US Patent 7178907 Fluid containment structure with coiled bag backpressure regulator

A fluid containment structure includes a containment vessel having an interior fluid chamber for fluid containment. A flexible bag is disposed within the containment vessel; the bag is vented to the external atmosphere outside the containment vessel. A spring is coupled to the bag to hold the bag in a coiled state until a back-pressure within the fluid chamber exerts sufficient force to commence uncoiling the bag against the spring pressure, allowing air from the external atmosphere to enter the bag and enlarge an interior bag space which is sealed from the interior fluid chamber.