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Fitness-for-Service Assessment?
Fitness-for-service (FFS) assessment is a multi-disciplinary approach to determine, as thebname suggests, whether equipment is fit for continued service. The equipment or system in question may contains flaws or other damage, or may be subjected to more severe operating conditions than anticipated by the original design. The outcome of a fitnessfor- service assessment is a decision to run as is, repair, re-rate, alter, or retire the equipment. A remaining life analysis may also be performed as part of the assessment, which can be used to set future inspection intervals and to budget for capital expenditures when existing equipment is to be retired.
A typical FFS assessment may involve several engineering disciplines, and it requires collecting data from a number of sources. Although one person may take a lead role in performing the assessment, he/she must rely on others to provide crucial data and expertise. Some of the areas of expertise that may be part of an FFS assessment are outlined below.
A typical FFS assessment may involve several engineering disciplines, and it requires collecting data from a number of sources. Although one person may take a lead role in performing the assessment, he/she must rely on others to provide crucial data and expertise. Some of the areas of expertise that may be part of an FFS assessment are outlined below.
- Stress Analysis. An accurate estimate of stresses acting on the component of interest is e to assessing structural integrity and remaining life.
- Metallurgy/Materials Engineering. An understanding of the performance of various materials subject to specific environments, temperatures, and stress levels is essential for ensuring safe and reliable operation.
- Nondestructive Examination (NDE). Flaws must be detected and sized before they can be assessed. The most suitable inspection technology depends on a variety of factors, including type of the flaws or damage present and the accessibility of the region of interest.
- Corrosion. An understanding of environmental degradation mechanism(s) that led to the observed damage is a prerequisite for FFS assessments. Moreover expertise in corrosion is useful for prescribing suitable remediation measures.
- Plant Operations. Interaction with plant personnel is usually necessary to understand the operating parameters for the equipment of interest. Information such as operating temperature & pressure, process environment, and startup/shutdown procedures are key inputs to a FFS assessment.
- Fracture Mechanics. This discipline is used to analyze cracks and other planar flaws.
- Probability and Statistics. This discipline is useful for data analysis and for probabilistic risk assessments.
Fitness-for-service assessments can range in complexity from simple screening evaluations to highly sophisticated computer simulations, including finite element analysis (FEA) and computational fluid dynamics (CFD). The necessary level of complexity varies from one situation to the next. In some cases, an advanced analysis is performed when a simple screening assessment is unable to demonstrate that the equipment in question is fit for continued service. Standardized FFS procedures typically include a range of assessment options that cover the full spectrum of complexity. The leading fitness-for-service standard for pressure equipment (pressure vessels, storage tanks and piping) has been published by American Petroleum Institute (API) and the American Society for Mechanical Engineers (ASME). The original version of this method, API 579, was published in 2000 by API. Both organizations collaborated in the creation of the revised edition, API 579-1/ASME FFS-1, which was published in 2007. The original API 579 document pertained primarily to refinery equipment, although the procedure was widely used outside of the petroleum industry. With the addition of the ASME brand name in the new version, there is an explicit recognition that this standard is suitable to a broad range of industries that rely on pressure equipment, including electric power, chemical, pipeline, and pulp & paper. The contents of the standard have been updated to reflect technological advances and the broader industry coverage.
Advantages of Fitness-for-Service Assessment
It goes without saying that safety is an important goal of any ethical company. Although there is an inherent risk in processing, transporting, and storing liquids & gases under pressure, it is important to reduce this risk to minimal levels. Design codes and standards are intended to ensure reliable operation of newly-constructed vessels, tanks and piping.
Fitness-for-service standards such as API 579-1/ASME FFS-1, can be used to assess whether or not it is safe to operate aging equipment that may have degraded in service. While improved safety is an obvious benefit of fitness-for-service assessment, there are substantial economic benefits to this technology that may be less apparent. For example, unplanned shutdowns are extremely costly in terms of lost production. Fitness-forservice assessments performed on key assets during a scheduled shutdown can greatly reduce the likelihood of unplanned outages. When flaws or other damage are detected, the decisions on how to deal with such imperfections have enormous economic implications. If flaws are discovered during normal operation, a fitness-for-service assessment can determine whether or not it is safe to operate the equipment until the next planned outage. If the outcome of the FFS assessment is favorable in such a case, then the operator can avoid a costly unplanned shutdown. Even during an outage, whether planned or not, it is desirable to avoid or postpone repairs, provided the FFS assessment indicates that the equipment can be safely operated until the next planned shutdown. Unscheduled retirement of components can be particularly costly, as long lead times for delivery of replacement components can result in extensive delays in production. Fitness-for-service assessments provide a rational basis to determine whether or not a damaged component can continue to operate until a replacement can be delivered. A lesser-known but significant economic benefit of FFS technology is that it can lead to improved yields. If the rate of life consumption of equipment can be accurately quantified through FFS assessment, a plant can be run more aggressively between shutdowns. Even if components are replaced more frequently due to accelerated life consumption, the increased output may generate significantly larger net profits for the
plant.
