Introduction
Nozzles are essential components of pressure vessels, serving as connection points for piping, instrumentation, and other equipment. While nozzle loads are typically considered to have a local effect on the vessel shell, there are scenarios where these loads can impact the overall pressure vessel stability. This article explores the implications of nozzle loads in static equipment design, the considerations outlined in EN 13445, and the method for combining multiple nozzle loads as per AD 2000.
Effects of Nozzle Loads
In standard pressure vessel analysis, nozzle loads—comprising forces and moments from connected piping—are generally treated as local effects. This assumption is based on numerous evaluations that have shown that the nozzle loads remain local. When modeling a loaded and pressurized nozzle using finite element analysis (FEA), it is clearly depicted that the stress arising is local, and the overall deformations remain only in an area close to the nozzle-to-shell junction. However, there are circumstances where the magnitude and configuration of nozzle loads may necessitate a broader evaluation, leading to stability concerns for the vessel as a whole.
EN 13445 Considerations on Nozzle Loads and Pressure Vessel Stability
EN 13445, the European standard for unfired pressure vessels, acknowledges that under certain conditions, nozzle loads can influence the global stability of the vessel. Specifically, the standard states that if a nozzle is sufficiently large and subjected to substantial loads, it can lead to global effects that need to be considered. If, for example, a radial load acts on a nozzle attached to a cylindrical shell, this load can be a shear load on the shell that needs to be considered. Moreover, the concept of considering the nozzle loads as global forces and moments transferred to the supporting structure is also discussed. For more information, refer to EN 13445-3, paragraph 5.3.4.2.5 and 22.4.6.
Combining Nozzle Loads as per AD 2000
The AD 2000 code, a German pressure vessel standard, is the first code that provides a structured approach for evaluating multiple nozzle loads acting simultaneously. This is crucial because pressure vessels often have multiple nozzles, each subjected to varying loads. AD 2000 S3/0 Annex 2 recommends:
Vectorial Summation of Forces and Moments: Combining forces and moments from different nozzles to assess the resultant effect on the vessel.
Interaction Diagrams: Using interaction diagrams to evaluate the combined impact of multiple loads, ensuring that the vessel remains within acceptable stress and deformation limits.
Reduction Factors: Applying appropriate reduction factors to account for uncertainties in load estimation and potential interactions between different nozzles.
By adopting the AD 2000 methodology, engineers can ensure that cumulative nozzle loads do not compromise vessel stability while maintaining compliance with design codes.
Conclusion
While nozzle loads are often treated as local effects in static equipment design, large nozzles and high loads can introduce global stability concerns. EN 13445 highlights the need for additional stability checks in such cases, while AD 2000 provides a systematic approach for evaluating and combining multiple nozzle loads. Proper assessment of these factors is essential for ensuring the safety, reliability, and compliance of pressure vessels in industrial applications.