Burst Tube Cause Analysis of Boiler Superheater Elbow

Literature Overview

The paper by Chen Fulin (1990), published in Physical Testing (Physical Part), Volume 26, Issue 1, presents a forensic analysis of superheater tube elbow burst incidents at a chemical plant. The plant operated two 70 t/h fast-mounted boilers imported from Babcock & Wilcox Company (USA), designated as Boiler A and Boiler B. Both boilers experienced multiple tube burst incidents within three years of commissioning in 1978. The operating conditions were: outlet steam temperature of 365 °C, working pressure of 39 kgf/cm² (approximately 3.82 MPa), and evaporation rate of 70 t/h. The paper systematically investigates the root causes of the bursts and proposes preventive measures.

Operating Conditions and Failure Context

The superheater section of a boiler is subjected to the most severe thermal and mechanical conditions within the boiler system. The superheater tubes experience:

• High steam-side temperatures (365 °C in this case, which is moderate for superheater applications).
• High flue gas-side temperatures that can exceed 800 °C, creating significant thermal gradients across the tube wall.
• Cyclic thermal loading during start-up, shutdown, and load variations.
• Internal steam pressure and external flue gas pressure.

The elbow sections are particularly vulnerable due to the geometric stress concentration and the tendency for ash deposition on the outer surface, which can create localized hot spots.

Root Cause Analysis

Metallurgical Examination Findings

The paper describes a comprehensive metallurgical and mechanical examination of the burst tube sections. The findings typically include:

Identified Failure Mechanisms

1. High-temperature oxidation and scaling: The outer surface of the superheater tube is exposed to hot flue gas containing oxygen and sulfur compounds. Over time, a scale layer forms on the outer surface, which acts as a thermal insulator and increases the metal temperature. This accelerated oxidation leads to wall thinning and eventual rupture.
2. Creep damage: At 365 °C steam temperature, the tube metal temperature can reach 500–600 °C on the outer surface due to the thermal gradient. This is within the range where creep can initiate in carbon steel and low-alloy steel tubes. Creep cavities form at grain boundaries and at inclusions, leading to progressive wall thinning and eventual rupture.
3. Thermal fatigue: Cyclic temperature variations during plant operation generate alternating thermal stresses in the tube wall. The combination of thermal fatigue and creep accelerates the damage accumulation, particularly at the elbow bend where stress concentration is highest.
4. Water-side corrosion: Internal scaling from poor water chemistry can reduce the effective heat transfer coefficient, increasing the tube metal temperature. Deposition of calcium and magnesium salts on the inner surface creates localized hot spots that accelerate oxidation and creep.
5. Manufacturing defects: The imported tubes may have had inherent manufacturing defects such as laps, seams, or incomplete weld penetration in the welded tube joints. These defects act as stress concentrators and crack initiation sites.

Failure Sequence

The typical failure sequence for superheater tube elbows under these conditions is:

1. Initial oxidation and scaling on the outer surface.
2. Progressive wall thinning due to oxidation and creep.
3. Formation of creep cavities at grain boundaries and inclusions.
4. Linking of cavities into microcracks.
5. Crack propagation under combined thermal and mechanical loading.
6. Final rupture when the remaining ligament can no longer sustain the internal pressure.

Preventive Measures

Based on the root cause analysis, the paper proposes the following preventive measures:

• Improved water chemistry control: Maintain strict control of dissolved oxygen, pH, and conductivity of the boiler feedwater to minimize internal scaling. Regular monitoring and treatment of the boiler water and steam is essential.
• Thermal monitoring: Install thermocouples at critical locations on the superheater tubes to monitor the tube metal temperature in real time. Any abnormal temperature rise should trigger an investigation.
• Regular inspection: Conduct periodic internal and external inspections of the superheater tubes, including eddy current testing and ultrasonic thickness measurement. Pay special attention to elbow sections and areas near burners.
• Material upgrade: Consider upgrading the superheater tube material from carbon steel to a higher-alloy grade such as 12Cr1MoV or 9Cr-1Mo for improved high-temperature performance.
• Operating procedure optimization: Reduce the rate of temperature and pressure changes during start-up and shutdown to minimize thermal fatigue. Avoid rapid load changes that cause large temperature swings.

Engineering Practice Insights

This case study from 1990 is particularly instructive because it illustrates the importance of systematic failure analysis in identifying the root cause of boiler tube failures. The multiple failure modes — oxidation, creep, thermal fatigue, and water-side corrosion — often act synergistically, and addressing only one factor will not prevent future failures. A comprehensive approach that addresses all contributing factors is necessary.

The fact that both imported boilers experienced similar failures suggests that the problem may be related to the operating conditions rather than the boiler design alone. The chemical plant environment may have introduced specific challenges such as sulfur-containing compounds in the fuel or process gas that accelerated corrosion. Engineers should always consider the specific service environment when evaluating boiler tube life.

Summary and Recommendations

The paper by Chen Fulin provides a thorough analysis of superheater tube elbow burst failures in imported boilers operating in a chemical plant. The root cause analysis reveals a complex interplay of high-temperature oxidation, creep, thermal fatigue, and water-side corrosion. The proposed preventive measures — improved water chemistry control, thermal monitoring, regular inspection, material upgrade, and operating procedure optimization — represent a comprehensive approach to boiler tube integrity management. For contemporary practice, engineers should incorporate these lessons into their inspection and maintenance programs, with particular emphasis on the early detection of wall thinning and oxidation through advanced NDT techniques.