Thermal Fatigue Cracking Analysis and Remediation of Reheater Header Connecting Pipe Elbows

Literature Overview

This paper, published in Thermal Power Generation (2007, Vol. 36, No. 10, pp. 66-68), is authored by Liu Fa-sheng, Chen Gang, and Wu Qi-xing from Huazhong University of Science and Technology. The study investigates repeated cracking of elbows located downstream of the fine-spray desuperheater in the reheat steam system of a 2-unit power plant boiler. The authors conducted component inspection, coal quality analysis, and cold-state reheat tests to diagnose the root cause, which was identified as thermal fatigue cracking resulting from overheating of the heat-absorbing surface due to deviation between the actual coal type and the design coal type, furnace exit flue gas temperature deviation, and frequent desuperheating water injection. Remedial measures including secondary air reverse-cut modification, improved feedwater and steam quality, coal quality improvement, and enhanced spray atomization were proposed.

System Description and Failure Mode

The reheat steam system in a coal-fired power plant boiler is designed to reheat the steam after it has expanded through the high-pressure turbine, improving the overall thermal efficiency of the Rankine cycle. The reheat steam flows through a series of pipe sections including headers, connecting pipes, elbows, and screen tubes, all of which are exposed to high-temperature flue gas. The fine-spray desuperheater is installed to control the reheat steam temperature by injecting water directly into the steam flow when the temperature exceeds the design limit.

The failure occurred at elbows downstream of the desuperheater in the No. 2 boiler reheat steam system. The elbows experienced multiple cracking events, severely affecting unit operation and reliability. The repeated nature of the failures indicates that the root cause was systemic rather than a one-time manufacturing defect, pointing to operational conditions that consistently exceeded the design limits of the elbow material.

Root Cause Analysis

The authors employed a systematic diagnostic approach involving three complementary methods:

The coal quality deviation was the primary contributing factor. When the actual coal differs from the design coal in terms of calorific value, ash content, ash fusion temperature, and flame characteristics, the furnace heat release profile changes, leading to higher-than-designed furnace exit flue gas temperatures. This directly increases the heat flux to the reheat section, causing the metal temperature of the connecting pipe elbows to exceed the design limit.

The frequent use of desuperheating water injection created an additional thermal cycling mechanism. Each injection event causes a rapid temperature drop in the steam and the pipe wall, followed by a temperature recovery when the injection stops. This thermal cycling, combined with the elevated mean temperature from coal quality deviation, produces a high thermal stress amplitude that drives thermal fatigue cracking.

The crack morphology observed during inspection was characteristic of thermal fatigue: multiple short cracks perpendicular to the pipe axis, initiated at the outer surface of the elbow where the temperature gradient is highest, and propagating inward. The cracks were likely initiated at stress concentration sites such as weld toes, surface defects, or material inclusions, and propagated under the cyclic thermal stress.

Material and Design Considerations

The reheat steam system operates at temperatures typically in the range of 540-620 degrees Celsius, depending on the power plant design and coal type. The connecting pipe elbows are typically fabricated from chromium-molybdenum low-alloy steels such as 12Cr1MoV (GB 5310) or equivalent materials (ASTM A213 T22, P22), which provide adequate creep strength and thermal fatigue resistance at these temperatures. However, when the operating temperature exceeds the design limit by even 30-50 degrees Celsius, the thermal fatigue life can decrease dramatically due to the exponential relationship between temperature and oxidation rate, as well as the increased thermal stress from differential expansion.

The desuperheater water injection introduces a particularly severe thermal shock mechanism. The rapid cooling of the pipe wall creates a transient thermal gradient through the wall thickness, with the inner surface cooling rapidly while the outer surface remains hot. This through-wall temperature gradient produces a bending stress in the pipe wall that, when superimposed on the membrane stress from internal pressure, can exceed the yield strength of the material. Repeated cycling of this thermal shock leads to fatigue crack initiation and propagation.

Remedial Measures and Engineering Solutions

The authors proposed a comprehensive set of remedial measures addressing both the root causes and the immediate failure mode:

1. Secondary air reverse-cut modification: Modifying the secondary air register geometry to alter the flame shape and heat release distribution in the furnace, reducing the furnace exit flue gas temperature and bringing it closer to the design value. This is a fundamental solution that addresses the root cause of overheating.
2. Improved feedwater and steam quality: Ensuring that the feedwater and steam meet the quality specifications required to prevent deposit formation on the heat transfer surfaces. Deposits act as thermal insulation, increasing the metal temperature and exacerbating the overheating problem.
3. Coal quality improvement and blending: Implementing coal blending strategies to ensure that the actual coal fed to the furnace more closely matches the design coal specifications in terms of calorific value, ash content, and flame characteristics. This may require sourcing alternative coal or blending multiple coal types.
4. Enhanced spray atomization: Improving the desuperheater nozzle design and operating conditions to achieve finer water droplet atomization. Better atomization leads to more uniform and rapid heat transfer from the steam to the water droplets, reducing the temperature gradient across the pipe wall and minimizing thermal shock.
5. Crack repair and replacement: For the immediately failed elbows, repair by welding or replacement with new elbows fabricated from appropriate material with proper post-weld heat treatment is necessary. The replacement elbows should be inspected for surface quality and residual stress to minimize future cracking risk.

Study Insights and Reflections

This case study is highly instructive for power plant engineers and demonstrates the importance of maintaining coal quality within design specifications and the cascading effects that coal quality deviation can have on boiler component integrity. The thermal fatigue cracking of reheat elbows is a well-recognized failure mode in coal-fired power plants, and this paper provides a clear example of how multiple factors—coal quality deviation, furnace exit temperature elevation, and frequent desuperheater use—can combine to produce a failure mode that would not occur under design conditions.

The systematic diagnostic approach employed by the authors—combining component inspection, coal analysis, and cold-state testing—is a model for failure analysis in power plant components. Each diagnostic method provided complementary information that, taken together, allowed the authors to confidently identify the root cause and propose targeted remedial measures. This multi-method approach should be adopted as standard practice for thermal fatigue failure investigations.

One additional consideration that could be explored is the implementation of online monitoring of reheat steam temperature and metal temperature, with automated alarms and interlocks to prevent operation beyond safe limits. Such monitoring systems, combined with predictive maintenance strategies based on thermal cycling data, could provide early warning of developing thermal fatigue damage and allow planned intervention before catastrophic failure occurs.

In summary, this paper presents a thorough thermal fatigue cracking analysis of reheater header connecting pipe elbows, identifying coal quality deviation, furnace exit temperature elevation, and frequent desuperheater water injection as the root causes, and proposing a comprehensive set of remedial measures including secondary air modification, coal quality improvement, and enhanced spray atomization to restore reliable operation.