Pressure vessels are integral components in various industries such as chemical processing, oil and gas, and power generation. These vessels are designed to withstand internal or external pressures during operation, and it is critical that the design and safety features of these vessels are accurately calculated. When designing static equipment, a common misconception is that pressure vessel calculations should be based solely on external pressure if that is the predominant condition. However, internal pressure verification must always be included, even when only external pressure is expected in the vessel’s operational environment. This article highlights the reasons why internal pressure calculations are essential and why the internal design pressure should always be at least equal to the external design pressure.
1. Pressure Testing of Vessels Is Almost Impossible Under External Pressure
One of the main reasons for considering internal pressure in the design process is that pressure testing under external pressure is typically impractical. Standard pressure tests for vessels are conducted by applying internal pressure, often using water, and visually inspecting the vessel’s exterior for leaks and signs of deformation, particularly at welds and components. However, testing under external pressure is much more challenging. External pressure is difficult to apply and maintain, and detecting failures such as weld issues is not easy, as there is no liquid inside the vessel to reveal leaks. Additionally, applying external pressure may cause the vessel to implode if there are even slight geometric imperfections, making this type of testing unsafe and unreliable.
2. Flange Design Assumptions Are Typically Based on Internal Pressure
Another important reason for including internal pressure verification in vessel analysis is that flange designs are typically based on internal pressure scenarios. Many pressure vessels incorporate flanged connections, which are primarily rated for internal pressure. Standardized pressure-temperature rating tables commonly provide flange ratings calculated for internal pressure conditions, not external pressure. These ratings specify the pressure levels at which flanges can maintain their integrity without failure. Generally, if a flange is designed to withstand internal pressure, it can also handle external pressure, as the forces and moments induced by bolts are typically lower under external pressure conditions. Therefore, designing the vessel with an internal design pressure at least equal to the external design pressure ensures that flanges will perform adequately under both internal and external conditions, enhancing the vessel’s overall safety and longevity.
3. Nozzle Verification Is Based on Internal Pressure
Nozzles are critical components in pressure vessels, facilitating the entry and exit of fluids or gases. Verification of nozzle strength and integrity is typically based on formulas that apply to internal pressure scenarios, as outlined in most vessel design codes. These formulas ensure that the nozzle, including its welds, flanges, and connections, can withstand the stresses imposed by the internal pressure. While external pressure may create some additional stresses, the primary design calculations for nozzles are based on internal pressure. If the internal pressure is not verified, the nozzle may not be adequately designed to handle the forces exerted on it, leading to potential failure or leaks. Therefore, including internal pressure verification in the calculations is crucial to ensuring the safety and functionality of the vessel’s nozzles.
4. Stability Formulas Are Based on Internal Pressure, Not External Pressure
Pressure vessel stability is a critical consideration, particularly regarding resistance to buckling or deformation. Design equations for assessing longitudinal and circumferential stresses due to internal pressure are well-established, accounting for material properties, geometry, and pressure loads to ensure the vessel can withstand internal forces. However, when subjected to external pressure, the vessel experiences different stress distributions, and internal pressure design formulas may not directly apply. Designing for external pressure requires specialized considerations, such as increasing shell thickness and incorporating appropriate supports and stiffening elements to prevent buckling. Therefore, incorporating internal pressure verification—ensuring the internal design pressure is at least equal to the external design pressure—provides a safer and more reliable approach, as it aligns with established design practices and enhances the vessel’s overall stability under various pressure conditions.
Conclusion
Pressure vessel analysis is a complex and multi-faceted process that requires careful consideration of both internal and external pressures. While external pressure may be a predominant concern in certain applications, it is essential to include internal pressure verification in all calculations. Internal pressure plays a critical role in ensuring that the vessel is structurally sound, flanges are designed for appropriate pressure loads, nozzles are adequately verified, and the overall stability of the vessel is maintained. Ignoring internal pressure verification can lead to unsafe designs and costly failures. Therefore, to ensure the safety, reliability, and compliance of pressure vessels, engineers must always incorporate internal pressure verification in their calculations. For engineers that use pressure vessel software to perform their pressure vessel evaluations, it needs to be pointed out that there are tools which implement local loads check per selected Code, such a VCLAVIS.com, PVElite and Codeware COMPRESS etc