Finite Element Analysis in PVP Design: What You Should Know
In pressure vessel and piping (PVP) design, various analysis methods are used to assess the safety and performance of systems. While hand...
What was earlier a rudimentary approach, characterized by a "build it and run it" perspective for ensuring the structural integrity of an asset, has transitioned towards a more precise analysis with more loading considerations—an outcome of Fitness-For-Service (FFS) advancements. Now, at a time when the development of pressure vessel equipment is increasingly complex, FFS evaluations have become pivotal in further safeguarding the equipment's life. Now, ensuring longevity against many factors is a process that works beyond mere code compliance.
The primary objective of FFS is to thoroughly assess flaws in pressurized or stressed components to evaluate how well they'll hold up over time. It's a more conservative process by nature, but the depth of the analysis can vary.
Additionally, adopting the API 579 standard has had a notable impact on the petroleum refining sector. It introduced specific guidelines for handling deteriorating equipment and often allows continued use without the need for repair, replacement, or a reduction in pressure rating.
As professionals continue to accept and practice the API 579 standard, using advanced tools to easily run FFS assessments has become more standardized. In this blog, we provide further insight into the growing applications of API 579 and some modern FFS solutions that expedite a system's stress analysis without jeopardizing compliance. Let's dive in.
comprehensive fFS for Industrial Damage
Various damage types like loss of thickness areas (LTAs), cracks, creep damage, and dents can be evaluated through Fitness-For-Service (FFS). FFS can assess components in service or recently in service, considering factors like crack growth and consequences of failure. In-depth analyses are categorized into three levels, with Level 1 being more conservative and quicker, Level 2 involving more complex computations, and Level 3 requiring detailed data and operational experience.
Levels of Depth |
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| Level 1 | They involve minimal data and experience, relying on tables and screening curves, which makes them quicker and more conservative than other levels. |
| Level 2 | Level 2 assessments, which involve more complex computations than Level 1 assessments, can lead to favorable outcomes and are often only implemented with specialized FFS software. |
| Level 3 | Require detailed data and substantial operational experience. Level 3 is rarely performed due to advanced equipment's complexity and potential for error. |
The need for deeper analyses of specific components and scenarios has increased as the industry shifts. At PRG, we align our solutions to reflect those needs.
Considerations for FFS Under Complex Loads
Under certain conditions or loading scenarios, designers and engineers must be cautious of a few intricates when performing FFS assessments:
- Local thin areas and pressure boundaries are generally easier to assess than crack-like flaws.
- Ductile materials can manage small cracks above 150°F, as these cracks resist rapid grain boundary attacks and are controlled by localized failures.
- Classifying smaller cracks and thin areas in pressurized environments is easier than in larger ones.
- Cracks and thin areas less than halfway through the wall are safer to classify than those deeper than 0.5T.
- Classifying cracks in thin geometries (less than 0.25 to 3/8”) is challenging due to the significant portion of thickness involved, often requiring a leak-before-break evaluation.
- Classifying thin areas is easier when the thinnest spots exceed 0.25” in diameter. For very localized thinning (less than 0.25”), consult API-579/FFS-1.
- Thick geometries with cracks have low resistance to propagation and are more brittle.
- Deep cracks in welds should be assessed for various factors like voids, penetration, residual stress, and crack depth. If critical data is missing, consider reducing KIC by 50% and JIC by 25%.
FFS, API 579 FAQs
Since the 1990s, we've encountered several questions and inquiries against the shifting standards and analysis types that determine cracks, dents, and overall LTAs for equipment. Here are some of the most common ones, answered by our experts at PRG:
What are the two failure modes for cracked geometries?
- Crack extends and propagates
- Geometry collapses
What is a leak before break criteria?
This refers to when the analysts assume that there is a through-wall crack equal to the part-through-wall crack length. When a thorough analysis of a through-wall crack shows no growth and the system remains intact without collapsing, it indicates that the system will leak before rupturing.
How can I get an accurate allowable for J or K?
Known as JIC and KIC, these are typically associated with a partial safety factor or are provided as lower bounds. Sometimes, when provided as a lower bound, it is considered that the safety factor is “implied.” API 579 /FFS-1 provides allowable values for base metal as a function of temperature from various sources. Any external data used should support values in FFS-1. For carbon steels, the user should approach it cautiously as the KIC values can drop rapidly as the temperature falls. JIC and KIC are not linearly proportional; JIC changes to the square of KIC changes. SENT or SENB testing can be performed to extract values for use when material samples are available.
While performing a non-linear analysis of the loading, the analyst can observe the crack tip driving force increase as the load increases. J values along the crack edge are printed for each converged solution.
What is a K (or J) threshold?
According to 9F.5.3 (d), the threshold crack growth K range is 1.8 ksi sqr. in. This is the J or K value, below which the crack growth per cycle is considered zero, ensuring that the system does not exceed the material's endurance limit.
Can J and K be used for fatigue prediction?
Generally, yes. For a given crack geometry, da/dN can be determined from K or J, leading to the identification of the local, temporal crack life.
When should I involve third-party reviews?
