Procedure 5-12 Design of Baffles

Baffles are frequently used in pressure vessels, either vertical or horizontal, to divide the interior volume into different compartments. These compartments may be used to segregate liquids or provide overflow weirs for the separation of liquids. Baffles may be stiffened or unstiffened. When welded across the entire cross section of the vessel, fliey must be checked that they are not unduly restricting the diametral expansion of the vessel. If flie unrestrained radial expansion of the vessel exceeds that of the baffle by more than Vi6 in. (Vs in. on the diameter), then a flexible type of connection between the vessel shell and the baffle should be utilized. Various flexible attachment designs are shown within the procedure. 

Baffles should always be designed in the corroded condition. It is typical for welded baffles to be designed with a full corrosion allowance on both sides. If the baffle is bolted in, flien one-half the full corrosion allowance may be applied to each side, the logic being that a bolted baffle is removable and therefore replaceable. 

The majority of baffles are flat and as a result are very inefficient from a strength standpoint. Deflection is flie governing case for flat plates loaded on one side. The preference is to have unstiffened baffles, and they should always be the first choice. This will be acceptable for small baffles. However, for larger baffles, as the baffle thickness becomes excessive, stiffeners offer a more economical design. Therefore stiffeners are frequently used to stiffen the baffle to prevent the thickness of the baffle from becoming excessive. The number, size, and spacing of stiffeners are dependent on the baffle thickness selected. There is a continual trade-off between baffle thickness and stiffener parameters. 

The design of a baffle with stiffeners is an iterative process. The procedure for the design of the stiffeners is first to divide the baffle into panel sections that are rigid enough to withstand the pressure applied on one side. Each individual panel is checked as a flat plate of the dimensions of the panel. The stiffeners are assumed to be strong enough to provide the necessary edge support for the panel. 

The stiffeners themselves are designed next. A section of the baffle is assumed as acting with the stiffener and as contributing to the overall stiffness. This combined section is known as the composite stiffener. The composite section is checked for stress and deflection. Both vertical and horizontal stiffeners can be added as required. 

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