Comparative strength assessment for elliptical bottoms with different designs of coupling reinforcement

Abstract

The stress state of elliptical bottoms with centrally located branch pipes reinforced with a monolithic insert or cover ring is analyzed and experimentally investigated. Five zones of maximum stresses are exposed and examined in the designs employed for reinforcement of couplings. The advantage of the cover-ring design is established.Couplings, which are intended for the intake and discharge of a working medium and which differ in terms of dimensions and structural modification, are widely used structural elements of pressure vessels. According to regulatory documents in force [1,2], the reinforcing components of the coupling can be built in the form of a cover ring or monolithic insert (Fig. 1).In designing cylindrical housings or bottoms with couplings in accordance with effective standards based on the principle of area compensation, the opening cut into the wall of a shell is considered reinforced if the metal removed is compensated by supplementary metal of the branch pipe and housing [1][2][3][4]. Considering this, designs of couplings with a cover ring or monolithic insert are equivalent. The principle of area compensation is inadequate, however, to ensure cyclic or brittle strength of the design. The strength of a design with respect to stress state, which may differ considerably in designs with a cover ring and monolithic insert, is confirmed in accordance with modern standards [2,4,5]. And, although the zone where the ring is positioned is treated as a monolithic part when the stress state of an inlet coupling with a cover ring is analyzed in engineering practice, they are, in reality different structures, whose stress states should be calculated in accordance with different computational schemes.The company IrkutskNIIkhimmash performs stress-state analysis, and conducts experimental investigations and comparative analyses of the stress states of elliptical bottoms with central branch pipes built with two types of reinforcement -with a monolithic insert and with a cover ring -for different ratios of the branch-pipe and bottom diameters d/D.An elliptical bottom with an inside diameter of 450 mm, wall thickness of 6 mm, and height-diameter ratio of 0.25 was selected for numerical and experimental investigations of the stress state. Ratios d/D = 0.22, 0.33, and 0.44 were used to evaluate the level of stresses. In the experimental models, the bottoms were connected in pairs via a cylindrical insert.