Combination pressure-vacuum relief and antipollution valve

Abstract From the Canadian patent

A combination venting and antipollution device for a shipboard liquid storage tank. A closure wall devides the vent conduit into a lower tank chamber in direst communication with the vent opening. A spring-urged vlave closure member opens or closes an opening through the closure wall. Low pressure in the tank moves the valve closure member down into a lower open position, to admit air or other fluid to the tank; high pressure in the tank moves the valve closure member up into an upper open position to vent air from the tank. A hydrostatic pressure chamber open to the atmosphere houses a sealed bellows, and a spill-lock balve secured to bellows normally opens to the vent a spill-lock chamber …