In pressure vessel stability – design, it is essential to account for various external loads to ensure structural integrity and safety. These loads include wind, seismic activities, live loads, snow loads, and friction factors (for horizontal vessels and heat exchangers). Each of these factors influences the pressure vessel stability and must be meticulously evaluated during the design process. Key considerations are exposed herein.
Wind Loads
Wind exerts lateral forces on pressure vessels, especially those installed outdoors. The magnitude of these forces depends on factors such as wind speed, vessel shape, height, and exposure to open terrains. Design codes, like ASCE 7 or EN1991-1-4, provide guidelines for assessing wind loads to ensure that vessels can withstand these forces without compromising structural integrity. Wind induced vibrations might also need to be considered, especially on tall columns with ratios H/D > 15.
Seismic Loads
Earthquakes can induce significant horizontal and vertical forces on pressure vessels. The impact varies based on the vessel’s location, local seismic activity, and soil characteristics. Design standards, such as ASCE 7 or EN1998-1, outline procedures for evaluating seismic loads, emphasizing the importance of considering regional seismic data and soil conditions to ensure vessel stability during seismic events. It is also important to consider the mass to height distribution when applying the seismic lateral forces.
Live Loads
Live loads refer to temporary or dynamic forces acting on static equipment, including maintenance activities, equipment loads, and operational fluctuations. These loads are variable and can change over time, necessitating a design that accommodates such variations to maintain structural integrity throughout the vessel’s operational life.
Snow Loads
In regions prone to snowfall, accumulated snow can add substantial weight to pressure vessels, particularly those with horizontal orientations. The additional load depends on factors like snow density and accumulation depth. Design codes provide guidelines to account for snow loads, ensuring that vessels can support this extra weight without structural failure. There are cases however (i.e in hot pressure vessels) that snow needs not be regarded, because it melts due to the vessel surface.
Friction Factors for Horizontal Vessels
For horizontal pressure vessels supported on saddles, friction between the supports and the foundation plays a vital role in stability. Frictional resistance helps prevent unwanted movements due to external forces like wind or seismic activities. However, it’s essential to balance friction to allow for necessary thermal expansions and contractions without inducing undue stress on the vessel. Such considerations may lead to application of teflon plates beneath the saddle base.
Soil Loads on Buried Vessels
Buried pressure vessels are subjected to external pressures from the surrounding soil, which can significantly impact their structural performance. The weight of the soil above the vessel exerts a vertical load, which depends on the depth of burial and soil density. This pressure can influence the vessel’s structural integrity, potentially leading to deformation or buckling if not properly accounted for. Moreover lateral earth pressure is applied as external pressure on the vessel’s sides. Understanding these pressures is crucial for designing vessels that can withstand such lateral forces without compromising their structural integrity.
Buoyancy Effects on Vessels Installed in Pits
Pressure vessels installed in pits, especially in areas with high groundwater levels, can be susceptible to buoyant forces that may cause them to float or become unstable. Designer may need to account for the presence of groundwater which exerts an upward buoyant force on the vessel. If this force exceeds the vessel’s weight, it can lead to flotation or displacement, particularly when the vessel is empty or partially filled. To counteract buoyant forces, vessels may require anchorage systems or ballasting to ensure they remain securely in place. Proper design and installation of these measures are essential to prevent movement or damage due to buoyancy.
Load Combinations
Designing pressure vessels requires evaluating combinations of these loads to ensure comprehensive safety. For instance, standards like EN13445, PD5500 and AD2000, as well as ASME VIIII DIV2, provide guidelines on considering and combining different loads, such as internal pressure with wind or seismic forces, to assess the vessel’s ability to withstand multiple simultaneous stresses.
Conclusion
Incorporating a comprehensive understanding of external factors and their interactions is crucial in pressure vessel analysis. Adherence to established design codes and standards ensures that all potential external forces are considered, leading to the safe and efficient operation of pressure vessels across various environmental conditions. To streamline this process, modern software applications, such as VCLAVIS.com, have been developed to assist mechanical engineers in evaluating various loadings on pressure vessels and simplifying the load combinations process.
