Optimum Design of Multi-Layered Vessels

Multi-layered pressure vessels are widely used in the field of high pressure technology. To enhance their load bearing capacity and life, different beneficial processes such as shrink-fit and autofrettage are usually employed. Shrink-fit process, increases load capacity but maximum interference is generally limited. Autofrettage, makes steep stress gradients moving away from bore but Bauschinger effect limits maximum feasible compression level. A combination of both, can conceivably give better stress distribution in layered vessels. The optimum design of a three-layer vessel for maximum life expectancy has been considered here, under the combined effects of autofrettage and shrink-fit. The numerical optimization procedure known as the Simplex search method is employed to get the optimum design. The layer thicknesses, shrink-fit pressures, and autofrettage percentages are treated as design variables. Under stress constraints, the operational sequences of the above processes, for assembly of the layered vessel have also been formulated so as to lead to best results. The fatigue life consideration is based on ASME code and standard for high pressure vessel technology defining the allowable final crack depth in multi-layered vessels. The proposed procedure is carried out on a number of examples. The results show that significant life enhancement can be achieved using the optimization procedure with proper combination of operations.