ASME Section VIII, Division 1 requires consideration of external loads under UG-22; but for decades, Division 1 did not offer a procedure for evaluation. Division 2’s 2007 rewrite added explicit consideration for external loads in the custom flange section (4.16), yet this method was not efficient to quickly evaluate typical rated flanges for these same loads.
As a result, engineers often used the Kellogg Equivalent Pressure Method, a leakage-based approach. While simple and conservative, it frequently underestimates the true capacity of many standard flanges.
That gap began to close with Dr. Warren Brown’s 2013 PVP paper, which introduced a more favorable methodology for evaluating standard ASME B16.5 and B16.47 weld neck flanges under pressure and external loads. By introducing (FM) — a factor representing flange capacity for external loads — the method aligned calculations more closely with physical performance.
The governing inequality is:
where (FM) is greater than zero for many flange sizes, resulting in higher allowable loads than the traditional Kellogg inequality that can be determined from above by setting FM=0.
That paper’s proposed method was simplified for the introduction of CC 2901, which quickly was incorporated into both UG-44(b) in Division 1 and 4.16.12 in Division 2. This simplification eliminated many of the Fm values that were included and presented only the more conservative (lower) Fm values in the method published in the CC and the codes.
Despite being written as “may be used,” in UG-44(b) and 4.16.12, the methods have often been enforced as mandatory since the codes themselves require consideration of the external loads. This caused significant disruption for many designers: weld neck flanges that previously satisfied other methods of evaluation, were not passing the new “optional” methods in the code.
The situation became more complex as ASME B31.3 piping code also adopted references to CC 2901, expanding its use beyond pressure vessels into piping systems. While considering leakage-based methods represented progress, they were often treated as mandatory and criticized as overly conservative.
In November 2023, a revised version of CC 2901 was released, 2901-1, addressing the most major concern. The new version expanded the FM values. The original edition applied the lowest FM across a group of flanges (e.g., all Class 150 weld necks NPS ≤ 12 were limited to FM = 1.2). The update assigns size-specific FM values included in Brown’s paper, many of which are significantly higher, reducing conservatism.
The 2025 Section VIII Division 1 and 2 code books are still based on the original CC 2901 and UG-44(b) and 4.16.12 are increasingly treated as the de facto method. This is perhaps a poor interpretation of code language; but it is the typical practice. This increases the industry need for the changes in CC2901-1 to be incorporated into these paragraphs.
Assuming a designer can persuade their customer and third-party reviewers to accept another method for a rated weld-neck under external loads, or the flange is not a weld-neck, what options exist?
In addition to CC 2901, several methods remain in use:
Koves Method (2005 PVP): Leakage-based; applies to B16.5 flanges ≤ 24”; considers moment but not axial load.
Kellogg Equivalent Pressure: Versatile and conservative; covers flange types not included in CC 2901 (e.g., slip-ons).
ASME VIII-2, §4.16 (Custom Flange): Stress-based; suitable when leakage analysis is outside scope.
EN 1591 / EN 13445 Annex G (Wölfel Method): Leakage-based; emphasizes bolt load variation and gasket behavior.
Each method has a different scope and basis. Selecting the right approach depends on flange type, loading conditions, and regulatory context.
To bridge the gap between code requirements and real-world flange behavior, engineers need tools that unify rule-based methods with finite element analysis (FEA). This is exactly what FlangePRO delivers.
FlangePRO is a purpose-built flange design tool that enables engineers and designers to quickly analyze and validate weld neck and hubbed slip-on flanges. It supports standard sizes from ASME B16.5 and B16.47 Series A/B, as well as fully custom, user-defined geometries.
With FlangePRO, users can:
Run Built-In Code Methods: Perform automated stress and leakage checks using ASME, EN, and PD 5500 standards.
Leverage Advanced FEA: Conduct both linear and nonlinear analysis with axisymmetric or 3D brick elements, accurately modeling interactions between flanges, gaskets, bolts, and nuts.
Compare Code and Simulation: Evaluate code-based results directly alongside FEA outcomes, ensuring consistency while highlighting potential leakage risks missed by simplified rules.
Automate Complex Tasks: Contact modeling, stress linearization, and result display are handled automatically, reducing manual effort and accelerating workflows.
By consolidating both standards-based checks and high-fidelity simulation, FlangePRO streamlines the entire design and validation process. Engineers gain faster turnaround, more defensible compliance, and greater confidence in flange performance under external loads.
The incorporation of leakage-based assessments into ASME codes is a milestone for the industry. While initial adoption of CC 2901 provided solutions and created challenges, its ongoing refinement — together with the rise of automated software tools — positions engineers to design flanges that are not only code-compliant but more reliable in practice.
With FlangePRO, engineers no longer face the dilemma of choosing between stress checks and leakage models. They can evaluate both frameworks simultaneously, gaining deeper insight into flange behavior under real-world loading conditions and advancing the industry toward truly performance-based standards.