Spherically dished head

In a true elliptical head the radii of curvature vary between adjacent points along a meridian. To simplify the calculations and fabrication, the ASME Code established the following various approximations. A 2 1 elliptical head can be assumed to consist of a spherically dished head with a radius of 90% and a knuckle radius of 17% of the shell diameter to which they are attached, as shown in Fig. 9.3. The smallest knuckle radius allowed for a hanged and dished head is 6% of the shell diameter and a spherical radius of 100% of the shell diameter. 

The equations for partial volumes of a liquid occupying spherically dished heads follow. 

FIGURE 2.15 Partial volume of a spherically dished head in the horizontal position. 

A spherically dished head with a 1 U-inch OD is spun from a l-inch plate. Determine the partial volume of liquid that is at the bottom portion of the head. The head is shown in Figure 2.16, with a liquid level 10.0 inches below the centerline. From the vessel head manufacturer s catalog, you can determine the following ... 

FIGURE 2.16 Spherically dished head in die hoizontal position showing the liquid level at 10.0 inches below the centerline. 

Although spherical vessels have a limited process application, the majority of pressure vessels are made with cylindrical shells. The heads may be flat if they are suitably buttressed, but preferably they are some curved shape. The more common types of heads are illustrated on Figure 18.16. Formulas for wall thicknesses are in Table 18.3. Other data relating to heads and shells are collected in Table 18.5. Included are the full volume V0 and surface S as well as the volume fraction V/V0 corresponding to a fractional depth H/D in a horizontal vessel. Figure 18.17 graphs this last relationship. For ellipsoidal and dished heads the formulas for V/V0 are not exact but are within 2% over the whole range. 

For torispherical (spherically dished) 0.885 PL, = SEj - 0.1 P + Cc For hemispherical head Same as for spherical shells with head n = La r a knuckle radius = 6% of inside crown radius and is not less than 3t... 

In addition to the hemispherical and ellipsoidal heads, the torispherical head has been used extensively for closures on a large variety of cylindrical vessels. It is shaped, not as an ellipsoid, but by the use of two radii. The crown radius is the radius of dish for the spherical crown which constitutes the major portion of the head, and the knuckle radius, sometimes referred to as the comer radius, is the radius joining the spherical crown to the cylindrical shell. Heads of this type require less forming than ellipsoidal dished heads, so forming costs are lower. 

The RPV assembly consists of a cylindrical shell, a spherically dished bottom head and a ring flange to which the upper closure head is bolted. The RPV and closure head are made as subassemblies from a combination of formed plates and forgings. 

In the case of an opening which is Ifwated entirely within the spherical portion of a torispherical dished head, the thickness to be used in Eq. 13.28 is calculated by Eq. 7.77 with a joint efficiency of 1.00 and a shape factor of 1.00. 

K = factor for ellipsoidal heads as determined from Eq. 9.6 L = spherical crown radius of flanged and dished heads L = effective length of cylindrical shell L, = effective length of conical section... 

Example 10.14. A spherically dished bead is to be bolted to the welding neck flange described in Example 10.9. The dished head is to be attached at the upper inside comer with the outside surface even with the ring s outside (see Fig. 10.23). What is the minimum required thickness of the flange ring when the spherical head is dished to a radius of L = 25 ... 

Dished or Basket Heads consist of a spherical segment normally dished to a radius equal to the inside diameter of the tank cylinder (or within a range of 6 inches plus or minus) and connected to the straight cylindrical flange by a knuckle whose inside radius is usually not less than 6 per cent of the inside diameter of the cylinder nor less than 3 times the thickness of the head plate. Basket heads closely approximate hemi-ellipsoidal heads. 

Figure 18.17. Types of heads for cylindrical pressure vessels, (a) Flat flanged KR = knuckle radius, SF= straight flange, (b) Torispherical (dished), (c) Ellipsoidal, (d) Spherical, (c) Conical, without knuckle, (f) Conical, with knuckle, (g) Nonstandard, one of many possible types in use.

Tanks may be constructed either in spherical or cylindrical shapes, although the cylindrical shape is more common. The most economical configuration is cylindrical with either dished, elliptical, or hemispherical heads. Orientation can be either horizontal or vertical. 

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