Allowable Stresses and Deflections

Nonpressure Parts of Vessel
All structural steel not part of the actual pressure vessel should be designed in accordance with AISC Manual of Steel Construction. A one-third increase over basic allowable Code stresses is permitted for stresses resulting from wind or earthquake loads.

For vessel skirts, and other cylindrical shells not subject to internal or external pressure, the longitudinal tensile stress under loading conditions, including wind or earthquake, should not exceed 85% of the yield stress at the operating temperature at the section under consideration. The efficiency of full-penetration, butt-weld joints should be taken as 1.0.

The compressive stress for carbon and low-allow steel shells operating at atmospheric temperature should not exceed the lesser of the following stresses:

• 2/3 of the minimum specified yield stress
• 2.32 x 10^6 t/R (psi)

t = shell thickness, in.
R = mean Radius, in.

The expression 2.32 x 10^6 t/R is based on modifying the classical buckling stress equation for a cylindrical shell given as CE t/r, with C=0.08 and E = 29,000,000 psi.

For carbon and low-alloy steel shells operating at high temperatures, and for other materials, the allowable compressive stress should not exceed the lesser of the following:

• For cylindrical shells designed before publication of the Summer 1983 Addenda of the ASME Code: 1/3 greater than the allowable compressive stress permitted by the applicable edition of the ASME Code.
• For cylindrical shells designed in accordance with the Summer 1983 Addenda or subsequent editions: the allowable compressive stress permitted by the ASME Code.

Lateral deflections of columns resulting from wind loads are typically limited by practical considerations of fluid sloshing off the trays in a vertical tower. The following are typical limits, per industry practice, on calculated deflections (delta) due to wind.

Actual displacements during earthquakes are hard to predict given the transient nature of earthquake ground motions. For this reason, strict limits are not set.

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