Third-party reviews of cracked geometry evaluations allow for a comprehensive assessment from an outside perspective. Crack evaluations usually involve more than “just” a calculation. Third-party reviews can be done by experts in the field, such as colleagues, outside consultants, or college professors trained in crack evaluation.
What causes much confusion with FFS crack calculations?
Welds that have yet to undergo post-weld heat treatment can be very complicated structures to evaluate from a crack growth point-of-view, especially when welds are overmatched. While ductile materials offer strength under singular load conditions, they are more susceptible to accelerated crack growth rates as compared to their base metal counterparts.
The API 579 equations can extract relatively low numbers for carbon steel weldments from the ASME charts and, as a result, provide low or inaccurate allowable values for J.
Comprehensive FFS with FEA Software
Driven by FEA, PRG provides software to streamline FFS and other analyses that safeguard an equipment's longevity. With FEPipe (for pressure vessels and piping) and NozzlePRO (for nozzles), users can perform reliable and coherent FFS assessments and their other capabilities.
FFS Analysis and Local Thinning Identification
In our experience, more advanced equipment calls for more in-depth analyses. That's why it's important to identify local thinning areas during the FFS analysis, a capability embedded in our software. This Level 3 type analysis considers the simulating areas of corrosion within the existing geometry while performing an ASME Section VIII-2 Part 5 FEA calculation based on the allowable stresses outlined in the code.
These are featured in the General Nozzles, Shells, and Plates templates in NozzlePRO and FEPipe. The primary purpose is to identify a thinning grid within the geometry, where users can adjust the geometry in the grid area according to their requirements.
This Level 3 type analysis locally thins the geometry to simulate the corroded area and performs a Finite Element Analysis following the allowable stress guidelines from API 579/FFS-1, which is similar to those outlines in ASME Section VIII-2 Part 5.
These are featured in NozzlePRO and in the General Nozzles, Shell, and Plates Template within FEPipe. The primary purpose is to superimpose the localized thinning grid (commonly obtained from an NDT report) on the geometry and adjust it according to their requirements.
Figure 1: Corroded section outside a reinforcing pad
more Precise flaw Detection
In both FEPipe and NozzlePRO, the LTAs of a specific component or area within the geometry are automatically determined via local thinning processors that display areas of concern.
Additionally, Level 1 and Level 2 calculations from API 579 can be performed with a click of a button. Level 3 calculations can also be performed from the same window using the allowable stresses from ASME Section VIII-2 Part 5.
FEPipe and NozzlePRO both offer the functionality to detect multiple flaws. Here are a few specific details on the process for each solution:
NozzlePRO
NozzlePRO can:
- Predict crack growth under specific stress conditions in components to assess when the crack will reach the half wall and through the wall for leaks. This swift calculation is based on observed crack growth in tested low-carbon steel components.
- Use Level 3 FFS analysis to assess flaws or cracks in pressurized or loaded components.
FEPipe
In FEPipe, the model is displayed as a 3D geometry, and local thin areas are identified on a grid. The model is ready for analysis once the geometry dimensions and properties have been entered and verified. This grid allows users to visualize thickness points on the model and pinpoint the exact instances of possible LTAs, flaws, and cracks.
Supported Analysis Types
Our solutions offer a range of advanced analysis capabilities designed to simplify the analysis process for the user. Here are some key insights into additional supported analyses from FEPipe:
- Nonlinear analysis for plasticity, large rotation, and large strain in assessing complex stress scenarios.
- WRC 107 assessments for combination loads are handled through "branch and run."
- Analysis for bends with trunnion models and structural or round attachments.
- Non-linear collapse and buckling analysis for load model perturbation.
- Stress Intensity Factor (SIF), Stress Severity Index (SSI), k values, and permissible loads calculations.
- Nonlinear pressure fatigue assessments for nozzles and olets.
- Both linear and nonlinear Leak-Before-Break (LBB) analyses.
- Automated nonlinear SSI and collapse calculations for bends, heads, branches, or saddles (on all shell geometries).
Maximizing Pressure Equipment Life & Safeguarding Assets
Fitness-For-Service (FFS) assessments are crucial in determining the reliability and safety of pressure vessel equipment. They go beyond basic code compliance, safeguarding the maintenance and use of equipment under diverse stress conditions. Careful maintenance and adherence to guidelines are essential not just for code compliance but for the longevity of equipment.
Opting for FFS tools that provide a robust, comprehensive approach to determining flaws, cracks, and overall equipment maintenance is becoming more of a need for professionals as the industry continues to evolve. With FEA software like NozzlePRO and FEPipe, users can expedite their analysis while safeguarding their equipment against the most updated ASME codes and API standards.
Additionally, we offer a range of courses and video tutorials from engineers covering flaw detection, crack analysis, and identifying local thin areas through our online learning management system, ThinkTank Academy.
These resources equip professionals with a deep understanding of FFS assessments, ensuring they are well-prepared to tackle complex challenges. At their own pace, ThinkTank Academy users can work through the step-by-step video lessons delivered by our team of experienced engineers who not only built the software but are also professional engineers with a combined 50 years of experience and knowledge of ASME code. Users receive a personalized certificate for each completed course to satisfy workplace requirements such as those outlined in Appendix 47 or for personal professional development.
