Effect of Imperfect Contact Between Adjacent Layers on the Integrity of Wrapped Vessels

Multilayer vessels are the designer’s solution to the inefficiency involved in operating monoblock vessels at pressures above 50 MPa. Multilayer wrapped vessels are commonly employed in high-pressure applications of up to 550 MPa. They are advantageous to monoblock cylindrical construction in several aspects, one of which is the prestressing of the layers in order to more efficiently distribute the stresses developed due to the inside pressure. Calculation of prestress in each layer, due to shrinkage of the weld, has been reported before. Nearly all such reports are based on the assumption that layers are in perfect contact with one another. In practice, however, occasions may arise, where due to imperfect rolling and/or variations in the thickness of a plate, there remains an isolated local gap between adjacent layers. In this investigation, the size of a gap is defined in terms of its circumferential arc length and maximum radial height. The effects of imperfect contact on prestresses are obtained, in terms of the size of the gap, by calculating the stresses sustained by the layer in order to bring the layer in perfect contact with the next layer. The foregoing task is achieved by deriving expressions for the stresses due to the closure of a gap as well as analysis by finite element approach. A method is proposed to calculate, for a given size gap, the necessary reduction in operating pressure in order to maintain the original factor of safety. Inversely, if the vessel must operate at the original design pressure, one can calculate the maximum allowable size of a gap (for a given percent reduction in factor of safety or strength) that would not affect the integrity of the vessel.