Opening Shapes and Sizes

The Code permits a pressure vessel to be designed with openings of any shape or size. Circular and elliptical openings are recommended because vessel manufacturers are familiar with these. An “obround” opening is formed by two parallel sides and semicircular ends. This shape provides good access for maintenance personnel because the headroom of this opening can be made large. Elliptical openings are usually found when attached piping joins the shell or head at an oblique angle. When a hillside nozzle is required, the opening in the shell or the head is elliptical. Although the Code does not restrict the size of a vessel opening, large openings require reinforcement, which may cause cost and fabrication problems. Good design practice dictates that when an opening becomes more than one-half the inside diameter of the shell, the design should employ a shell-reducing section instead of a nozzle.

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