Supporting structures play a crucial role in the stability and functionality of pressure vessels, which are widely used in industries such as oil and gas, chemical processing, and power generation. Among the various support types, leg supports are commonly employed for vertical vessels to ensure proper load distribution and structural integrity. This article discusses the types of leg supports, including braced and non-braced legs, as well as legs mounted on cylindrical shells and torispherical heads and how leg supports are implemented in pressure vessel calculations.
Types of Leg Supports
Leg supports are commonly used for smaller vertical static equipment. They transfer the weight of the vessel, its contents, and additional external forces to the foundation. The selection and design of leg supports depend on factors such as load conditions, vessel geometry, and seismic or wind loading requirements.
Braced vs. Non-Braced Legs
Braced Legs: These supports include diagonal or horizontal bracing elements to provide additional stability. Bracing reduces bending stresses in the legs and the shell by redistributing the forces, making them ideal for tall vessels or those subjected to significant external loads such as wind or seismic forces. Braced legs are commonly used when higher structural rigidity is needed.
Non-Braced Legs: These are simple, standalone legs attached directly to the vessel without additional reinforcement. They are suitable for smaller vessels with relatively lower loads. However, since they experience higher bending stresses, careful analysis is necessary to prevent excessive deformation or failure. It is also worth while mentioning that non-braced leg base plates are thicker than in braced legs, due to the existence of bending stresses.
Leg Attachment Methods
Legs can be attached to pressure vessels in two primary ways:
Attachment to Cylindrical Shells: This is a common method where the legs are welded directly onto the cylindrical portion of the vessel. This approach requires careful stress analysis to account for localized stress concentrations and potential bending moments.
Attachment to the Spherical Portion of Torispherical Heads: Legs can also be mounted on the curved sections of torispherical heads. This method introduces additional complexities due to the varying curvature and requires reinforcement to distribute the load effectively.
Support for Spherical Vessels: Although spherical vessels can also be supported using legs, this approach involves different considerations and is beyond the scope of this discussion
Legs on Cylindrical Shells
When legs are attached to a cylindrical shell, they induce localized stresses, which must be carefully analyzed. The forces from the legs introduce bending moments in the shell, leading to stress concentrations that could compromise the vessel’s structural integrity. To mitigate this:
Reinforcing pads are often added around the leg attachment points to distribute the load and reduce stress concentration.
The attachment welds must be designed with sufficient strength to prevent cracking or fatigue failure (fillet welds are typically subjected to shear).
WRC analysis needs to be used to assess stress distribution and ensure compliance with design codes such as ASME Section VIII. Similar considerations are made on EN13445, PD5500 and AD2000.
When a cylindrical shell is thin and the local leg to shell attachment fails to pass the analysis, there are procedures (i.e as per Denis R. Moss, design of bins) that further support the cylindrical shell using internal or external rings.
Legs on Torispherical Heads
When attaching legs to torispherical heads, the connection is typically made on the spherical portion of the head. This method introduces additional complexities due to the curved geometry and varying stress distribution. Key considerations include:
Adding reinforcing plates to distribute forces more effectively and prevent excessive deformation of the head.
Perform WRC analysis which is commonly used to assess stress distribution and ensure compliance with design codes such as ASME Section VIII. Similar considerations are made on EN13445, PD5500 and AD2000.
It needs to be pointed out that attachment of legs on heads is inherently strong because the torispherical end behaves as an arch in engineering terms, allowing it to efficiently sustain significant loads. The legs are typically shorter (reducing the possibility of buckling), but the resulting compressive forces are a bit larger than legs on shell, because the legs on head are positioned at a smaller leg circle.
Implementing leg supports in pressure vessel analysis
In modern engineering design, verification of leg supports on vessels is done via software tools. One essential feature for pressure vessel support analysis is a comprehensive leg beam library. Having an extensive database of beam profiles allows engineers to quickly select and evaluate suitable leg configurations based on material properties, load capacities, and structural behavior. A well-integrated leg beam library enhances design accuracy, reduces calculation time, and helps in meeting compliance requirements by enabling engineers to perform detailed stress and deflection analyses with ease.
Conclusion
The design of leg supports affects pressure vessel stability, safety, and longevity. Whether using braced or non-braced legs, or attaching legs to cylindrical shells or torispherical heads, careful analysis and proper reinforcement are essential to prevent excessive stress and structural failure. Adhering to industry standards and conducting detailed calculations will ensure that pressure vessels operate safely under various loading conditions. Additionally, modern software solutions like VCLAVIS.com provide state-of-the-art leg support calculations, including a comprehensive leg beam library, thus helping engineers achieve optimal design accuracy and efficiency in their projects.
