Fitness-for-service assessment is a multi-disciplinary engineering approach that is used to determine if the equipment is fit to continue operation for some desired future period. –American Society of Mechanical Engineers
Often times, major equipment in process plant are exposed to corrosive environments and/or elevated temperatures and may possess sustained damage, defects or flaws. These equipment cost exponentially high and thus many company’s want to predict if the equipment is fit to use, if yes, for how long can it be used. In many cases, unnecessary weld repairs can create more issues than solve the problem, FFS assessment determines if these weld repairs are necessary for the equipment. Common degradation mechanisms include corrosion, localized corrosion, pitting and crevice corrosion, hydrogen attack, embrittlement, fatigue, high-temperature creep, and mechanical distortion.
Fitness for service assessment is a multidisciplinary engineering analysis that ensures all process and plant equipment such as pressure vessels, piping, and tanks operate safely and reliably for the desired period of operation and until the next turnaround or planned shutdown occurs in the future with useful remaining life. – TCR Advanced Engineering Pvt. Ltd
There are various standardized methods and design codes that one can use to determine the fitness of an asset. A crucial method is API RP 579-1/ASME FFS-1, Fitness-For-Service. API RP 579-1/ASME FFS-1 provides guidelines regarding the methodology of inspection that a material or piece of equipment is safe to use. These methodologies require huge amounts of data, calculations and inspections to find the actual fitness of an asset.
There are two major methods/design codes to perform Fitness For Service Assessment:
The API 579 relate to ASME code design in terms of allowable stresses and factors of safety. A wide range of defect and damage types typically found during in-service inspection of refinery and petrochemical equipment is covered, with corrosion and locally thinned areas given prominence.
Level 1 – inspectors to use on site for rapid decisions with the less number of data and calculation
Level 2 – qualified engineers and requires simple data and analysis
Level 3 – requires detailed data, computer analysis and considerable technical knowledge and expertise in FFS assessment procedures
BS 7910 is published by British Standards for application to metallic structures across a range of industries and is therefore more general in its approach than API 579.
Level 1 – screening procedure and the most conservative.
Level 2 – material specific and estimates the interaction between fracture and plasticity.
Level 3 – direct calculation of plasticity effects.