Many industrial installations include pressure vessels which contain high pressure gas and particulate material. Given the rupture of such a vessel, it is established that provided the particulate acceleration is not greater than the acceleration of a detached endcap, the end-cap velocity will be similar to that generated by rupture of a gas filled vessel with the same available gas expansion energy. In contrast, if the particulates accelerate to overtake the end-cap, the momentum transfer resulting from the bombardment of the end-cap by the particulates generates a higher end-cap velocity. The latter can be similar to that generated when the rupturing vessel contains only high pressure gas and a consequent greater gas expansion energy than available when the vessel contains both high pressure gas and particulates. The velocity of a ＇rocket＇ missile resulting from failure of a gas-pressurised vessel containing particulate material is greater than that generated by failure of a similar vessel containing only high pressure gas.