Horizontal Vessels: Structural Design

In-Service Conditions
Circumferential stresses from internal pressure usually control the design of cylindrical pressure vessels. In horizontal vessels, wind or seismic loads rarely affect the shell stress calculation. A horizontal vessel on a saddle support acts as a beam with the following special circumstances:
• The loading conditions are different for a full or partially filled vessel.
• The circumferential stresses in the vessel vary according to the included angle of the saddle.
• The load due to the weight of the vessel is combined with other loads.

The following loads for horizontal vessels need to be considered:
• Support reaction at each saddle. Design the vessel for a full waterload reaction.
• Internal pressure. Since the longitudinal stress in the vessel shell is only one half of the circumferential stress, approximately one half of the actual plate thickness in the longitudinal direction is available to resist the load of the vessel weight and contents.
• Wind and seismic loads. These are important for designing the support system, typically consisting of a saddle, anchor bolts, and a concrete pier. Long vessels with very small thickness/radius values are subject to distortion from wind pressure. However, a vessel designed to 1 psi or more external pressure can successfully resist external loads encountered in normal service.

Maximum longitudinal stresses are found by combining:
• Operating weight
• Operating internal pressure (or vacuum)

Maximum circumferential stress must be checked at the saddle support locations, and stiffeners may be required.

Pre-Service Conditions
The orientation of a horizontal vessel during fabrication and in service are typically the same, so no major change in shell stresses should occur. However, if the lifting points of the vessel are not located near the final support points, then a pre-service shear and bending moment diagram should be drawn and the maximum stresses considered. Attaching lifting lugs near the support saddle or near the shell-head intersection is recommended in order to avoid additional stiffening of the vessel for pre-service loads.

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