Optimum Design of Compound Cylinders Used for Storing Pressurized Fluids

In the classical design of thick cylinders, if the internal fluid pressure approaches the safe working stress limit of the material, the thickness of the cylinder approaches infinite value. To overcome this difficulty, compound cylinders are used, where another cylinder is shrinkfitted on the inner cylinder. Designing a shrink fit assembly is tricky because the stress developed in the cylinders is a function of internal fluid pressure, shrinkage pressure and the dimensions of the cylinders. Also the shrinkage pressure is a function of the amount of interference and dimensions of the cylinders. That is, unless the shrinkage pressure is known, stresses developed cannot be computed and to compute shrinkage pressure, dimensions of both the cylinders must be known. Hence a cumbersome trial and error method is to be used. In the optimum design of compound cylinders, the thickness of both the cylinders should be just sufficient to withstand the hoop stresses developed. That means the maximum hoop stress produced in both cylinders should be equal. In this, shrinkage (contact) pressure plays an important role. The shrinkage pressure can be such that the limiting compressive stress is produced in inner cylinder. But when subjected to internal pressure, causes unequal stresses in both cylinders. That is, in one of the cylinders, the stress level can be maximum allowable and in other, less than maximum allowable. This paper describes the method of determining the optimum dimensions of both the cylinders made of specified material and to withstand a specified internal pressure so that the volume (and weight) is minimum. The results obtained are verified by using COSMOS and ANSYS finite element analysis packages.