Vertical Vessel Skirts
The design procedure outlined here is based on cylindrical skirts. (Tapered designs are a special problem and beyond the scope of this manual.) Design of the skirt consists of determining the operating weight of the vessel and the controlling bending moment due to wind or earthquake. The skirt thickness should be checked at three locations:
1. Top of the skirt where the allowable stress may be reduced by the temperature of the vessel, and by the efficiency, E, of the weld between the skirt and the shell. (See Standard Drawing GD-C78876, Standard Skirt and Base Details for Vertical Vessels, located in the Standard Drawings and Forms Section.)
E = 0.5 for Type A Attachment per Standard Drawing GD-C78876
E = 1.0 for Type B Attachment per Standard Drawing GD-C78876
Type B attachments are preferred for all cases: In dished heads, there is a high stress concentration at the knuckle which would be augmented by an attachment such as a Type B skirt. Brittle failures starting at the highly stressed inside face of the knuckle have occurred in large vessels with thick heads. For this reason, the maximum thickness for dished heads should be 5/8 inch after forming. Since the thickness of dished heads is limited, the use of the Type B skirt attachment is recommended.
2. Sections with the greatest number and/or size of skirt openings. (manways, pipe penetrations, etc.)
3. Base of skirt where the maximum load exists.
The stress in the skirt is represented by:
Use any consistent units for this stress calculation.
In most practical applications, the ratio of R/t is greater than 10. Hence, the area A and the moment of inertia I are expressed in the simplified version shown above.
The equation for the stress in a skirt then becomes:
Because the compressive stress in the skirt is larger than the tensile stress, the compressive stress usually controls the design and is kept below the allowable stresses defined in Section 444 above. The thickness of the skirt should be ¼ inch minimum.
The base of skirts should normally be checked on the basis of allowable stresses corresponding to ambient temperature. Special attention should be given to the added effect of thermal stresses in those vessels employing Type A skirt attachment where the vessel “design temperature” is 450°F or above, and the skirt height is
3 feet or less.
If a Type B attachment is used, special attention should be given to the thermal stresses for temperatures above 650°F and skirt heights under 3 feet.
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