Pressure Vessel Flat Plate Closures

The unstayed flat head, or cover, is a common type of closure for vessels. It may be integrally formed with the shell, or welded to it as shown in Figure 100-9. It can also be attached by bolts or some quick opening device. Plates may be arbitrarily classified into three groups:
1. Thick plates in which the shear stress is the most important.
2. Medium thickness plates in which bending stresses are the most important.
3. Thin plates whose strength depends mainly on the direct tension produced by the stretching of middle plane.

Most actual flat plate closures for pressure vessels are included in the second group.

The stresses in circular flat plates of constant thickness are calculated based upon the assumption that the edges are either simply supported, or fully fixed. In actual designs, neither of these edge conditions is actually realized, the actual condition being somewhere between.

With edges assumed simply supported, the maximum stress is located at the center and equals:

Pi = Internal pressure
D = Diameter
t = Thickness

With fully fixed edges, the maximum stress is radial and located at the edge:

where D is the diameter of the plate, P the pressure, and t the thickness of the plate.

The basic equations used by the ASME Code introduce a variable “C” factor depending on the details of the corner construction. The maximum deflection is limited to one half of the thickness, and all stresses are kept within the elastic limit.

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