Design Investigation into the Stress at the Base of a Nozzle of a Pressure Vessel

A pressure vessel typically consists of large cylindrical and / or spherical containers with nozzles through which the reactants flow in and out. . While plain cylindrical or spherical containers can be analyzed for internal pressure using thin/ thick cylinder formulae, the ones with nozzles are difficult to analyze. This is in view of complicated stress concentrations that arise at the interface of the nozzle and pressure vessel junction. The calculations have become complicated because of forces that arise at the free end of the nozzle. The forces include those of piping, wind forces, earth quake forces in addition to the internal pressure. In spite of these, strict adherence to safety codes is to be followed. ASME, Section VIII specifies the stress limits to be adhered to. One of the criterions is the stress intensity, which is not possible to compute by simple analytical procedures. FEM can be used for computing the deformation and stress at the nozzle-vessel junction in the structure and also at all other points on the pressure vessel. Quite often the geometric models are imported from CAD files for mesh generation with tetrahedral elements. Engineers generally use shell elements or tetrahedral elements while modeling the reactor vessels. But, precise estimation of stress intensity is not possible with these elements for a structure with nozzles. A method is developed for a precise structured modeling and for estimating the stress intensities at the junction of nozzles and pressure vessels. The structure of a reactor vessel of diameter 1900mm and length of 3600mm with a nozzle will be modeled and analyzed. The pressure load is 7 MPa. Stresses have been estimated. The induced stresses are compared with allowable stress. Based on the induced stress in the Pressure vessel three additional design variants have been studied to bring the stresses within allowable limits.