Modern metal pressure vessels are designed and constructed according to ASME Boiler and Pressure Vessel code to obtain safe operation through predictable material properties, specified stress limits and fabrication quality requirements. However, once the pressure vessel is stamped to signify it conforms to the code rules, the equipment is no longer covered by ASME Boiler and Pressure Vessel codes.
Pressure vessels in service are often exposed to corrosive environments and/or elevated temperatures. Under these conditions, the material used in the vessel can degrade or age with time in service. As the pressure vessel becomes older, the plant operator must decide if it can continue to operate safely and reliably to avoid injuries to personnel and the public, environmental damage, and unexpected shutdowns. API 579 Fitness-for-Service (FFS) assessment procedures provide a means for helping the plant operator make these decisions based on sound, established engineering principles
Common reasons for assessing the FFS of equipment include:
- Maintaining the safety of plant personnel and the public.
- Complying with OSHA 1910 process safety management (PSM) rules.
- Protecting the environment for accidental release of damaging substances.
- Reliably operating aging facilities.
- Maintaining safe and reliable operations with increased run lengths and decreased shutdown periods.
- Determining the feasibility of increasing the severity of operations.
- Rationalizing the damage found by more rigorous in-service inspections than found by inspections performed during original construction.
What is FFS for?
The procedures given in FFS are aimed at equipment operating in the petroleum and chemical industry. They apply to the following items:
- Components designed and constructed to the ASME Boiler and Pressure Vessel Code, Section I, and Section VIII, Divisions 1 and 2.
- Piping designed to ASME B31.1 and 31.3 piping codes.
- Storage tanks designed and constructed to API 620 and 650.
What do we expect as a result of FFS?
FFS addresses flaws commonly found in pressure vessels, piping, and storage tanks, and both present integrity and remaining life of components.
The main products of FFS are:
- A decision to run, alter, repair, monitor, or replace the equipment, and
- Guidance on inspection interval for the equipment.
What is FFS?
FFS is a multi-disciplinary engineering analysis that helps to determine if the dimensions of damage in the pressure vessel component exceed the limits for safe operation, and allows the owner to decide whether to continue operating the vessel, to monitor the damage, to repair the damage or retire the vessel. Guidance is also provided for determining when next and how to inspect the damage. The analysis for FFS assessment is based on the following information:
- Knowledge of damage mechanisms and material behavior
- Access to operating condition data; understanding parameters controlling damage mechanisms
- NDT (flaw sizing and location)
- Material properties (environmental effects)
- Stress analysis (often finite element analysis)
- Data analysis (engineering reliability models)
FFS assessment requires both knowledge of past operating conditions and a forecast of future operating conditions. Interactions with operations personnel is required to obtain these data.
The owner-user has overall responsibility for the assessment procedures. The inspector has to make sure that requirements of the applicable API inspection codes are satisfied and provide inspection data. The owner-user must show that the flaw or damage will not reach unacceptable dimensional limits before the specified next service period for the component under the specified service conditions.
History of FFS
National Board of Boilers and Pressure Vessel Inspectors was created in 1919 to promote greater safety to life and property through uniformity in construction, installation, repair, maintenance, and inspection of pressure equipment. For many years, NB-23 provided rules and guidance for inspectors trained and certified by the National Board to carry out in-service inspections and approve repair and alteration of pressure vessels and pressure relief equipment. However, in last 35 years, increased scientific analyses and engineering experience have confirmed that pressurized equipment constructed to the Boiler and Pressure Vessel rules can safely tolerate substantial damage, depending on the local conditions. Additionally, substantial experience has been collected through industry standards aimed at safe operation of pressurized vessels and tanks, including API 510 Pressure Vessel Inspection Code, API 570 Piping Inspection Code, and API 653 Tank Inspection, Repair, Alteration, and Reconstruction.
These developments have been the springboard for documents for assessing the limits of in-service damage. One such document is API 579/ ASME FFS Fitness for Service (FFS) that describes the procedures and rules for assessing if equipment with damage can be safely operated for a specific duration, or to prescribe the limits of tolerable damage for a particular piece of equipment under its operating conditions.
First published in 2000, FFS represented a major advance in standardizing the methods for assessing the structural significance of cracks, mechanical damage, corrosion and fire or overheating damage in pressure equipment. FFS is endorsed and accepted by the National Board which recognizes it as a powerful engineering tool for reconciling the often conflicting interests of owners who want to safely operate equipment with some corrosion or other damage, and inspectors who want the equipment removed from service because of the same damage symptoms.
