When evaluating static equipment, one critical consideration is how to accurately calculate the stresses on the saddle supports of horizontal vessels and heat exchangers. This calculation task is vital in pressure vessel design, for ensuring the safety, functionality, and longevity of the equipment used in industries such as oil and gas, chemical manufacturing, and power generation. There are different methodologies for calculating these stresses, and two of the most well-known approaches are LP Zick’s methodology and the EN 13445 method, which is influenced by AD 2000.

 

This article explores why the ASME VIII Division 2 code adopts the LP Zick method for saddle design, rather than the EN 13445 methodology, and highlights the key differences between these two approaches.

 

The LP Zick Methodology: A Historical Approach

The LP Zick methodology was developed in the early 1950s and has become one of the standard approaches for analyzing the stresses on saddle-supported horizontal pressure vessels in ASME Section VIII Division 2. Zick’s method is based on beam theory and assumes that the pressure vessel shell behaves as a rigid beam supported by saddles. This approach simplifies the complex reality of pressure vessel behaviour under load by using empirical formulas to calculate the bending stresses, shear stresses, and local shell stresses caused by the saddle supports.

 

While the Zick method provides a simplified calculation approach that is easy to implement, it has limitations. For example, it does not account for shell flexibility or the non-uniform distribution of stresses at the saddle points, which can lead to less accurate or over-conservative results in certain conditions.

 

The EN 13445 Methodology: A More Detailed Approach

In contrast to the Zick method, the EN 13445 methodology is more sophisticated and is based on shell theory and modern stress analysis techniques. This methodology has its roots in AD 2000, a German standard for the design of pressure vessels, and is widely used in European countries.

 

AD 2000 laid the foundation for detailed stress analysis of saddle supports using finite element analysis (FEA)-inspired methods and a more accurate representation of the stress distribution across the saddle support region. EN 13445 effectively borrowed this method from AD 2000, refining and incorporating it into the European pressure vessel code.

 

Why ASME VIII Div. 2 Uses the Zick Method

The ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 2, which governs the design and construction of pressure vessels in the United States, uses the LP Zick method rather than the EN 13445 method. There are several key reasons for this choice:

 

Historical Context and Regional Code Traditions

The Zick method has been in use in ASME codes for decades, making it deeply ingrained in the U.S. engineering tradition. It has been widely adopted and is well-understood by engineers in the U.S. and other countries that follow ASME standards. The method’s simplicity and empirical nature made it an attractive option when it was first introduced in the 1950s and continue to make it a practical choice for engineers today.

 

On the other hand, EN 13445 is part of the European unfired pressure vessel standard and reflects the German approach to pressure vessel analysis. While it is a more detailed and comprehensive method, it is not as widely adopted in the U.S. industry.

 

Simplicity and Conservatism

The Zick method provides a simplified yet conservative approach to saddle design. It’s empirical, which means it relies on observed data and practical experience, and uses simplified assumptions like a rigid vessel shell and parabolic load distribution. This simplicity makes it easy to implement and understand which is essential for quick preliminary designs or for vessels with less complex loading conditions.

 

ASME Design Philosophy

The ASME code emphasizes practical application, ease of use, and industry familiarity. The Zick method aligns with these principles by offering a relatively simple and fast way to perform calculations while still providing conservative safety margins. It is integrated into engineering tools and design software that engineers use routinely. While EN 13445 provides more accuracy and takes into account shell flexibility, ASME Section VIII Div. 2 permits the use of finite element analysis (FEA) for more complex cases, which can handle the advanced stress distributions and local effects more accurately. Therefore, Zick’s method remains in use for standard cases, with the flexibility to use advanced FEA tools for more detailed analysis when necessary.

 

Conclusion

While the EN 13445 and AD 2000 methods provide more advanced and precise approaches for saddle design, ASME Section VIII Division 1 remains aligned with the LP Zick method. This adherence to the Zick method reflects ASME’s design philosophy, emphasizing simplicity, practicality, and conservatism for routine applications. Designers working within Division 1 should respect this tradition and consistently apply Zick’s method for saddle calculations as outlined in Division 2. Likewise, pressure vessel calculation software supporting multiple codes must ensure that Zick’s method is used for ASME Division 1 vessels, while the appropriate EN 13445 method is used for European vessels and the AD 2000 method for German vessels, avoiding any mixing of these methodologies. This ensures accuracy and compliance with each code’s intent. Such software found in the industry, like VCLAVIS.com, strictly applies the relevant methods for each code, maintaining the integrity and spirit of each code without mixing them.