Fitness-for-service standards such as API 579-1/ASME FFS-1, can be used to assess whether or not it is safe to operate aging equipment that may have degraded in service. While improved safety is an obvious benefit of fitness-for-service assessment, there are substantial economic benefits to this technology that may be less apparent. For example, unplanned shutdowns are extremely costly in terms of lost production. Fitness-forservice assessments performed on key assets during a scheduled shutdown can greatly reduce the likelihood of unplanned outages. When flaws or other damage are detected, the decisions on how to deal with such imperfections have enormous economic implications. If flaws are discovered during normal operation, a fitness-for-service assessment can determine whether or not it is safe to operate the equipment until the next planned outage. If the outcome of the FFS assessment is favorable in such a case, then the operator can avoid a costly unplanned shutdown. Even during an outage, whether planned or not, it is desirable to avoid or postpone repairs, provided the FFS assessment indicates that the equipment can be safely operated until the next planned shutdown. Unscheduled retirement of components can be particularly costly, as long lead times for delivery of replacement components can result in extensive delays in production. Fitness-for-service assessments provide a rational basis to determine whether or not a damaged component can continue to operate until a replacement can be delivered. A lesser-known but significant economic benefit of FFS technology is that it can lead to improved yields. If the rate of life consumption of equipment can be accurately quantified through FFS assessment, a plant can be run more aggressively between shutdowns. Even if components are replaced more frequently due to accelerated life consumption, the increased output may generate significantly larger net profits for the
plant.
Levels of Assessment
The API/ASME fitness-for-service standard provides three levels of assessment:
- Level 1. This is a basic assessment that can be performed by properly trained inspectors or plant engineers. A Level 1 assessment may involve simple hand calculations.
- Level 2. This assessment level is more complex than Level 1, and should be performed only by engineers trained in the API/ASME FFS standard. Most Level 2 calculations can be performed with a spreadsheet.
- Level 3. This is the most advanced assessment level, which should be performed only by engineers with a high level of expertise and experience. A Level 3 assessment may include computer simulation, such as finite element analysis (FEA) or computational fluid dynamics (CFD).
These three assessment levels represent a trade-off between simplicity and accuracy. The simplified assessment procedures are necessarily more conservative than more sophisticated engineering analyses. In some cases, the component being evaluated may fail a Level 1 assessment but pass a Level 2 or Level 3 component because of the conservative simplifying assumptions in the former. In certain situations, the API/ASME standard does not permit a Level 1 assessment. For example, Level 1 assessments are not applicable to pressure equipment subject to significant supplemental loads, such as dead loads, wind loads, thermal expansion loads, and seismic loads. With Level 1 assessments, the specified procedures must be followed exactly, and there is little or no room for interpretation. Level 2 procedures provide some latitude to exercise sound engineering judgment. For Level 3 assessments, the API/ASME standard provides a few overall guidelines, but the details of the assessment are left to the user. The lack of specificity in Level 3 is by design. There is no practical way to codify stepby- step procedures for advanced engineering analyses because every situation is different, and there a wide range of approaches that may be suitable for a given situation. As one might expect, the cost of a fitness-for-service assessment tends to increase with complexity. Sophisticated computer modeling that is performed as part of a Level 3 assessment is obviously more expensive than a simple hand calculation. Moreover, Level 1 assessments may have less onerous inspection requirements than higher-level evaluations. When compared with the potential savings, however, the cost of an assessment, even at Level 3, is often insignificant. If a complex engineering analysis allows a plant to avoid a catastrophic failure or an unplanned shutdown, then it is certainly a good investment.
Application of Fitness-for-Service Technology
Pipeline Corrosion
Pipelines experience various degradation mechanisms, including corrosion, cracking, dents, and gouges. The API/ASME fitness-for-service assessment can be applied to each of these damage mechanisms. Most pipelines are examined periodically with in-line inspection (ILI). A variety of ILI inspection technologies are commercially available. The most suitable technology depends on the contents of the pipeline and the type of damage one wishes to detect. ILI technology that measures wall thickness is similar to FTIS, but on a different scale in terms of pipe diameter and length of pipe to be inspected. Quest Reliability is currently developing software called LifeQuest PipeTM, which has been adapted from the fired heater software described above. Figure 6 is a color plot of thickness data obtained from a pipeline inspection. The LifeQuest Pipe software analyzes these data with a Level 2 assessment of local metal loss. The remaining strength factor (RSF) is computed for short segments of pipe, usually on the order of 1 m in length. The RSF values are used to rank individual pipe segments for corrosion damage. The segments with the lowest RSF values correspond to the most highly corroded areas. Using proprietary corrosion rate models, the LifeQuest Pipe software can also estimate the remaining life for each segment.
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