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.
Continued use of a vessel not of original design may need to be accepted by the Insurer that may then pose limitations on insurance coverage. Depending on the situational use of vessel the Insurer may require a bond-rider to be posted by the Engineering Co. that performed the FFS showing vessel is safe for continued operation. Canada and USA are very different when it comes to lawsuits when vessels blow-up and kill people.
You wrote " CSA B51 does not impose any additional design requirements for Div 1 construction, it simply defines what needs to be registered and what is considered a pressure vessel versus not. "
We disagree, and I don't want to belabor this, but CSA B51 does impose additional design requirements further to what ASME requires.
The line between where 'degradation' ends and alteration begins seems undefined and it seems that mandated safety requirements differ depending on location. Requirements in the USA are markedly different and less defined than those in Canada. 'Alteration' is defined in CSA B51, and 'degraded condition' is not.
Yes, any degraded condition that goes beyond the tolerances of the original Code of Construction must be demonstrated to meet certain safety standards. That is the basis of the creation of API 579-1/ASME FFS-1. This FFS standard is accepted in Canada, I have personally been involved in submittals of FFS reports to regulatory agencies in three different provinces, all of which were accepted. Both the USA & Canada have a common approach to pressure vessel safety. Although Canadian regulatory agencies do a far more thorough review of the FFS submittals the basic approach is the same.
With respect to the pressure vessel mentioned in this article, protection against gross plastic collapse, local strain limit failure, and ratcheting were all demonstrated per the criteria found in API 579-1/ASME FFS-1 with margins based on it being a Div 1 vessel. These demonstrate safe operation even if the bulge is not explicitly compliant with the original code of construction. A purposeful alteration on the other hand would require more explicit compliance with all the Original Code of Construction rules, regardless of if advanced analysis demonstrated sufficient design margin.
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