1.4 Closures for Vessels

  • The ends of a cylindrical vessel are closed by heads of various shapes. Heads are typically curved rather than flat. The reason is that curved configurations are stronger and allow the heads to be thinner, lighter and less expensive than flat heads. The principal types used are:

    • Flat plates and formed flat heads

    • Hemispherical heads

    • Ellipsoidal heads

    • Torispherical heads

    Hemispherical, ellipsoidal, and torispherical heads are collectively referred to as domed heads. They are formed by pressing or spinning; large diameters are fabricated from formed sections. Torispherical heads are often referred to as dished ends. Typical heads are shown in Fig.(7).

    Formed domed heads are made with a short, straight cylindrical section, called straight-flange or skirt.

  • Torispherical head: Inside corner radius is at least equal to 3 times the metal thickness; and in no case less than 6% of the inside diameter. Also the radius of the dish may be made equal or less than the diameter of the head.

    • Inside corner radius (\(r\))—referred to as the “knuckle radius”.

    • Crown radius (\(R\))—radius of dish.

    • If the radius of dish is greater than the shell outside diameter, the head is known as a “flanged and shallow dished head”. If the radius of dish is equal to or less than the outside diameter, the head is known as “flanged and standard dished”.

    • Vessels with flanged and shallow dished heads are primarily used for horizontal storage tanks.

  • Ellipsoidal head

    • If the ratio of major to minor axis is 2:1, the strength of the head is approximately equal to the strength of seamless cylindrical shell having the corresponding inside and outside diameters. For this reason, most manufacturers have standardized on elliptical dished heads having a 2:1 ratio of axis. The inside depth of the dish is half of the minor axis and is equal to one fourth of the inside diameter of the head.

    • The ellipsoidal head is more expensive to fabricate than the torispherical, but is thinner at the same pressure.

    • The diameter of a circular plate required for forming an ellipsoidal head is approximately 22% greater than the internal diameter of the finished vessel.

    • At pressures below 150 psig the torispherical head is generally the cheapest. From 150 psig to 500 psig, the ellipsoidal is usually selected.

  • Hemispherical head

    • For a given thickness, hemispherical heads are the strongest of the formed heads.

    • These heads can be used to resist approximately twice the pressure rating of an elliptical dished head or cylindrical shell of the same thickness and diameter.

    • If large access openings for man-ways or nozzles for piping are to be cut into formed closures, the hemispherical head may be used to advantage because its greater strength will make less reinforcement required.

  • Flat heads and formed flat heads Flat plates are used as covers for man-ways and as the channel covers of heat exchangers. Formed flat ends, known as flange-only ends, are manufactured by turning over a flange with a small radius on a flat plate. The corner radius reduces the abrupt change of shape, at the junction with the cylindrical section, which reduces the local stresses to some extent: flange-only heads are the cheapest type of formed head to manufacture, but their use is limited to low-pressure and small-diameter vessels.

  • During spinning of the heads, thinning out of the plate occurs at the corner radius. Therefore, for heads having an outside diameter of under 150 inch, plate thickness must be increased by 1/16 inch for plates up to 1 inch thickness and 1/8 inch for plates 1 to 2 inch thickness, if the minimum thickness is to be maintained throughout the corners.