Flat plates, covers, and flanges are used extensively in pressure vessels. Circular plates are used for most applications; however, there are some applications where the flat plate is obround, square, rectangular, or some other shape. When a flat plate or cover is used as the end closure or head of a pressure vessel, it may be an integral part of the vessel by virtue of having been formed with the cylindrical shell or welded to it or it may be a separate component that is attached by bolts or some quick-opening mechanism utilizing a gasketed joint attached to a companion flange on the end of the shell.

Flat plates and covers may contain no openings, a single opening, or multiple openings. To satisfy the loadings and allowable stresses, the plate may need to be of an increased thickness or it may require reinforcement from attachments. The equations for the design of unstayed plates and covers based on a uniform thickness with a uniform pressure loading over the entire surface are described in UG-34 of VIII-1 and Article 4.6 of VIII-2. For flat plates and covers with either single or multiple openings, design requirements are given in UG-39 of VIII-1 and in Article 4.6 of VIII-2. See Chapter 7 and Part UHX of VIII-1 and Article 4.18 of VIII-2 for design requirements of tubesheets with multiple openings as used in shell-and-tube heat exchangers.

For the design of flat plates and covers which are attached by bolting that causes an edge moment due to the gasket and bolt loading action, both gasket seating loads and operating loads shall be considered in a similar manner to that required for determining the acceptability of a bolted, flanged joint. Since the loadings and dimensions required for analysis of bolted, flat plates and flanges in both VIII-1 and VIII-2 are very similar, if not the same, they will be treated together. Spherically-dished covers are considered in section 4.7 and in Appendix 1–6 of VIII-1 and in Article 4.7 of VIII-2.