These heads have a dish with a fixed radius (r1), the size of which depends on the type of torispherical head. The transition between the cylinder and the dish is called the knuckle. The knuckle has a toroidal shape.
Note: The inside crown radius to which an unstayed formed
head is dished shall be not greater than the outside diameter
of the skirt of the head. The inside knuckle radius of
a torispherical head shall be not less than 6% of the
outside diameter of the skirt of the head but in no case
less than three times the head thickness.
Torispherical Heads. The required thickness and the design pressure of a torispherical head is calculated by the following formulas:
The following equations apply:
P = Design Pressure
L = inside spherical or crown radius
S = maximum allowable stress value (provided from ASME BPVC Tables
HF-300.1 and HF-300.2