This case study examines the structural integrity of an industrial pressure vessel subjected to cyclic thermal and pressure loading. In processes where such vessels experience significant temperature and pressure fluctuations, ensuring the longevity and safety of these critical components is paramount. The primary objective of this study is to assess the vessel’s ability to withstand repeated thermal and pressure cycles over its intended operational lifespan while maintaining compliance with relevant industry standards.
The pressure vessel operates in a demanding environment, cycling between 50°F (10°C) and 320°F (160°C), with pressures ranging from 0 psig (0 barg) to 101.5 psig (7 barg). Additionally, hot steam at 392°F (200°C) enters the inlet nozzle N4.1, causing uneven thermal loading. This cyclic loading poses potential risks to the structural integrity of the vessel. This case study aims to provide a comprehensive analysis of the thermal stresses and fatigue effects on the system, ultimately determining if the current design meets the required safety standards or if modifications are necessary.
Thermal and Pressure Cycles in Industrial Applications
The specific technical challenge addressed in this case study is the assessment of the pressure vessel’s structural integrity under the following conditions:
- Cyclic thermal loading from 50°F (10°C) to 320°F (160°C).
- Cyclic pressure loading from 0 psig (0 barg) to 101.5 psig (7 barg).
- A total of 90,000 thermal and pressure cycles over the operational lifespan.
- Uneven thermal loading due to high-temperature steam entering an inlet nozzle.
The aim is to comply with ASME VIII Division 2, Part 5.5 guidelines for protection against cyclic loading failure. The analysis must demonstrate that the vessel can withstand the specified operational conditions without failure due to fatigue.
A Systematic Approach to Fatigue Analysis Using Finite Element Techniques
To analyze the problem and assess the pressure vessel’s structural integrity, the following methodology was employed:
- A comprehensive FEA was conducted on the entire vessel, including nozzles and non-structural attachments.
- The analysis focused on thermal transient and static stress evaluations to assess thermal stresses and fatigue resistance.
- Stress Analysis:
- Stress distributions were computed for the entire thermal and pressure cycle.
- Peak stress locations and magnitudes were identified.
- Stress ranges were calculated and compared against allowable limits.
- Fatigue Assessment:
  – ASME VIII Division 2, Part 5.5 guidelines were followed for fatigue analysis.
  – Fatigue Strength Reduction Factors (FSRF) were determined for critical locations.
Measures for Fatigue Resistance in Critical Components
Based on the comprehensive analysis of the industrial pressure vessel, the proposed solution focuses on ensuring the structural integrity of a critical weld on the inlet nozzle. The recommendation is to implement a full-penetration weld with a machined surface condition, corresponding to the highest weld quality level as specified in relevant industry standards such as ASME VIII Division 2.
This high-quality weld is crucial for achieving a fatigue strength reduction factor (FSRF) of 1, which is necessary to withstand the demanding cyclic loading conditions over the vessel’s design life of 90,000 cycles. The highest calculated stress range at the fatigue-susceptible location is approximately 127.5 MPa, which is deemed acceptable with the proposed weld quality.
By implementing this solution, the vessel is expected to maintain its structural integrity throughout its operational lifespan, effectively mitigating the risk of fatigue failure. This approach aligns with industry standards and best practices for pressure vessel design and fabrication, ensuring both safety and reliability in high-stress industrial applications where cyclic thermal and pressure loading are present.
Ensuring Long-Term Safety and Reliability in Vessel Design
This case study of the industrial pressure vessel demonstrates the critical importance of thorough fatigue analysis in high-temperature, high-pressure cyclic applications. Key findings and outcomes include:
- The current vessel design, with the proposed weld quality requirement, successfully meets ASME VIII Division 2 standards for fatigue resistance under the specified operational conditions.
- The analysis highlights the importance of considering both thermal and pressure cycling in assessing the long-term integrity of pressure vessels.
- The study underscores the value of advanced FEA techniques in validating designs for complex loading scenarios and ensuring compliance with industry standards.
By confirming the structural integrity of the pressure vessel under these demanding conditions, this case study provides confidence in its long-term performance and safety.Â
