The present invention discloses a fluid relief device for relieving pressure of a fluid stored in a container or tank, or fluid used in a piece of equipment. In one aspect such a device has a body member with a primary channel therethrough in communication with a container of a contained fluid under pressure. When the pressure of the contained fluid under pressure exceeds a predetermined level, the pressure of the contained fluid moves a cover which is sealingly in place to close off the primary channel. During normal conditions the force of the contained fluid is insufficient to move the cover and leakage of the contained fluid past the cover is inhibited by the provision of a barrier fluid supplied into a barrier fluid chamber to oppose the contained fluid, the barrier fluid at a pressure greater than the normal pressure of the contained fluid. Seals may be provided on one or two sides of the barrier fluid chamber--a first seal to seal the primary channel-chamber interface and a second seal to seal the chamber-exterior interface. Fluid under pressure supplied to the chamber may leak past the first seal into the container. In one aspect an inert barrier fluid, e.g. nitrogen is used. Devices according to this invention may be used with pressure relief devices and vacuum relief devices.