A storage tank isolation system for a storage tank includes a vertical riser attached to a top of the storage tank and having a cover with an aperture and a connecting member extending from a side of the vertical riser. A distal end of the connecting member is parallel to the top of the storage tank and attached to a storage hatch. The storage tank isolation system includes a valve assembly, which includes a stem extending through the aperture in the cover, a valve plate attached to the stem, and a valve seat sealed to the vertical riser at a position below a connection to the connecting member. The valve plate is moves with the stem from an open position in which the valve plate is held within the vertical riser at a position equal to or above the connection to the connecting member and a closed position.