API 579-1/ASME FFS-1
Fitness-for-service assessment is a multi-disciplinary engineering approach that is used to determine if equipment is fit to continue operation for some desired future period. The equipment may contain flaws, have sustained damage, or have aged so that it cannot be evaluated by use of the original construction codes. API 579-1/ASME FFS-1 is a comprehensive consensus industry recommended practice that can be used to analyze, evaluate, and monitor equipment for continued operation. The main types of equipment covered by this standard are pressure vessels, piping, and tanks. This course is timely, emphasizing the practical application of a recently updated standard.
The American Petroleum Institute prepared API 579 specifically for assessing equipment in the refining and petrochemicals sectors designed to ASME codes. The procedures and supporting data relate to ASME design specifications and materials and are consistent with the design philosophy 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 are covered, with corrosion and locally thinned areas given prominence. Defect and damage types specifically considered include:
- General metal loss
- Local metal loss and gouges
- Pitting corrosion
- Blisters and laminations
- Weld misalignment, dents and shell distortions
- Crack-like flaws
- Creep damage
- Fire damage
- 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.
API 579 has modular organisation based around each defect/damage type. The procedures are largely self contained within each module and derived from recognised authoritative sources. There are extensive annexes containing materials data, design formulae and reference solutions. Each module generally has three levels of assessment.
Level 1 is aimed at inspectors for use on site for quick decisions with the minimum of data and calculation.
Level 2 is intended for qualified engineers and requires simple data and analysis.
Level 3 is an advanced assessment requiring detailed data, computer analysis and considerable technical knowledge and expertise in FFS assessment procedures.
API 579 recognises the need of plant inspectors and engineering personnel on site to be able to undertake a quick initial assessment of defects and damage detected during plant examination. The level 1 procedures are designed for this purpose. Personnel with a broad engineering knowledge and experience can use these procedures with ease, although they may be simplistic and very conservative in some cases.
A more refined FFS assessment can always be made using the level 2 or 3 procedures. The degree of conservatism becomes progressively less as levels increase but this is compensated by the increased knowledge that is available aboutthe equipment, the defect/damage and the margins in hand. Application of level 2 and 3 procedures is usually a more complex process requiring greater specialist knowledge and experience.
Accordingly, API gives guidance for the knowledge and experience of engineers considered competent to undertake FFS assessments to each level. It recognises the need for adequate education and training in FFS assessment so that companies may have confidence in their staff making safe and correct judgements. Whilst qualifications and accreditation of welders, non-destructive testing personnel and plant inspectors have been in existence for some time, there is now perhaps a need to extend these schemes to cover fitness-for service assessment in a more formal way.
The safe use of FFS assessment must depend on having an adequate level of competency, training, information and support necessary to make technical judgements about potentially hazardous equipment. Industry will always like quick simplified procedures that can be used on site without detailed information, analysis and specialist knowledge. Expert systems may be the means to reconcile these aims.