Investigation of Nozzle on Knuckle Region of Dished Head

Abstract

Abstract ASME Code Section VIII Division 1 [1] provides rules for the shape of openings, size of openings, strength and design of openings, however, the existing rules do not provide any restrictions on the specific location of the nozzle on the dished head knuckle region. Many corporate guidelines/ user design requirements meant for pressure vessel design and specification suggest avoiding placement of any type of nozzle in the knuckle area of a dished head and generally state in their design specification to limit the placement of a nozzle including its reinforcement within the crown area. This applies to Torispherical and Ellipsoidal dished heads. Code [1] rule UG-37(a) provides the benefit in reinforcement by reducing the required thickness (tr) of the dished head when the nozzle is in the spherical portion of the dished head for the Ellipsoidal and Torispherical dished head. High stresses occur in the knuckle region of the dished head due to the edge bending effect caused as the cylinder and head try to deform in different directions. For various reasons the user design requirements insist on placing the nozzle in the knuckle region, further compounding the complexity of the stress pattern in the knuckle area. The work carried out in this paper was an attempt to check whether it is safe to locate a nozzle in the knuckle region of the dished head since the knuckle portion is generally subjected to higher stresses in comparison to the crown portion of a dished head and the Code [1] and [2] does not impose any restrictions for the placement of nozzles in the knuckle region. Also, in this paper an attempt was made to evaluate the induced stresses when equivalent sizes of nozzles are placed in the crown as well as the knuckle portion of the dished head.