AutoPIPE - Warning Code 589 - FLANGE (nozzle XX): the rating class is less than required rating class (XX)

Question:

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

589 FLANGE (nozzle XX): the rating class is less than required rating class (25).

Why and how to avoid it?

Answer:

This warning indicates the selected flange rating is inadequate for the computed design conditions per the applicable code and material limits. It commonly occurs when a flange rating is manually forced to a lower class than required, or when additional pressure sources (e.g., static head at the nozzle) raise the effective design pressure above the allowable for the selected PN/Class.

How to Verify or Avoid the Warning

1. Open Nozzle Properties
2. Navigate to Nozzle Neck
3. Select Edit Flange
4. Check whether the Rating has been forced
5. If forced, update it to a sufficient rating class

If the rating is not forced, AutoPIPE Vessel will automatically select an adequate flange class, and no warning will be issued.

Why the reported flange maximum allowable pressure values are not matching with code Tables?

This is related to the method used by AutoPIPE Vessel to determine the allowable pressure of an EN 1092 flange.

AutoPIPE Vessel does not simply use the published pressure-temperature rating as a fixed allowable pressure. Instead, the permissible pressure is recalculated based on the nominal design stress of the selected flange material at the applicable temperature.

EN 1092 provides a calculation approach in which the allowable pressure depends on the material's nominal design stress. AutoPIPE Vessel applies this approach when checking the suitability of the selected PN rating.

For example, EN 1092-1:2018, Annex F, Clause F.2.2 provides the following relationship for materials with time-independent nominal design stress:

The nominal design stress (ft) is calculated in accordance with EN 13445-3, using the applicable safety factors and material properties such as yield strength and tensile strength.

Note: The material properties and corresponding design stress can depend on the flange thickness. Therefore, flanges with the same material and PN rating may have different calculated maximum allowable pressure values depending on their thickness.

AutoPIPE Vessel calculates the maximum allowable pressure value and compare it with the design pressure of the vessel.

Since 1.739 MPa > 1.6 MPa, PN16 is insufficient → Warning 589 is triggered.

Pressure-Temperature Table vs. Material-Specific Calculation:

The pressure-temperature rating tables in EN 1092 may provide similar maximum allowable pressure values for materials belonging to the same material group. These tabulated values provide a standardized pressure-temperature rating for the applicable material group.

However, individual materials within the same material group can have different mechanical properties and, consequently, different allowable design stresses.

AutoPIPE Vessel considers the allowable stress of the specific material selected for the flange. The maximum allowable pressure is then calculated using the nominal design stress ft of that material at the applicable design temperature.

Conclusion

Warning 589 serves as an important check to ensure flange adequacy under actual operating pressures, including static head effects. To avoid the warning:

- Do not force an undersized flange rating, or
- Increase the flange class to meet the computed pressure requirements