COMPRESS provides an option to limit the deflection of vertical vessels due to wind. By default, COMPRESS limits the deflection to 6 inches per 100 feet of vessel height (6" / 100' or 5 mm per 1 meter in metric units). The user can specify a different deflection limit or the option may be disabled. This activity is performed on the Wind Code dialog; see the screen print below.

If the "Limit top deflection to" switch is checked and COMPRESS is in design (not rating) mode, skirt and shell thicknesses are increased to meet the specified deflection limit. Individual skirt and shell thicknesses are increased by a standard plate thickness increment, one component at a time, beginning at the base of the vessel. The deflection is re-calculated after each component thickness increase and compared to the required deflection limit. If the deflection limit is not met, the next component thickness is increased; this process is repeated until the top of the vessel is reached. If the deflection limit is still not met the process is repeated, beginning at the bottom of the vessel (skirt).

Note that vertical vessels act as cantilever beams so the calculated deflection acts accordingly. Because of the flexibility of the shell components, it is usually not possible to meet the deflection limit by support skirt thickness (or base diameter) increases alone.

The numerical method used by COMPRESS to calculate the deflection of vertical vessels is also used when calculating the fundamental period of vibration. For further details on this method please see "Vibrations of Vertical Pressure Vessels" by C.E. Freese.