Internal Loads

Pressure vessels normally contain various internal components that are attached directly to a vessel’s shell, such as distributor trays, catalyst support grids, baffles, and demister pads, etc. These internal components apply loads to the shell, and thereby develop stresses that must be added to those resulting from the internal pressure. The weight of the internal components plus the weight of liquid or catalyst supported by the component must be considered. In addition, the pressure drop across the component will apply an additional load to the shell that must be considered separately from the influence of the pressure drop on the design pressure.

The internal loads in vertical pressure vessels are usually downward, developing a compressive stress in the vessel’s shell that counteracts the longitudinal tensile stress developed by the internal pressure. Therefore, it is rare that internal loads affect the design of a vertical vessel. An exception could be encountered with an upflow vertical vessel, if a high pressure drop occurs across an internal component. This would develop a tensile stress in the shell that would add to the longitudinal stress developed by the internal pressure. Note that the weight of the internal contents of a vessel (i.e., internal components, catalyst, and fluids, etc.) will affect the design of the vessel’s support, both by directly increasing the compressive stress and indirectly by amplifying the overturning moment in the event of an earthquake.

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