Zhuojin Pipe Fitting Co., LtdThermal Fatigue Crack Analysis of Reheater Header Connection Pipe Elbows
Failure Description and Investigation Approach
This paper, published in Thermal Power Generation (2007, Vol. 36, No. 10, pp. 66–68) by Liu Fasheng, Chen Gang, and Wu Qixing of Huazhong University of Science and Technology, investigates the repeated cracking of elbows in the rear section of a trace-amount spray desuperheater in the reheater system of a power plant boiler (Unit #2). The failure significantly impacted unit availability and required a systematic root cause analysis. The investigation included component inspection, coal quality analysis, and cold-state reheater testing.
Root Cause Analysis
The analysis concluded that the elbow cracking was a typical case of thermal fatigue fracture. The following table summarizes the causal chain and contributing factors.
| Factor | Description | Evidence |
|---|---|---|
| Coal quality deviation | Actual coal type differed from design coal, leading to higher furnace exit flue gas temperature | Coal quality analysis showed higher volatile matter and calorific value than design coal |
| Reheater over-temperature | Elevated flue gas temperature caused overheating of reheater surfaces | Cold-state testing confirmed higher-than-design outlet temperatures |
| Frequent desuperheater operation | The trace-amount spray desuperheater was frequently activated to control steam temperature | Operating logs showed frequent on-off cycling of the desuperheater spray valve |
| Thermal fatigue cracking | Repeated thermal cycling caused crack initiation and propagation at the elbow | Metallographic examination revealed thermal fatigue cracks with characteristic striations |
The mechanism is clear: the deviation in coal quality led to a higher furnace exit flue gas temperature than designed, which in turn caused the reheater outlet steam temperature to exceed the design limit. To compensate, the trace-amount spray desuperheater was frequently activated, subjecting the downstream elbows to repeated thermal cycling. Each cycle of heating and cooling generated thermal stresses that exceeded the fatigue limit of the material, leading to crack initiation at stress concentration sites (typically at the outer bend of the elbow) and subsequent crack propagation.
Remedial Measures and Design Recommendations
The authors proposed a comprehensive set of corrective measures:
- Secondary air reverse-cut modification: Modifying the secondary air damper arrangement to redirect the combustion zone and reduce the furnace exit flue gas temperature.
- Improved water and steam quality: Enhancing feedwater and steam purity to reduce scaling and fouling that can exacerbate overheating.
- Coal quality management: Implementing stricter coal quality control to ensure that the actual coal matches the design coal specification.
- Desuperheater nozzle optimization: Improving the atomization performance of the desuperheater spray nozzles to achieve more uniform cooling and reduce thermal cycling intensity.
- Material upgrade consideration: For future similar designs, consider using higher-grade materials (e.g., 12Cr1MoV or P91) for the reheater header connection elbows to improve thermal fatigue resistance.
| Measure | Implementation Complexity | Expected Effectiveness | Timeline |
|---|---|---|---|
| Secondary air reverse-cut modification | High (requires boiler modification) | High (addresses root cause) | 6–12 months |
| Water and steam quality improvement | Medium (requires treatment system upgrade) | Medium (reduces scaling) | 3–6 months |
| Coal quality control | Low (requires procurement and blending) | Medium (reduces temperature deviation) | Immediate |
| Nozzle atomization improvement | Medium (requires nozzle replacement) | Medium (reduces thermal cycling) | 1–3 months |
| Material upgrade | High (requires fabrication and installation) | High (improves fatigue life) | 12–24 months |
Engineering Practice Insights
This case study highlights the importance of understanding the interaction between fuel quality, boiler design, and component fatigue life. The root cause was not a fabrication defect or material failure; it was a process deviation that cascaded into a mechanical failure. This is a classic example of why root cause analysis must go beyond the immediate failure mechanism to identify the systemic causes. For engineers involved in boiler design and operation, the key lessons are:
- Always design reheater systems with adequate margin for coal quality variations.
- Implement real-time monitoring of reheater outlet temperature with automatic alarm and shutdown setpoints.
- Design desuperheater systems to minimize thermal cycling intensity, using larger diameter spray nozzles and more uniform spray patterns.
- Consider thermal fatigue in the design of reheater header connection components, especially when desuperheater systems are present.
Summary
The cracking of reheater header connection pipe elbows was caused by thermal fatigue resulting from coal quality deviation, reheater over-temperature, and frequent desuperheater operation. The proposed remedial measures address both the root cause (coal quality and boiler design) and the contributing factors (desuperheater design and operation). This case study serves as a valuable reminder that component failures in power plants are often systemic rather than isolated, and that effective root cause analysis requires a holistic view of the process, design, and operational factors.