Hemispherical closure, dished

In years past, the ellipsoidal dished head was so much more available than the hemispherical dished head that the hemispherical head was limited primarily to small-diameter vessels. But vessel closure manufacturers have increased the nimiber of dies stocked for hemispherical head production and these are now becoming more widely used. If it were not for the cost of forming, the engineer would usually choose the hemispherical closure for the top head of a tall vertical vessel since it has the least weight and the lowest discontinuity stresses and is therefore the strongest. 

The code provides design requirements for closures which are flat, elhpsoidal, spherically dished, hemispherical, conical (without transition knuckles), conic convex to pressure, toriconical concave to pressure, and toriconical convex to pressure. 

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. 

If the hemispherical dished head is not feasible, the ellipsoidal dished should be preferred over the torispherical dished head for tall vertical vessels, since the cost of the top closure is only a small part of the total cost of the vessel. For a battery of horizontal storage tanks or small vertical vessels, the torispherical dished head may be the more economical choice. Closures with conical, tori-conical, flanged and dished (not ASME) or flat shapes have no place in the design of tall vertical towers unless the process requires such special shapes for some particular reason. 

U he real ntwl for the use of formed clt)sures on cylindrical vessels artwe with the development of the power steam b< iler early in the nineteenth century. As a result of the frequent occurrence of boiler explosions, the British House of Commons in 1817 made the recommendation that the heads of cylindrical boilers be hemispherical (12). Since then a wide variety of formed closures termed heads have been developed, standardised, and extensively used in the fabrication of prowss pressure ve sls. The develppment of the thermal cracking process in the petroleum industry during the period from 1915 to 1930 resulted in the construction of thousands of prewure vessels with formed heads operating in the range of from 100 to 400 psi. The heads of t he rly vessels usually were of the torispherical-dish type with a small knuckle radius. 

Selection of Ellipticdl, Torisphericoi, and Hemispherical Dished Closures... 

STRESSES IN THE SHELL AT ITS JUNCTION WITH HEMISPHERICAL DISHED CLOSURES... 

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