What to Expect With a Fitness-For-Service Evaluation
Inspections of a pressure vessel, tank, or related equipment should be performed on a regular basis to ensure their continued operation is safe. But what happens when the inspection uncovers damage such that its continued safe operation is suspect? When the cost of repairing or replacing the vessel or equipment is not trivial a fitness-for-service evaluation may be performed. But what is a fitness-for-service evaluation and what can you expect if you have one performed?
What is a Fitness-for-Service Evaluation?
A fitness-for-service evaluation is an inspection and engineering assessment done in accordance with the standard API 579-1/ASME FFS-1, “Fitness-for-Service”. API 579-1/ASME FFS-1 provides consensus methods for the reliable assessment of the structural integrity of equipment containing flaws or damage. The guidelines provided in API 579-1/ASME FFS-1 can be used to make decisions on whether or not the equipment can be used as-is, requires repair, or should be replaced. Fitness-for-service evaluations performed in accordance with this Standard are recognized and referenced by API 510 (“Pressure Vessel Inspection Code”), API 570 (“Piping Inspection Code”), API 653 (“Tank Inspection, Repair, Alteration, and Reconstruction”), and by NB-23 (“National Board Inspection Code”) as suitable for evaluating the structural integrity of pressure vessels and related equipment.
Types of Flaws and Damage
API 579-1/ASME FFS-1 is divided into several parts describing how to assess various flaws or damage. These are:
Brittle fracture
General metal loss
Local metal loss
Pitting corrosion
Hydrogen blisters and hydrogen damage
Weld misalignment and shell distortions
Crack-like flaws
Operation in creep range
Fire damage
Dents, gouges, and dent-gouge combinations
Laminations
Assessment Levels
Three levels of assessment are provided in each of these parts. A Level 1 Assessment can be performed when there is a minimum amount of information on the flaw or damage. A Level 1 Assessment is the easiest to perform but is also the most conservative. If the assessment does not provide acceptable results, a more rigorous evaluation can be performed, proceeding sequentially to a Level 2 or Level 3 Assessment.
A Level 1 Assessment is the most conservative and can be performed by an inspector or an engineer. It also requires the least amount of information. Level 2 and Level 3 Assessments are to be performed by an engineer experienced and knowledgeable in performing FFS evaluations. With a Level 2 Assessment more information or data will be required and calculations typically performed. A Level 3 Assessment is the most detailed assessment and usually requires a rigorous analysis method such as finite element analysis (FEA).
Assessment Procedure
Each of the parts in API 579-1/FFS-1 give detailed information on how to perform a Level 1, Level 2, and Level 3 Assessment. Discussion of each of these parts and their specific requirements and procedures are beyond the scope of this article. However, the procedures for performing assessments for the different types of flaws and damage are similar. The general procedure is as follows:
Identification of flaw and damage mechanism
Determination of applicability and limitations of FFS assessment procedures
Gathering of information
Assessment technique and acceptance criteria
Remaining life assessment
Remediation
In-service monitoring
Documentation
Identification of Flaw Type and Damage Mechanism
The first step in an FFS evaluation is to identify the flaw type and cause of damage. A likely cause of damage may be determined through an examination of the original design data, fabrication procedures, maintenance history, environmental conditions, etc.
Applicability and Limitations
Once the flaw type is identified, the applicability and limitations of the appropriate Part of API 579-1/ASME FFS-1 will be reviewed to decide on whether to proceed with the assessment.
Gathering of Information
The next step consists of gathering information. This includes taking measurements to quantify the flaw or damage, reviewing equipment design data, material specifications, maintenance records, operational history, and expected future operating conditions.
Assessment Technique and Acceptance Criteria
After the pertinent information and data has been obtained, the required technique or procedure for conducting the FFS assessment and its acceptance criteria is identified. The details for each of these is provided in the applicable Part of API 579-1/ASME FFS-1.
Calculations, Remaining Life Assessment, and Remediation
At this point, the required calculations, analyses, or assessments are made and a remaining life determined. Based on the results of the assessments, an appropriate remediation will be determined. This could be a determination that the equipment can be continued to be operated in its current condition, or may require repairing or replacing the equipment.
In-Service Monitoring
In-service monitoring may be recommended or required where a remaining life and inspection interval cannot be adequately established, or when the remaining life is short. API 579-1/ASME FFS-1 provides methods for in-service monitoring based on the type of flaw or damage mechanism.
Documentation
The last part of conducting an FFS evaluation is providing documentation, typically a report providing the details of the assessment and remediation. Requirements on what information is to be documented are provided in the appropriate Part of API 579-1/ASME FFS-1.
In conclusion, a fitness-for-service assessment performed in accordance with API 579-1/ASME FFS-1 can be valuable in evaluating the structural integrity of flawed or damaged equipment in-service. The results of the assessment will provide information necessary to make informed repair-replace decisions.