There are a variety of conditions an in-service component (e.g. vessel, tank, piping) can be found in. The purpose of Fitness-for-Service (FFS) is to evaluate the integrity of an in-service component given a certain degraded condition and rate it for future service considering potential for any additional degradation. A degraded condition does not have to just be based on corrosion. Sometimes a component can experience a large deformation due to unexpected one-time loads resulting in stresses greater than yield. When such a large deformation event occurs it is important to inspect the vessel to make sure no cracking occurred during the deformation event. If it is found that the impacted area is defect-free then the next step is to determine if the component can operate in the deformed shape.
One example of a non-corrosion related degraded condition is a bulged tank. Figure 1 shows an example of a tank that bulged due to an over-pressure event. This tank was modeled using the FEA program Abaqus. The general bulged shape in the model can be seen in Figure 2. An elastic-plastic analysis was performed with the mesh shown in Figure 3. The global model used shell elements to save on computational time without sacrificing accuracy for the tank shell while the sub-model on the critical nozzle was created using 3D brick elements for maximum fidelity. This analysis was able to demonstrate that the tank was qualified for continued operation - even with an additional corrosion allowance.
Significant schedule and cost savings can be realized if analysis is performed to demonstrate the current and future integrity of components that otherwise would be replaced based on differences from their installed condition.
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Charles Becht V is a mechanical and nuclear senior engineer with expertise in FEA and thermohydraulics modeling and analysis. His experience ranges from developing new designs meeting code requirements, failure investigation and repair, and FFS assessments. In the hydraulic area, his work includes the study of NRC issues related to entrapped gases in liquid lines, for which he developed hydraulics models and analysis of the effects of pump start-up, and two-phase transients. In the stress area, Mr. Becht's work includes the investigation of the failure of one of the largest mobile cranes in the United States, developing the structural model for the crane failure which was benchmarked against observations of the event. In addition Mr. Becht has worked on several rehabilitation projects on furnaces damaged by over pressure events. Responsibilities for the furnace repair have included oversight of field personnel, development of structural reinforcement, and analysis of the completed repairs. Fitness for Service analyses included analyzing both degraded structures and improperly fabricated structures, including buried cooling water pipe in nuclear facilities and large shiploader structures exposed to salt spray and sulfurous coke.
Mr. Becht received his Masters of Science in Nuclear Engineering from the Georgia Institute of Technology and his Bachelors of Science in Mechanical Engineering from Bucknell University.
Very informative and not heavy on technical jargon, which is greatly appreciated. Well done!
Given that 'alterations' need to meet requirements of both the original and current code of construction, justifying the 'new' strain hardened material and irregular cylinder would be an interesting exercise. Is / was this an ASME pressure vessel? In what jurisdiction is the vessel installed?
This pressure vessel was an ASME vessel, specifically Section VIII Division 1 stamped. The bulge occurred during an over-pressure event and therefore is not an alteration but rather a degraded condition. The two primary reference codes for pressure vessel post construction lifecycle are NBIC & API-510, both of these Codes refer to API 579-1/ASME FFS-1 [NBIC under Part 2 - Section 4.4, & API-510 under Section 7.5] for the evaluation of a degraded condition to determine inspection intervals and if the condition is acceptable for continued operation even if the condition does not explicitly meet all of the original Code of Construction requirements. Within the USA I'm not aware of a single state which does not accept a properly performed API 579-1/ASME FFS-1 evaluation.
Any 'degraded condition', if exceeding what original design parameters permit, necessitates supplemental calculations pursuant to an alteration of the original design. Requirements for alterations need to meet mandated safety requirements. From your response, it's understood that this vessel is located in the USA.
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