Structure of API 579
The Code is organized in 13 Parts and 10 Annexes:
Part 1: Introduction
Part 2: Fitness-for Service Engineering Assessment Procedure
Part 3: Assessment of Existing Equipment for Brittle Fracture
Part 4: Assessment of General Metal Loss
Part 5: Assessment of Local Metal Loss
Part 6: Assessment of Pitting Corrosion
Part 7: Assessment of Hydrogen Blisters and Hydrogen Damage HIC and SOHIC
Part 8: Assessment of Weld Misalignment and Shell Distortions
Part 9: Assessment of Crack-like Flaws
Part 10: Assessment of Components Operating in Creep Regime
Part 11: Assessment of Fire Damage
Part 12: Assessment of Dent, Gouges, and Dent-Gouge Combinations
Part 13: Assessment of Laminations
Annex A – has thickness, MAWP and membrane stress equations.
Annex B – is an overview of how to analyze stresses.
Annex C – is a collection of stress intensity factor solutions.
Annex D – is a compendium of reference stress solutions.
Annex E – covers residual stresses mostly from welding.
Annex F – shows how to incorporate material properties in the assessment.
Annex G – discusses damage/ deterioration and failure modes.
Annex H – emphasizes the importance of validating assessment results.
Annex I – is a glossary of terms and definitions.
Annex J – is presently blank.
Annex K – reviews concepts related to crack opening areas.
API 579 is largely self-contained so users don’t have to refer to many other documents. One exception to this principle is that materials data need to be obtained from Section II of the ASME Boiler and Pressure Vessel Code.
General Evaluation Procedure
FFS assessment consists of the following procedural steps:
Step 1: Identify the type of flaw and the material damage mechanism.
Step 2: Determine the applicability and limitations of the FFS assessment procedures.
Step 3: Define the data requirements. The data needed for all assessments are – original equipment design data, maintenance and operational history, required data/ measurements for FFS assessment, and recommendations for inspection techniques and sizing requirements.
Step 4: Apply the assessment techniques and acceptance criteria.
Step 5: Evaluate the remaining life or the limiting flaw size and establish an inspection interval based on results of the evaluation.
Step 6: Apply the required remediation methods to the extent possible and practical to slow the flaw growth rate.
Step 7: Employ in-service monitoring procedures when remaining life and inspection intervals cannot be adequately established.
Step 8: Document all information used and decisions made in Steps 1 through 7, and store the documentation with inspection records.
These eight steps should be included in each FFS assessment of a specific flaw and component combination.
Three Assessment Levels
FFS has three assessment levels – each higher level assessment uses more detailed stress information, more accurate characterization of the type of damage, and more specific materials’ property data to generate increasingly less conservative assessment results.
Level 1 – Most conservative assessment (rationale similar to NBIC 23)
- Basic damage sizing (inspection) and component information
- Can be done by a trained “FFS inspector”
Level 2 – More detailed assessment produces less conservative results; Level 1 requirements plus:
- Accurate damage sizing and growth rate
- More comprehensive knowledge of damage mechanism rate controlling parameters
- Qualified engineering skill in FFS methods and procedures
Level 3 – Most detailed and least conservative results; Level 2 requirements plus:
- Highest detail in damage sizing; materials and component service data
- Local material properties
- Numerical stress and temperature analysis techniques, including finite element methods
For practitioners, Level 1 FFS inspectors should have relevant FFS knowledge and training and may have API 510, 570 or 653 certification. FFS engineers are expected to have an engineering degree and at least two years of relevant mechanical or materials engineering training and experience.
Assessment methods generally use one or more of the three fundamental acceptance criteria: allowable stress, remaining strength factor (RSF), and failure assessment diagram (FAD). Majority of FFS is based on RSF and FAD acceptance criteria. RSF is the ratio of the limit of plastic collapse load for a damaged component to that of an identical, undamaged component. FAD evaluates whether a crack like flaw of known shape and size acted on by the postulated stress conditions will fail either due to unstable crack growth (brittle fracture) depending on the metal toughness, or due to exceeding the limit load for plastic collapse (ductile fracture), on the y and x axes of the FAD, respectively.
API 579 FFS describes methods and procedures intended to supplement the requirements in API 510, API 570, API 653, and NB-23 to provide post-construction code rules for assessing the FFS of equipment designed and constructed to recognized codes and standards. The standard has broad application since the assessment procedures are based on allowable stress methods and plastic collapse loads for non-crack like flaws, and the Failure Assessment Diagram (FAD) approach for crack-like flaws. FFS assessment procedures in the standard can be used to evaluate corrosion and other service-related damage encountered in pressure vessels, piping and storage tanks.
Extracted and edited from “API 579-1/ASME FFS-1 Fitness for Service – Post Construction Code for Pressure Equipment with Flaws and Corrosion Damage” by David Bennett and Nathan Mixon; and “Process Equipment Fitnes-for-Service Assessments Using API RP 579” by Carl Jaske.
This article appeared in a May 2016 issue of the Pressure Vessel Newsletter, a monthly publication. For more pressure vessel related articles from the previous issues of the newsletter, please contact Ramesh Tiwari at [email protected]
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