In pressure vessel design software, understanding the effects of local loads on nozzles is essential, particularly due to their influence on standard flanges. Nozzles, as points of attachment for piping or instrumentation, introduce local stresses that can significantly affect the mechanical integrity of the connected flange and the surrounding vessel wall.
In recent years, ASME Section VIII, Division 1, UG-44 (as well as Division 2) has incorporated an approach, commonly known as the Kellogg method, to provide a systematic means of verifying standard flanges under nozzle loads, which are typically dictated by licensor specifications. UG-44 considers flange size and rating, together with applied moments and axial forces, which are converted into an “equivalent pressure” and then checked against the rated pressure of the flange.
Despite the rigor of this method, practical experience has highlighted several challenges in its application. Because UG-44 is not mandatory, many designers choose not to apply it rigorously – particularly for flanges such as 150# flanges of small diameter. Before the introduction of the Kellogg method into the Code, few incidents were reported of flanges leaking under nozzle loads, so there was little perceived concern. However, once UG-44 was adopted, many of these small flanges suddenly failed to pass calculations, and upgrades in rating (e.g., from 150# to 300#) often became necessary.
That is why some designers, confident in their field experience, elect not to apply UG-44 to avoid unnecessary rating changes. While this may be defensible in certain cases, it is important not to underestimate the effect of nozzle loads on flanges – particularly in lethal service. Field observations have shown cases where flanges experienced higher-than-anticipated stress concentrations, potentially leading to premature fatigue or leakage under operating conditions.
To address these difficulties, the ASME committee issued Code Case 2901-1, which introduces reduced moment factors for nozzle loads on small flanges. This adjustment provides a more balanced solution: it improves the calculation factors, avoids overly conservative results, and still maintains adequate safety margins. By applying these reduced factors, designers can more effectively manage cases where strict UG-44 compliance would otherwise mandate unnecessary flange rating increases.
In conclusion, while UG-44 remains a robust framework for evaluating nozzle effects, both practice and field experience have demonstrated its limitations in smaller flange applications. Code Case 2901-1 offers a practical alternative, striking a balance between safety and reliability. Designers should remain attentive to these considerations to ensure that local nozzle loads are adequately accounted for in pressure vessel analysis.
