An anti-pressure system for a vessel which is subject to internal pressure created by heating or boiling of its contents, the system comprising first and second spaced-apart outlets for vapour to exit the vessel, each outlet having a first side communicating with the vessel interior and a second side communicating with the vessel exterior. The invention provides a reliable method of protecting a normally un-pressurized vessel, such as an autoclave, distillation vessel, boiler or the like, from dangerous pressure build up, resulting from its vapour outlet being blocked, regardless of what other pressure release devices (if any) may be present on the vessel.