1. Nature and Importance of Thermal Damage in Refinery Equipment: In refineries, metallic equipment is constantly subjected to harsh operating conditions. When this equipment suffers thermal exposure due to fires, uncontrolled temperature excursions, insulation failure, or direct flame impingement, a series of complex changes occur. These changes are far more significant than what is visible to the naked eye. While discoloration, surface scaling, or initial scorching are typically the first indicators, the impact of temperature on metal primarily manifests in the internal layers and the microstructure, where mechanical properties such as strength, toughness, and pressure resistance are compromised. For this reason, identifying thermal damage—from the simplest initial Technical Inspection to the most advanced testing—is of vital importance.

2. Evolution of Surface and Microstructural Changes due to Heat: When metal is exposed to high temperatures, severe oxidation usually occurs at the surface, leading to the formation of oxide scales that gradually detach; this is a clear sign that the metal has exceeded its design temperature limits. However, the primary damage occurs in the internal layers. As temperatures rise, steels undergo changes such as grain growth, carbide transformation, and phase transformations, all of which lead to a reduction in the metal’s mechanical properties. If the equipment has also been under mechanical stress, these processes can lead to phenomena such as creep and the formation of microscopic voids at grain boundaries. Such changes are only observable through precise microscopic examinations, proving that a “healthy appearance” is never a sufficient criterion for judging the integrity of a component.

3. Methods for Detection and Damage Assessment: Post-incident technical inspection begins with a visual assessment; this is the stage where the heat-affected zone is defined and signs such as deformation, buckling, and surface scorching are identified. From there, the main work begins using methods capable of revealing internal damage. Hardness testing is one of the first tools employed, as a reduction in hardness is often an indicator of degraded strength. Subsequently, to examine the microstructure, Replica Metallography is used to analyze the metal’s internal changes without needing to cut or remove the component; these changes serve as an index for the severity and duration of the thermal exposure. If there are concerns regarding surface or sub-surface cracks, non-destructive testing (NDT) methods such as Dye Penetrant (PT), Magnetic Particle (MT), or Ultrasonic Testing (UT) are utilized to detect cracks invisible to the naked eye. These cracks, especially those caused by thermal shock, can pose serious risks during future operations.

4. Engineering Conclusion and Fitness-For-Service (FFS) Assessment: After gathering technical inspection data, the stage of engineering analysis and judgment begins. At this point, engineers compare the current state of the equipment with original design data, codes, and historical criteria to decide whether the equipment is still fit for safe service. Sometimes the damage is limited, and the equipment can be returned to service with corrective actions or derating (reduced pressure/temperature). However, if investigations reveal deep-seated metallurgical changes or extensive creep cracking, continued operation can be hazardous. In such cases, replacement or decommissioning is the only safe option. The importance of this stage lies in the fact that many thermal damage effects appear late, and if the engineering decision is not made with sufficient precision, there is a risk of sudden, catastrophic failure in the future.

5. Conclusion: The Importance of Thermal technical Inspection in Refinery Units: The necessity of inspecting heat-affected equipment stems from the fact that thermal effects are not always visible, and many dangerous changes occur deep within the metal structure. The combination of visual inspection, non-destructive testing, metallurgical methods, and engineering analysis constitutes a comprehensive toolkit that ensures equipment is returned to service safely. In such a process, the role of experienced and specialized inspectors is critical, as the final decision regarding the fitness-for-service is based on a synthesis of empirical data, material behavior knowledge, and international standards.

Author: Zahra Shirband – International Relations Expert ISQI

Sources:

1. API 579-1/ASME FFS-1, Fitness-For-Service
2. API 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry
3. API 510, Pressure Vessel Inspection Code
4. API 570, Piping Inspection Code
5. ASME Boiler and Pressure Vessel Code, Section VIII
6. ASME PCC-2, Repair of Pressure Equipment and Piping