Hemispherical heads, external pressure

The Pressurizer is a pressure vessel of slim cylindrical shape with hemispherical heads. The pressure control function is carried out in the upper part, which is externally insulated to limit heat losses from the steam and hot water plena. 

External-pressure failure of shells can result from overstress at one extreme or n om elastic instability at the other or at some intermediate loading. The code provides the solution for most shells by using a number of charts. One chart is used for cylinders where the shell diameter-to-thickness ratio and the length-to-diameter ratio are the variables. The rest of the charts depic t curves relating the geometry of cyhnders and spheres to allowable stress by cui ves which are determined from the modulus of elasticity, tangent modulus, and yield strength at temperatures for various materials or classes of materials. The text of this subsection explains how the allowable stress is determined from the charts for cylinders, spheres, and hemispherical, ellipsoidal, torispherical, and conical heads. 

P = design internal pressure, psi P = allowable external pressure, psi Px = design external pressure, psi R = outside radius of spheres and hemispheres, crown radius of torispherical heads, in, t = thickness of cylinder, head or conical section, in. t, = equivalent thickness of cone, in. oc = half apex angle of cone, degrees... 

Formed t losures under external pn ssure ai( subject to failur l y elastic insUibility as are shells. Equation 1.33 applies in the case of hemispherical or torispherical heads and gives the theoretical pressure at which collapse wouhl occur because of elastic insUibility. 

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