Preliminary Analysis of Superheater Elbow Cracking in Sulfur Recovery Unit Waste Heat Boiler

Literature Overview

This failure analysis paper by Li Yujun and Jiang Shiliang from the Henan Boiler and Pressure Vessel Safety Inspection Research Institute and the Tianjin Petrochemical Pressure Vessel Inspection Research Center (published in Pressure Vessel, Vol. 28, No. 9, 2011, pp. 52–55) investigates the cracking of superheater elbows in a waste heat boiler of a sulfur recovery unit in a petrochemical enterprise. The analysis employed macroscopic inspection, thickness measurement, spectroscopic analysis, metallographic examination, hardness testing, and chemical analysis of furnace ash and scale. This case is particularly relevant given the critical role of sulfur recovery units in petrochemical processing and the severe operating conditions of waste heat boilers.

Service Background

The sulfur recovery unit plays a vital role in the overall production system of a petrochemical enterprise, recovering elemental sulfur from acid gas streams and reducing environmental emissions. The waste heat boiler is a key component of this unit, recovering thermal energy from the high-temperature gas stream exiting the sulfur recovery reactor. The superheater section of the waste heat boiler heats process water to generate high-pressure steam, and the superheater elbows are subjected to particularly severe conditions:

• High operating temperatures (typically 400–600°C depending on the specific design)
• Presence of sulfur compounds and other corrosive species in the gas stream
• Thermal cycling due to startup/shutdown and load variations
• Potential for ash deposition and erosion from particulates in the gas stream
• Mechanical loading from thermal expansion and pressure

Examination and Analysis

The authors conducted a comprehensive examination of the cracked superheater elbows, employing multiple analytical techniques to identify the failure mechanism.

Examination Results Summary

Detailed Analysis of Findings

Macroscopic inspection: The cracks were predominantly located at the outer wall of the elbows, which is the region subjected to the highest tensile stress due to thermal expansion and the geometric stress concentration at the bend. The presence of ash and scale deposits on the outer surface indicated that the gas stream contained particulates that deposited on the pipe surface, potentially creating localized corrosion cells.

Thickness measurement: The wall thinning observed at the outer wall of the elbows was consistent with a combination of erosion from gas-borne particulates and corrosion. The degree of thinning varied along the length of the elbow, with the maximum thinning occurring at the point of maximum curvature.

Spectroscopic analysis: The chemical composition of the pipe material was found to be consistent with the specified grade, indicating that the material itself was not the primary cause of failure. This ruled out material substitution or manufacturing defects as root causes.

Metallographic examination: The microstructural examination revealed significant changes in the near-surface regions of the pipe wall. In the heat-affected zone (HAZ) of the welded elbows, grain coarsening and possible temper embrittlement were observed. In the base metal near the outer wall, intergranular attack and microcracking were evident, consistent with high-temperature corrosion mechanisms.

Hardness testing: Elevated hardness was observed in certain regions of the pipe wall, particularly in the HAZ and near the outer surface. This hardness increase could be attributed to microstructural changes such as grain coarsening, carbide precipitation, or strain hardening from thermal cycling.

Furnace ash and scale analysis: The chemical analysis of the deposited ash and scale revealed the presence of sulfur compounds (sulfates, sulfides), iron oxides, and other corrosion products. The presence of sulfur compounds is particularly significant, as they can cause sulfidation corrosion at elevated temperatures, leading to the formation of iron sulfide scales that may be non-protective and can spall off, exposing fresh metal to further attack.

Failure Mechanism Analysis

Based on the comprehensive examination results, the authors identified the following failure mechanisms contributing to the superheater elbow cracking:

1. High-temperature corrosion: The presence of sulfur compounds in the gas stream caused sulfidation corrosion at elevated temperatures. This corrosion preferentially attacked the outer wall of the elbows, leading to wall thinning and the formation of non-protective iron sulfide scales.
2. Erosion: Gas-borne particulates (ash and dust) impinged on the outer wall of the elbows at high velocity, causing mechanical erosion. The erosion was most severe at the outer wall where the flow velocity was highest and where the wall was already weakened by corrosion.
3. Thermal fatigue: The cyclic temperature changes during normal operation and startup/shutdown caused thermal fatigue cracking. The thermal stress was highest at the outer wall of the elbows due to the geometric stress concentration and the temperature gradient through the wall thickness.
4. Weld-related degradation: The HAZ of the welded elbows was susceptible to temper embrittlement and grain coarsening at elevated temperatures. This degradation reduced the toughness and fatigue resistance of the weld region, making it more susceptible to cracking.

Interaction of Failure Mechanisms

The cracking of the superheater elbows was not caused by a single mechanism but by the interaction of multiple degradation mechanisms:

• Corrosion and erosion acted synergistically, with corrosion weakening the metal and erosion removing protective scales, exposing fresh metal to further corrosion.
• Thermal fatigue initiated cracks at stress concentration sites, which were then propagated by corrosion-assisted mechanisms.
• Weld-related degradation created weak regions that were preferentially attacked by corrosion and fatigue.

Preventive and Control Measures

Based on the failure analysis, the authors proposed the following preventive and control measures:

1. Material upgrade: Consider upgrading the pipe material to a higher-grade alloy with improved high-temperature corrosion resistance, such as 9Cr-1Mo steel or stainless steel cladding.
2. Weld quality improvement: Implement stricter welding procedures and post-weld heat treatment (PWHT) to minimize residual stresses and improve the microstructure of the HAZ.
3. Temperature control: Optimize the operating temperature to minimize the rate of high-temperature corrosion and temper embrittlement. This may require process modifications or the addition of temperature monitoring and control systems.
4. Gas cleaning: Improve the cleaning of the gas stream before it enters the waste heat boiler to reduce the concentration of sulfur compounds and particulates.
5. Inspection and monitoring: Implement a comprehensive inspection program including regular thickness measurements, visual inspections, and non-destructive testing (NDT) to detect early signs of degradation.
6. Erosion protection: Consider applying erosion-resistant coatings or overlays to the outer wall of the elbows, or redesigning the flow path to reduce impingement.

Study Insights

This failure analysis case study highlights the complexity of failure mechanisms in high-temperature pressure vessels and piping systems. The superheater elbows in a sulfur recovery unit waste heat boiler are subjected to a unique combination of degradation mechanisms—corrosion, erosion, thermal fatigue, and weld-related degradation—that interact in complex ways. The comprehensive analytical approach employed in this study, combining macroscopic, microscopic, chemical, and mechanical analyses, is essential for identifying the true root cause of failure in such complex scenarios.

For practitioners, the key lessons are:

• Failure in high-temperature service is rarely caused by a single mechanism; a comprehensive analysis is required to identify all contributing factors.
• Weld quality is critical in high-temperature applications, as the HAZ is often the weakest link in the system.
• The presence of corrosion products and ash deposits provides valuable information about the operating conditions and degradation mechanisms, and should always be included in the failure analysis.
• Preventive measures should address all identified failure mechanisms, not just the most obvious one. A multi-pronged approach combining material selection, process optimization, and inspection is the most effective strategy for ensuring long-term reliability.

The paper also underscores the importance of preventive maintenance and condition monitoring in critical process equipment. The sulfur recovery unit is essential for environmental compliance and process efficiency, and the failure of superheater elbows can lead to significant production losses and safety risks. A proactive approach to inspection and maintenance, informed by the lessons of failure analysis, is the most effective way to ensure the long-term reliability of these critical components.