Fitness For Service
Fitness for Service (FFS) is a critical aspect of engineering consulting, particularly in evaluating the structural integrity and safety of industrial equipment and components. This assessment methodology is crucial for determining whether equipment can continue to operate safely and efficiently despite any defects or damage that may have occurred during its service life.
Static equipment and piping are typically designed for a specific lifespan, often around 25 years. However, during this prolonged use, materials may deteriorate due to various factors such as creep from prolonged high-temperature loading, corrosion and erosion related to process characteristics, and fatigue due to cyclic operations or vibrations.
Additionally, damage may occur due to occasional events such as impacts, overheating, severe wind loads, or human errors. Both the effects of prolonged use and occasional damages can be assessed using the FFS code API 579 / ASME FFS-1, which provides a comprehensive framework for evaluating the condition of in-service components.
FFS assessments are categorized into three levels of increasing complexity and detail. A Level 1 assessment typically employs simple analytical criteria to pass or fail components, providing a conservative evaluation with minimal data requirements. Level 2 assessments are similar but include more detailed analysis of applicable damage, offering a more accurate assessment of the component’s condition.
Both Level 1 and Level 2 assessments require that the damage be located away from structural discontinuities. When these assessments fail to provide conclusive results, or when damage is close to or at structural discontinuities, a Level 3 assessment can be performed. This most advanced level utilizes numerical analysis techniques, such as finite element analysis (FEA), to assess damage in complex geometries subjected to general loading conditions.
Throughout these analyses, critical points in the system are identified, which require priority during inspections. These inspections may be visual or employ non-destructive testing (NDT) methods, and identifying regions of increased concern in detail enhances the likelihood of localizing material deterioration during inspection.
Dynaflow Research Group excels in providing specialized FFS assessments according to API 579 / ASME FFS-1 standards at all assessment levels.
We possess extensive knowledge of API 579 and other related codes, backed by decades of experience in mechanical and thermal analyses using FEA and piping analysis to compute loads on piping and equipment nozzles. Our expertise extends to the design and analysis of pressure vessels, boilers, heat exchangers, and other critical industrial components.
When the design life of equipment has been reached, we can re-assess it to determine its remaining service life, especially when operating conditions have led to creep, fatigue, or corrosion. In cases where prolonged operation is not feasible, we can explore options to de-rate the equipment, ensuring continued safe operation within adjusted parameters.
By leveraging our comprehensive understanding of FFS methodologies and advanced analytical tools, we provide valuable insights into equipment condition, optimize maintenance strategies, extend service life, and prevent catastrophic failures in industrial settings.
FITNESS FOR SERVICE APPLICATIONS
Fitness for Service Project Examples
Local wall corrosion of piping and pipe fittings occurs over the system’s lifetime. The reduced wall thickness of the component can result in the system being deemed unsafe and unfit for service. In these scenarios the corroded component can be replaced, or an API-579 part 5 assessment can be performed. If the assessment is passed, the fitness for service of that component is confirmed and it is possible to extend its lifetime. If not, the component needs to be replaced, repaired, treated, or retired.
There are 3 assessment levels to an API-579 part 5 analysis. The level of assessment depends on the type of component and the proximity of the local metal loss to structural discontinuities. Each level increases the complexity of the calculation. Level 3 assessment requires finite element analysis. When the local metal loss is at a major structural discontinuity and supplemental loads are applicable, special consideration is required. Â The flexibility and plastic collapse capability are significantly affected as well as the stress distribution in that component.
Dynaflow Research Group regularly performs API-579, level 1, 2, and 3 assessments resulting in either extended service lifetime or a recommendation for component replacement.
In the case of local metal loss near a tee intersection an assessment according to API 579 part 5, level 3, is required. The tee with local metal loss is then explicitly modeled in FEA. With this approach, the altered stress distribution and stress levels are accurately captured. The remaining service life is then determined based on the corrosion rate of the component or using the MAWP approach. The thickness approach can only be used if the thickness of the component is characterized by a single thickness. When a thickness profile is required the MAWP approach must be used.
In older piping and equipment dents may be present. This may be caused for example by loading conditions or by accidents during maintenance or inspections. Dents in equipment may cause reduced allowable working pressure and may disqualify the component from its operation. This kind of damage may also raise uncertainty regarding the component’s toughness.
In those cases, an analysis conforming to API 579 part 12 can be used to quantify the damage and determine the component’s fitness for service. The location and size of the dent need to be determined. In the case of level 1 and 2 assessments, only the maximum depth of the dent is required. In a level 3 assessment, more detailed measurements are required to determine the dent profile in axial and circumferential directions.
When the dents are sufficiently far from Major Structural Discontinuities (MSD) and welds, a level 1 assessment of the dent is applicable. When the dent then satisfies the maximum depth criteria, no additional analyses are required. The next step is to verify that the MAWP, based on the thickness corrected with future corrosion allowance and past thickness loss, is equal to or larger than the original MAWP of the component. The assessment is then complete.
When the dents are far from MSD and welds, but are located in a region under cyclic loading, a level 2 assessment is applicable. In addition to criteria similar to level 1, the acceptable number of cycles is determined. This includes the past and future anticipated number of cycles.
When level 1 and 2 assessment criteria are not satisfied, a level 3 assessment can be performed. In a level 3 assessment, numerical analysis is required to verify that the component with the dent is fit for the applicable failure mechanisms, including plastic collapse, local failure, buckling, cyclic loading, or creep and creep fatigue.
