A valve for relieving pressure from within a cargo tank through an opening in a cargo tank wall, which comprises:
- (a) a chamber housing coupled to the cargo tank at a position centrally disposed over the tank wall opening having a flow chamber extending axially throughout and having a first opening at its lower end in flow communication with the interior of the cargo tank and a second opening at its upper end in flow communication with the exterior of the cargo tank, wherein the first opening and the second opening define a first annular valve seat and a second annular valve seat, respectively;
- (b) a first vent head positioned for engagement with the first annular valve seat and movable between a normally closed position in sealing engagement with the first annular valve seat blocking flow of fluid past the first annular valve seat and an open position above the first annular valve seat enabling flow of fluid from the cargo tank past the first annular valve seat and into the flow chamber;
- (c) a second vent head positioned for engagement with the second annular valve seat and movable between a normally closed position in sealing engagement with the second annular valve seat blocking flow of fluid past the second annular valve seat and an open position above the second annular valve seat enabling flow of fluid from the flow chamber to the exterior of the cargo tank;
- (d) a means for yieldably biasing the first vent head toward the normally closed position to maintain a positive closure seal between the first vent head and the first annular valve seat, wherein the means provides a predetermined closing force which, when overcome by the pressure within the cargo tank, enables movement of the first vent head against the biasing force to the open position;
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- (e) a means for yieldably biasing the second vent head toward the normally closed position to maintain a positive closure seal between the second vent head and the second annular valve seat, wherein the means provides a predetermined closing force which, when overcome by the pressure within the flow chamber, enables movement of the second vent head against the biasing force to the open position.