AutoPIPE - Warning Number 369 FLANGE 25.01: the hub thickness ratio is > 4.5.

Question:

When analyzing a model in AutoPIPE Vessel the following warning number was reported.

369 FLANGE 25.01: the hub thickness ratio is > 4.5.

Why and how to avoid it?

Answer:

This warning is generated when the flange hub taper ratio,

Xg = g1/g0

is greater than 4.5. In the displayed calculation report:

Xg = 15.4/3.4 = 4.529 > 4.5

therefore the warning is issued.

There is no strict mathematical limit, but there is a practical limit of g₁ / g₀ ≤ 5 when calculating a welding neck flange according to the Taylor-Forge method (integrated into ASME Section VIII Div. 1, Appendix 2 and other similar design codes).

1. The Limit of the Calculation Charts (1 ≤ g₁ / g₀ ≤ 5):

The Taylor-Forge method relies on geometric factors (F, V, f) to determine the stresses in the tapered hub. The analytical formulas and official charts provided by ASME to determine these factors are strictly limited to a g₁ / g₀ range of 1 to 5.
g₁ / g₀ = 1: Represents a straight hub (uniform thickness).
g₁ / g₀ = 5: Represents the maximum taper taken into account by the standard method.

2. Optimal Design Recommendations:

In most pressure vessel and heat exchanger applications, the g₁ / g₀ ratio is typically well below 1.5. A significantly higher ratio generally indicates an excessively thick hub section, which may lead to increased local stress concentrations and an inefficient flange design.

For this reason, AutoPIPE Vessel reports a warning when the ratio exceeds 4.5, providing an early indication that the flange geometry is approaching the practical limits of the design methodology.

So we can consider that the limit of 4.5 is just a recommended good practice.

How to Avoid the Warning

Review the flange geometry and reduce the hub taper ratio so that:

g₁ / g₀ ≤ 4.5

This can typically be achieved by:

- Reducing the hub thickness (g₁), or
- Increasing the small-end thickness (g₀), where appropriate.

The warning is intended as a recommended design guideline and good engineering practice rather than a strict code violation.