Cylindrical shells, stress attachments

From basic theoretical considerations for thin-walled vessels, the minimum thickness of a closure for a cylindrical shell is obtained when the closure has a hemispherical shape. In this case, the membrane theory predicts that, using the same construction materials and allowable stresses, a hemispherical closure needs to be only half as thick as the cylindrical shell to which it is attached. 

The geometry of the head is selected based on the function as well as on economic considerations, and methods of forming and space requirements. The elliptical and torispherical heads are most commonly used. The carbon steel hemispherical heads are not so economical because of the high manufacturing costs associated with them. They are thiimer than the cylindrical shell to which they are attached, and require a smooth transition between the two to avoid stress concentration effects. 

G. W. Wat Is and H. A. Lang (129) analyzed the stresses in a pressure vessel with a hemispherical head. The methcxl used was the same used by these authors for flal-plate closures and conical closures, described in (Chapter 6. Dimensionless equations having the form of Eqs. 6.113 and 6.114 were u.sed. However, for a hemispherical head attached lo a cylindrical shell, the coeflieieiit bi2 = = 0. 

One of the most common methods of supporting vertical pressure vessels is by means of a rolled cylindrical or conical shell called a skirt. The skirt can be either lap-, fillet-, or butt-welded directly to the vessel. This method of support is attractive from the designer s standpoint because it minimizes the local stresses at the point of attachment, and the direct load is uniformly distributed over the entire circumference. The use of conical skirts is more expensive from a fabrication standpoint, and unnecessary for most design situations. 

---