Page 249 - Pressure Vessel Design Manual

Special Designs 227

PROCEDURE 4-6
DESIGN OF BAFFLES [12]

Baffles are frequently used in pressure vessels, either ver- The design of a baffle with stiffeners is an iterative pro-
tical or horizontal, to divide the interior volume into differ- cess. The procedure for the design of the stiffeners is first to
ent compartments. These compartments may be used to divide the baffle into “panel” sections that are rigid enough
segregate liquids or provide overflow weirs for the separation to withstand the pressure applied on one side. Each indivi-
of liquids. Baffles may be stiffened or unstiffened. When dual panel is checked as a flat plate of the dimensions of the
welded across the entire cross section of the vessel, they panel. The stiffeners are assumed to be strong enough to
must be checked that they are not unduly restricting the provide the necessary edge support for the panel.
diametral expansion of the vessel. If the unrestrained radial The stiffeners themselves are designed next. A section of
expansion of the vessel exceeds that of the baffle by more the baffle is assumed as acting with the stiffener and as
than l/i6 in. (% in. on the diameter), then a “flexible” type of contributing to the overall stiffness. This combined section
connection between the vessel shell and the baffle should be is known as the composite stiffener. The composite section is
utilized. Various flexible attachment designs are shown checked for stress and deflection. Both vertical and horizon-
within the procedure. tal stiffeners can be added as required.
Baffles should always be designed in the corroded condi- If required, an alternate design is assumed based on a
tion, It is typical for welded baffles to be designed with a full thicker or thinner baffle and checked until a satisfactory
corrosion allowance on both sides. If the baffle is bolted in, design is found. There is no “right” answer; however, it
then one-half the full corrosion allowance may be applied to should be noted that the thinner the baffle, the greater
each side, the logic being that a bolted baffle is removable the number of stiffeners. The lightest overall weight
and therefore replaceable. is probably the “best” design but may not be the
The majority of baffles are flat and as a result are very least expensive due to the welding costs in attaching the
inefficient from a strength standpoint. Deflection is the gov- stiffeners.
erning case for flat plates loaded on one side. The preference One alternative to a flat baMe with stiffeners is to go to a
is to have iinstiffened baffles, and they should always be the curved baffle. A curved baffle works best as a vertical baffle
first choice. This will be acceptable for small baffles. in a vertical vessel. The curved baffle takes pressure from
However, for larger baffles, as the baffle thickness becomes either side wall. If the pressure is on the concave side the
excessive, stiffeners offer a more economical design. baffle is in tension. If the pressure is on the convex side, the
Therefore stiffeners are frequently used to stiffen the baffle baffle is in compression.
to prevent the thickness of the baffle from becoming exces- There are various tables given in this procedure for flat
sive. The number, size, and spacing of stiffeners are depen- plate coefficients. Flat plate coefficients are utilized to deter-
dent on the baMe thickness selected. There is a continual mine the baffle thickness or a panel thickness. Each table is
trade-off between baffle thickness and stiffener parameters. specific for a given condition and loading.