Challenges of Implementing Torque on Support Structures

Designing vertical pressure vessels involves careful consideration of loads acting on the structure, including internal pressure, wind, seismic forces, and operational loads, as well as Pressure Vessel Stability in total. One aspect that presents significant complexity is the implementation of torque (or torsional moment) on the vessel’s support structures, such as brackets, legs, skirts, and ring support. The current article examines the effect of torque in pressure vessel design and its consideration in stability checks.

Brackets and Legs: These components are primarily designed to handle axial and lateral forces rather than torsional moments. The introduction of torque would require complex structural reinforcements to mitigate induced stresses and avoid localized failures. Brackets and legs generally experience bending and shear forces rather than significant torsional effects; however, torque effect is very difficult to be examined or modeled in exact mathematical formulas.

Skirts: Skirt supports, commonly used for high-pressure and large-diameter vessels, are effective in transferring loads to the foundation. However, designing skirts to resist torque is challenging due to the difficulty in distributing torsional stresses uniformly along the skirt wall, which is often interrupted by openings or cutouts. Most skirt designs prioritize axial compression and bending rather than torsional resistance. On the other hand, skirt base rings exhibit significant resistance to torque due to their circular geometry, which provides a high torsional section modulus, enhancing their ability to withstand torsional loads. Therefore, torque effect is either difficult to be examined on the skirt shell, or negligible on the skirt base ring.

Ring Supports: These supports are often utilized in horizontal vessels and large vertical vessels to distribute loads efficiently. Due to their circular geometry, ring supports exhibit high torsional stiffness and can resist significant torque around the axis parallel to the vessel shell. This inherent resistance results from their large torsional section modulus, allowing them to effectively counteract torsional forces without excessive deformation. Therefore, torque effect is negligible on the support ring

Lack of Code-Specific Formulas for Torque Consideration

Due to the complexities involved in implementing torque on vessel supports, major design Codes (such as ASME BPVC Section VIII and EN13445) do not provide explicit formulas for calculating torque effects on support structures or anchor bolts. This omission is primarily due to:

v  Structural Challenges: Designing support structures to resist torque would require highly specialized calculations, including finite element analysis (FEA), rather than simple formula-based approaches.

v  Lack of Practical Impact: The contribution of torsional shear stress to the overall stability of the vessel is usually minimal compared to other governing forces such as wind, seismic loads, and dead weight.

v  Design Optimization: Most static equipment support systems are inherently resistant to minor torsional effects due to their existing design for axial and lateral loads, making additional calculations unnecessary.

Negligibility of Torque-Induced Shear in Pressure Vessel Stability

In practice, torque-induced shear stress is often negligible when compared to other forces acting on the vessel. Stability assessments primarily consider overturning moments, uplift forces, and bending stresses, as these have a much greater impact on the structural integrity of the vessel. Since torque rarely governs the design of support structures, Codes prioritize stability against axial and lateral forces instead.

Evaluating Vessel Shell Under Torque

While support structures are not explicitly designed for torsional loads, the pressure vessel shell itself can be evaluated under torque. Various formulas exist to determine the effects of torsional stress on cylindrical shells, which can be useful when assessing localized stress concentrations due to operational factors such as piping connections, asymmetric loading, or eccentric force application.

Conclusion

The complexity of implementing torque in vertical pressure vessel support structures, combined with its generally negligible impact on overall stability, explains why major design Codes do not provide specific formulas for such considerations. Instead, the focus remains on axial, bending, and lateral forces, ensuring the vessel remains stable under expected loading conditions. However, when necessary, the vessel shell can be analyzed for torsional effects using established formulas for cylindrical structures.