Flange Leakage Assessment

While ASME B31 codes evaluate the structural integrity of the steel pipe wall, they do not explicitly guarantee that flanged joints will remain leak-tight under heavy external loads. Large bending moments can pry flanges apart, unseating the gasket and causing loss of containment.

To evaluate joint integrity, the Pipe Stress FEA engine utilizes the Kellogg Equivalent Pressure Method, with optional support for the modernized ASME Section VIII, Div 1 UG-44(b) moment factors.

4.4.1 The Kellogg Equivalent Pressure Method

The Kellogg method evaluates external forces by converting them into an equivalent internal fluid pressure ($P_{eq}$). This equivalent pressure represents the internal pressure that would cause the same longitudinal stress in the flange hub as the applied bending moments and axial forces.

The engine extracts the local resultant bending moment ($M_{res}$) and the absolute axial force ($F_{ax}$) at the flange node:

$M_{res} = \sqrt{M_y^2 + M_z^2}$

Equivalent Pressure from Bending ($P_{eq, M}$)

The equivalent pressure caused by the resultant bending moment is calculated using the effective gasket diameter ($G$):

$P_{eq, M} = \frac{16 M_{res}}{\pi G^3}$

Equivalent Pressure from Axial Force ($P_{eq, F}$)

The equivalent pressure caused by the axial tension or compression is calculated as:

$P_{eq, F} = \frac{4 F_{ax}}{\pi G^2}$

(Note: To ensure a conservative analysis, the engine utilizes the absolute magnitude of the axial force, treating both tension and compression as potential prying loads).

4.4.2 ASME UG-44(b) Moment Factor

Historically, the standard Kellogg method has been criticized as overly conservative because it assumes the flange acts as a perfectly rigid body.

If the user selects the UG-44 leakage method in the flange properties, the engine applies the ASME Section VIII Div 1 UG-44(b) moment factor ($F_m$). This factor accounts for the true flexibility of standard ASME B16.5 / B16.47 flanges, reducing the artificial conservatism of the bending equivalent pressure:

$P_{eq, M_{adjusted}} = \frac{P_{eq, M}}{F_m}$

The $F_m$ Factor

The $F_m$ factor is a predetermined constant based on the flange class. For standard ASME B16.5 Carbon Steel flanges, it is typically set to $1.2$.

4.4.3 Final Leakage Evaluation

Once the equivalent pressures are calculated, they are added to the actual internal design pressure ($P_{design}$) of the pipeline to find the Total Equivalent Pressure ($P_{total}$):

$P_{total} = P_{design} + P_{eq, M_{adjusted}} + P_{eq, F}$

This total pressure is then evaluated against the flange's rated pressure capacity ($P_{rating}$) at the design temperature. The engine outputs a Leakage Ratio:

$\text{Leakage Ratio} = \frac{P_{total}}{P_{rating}}$

If this ratio exceeds $1.0$, the flange is flagged as failing the leakage assessment, indicating that the bending moments are too severe for the selected flange class.