Three-dimensional analysis of the stress state of connection subassemblies between high-pressure units

Abstract

High-pressure units (vessels) are used in the chemical and petrochemical industry, and also in related branches. Various designs of these units are employed, depending on the purpose, operating conditions, and other requirements [1]. The internal excess pr~e, which may reach 100 MPa, is the determining load in the structural standards and computational methods developed for these units.The connection subassemblies on the housing or bottom of a high-pressure unit are some of its most critical structural components. The pipe connector is considered the reinforcing element of the hole; in that ease, the adapter between the pipe connector and the housing or bottom of the unit is a complex structural connection between shells that are usually thick-walled. An appreciably nonuniform stress state, frequently with a high level of stress concentration, which must be rather accurately determined to ensure the unit's required reliability, develops in the connection zone.The design of connection subassemblies, especially for a eylinclrieal housing, is classed among complex problems of the meehanies of a deformable body. Complete computational analysis of connection subassemblies is possible only with the use of numerical methods, the most general of which is a three-dimensional analysis of these connections; this analysis does, however, require use of an adequate applied procedure.Let us examine applied developments in the field of stress analysis in intersecting shells [2]. Three-dimeusional analysis of connection subassemblies can be performed using the finite-element method (FEM) in a mixed variational formulation [3]. This approach makes it possible to develop a highly economical and effective computational procedure for computational investigation of elastic stresses in connection subassemblies. General positions of the computational investigation of connections between intersecan~ shells are based on the following applied procedure of three-dimensional analysis [2]:--use of curvilinear coordinate systems linked to the surfaces of the shells; -use of a modified mixed variational formulation of the FEM with independent approximation of the displacement and strain fields to build effective finite-element models; and, -use of a rational computational algorithm that takes into account characteristic features of the geometry of the computed entity.The solving system of FEM equations for three-dimeusioual analysis assumes the form Ks = e, K= F= F', <1)where K, F, and 8 are the stiffness matrix, load vector, and vector of nodal displacements of the finite-element model of the connection, and ICe and F r are the stiffness matrix and vector of the equivalent nodal loading of the element. The characteristics of an element (K e, b "e) can be determined in the eurvilinear coordinate system of the corresponding shell (for example, in cylindrical coordinate systems for a connection subassembly on the housing of the unit). As compared with the traditional approach (use of a Cartesian coordinate system), this offers the following advan...