Horizontal Vessels Saddle Supports and Stiffening Rings

Horizontal vessels are typically supported on two saddles. The use of two saddles is preferred both statically and economically over a system with more supports.

The vessel designer should follow these steps:

1. Locate the position of saddle supports.
2. Calculate the maximum support reactions and corresponding allowable shell stresses.
3. Provide circumferential stiffening rings if required.

The location of the saddle is sometimes governed by the position of openings, sumps, etc., on the vessel. When no openings dictate saddle location, the statically optimal points should be chosen, i.e., where the positive and negative bending moments are nearly equal. Thin walled vessels with a large diameter are best supported near the heads, to utilize the stiffening effect of the heads. The distance between the head tangent line and the saddle should in no case be more than 20% of the tangent-to-tangent length, L. Note: one end of the horizontal vessel typically contains a sliding support to facilitate thermal expansion. See Standard Drawing GA-C99694 “Standard Details of Support Feet for Horizontal Vessels.”

Once the locations of the supports are determined, the calculation of the maximum support reaction is a matter of simple statics. The following load conditions must be considered, along with the corresponding allowable stress.
• Erection/fabrication
• Hydrotest
• Operation

Use the maximum support reaction to determine whether a vessel requires stiffening at the supports. A criterion for making that decision and for sizing stiffeners is presented in Appendix B, “Design of Supports and Stiffening Rings for Thin Wall Horizontal Vessels.”

To use Appendix B, the designer needs the following information:
• Support reactions
• Vessel wall thickness, excluding corrosion allowance
• Shell material
• Operating temperature

Appendix B also gives an alternative method for stiffening the shell at points of the vessel support by using internal struts. However, the use of internal struts should be limited to large vessels operating at low internal pressures, since internal struts restrain the radial growth of the shell. This restraint may produce circumferential distortion causing failure of internal weld connections.

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