292-1293), addresses the recurring leakage problem in the upper zone serpentine tube elbows of the second catalytic waste heat boiler No. 2 economizer. The study combines field statistics with metallurgical analysis to identify the root causes of frequent leaks and proposes preventive measures.

Field Statistics and Failure Pattern Analysis

The authors conducted a comprehensive statistical survey of the leakage incidents, which revealed several important patterns:

Leaks occurred predominantly in the upper zone of the economizer, where gas temperatures are highest.
The affected elbows were located at specific positions along the serpentine tube runs, suggesting a relationship between tube orientation, flow direction, and corrosion mechanism.
Both internal and external surface corrosion were observed, but with different severity levels and mechanisms.
The frequency of leaks increased with operating time, indicating progressive degradation rather than random manufacturing defects.

Corrosion Mechanism Analysis

The internal corrosion was attributed to:

High-temperature flue gas containing SO₂, SO₃, and water vapor.
Formation of sulfuric acid dew point corrosion on the tube inner surfaces.
Possible acid dew point temperatures ranging from 130°C to 180°C, depending on SO₃ concentration.
Localized attack at the inner curvature of elbows where gas flow impingement and turbulence are most severe.

The external corrosion was linked to:

Condensation of moisture on the outer tube surfaces during transient operating conditions.
Sulfuric acid formation from atmospheric SO₂ and moisture.
Potential chloride contamination from maintenance activities or ambient environment.

Technical Parameters and Material Considerations

Parameter	Typical Value	Impact on Corrosion
Gas temperature at economizer inlet	320-420°C	Above dew point during normal operation
Gas temperature at economizer outlet	180-250°C	Approaching dew point
SO₃ concentration in flue gas	50-200 ppm	Determines acid dew point
Tube material	Carbon steel or low-alloy steel	Susceptible to acid corrosion
Wall thickness	6-10 mm	Provides initial corrosion allowance
Elbow radius	1.5D to 3D	Affects flow impingement

Preventive Measures Recommended

The paper proposes a multi-pronged approach to prevent future leaks:

Material upgrade: Replace carbon steel tubes with corrosion-resistant alloys such as 321 stainless steel, 347 stainless steel, or superalloys for the upper zone where temperatures exceed the acid dew point.
Temperature control: Maintain gas outlet temperatures above the acid dew point by adjusting fuel firing rates or economizer surface area.
Coating application: Apply internal coatings such as ceramic coatings or aluminized coatings to provide a protective barrier against acid attack.
Enhanced monitoring: Implement ultrasonic thickness monitoring programs at critical elbow locations, with inspection intervals based on corrosion rate data.
Design modification: Consider increasing the bend radius or adding flow straighteners to reduce impingement erosion at elbow inner surfaces.

Integration with Engineering Practice

This case study is highly relevant to engineers designing waste heat recovery systems in catalytic cracking units. The serpentine tube arrangement, while space-efficient, creates challenging flow conditions at bends where erosion-corrosion is most severe. The upper zone of the economizer represents a critical design boundary where the transition from high-temperature gas to lower-temperature gas creates conditions favorable for condensation and subsequent corrosion.

From a welding perspective, the elbows in serpentine tube systems are typically formed by hot bending or cold bending of seamless pipe, followed by welding to straight tube sections. The welding process introduces additional vulnerabilities:

Residual stresses from welding can accelerate stress corrosion cracking.
Microstructural changes in the HAZ may reduce corrosion resistance.
Incomplete weld penetration or porosity can serve as crack initiation sites.

Study Insights and Reflections

This paper demonstrates the importance of a systems approach to failure analysis. The authors did not simply identify the corrosion mechanism but also considered the operational context, design parameters, and maintenance history. The statistical approach to identifying failure patterns is particularly valuable, as it provides a basis for risk-based inspection and maintenance planning.

One area that could be further explored is the role of welding quality in the overall failure rate. While the paper focuses on corrosion mechanisms, the condition of welds at tube-to-tee and tube-to-elbow joints could significantly influence the onset and progression of leaks. Future investigations should incorporate weld quality assessment as a variable in the failure analysis.