An ovoid flexible pressure vessel is described. At least one hollow pressure cell, formed of resilient material, a passageway, a valving means, a capillary tube, hoop winding, high-strength braiding material and at least one reinforcing ring are provided. The ovoid flexible pressure vessel has a pressure relief device comprising a reduction in thickness of the hollow pressure cell at a predetermined location whereby, when the hollow pressure cell is subjected to an overpressure condition it will fail at the predetermined location. Further pressure release devices include the following: a reduction in thickness of the cell, an indentation, a projecting member, a weakened section of the passageway, a weakening or an absence of high-strength braiding material or hoop winding at a predetermined location along the passageway, a weakening or spreading of fibers in either of the reinforcing panels or in either flexible blankets.