Abstract: A gauge hatch is provided that has a cover structure that incorporates a pallet/diaphragm combination and that is cooperatively associated with a pressure responsive liquid seal subassembly. The gauge hatch also incorporates a vent neck and a radially adjacent wall that defines with the vent neck a circumferentially extending reservoir. An exterior preferably radially extending arm permits the cover structure to be opened and inclined at an angle relative to the vent neck. Optionally, the cover may be kept in a closed position. The diaphragm normally seats upon the vent neck. A sealing liquid in the reservoir has a closed chamber over the reservoir liquid. When the arm is not held in a cover closed position, and up until a set vent neck pressure is reached, the liquid seal subassembly eliminates low level fugitive vapor emissions escaping from the diaphragm/seat region.

Abstract: A vent valve is provided wherein a pallet/diaphragm type valve is cooperatively and coactively associated with a pressure responsive liquid seal. The assembly eliminates low level fugitive vapor emissions escaping from the diaphragm/seat region up to a predeterminable liquid seal set pressure which preferably is less than or equal to the set pressure for diaphragm opening. A liquid reservoir with a sealed chamber is provided for the liquid seal subassembly. Fugitive emissions from a tank or the like that is associated with the vent valve enter the sealed chamber and are there contained until the pressure therein is sufficient to displace the sealing liquid to an extent sufficient to open the sealed chamber. The valve assembly can be used as a pressure vent, as a vacuum vent, and as a combination pressure and vacuum vent. Optional features, such as a constant liquid level controller, a sight glass, a signaling device, or the like can be provided.